JP2007086817A - Image formation apparatus and image formation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce detection errors and to appropriately set density control factors in a density control technique by which the density of a toner image formed as a patch image is detected and a density control is performed based on the detected result. <P>SOLUTION: The density control factor is optimized based on a variation rate of the patch image densities against a varied density control factor. The detected results of the patch image densities are corrected based on information on an image carrier acquired before the formation of the patch images. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for stabilizing image density in an electrophotographic image forming apparatus such as a printer, a copying machine, and a facsimile machine.

  In image forming apparatuses such as copiers, printers, and facsimile machines that apply electrophotographic technology, the image density of the toner image differs due to individual differences, changes over time, and changes in the surrounding environment such as temperature and humidity. There is. In view of this, various techniques for stabilizing the image density have been proposed. As such a technique, for example, a test small image (patch image) is formed on an image carrier, and a density control factor that affects the density of the image is optimized based on the density of the patch image. There is. This technique forms a predetermined toner image on an image carrier while variously changing and setting a density control factor, and also uses the toner image on the image carrier or another transfer medium such as an intermediate transfer medium. The image density is detected using the toner image transferred to the patch image as the patch image, and a desired image density is obtained by adjusting the density control factor so that the patch image density matches the preset target density. It is what.

  Various techniques for measuring the density of a patch image (hereinafter referred to as “patch sensing technique”) have been proposed in the past, but the technique using optical means is the most common. That is, the surface area of the image carrier or transfer medium on which the patch image is formed is irradiated with light, and the light reflected or transmitted from the surface area is received by an optical sensor, and the patch image density is obtained based on the light quantity. ing.

  In an image forming apparatus that adjusts the density control factor based on the patch image density, the density of the formed patch image is accurately detected in order to obtain a good quality image by appropriately setting the density control factor. How to do is an important issue. However, in the conventional patch sensing technology described above, the density of the formed image is not directly measured, but from a toner image temporarily carried on the surface of the image carrier or transfer medium as a patch image. Since the amount of emitted light is detected and the image density is only indirectly estimated from the detection result, the sensor output may not necessarily reflect the final image density correctly. In addition, there may be a gap between the sensor output and the final image density due to variations in sensor characteristics and detection errors.

  Further, when the image density of a toner image formed on an image carrier such as a photosensitive member or a transfer medium is measured by a density sensor as described above, the measurement result is simply the amount of toner attached to the image carrier. Rather than being determined, the measurement result may vary depending on the surface state of the image carrier, such as reflectivity or surface roughness. For example, when the surface color of the image carrier changes as the cumulative number of printed sheets of the image forming apparatus increases, the output from the density sensor fluctuates according to the change in the surface color even if the toner adhesion amount is the same. Thus, accurate concentration measurement becomes difficult. Further, when the surface state of the image carrier is not uniform, the influence of the surface state cannot be ignored.

  Thus, if the sensor output does not correctly reflect the final image density, the density control factor is adjusted based on the image density that is erroneously estimated from the sensor output. As a result, the concentration control factor is set to be out of the optimum value. In particular, in the state where the toner is attached at a relatively high density as in the case of forming a solid image, for example, the final change in the image density is small with respect to the increase / decrease of the toner adhesion amount, so that the sensor output is slightly shifted. Even if it exists, the value of the density control factor that is set changes greatly. As a result, the density control factor is set to a state that is significantly different from the optimum value, and the image quality is impaired. There was a case where a malfunction occurred.

  For example, in the case of a high density image such as a solid image, when the image density obtained from the sensor output is estimated to be lower than the actual image density, the apparatus adjusts the density control factor to further increase the image density. As a result, an excessive toner adhesion amount may cause transfer / fixing failure, or the toner consumption amount may be abnormally increased. In addition, by repeating image formation under conditions where the toner adhesion amount becomes higher than necessary, the history of previous image formation may affect the image to be formed later, and the life of the apparatus may be significantly shortened. There is.

  Furthermore, since the image density of the patch image to be formed is determined by a combination of various factors, in order to individually optimize a plurality of density control factors that affect the image density based on the image density. Complex processing is required. For this reason, the conventional density control technique has problems such as an increase in apparatus cost due to such complicated operations, and a long processing time and a reduction in image formation throughput. Therefore, establishment of a technique capable of optimizing the concentration control factor with a simpler method and surely is desired.

  An object of the present invention is to provide an image forming apparatus and an image forming method capable of stably forming a toner image having good image quality by setting an appropriate density control factor based on the image density of the toner image. It is.

  In order to achieve the above object, according to the present invention, prior to obtaining the image density of the toner image on the image carrier, information relating to the image carrier is stored in advance as correction information to obtain the image density of the toner image. In this case, the output from the density sensor is not used as it is to obtain the image density, but the sensor output is corrected by the correction information. As a result, the influence of the surface state of the image carrier is canceled, and a correction value reflecting only the image density of the toner image is obtained. Then, by obtaining the image density of the toner image based on the correction value, the image density of the toner image can be measured with high accuracy, and an image can be formed with a stable density based on the measurement result.

  Further, the influence of the surface state of the image carrier on the output from the density sensor varies depending on the density of the toner image formed on the image carrier as will be described later. That is, when a relatively low density toner image is formed on the image carrier, a part of the light from the light emitting element passes through the toner image and is reflected by the image carrier, and then the image carrier again. Since the light passes through the body and is received by the light receiving element, the output from the density sensor varies relatively greatly depending on the surface state of the image carrier. On the other hand, not only light that passes through the toner image and enters the image carrier as the toner image becomes darker, but also light that passes through the image carrier again after being reflected by the image carrier and enters the light receiving element. As a result, the influence of the surface state of the image carrier on the output from the density sensor is reduced. Therefore, if the image density of the toner image is uniformly determined based on the correction information without considering the density of the toner image, there is a certain limit in the accuracy. On the other hand, the measurement accuracy of the image density is further improved by correcting the correction information according to the density of the toner image on the image carrier as in the present invention.

  Here, the correction information may be obtained based on a signal output from the density sensor before the toner image is formed on the image carrier, and the correction information thus obtained may be stored in the storage unit. . In obtaining correction information, the sampling data itself that constitutes the signal output from the density sensor before forming the toner image on the image carrier may be used as correction information. May overlap. In order to remove such spike noise, it is effective to cancel, for example, the upper level and / or the lower level of the sampling data and replace the canceled data with the average value of the remaining sampling data.

  Further, as the toner image becomes darker as described above, the influence of the surface state of the image carrier on the output from the density sensor is reduced, so that the correction amount based on the correction information is reduced as the toner image becomes darker. Therefore, the image density of the toner image can be obtained with high accuracy.

<First Embodiment>
(1) Configuration of Apparatus FIG. 1 is a diagram showing a first embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This image forming apparatus forms a full color image by superposing four color toners of yellow (Y), cyan (C), magenta (M), and black (K), or uses only black (K) toner. This is an apparatus for forming a monochrome image. In this image forming apparatus, when an image signal is given to the main controller 11 from an external device such as a host computer in response to an image formation request from a user, the engine controller 10 causes the engine unit EG to respond to a command from the main controller 11. Are controlled to form an image corresponding to the image signal on the sheet S.

  In the engine section EG, the photosensitive member 2 is provided so as to be rotatable in the arrow direction d1 in FIG. A charging unit 3, a rotary developing unit 4, and a cleaning unit 5 are arranged around the photosensitive member 2 along the rotational direction d1. The charging unit 3 is applied with a charging bias from the charging controller 103 and uniformly charges the outer peripheral surface of the photoreceptor 2 to a predetermined surface potential.

  Then, the light beam L is irradiated from the exposure unit 6 toward the outer peripheral surface of the photosensitive member 2 charged by the charging unit 3. The exposure unit 6 exposes the light beam L onto the photoconductor 2 in accordance with a control command given from the exposure control unit 102 to form an electrostatic latent image corresponding to the image signal on the photoconductor 2. For example, when an image signal is given from an external device such as a host computer to the CPU 111 of the main controller 11 via the interface 112, the CPU 101 of the engine controller 10 sends a control signal corresponding to the image signal to the exposure control unit 102 at a predetermined timing. In response to this, a light beam L is irradiated onto the photosensitive member 2 from the exposure unit 6, and an electrostatic latent image corresponding to the image signal is formed on the photosensitive member 2. When forming a patch image, which will be described later, as necessary, a control signal corresponding to a predetermined pattern of patch image signals is given from the CPU 101 to the exposure control unit 102, and the electrostatic image corresponding to the pattern is output. A latent image is formed on the photoreceptor 2.

  The electrostatic latent image thus formed is developed with toner by the developing unit 4. In other words, in this embodiment, the developing unit 4 is configured to be detachably attached to the support frame 40 that is rotatably provided about the shaft center, a rotation drive unit (not shown), and the support frame 40, and each color toner. Is provided with a yellow developing unit 4Y, a cyan developing unit 4C, a magenta developing unit 4M, and a black developing unit 4K. The developing unit 4 is controlled by the developing device controller 104 as shown in FIG. Based on a control command from the developing device control unit 104, the developing unit 4 is driven to rotate, and the developing devices 4Y, 4C, 4M, and 4K selectively face the photoconductor 2 at a predetermined developing position. The toner of the selected color is applied to the surface of the photoreceptor 2. As a result, the electrostatic latent image on the photoreceptor 2 is visualized with the selected toner color. FIG. 1 shows a state in which the developing device 4Y for yellow is positioned at the developing position.

  These developing units 4Y, 4C, 4M, and 4K all have the same structure. Therefore, the configuration of the developing device 4K will be described in more detail with reference to FIG. 3, but the structures and functions of the other developing devices 4Y, 4C, and 4M are the same.

  FIG. 3 is a cross-sectional view showing a developing device of the image forming apparatus. In the developing device 4K, a supply roller 43 and a developing roller 44 are axially attached to a housing 41 that accommodates the toner TN. When the developing device 4K is positioned at the developing position, the developing roller 44 is While being in contact with the photosensitive member 2 or opposed to each other with a predetermined gap, these rollers 43 and 44 are engaged with a rotation driving unit (not shown) provided on the main body side and rotated in a predetermined direction. To do. The developing roller 44 is formed in a cylindrical shape from a metal or alloy such as copper, aluminum, iron, and stainless steel so that a developing bias described later is applied. These materials are appropriately subjected to surface treatment (for example, oxidation treatment, nitridation treatment, blast treatment, etc.). Then, the two rollers 43 and 44 rotate while being in contact with each other, whereby the black toner is rubbed against the surface of the developing roller 44 and a toner layer having a predetermined thickness is formed on the surface of the developing roller 44.

  In the developing device 4K, a regulating blade 45 for regulating the thickness of the toner layer formed on the surface of the developing roller 44 to a predetermined thickness is disposed. The regulation blade 45 is composed of a plate-like member 451 such as stainless steel or phosphor bronze and an elastic member 452 such as rubber or resin member attached to the tip of the plate-like member 451. The rear end portion of the plate member 451 is fixed to the housing 41, and the elastic member 452 attached to the front end portion of the plate member 451 is a rear end portion of the plate member 451 in the rotation direction d3 of the developing roller 44. It arrange | positions so that it may be located in the upstream rather than. Then, the elastic member 452 elastically contacts the surface of the developing roller 44, and the toner layer formed on the surface of the developing roller 44 is finally restricted to a predetermined thickness.

  Each toner particle constituting the toner layer on the surface of the developing roller 44 is charged by being rubbed with the supply roller 43 and the regulating blade 45, and here, the toner will be negatively charged. Toner that is positively charged by appropriately changing the potential of each part of the apparatus can also be used.

  The toner layer formed on the surface of the developing roller 44 in this way is sequentially conveyed to a position facing the photoreceptor 2 on which the electrostatic latent image is formed by the rotation of the developing roller 44. When the developing bias from the developing device controller 104 is applied to the developing roller 44, the toner carried on the developing roller 44 partially adheres to each surface portion of the photoreceptor 2 according to the surface potential. Thus, the electrostatic latent image on the photoreceptor 2 is visualized as a toner image of the toner color.

  As the developing bias applied to the developing roller 44, a DC voltage or a voltage obtained by superimposing an AC voltage on the DC voltage can be used. In particular, the photosensitive member 2 and the developing roller 44 are spaced apart from each other, and toner is supplied between them. In a non-contact development type image forming apparatus that develops toner by flying, in order to fly toner efficiently, a voltage waveform in which an AC voltage such as a sine wave, a triangular wave, or a rectangular wave is superimposed on a DC voltage is used. Is preferred. The magnitude of the DC voltage and the amplitude, frequency, duty ratio, etc. of the AC voltage are arbitrary. Hereinafter, in the present specification, the DC will be applied regardless of whether the developing bias has an AC component. The component (average value) is referred to as a direct current developing bias Vavg.

  Here, an example of a preferable developing bias in the non-contact developing type image forming apparatus will be shown. For example, the waveform of the development bias is obtained by superimposing a rectangular wave AC voltage on a DC voltage, and the frequency of the rectangular wave is 3 kHz and the amplitude Vpp is 1400V. As will be described later, in the present embodiment, the developing bias Vavg can be changed as one of the density control factors. However, the variable range takes into consideration the influence on the image density, the characteristic variation of the photoconductor 2, and the like. Thus, for example, (−110) V to (−330) V can be set. These numerical values and the like are not limited to the above, and should be appropriately changed according to the apparatus configuration.

  Further, as shown in FIG. 2, each of the developing devices 4Y, 4C, 4M, and 4K is provided with memories 91 to 94 for storing data relating to the manufacturing lots and usage histories of the developing devices, characteristics of the built-in toner, and the like. Yes. Further, connectors 49Y, 49C, 49M, and 49K are provided in the developing devices 4Y, 4C, 4M, and 4K, respectively. If necessary, these are selectively connected to the connector 108 provided on the main body side, and data is transmitted and received between the CPU 101 and each of the memories 91 to 94 via the interface 105 to relate to the developing device. It manages various information such as consumables management. In this embodiment, the main body side connector 108 and each developing device side connector 49Y and the like are mechanically fitted to each other to exchange data. However, for example, electromagnetic means such as wireless communication is used. Data transmission / reception may be performed without contact. The memories 91 to 94 for storing data unique to each of the developing devices 4Y, 4C, 4M, and 4K are nonvolatile memories that can store the data even when the power is off or the developing device is removed from the main body. As such a nonvolatile memory, for example, a flash memory, a ferroelectric memory, an EEPROM, or the like can be used.

  Returning to FIG. 1, the description of the device configuration will be continued. The toner image developed by the developing unit 4 as described above is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TR1. The transfer unit 7 includes an intermediate transfer belt 71 stretched between a plurality of rollers 72 to 75, and a drive unit (not shown) that rotates the intermediate transfer belt 71 in a predetermined rotational direction d2 by rotationally driving the roller 73. It has. Further, a secondary transfer roller 78 is provided at a position facing the roller 73 with the intermediate transfer belt 71 interposed therebetween, and configured to be able to contact and separate with respect to the surface of the belt 71 by an electromagnetic clutch (not shown). Yes. When a color image is transferred to the sheet S, the color toner images formed on the photoreceptor 2 are superimposed on the intermediate transfer belt 71 to form a color image, and the color image is taken out from the cassette 8 to be intermediate. The color image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2 between the transfer belt 71 and the secondary transfer roller 78. Further, the sheet S on which the color image is formed in this way is conveyed via the fixing unit 9 to a discharge tray portion provided on the upper surface portion of the apparatus main body.

  Note that the surface potential of the photoreceptor 2 after the toner image is primarily transferred to the intermediate transfer belt 71 is reset by a neutralizing unit (not shown), and the toner remaining on the surface is removed by the cleaning unit 5. Then, the charging unit 3 receives the next charging.

  If further images need to be subsequently formed, the above operation is repeated to form the required number of images, the series of image forming operations is completed, and the apparatus remains in a standby state until a new image signal is given. However, in this apparatus, the operation is shifted to the stop state in order to suppress the power consumption in the standby state. That is, the rotation of the photosensitive member 2, the developing roller 44, the intermediate transfer belt 71 and the like is stopped, and the operation of the apparatus is stopped by stopping the application of the developing bias to the developing roller 44 and the charging bias to the charging unit 3. It becomes a state.

  Further, a cleaner 76, a density sensor 60, and a vertical synchronization sensor 77 are disposed in the vicinity of the roller 75. Among these, the cleaner 76 can be moved toward and away from the roller 75 by an electromagnetic clutch (not shown). Then, the blade of the cleaner 76 abuts on the surface of the intermediate transfer belt 71 that is stretched over the roller 75 while moving to the roller 75 side, and the toner that remains on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer. Remove. The vertical synchronization sensor 77 is a sensor for detecting the reference position of the intermediate transfer belt 71, and is used to obtain a synchronization signal output in association with the rotational driving of the intermediate transfer belt 71, that is, a vertical synchronization signal Vsync. Functions as a vertical sync sensor. In this apparatus, the operation of each part of the apparatus is controlled based on the vertical synchronization signal Vsync in order to align the operation timing of each part and accurately superimpose the toner images formed in the respective colors. Further, the density sensor 60 is provided so as to face the surface of the intermediate transfer belt 71 and is configured as described later to measure the optical density of the patch image formed on the outer peripheral surface of the intermediate transfer belt 71. Therefore, in this embodiment, the intermediate transfer belt 71 corresponds to the “image carrier” of the present invention.

  In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing an image signal given from an external device such as a host computer via the interface 112, and reference numeral 106 is executed by the CPU 101. A ROM for storing calculation data, control data for controlling the engine unit EG, and the like, and a reference numeral 107 are RAMs for temporarily storing calculation results in the CPU 101 and other data.

  FIG. 4 is a diagram showing the configuration of the density sensor. The density sensor 60 includes a light emitting element 601 such as an LED for irradiating light to a winding area 71 a wound around a roller 75 in the surface area of the intermediate transfer belt 71. Further, the density sensor 60 includes a polarization beam splitter 603, an irradiation light amount monitoring light receiving unit 604, and an irradiation light amount in order to adjust the irradiation light amount of the irradiation light in accordance with the light amount control signal Slc provided from the CPU 101 as will be described later. An adjustment unit 605 is provided.

  As shown in FIG. 4, the polarization beam splitter 603 is disposed between the light emitting element 601 and the intermediate transfer belt 71, and the light emitted from the light emitting element 601 is incident on the intermediate transfer belt 71. It is divided into p-polarized light having a polarization direction parallel to the plane and s-polarized light having a perpendicular polarization direction. The p-polarized light enters the intermediate transfer belt 71 as it is, while the s-polarized light is extracted from the polarization beam splitter 603 and then incident on the light receiving unit 604 for monitoring the amount of irradiated light. A signal proportional to the irradiation light amount is output to the irradiation light amount adjustment unit 605.

  The irradiation light quantity adjustment unit 605 irradiates the intermediate transfer belt 71 from the light emitting element 601 by feedback controlling the light emitting element 601 based on the signal from the light receiving unit 604 and the light quantity control signal Slc from the CPU 101 of the engine controller 10. The irradiation light quantity is adjusted to a value corresponding to the light quantity control signal Slc. Thus, in this embodiment, the irradiation light quantity can be changed and adjusted in a wide range and appropriately.

  Further, in this embodiment, the input offset voltage 641 is applied to the output side of the light receiving element 642 provided in the light receiving unit 604 for monitoring the irradiation light quantity, and the light emitting element as long as the light quantity control signal Slc does not exceed a certain signal level. 601 is configured to be kept off. The specific electrical configuration is as shown in FIG.

FIG. 5 is a diagram showing an electrical configuration of the light receiving unit 604 employed in the density sensor 60 of FIG. In this light receiving unit 604, the anode terminal of the light receiving element PS such as a photodiode is connected to the non-inverting input terminal of the operational amplifier OP constituting the current-voltage (I / V) conversion circuit, and grounded via the offset voltage 641. Connected to potential. The cathode terminal of the light receiving element PS is connected to the inverting input terminal of the operational amplifier OP, and is connected to the output terminal of the operational amplifier OP via the resistor R. Therefore, when light is incident on the light receiving element PS and the photocurrent i flows, the output voltage VO from the output terminal of the operational amplifier OP is
VO = i ・ R + Voff (1-1)
(However, Voff is the offset voltage value)
Thus, a signal corresponding to the amount of reflected light is output from the light receiving unit 604. The reason for this configuration will be described below.

  FIG. 6 is a diagram showing light quantity control characteristics in the density sensor of FIG. When the input offset voltage 641 is not applied, the light quantity characteristic as shown by the broken line in FIG. 6 is shown. That is, when the light quantity control signal Slc (0) is given from the CPU 101 to the irradiation light quantity adjustment unit 605, the light emitting element 601 is turned off. When the signal level of the light quantity control signal Slc is increased, the light emitting element 601 is turned on and the intermediate transfer belt. The amount of light irradiated on 71 also increases in proportion to the signal level. However, the light quantity characteristic may be shifted in parallel like the one-dot chain line or the two-dot chain line shown in FIG. 6 depending on the influence of the ambient temperature, the configuration of the irradiation light quantity adjustment unit 605, and the like. In this case, the light emitting element 601 may be lit even though the CPU 101 gives a turn-off command, that is, a light amount control signal Slc (0).

  On the other hand, as in the present embodiment, a dead zone (signal levels Slc (0) to Slc (1)) is provided by applying the input offset voltage 641 and shifting it in advance to the right-hand side in FIG. (Solid line) can be reliably turned off by giving a turn-off command from the CPU 101, that is, a light quantity control signal Slc (0), and malfunction of the apparatus can be prevented beforehand.

  On the other hand, when the light amount control signal Slc exceeding the signal level Slc (1) is supplied from the CPU 101 to the irradiation light amount adjustment unit 605, the light emitting element 601 is turned on, and the intermediate transfer belt 71 is irradiated with p-polarized light as irradiation light. Then, the p-polarized light is reflected by the intermediate transfer belt 71, and the reflected light amount detection unit 607 detects the p-polarized light amount and the s-polarized light amount among the light components of the reflected light, and a signal corresponding to each light amount is sent to the CPU 101. Is output.

  As shown in FIG. 4, the reflected light amount detection unit 607 receives the polarized beam splitter 671 disposed on the optical path of the reflected light and the p-polarized light passing through the polarized beam splitter 671, and corresponds to the light amount of the p-polarized light. And a light receiving unit 670s that receives the s-polarized light divided by the polarization beam splitter 671 and outputs a signal corresponding to the light quantity of the s-polarized light. In the light receiving unit 670p, the light receiving element 672p receives the p-polarized light from the polarization beam splitter 671, amplifies the output from the light receiving element 672p by the amplifier circuit 673p, and then the amplified signal is a signal corresponding to the light quantity of the p-polarized light. Is output from the light receiving unit 670p. Similarly to the light receiving unit 670p, the light receiving unit 670s includes a light receiving element 672s and an amplifier circuit 673s. For this reason, the light quantity of two different component light (p polarized light and s polarized light) among the light components of reflected light can be calculated | required independently.

  In this embodiment, output offset voltages 674p and 674s are applied to the output sides of the light receiving elements 672p and 672s, respectively. As shown, it is offset to the plus side.

  FIG. 7 is a graph showing how the output voltage changes with respect to the amount of reflected light in the density sensor of FIG. Since the specific electrical configuration of each of the light receiving units 670p and 670s is the same as that of the light receiving unit 604, the description thereof is omitted here. In the light receiving units 670p and 670s configured as described above, similarly to the light receiving unit 604, even when the amount of reflected light is zero, the output voltages Vp and Vs have values of zero or more and are reflected. The output voltages Vp and Vs also increase in proportion to the increase in the amount of light. By applying the output offset voltages 674p and 674s in this way, the influence of the dead zone of FIG. 6 can be surely eliminated, and an output voltage corresponding to the amount of reflected light can be output.

  The signals of these output voltages Vp and Vs are input to the CPU 101 via an A / D conversion circuit (not shown), and the CPU 101 sets these output voltages Vp and Vs as needed at predetermined time intervals (this embodiment). In this case, sampling is performed every 8 msec). Then, at an appropriate timing, for example, when the apparatus power is turned on or immediately after any unit is replaced, the CPU 101 optimizes the density control factor that affects the image density such as the developing bias and exposure energy. To stabilize the image density. More specifically, an image forming operation is executed while changing the above-described density control factor in multiple stages for each toner color using image data stored in advance in the ROM 106 corresponding to a predetermined patch image pattern as an image signal. Then, a small test image (patch image) corresponding to the image signal is formed, the image density is detected by the density sensor 60, and a condition for obtaining a desired image density is found based on the result. Hereinafter, the optimization process of the concentration control factor will be described.

(2) Optimization Processing FIG. 8 is a flowchart showing an outline of concentration control factor optimization processing in this embodiment. This optimization processing is performed in the following six sequences in the order of processing: initialization operation (step S1); pre-operation (step S2); derivation of control target value (step S3); development bias setting (step S4); exposure energy It consists of setting (step S5) and post-processing (step S6), and the details of the operation will be described below for each of the above sequences.

(A) Initialization Operation FIG. 9 is a flowchart showing the initialization operation in this embodiment. In this initialization operation, as a preparatory operation (step S101), the developing unit 4 is rotationally driven and positioned at a so-called home position, and the cleaner 76 and the secondary transfer roller 78 are separated from the intermediate transfer belt 71 by an electromagnetic clutch. Move to. Then, in this state, the driving of the intermediate transfer belt 71 is started (step S102), and then the photosensitive member 2 is started by starting the rotational driving and neutralization operation of the photosensitive member 2 (step S103).

  When the vertical synchronization signal Vsync indicating the reference position of the intermediate transfer belt 71 is detected and its rotation is confirmed (step S104), predetermined bias application is started to each part of the apparatus (step S105). That is, the charging control unit 103 applies a charging bias to the charging unit 3 to charge the photosensitive member 2 to a predetermined surface potential, and subsequently, a predetermined primary transfer from a bias generation unit (not shown) to the intermediate transfer belt 71 is performed. Apply a bias.

  From this state, the intermediate transfer belt 71 is cleaned (step S106). That is, the cleaner 76 is brought into contact with the surface of the intermediate transfer belt 71, and in this state, the intermediate transfer belt 71 is rotated almost once to remove the toner and dirt remaining on the surface. Then, the secondary transfer roller 78 to which the cleaning bias is applied is brought into contact with the intermediate transfer belt 71. This cleaning bias has a polarity opposite to that of the secondary transfer bias applied to the secondary transfer roller 78 during execution of a normal image forming operation. Therefore, the toner remaining on the secondary transfer roller 78 remains on the intermediate transfer belt 71. The surface moves to the surface, and is further removed from the surface of the intermediate transfer belt 71 by the cleaner 76. When the cleaning operation of the intermediate transfer belt 71 and the secondary transfer roller 78 is thus completed, the secondary transfer roller 78 is separated from the intermediate transfer belt 71 and the cleaning bias is turned off. Then, after waiting for the next vertical synchronization signal Vsync (step S107), the charging bias and the primary transfer bias are turned off (step S108).

  In this embodiment, the CPU 101 is not limited to executing the optimization process of the concentration control factor, but allows the CPU 101 to execute this initialization operation independently of other processes as necessary. That is, when the next operation is subsequently executed (step S109), the initialization operation is terminated in a state where the above steps S108 are executed, and the next operation is started. On the other hand, when the next operation is not scheduled, as a stop process (step S110), the cleaner 76 is separated from the intermediate transfer belt 71, and the neutralization operation and the rotation drive of the intermediate transfer belt 71 are stopped. In this case, the intermediate transfer belt 71 is desirably stopped in a state where the reference position is located immediately before the position facing the vertical synchronization sensor 77. This is because when the intermediate transfer belt 71 is rotationally driven in the subsequent operation, the rotational state is confirmed by the vertical synchronization signal Vsync. However, if the above operation is performed, the vertical synchronization signal Vsync is detected immediately after the start of driving. This is because the presence or absence of an abnormality can be determined in a short time depending on whether or not it is performed.

(B) Pre-Operation FIG. 10 is a flowchart showing the pre-operation in this embodiment. In this pre-operation, two processes are simultaneously performed as a pre-process prior to the formation of a patch image described later. In other words, in order to perform the optimization process of the density control factor with high accuracy, each of the developing units 4Y, 4C, 4M, and 4K is provided in parallel with the adjustment of the operation condition of each part of the apparatus (pre-operation 1). Further, the idling process (pre-operation 2) of the developing roller 44 is performed.

(B-1) Operation condition setting (pre-operation 1)
In the left flow (pre-operation 1) shown in FIG. 10, the density sensor 60 is first calibrated (steps S21a and S21b). In calibration (1) in step S21a, the output voltages Vp and Vs of the light receiving units 670p and 670s when the light emitting element 601 of the density sensor 60 is in the off state are detected and stored as dark outputs Vp0 and Vs0. Next, in the calibration (2) of step S21b, the light amount control signal Slc given to the light emitting element 601 is changed so that two kinds of lighting states of low light amount and high light amount are changed, and the output of the light receiving unit 670p with each light amount. The voltage Vp is detected. From these three values, the output voltage Vp in a state where the toner is not attached becomes a predetermined reference level (in this embodiment, the value obtained by adding the above-described dark output Vp0 to 3V). Find the reference light intensity. Thus, the level of the light amount control signal Slc is calculated so that the light amount of the light emitting element 601 becomes the reference light amount, and the value is set as the reference light amount control signal (step S22). Thereafter, when it is necessary to turn on the light emitting element 601, the CPU 101 outputs this reference light amount control signal to the irradiation light amount adjustment unit 605, so that the light emitting element 601 is always feedback controlled to emit light with the reference light amount. The

  Further, the output voltages Vp0 and Vs0 when the light emitting element 601 is in the extinguished state are stored as “dark output” of the present sensor system, and each output voltage Vp, By subtracting this value from Vs, it is possible to eliminate the influence of the dark output and detect the density of the toner image with higher accuracy.

  The output signal from the light receiving element 672p when the light emitting element 601 is turned on depends on the amount of light reflected from the intermediate transfer belt 71, but the surface state of the intermediate transfer belt 71 is not necessarily optically uniform as will be described later. Therefore, when obtaining the output in this state, it is desirable to take the average value of the output over one turn of the intermediate transfer belt 71. On the other hand, it is not necessary to detect the output signal for one rotation of the intermediate transfer belt 71 as described above when the light emitting element 601 is turned off, but in order to reduce the detection error, the output signals at several points are averaged. preferable.

  In this embodiment, since the surface of the intermediate transfer belt 71 is white, the reflectance of light is high, and when any color toner adheres to the belt 71, the reflectance decreases. Therefore, in this embodiment, the output voltages Vp and Vs from the light receiving unit decrease from the reference level as the toner adhesion amount on the surface of the intermediate transfer belt 71 increases, and the magnitudes of these output voltages Vp and Vs. Therefore, it is possible to estimate the toner adhesion amount, and hence the image density of the toner image.

  Further, in this embodiment, based on the fact that the reflection characteristics are different between the color (Y, C, M) toner and the black (K) toner, the density of the patch image by the black toner described later is determined from the patch image. The density of the patch image with color toner is obtained based on the light quantity ratio of p-polarized light and s-polarized light while obtaining the image density accurately over a wide dynamic range. It is possible to ask.

  Now, returning to FIG. 10, the description of the pre-operation will be continued. The surface state of the intermediate transfer belt 71 is not necessarily optically uniform, and as the toner is used, it may gradually become discolored or smudged as it is used. In this embodiment, in order to prevent an error in the density detection of the toner image due to such a change in the surface state of the intermediate transfer belt 71, in this embodiment, a background profile for one rotation of the intermediate transfer belt 71, that is, a toner image is used. Information on the density of the surface of the intermediate transfer belt 71 in a state where it is not carried is acquired. Specifically, the light emitting element 601 emits light with the previously obtained reference light amount, and the intermediate transfer belt 71 is rotated once while sampling the output voltages Vp and Vs from the light receiving units 670p and 670s (step S23). Sample data (number of samples in the present embodiment: 312) is stored in the RAM 107 as a base profile. As described above, by grasping in advance the density of each surface portion of the intermediate transfer belt 71, the density of the toner image formed thereon can be estimated more accurately. This point will be described in detail in a later embodiment.

  By the way, the output voltages Vp and Vs from the density sensor 60 described above are caused by a change in reflectance due to minute dirt and scratches on the roller 75 and the intermediate transfer belt 71, and electrical noise mixed in the sensor circuit. Spike-shaped noise may be superimposed.

  FIG. 11 is a diagram illustrating an example of the base profile of the intermediate transfer belt. When the amount of light reflected from the surface of the intermediate transfer belt 71 is sampled and plotted by the density sensor 60 over one or more rounds, the output voltage Vp from the sensor 60 is equal to that of the intermediate transfer belt 71 as shown in FIG. In addition to periodically changing in accordance with the circumference or the rotation period thereof, narrow spike noise may be superimposed on the waveform. This noise may include both a component synchronized with the rotation period and an irregular component not synchronized with this. FIG. 11B is an enlarged view of a part of such a sample data string. In this figure, two data marked with symbols Vp (8) and Vp (19) out of each sample data are larger than other data due to noise superposition, while symbols Vp (4) and Vp ( The two data marked with 16) are much smaller than the others. Although the p-polarized component of the two sensor outputs has been described here, the s-polarized component can be considered in the same manner.

  The detection spot diameter of the density sensor 60 is, for example, about 2 to 3 mm, and discoloration and dirt of the intermediate transfer belt 71 are generally considered to occur in a larger range. It can be seen that it is affected. In this way, the density of the background profile or patch image is obtained based on the sample data with the noise still superimposed, and if the density control factor is set from the result, it is not always possible to set each density control factor to the optimum state. On the other hand, the image quality may deteriorate.

  Therefore, in this embodiment, as shown in FIG. 10, after the sensor output is sampled for one turn of the intermediate transfer belt 71 in step S23, spike noise removal processing is executed (step S24).

  FIG. 12 is a flowchart showing spike noise removal processing in this embodiment. In this spike noise removal process, a continuous part of the acquired “raw” sample data sequence (ie, a length corresponding to 21 samples) is extracted (step S241), After removing the data corresponding to the upper three and lower three of the 21 sample data included in the section (steps S242 and S243), the arithmetic average of the remaining 15 data is obtained (step S244). ). Then, the average value is regarded as an average level in this section, and the “corrected” sample data string from which noise is removed is obtained by replacing the six data removed in steps S242 and S243 with the average value (step S245). ). Furthermore, if necessary, the above steps S241 to S245 are repeated for the next section, and spike noise is similarly removed (step S246). Spiking noise removal by the above process will be described in more detail with reference to FIG. 13, taking the data string shown in FIG. 11B as an example.

  FIG. 13 is a diagram showing how spike noise is removed in this embodiment. In the data string of FIG. 11 (b), the influence of noise appears in two large data Vp (8) and Vp (19) that protrude from other data, and small data Vp (4) and Vp (16) that protrude from the other data. It seems to be. In the spike noise removal process, the top three of the sample data are removed (step S242 in FIG. 12), and therefore, three pieces of data Vp (8 including two pieces of data that are considered to contain noise are included among these data. ), Vp (14) and Vp (19) are removed. Similarly, three data Vp (4), Vp (11) and Vp (16) including two data that are considered to contain noise are also removed (step S243 in FIG. 12). Then, as shown in FIG. 13, these six data are replaced with the average value Vpavg (indicated by a hatched circle) of the other 15 data, so that spike noise included in the original data string is obtained. Is removed.

  When performing this spike noise removal, the number of samples to be extracted and the number of data to be removed are not limited to the above, and may be any number, but depending on how they are selected, a sufficient noise removal effect can be obtained. In addition, there is a risk that the error may be increased on the contrary, so it is desirable to determine carefully based on the following viewpoints.

  In other words, if a data string in a section that is too short relative to the frequency of noise is extracted, the probability that no noise is included in the section in which the noise removal process is performed increases, and the number of arithmetic processes increases, which increases efficiency. Not right. On the other hand, if a data string of a very wide section is extracted, significant fluctuations in the sensor output, that is, fluctuations reflecting the density change of the detection target will be averaged, and the original density profile Cannot be obtained correctly.

  Further, since the frequency of noise occurrence is not constant, the data Vp (11) and Vp (14) in the above example can be obtained by simply removing a predetermined number of upper and lower data from the extracted data string. There is a possibility that even data that does not contain noise is removed as shown in FIG. Among these, even if some data not including noise is removed, as shown in FIG. 13, the difference between the data Vp (11), Vp (14) and the average value Vpavg is relatively small. The error due to the replacement of these data with the average value Vpavg is small. On the other hand, when data including noise is left without being removed, there is a possibility that the error may be increased by replacing other data with an average value obtained including the data. Accordingly, it is desirable that the ratio of the number of data to be removed with respect to the number of samples of the extracted data is determined so as to be equal to or slightly larger than the frequency of noise generated in an actual apparatus.

  In this embodiment, as shown in FIG. 11A, the frequency of data shifted to the larger side and the data shifted to the smaller side due to the influence of noise is approximately the same, and the frequency of occurrence of noise itself is the same. Based on the experimental fact that it was about 25% or less (5 samples or less out of 21 samples), the spike noise removal processing is configured as described above.

  In addition to the above, various methods are conceivable as processing methods for removing spike noise. For example, spike-like noise can also be removed by applying a conventionally known low-pass filter process to “raw” sample data obtained by sampling. However, in the conventional filter processing, the sharpness of the noise waveform can be reduced, but as a result, not only the data including noise but also the surrounding data are changed from the original value, which occurs. Depending on the form of noise, a large error may be caused.

  On the other hand, in the present embodiment, the number of upper / lower data corresponding to the frequency of occurrence of noise in each sample data is replaced with an average value, while other data is left as it is. The possibility of error is low.

  The spike noise removal process is performed not only when obtaining the background profile described above, but also for sample data obtained as the amount of reflected light when obtaining the image density of the toner image as will be described later.

(B-2) Emptying of developing device (pre-operation 2)
If image formation is performed after the power-off state or the image formation operation is not performed even when the power is on and the operation is stopped for a long time, periodic density unevenness may appear in the image. Conventionally known. In the present specification, this phenomenon is referred to as a neglected banding phenomenon. The inventor of the present application recognizes that this neglected banding phenomenon is caused by the fact that the toner is left on the developing roller 44 of each developing device for a long period of time. It has been found that the toner layer on the surface of the developing roller 44 is not uniform and the toner layer on the developing roller 44 gradually becomes non-uniform. For example, in the developing device 4K of the present embodiment shown in FIG. 3, when the rotation of the developing roller 44 is stopped, the supply roller 43 or the regulating blade 45 is in contact with a part of the surface thereof. Further, a portion of the surface located inside the housing 41 is covered with a large amount of toner, whereas a portion exposed to the outside of the housing 41 is in the atmosphere while carrying a thin toner layer. The surface state of the developing roller 44 is non-uniform in the circumferential direction, such as being exposed.

  When the density control factor is newly optimized prior to the next image formation after the apparatus has been in a non-operating state for a long time with the surface of the developing roller 44 being non-uniform, the banding is left as it is. The density unevenness of the patch image caused by the phenomenon may affect the optimization processing.

  Therefore, in the image forming apparatus of this embodiment, each developing roller 44 is idled in order to eliminate the neglected banding phenomenon prior to forming the patch image. Specifically, as shown in the flow on the right side of FIG. 10 (pre-operation 2), first, the yellow developing device 4Y is disposed at the developing position facing the photoreceptor 2 (step S25), and the DC developing bias Vavg is made variable. After the absolute value is set to the minimum value in the range (step S26), the developing roller 44 is rotated at least one turn by the rotation drive unit on the main body side (step S27). Then, while rotating the developing unit 4 to switch the developing device (step S28), the other developing devices 4C, 4M, and 4K are sequentially positioned at the developing position, and the developing roller 44 provided for each is similarly set to 1. Rotate more than one lap. Thus, by rotating each developing roller 44 one or more times, the toner layer on the surface of the developing roller 44 is peeled off once by the supply roller 43 and the regulating blade 45 to be re-formed. Since the toner layer that has been re-formed in this way and is in a more uniform state is used for image formation, density unevenness due to the neglected banding phenomenon is less likely to occur.

  In the pre-operation 2, the absolute value of the DC developing bias Vavg is minimized in step S26. The reason is as follows.

  As will be described later, the DC developing bias Vavg as a density control factor that affects the image density increases as the absolute value | Vavg | increases. This is because, as the absolute value | Vavg | of the DC developing bias increases, the area exposed to the light beam L in the electrostatic latent image on the photoreceptor 2, that is, the surface area where the toner is to be adhered to the developing roller 44. This is because the potential difference is increased and toner movement from the developing roller 44 is further promoted. However, it is not preferable that such toner movement occurs when the background profile of the intermediate transfer belt 71 is acquired. This is because when the toner moved from the developing roller 44 to the photosensitive member 2 is transferred onto the intermediate transfer belt 71 in the primary transfer region TR1, the amount of reflected light from the intermediate transfer belt 71 is changed. It is because it becomes impossible to ask.

  In this embodiment, as will be described later, the DC development bias Vavg can be changed and set in multiple stages within a predetermined variable range as one of the density control factors. Therefore, the DC developing bias Vavg is set to a value that minimizes the absolute value in the variable range, and a state in which the toner hardly moves from the developing roller 44 to the photosensitive member 2 is realized. The toner adhesion is minimized. For the same reason, in an apparatus having an AC component in the developing bias, the amplitude is preferably set smaller than that during normal image formation. For example, as described above, in an apparatus in which the amplitude Vpp of the developing bias is 1400V, the amplitude Vpp is preferably about 1000V. Even in an apparatus that uses parameters other than the DC developing bias Vavg, for example, the duty ratio of the developing bias or the charging bias as the density control factor, the density control factor is appropriately set so as to realize the above-described conditions in which toner movement is less likely to occur. It is preferable to set.

  In this embodiment, the pre-operation 1 and the pre-operation 2 are simultaneously executed in parallel to shorten the processing time. That is, in the pre-operation 1, at least one turn of the intermediate transfer belt 71 is required to acquire the background profile, and more preferably, three turns including two turns for sensor calibration are required. It is preferable to rotate each developing roller 44 as much as possible, and since these operations can be performed independently of each other, the time required for each processing can be secured by performing these operations in parallel. Thus, the time required for the entire optimization process can be shortened.

(C) Derivation of Control Target Value In the image forming apparatus of this embodiment, as will be described later, two types of toner images are formed as patch images, and each density is set so that the density becomes a predetermined density target value. Although the control factor is adjusted, this target value is not fixed, but is changed according to the operating state of the apparatus. The reason is as follows.

  As described above, in the image forming apparatus of this embodiment, the image density is determined by detecting the amount of reflected light from the toner image that has been visualized on the photoreceptor 2 and primarily transferred onto the surface of the intermediate transfer belt 71. I have an estimate. As described above, the technique for obtaining the image density from the reflected light amount of the toner image has been widely used. However, as described in detail below, the reflected light amount from the toner image carried on such an intermediate transfer belt 71 is used. (Or the sensor outputs Vp and Vs from the corresponding density sensor 60) and the optical density (OD value) of the toner image formed on the sheet S as the final transfer material are uniquely determined. However, it changes slightly depending on the state of the device and toner. Therefore, as in the prior art, even if each density control factor is controlled so that the amount of reflected light from the toner image is constant, the density of the image finally formed on the sheet S varies depending on the state of the toner. Will end up.

  As described above, one of the causes that the sensor output and the OD value on the sheet S do not coincide with each other is the toner fused on the sheet S through the fixing process and the toner is not fixed and simply adhered to the surface of the intermediate transfer belt 71. That is, the state of reflection is different from that of the only toner.

  FIG. 14 is a schematic diagram showing the relationship between the toner particle size and the amount of reflected light. As shown in FIG. 14A, in the image Is finally obtained on the sheet S, the toner Tm melted by heating and pressurization in the fixing process is fused to the sheet S. . Therefore, the optical density (OD value) reflects the amount of reflected light when the toner is fused, but the magnitude is mainly expressed by the toner density on the sheet S (for example, the toner mass per unit area). Can be determined).

  On the other hand, in the toner image on the intermediate transfer belt 71 that has not undergone the fixing process, each toner particle is simply adhered to the surface of the intermediate transfer belt 71. Therefore, even if the toner density is the same (that is, the OD value after fixing is the same), for example, the toner T1 having a small particle diameter shown in FIG. In the state where the toner T2 having a large particle diameter shown in (c) adheres at a lower density and the surface of the intermediate transfer belt 71 is partially exposed, the amount of reflected light is not necessarily the same. In other words, even if the amount of reflected light from the toner image before fixing is the same, the image density (OD value) after fixing is not always the same. In general, when the amount of reflected light is the same, the image density after fixing tends to increase when the ratio of the large-diameter toner in the toner particles constituting the toner image is high. . As described above, the correspondence between the OD value on the sheet S and the amount of reflected light from the toner image on the intermediate transfer belt 71 varies depending on the state of the toner, particularly the particle size distribution.

  FIG. 15 is a diagram illustrating the correspondence between the toner particle size distribution and the change in the OD value. Ideally, the toner particles contained in each developing device for forming a toner image have all the particle diameters equal to the design center value. However, as shown in FIG. 15 (a), the particle diameter actually has a distribution of various modes, and the mode varies depending on the type of toner and the manufacturing method, and the same specification. Even the manufactured toner is slightly different for each production lot and product.

  Since these toners having various particle sizes have different masses and charge amounts, when image formation is performed using toner having such a particle size distribution, these toners are not consumed uniformly. The toner having a suitable particle size is selectively consumed by the apparatus, while the other toners are not consumed so much and remain in the developing unit. Therefore, as the toner consumption progresses, the particle size distribution of the toner remaining in the developing device also changes.

  As described above, since the amount of reflected light from the toner image before fixing varies depending on the particle size of the toner constituting the image, even if each density control factor is adjusted so that the amount of reflected light is always constant, the sheet S The image density after being fixed on top is not always constant. FIG. 15B shows a sheet when image formation is performed while controlling each density control factor so that the amount of reflected light from the toner image is constant, that is, the output voltage from the density sensor 60 is constant. The change in the optical density (OD value) of the image on S is shown. For example, when the toner particle diameters are well aligned around the design center value as shown by a curve a in FIG. 15A, the inside of the developing device is shown as a curve a in FIG. Even if the toner consumption increases, the OD value is maintained at the target value. On the other hand, for example, when a toner having a wider particle size distribution is used as shown by the curve b in FIG. 15A, the design center is initially set as shown by the curve b in FIG. Although toner with a particle size near the value is mainly consumed and an OD value almost as the target value is obtained, the percentage of such toner decreases as the toner consumption proceeds, and instead a toner with a larger particle size is displayed. Since it is used for formation, the OD value gradually increases. Further, as indicated by the dotted lines in FIG. 15A, the median value of the distribution may deviate from the design value from the beginning depending on the production lot of the toner or the developing device. As shown by the dotted lines in FIG. 15B, the OD value also shows various changes as the toner consumption increases.

  Factors that influence the characteristics of the toner as described above depend on, for example, the dispersion state of the pigment in the toner base particles and the mixing state of the toner base particles and the external additive in addition to the above-described toner particle size distribution. There is a change in chargeability of toner. As described above, since the toner characteristics are slightly different for each product, the image density on the sheet S is not always constant, and the degree of density change varies depending on the toner used. Therefore, in a conventional image forming apparatus that controls each density control factor so that the output voltage from the density sensor is constant, fluctuations in image density due to variations in toner characteristics are unavoidable, and satisfactory image quality is always obtained. I couldn't.

  Therefore, in this embodiment, an evaluation value of an image density (to be described later) that is calculated based on an output from the density sensor 60 for each of two types of patch images, which will be described later, according to the operating status of the apparatus, and serves as a scale representing the image density. ) And the respective density control factors are adjusted so that the evaluation value obtained for each patch image becomes this control target value, so that the image density on the sheet S is kept constant. I have to.

  FIG. 16 is a flowchart showing a control target value derivation process in this embodiment. In this process, for each toner color, the usage status of the toner, specifically, initial characteristics such as the particle size distribution of the toner filled in the developing unit, and the amount of toner remaining in the developing unit, The control target value commensurate with First, one of the toner colors is selected (step S31). As information for the CPU 101 to estimate the usage status of the toner, toner individuality information regarding the selected toner color and dots indicating the number of dots formed by the exposure unit 6 are displayed. Information on the count value and the developing roller rotation time is acquired (step S32). Here, a case where the control target value corresponding to the black color is obtained will be described as an example, but the same applies to other toner colors.

  “Toner individuality information” is information representing the characteristics of the toner filled in the developing device 4K. In this apparatus, the characteristics of the toner are classified into eight types in view of the fact that the various characteristics such as the particle size distribution of the toner differ depending on the production lot. Then, one table to be referred to when determining the control target value is selected from a plurality of lookup tables described later, depending on which type the toner in the developing device belongs to.

  The “dot count value” is information for estimating the amount of toner remaining in the developing device 4K. The most convenient method for estimating the remaining amount of toner is to calculate from the integrated value of the number of formed images. However, since the amount of toner consumed by forming one image is not constant, this method is accurate. It is difficult to know the remaining amount. On the other hand, the number of dots formed on the photoconductor 2 by the exposure unit 6 represents the number of dots visualized by the toner on the photoconductor 2, and thus more accurately reflects the amount of toner consumption. Become. Therefore, in this embodiment, the number of dots when the exposure unit 6 forms the electrostatic latent image on the photoreceptor 2 to be developed by the developing device 4K is counted and stored in the RAM 107, and this dot count is stored. The value is a parameter indicating the remaining amount of toner in the developing device 4K.

  Further, the “developing roller rotation time” is information for estimating the characteristics of the toner remaining in the developing device 4K in more detail. As described above, a toner layer is formed on the surface of the developing roller 44, and development is performed by moving a part of the toner onto the photoreceptor 2. At this time, on the surface of the developing roller 44, the toner that has not contributed to the development is conveyed to a contact position with the supply roller 43, and is peeled off by the roller 43 to form a new toner layer. The toner is fatigued by repeated adhesion and peeling to the developing roller 44, and its characteristics gradually change. Such a change in toner characteristics progresses as the developing roller 44 continues to rotate. Therefore, for example, even if the remaining amount of toner in the developing device 4K is the same, the characteristics of the fresh toner that has not been used and the old toner that has repeatedly adhered and peeled off may differ. The density of the image formed using is not necessarily the same.

  Therefore, in this embodiment, the state of the toner built in the developing device 4K is determined based on a combination of two parameters: a dot count value indicating the remaining amount of toner and a developing roller rotation time indicating the degree of toner characteristic change. The image quality is stabilized by estimating and finely setting the control target value according to the state.

  Note that these pieces of information are also used to manage the wear status of each part of the apparatus and improve maintainability. That is, one dot count corresponds to a toner amount of 0.015 mg, and the consumption amount is almost 180 g at 122,000 dot count, which means that most of the toner stored in each developer is used up. Regarding the rotation time of the developing roller, the integrated value of 10600 sec corresponds to 8000 sheets in continuous printing at A4 size, and it is not preferable from the viewpoint of image quality to continue image formation. Therefore, in this embodiment, when any of these pieces of information reaches the above value, a message notifying the toner end is displayed on a display unit (not shown) to prompt the user to replace the developing device. I am doing so.

  Now, a control target value corresponding to the status is determined from each piece of information regarding the operating status of the apparatus thus obtained. In this embodiment, an optimum control target value corresponding to the toner individuality information indicating the toner type and the characteristics of the remaining toner estimated from the combination of the dot count value and the developing roller rotation time is obtained experimentally in advance. This value is stored in the ROM 106 of the engine controller 10 as a lookup table for each toner type. Based on the toner individuality information, the CPU 101 selects one table to be referred to corresponding to the toner type from among these lookup tables (step S33), and the dot count value and developing roller rotation time at that time point are selected. A value corresponding to the combination is read from the table (step S34).

  Further, in the image forming apparatus of this embodiment, the user can perform a predetermined operation input through an operation unit (not shown) so that the density of an image to be formed can be increased or decreased within a predetermined range according to preference or as necessary. It is configured. That is, each time the user increases or decreases the image density by one step, a predetermined offset value, for example, 0.005 per step is added to or subtracted from the value read from the lookup table. The control target value Akt for the black color at that time is set and stored in the RAM 107 (step S35). In this way, the control target value Akt for the black color is obtained.

  FIG. 17 is a diagram illustrating an example of a lookup table for obtaining a control target value. This table is a table that is referred to when a toner having a black color and a characteristic belonging to “type 0” is used. In this embodiment, eight types of tables corresponding to eight types of toner characteristics are prepared for each toner color corresponding to two types of patch images for high density and low density described later, respectively. It is stored in a ROM 106 provided in the engine controller 10. Here, FIG. 17A is an example of a table corresponding to a high-density patch image, and FIG. 17B is an example of a table corresponding to a low-density patch image.

  If the toner individuality information indicates, for example, “type 0”, the table of FIG. 17 corresponding to the toner individuality information “0” is selected from the eight types of tables in step S33. Then, a control target value Akt is obtained based on the acquired dot count value and developing roller rotation time. For example, for a high-density patch image, if the dot count value is 1500,000 counts and the developing roller rotation time is 2000 seconds, a value 0.984 corresponding to the combination of these is referred to in this case with reference to FIG. Is the control target value Akt. Further, for example, when the user sets the image density one step higher than the standard state, a value 0.989 obtained by adding 0.005 to this value becomes the control target value Akt. Similarly, the control target value for the low density patch image can also be obtained.

  The control target value Akt obtained in this way is stored in the RAM 107 of the engine controller 10, and the evaluation value obtained based on the reflected light amount of the patch image is set as this control target value in the subsequent setting of each density control factor. Make sure they match.

  As described above, the control target value for one toner color is obtained by executing steps S31 to S35. By repeating the above processing for each toner color (step S36), the control target value for all toner colors is obtained. The values Ayt, Act, Amt and Akt are determined. Here, the subscripts y, c, m, and k represent the toner colors, that is, yellow, cyan, magenta, and black, respectively, and the subscript t represents the control target value.

(D) Development Bias Setting In this image forming apparatus, a DC development bias Vavg applied to the developing roller 44, energy per unit area of the exposure beam L for exposing the photosensitive member 2 (hereinafter simply referred to as “exposure energy”) E, The image density is controlled by adjusting these. Here, the variable range of the DC developing bias Vavg is changed from the low level side to 6 levels from V0 to V5, and the variable range of the exposure energy E is changed from the low level side to 4 levels from levels 0 to 3, respectively. However, the variable range and the number of divisions thereof can be appropriately modified according to the specifications of the device. In the above-described apparatus in which the variable range of the DC developing bias Vavg is (−110) V to (−330) V, the lowest level V 0 has the smallest absolute value of the voltage (−110) V, The highest level V5 corresponds to (−330) V having the largest absolute voltage value.

  FIG. 18 is a flowchart showing the developing bias setting process in this embodiment. FIG. 19 shows a high-density patch image. In this processing, first, the exposure energy E is set to level 2 (step S41), and then the DC development bias Vavg is increased by one level from the minimum level V0, and a solid image as a high-density patch image at each bias value. Is formed (steps S42 and S43).

  Corresponding to the DC developing bias Vavg changed and set in six stages, as shown in FIG. 19, six patch images Iv0 to Iv5 are sequentially formed on the surface of the intermediate transfer belt 71. Five patch images Iv0 to Iv4 are formed to a length L1. This length L1 is configured to be longer than the circumferential length of the cylindrical photosensitive member 2. On the other hand, the last patch image Iv5 is formed to have a length L3 shorter than the circumferential length of the photoreceptor 2. The reason for this will be described in detail later. Further, when the DC developing bias Vavg is changed and set, there is a slight time delay until the potential of the developing roller 44 becomes uniform, so that each patch image is formed with an interval L2 in consideration of this time delay. . Of the surface of the intermediate transfer belt 71, the area where the toner image can actually be carried is the image forming area 710 shown in the figure. However, since the shape and arrangement of the patch image are configured as described above, the image forming area The number of patch images that can be formed in the area 710 is about three, and six patch images are formed over two rotations of the intermediate transfer belt 71 as shown in FIG.

  Here, the reason why the length of the patch image is set as described above will be described with reference to FIGS. 1 and 20. FIG. 20 is a diagram showing fluctuations in image density that occur in the photoreceptor cycle. As shown in FIG. 1, the photoreceptor 2 is formed in a cylindrical shape (the circumference is L0), but due to manufacturing variations, thermal deformation, etc., the shape is not a perfect cylinder. In such a case, a periodic fluctuation corresponding to the peripheral length L0 of the photoreceptor 2 may occur in the image density of the formed toner image. This is because, in a contact development type apparatus in which toner development is performed in a state where the photosensitive member 2 and the developing roller 44 are in contact with each other, the contact pressure between the two varies, and the two are spaced apart to perform toner development. In the non-contact development type apparatus, the intensity of the electric field that causes the toner to fly between the two changes, and in either apparatus, the probability that the toner moves from the developing roller 44 to the photosensitive member 2 is periodically determined by the rotation cycle of the photosensitive member 2. This is because it will fluctuate.

  As shown in FIG. 20A, the width of the density fluctuation is large particularly when the absolute value | Vavg | of the DC developing bias Vavg is relatively low, and decreases as the value | Vavg | increases. For example, when a patch image is formed by setting the absolute value | Vavg | of the DC developing bias to a relatively small value Va, the image density OD varies depending on the position on the photoreceptor 2 as shown in FIG. It changes within the range of the width Δ1. Similarly, even when a patch image is formed with another DC developing bias, the image density varies within a certain range as indicated by the hatched portion in FIG. As described above, the density OD of the patch image varies depending not only on the magnitude of the DC developing bias Vavg but also on the position on the photoreceptor 2. Therefore, in order to obtain the optimum value of the DC developing bias Vavg from the image density, it is necessary to eliminate the influence of density fluctuation corresponding to the rotation cycle of the photosensitive member 2 on the patch image.

  Therefore, in this embodiment, a patch image having a length L1 exceeding the circumferential length L0 of the photosensitive member 2 is formed, and an average value of the density obtained for the length L0 as described later is used as the image density of the patch image. It is said. This effectively suppresses the influence of density fluctuations corresponding to the rotation period of the photoreceptor 2 on the density of each patch image. As a result, the optimum value of the DC developing bias Vavg is set based on the density. It is possible to ask appropriately.

  In this embodiment, as shown in FIG. 19, among the patch images Iv0 to Iv5, the length L3 of the last patch image Iv5 formed with the DC developing bias Vavg as the maximum is set to the circumference of the photoreceptor 2. It is smaller than the length L0. As shown in FIG. 20B, the patch image formed under a condition where the absolute value | Vavg | of the DC developing bias is large has a small density fluctuation corresponding to the rotation period of the photosensitive member 2 and thus is sensitive as described above. This is because it is not necessary to obtain an average value over the body cycle. By doing this, it is possible to shorten the time required for forming and processing a patch image and to reduce the amount of toner consumed in forming the patch image.

  As described above, in order to eliminate the influence of the density fluctuation caused by the photoconductor cycle on the optimization process of the density control factor, the patch image should be formed longer than the circumference L0 of the photoconductor 2. Although it is desirable that not all patch images have such a length, the number of patch images to have such a length depends on the level of density fluctuation that appears in each device and the level of image quality required. Should be determined accordingly. For example, when the influence of the density fluctuation in the photosensitive member cycle is relatively small, only the patch image Iv0 formed under the condition that the DC developing bias Vavg is minimum is set to the length L1, and the other patch images Iv1 to Iv5 are used. You may make it form in length L3 shorter than this.

  Conversely, all the patch images may be formed with the length L1, but in this case, there is a problem that the processing time and the toner consumption amount increase. Further, it is not preferable from the viewpoint of image quality that the density fluctuation corresponding to the photosensitive member cycle appears even when the DC developing bias Vavg is maximized. At least when the maximum value is set, such density fluctuation does not appear. It is natural to define a variable range of the DC developing bias Vavg. When the variable range of the DC developing bias Vavg is set as described above, such a density variation does not appear at least at the maximum value, and therefore the length of the patch image in this case does not need to be L1.

  Returning to FIG. 18, the description of the developing bias setting process will be continued. For the patch images Iv0 to Iv5 thus formed with the respective DC developing biases, the voltages Vp and Vs output from the density sensor 60 corresponding to the amount of light reflected from the surface are sampled (step S44). In this embodiment, 74 points (corresponding to the circumferential length L0 of the photoreceptor 2) in the patch image Iv0 to Iv4 of length L1, and 21 points (corresponding to the circumferential length of the developing roller 44) in the patch image Iv5 of length L3. The sample data of the output voltages Vp and Vs from the concentration sensor 60 is obtained at a sampling period of 8 msec. Then, after removing spike noise from the sample data (step S45) in the same manner as when the background profile was derived (FIG. 10), each patch from which the dark output of the sensor system and the influence of the background profile were removed from the data. An “evaluation value” of the image is calculated (step S46). However, with respect to the patch images Iv0 to Iv4 having the length L1, the spike noise is removed by removing 10 samples in order from the largest value and the smallest value among the 74 samples.

  As described above, the density sensor 60 in this apparatus exhibits the characteristic that the output level is the highest when no toner is attached to the intermediate transfer belt 71 and the output decreases as the toner amount increases. Further, since an offset due to dark output is added to this output, it is difficult to treat the output voltage data from this sensor as information for evaluating the toner adhesion amount. Therefore, in this embodiment, the obtained data is processed and converted into data reflecting the amount of toner adhesion, that is, an evaluation value, so that the subsequent processing can be easily performed.

The calculation method of the evaluation value will be described more specifically by taking a patch image with a black toner color as an example. Of the six patch images developed with black toner, the evaluation value Ak (n) of the nth patch image Ivn (where n = 0, 1,..., 5) is expressed by the following formula:
Ak (n) = 1- {Dp_avek (n) -Vp0} / {Tp_ave-Vp0} (1-2)
Calculate based on Here, the meaning of each term of the above formula is as follows.

  First, Dp_avek (n) is output from the density sensor 60 as the output voltage Vp corresponding to the p-polarized component of the reflected light from the nth patch image Ivn, and the average of each sampled sample data after noise removal is obtained. Value. That is, for example, the value Dp_avek (0) corresponding to the first patch image Iv0 is detected as the output voltage Vp from the density sensor 60 in the length L0 of the patch image, and then subjected to spike noise removal processing. This is an arithmetic average of 74 sample data stored in the RAM 107. Note that the subscript k of each term in the above expression represents a value for the black color.

  Vp0 is a dark output voltage from the light receiving unit 670p acquired in the previous pre-operation 1 with the light emitting element 601 turned off. Thus, by subtracting the dark output voltage Vp0 from the sampled output voltage, it is possible to eliminate the influence of the dark output and obtain the toner image density with higher accuracy.

  Further, Tp_ave is the same as the 74 pieces of sample data used for calculating Dp_avek (n) detected on the intermediate transfer belt 71 among the background profile data previously obtained and stored in the RAM 107. It is an average value of each sample data detected at the position.

  That is, the evaluation value Ak (n) for the nth patch image Ivn in the black color is the average value of the sensor output Vp obtained from the surface of the intermediate transfer belt 71 before the toner adheres, and the toner adheres. After subtracting the dark output of the sensor from each of the average values of the sensor output Vp obtained from the patch image Ivn, the ratio between the two is taken and the value is subtracted from 1. Therefore, when no toner as a patch image adheres to the intermediate transfer belt 71, Dp_avek (n) = Tp_ave and the evaluation value Ak (n) becomes zero, while the surface of the intermediate transfer belt 71 is completely made of black toner. In the state where the reflectance is zero because of the cover, Dp_avek (n) = Vp0 and the evaluation value Ak (n) = 1.

  As described above, when the evaluation value Ak (n) is used instead of the value of the sensor output voltage Vp as it is, the influence of the surface state of the intermediate transfer belt 71 is canceled and the image density of the patch image is measured with high accuracy. Can do. Further, since the correction is made according to the density of the patch image on the intermediate transfer belt 71, the accuracy of measuring the image density can be further improved. Further, the density of the patch image Ivn is normalized and expressed by a value from a minimum value 0 representing a state where no toner is adhered to a maximum value 1 representing a state where the surface of the intermediate transfer belt 71 is covered with high-density toner. Therefore, it is convenient to estimate the density of the toner image in the subsequent processing.

Note that toner colors other than black, that is, yellow color (Y), cyan color (C), and magenta color (M) have a higher reflectance than the black color even when the toner covers the surface of the intermediate transfer belt 71. Since the amount of reflected light is not zero, the evaluation value obtained as described above may not accurately represent the density. Therefore, in this embodiment, instead of the output voltage Vp corresponding to the p-polarized component as the sample data used for obtaining the evaluation values Ay (n), Ac (n), Am (n) for these toner colors, a dark output will be generated from now on. A value Dps obtained by dividing the value obtained by subtracting Vp0 by the value obtained by subtracting the dark output Vs0 from the output voltage Vs corresponding to the s-polarized component, ie,
Dps = (Vp−Vp0) / (Vs−Vs0) (1-3)
Is used as sample data at each position, so that it is possible to accurately estimate the image density of these toner colors. Further, as in the case of the black color, the influence of the surface state of the intermediate transfer belt 71 is canceled by considering the sensor output obtained from the surface of the intermediate transfer belt 71 before the toner adheres, and the intermediate transfer Since correction is performed according to the density of the patch image on the belt 71, the accuracy of image density measurement can be improved.

For example, for cyan (C), the evaluation value Ac (n) is given by
Ac (n) = 1- {Dps_avec (n) -Dps (color)} / {Tps_ave-Dps (color)} (1-4)
It can ask for. Here, Dps_avec (n) is an average after noise removal of the value Dps shown in the above equation (1-3) obtained based on the sensor outputs Vp and Vs at each position of the nth patch image Ivn in cyan. Value. Dps (color) is the value Dps corresponding to the sensor outputs Vp and Vs in a state where the surface of the intermediate transfer belt 71 is completely covered with the color toner, and this value Dps is the minimum value that can be taken. . Further, Tps_ave is an average value of the values Dps obtained based on the sensor outputs Vp and Vs sampled as the background profile at each position on the intermediate transfer belt 71.

  By defining the evaluation value corresponding to the color toner as described above, as in the case of the black color described above, the toner does not adhere to the intermediate transfer belt 71 (at this time, Dps_avec (n) = Tps_ave ) To normalize the density of the patch image Ivn with a value from a minimum value 0 representing the belt 71 to a maximum value 1 representing the state in which the belt 71 is completely covered with toner (Dps_avec (n) = Dps (color) at this time). Can be expressed.

  When the density of each patch image (more accurately, the evaluation value) is obtained in this way, the optimum value Vop of the DC developing bias Vavg is calculated based on the value (step S47). FIG. 21 is a flowchart showing the DC developing bias optimum value calculation process in this embodiment. Since the contents of this process are the same regardless of the toner color, the evaluation value suffixes (y, c, m, k) corresponding to the toner color are omitted in FIG. 21 and the following. Needless to say, the target value is different for each toner color.

  First, the variable n is set to 0 (step S471), and the evaluation value A (n), that is, A (0) is compared with the previously obtained control target value At (for example, Akt in the black color) (step S471). S472). At this time, if the evaluation value A (0) is equal to or greater than the control target value At, it means that an image density exceeding the target density is obtained at the minimum value V0 of the DC developing bias Vavg, and is higher than this. The developing bias need not be studied, and the processing ends with the DC developing bias V0 at this time set to the optimum value Vop (step S477).

  On the other hand, when the evaluation value A (0) does not reach the target value At, the evaluation value A (1) for the patch image Iv1 formed at the DC developing bias V1 that is one level higher is read and the evaluation value A (0 ) And a determination is made as to whether or not the difference is equal to or less than a predetermined value Δa (step S473). Here, when the difference between the two is equal to or less than the predetermined value Δa, the DC developing bias V0 is set to the optimum value Vop as described above. The reason for this will be described in detail later.

  On the other hand, if the difference between the two is larger than the predetermined value Δa, the process proceeds to step S474, and the evaluation value A (1) is compared with the control target value At. At this time, if the evaluation value A (1) is greater than or equal to the target value At, the target value At is greater than the evaluation value A (0) and less than or equal to A (1), that is, A (0) <At ≦ A (1). Therefore, the optimum value Vop of the DC developing bias for obtaining the target image density exists between the DC developing biases V0 and V1. That is, V0 <Vop ≦ V1.

  In such a case, the process proceeds to step S478, and the optimum value Vop is obtained by calculation. Various calculation methods are conceivable. For example, a change in the evaluation value with respect to the DC developing bias Vavg is approximated to an appropriate function in the interval V0 to V1, and the DC value is such that the value of the function becomes the target value At. The developing bias Vavg may be set to the optimum value Vop. Of these, the method of approximating the change of the evaluation value with a straight line is the simplest, but the optimum value Vop can be obtained with sufficient accuracy by appropriately selecting the variable range of the DC developing bias Vavg. Of course, other methods, for example, a more accurate approximation function may be introduced to calculate the optimum value Vop, but this is not always practical in consideration of detection errors and variations of the apparatus.

  On the other hand, if the target value At is larger than the evaluation value A (1) in step S474, n is incremented by 1 (step S475), and until n reaches the maximum value (step S476), the above-described steps S473- The optimum value Vop of the DC developing bias is obtained by repeating S475, but the optimum value Vop was not obtained even if n was the maximum value (n = 5) in step S476, that is, evaluation corresponding to six patch images. If none of the values has reached the target value, the DC developing bias V5 having the maximum density is set as the optimum value Vop (step S477).

  Thus, in this embodiment, each of the evaluation values A (0) to A (5) corresponding to the patch images Iv0 to Iv5 is compared with the target value At, and the target density is determined based on the magnitude relationship. The DC developing bias optimum value Vop for obtaining is obtained. As described above, in step S473, the difference between the evaluation values A (n) and A (n + 1) corresponding to two consecutive patch images is calculated. When it is equal to or less than the predetermined value Δa, the DC developing bias Vn is set to the optimum value Vop. The reason will be described below.

  FIG. 22 is a graph showing the relationship between the DC development bias and the evaluation value for the solid image. A curve a in FIG. 22A shows the original relationship without detection error. As described above, the evaluation value for the solid image increases as the absolute value | Vavg | of the DC developing bias increases, but the rate of change decreases in a region where the DC developing bias Vavg is relatively large, and gradually becomes saturated. Indicates. This is because if the toner adheres to a certain high density, the image density does not increase much even if the toner adhesion amount is increased further. As the change in the image density becomes smaller in this way, the change in the evaluation value also becomes smaller. Therefore, the slope of the curve a becomes smaller as the DC developing bias | Vavg | becomes larger. In the following, the curves a and b indicating the correspondence between the DC developing bias Vavg and the evaluation value shown in FIG. 22A are simply referred to as “evaluation value curve”.

  Under such a relationship, when the evaluation value for the patch image is obtained based on the sensor outputs Vp and Vs as described above, if the detection error is not included in the sensor output, each value V0 of the DC developing bias Vavg. , V1,..., The evaluation values for the patch images should take the values indicated by the white circles in FIG. On the other hand, due to variations in the characteristics of the density sensor 60 and the like, detection errors may be included in the sensor outputs Vp and Vs. For example, when the sensor output Vp tends to be output slightly shifted to the higher potential side than the original value, the evaluation value obtained based on the output Vp is indicated by the curve b and the diagonal line in FIG. As indicated by the circle, the value is slightly smaller than the original value. In addition, the evaluation value obtained based on the sensor output may not match the original image density due to the above-described variation in toner characteristics. As described above, when the image density of the patch image is obtained indirectly based on the sensor output, a wrinkle may occur between the result and the actual image density.

  Now, a case where the optimum value Vop of the DC developing bias Vavg is obtained based on the evaluation value of the patch image thus obtained will be considered. FIG. 22B is a partially enlarged view of the graph shown in FIG. As the DC developing bias Vavg, a value when the evaluation value for the solid image coincides with the control target value At may be set as the optimum value. If there is no detection error, as shown in FIG. The DC developing bias Vt corresponding to the intersection of the evaluation value curve a and the straight line c representing the control target value At may be set to the optimum value. In this example, the optimum value of the DC developing bias should be an intermediate value between the DC developing biases V3 and V4.

  However, in actuality, the evaluation value obtained from the sensor output inevitably includes a detection error.For example, when the evaluation value tends to be lower than the original value due to variations in sensor characteristics as described above, The evaluation value curve is as shown by a curve b in FIG. Therefore, in this case, if the DC developing bias Vf corresponding to the intersection of the curve b and the straight line c is the optimum value, the value Vf is greatly different from the original optimum value Vt.

  As described above, in the region where the change in image density with respect to the DC developing bias Vavg is small, that is, the slope of the evaluation value curve is small, the DC developing bias Vavg obtained as the optimum value greatly fluctuates even with a slight detection error. . Although the image density does not change greatly due to such fluctuation, when the absolute value | Vavg | of the DC developing bias is set larger than necessary, the following problems may occur. That is, even if the change in the image density is small, the toner adhesion amount is increased, so that the toner in each developer unit is consumed intensively, and the trouble of replacing the developer unit becomes complicated, and the running cost of the apparatus is reduced. To rise. Further, since the amount of toner constituting the toner image increases, a transfer failure occurs in the transfer process from the photosensitive member 2 to the intermediate transfer belt 71 or from the intermediate transfer belt 71 to the sheet S, or there is sufficient toner in the fixing process. This causes deterioration of image quality, such as fixing failure without being melted. Furthermore, the development is performed in a state where a voltage higher than necessary is applied to the developing roller 44, so that charges remain on the surface of the developing roller 44 and hinder the formation of a uniform toner layer. There is a case where image quality deterioration such as the influence of the image formed earlier appears in the later image. As described above, it is not preferable to apply the DC developing bias Vavg higher than necessary to the developing roller 44 in the region where the slope of the evaluation value curve is small.

  In this embodiment, the evaluation value for each patch image obtained from the sensor output is used as an index representing the toner density. However, when obtaining the optimum value of the DC developing bias Vavg, only the evaluation value itself is used. In addition, the influence of the detection error or the like in the optimization process of the DC developing bias Vavg is suppressed by taking into account the rate of change with respect to the developing bias Vavg.

  FIG. 23 is a graph showing the evaluation value with respect to the DC developing bias and the rate of change thereof. Since the evaluation value gradually saturates as the DC developing bias | Vavg | increases as shown by the curve a in FIG. 23A, the rate of change is as shown in FIG. 23B. Decreases monotonically with increasing | Vavg |. Here, when the optimum value of the DC developing bias Vavg is obtained from the evaluation value curve based on the curve b including the detection error, as described above, the optimum value is originally Vt, but this is because of the detection error. It is obtained as a greatly different value Vf. On the other hand, as shown in FIG. 23 (b), a curve representing the rate of change of the evaluation value with respect to the DC developing bias Vavg (hereinafter referred to as “change rate curve”) can be obtained even if the evaluation value curve slightly changes due to a detection error. The change is small. This is because, as shown in FIG. 23 (a), the fluctuation of the evaluation value curve caused by the detection error or the like appears as a shift of the original curve in either direction, and the shape of the curve itself is large. Although it is difficult to change, the change rate curve is obtained by differentiating the evaluation value curve, and therefore, the shape thereof hardly changes even when the evaluation value curve shifts.

  Therefore, as shown in FIG. 23B, a predetermined target value for the rate of change of the evaluation value, that is, a value Δt corresponding to the “effective rate of change” according to the present invention is determined, and the DC developing bias Vavg is set. On the other hand, the DC development bias Vd when the rate of change of the evaluation value monotonously decreasing substantially matches the target value Δt is obtained, and the DC development bias is obtained based on this value Vd and the optimum value obtained from the evaluation value curve. What is necessary is just to obtain | require the optimal value of Vavg. For example, if the difference between the value obtained from the evaluation value curve and the value Vd obtained from the rate of change curve is not so large, either one of these values or a value obtained based on these values (for example, The average value of both) may be the optimum value of the DC developing bias Vavg. However, if the difference between the two is large, in order to solve the above-described problems, the smaller amount of toner adhesion, that is, the smaller value of the DC developing bias | Vavg | is set as the optimum value of the DC developing bias Vavg. It is preferable to do this. In this way, even when the value obtained from the evaluation value curve is greatly different from the original value Vt due to a detection error, such as the value Vf shown in FIG. 23A, the optimum value of the DC developing bias Vavg is obtained. Becomes the value Vd obtained from the change rate curve, and therefore it is possible to derive a value substantially close to the original value Vt.

  As described above, in the actual apparatus, the DC developing bias Vavg is not continuously changed as described above, but the DC developing bias Vavg is discretely changed in six stages of V0 to V5. Yes. Therefore, as shown in FIG. 24A, six evaluation values are derived corresponding to the image density of each patch image, and an evaluation value curve is obtained by performing linear interpolation between them. FIG. 24 is a diagram showing an evaluation value curve and a change rate thereof in this embodiment. In addition, as the evaluation values are obtained discretely in this way, the rate of change is obtained as a difference Δ between the evaluation values corresponding to two patch images having different DC development bias Vavg by one step. That is, as described above, Δ = A (n + 1) −A (n).

  Then, based on the evaluation value curve of FIG. 24A, the DC development bias Vc when the evaluation value substantially coincides with the control target value At is set to the optimum value, but the DC development below this value Vc. When the difference Δ described above in the bias Vavg is equal to or less than the predetermined effective change rate Δa, the DC developing bias at that time is set to the optimum value Vop even if the evaluation value does not reach the control target value At. ing. That is, in the example of FIG. 24B, Vop = V3. Thus, in this embodiment, this value Δa corresponds to the “effective change rate” of the present invention. This value Δa is such that when there are two images whose evaluation values differ by Δa, the difference in density between the two cannot be easily discerned with the naked eye, or the difference in density between the two is acceptable in the apparatus. It is desirable to choose as follows.

  This prevents the DC developing bias Vavg from being set to a value higher than necessary even though there is almost no increase in image density due to the detection error of the density sensor 60 and the like. While effectively suppressing the occurrence of various problems, an image density close to a predetermined value can be obtained.

  On the other hand, in the region where the difference Δ is larger than the effective change rate Δa, the gradient of the evaluation value curve is large, so that the fluctuation of the DC developing bias Vavg accompanying the shift of the evaluation value curve due to the detection error is small. The optimum value Vop of the DC developing bias Vavg may be obtained from the above. Here, the “evaluation value” obtained from the sensor output value is used as the index representing the image density. However, the image density value itself or another index indicating the image density may be used in the same manner. Can do.

  As described above, the optimum value Vop of the DC developing bias Vavg at which a predetermined solid image density is obtained is set to any value in the range from the minimum value V0 to the maximum value V5. In this image forming apparatus, from the viewpoint of improving the image quality, the surface potential and the DC developing bias of the portion (non-image portion) where the toner is not attached corresponding to the image signal in the electrostatic latent image on the photoreceptor 2. The potential difference from Vavg is always constant (for example, 325 V), and when the optimum value Vop of the DC developing bias Vavg is obtained as described above, the charging given from the charging control unit 103 to the charging unit 3 accordingly. The magnitude of the bias is also changed, and the potential difference is kept constant.

(E) Exposure energy setting Subsequently, the exposure energy E is set to its optimum value. FIG. 25 is a flowchart showing exposure energy setting processing in this embodiment. As shown in FIG. 25, the processing content is basically the same as the development bias setting processing (FIG. 18) described above. That is, first, the DC developing bias Vavg is set to the optimum value Vop obtained previously (step S51), and then a patch image is formed at each level while increasing the exposure energy E by one level from the minimum level 0 (step S51). Steps S52 and S53). The sensor outputs Vp and Vs corresponding to the amount of reflected light from each patch image are sampled (step S54), spike noise is removed from the sample data (step S55), and an evaluation value representing the density of each patch image is obtained. The optimum value Eop of the exposure energy is obtained based on the result (step S56).

  In this processing (FIG. 25), the processing content is different from the above-described development bias setting processing (FIG. 18). It is the calculation process to be obtained, and the other processes are almost common. Therefore, here, the difference will be mainly described.

  In this image forming apparatus, an electrostatic latent image corresponding to an image signal is formed by exposing the surface of the photosensitive member 2 with the light beam L. For example, an exposed area such as a solid image is relatively wide. In the density image, the potential profile of the electrostatic latent image does not change much even if the exposure energy E is changed. On the other hand, for example, in a low density image in which exposed areas are scattered on the surface of the photoreceptor 2 like a fine line image or a halftone image, the potential profile changes greatly depending on the exposure energy E. Such a change in the potential profile causes a change in the density of the toner image. That is, the change in the exposure energy E does not significantly affect the high density image, but greatly affects the density of the low density image.

  Therefore, in this embodiment, first, a solid image in which the influence of the exposure energy E on the image density is small is formed as a high density patch image, and the optimum value of the DC developing bias Vavg is obtained based on the density, while the exposure energy E When obtaining the optimum value, a low density patch image is formed. Therefore, in this exposure energy setting process, a patch image having a pattern different from the patch image (FIG. 19) formed in the DC development bias setting process is used.

  Although the influence of the exposure energy E on the high density image is small, if the variable range is too wide, the density change of the high density image also becomes large. In order to prevent this, the variable range of the exposure energy E is an electrostatic latent image corresponding to a high density image (for example, a solid image) when the exposure energy is changed from the minimum (level 0) to the maximum (level 3). It is preferable that the change in the surface potential is within 20V, more desirably within 10V.

  FIG. 26 is a diagram showing a low-density patch image. As described above, in this embodiment, the exposure energy E is changed and set in four stages. Here, a total of four patch images Ie0 to Ie3 are formed, one for each level. Yes. Also, as shown in FIG. 26, the pattern of the patch image used here is composed of a plurality of thin lines that are spaced apart from each other, and more specifically, is a 1-dot 10-off 1-dot line pattern. The pattern of the low-density patch image is not limited to this, but if a pattern in which lines or dots are isolated from each other as described above, a change in exposure energy E can be more reflected in a change in image density. Therefore, it is possible to obtain the optimum value with higher accuracy.

  Further, the length L4 of each patch image is set to be smaller than the length L1 (FIG. 19) of the high-density patch image. This is because, in this exposure energy setting process, the DC developing bias Vavg is already set to the optimum value Vop, and under this optimum condition, density unevenness does not occur in the two cycles of the photoconductor (conversely, If such density unevenness occurs in the state, Vop is not an optimum value as the DC developing bias Vavg). However, on the other hand, there may be density unevenness due to deformation of the developing roller 44. Therefore, it is preferable to use an average value for the length corresponding to the circumferential length of the developing roller 44 as the density of the patch image. Therefore, the circumferential length L4 of the patch image is set to be larger than the circumferential length of the developing roller 44. If the moving speeds (peripheral speeds) of the surfaces of the developing roller 44 and the photosensitive member 2 are not the same in the non-contact developing type apparatus, the peripheral speed ratio is taken into consideration and the developing roller 44 corresponds to one round. A patch image having such a length may be formed on the photoreceptor 2.

  Further, the interval L5 between the patch images may be smaller than the interval L2 shown in FIG. This is because the energy density of the light beam L from the exposure unit 6 can be changed in a relatively short time, and particularly when the light source is composed of a semiconductor laser, the energy density is very short. It is because it can be changed. By configuring the shape and arrangement of each patch image in this way, it is possible to form all the patch images Ie0 to Ie3 on one turn of the intermediate transfer belt 71 as shown in FIG. Accordingly, the processing time is also shortened.

  For the low-density patch images Ie0 to Ie3 formed in this way, an evaluation value representing the image density is obtained in the same manner as in the case of the high-density patch image described above. Then, based on the evaluation value and the control target value derived from the low-density patch image lookup table (FIG. 17B) prepared separately from the above-described high-density patch image, exposure energy is obtained. The optimum value Eop is calculated. FIG. 27 is a flowchart showing an optimum value calculation process for exposure energy in this embodiment. Also in this process, as in the optimum developing bias value calculation process shown in FIG. 21, the evaluation values are compared with the target value At in order from the patch image formed at the low energy level so that the evaluation value matches the target value. The optimum value Eop is determined by determining the value of the exposure energy E (steps S571 to S577).

  However, in the range of normally used exposure energy E, the saturation characteristic (FIG. 20B) seen in the relationship between the solid image density and the DC developing bias does not appear between the fine line image density and the exposure energy E. Processing corresponding to step S473 in FIG. 21 is omitted. In this way, the optimum value Eop of the exposure energy E that can obtain a desired image density is obtained.

(F) Post-processing As described above, the optimum values for the DC developing bias Vavg and the exposure energy E are obtained, and thereafter, it becomes possible to form an image with a predetermined image quality. Therefore, at this time, the density control factor optimization process may be terminated, the rotational drive of the intermediate transfer belt 71 and the like may be stopped, and the apparatus may be shifted to a standby state, and other density control factors may be controlled. Some adjustment operation may be performed as much as possible, and the content of the post-processing is arbitrary as described above, and thus the description thereof is omitted here.

(3) Effect As described above, in this embodiment, for a patch image formed while changing the DC developing bias Vavg in six stages, an evaluation value corresponding to the image density is obtained and the rate of change is obtained, and the evaluation is performed. The absolute value | Vavg | of the DC development bias when the value substantially coincides with the control target value At and the DC development bias when the rate of change is equal to or less than the effective change rate Δa, that is, to the photosensitive member 2 is smaller. The value with the smaller toner adhesion amount is set as the optimum value Vop of the DC developing bias. Therefore, even if the obtained evaluation value includes an error due to the characteristic variation of the density sensor 60 or the characteristic variation of the toner, it is possible to prevent the value from being greatly different from the original optimum value. Yes.

  In this way, it is possible to set the DC developing bias Vavg to almost the optimum value while suppressing the influence of the detection error. In this image forming apparatus, the toner consumption amount is excessive, or the transfer / fixing failure is caused. Occurrence of problems is also suppressed, and as a result, it is possible to stably form a toner image with good image quality.

(4) Others In the above-described embodiment, the density sensor 60 is disposed opposite to the surface of the intermediate transfer belt 71 so as to detect the density of the toner image as the patch image primarily transferred to the intermediate transfer belt 71. However, the present invention is not limited to this. For example, a density sensor may be arranged facing the surface of the photoconductor 2 to detect the density of the toner image developed on the photoconductor 2.

  In the embodiment described above, the DC development in which the difference Δ shown in FIG. 24B is equal to or less than the effective change rate Δa before finding the DC development bias Vc at which the evaluation value shown in FIG. 24A becomes the control target value At. When the bias V3 is found, the value V3 is set as the optimum value Vop of the DC developing bias (FIG. 21). However, for example, as shown in FIG. 24, when the difference between the optimum value Vc obtained from the evaluation value curve and the optimum value V3 obtained from the rate of change thereof is relatively small, either of them may be set as the optimum value Vop. Absent. Therefore, the order of steps S473 and S474 in FIG. 21 may be interchanged. In such a case, the optimum value Vop of the DC developing bias when Vc and V3 are in the relationship illustrated in FIG. 24 is Vc.

  In the above-described embodiment, the optimum value Vop of the DC developing bias Vavg is obtained based on both the evaluation value curve and the change rate thereof. However, the optimum value Vop may be obtained based only on the change rate curve. . That is, there are cases where the optimum value of the density control factor can be obtained only by obtaining an image forming condition in which the change rate of the toner density substantially matches the predetermined effective change rate. For example, as shown in FIG. 23, when the correspondence between the evaluation value and the change rate, more generally, the toner density of the detected patch image and the change rate is known in advance, either one is obtained. Since the other can be obtained, the concentration control factor can be optimized from only one of them.

  In the conventional image forming apparatus, the density control factor is optimized based only on the detected toner density. However, as described above, the detection result may include an error. Focusing on the change rate of the toner density as in the present invention can eliminate the influence of the detection error and optimize the density control factor with higher accuracy. In particular, in a device in which the correspondence between the toner density and the density control factor has been clarified in advance, and the rate of change of the toner density with respect to the density control factor is large in the vicinity of the density target value, the density is required with sufficient accuracy. It is possible to optimize the control factor.

  The procedure of the optimization process of the concentration control factor in the above-described embodiment shows an example, and other procedures may be used. For example, in the present embodiment, the pre-operation 1 and the pre-operation 2 are started at the same time, but these may not necessarily be executed at the same time. The image density control target value may be obtained at least when the optimum value Vop of the DC developing bias is obtained. The control target value is obtained at a timing different from that of the present embodiment, for example, before the pre-operation. Also good.

  In the above-described embodiment, each sample data obtained by sampling the output of the density sensor 60 for one rotation of the intermediate transfer belt 71 is stored as a background profile of the intermediate transfer belt 71. However, a patch image is formed later. Only sample data from a position corresponding to the position may be stored, and in this way, the amount of data to be stored can be reduced. In this case, if the formation positions of the patch images on the intermediate transfer belt 71 are matched as much as possible, the calculation can be performed using the common base profile for each patch image, which is more effective. .

  In the above-described embodiment, the DC development bias and the exposure energy as the density control factors for controlling the image density are variable. However, only one of these may be variable to control the image density. A concentration control factor may be used. Furthermore, in the above-described embodiment, the charging bias is configured to change following the DC developing bias. However, the present invention is not limited to this, and the charging bias is fixed or independent of the DC developing bias. You may make it changeable.

Second Embodiment
FIG. 28 is a diagram illustrating a light amount control signal conversion unit in the second embodiment. In the apparatus according to the first embodiment (FIG. 4), the light quantity control signal Slc is output from the CPU 101 and directly input to the irradiation light quantity adjustment unit 605 of the density sensor 60, whereas the apparatus according to the second embodiment is provided with the CPU 101. Is different from the first embodiment in that a light amount control signal conversion unit 200 is provided between the irradiation light amount adjustment unit 605 and the irradiation light amount adjustment unit 605.

  The light quantity control signal conversion unit 200 supplies a light quantity control signal Slc having voltage values corresponding to two types of digital signals DA1 and DA2 output from the CPU 101 for light quantity control to the irradiation light quantity adjustment unit 605 of the density sensor 60. Is. The light quantity control signal converter 200 is provided with two D / A (digital / analog) converters 201 and 202 for converting two digital signals DA1 and DA2 from the CPU 101 into analog signal voltages VDA1 and VDA2, respectively. . These analog signals VDA1 and VDA2 are input to the arithmetic unit 210 via the buffers 203 and 204, respectively.

  In this embodiment, each of the D / A converters 201 and 202 has an 8-bit resolution and operates with a single + 5V power supply. In other words, these output voltages VDA1 and VDA2 take 256 discrete values from 0V to + 5V in accordance with the value (0 to 255) of the 8-bit digital signal DA1 or DA2 from the CPU 101. For example, when the digital signal DA1 from the CPU 101 is 0, the output voltage VDA1 of the D / A converter 201 is 0V. Whenever the value of the digital signal DA1 increases by 1, the output voltage VDA1 increases by the minimum voltage step ΔVDA = (5/255) V. When the digital signal DA1 is 255, the output voltage VDA1 of the D / A converter 201 is increased. Becomes + 5V. The same applies to the output voltage VDA2 of the D / A converter 202. Thus, both the output voltage VDA1 of the D / A converter 201 and the output voltage VDA2 of the D / A converter 201 can take 256 discrete values corresponding to an 8-bit digital signal.

  Here, in order to finely control the amount of light emitted by the light emitting element 601, it is desirable that the light amount control signal Slc can be set in multiple steps in fine steps. If the number of bits of the digital signals DA1 and DA2 is increased, finer settings can be made, but this is not practical in terms of apparatus cost. That is, it is necessary to use a D / A converter 201, 202 having a larger number of input bits and higher resolution, but such a device is expensive. In particular, for a CPU, it is necessary to use a product with a data bit length of 16 bits in order to handle data exceeding 8 bits. Such a product is very expensive compared to a product with a data bit length of 8 bits. turn into.

  Therefore, in this embodiment, the calculation unit 210 performs a predetermined calculation on the output voltages of the two D / A converters 201 and 202, and the calculation result is used as the light quantity control signal Slc, thereby obtaining the data bit length. Is limited to 8 bits, and the light quantity can be controlled with high resolution while suppressing the apparatus cost.

The arithmetic unit 210 is a subtraction circuit composed of four resistors 211 to 214 and an operational amplifier 215. Of the four resistors 211 to 214, the two resistors 211 and 214 have the same resistance value R1, and the other two resistors 212 and 213 have the same resistance value R2 (where R2> R1). have. In such a configuration, the output voltage Vout output from the calculation unit 210 is expressed by the following formula:
Vout = VDA1− (R1 / R2) VDA2 (2-1)
It is represented by This output voltage Vout is input to the irradiation light quantity adjustment unit 605 of the density sensor 60 as the light quantity control signal Slc.

  In the above equation (2-1), when the value VDA1 increases by ΔVDA, the output voltage Vout also increases by ΔVDA. On the other hand, when the value VDA2 increases by ΔVDA, the output voltage Vout decreases by (R1 / R2) ΔVDA. That is, when the value of the digital signal DA1 given from the CPU 101 to the D / A converter 201 is changed by 1, the output voltage Vout changes by ΔVDA, whereas the value of the signal DA2 given to the D / A converter 202 is 1. When the output voltage Vout is changed only by the amount, the output voltage Vout changes by (R1 / R2) ΔVDA. Therefore, the light quantity control signal Slc can be adjusted by the minimum voltage step (R1 / R2) ΔVDA by appropriately setting the combination of the values of the signals DA1 and DA2. For example, if the resistance values R1 and R2 are determined so that (R1 / R2) = 1/4, the light amount control signal Slc can be set within the range of 0 to +5 V and the minimum voltage by the combination of the values of the signals DA1 and DA2. An arbitrary value can be set in the step (ΔVDA / 4). This is an improvement in resolution corresponding to 2 bits as compared with the case where setting is performed only with the value of the 8-bit digital signal DA1.

  FIG. 29 is a principle diagram illustrating a method for setting the light amount control signal. Here, the case where (R1 / R2) = 1/4 is described as an example. First, with only the 8-bit digital signal DA1 from the CPU 101, the output signal Vout can be set only in increments of the minimum voltage step ΔVDA as shown by white circles in FIG. For example, when the value of the signal DA1 is (X-1), the output signal Vout is Vout (x-1) as shown in FIG. 29, whereas the value of the signal DA1 increases by 1 to become X. For example, the output signal Vout becomes Voutx which is larger than this by ΔVDA, and the output signal Vout cannot be set to an intermediate value between them.

  Here, if the value of the signal DA1 is X and the value of the signal DA2 is increased from 0 by 1, the output signal Vout decreases from Voutx by (ΔVDA / 4). That is, as shown by the black circles in FIG. 29, by setting the value of the signal DA2 in the range from 0 to 3, the value of the intermediate output signal Vout from Vout (x-1) to Vout can be taken. become able to. That is, the light amount control signal Slc can be set with a higher resolution (four times in this example) than when only the signal DA1 is used.

  If the value of the signal DA1 is fixed and the output voltage Vout is adjusted only by the signal DA2, the output voltage can be set in fine steps, but on the other hand, the variable range of the output voltage itself becomes narrow. As described above, a variable range can be obtained by combining the signal DA1 for setting the output voltage Vout to an approximate value in a relatively coarse step and the signal DA2 for interpolating the voltage step in a finer step. This makes it possible to achieve both a wide area and high resolution.

  Thus, the increment of the output voltage Vout can be arbitrarily set by the value of the ratio (R1 / R2) between the resistance values R1 and R2. Therefore, from the viewpoint of improving the resolution, it is preferable to make the value (R1 / R2) as small as possible. However, the variable range of the output voltage Vout by the signal DA2 also becomes small according to the value of this ratio. In order to interpolate the voltage step ΔVDA corresponding to the minimum step 1 of the signal DA1 by adjusting the signal DA2, it is not preferable to make the range of the output voltage Vout adjustable by the signal DA2 smaller than ΔVDA. More specifically, since the data bit length of the signal DA1 is 8 bits, if the value (R1 / R2) is made smaller than (1/256), the output voltages Vout are Vout (x-1) and Voutx. Cannot be evenly interpolated.

  In an actual apparatus, the resistance values R1 and R2 may be determined according to the bit length of data handled by the apparatus and the resolution necessary for setting the light quantity. In this embodiment, R1 = 1 kΩ and R2 = 64.9 kΩ, thereby realizing a resolution equivalent to almost 14 bits while the data bit length is 8 bits.

  FIG. 30 is a flowchart showing a reference light amount setting process in the second embodiment. FIG. 31 is a diagram for explaining the principle of the reference light quantity setting process. This reference light quantity setting process includes “sensor calibration (1), (2)” (steps S21a, S21b) and “reference light quantity control” among the operation steps of the pre-operation 1 in the first embodiment shown in FIG. Instead of each step of “Signal setting” (step S22), this is executed in the apparatus of the second embodiment. Specifically, it is a process of setting the values of the signals DA1 and DA2 so that the light amount control signal Slc for causing the light emitting element 601 to emit light with a predetermined reference light amount is given to the irradiation light amount adjustment unit 605. Other apparatus configurations and operations of the second embodiment are the same as those of the first embodiment.

  As shown in FIG. 30, in this reference light quantity setting process, dark output is first detected as in the first embodiment (step S211). Here, the output voltages Vp and Vs of the light receiving elements 670p and 670s are detected with the light emitting element 601 turned off. In the following description, instead of analog values of the output voltages Vp and Vs of the two light receiving elements, detection values Dp and Ds obtained by converting these voltage values into 10-bit digital values by an A / D conversion circuit (not shown), respectively. Will be used.

  Thus, the values Dp and Ds detected when the light emitting element 601 is turned off are stored as dark output values Dp0 and Ds0, respectively. These values are digital values corresponding to the analog values described as the dark outputs Vp0 and Vs0 in the first embodiment. In order to reduce the detection error, the voltage detection is performed at 22 samples at 8 msec intervals, and the average thereof is set as the dark output values Dp0 and Ds0.

  Next, the light emitting element 601 emits light with a low amount of light, and the detection value Dp corresponding to the p-polarized component at that time is detected (step S212). At this time, in order to cause the light emitting element 601 to emit light with a low light amount, the value DATEST1 of the signal DA1 output from the CPU 101 to the D / A converter 201 is set to 56, while the value of the signal DA2 output to the D / A converter 202 is set to 56. 0. In this state, the detection value Dp of 312 samples is acquired, and the average value is Pave1.

  Next, the light emitting element 601 emits light with a high amount of light, and a detection value Dp corresponding to the p-polarized component at that time is detected (step S213). The value DATEST2 of the signal DA1 at this time is set to 67 so that the amount of light is higher than in the previous step. The value of the signal DA2 is again 0 here. In this state, the detection values Dp of 312 samples are acquired in the same manner, and the average value is set as Pave2.

  Note that the values DATEST1 and DATEST2 of the signal DA1 for causing the light emitting element 601 to emit light with a low light amount and a high light amount are not limited to the above numerical values, but light emission occurs in the relationship between the light emission amount of the light emitting element 601 and the signal DA1. These values are preferably set to numerical values belonging to a region where the light emission amount of the element 601 is proportional to the value of the signal DA1. In this way, calculation can be performed by linear interpolation.

Then, as data for use in the calculation described later, the rate of change of the detected value Dp with respect to the value of the signal DA1:
ΔDp = (Pave2-Pave1) / (DATEST2-DATEST1) (2-2)
Is obtained (step S214).

  Here, the following calculation method differs depending on the magnitude relationship between the target value Dpt corresponding to the detection value Dp when the light emitting element 601 emits light with the reference light amount and the value Pave2 obtained above (step). S215). The target value Dpt here is a value corresponding to an analog value obtained by adding the dark output Vp0 to 3V, as in the case of the first embodiment. Note that a linear relationship is established between the detected value Dp and the value of the signal DA1, and the value ΔDp previously obtained corresponds to the slope of this straight line.

(1) Pave2 ≧ Dpt: Step S216 (FIG. 31 (a))
In this case, as shown in FIG. 31 (a), since the target value Dpt is in the middle of the actually measured values Pave1 and Pave2, the set values DA10 and DA20 of the signals DA1 and DA2 for obtaining the target light quantity are interpolated. Can be sought. First, the value of DA1 when the detected value Dp is equal to or greater than the target value Dpt and is closest to the target value Dpt is set as the set value DA10 of DA1. Then, the value DA20 of the signal DA2 is obtained so that the detected value Dp is closest to the target value Dpt in combination with the set value DA10.

Specifically, the following formula:
DA10 = DATEST2-INT [(Pave2-Dpt) / ΔDp] (2-3)
DA20 = [(Pave2−Dpt) mod ΔDp] / (ΔDp / 64.9) (2-4)
To obtain set values DA10 and DA20. Here, INT [x] represents an operator for obtaining a maximum integer not exceeding x, and [x
mod y] represents an operator for obtaining a remainder when x is divided by y.

(2) Pave2 <Dpt: Step S217 (FIG. 31 (b))
In this case, as shown in FIG. 31 (b), the target value Dpt is not in the middle of the actually measured values Pave1 and Pave2, so the set values DA10 and DA20 of the signals DA1 and DA2 for obtaining the target light quantity are extrapolated. Ask for. The basic method is the same as above, but the calculation formula is slightly different, and the following formula:
DA10 = DATEST2 + INT [(Dpt−Pave2) / ΔDp] +1 (2-5)
DA20 = {ΔDp − [(Dpt−Pave2) mod ΔDp]} / (ΔDp / 64.9) (2-6)
To obtain set values DA10 and DA20.

  In the subsequent operation, in order to cause the light emitting element 601 to emit light with the reference light amount, the signals DA1 and DA2 output from the CPU 101 to the D / A converters 201 and 202 may be set to the set values DA10 and DA20, respectively. In this way, the light amount control signal Slc corresponding to the reference light amount is given to the irradiation light amount adjustment unit 605, and thereby the light emitting element 605 emits light with the reference light amount. Note that since the light amount of the light emitting element 601 is not stable immediately after the light amount control signal Slc is changed, it is preferable to detect the light amount after a predetermined time has elapsed after the change. In this embodiment, when the value of the signal DA1 or DA2 is changed, only the detection value that has passed 100 msec or more after the change is made valid.

  Needless to say, the numerical values such as the resistance value and the set value described above are merely examples, and are not limited to these numerical values.

<Third Embodiment>
Next, a third embodiment of the image forming apparatus according to the present invention will be described. The configuration of the image forming apparatus of this embodiment is such that the image forming apparatus of the first embodiment described above is further provided with a light amount control signal conversion unit 200 of the second embodiment. However, as will be described later, the apparatus configuration is partially different. Along with this, the contents of the processing in the concentration control factor optimization processing are also partially different. Here, the differences between the device configuration and the concentration control factor optimization process in the present embodiment and the first or second embodiment described above will be described below. Description of common parts is omitted.

(1) Difference in Apparatus Configuration In the first embodiment described above, the density sensor 60 (FIG. 4) includes a light receiving unit 670p that receives p-polarized light component of reflected light from the intermediate transfer belt 71, and an s-polarized light component. The light receiving unit 670s that receives light has been described as having the same configuration. On the other hand, in the third embodiment, the gains of the amplifier circuits 673p and 673s of both light receiving units are set to different values. This is because the reflected light received by the light receiving unit 670s as the s-polarized component is scattered light, so the output voltage Vs corresponding to the s-polarized component is lower in level than the output voltage Vp corresponding to the p-polarized component, This is to compensate for the narrow dynamic range of the signal. In other words, by increasing the gain of the amplifier circuit 673s corresponding to the s-polarized component, the dynamic range of the output voltage Vs is widened, and density detection can be performed with higher accuracy.

  Specifically, the gain of the amplifier circuit 673s is set to Sg times the gain of the amplifier circuit 673p (where Sg> 1). The gain magnification Sg may be appropriately determined according to the optical characteristics of the intermediate transfer belt 71, the sensitivity of the light receiving elements 672p, 672, and the like. However, as will be described later, both sensors at the maximum density of the color toner are used. If the output voltages Vp and Vs are equal to each other, it is convenient for later calculations. Accordingly, when various calculations are performed using the detection values of both the output voltages Vp and Vs from the concentration sensor 60, first, detection corresponding to the output voltage Vp is performed in order to make the ranges of both detection values uniform. It is necessary to multiply the value by Sg.

(2) Optimization processing execution timing and processing contents to be executed In the apparatus according to the first embodiment, a series of optimization shown in FIG. 8 is performed at the timing after the apparatus power is turned on or immediately after any unit is replaced. It was configured to execute the digitization process. On the other hand, in the apparatus of the third embodiment, immediately after the power is turned on, when a new photoconductor 2 is mounted, or when any of the developing cartridges is replaced, the same optimization process as described above is executed. However, optimization processing is not required when the removed developer is reattached, so if the removed developer and the installed developer are the same, optimization processing is performed. Absent. In order to determine the identity of the developing devices, information unique to the developing device, for example, a manufacturing number may be stored in advance in the memory 91 provided in each developing device 4Y.

  Further, when the developing roller rotation speed and the dot count value counted for each developing device are referred to as information indicating the operating state of the developing device, and as a result, it is necessary to change the control target value used for density control The optimization process shown in FIG. 8 is executed. The reason for this is as follows. That is, also in this image forming apparatus, as in the first embodiment described above, the control target value of the patch image density when the density control factor is optimized differs depending on the usage state of the developing device.

  Therefore, by executing the optimization process at a certain time, the image density can be adjusted based on the control target value at that time. However, as the image formation is repeated from that time, the state of the toner in the developing device changes, and the image density gradually changes. In order to suppress such fluctuations in image density, not only when the power is turned on or when the unit is replaced, but also at an appropriate timing, for example, while a large number of images are being continuously formed. It is desirable to readjust the image density.

  There are various timings at which this readjustment is performed. For example, one of the rational methods is to execute the readjustment at a timing at which the control target value needs to be changed. In this way, when the necessity of changing the control target value occurs according to the change in the toner characteristics, the change can be immediately reflected in the image forming conditions to stabilize the image density. Because. The control target value is set based on the developing roller rotation speed and the dot count value counted for each developing device.

  Therefore, in this embodiment, when the developing roller rotation speed or the dot count value corresponding to the developing device of any of the four developing devices reaches a predetermined threshold value, the image density is readjusted. Like to do. Since the apparatus is in an operating state, the initialization operation in step S1 can be omitted from the optimization process shown in FIG. Thus, by omitting the initialization operation and only adjusting the image density, the processing time can be shortened and the waiting time of the user can be shortened.

  In addition, regarding the configuration of the apparatus, the engine controller 10 side rather than the main controller 11 has information such as whether or not the mounted developing device is the same as that taken out, and information such as the timing at which the control target value should be changed. It is easier to grasp by. Therefore, the CPU 101 of the engine controller 10 processes individual information on the developing device and information on the operation status. When it is determined that the image density needs to be adjusted based on the information, the CPU 101 notifies the main controller. 11 is notified to the CPU 111, and the CPU 111 that has received the notification shifts each part of the apparatus to an appropriate operation state for density adjustment.

(3) Base Profile Sampling Position of the Intermediate Transfer Belt 71 In the first embodiment, in order to eliminate the influence of the surface state of the intermediate transfer belt 71 on the toner image density detection result, one rotation of the intermediate transfer belt 71 is required. The substrate profile was determined for. On the other hand, in this embodiment, the base profile is obtained only for the area where the patch image is to be formed later on the surface of the intermediate transfer belt 71. In this way, the amount of data to be stored is reduced, and memory resources are saved.

  The patch image Iv0 shown in FIG. 19 will be described as an example. As described above, the length L1 of the patch image Iv0 is a length corresponding to the circumferential length L0 of the photoreceptor 2. Then, 74 points of the patch image Iv0 thus formed are sampled by the density sensor 60, and the density of the patch image Iv0 is obtained based on the result. Therefore, if the background profile is obtained at least at the same position as the 74 points at which the density is sampled in the patch image Iv0, the density of the patch image can be obtained without being affected by the surface state of the intermediate transfer belt 71. Is possible. Specifically, this is done as follows.

  FIG. 32 is a diagram showing the relationship between the base profile detection position and the patch image in this embodiment. First, sampling for obtaining the background profile of the surface of the intermediate transfer belt 71 by the density sensor 60 is output from the vertical synchronization sensor 77 in association with the rotational drive of the intermediate transfer belt 71 as shown in FIG. It starts after a certain time ts from the change of the vertical synchronization signal Vsync (FIG. 32 (a)). In the figure, the number with # indicates the sampling position. Then, 74 pieces of sample data detected from the third sampling position # 3 to the 76th sampling position # 76 are stored as valid data.

  Next, a patch image Iv0 is formed on the intermediate transfer belt 71. The patch image Iv0 is formed so as to cover at least sampling positions # 3 to # 76 as shown in FIG. More specifically, it is formed between sampling positions # 1 to # 78. Then, when detecting the density of the patch image Iv0, sampling is performed at the same sampling position where the background profile is detected, that is, sampling positions # 3 to # 76. Based on the 74 pieces of sample data for the background profile and the patch image Iv0 obtained in this way, the patch image density excluding the influence of the surface state of the intermediate transfer belt 71 can be obtained.

  In this way, it is not necessary to store the background profile sampling data for sampling positions (before # 2 and after # 77) outside the range where the density detection of the patch image Iv0 is performed, thus saving memory resources. be able to.

  The same applies to other patch images Iv1 and the like. In this embodiment, among the 312 sampling positions # 1 to # 312 on the circumference of the intermediate transfer belt 71, the following sampling positions are assigned to each patch image as a block corresponding to each patch image.

Iv0, Iv3: # 3 to # 76 (74 points)
Iv1, Iv4: # 119 to # 192 (74 points)
Iv2: # 235- # 308 (74 points)
Iv5: # 235- # 255 (21 points)
Ie0: # 56 to # 76 (21 points)
Ie1: # 119 to # 139 (21 points)
Ie2: # 182 to # 202 (21 points)
Ie3: # 245 to # 265 (21 points)
As described above, when the formation positions of the patch images are set so that the sampling positions are shared as much as possible, only 232 pieces of sample data need to be stored as the base profile. Furthermore, if only the sum or average value of the sample data in each block is stored as a representative value corresponding to each patch image, the number of data to be stored can be further reduced. In this case, the evaluation value is calculated based on the representative value in the block corresponding to each patch image.

(4) Development bias setting This is a process replacing “(D) Development bias setting” in the first embodiment. In this embodiment, by setting the developing bias setting parameter Pv that can take an integer value of 0 to 255, the DC developing bias Vavg can be set in 256 steps within the range of (-50) V to (-400) V. It has become. That is,
Vavg = − (50 + Pv × 350/255) [V] (3-1)
It is expressed. For example, if Pv = 0, Vavg = (− 50) V, and if Pv = 100, Vavg = (− 187.3) V. Hereinafter, the value of the development bias Vavg corresponding to the development bias setting parameter Pv is referred to as Vavg (Pv). In the above example, Vavg (0) = (− 50) V and Vavg (100) = (− 187.3) V. The image density increases as the developing bias setting parameter Pv increases.

  In this embodiment, the exposure energy can be set in eight stages from the lowest level E (0) to the highest level E (7). The image density is lowest at exposure energy E (0), and is highest at energy E (7).

  FIG. 33 is a flowchart showing the developing bias setting process in this embodiment. In this development bias setting process, first, the exposure energy is set to E (4) (step S401), and then each bias value is changed while changing the DC development bias Vavg by sequentially changing and setting the development bias setting parameter Pv. In step S402, a patch image is formed. The pattern and shape of the patch image to be formed are the same as those in the first embodiment shown in FIG. The values of the development bias setting parameter Pv (n) corresponding to the patch image Ivn are as follows: Pv (0) = 44 (corresponding to Vavg = −110V); Pv (1) = 76; Pv ( 2) = 108; Pv (3) = 140; Pv (4) = 172; Pv (5) = 204 (corresponding to Vavg = −330V).

For each patch image thus formed, the density sensor 60 detects the amount of reflected light for a predetermined number of samples (step S403), and after removing spike noise from the sample data (step S404), the patch image Ivn An evaluation value A (n) for is calculated (step S405). These arithmetic processes are the same as those in the first embodiment. Based on the obtained evaluation value, the optimum value Pvop of the developing bias setting parameter Pv that gives the optimum developing bias Vop is calculated (step S406). Between this optimum value Pvop and the optimum DC development bias Vop,
Vop = Vavg (Pvop) (3-2)
There is a relationship. Therefore, the optimum DC developing bias Vop can be obtained by obtaining the optimum value Pvop of the developing bias setting parameter Pv. In this embodiment, as will be described in detail below, different calculation methods are used for the color toner and the black toner.

  FIG. 34 is a flowchart showing the optimum value calculation processing of the developing bias setting parameter for the color toner in this embodiment. In this optimum value calculation process, first, the variable n is set to 0 (step S481), and the evaluation value A (0) of the patch image Iv0 is compared with the target value At (step S482). As a result, if the evaluation value A (0) is equal to or greater than the target value At (YES), the process jumps to step S487 to set the development bias setting parameter value Pv (0) when the patch image Iv0 is formed to the optimum value Pvop. Finish the calculation as This corresponds to a case where a sufficient image density is obtained despite the development bias parameter Pv being set to such a low value.

On the other hand, if NO in step S482, the process proceeds to a processing loop consisting of steps S483 to S486, and the optimum value of the developing bias parameter Pv is obtained as follows. That is, if the evaluation value A (n) for the patch image Ivn is equal to the target value At for the variable n (step S483), the process jumps to step S487, and the development bias parameter Pv (n) at that time is set. The optimum value is Pvop. Otherwise, the evaluation value A (n) for the patch image Ivn and the evaluation value A (n + 1) for the patch image Iv (n + 1) formed under the condition of one step higher density than this. It is determined whether there is a target value At between them (step S484). If there is a target value At between the two evaluation values, the process jumps to step S488, and an optimum value Pvop is obtained by interpolation based on the following calculation formula:
Pvop = {At−A (n)} / {A (n + 1) −A (n)} × {Pv (n + 1) −Pv (n)} + Pv (n) (3-3)
However, the calculation result shall be rounded off to an integer.

  If there is no target value At between the two evaluation values, the variable n is incremented (step S485), and the above process is repeated to obtain the optimum value Pvop. However, when the variable n reaches the maximum value 5 without finding the optimum value (step S486), the developing bias parameter Pv (n) at that time, that is, Pv (5) is set as the optimum value Pvop. When this process is performed for each color of yellow, cyan, and magenta, the optimum value Pvop of the development bias setting parameter is set to any value between Pv (0) and Pv (5) for each color. When the CPU 101 outputs this value Pvop to the developing device controller 104 (FIG. 2), the optimum developing bias Vop corresponding to the value is applied from the developing device controller 104 to the developing roller 44.

  FIG. 35 is a flowchart showing the optimum value calculation processing of the developing bias setting parameter for black toner in this embodiment. In a patch image with black toner, the saturation of the evaluation value with respect to the toner adhesion amount described in the first embodiment is more likely to occur than in the case of color toner. Therefore, in this embodiment, the optimum value of the developing bias setting parameter is obtained for black toner in consideration of the change rate of the evaluation value as in the first embodiment. That is, when the difference between the evaluation value A (n + 1) for the patch image Iv (n + 1) and the evaluation value A (n) for the patch image Ivn is equal to or smaller than Δa in step S493, the process jumps to step S497. The developing bias parameter Pv (n) when the patch image Ivn is formed is set to the optimum value Pvop.

  The other processing contents are almost the same as those of the color toner. Also, the same formula (3-3) as that for the color toner can be applied to the calculation formula in step S498. In this way, the development bias setting parameter Pv that gives the optimum development bias Vop is obtained for the four toner colors (Y, M, C, K).

(5) Exposure energy setting This is a process that replaces “(E) Exposure energy setting” in the first embodiment. As described in the section “(4) Development bias setting” in the present embodiment, the exposure energy can be set in eight stages E (0) to E (7) in the apparatus of the third embodiment. Specifically, by setting the exposure energy setting parameter Pe to any of 0 to 7, the exposure energy of the light beam L emitted from the exposure unit 6 is set to E (Pe). In the exposure energy setting process of this embodiment, four of the four exposure energies: E (0); E (2); E (4); E (7) are patch images under the optimum developing bias Vop. And a parameter Pe that gives an optimum value of the exposure energy based on the image density is obtained for each toner color. Since the processing content is basically the same as the exposure energy setting processing (FIG. 25) of the first embodiment, the description is omitted. However, instead of directly calculating the optimal exposure energy Eop in step S57, the optimal The optimum value of the exposure energy setting parameter Pe that gives the exposure energy Eop is obtained.

  As described above, the image forming apparatus of the third embodiment has a partially different configuration and operation from the apparatus of the first embodiment. However, even with the configuration as described above, similarly to the apparatus of the first embodiment, it is possible to perform image formation by setting the DC developing bias Vavg and the exposure energy E to optimum values, and a toner image with good image quality can be obtained. It is possible to form stably.

  In the first and second embodiments, processing contents that are different from each other may be exchanged for those having the same purpose. For example, in the apparatus of the first embodiment, the development bias setting process (FIGS. 33 to 35) in the third embodiment may be applied instead of the development bias setting process (FIGS. 18 and 21), or vice versa. .

<Fourth embodiment>
Next, the reason why it is important to consider the surface state of the image carrier in order to accurately obtain the image density of the patch image formed on the image carrier such as the photoreceptor 2 or the intermediate transfer belt 71 will be described. To do. A specific embodiment for measuring the image density of a toner image with high accuracy regardless of the surface state of the image carrier will be described. FIG. 36 is a diagram showing sensor output values obtained at each sampling position before and after the formation of a patch image (toner image) on an image carrier having a uniform surface state. FIG. 37 is a diagram showing sensor output values obtained at each sampling position before and after the formation of a patch image (toner image) on an image carrier having an uneven surface state.

  Many density sensors used in image forming apparatuses irradiate light from a light emitting element toward an image carrier, receive light reflected from the image carrier by a light receiving element, and output an analog signal corresponding to the amount of light received. It is configured to output. In the image forming apparatus, the image density is measured based on the sensor output value obtained by converting the analog signal into a digital signal. Here, assuming that the reflectance and surface roughness are constant over the entire surface of the image carrier and that the surface state of the image carrier is uniform, a toner image such as a patch image is formed on the image carrier. The previous sensor output value becomes a constant value T regardless of the sampling position, for example, as shown in FIG. For example, when patch images having different densities OD1 to OD3 are formed on the image carrier, the sensor output values change by an amount corresponding to the image density at the first to third patch positions, respectively. D2 and D3 ((b) in the figure). Here, since the surface state of the image carrier is uniform, the sensor output values D1, D2, and D3 are constant values at the respective patch positions.

  However, in the actual image forming apparatus, the surface state of the image carrier is not uniform, and the sensor output value is, for example, as shown in FIG. 37 (a) before forming a toner image such as a patch image on the image carrier. It fluctuates according to the sampling position. Further, when a plurality of patch images having different densities OD1 to OD3 are formed on the image carrier, the sensor output values change by an amount corresponding to the image density at the first to third patch positions (same as above). FIG. 5B shows the details of each patch position. The sensor output value fluctuates in accordance with the sampling position even in the same patch area. This is considered to be influenced by the surface state of the image carrier.

  Moreover, as can be seen by comparing FIG. 10A and FIG. 10B, the amount of variation at each patch position becomes smaller as the patch image becomes darker. In other words, the influence of the surface state at each patch position becomes weaker as the patch image becomes darker. In order to make this clearer, plotting sensor output values when uniform density images are formed on the entire surface of the image carrier at different densities OD1 to OD3, for example, the result shown in FIG. 38 is obtained.

FIG. 38 is a graph showing sensor output values before an image is formed on the image carrier and sensor output values when three density images having a uniform density are formed on the image carrier. “Tave”, “Dave_1”, “Dave_2”, and “Dave_3” in FIG.
“Tave”: Average sensor output value before forming an image on the image carrier,
“Dave_1”: Average sensor output value when an image of density (OD1) is formed,
“Dave_2”: Average sensor output value when an image of density (OD2) is formed,
“Dave_3”: Average sensor output value when an image of density (OD3) is formed,
Is shown. Here, these “Tave”, “Dave_1”, “Dave_2”, and “Dave_3” are almost the same as “T”, “D1”, “D2”, “D3” in FIG. , "Dave_2" and "Dave_3" are obtained, a value obtained by canceling the influence of the surface state of the image carrier is obtained, and each image density can be accurately detected.

  As can be seen from the figure, the influence of the surface state of the image carrier on the sensor output value varies depending on the density of the toner image formed on the image carrier. That is, when a relatively low density toner image is formed on the image carrier, a part of the light from the light emitting element passes through the toner image and is reflected by the image carrier, and then the image carrier again. Since the light passes through the body and is received by the light receiving element, the output from the density sensor varies relatively greatly depending on the surface state of the image carrier. On the other hand, not only light that passes through the toner image and enters the image carrier as the toner image becomes darker, but also light that passes through the image carrier again after being reflected by the image carrier and enters the light receiving element. As a result, the influence of the surface state of the image carrier on the output from the density sensor is reduced. Therefore, a sensor output value (indicating the surface state of the image carrier) before forming an image on the image carrier is obtained in advance as correction information and formed in a certain surface area on the image carrier, for example, the sampling position x1. When the image density of the toner image is actually detected, the sensor output value at the sampling position x1 is uniformly corrected by the correction information without considering the density of the toner image at all, and the toner image of the toner image is corrected based on the correction value. Obtaining the image density has a certain limit in its accuracy.

  In contrast, when the image density of the toner image formed at the sampling position x1 is actually detected, not only the detected value is corrected based on the correction information, but also the correction information is corrected according to the density of the toner image. By doing so, the measurement accuracy of the image density can be further improved.

  Furthermore, the inventor of the present application has found that the variation amount of the sensor output value is proportionally smaller as the image density on the image carrier increases. Based on this, it has been found that values “Dave_1”, “Dave_2”, and “Dave_3” can be obtained by canceling the influence of the surface state of the image carrier by calculating as follows. Hereinafter, this will be described in detail with reference to FIG.

  FIG. 39 is a diagram illustrating a relationship between sensor output values before and after the formation of the first patch image (toner image). In the figure, reference numeral x1 is a sampling position indicating the position of the surface region on the image carrier, and sensor output values at the sampling position x1 before and after the formation of the first patch image are T (x1) and D (x1), respectively. It has become. Further, the symbol D0 in the figure indicates a so-called dark output value obtained by converting an analog signal output from the light receiving element into a digital signal with the light emitting element of the density sensor turned off. The reason why the dark output value D0 is obtained in this way is to remove the influence of the dark output component by subtracting the dark output value D0 from the sensor output value to increase the density measurement accuracy. That is, D0 is a reference value related to the amount of light received by the sensor.

Here, as described above, as the density of the first patch image on the image carrier increases, the fluctuation amount of the sensor output value decreases proportionally, and therefore, the following equation (Tave−D0) / (T (x1) -D0)
= (Dave_1-D0) / (D (x1) -D0) (4-1)
It is considered that the relationship shown in FIG. The left side of the equation (4-1) shows the relationship before the toner image is formed. The average sensor output value Tave and the sensor before the toner image is formed on the image carrier after the dark output value D0 is removed. The ratio to the output value T (x1) is shown. On the other hand, the right side shows the relationship when the toner image having the same density as the first patch image is uniformly formed, and the average value Dave_1 (that is, the average value of the sensor output when the toner image is uniformly formed on the image carrier) The ratio between the sensor output value D (x1) and the value obtained by canceling the influence of the surface state of the image carrier is shown. These ratios are considered to be equal.

Furthermore, when transforming equation (4-1),
(Dave_1−D0) = (D (x1) −D0) × {(Tave−D0) / (T (x1) −D0)} (4-2)
Is obtained. Therefore, the dark output value D0, the average sensor output value Tave before the toner image is formed on the image carrier, and the sensor output value T (x1) in the surface area x1 are obtained before the patch image is formed. By detecting the sensor output value D (x1) in the surface area x1 on which the first patch image is formed when the patch image is formed, and substituting each value into the above equation (4-2), the image carrier A sensor output value obtained by removing both the influence of the surface state and the influence of the dark output component is obtained as the correction value C (x1), and the image density of the first patch image is obtained based on the correction value C (x1) (= Dave_1−D0). Can be obtained accurately.

  FIG. 39 shows only the case where the first patch image is formed, but the same applies to the second and third patch images.

  In the above description, a case has been described in which a signal from the light receiving element of the density sensor is A / D converted to obtain a sensor output value, and an image density of the patch image is obtained based on the single sensor output value. As in the first embodiment and the third embodiment, the reflected light from the image carrier is divided into two light components, and sensor output values are obtained based on the light amounts of these light components, and these two sensor output values are obtained. Based on this, the image density of the patch image may be obtained. In particular, when the patch image is formed of black toner, the former density measurement is suitable, and when the patch image is formed of color toner, the latter density measurement is suitable.

  Next, the operation of the image forming apparatus according to the fourth embodiment will be described. Note that the mechanical and electrical configuration of the image forming apparatus according to the embodiment described below is the same as that of the first embodiment, and thus description thereof is omitted.

  FIG. 40 is a flowchart showing a concentration control factor optimization process executed in the fourth embodiment. In this image forming apparatus, the CPU 101 determines the optimum value of the density control factor by controlling each part of the apparatus according to a program stored in advance in the ROM 106 at the timing described above.

  First, before transferring the patch image to the intermediate transfer belt 71 corresponding to the “image carrier” of the present invention, steps S71 to S73 are executed to obtain information on the intermediate transfer belt 71 as correction information. That is, in the first step S71, dark output voltages Vp0 and Vs0 are detected, and values obtained by A / D conversion of them are stored in the RAM 107 as dark output values Dp0 and Ds0, respectively. Here, “dark output voltages Vp0, Vs0” means that a light amount control signal Slc (0) corresponding to a turn-off command is output to the irradiation light amount adjustment unit 605 to turn off the light emitting element 601. This is an output voltage indicating the amount of s-polarized light, and means a dark output component of p and s-polarized light. Then, as will be described later, by subtracting the dark output values Dp0 and Ds0 from the actually detected sensor output values, the adverse effects of the dark output components are eliminated, and more accurate measurement is possible. As described above, in this embodiment, the dark output values Dp0 and Ds0 are obtained as reference values related to the amount of light received by the sensor, which corresponds to the “reference value detection step” of the present invention.

  Next, a signal Slc (2) having a signal level exceeding the dead zone is set as the light quantity control signal Slc, and the light quantity control signal Slc (2) is given to the irradiation light quantity adjustment unit 605 to turn on the light emitting element 601 (step S72). . Then, the light from the light emitting element 601 is irradiated onto the intermediate transfer belt 71, and the light quantity of p-polarized light and s-polarized light reflected by the intermediate transfer belt 71 is detected by the reflected light quantity detection unit 607, and the received light quantity is changed to each received light quantity. Corresponding output voltages Vp and Vs are A / D converted and input to the CPU 101 as sensor output values. Then, the CPU 101 calculates correction information based on the sensor output value and stores it in the RAM 107 (step S73; correction information detection step).

  FIG. 41 is a flowchart showing correction information calculation processing. In this correction information calculation process (step S73), when a predetermined time elapses after the vertical synchronization signal Vsync is output (step S731), sensor output values Tp (x) and Ts (x) for p-polarized light and s-polarized light. The sensor output value for one cycle of the intermediate transfer belt 71 before forming the patch image is detected, the following three types of profiles are obtained as correction information, and stored in the RAM 107 (step S732).

Profile of p-polarized light: Tp (x) -Dp0
Profile of s-polarized light: Ts (x) -Ds0
Profile of ps ratio: Tps (x)
Note that Tps (x) is the ratio of p-polarized light and s-polarized light at each sampling position x, that is,
Tps (x) = Sg × {(Tp (x) −Dp0) / (Ts (x) −Ds0)}
It is. Here, symbol Sg indicates a gain magnification relating to s-polarized light. In this embodiment, the gains of the amplifier circuits 673p and 673s are set so that the sensor output values at the maximum density of the color toner are the same. (FIG. 42). For this reason, the sensor output value also changes greatly according to the change in the image density. In particular, for color toner, the ps ratio Tps (x) decreases as the image density increases and becomes “1” at the maximum density.

Also, the average sensor output value of p-polarized light and ps ratio, that is,
Average sensor output value of p-polarized light: Tp_ave−Dp0
Average sensor output value of ps ratio: Tps_ave-Dps (color)
Are obtained and stored in the RAM 107 (step S733). Here, the symbol Dps (color) means the following contents. As described above, the ps ratio is set to “1” when the color toner maximum density is detected. However, in actuality, variations in components constituting the sensor, output detector accuracy at the time of setting, and adjustment are set. Depending on the adjustment accuracy by the method or the like, there is a case where it cannot be strictly set to “1”. The output when the maximum density of each toner is detected varies with respect to “1” depending on the specification, color, lot, and the like of the toner used. At this time, if the maximum density is detected as being fixed to “1”, the color toner detection accuracy and correction accuracy may be reduced. Therefore, the maximum density detection value of each color toner by the sensor is not simply fixed to “1” but can be set as Dps (color), thereby improving the accuracy of color toner detection based on the ps ratio. That is, Dps (color) is a reference value related to the amount of light received by the sensor when color toner is detected, and corresponds to D0 in equation (4-2).

  When correction information is obtained in this way, the process proceeds to step S74 in FIG. 40 to perform patch sensing processing. FIG. 43 is a flowchart showing the patch sensing process. In this patch sensing process (step S74), a patch image corresponding to the patch image signal stored in advance in the ROM 106 is formed on the photosensitive member 2 while changing the density control factor in multiple stages, and the patch image is intermediately transferred. Transfer is performed on the belt 71 (step S741).

As in the correction information calculation process (step S73), after a predetermined time has elapsed since the vertical synchronization signal Vsync was output (step S742), the patch image is moved to the sensing position of the density sensor 60. Every time it comes, steps S743 to S748 are executed to obtain correction values for all patch images. That is, in step S743, it is determined whether the patch image is formed with black toner (K) or color toner (Y, M, C). A sensor output value Dp (x) at the sampling position x corresponding to the formed surface region is detected (step S744; output detection step). And the formula corresponding to formula (4-2),
Cp (x) = (Dp_ave−Dp0) = (Dp (x) −Dp0) × {(Tp_ave−Dp0) / (Tp (x) −Dp0)} (4-2A)
Based on this, a correction value Cp (x) is calculated (step S745, see FIG. 44). That is, the p-polarized average sensor output value (Tp_ave−Dp0), the sensor output value (Tp (x) −Dp0), and the dark output value Dp0 stored in the RAM 107 are read out as described above. The sensor output value Dp (x) detected in this way is substituted into the above equation (4-2A) to correct the sensor output value Dp (x) to calculate the correction value Cp (x) (correction value calculation step) .

On the other hand, if it is determined in step S743 that the toner is color toner, the sensor output values Dp (x) and Ds (x) at the sampling position x corresponding to the surface area where the patch image is formed are detected (step S743). S746). And the formula corresponding to formula (4-2),
Cps (x) = Dps_ave = (Dps (x) −Dps (color)) × {(Tps_ave−Dps (color)) / (Tps (x) −Dps (color))} + Dps (color) (4-2B )
The correction value Cps (x) is calculated based on (step S747, see FIG. 45). That is, the average sensor output value {Tps_ave−Dps (color)} of the ps ratio stored in the RAM 107, the ps ratio value {Tps (x) −Dps (color)} at the sampling position x, and the reference value Dps ( color) is read out and substituted into the above equation (4-2B) together with the sensor output values Dp (x) and Ds (x) ps ratio Dps (x) detected as described above to correct the ps ratio. Then, the correction value Cps (x) is calculated (correction value calculation step).

  If such a detection operation (steps S744 and S746) and calculation processing (steps S745 and S747) are performed on all patch images, that is, if “YES” is determined in the step S748, the process proceeds to a step S75 in FIG. The image density of each patch image is calculated based on the correction values Cp (x) and Cps (x). Then, the optimum value of the density control factor is determined based on these image densities (step S76; density derivation step).

  As described above, according to this embodiment, prior to obtaining the image density of the patch image (toner image) formed on the intermediate transfer belt 71, three types of profiles indicating the surface state of the intermediate transfer belt 71 are obtained. When the image density of the patch image is stored in advance as correction information, the sensor output value detected by the density sensor 60 is not used as it is, but the sensor output value is calculated based on the correction information. Since the correction is made, the influence of the surface state of the intermediate transfer belt 71 can be canceled and the image density of the patch image can be measured with high accuracy, and an image can be formed with a stable density based on the measurement result. It becomes.

  In the above embodiment, the image density of the patch image is obtained in consideration of the density of the patch image. In other words, since the correction information is corrected according to the density of the patch image on the intermediate transfer belt 71, the measurement accuracy of the image density can be further improved. In addition, as a method for obtaining the correction value, two types of processes, that is, a process for obtaining the correction value Cp (x) by executing steps S744 and S745 and a process for obtaining the correction value Cps (x) by executing steps S746 and S747. Are prepared and selectively executed according to the toner color that forms the patch image. Therefore, the image density of the patch image can be obtained by the optimum processing corresponding to each toner color, and the image density measurement accuracy can be obtained. It is advantageous in improving the quality.

  By the way, the output voltages Vp and Vs from the density sensor 60 described above are caused by a change in reflectance due to minute dirt and scratches on the roller 75 and the intermediate transfer belt 71, and electrical noise mixed in the sensor circuit. Spike-shaped noise may be superimposed. Therefore, it is desirable to execute spike noise removal as in the first and third embodiments.

Note that although the density of the patch image itself is obtained based on the correction values Cp (x) and Cps (x) in step S75 in FIG. 40, it may be converted into a value indicating the density instead of the density value. . For example, an evaluation value A indicating the image density of a patch image formed with black toner is expressed by the following equation: Evaluation value A = 1−Cp (x) / Tp_ave
On the other hand, an evaluation value A that indicates the image density of a patch image formed with color toner is calculated as follows: Evaluation value A = 1− {Cps (x) −Dps (color)} / {Tps_ave−Dps (color )}
You may make it calculate | require based on. These evaluation values are obtained by normalizing detection values for patch images using correction information indicating the surface state of the intermediate transfer belt 71 as a scale representing the toner adhesion amount for each color. Like the image density, the evaluation value varies depending on the toner individuality information and the operation status of the apparatus (for example, toner usage status), but the relationship between the evaluation value and the image density in each situation is obtained in advance through experiments and stored in a table. It is possible to keep it. Therefore, the evaluation value is suitable as a scale representing the image density with the detection error corrected.

  In the fourth embodiment, the density of the patch image formed with the color toner is obtained based on the ratio between the p-polarized light and the s-polarized light. However, the density of the patch image is obtained based on the difference between the p-polarized light and the s-polarized light. You may do it. Hereinafter, a description will be given with reference to FIGS. 46 to 48.

  First, prior to transferring the patch image to the intermediate transfer belt 71 corresponding to the “image carrier” of the present invention, as in the fourth embodiment, steps S71 to S73 are executed to obtain information on the intermediate transfer belt 71. Obtained as correction information. However, as will be described later, since the density of the color patch image is obtained based on the difference between p-polarized light and s-polarized light, correction information is calculated according to the operation flow shown in FIG.

  FIG. 46 is a flowchart showing correction information calculation processing. In this correction information calculation processing, sampling of the p-polarized and s-polarized sensor output values Tp (x) and Ts (x) is started when a predetermined time has elapsed since the vertical synchronization signal Vsync was output (step S731). Then, the sensor output value for one cycle of the intermediate transfer belt 71 before the patch image is formed is detected, the following three types of profiles are obtained as correction information, and stored in the RAM 107 (step S734).

Profile of p-polarized light: Tp (x) -Dp0
Profile of s-polarized light: Ts (x) -Ds0
ps difference profile: Tp_s (x)
Tp_s (x) is a difference between p-polarized light and s-polarized light at each sampling position x, that is,
Tp_s (x) = Sg × {Tp (x) −Dp0} − {Ts (x) −Ds0}
It is. Also in this embodiment, the gains of the amplifier circuits 673p and 673s are set so that the sensor output values at the maximum density of the color toner are the same (FIG. 42). For this reason, the sensor output value also changes greatly according to the change in the image density. In particular, the ps difference Tp_s (x) for the color toner decreases as the image density increases.

Also, the average sensor output value of p-polarized light and ps difference, that is,
Average sensor output value of p-polarized light: Tp_ave−Dp0
Average sensor output value of ps difference: Tp_s_ave = {Sg × Σ [Tp (x) −Dp0] −Σ [Ts (x) −Ds0]} / (sampling number)
Are obtained and stored in the RAM 107 (step S735).

When correction information is obtained in this way, patch sensing processing shown in FIG. 47 is performed. FIG. 47 is a flowchart showing the patch sensing process. In this patch sensing process, the same steps as the patch sensing process (FIG. 43) in the fourth embodiment are executed except for the method of calculating the correction value for color. That is, in step S741, a patch image is formed on the photoreceptor 2 while changing the density control factor in multiple stages, and the patch image is transferred to the intermediate transfer belt 71. Further, after a predetermined time has elapsed since the vertical synchronization signal Vsync was output (step S742), when the patch image formed of black toner (K) moves to the sensing position of the density sensor 60, the patch image The sensor output value Dp (x) at the sampling position x corresponding to the surface region where is formed is detected (step S744; output detection step). And the formula corresponding to formula (4-2),
Cp (x) = (Dp_ave−Dp0) = (Dp (x) −Dp0) × {(Tp_ave−Dp0) / [Tp (x) −Dp0]} (4-2A)
Based on this, a correction value Cp (x) is calculated (step S745, see FIG. 44). That is, the p-polarized average sensor output value (Tp_ave−Dp0), the sensor output value (Tp (x) −Dp0), and the dark output value Dp0 stored in the RAM 107 are read out as described above. The sensor output value Dp (x) detected in this way is substituted into the above equation (4-2A) to correct the sensor output value Dp (x) to calculate the correction value Cp (x) (correction value calculation step) .

On the other hand, when the patch image formed with the black toner (K) moves to the sensing position of the density sensor 60, the sensor output value Dp (x at the sampling position x corresponding to the surface area where the patch image is formed. ), Ds (x) is detected (step S746). And the formula corresponding to formula (4-2),
Cp_s (x) = Dp_s_ave = Dp_s (x) × (Tp_s_ave / Tp_s (x)) (4-2C)
The correction value Cp_s (x) is calculated based on (step S749, see FIG. 48). That is, the ps difference average sensor output value (Tp_s_ave) and the ps difference value (Tps (x)) at the sampling position x stored in the RAM 107 are read out, and the sensor output value Dp (x ) And Ds (x) and the ps difference Dp_s (x) are substituted into the above equation (4-2C) to correct the ps difference to calculate a correction value Cp_s (x) (correction value calculation step).

  Such detection operation (steps S744 and S746) and calculation processing (steps S745 and S749) are executed for all patch images, that is, if “YES” is determined in step S748, the image density of each patch image is determined. Calculation is based on the correction values Cp (x) and Cp_s (x). Then, the optimum value of the density control factor is determined based on these image densities.

  Note that it is the same as in the fourth embodiment that it is desirable to perform spike noise removal, and that it may be converted into a value indicating the density instead of the density value.

<Fifth Embodiment>
By the way, in the non-contact developing type image forming apparatus, the developing roller 44 and the photosensitive member 2 are arranged to face each other with a gap therebetween. The size of the gap depends on manufacturing variations of the apparatus and deformation due to thermal expansion. Due to the above, each device, and even a single device, varies slightly depending on the position or with time. If there is such a gap variation, the intensity of the alternating electric field that causes the toner to fly also varies. As a result, the image density of the toner image may fluctuate greatly. Therefore, a patch processing technique suitable for a non-contact development type image forming apparatus was examined.

  FIG. 49 is a diagram illustrating a development position in the non-contact development type image forming apparatus. FIG. 50 is a diagram illustrating an example of the waveform of the developing bias. In this apparatus, the developing roller 44 provided in one developing device (for example, the yellow developing device 4Y in FIG. 1) arranged at a position facing the photoconductor 2 and the photoconductor 2 are arranged to face each other with a gap G therebetween. ing. Then, a development bias is applied from the development control unit 104 to the development roller 44. As shown in FIG. 50A, the developing bias is an alternating voltage having a waveform in which a rectangular wave voltage having an amplitude Vpp is superimposed on the DC component Vavg. As will be described later, by applying a developing bias having such a waveform, the flying amount of the toner can be controlled by the amplitude Vpp, and the image density can be controlled by the DC component Vavg.

  Note that the waveform of the alternating voltage as the developing bias is not limited to this, and for example, a sine wave or a triangular wave may be superimposed on the DC component. For example, as shown in FIG. 50B, a waveform whose duty ratio is not 50% may be used. In this case, as the DC component Vavg, a weighted average voltage, that is, a value obtained by averaging instantaneous values of a voltage waveform whose amplitude changes with time over a certain time range and converting it into a DC voltage value can be used.

  With respect to the duty ratio of the developing bias, a direction in which toner adhesion to the photosensitive member 2 is promoted, that is, a period in which a negative (upper side in the figure) voltage is applied to one period (symbol t0) in the waveform of FIG. It has been found through experiments by the inventors that the density of the fine line image increases as the duty of (symbol t1), that is, (t1 / t0) is made smaller than 50%. More specifically, when the duty ratio is changed while the amplitude Vpp of the developing bias is kept constant, and the direct current component Vavg is adjusted so that the density of the solid image at that time is constant, the density of the thin line image is The inventor has obtained the knowledge that there is duty dependency and the density of the fine line image is higher as the duty ratio is smaller. In addition, when the flying property of the toner decreases due to the change of the apparatus with time or the deterioration of the toner, the quality of the fine line image is likely to deteriorate. Therefore, in order to continuously form a fine line image with more stable image quality, it is preferable to make the period during which the negative voltage is applied smaller than 50%, and the duty ratio (t1 / t0) of the developing bias is 30 to 48%, More desirably, the content is about 35 to 45%.

  Returning to FIG. 49, when an alternating voltage as a developing bias is applied to the developing roller 44, an alternating electric field is generated at the developing position DP sandwiched between the developing roller 44 and the photoreceptor 2. Due to the action of this electric field, a part of the toner TN carried on the developing roller 44 is released from the developing roller 44 and flies to the developing position DP and reciprocates (reference T3). The toner thus flying adheres to each part of the photoconductor 2 according to the surface potential, whereby the electrostatic latent image on the photoconductor 2 is developed with the toner.

  Here, in the development process performed as described above, there is an appropriate range for the amount of toner that is allowed to fly to the development position DP. FIG. 51 is a diagram showing the relationship between the toner density on the photoreceptor 2 and the optical density of the toner image. As shown in FIG. 51, when the density of the toner constituting the toner image is increased, the optical density is increased. However, if the toner is in a densely adhered state, the optical density does not change much even if the amount of adhered toner is further increased, and exhibits saturation characteristics in a region where the toner density is high as shown in FIG. In other words, in such a state where the toner adheres at a high density, even if there is some variation in the amount of toner adhering to the photoreceptor 2, the image density hardly changes. Since the density of the toner adhering to the photoconductor 2 as a toner image depends on the amount of toner flying to the development position DP, this characteristic varies slightly if the amount of toner flying is increased to some extent. This shows that the density change of the obtained toner image can be reduced.

  In a non-contact development type image forming apparatus, in order to form a toner image with little density unevenness and high image contrast, image formation should be performed under such conditions that a toner flying amount with little change in image density can be obtained. Is preferred. This is because it is inevitable that the non-contact development type apparatus causes a certain amount of fluctuation in the gap G for manufacturing reasons. However, this makes it possible to suppress fluctuations in image density due to gap fluctuation. Because. However, if the amount of toner to be adhered is excessively large, toner consumption becomes violent, and there is a possibility that the transfer / fixing process described later may be hindered. Therefore, the upper limit of the toner amount is regulated by these requests.

  In this embodiment, the configuration shown in the following (1) and (2) ensures a necessary and sufficient amount of toner flying, and, as will be described later, controls the DC developing bias and exposure energy to control the image. The density is adjusted.

  (1) The regulating blade 45 regulates the thickness of the toner layer on the developing roller 44 to about two toner layers. Of the toner TN constituting the toner layer, the toner (reference numeral T4 shown in FIG. 49) that is in direct contact with the developing roller 44 has a strong mirror image force acting on the developing roller 44, so that it is difficult to fly. Therefore, the thickness of the toner layer is set to about two toner layers, and the amount of toner that is more likely to fly without directly contacting the developing roller 44 is increased. When toner that is easy to fly is present, the toner can fly from the developing roller 44 with a relatively small force, and the toner on the developing roller 44 is reciprocated according to an alternating electric field. There is also an effect of causing the toner T4 to fly by colliding with T4. Therefore, a sufficient amount of toner can be supplied to the development position DP.

  (2) The amplitude Vpp of the developing bias is made as large as possible without causing discharge at the developing position DP. In the image forming apparatus of the non-contact development type as in this embodiment, it is possible to control the toner flying amount by changing the electric field intensity generated at the development position DP, but the fluctuation of the gap G (FIG. 49). The intensity of the alternating electric field also changes depending on Therefore, by setting the alternating voltage amplitude Vpp as high as possible, a sufficient amount of toner can be ejected even when the gap G is large and the electric field is weak. However, if the voltage is set too high, a discharge occurs between the developing roller 44 and the photosensitive member 2 and the image quality is remarkably deteriorated. Therefore, it is necessary to set the voltage so that such a discharge does not occur. In this third embodiment, the design center value of the gap G is 150 μm, but the gap when the developing roller 44 and the photosensitive member 2 are closest to each other is 80 μm, and the amplitude Vpp of the developing bias is set to 1500 V. The frequency is 3 kHz. Further, the duty ratio of the developing bias is 40%.

  In order to stably form a toner image with good image quality, the image forming apparatus according to the fifth embodiment forms a predetermined patch image at an appropriate timing such as when the power is turned on, and based on the image density. Patch processing that optimizes image forming conditions. Specifically, the CPU 101 of the engine controller 10 executes a program stored in advance, and performs the processing shown in FIG. 52 for each toner color. FIG. 52 is a flowchart showing patch processing of the image forming apparatus. The outline of this patch processing is as follows.

  In the process shown on the left side of FIG. 52, the energy per unit area (hereinafter simply referred to as “exposure energy”) E of the exposure beam L is temporarily set to a constant value, for example, the median value in the variable range (step S81), for example, a solid image is formed as a high-density patch image under each bias condition while changing and setting the DC component of the developing bias (hereinafter referred to as “DC developing bias”) Vavg (steps S82 to S85). Then, the image density of each patch image formed in this way is detected by the density sensor 60 (step S86), and when the density substantially matches the preset target value, in this embodiment, the optical density OD = 1.3. A bias value is obtained and set as the optimum developing bias.

  Subsequently, the process on the right side of FIG. 52 is executed. That is, the DC developing bias Vavg is set to the optimum developing bias obtained previously (step S91), and a pattern of, for example, 1 on 10 off is formed as a low-density patch image under each energy condition while changing the exposure energy E. In this way, a fine line image composed of a plurality of one-dot lines spaced apart from each other is formed (steps S92 to S95). Then, the image density of each patch image formed in this way is detected by the density sensor 60 (step S96), and when the density substantially matches the preset target value, in this embodiment, the optical density OD = 0.22. The exposure energy is obtained and the value is set as the optimum exposure energy.

  The reason for this will be described with reference to FIG. FIG. 53 is a diagram showing an example of the surface potential profile of the photoreceptor 2 when an electrostatic latent image corresponding to a solid image and a fine line image is formed. When the photosensitive member 2 charged to a uniform surface potential Vu is partially exposed by the light beam L, the charge of the portion is neutralized and an electrostatic latent image is formed on the surface of the photosensitive member 2, but a solid image is formed. In such a high-density image, a relatively wide range of the surface of the photoconductor 2 is exposed, so that the surface potential profile is a well type that is lowered to about the residual potential Vr determined by the characteristics of the photoconductor 2. On the other hand, in an image for low density such as a thin line image, the exposed area is narrow, so the surface potential Vsur has a sharp dip profile. In the figure, an example of only one line is shown as the low-density image, but the same applies to a plurality of lines that are spaced apart from each other.

  When the electrostatic latent image having such a potential profile is conveyed to the developing position DP facing the developing roller 44 carrying the toner, the toner flying back and forth at the developing position DP is transferred to the developing roller 44. The photosensitive member 2 adheres to any one of the parts in accordance with the DC potential of each part. At this time, as the potential difference between the DC developing bias Vavg and the surface potential Vsur of the photosensitive member 2 increases, toner transfer from the developing roller 44 to the photosensitive member 2 is promoted. Therefore, the photosensitive member 2 increases as the potential difference, that is, the contrast potential Vcont increases. The density of the toner adhering to the toner increases, and the image density increases accordingly.

  Considering the case where the exposure energy is changed, as shown by the dotted line in FIG. 53, the change in the surface potential profile is small in the solid image, whereas the depth or width of the dip in the thin line image, or Both will change greatly. Thus, the influence of the exposure energy on the potential profile of the electrostatic latent image is small for the solid image and large for the thin line image. Therefore, the density of the toner image to be developed also changes greatly due to the exposure energy E in the thin line image, while the change in the solid image is small.

  On the other hand, when the DC developing bias Vavg is changed, the contrast potential Vcont changes, so that the image density greatly changes in both the solid image and the thin line image.

  As described above, the influence on the image density of the solid image and the thin line image is different between the two parameters, that is, the DC developing bias Vavg and the exposure energy E. That is, the image density of the thin line image is greatly influenced by both the DC development bias Vavg and the exposure energy E, whereas the image density of the solid image changes greatly depending on the DC development bias Vavg, but changes very much depending on the exposure energy E. do not do.

  This will be described in more detail with reference to FIG. FIG. 54 is a diagram showing isodensity curves for a solid image and a fine line image. More specifically, the solid image and the fine line image are changed while changing the combination (Vavg, E) of the DC developing bias Vavg and the exposure energy E. When the image is formed, a combination in which each image density matches the target density (OD = 1.3 and OD = 0.22) is shown.

  As described above, since the influence of the exposure energy E on the density of the solid image is small, the isodensity curve indicating the optical density OD = 1.3 in the solid image has an inclination close to vertical as shown by the solid line in FIG. is doing. The meaning is as follows. That is, when the combination (Vavg, E) of the DC developing bias Vavg and the exposure energy E is on this curve, an image density of the target value OD = 1.3 is always obtained if a solid image is formed under this condition. Here, since the slope of the curve is almost vertical in the exposure energy region of EA or more shown in FIG. 54, if the DC developing bias Vavg is set to the potential VA shown in FIG. 54, the value of the exposure energy E in this region. Regardless of this, a solid image of the target density is obtained. The equal density curve is curved below the exposure energy EA because the surface potential Vsur of the photosensitive member 2 is not sufficiently lowered to the residual potential Vr by exposure with such weak energy, and the magnitude of the energy is small. This is caused by the change in the depth of the latent image.

  Therefore, a patch image for high density with various DC developing bias Vavg under exposure energy E equal to or higher than EA (in this embodiment, the median value in the variable range is set to be larger than EA). As a solid image is formed, and a bias potential VA is obtained such that the density becomes a target value (OD = 1.3), thereby obtaining an optimum value of the DC developing bias Vavg for obtaining a desired image density with the solid image. Can be requested. As described above, in the solid image, the exposure energy E may be an arbitrary value equal to or greater than EA.

  On the other hand, in the thin line image, the image density changes depending on both the exposure energy E and the DC developing bias Vavg, and the equal density curve becomes a downward-sloping curve as shown by the broken line in FIG.

  In order to obtain the target image density in both the solid image and the thin line image, the DC developing bias Vavg and the exposure energy E are set so as to be a combination corresponding to the intersection of the two curves in FIG. Good. Here, the value of the DC development bias Vavg corresponding to this intersection point is a bias that can obtain a solid image of the target density, as is apparent from the fact that the isodensity curve corresponding to the solid image has a substantially vertical inclination. This is almost the same as the value already obtained as the potential VA. That is, it can be seen that the optimum DC development bias VA for the solid image obtained previously was the optimum development bias Vop in this apparatus that can obtain the target density even for the thin line image. Accordingly, a thin line image as a low density patch image is formed with various exposure energies E while giving this optimum value Vop as the DC developing bias Vavg, and the density becomes a target value (OD = 0.22). By obtaining the exposure energy Eop, it is possible to obtain image forming conditions (Vop, Eop) that satisfy the target density for both solid images and thin line images.

  In determining the variable ranges of the DC developing bias Vavg and the exposure energy E, it is a matter of course that a desired image density can be obtained for both solid images and fine line images within the range of possible combinations. Such matters are also considered.

  That is, if the contrast potential (Vcont shown in FIG. 53) is extremely increased or decreased to obtain a desired image density, the image blurs (if the contrast potential Vcont is too high, for example, when a solid image of about 1 cm square is formed, the image is displayed. Cause image quality deterioration due to other factors such as toner scattering and distortion (when the contrast potential Vcont is low, for example, when a solid image of about 1 cm square is formed, the image is distorted in a diamond shape instead of a square). In addition, since the residual potential Vr of the photosensitive member 2 has variations due to its temperature and manufacturing variation, the variable range of the DC developing bias Vavg covers the variation of the photosensitive member 2 and the contrast potential Vcont is set to a predetermined value. It is necessary to determine a range that can be within the range. In this embodiment, the variable range of the DC developing bias Vavg is set to (−110 V) to (−330 V).

  Further, according to the knowledge of the inventors, it is known that the potential difference between the surface potential Vu of the unexposed area (non-image portion) of the surface of the photoreceptor 2 and the DC developing bias Vavg also affects the image quality. . For example, when the potential difference is increased, the fogging of the toner to the non-image area and the reproducibility of the isolated dot line are reduced. On the other hand, when this potential difference becomes small, soiling tends to occur. Therefore, in this embodiment, the charging bias from the charging control unit (FIG. 2) is changed in conjunction with the change of the DC developing bias Vavg, whereby the potential difference between the two (| Vu | − | Vavg |). Is maintained at a constant value (350 V).

  Further, although the depth of the electrostatic latent image in the solid image is little changed by the exposure energy E, it does not change at all. Therefore, if the variable range of the exposure energy E is excessively large, the change in the exposure energy E causes the solid image to change. The density also fluctuates, making it difficult to find optimal image forming conditions. Therefore, when the exposure energy E is changed from the minimum value to the maximum value in the variable range in order to make the density change of the solid image negligible even if the exposure energy E changes, the solid image of the electrostatic latent image becomes solid. The variable range of the exposure energy E should be determined so that the change in the surface potential of the region corresponding to the image is within 20V, more preferably within 10V.

  Needless to say, these values are determined according to the configuration of the present embodiment, and should be appropriately changed according to the configuration of the apparatus.

  As described above, in this embodiment, the toner layer carried on the developing roller 44 is made thicker than the toner layer and the developing bias amplitude Vpp is set as high as possible in order to make the toner fly easily. The toner density at the development position DP is sufficiently increased in advance, and the image density is adjusted by controlling two parameters (DC development bias Vavg and exposure energy E) constituting the image forming conditions.

  When optimizing these parameters, a solid image is formed as a high-density patch image while changing the DC developing bias Vavg to various values while temporarily setting the exposure energy E to a constant value. The optimum value Vop of the DC developing bias is obtained based on the image density. Then, under the optimum DC developing bias Vop thus obtained, a thin line image is formed as a low-density patch image while changing the exposure energy E to various values, and the optimum value Eop of the exposure energy based on the image density. Seeking.

  As described above, in the image forming apparatus of this embodiment, the optimum values can be obtained individually and reliably for each parameter by a relatively simple process, and the image forming conditions optimized in this way are obtained. By performing image formation under the above, it is possible to stably form a toner image with good image quality.

<Sixth Embodiment>
Next, a sixth embodiment of the image forming apparatus according to the present invention will be described. The apparatus of this embodiment is partly different in the configuration of the developing device compared to the fifth embodiment, but the other configurations and operations are the same, so the description thereof is omitted here. FIG. 55 is a view showing a sixth embodiment of the image forming apparatus according to the present invention. In this embodiment, the developing roller 44 includes a metal roller 441 and a resistance layer 442 formed on the surface thereof. The resistance layer 442 is formed of, for example, a resin layer in which conductive powder is dispersed. Here, a metal powder such as aluminum, carbon black or the like can be used as the conductive powder, and a phenol resin, a urea resin, a melamine resin, a polyurethane resin, a nylon resin, or the like can be used as the resin layer. Further, the specific resistance of the resistance layer 442 is preferably 10 ⁴ Ωcm or more.

  Thus, the provision of the resistance layer 442 prevents the toner TN and the metal roller 441 from coming into direct contact with each other, whereby the mirror image force acting on the toner TN is reduced, and the toner from the developing roller 44 is reduced. Flight performance is improved. Accordingly, in this embodiment, as shown in FIG. 55, the regulating blade 45 regulates the thickness of the toner layer on the developing roller 44 to approximately one toner layer. This is because the provision of the resistance layer 442 makes it easy for the toner T5 that is in direct contact with the developing roller 44 to fly, as shown in FIG. This is because a sufficient amount of toner can be made to fly to the development position DP.

  Even in the apparatus configured as described above, the same processing (FIG. 52) as that of the apparatus of the first embodiment is performed, whereby the optimum values of the DC developing bias Vavg and the exposure energy E are individually obtained by simple processing. In this way, by performing image formation under the optimized image forming conditions, it is possible to stably form a toner image with good image quality.

  As described above, the apparatuses of the fifth and sixth embodiments described above are configured to increase the amount of toner flying at the development position DP, although the methods are different from each other. It can be suitably applied. This technique is also effective in an apparatus in which the toner flying amount is increased by other methods. In addition to the above, various methods are conceivable for increasing the toner flying amount.

For example, when titanium oxide is used as an external additive for the toner, it is possible to effectively reduce the so-called intermolecular force acting between the toner particles and the surface of the developing roller 44, and as a result, toner flying properties. Will improve. Further, there is toner fluidity as an index for evaluating the magnitude of the intermolecular force between the toner and the developing roller 44. The toner having higher toner fluidity can reduce the intermolecular force, and the standard of fluidity suitable for the toner used in the present invention is 25 ° or less in the angle of repose. Further, the fluidity of the toner depends on the coverage of the external additive on the toner base particles, and by setting the coverage to 1 or more, the intermolecular force can be reduced and the fluidity can be enhanced. Here, the coverage of the external additive is defined by the following formula:
(Coverage) = (D · ρ1 · w) / (d · ρ2 · W · π) (6-1)
In the above formula, D and d are the volume average particle diameters of the toner base particles and the external additive, ρ1 and ρ2 are the true specific gravity of the toner base particles and the external additive, and W and w are the toner base particles and the external additive, respectively. The mass of π is pi.

  In addition, since the image power increases as the particle size decreases for the same charge amount, it is also effective to use toner having a relatively large particle size in order to reduce the image power. According to the experiments by the inventors, it was found that a sufficient toner flying amount can be ensured by using a toner having a volume average particle diameter of 8 μm or more.

  In the fifth and sixth embodiments described above, the value of the exposure energy E is provisionally set to the median value in the variable range when forming a patch image for obtaining the optimum value of the DC developing bias Vavg. The value of the exposure energy at this time is not limited to this and is arbitrary. However, if the exposure energy is too large, the amount of toner adhering to the latent image increases and the amount of toner consumption increases. If the exposure energy is too small, not only the fine line image but also the density of the solid image changes depending on the exposure energy, and it becomes difficult to accurately determine the optimum image forming conditions. It is preferable that the value be equal to or greater than the sign EA shown in 54 and not too large.

<Others>
The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, it can be configured as follows.

  In each of the above-described embodiments, a solid image is used as a high-density patch image, and a thin line image composed of a plurality of one-dot lines that are spaced apart from each other is used as a low-density patch image. The images that can be formed are not limited to these, and may be images having other patterns. These should be changed as appropriate according to the characteristics of the toner used and the sensitivity of the density sensor. Further, the target density of each patch image is not limited to the above numerical value, and may be appropriately changed.

  In the above embodiment, the present invention is applied to an image forming apparatus in which the intermediate transfer belt 71 is an “image carrier” of the present invention. However, the application target of the present invention is not limited to this. The present invention can also be applied to an image forming apparatus using a transfer drum as a carrier, an image forming apparatus configured to measure the image density of a patch image formed on a photoreceptor, and an image of a photoreceptor or a transfer medium. The present invention can be applied to all image forming apparatuses and methods for obtaining the image density of a toner image formed on a carrier.

  In the above embodiment, the image forming apparatus is capable of forming a color image using four color toners, but the application target of the present invention is not limited to this, and an image that forms only a monochrome image. Naturally, it can also be applied to a forming apparatus. The image forming apparatus according to the above embodiment is a printer that forms an image provided from an external device such as a host computer on a sheet S such as copy paper, transfer paper, paper, and an OHP transparent sheet. Can be applied to all electrophotographic image forming apparatuses such as copying machines and facsimile machines.

  As described above, the present invention can be applied to electrophotographic image forming apparatuses such as printers, copiers, and facsimile machines, and the image density can be adjusted by adjusting the density control factor that affects the image density. It is possible to stabilize and improve the image quality.

1 is a diagram illustrating a first embodiment of an image forming apparatus according to the present invention. FIG. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus in FIG. 1. FIG. 3 is a cross-sectional view illustrating a developing device of the image forming apparatus. The figure which shows the structure of a density sensor. The figure which shows the electrical constitution of the light reception unit employ | adopted in the density | concentration sensor of FIG. The figure which shows the light quantity control characteristic in the density sensor of FIG. The graph which shows the mode of a change of the output voltage with respect to the reflected light quantity in the density sensor of FIG. The flowchart which shows the outline | summary of the optimization process of the concentration control factor in 1st Embodiment. 5 is a flowchart showing an initialization operation in the first embodiment. The flowchart which shows the pre operation in 1st Embodiment. FIG. 4 is a diagram illustrating an example of a base profile of an intermediate transfer belt. The flowchart which shows the spike noise removal process in 1st Embodiment. The figure which shows the mode of the spike noise removal in 1st Embodiment. FIG. 4 is a schematic diagram illustrating a relationship between a toner particle size and a reflected light amount. The figure which shows the response | compatibility with the particle size distribution of toner and the change of OD value. The flowchart which shows the derivation process of the control target value in 1st Embodiment. The figure which shows the example of the look-up table which calculates | requires a control target value. 6 is a flowchart showing development bias setting processing in the first embodiment. The figure which shows the patch image for high concentration. FIG. 6 is a diagram illustrating fluctuations in image density that occur in the photoreceptor cycle. 6 is a flowchart showing optimum DC bias calculation processing in the first embodiment. The graph which shows the relationship between the evaluation value about a direct-current developing bias and a solid image. The graph which shows the evaluation value with respect to DC developing bias, and its change rate. The figure which shows the evaluation value curve in 1st Embodiment, and its change rate. 6 is a flowchart showing exposure energy setting processing in the first embodiment. The figure which shows the patch image for low concentration. 6 is a flowchart illustrating an exposure energy optimum value calculation process according to the first embodiment. The figure which shows the light quantity control signal conversion part in 2nd Embodiment. The principle figure explaining the setting method of a light quantity control signal. The flowchart which shows the reference light quantity setting process in 2nd Embodiment. The figure explaining the principle of a reference light quantity setting process. The figure which shows the relationship between the base profile detection position and patch image in 3rd Embodiment. 10 is a flowchart showing development bias setting processing in the third embodiment. 10 is a flowchart showing an optimum value calculation process of a developing bias setting parameter for color toner according to a third embodiment. 10 is a flowchart showing an optimum value calculation process of a developing bias setting parameter for black toner according to a third embodiment. The figure which shows the sensor output value obtained in each sampling position before and after formation of the patch image (toner image) to the image carrier whose surface state is uniform. The figure which shows the sensor output value obtained in each sampling position before and after formation of the patch image (toner image) to the image carrier with the nonuniform surface state. The figure which shows the sensor output value obtained in each sampling position before and after formation of the density uniform image (toner image) to the image carrier whose surface state is non-uniform. The figure which shows the relationship of the sensor output value before and behind formation of a 1st patch image (toner image). 10 is a flowchart showing density control factor optimization processing executed in the fourth embodiment of the image forming apparatus according to the present invention; The flowchart which shows the calculation process of correction information. The graph which shows the mode of a change of the sensor output value with respect to the image density of a color toner. The flowchart which shows a patch sensing process. The figure which shows the relationship of the sensor output value before and after formation of the patch image (toner image) formed with black toner. The figure which shows the relationship of the sensor output value before and after formation of the patch image (toner image) formed with a color toner. The flowchart which shows the calculation process of correction information. The flowchart which shows a patch sensing process. The figure which shows the relationship of the sensor output value before and after formation of the patch image (toner image) formed with a color toner. FIG. 3 is a diagram illustrating a development position in a non-contact development type image forming apparatus. The figure which shows the example of the waveform of a developing bias. FIG. 4 is a diagram illustrating a relationship between toner density on a photoreceptor and optical density of a toner image. 10 is a flowchart showing patch processing in the fifth embodiment of the image forming apparatus according to the present invention; The figure which shows the example of the surface potential profile of the photoconductor at the time of forming the electrostatic latent image corresponding to a solid image and a thin line image. The figure which shows the equal density curve with respect to a solid image and a thin line image. FIG. 10 is a diagram illustrating a sixth embodiment of an image forming apparatus according to the present invention.

Explanation of symbols

  2 ... photosensitive member, 4K, 4C, 4M, 4Y ... developer, 6 ... exposure unit, 10 ... engine controller (control means), 44 ... developing roller, 60 ... density sensor, 71 ... intermediate transfer belt (image carrier) 104: Developing device controller 107: RAM (storage unit) 442: Resistance layer 601: Light emitting element 672p, 672s ... Light receiving element E: Exposure energy x, x1: Sampling position

Claims (8)

A density sensor that irradiates light toward the image carrier, receives reflected light from the image carrier, and outputs a signal corresponding to the amount of light received;
Before forming a toner image on the image carrier, information relating to the image carrier is stored in advance as correction information, and the density sensor is used to determine the image density of the toner image formed on the image carrier. Control means for correcting the output from the image based on the correction information, and determining the image density of the toner image based on the correction value,
The image forming apparatus according to claim 1, wherein the control unit corrects the correction information in accordance with a density of a toner image on the image carrier.   The image forming apparatus according to claim 1, wherein the control unit obtains the correction information based on a signal output from the density sensor and stores the correction information in a storage unit before forming a toner image on the image carrier.   The control means cancels an upper level and / or a lower level among sampling data constituting a signal output from the density sensor before forming a toner image on the image carrier, and the cancel data is transferred to the remaining data. The image forming apparatus according to claim 2, wherein the correction information is obtained by replacing with an average value of sampling data.   The image forming apparatus according to claim 1, wherein the control unit reduces a correction amount based on the correction information as a toner image becomes darker. Before forming a toner image on the image carrier, information about the image carrier is obtained as correction information, and when obtaining the image density of the toner image formed on the image carrier, an output from the density sensor is obtained. Is corrected by the correction information, and an image forming method for obtaining an image density of the toner image based on the correction value,
An image forming method, wherein the correction information is corrected according to the density of the toner image. Before forming a toner image on the image carrier, each of the plurality of surface regions x (x = x1, x2,...) On the image carrier is irradiated with light and light from the surface region. A correction information detection step of obtaining a detection value T (x) by detecting a value related to the amount of received light,
An output detecting step of irradiating the toner image formed on the surface region x1 of the image carrier with light and receiving light from the toner image to detect a value D (x1) related to the amount of received light;
A correction value calculation step of correcting the detection value D (x1) based on the following formula to obtain a correction value C (x1);
C (x1) = D (x1) × {Tave / T (x1)}
However, Tave is an average value of the detection values T (x).
And a density deriving step for obtaining an image density of the toner image based on the correction value C (x1). An image for detecting an image density of a toner image formed on the image carrier by a density sensor having a light emitting element for irradiating light toward the image carrier and a light receiving element for receiving reflected light from the image carrier. In the forming method,
A reference value detection step for obtaining a reference value D0 related to the amount of light received by the light receiving element;
Before forming a toner image on the image carrier, each of the plurality of surface regions x (x = x1, x2,...) On the image carrier is irradiated with light from the surface region. A correction information detection step of obtaining a detection value T (x) by receiving light and detecting a value related to the amount of received light;
An output detecting step of irradiating the toner image formed on the surface region x1 of the image carrier with light and receiving light from the toner image to detect a value D (x1) related to the amount of received light;
A correction value calculation step of correcting the detection value D (x1) based on the following formula to obtain a correction value C (x1);
C (x1) = {D (x1) −D0} × {(Tave−D0) / (T (x1) −D0)}
However, Tave is an average value of the detection values T (x).
And a density deriving step for obtaining an image density of the toner image based on the correction value C (x1). The correction information detection step includes
Before forming a toner image on the image carrier, each of the plurality of surface regions x (x = x1, x2,...) On the image carrier is irradiated with light from the surface region. A sub-process for receiving light and outputting a signal corresponding to the amount of light received;
A sub-step of canceling an upper level and / or a lower level of sampling data constituting the signal, and substituting the canceled data with an average value of the remaining sampling data to obtain the detected value T (x). Item 8. The image forming method according to Item 6 or 7.

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