JP5304618B2 - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a toner quantity detection technology configured to detect a toner quantity of an image on an image carrier based on a detection value obtained by an optical density detection means through the use of property information and configured such that the dark current of the optical density detection means may not affect the detection performance, and to provide an image forming apparatus configured to perform the image density control using the toner quantity detection technology and always stably output excellent images. <P>SOLUTION: There is provided the image density detection control to highly accurately detect the toner quantity of patch images showing all the density regions by, prior to the formation of the patch images, measuring an offset voltage generated by the dark current of a light-receiving element, acquiring property information adaptable to the offset voltage, and then, previously setting the acquired property information. Further, there is provided the image forming apparatus including the image density control part which is excellent in image stabilizing performance, configured to always output images of high image quality by adjusting image forming conditions based on the highly accurately detected toner quantity. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an image forming apparatus using an electrophotographic system such as a copying machine, a printer, a facsimile, or a composite machine thereof, and in particular, an image forming apparatus that detects an image forming characteristic in order to maintain an optimum image quality. About.

  In an image forming apparatus such as a copying machine, fluctuations in image density are likely to occur depending on various conditions such as the use environment and the number of copies. Therefore, when a predetermined timing is reached, a reference patch image for image density correction, such as a toner patch, is formed on the surface of the image carrier such as a photosensitive drum or an intermediate transfer belt, and the toner amount ( Density) is detected, and based on the result, image forming conditions are adjusted to stabilize the image density.

  The following documents are disclosed as so-called image density control techniques for detecting the toner amount (density) of a patch image and stabilizing the image density based on the result.

  In the technique described in Patent Document 1, a large number of patch forming surfaces for forming a patch image are set on an image carrier, and each patch forming surface from which an image (toner) has been removed is detected. Base correction control is executed to set the output condition of the light emitting part of the optical density detecting means so as to make the output of the light receiving part constant. Further, the density of the patch image formed on the image carrier is detected by driving the light emitting portion of the optical density detecting means under the output condition set by the base correction control. The problem of density detection error of the patch image due to the deviation of the surface reflectivity of the image carrier itself is solved.

  In Patent Document 2, the density of a patch image is acquired with reference to a correspondence table in which the output of the optical density detection means is stored in advance, and the image carrier is stained by the dirt on the surface of the image carrier (intermediate transfer member). The problem that the relationship between the density of the patch image formed with black toner and the output of the optical density detection means changes and a large error occurs in the density detection of the patch image is described, and a countermeasure against it.

  As a countermeasure, in the case of an image carrier that has been used for a long time, the correction output X is calculated from the output a of the optical density detection means using the following equation, and the correction output X is referred to the correspondence stored in advance. In this way, the density of the patch image is acquired.

X = (ac) x (b1-c) / (b2-c) + c
Here, a is the output of the optical density detection means, b1 is the output of the optical density detection means when the surface of the initial image carrier on which no patch image is formed, and b2 is the patch image formed. The output of the optical density detection means when detecting the surface of the image carrier after no long-term use, c is the dark current value of the optical density detection means.

JP 2007-249032 A JP-A-11-133700

FIG. 20A shows the detection characteristics of the optical density detection means used in the color image forming apparatuses of Patent Document 1 and Patent Document 2. The horizontal axis represents the toner amount of the patch image (toner mass g / m ² per unit area), and the vertical axis represents the detection output (V) of the optical density detection means.

  As shown in FIG. 20A, the output of the Y color, M color, and C color patch images increases in direct proportion to the toner amount, and has a constant sensitivity for a wide range of toner amounts. On the other hand, a black (K color) patch image has a relatively low sensitivity and a detection characteristic in which the output decreases in inverse proportion to the toner amount.

  FIG. 20B shows detection characteristics when the output gain is increased when detecting a K color image. As shown in the figure, the ratio of the change amount of the output to the change amount of the toner amount is significantly lower in the high toner amount region (A region) than in the low toner amount region (B region). Further, even if the toner amount is further increased in the high toner region, the output of the optical density detecting means tends to gradually approach a constant value of C1. C1 is an output voltage due to the dark current of the light receiving element. The dark current is a component that does not depend on the amount of light incident on the light receiving element.

  However, the dark current changes depending on the temperature of the light receiving element and variations in the element itself. FIG. 20B shows a characteristic curve of the optical density detecting means when different dark currents are generated. K color (1) is a characteristic curve when the dark current is c1, and K color (2) is a characteristic curve when the dark current is c2.

  As shown in the figure, when the patch image with the toner amount b1 is detected by the optical density detection means, the optical density detection means outputs a1 according to the characteristic curve of K color (1) when the dark current is c1. On the other hand, when the dark current is c2, the output of the optical density detection means outputs a2 according to the characteristic curve of K color (2). As described above, the optical density detection means detects different output values for patch images of the same toner amount (density) according to changes in dark current.

  In the techniques described in Patent Document 1 and Patent Document 2, since the toner amount of the patch image is acquired from the output value of the optical density detection means based on the characteristic curve stored in advance, the toner amount of the patch image is detected (acquired). ) Causes an error due to the dark current of the optical density detection means.

  For example, it is assumed that a characteristic curve of K color (1) is stored in advance and a patch image having the toner amount b1 is detected. As shown in FIG. 20B, when the dark current is c1, the optical density detection means outputs a1, and acquires the toner amount b1 based on the characteristic curve of K color (1). On the other hand, when the dark current is c2, the optical density detection means outputs a2. However, as shown in FIG. 20C, the toner amount b2 is acquired based on the characteristic curve of K color (1). That is, when the dark current changes from the reference value c1 to c2, a detection miscalculation of (b2-b1) occurs with respect to the original toner amount b1.

  An object of the present invention is to detect the toner amount of a patch image on an image carrier from the detection value of the optical density detection means using the characteristic information, and the toner whose dark current of the optical density detection means does not affect the detection performance In addition to providing an amount detection technique, an object is to provide an image forming apparatus capable of stably outputting an excellent image at all times by performing image density control using the toner amount detection technique.

  The object of the present invention is achieved by the following configuration.

1. An image forming unit that forms an image on the image carrier;
An optical density detector that has a light emitting element and a light receiving element disposed opposite to the image carrier and detects the reflection density of the image carrier surface by lighting the light emitter at a preset reference light emission setting value. Means,
A storage unit that stores predetermined characteristic information that associates a detection value of the optical density detection unit with a toner amount;
The image forming unit is controlled to form a patch image on the image carrier, the optical density detection unit is controlled to detect a detection value of the patch image, and the patch characteristic image is detected using the predetermined characteristic information. A control unit that acquires a toner amount of the patch image based on a detection value;
Prior to the formation of a patch image, the control unit measures an offset voltage generated by a dark current of the light receiving element, acquires characteristic information matching the offset voltage, and sets the characteristic information as the predetermined characteristic information An image forming apparatus.

  2. The control unit controls the optical density detecting unit to detect a non-image surface on the image carrier prior to formation of a patch image, and the detection value of the optical density detecting unit matches a predetermined value. 2. The image forming apparatus according to 1, wherein a light emission setting value of the light emitting element is acquired and the light emission setting value is set as the reference light emission setting value.

  3. 3. The image forming apparatus according to 2, wherein the control unit changes the reference light emission value according to a change in the offset voltage.

  4). The image forming apparatus according to any one of 1 to 3, wherein the control unit adjusts an image forming condition of the image forming unit so that a toner amount of the patch image is set to a target value. .

5. An image forming unit that forms an image on the image carrier;
An optical density detecting means having a light emitting element and a light receiving element disposed to face the image carrier and lighting the light emitting element under a predetermined light emission condition set in advance to detect a reflection density on the surface of the image carrier. When,
A storage unit that stores predetermined characteristic information that associates the detection value of the patch image by the optical density detection unit with the toner amount of the patch image;
The image forming unit is controlled to form a patch image on the image carrier, and when the detected value of the patch image detected by the optical density detecting means is within the calculated use range, the detected value is set to the predetermined characteristic. When the toner amount of the patch image is calculated with reference to the information, and the image forming condition of the image forming unit is adjusted based on the toner amount of the patch image, while the detection value of the patch image is outside the calculated use range A control unit that changes an image forming condition of the image forming unit so that a detection value of the patch image is within the calculated use range,
The control unit measures an offset voltage generated by a dark current of the light receiving element prior to formation of a patch image, and changes the calculated use range according to the offset voltage.

6). The patch image is a plurality of patch images showing different gradation values;
The control unit calculates a toner amount of a virtual patch image showing a gradation value having a higher density than the plurality of patch images based on a toner amount of the patch image, and the toner amount of the virtual patch image matches a target value. 6. The image forming apparatus as described in 5 above, wherein an image forming condition of the image forming unit is adjusted.

  7). 7. The image forming apparatus according to 5 or 6, wherein the gradation value of the virtual patch image is a maximum density that can be instructed to the image forming unit.

  8). The calculation use range is an upper limit that is a boundary between regions where unevenness of an unacceptable level occurs in the toner amount of the patch image, and a boundary between regions where the detection sensitivity of the optical density detection unit decreases to an unacceptable level. 8. The image forming apparatus according to any one of 5 to 7, wherein the image forming apparatus is a detection range of the optical density detector defined by a lower limit.

  In the present invention, the offset voltage of the optical density detection unit is measured, characteristic information matching the offset voltage is acquired, the characteristic information is set as predetermined characteristic information, and the optical density detection unit is detected using the predetermined characteristic information. By acquiring the toner amount of the patch image based on the detection value of the patch image, it is possible to provide an image density detection control technique for detecting the toner amount with high accuracy for the patch image in the entire density range. Furthermore, by adjusting the image forming conditions based on the toner amount of the patch image detected by the image density detection control, it is possible to provide an image density detection control capable of always outputting a high-quality image and an image forming apparatus including the control. To.

1 is an overall configuration diagram of an image forming apparatus according to the present invention. It is an expansion schematic diagram of an image formation part. It is an expansion schematic diagram of the optical density detection means which concerns on this invention. 3 is a control block diagram related to control of the image forming apparatus. FIG. It is a control block diagram related to image density stabilization control of each color image forming unit. It is a control block diagram related to image density detection control according to the present invention. It is a control flow figure showing Voff measurement control which measures the Voff voltage concerning the present invention. It is a control flowchart which shows the light emission amount setting control which concerns on 1st Embodiment of this invention. It is a control flow figure showing image density detection control concerning a 1st embodiment concerning the present invention. It is the characteristic curve which graphed an example of characteristic table Tbj. It is an example of a correspondence table associating each Voff range with a characteristic table Tbj (Tb1, Tb2,... TbA) matching the range. When the offset voltage Voff is Voff = a1, Voff = a5, or Voff = aA, it is a characteristic curve showing the characteristic tables Tb1, Tb5, TbA selected in step S200. It is a control flowchart which shows the light emission amount setting control which concerns on 2nd Embodiment. It is an example of the correction table CTb used for image density detection control according to the second embodiment. Characteristic curves KC1, KC5, and KCA that graph the correspondence relationships between the characteristic tables Tb1, Tb5, and TbA are shown. It is a control flowchart which shows the image density detection control of 3rd Embodiment which concerns on this invention, and image density stabilization control. 6 is a schematic diagram showing a first patch image Pa1 and a second patch image Pa2 formed on the intermediate transfer member 70. FIG. It is a characteristic view showing virtual patch density detection control according to the present invention. FIG. 6 is a schematic diagram showing a characteristic curve KC of the optical density detection means 8 when the offset voltage Voff changes, and a calculated usage range AR that is changed in accordance with the change of the offset voltage Voff. It is a related figure which shows the influence of the dark current in the detection characteristic of the optical density detection means of a prior art.

  Embodiments of the present invention will be described below. The description in this column does not limit the technical scope of the claims or the meaning of terms. In addition, the following assertive description in the embodiment of the present invention shows the best mode, and does not limit the meaning or technical scope of the terms of the present invention.

[Image forming apparatus]
FIG. 1 is a schematic configuration diagram showing a color image forming apparatus as an embodiment of the image forming apparatus of the present invention.

  The printer unit 101 of the image forming apparatus 100 is called a tandem full-color image forming apparatus. It includes a plurality of sets of image forming units 10Y, 10M, 10C, and 10K, an intermediate transfer body unit 7, a paper feeding / conveying means 21, and a fixing device 24. A scanner unit 103 is disposed above the main body unit 101 of the image forming apparatus.

  FIG. 2 is an enlarged view of the image forming units 10Y, 10M, 10C, and 10K. The image forming units 10Y, 10M, 10C, and 10K will be described below.

  The image forming unit 10Y that forms a yellow image includes a drum-shaped photoreceptor 1Y, a charging unit 2Y, an image exposing unit 3Y, a developing unit 4Y, and a roller-shaped primary transfer unit 5Y disposed around the photoreceptor 1Y. And a cleaning means 6Y. The image forming unit 10M that forms a magenta image includes a drum-shaped photosensitive member 1M, a charging unit 2M, an image exposing unit 3M, a developing unit 4M, and a primary transfer unit that are arranged around the photosensitive unit 1M. Means 5M and cleaning means 6M are provided. The image forming unit 10C for forming a cyan image has a drum-shaped photoconductor 1C, a charging unit 2C arranged around the photoconductor 1C, an image exposure unit 3C, a developing unit 4C, and a primary transfer unit as a primary transfer unit. Means 5C and cleaning means 6C are provided. The image forming unit 10K that forms a black image includes a drum-shaped photoconductor 1K, a charging unit 2K, an image exposure unit 3K, a developing unit 4K, and a primary transfer unit 5K as a primary transfer unit arranged around the photoconductor 1K. And a cleaning means 6K.

  An embodiment of each part of the electrophotographic process around each of the photoreceptors 1Y, 1M, 1C, and 1K in each of the image forming units 10Y, 10M, 10C, and 10K will be described in more detail with reference to FIG.

  The system speed of the image forming apparatus 100 is 300 mm / sec. Each of the photoreceptors 1Y, 1M, 1C, and 1K is an OPC having a diameter of 60 mm, and is charged to a negative polarity by each charging unit 2Y, 2M, 2C, and 2K.

  Each of the charging means 2Y, 2M, 2C, and 2K is a scorotron type corona discharge electrode that can control the voltage of the photosensitive member to an arbitrary charging voltage by switching the grid voltage, and charging power sources 2Y1, 2M1, and 2C1. 2K1.

  The developing devices 4Y, 4M, 4C, and 4K are two-component developing devices and are loaded with a two-component developer composed of toner and a carrier. The toner is negatively charged due to mutual friction with the carrier.

  Each of the developing devices 4Y, 4M, 4C, and 4K includes a developer carrier 4Y1, 4M1, 4C1, and 4K1, a stirring unit 4Y4, 4M4, 4C4, and 4K4, a toner concentration detection unit 4Y5, 4M5, 4C5, and 4K5, and a developing unit. It is comprised with containers 4Y6, 4M6, 4C6, 4K6.

  Each stirring means 4Y4, 4M4, 4C4, 4K4 is composed of two rotating shafts and screws. Then, the rotation of the stirring means causes the toner replenished from a toner replenishing section (not shown) to be evenly mixed with the developer already contained in the developing device, and the mutual friction between the toner and the carrier is promoted. Improvement of charge amount is promoted.

  The toner density detection means 4Y5, 4M5, 4C5, and 4K5 are disposed at the bottom of the developing containers 4Y5, 4M5, 4C5, and 4K5 so as to face one of the stirring means 4Y4, 4M4, 4C4, and 4K4. The toner concentration detection means 4Y5, 4M5, 4C5, and 4K5 detect the toner concentration of the developer stored in each developing device. When the toner density detected by the toner density detecting means 4Y5, 4M5, 4C5, and 4K5 is below the reference value by the control described later, the above-described toner replenishing mechanism is controlled to operate. The agent is always maintained at a toner concentration near the reference value.

  On each developer carrier 4Y1, 4M1, 4C1, 4K1, a two-component developer layer regulated to an appropriate amount is formed. By rotation of each developer carrier 4Y1, 4M1, 4C1, 4K1, an appropriate amount of developer is conveyed to a development area facing the photoreceptors 1Y, 1M, 1C, 1K.

  Each developer carrier 4Y1, 4M1, 4C1, 4K1 is connected to a developing bias power source 4Y2, 4M2, 4C2, 4K2. The developing bias power supplies 4Y2, 4M2, 4C2, and 4K2 output a bias voltage in which an AC voltage is superimposed on a DC voltage. By appropriately changing the DC component and AC component of the bias voltage, development characteristics such as fog and image density can be adjusted.

The primary transfer means 5Y, 5M, CM, and 5K are composed of a primary transfer roller covered with a semiconductive sponge (registered trademark), and the resistance value of the primary transfer roller is 1 × 10 ⁷ Ω. The primary transfer units 5Y, 5M, CM, and 5K are connected to primary transfer power supplies 5Y1, 5M1, 5C1, and 5K1, and a bias voltage is applied thereto. By applying this bias voltage, the images on the photoreceptors 1Y, 1M, 1C, and 1K are transferred to an intermediate transfer member as the image carrier of the present invention. The primary transfer power supplies 5Y1, 5M1, 5C1, and 5K1 are constant current systems that mainly control output current.

  Photosensitive member cleaning means 6Y, 6M, 6C, and 6K are disposed downstream of the primary transfer means 5Y, 5M, CM, and 5K, and cannot be transferred to the endless belt-like intermediate transfer body 70 by the primary transfer means. Residual toner remaining on the photoreceptors 1Y, 1M, 1C, and 1K is cleaned. The remaining toner is removed from the photoreceptor by cleaning blades 6Y1, 6M1, 6C1, and 6K1 that are supported and fixed to the cleaning casing so that the edges always abut against each other. The removed residual toner falls to the conveying screws 6Y1, 6M2, 6C3, and 6K4. Then, after the conveyance screws 6Y1, 6M2, 6C3, and 6K4 are rotated to the back side of the image forming apparatus main body, they are accommodated in a storage container through a conveyance mechanism (not shown).

  Returning to FIG. 1, the intermediate transfer member unit will be described below.

  The intermediate transfer body unit 7 is arranged on the left side of the image forming units 10Y, 10M, 10C, and 10K of the image forming units 10Y, 10M, 10C, and 10K arranged in the vertical direction. The intermediate transfer member unit 7 includes an endless belt-like intermediate transfer member 70 (which is a transfer member) stretched around rollers 71, 72, 73, 74, 76, and 77, and a primary transfer member. Means 5Y, 5M, 5C, 5K, secondary transfer means 5A, cleaning means 6A, and optical density detection means 8 are provided.

  The color images formed by the image forming units 10Y, 10M, 10C, and 10K are sequentially transferred onto the rotating intermediate transfer body 70 by the primary transfer units 5Y, 5M, 5C, and 5K, and are synthesized. Is formed.

  The primary transfer unit 5K is always in pressure contact with the photoreceptor 1K during the image forming process. The other primary transfer units 5Y, 5M, and 5C are in pressure contact with the corresponding photoreceptors 1Y, 1M, and 1C, respectively, only during color image formation.

  When a patch image of each color, which will be described in detail later, passes through the areas of the primary transfer means 5Y, 5M, 5C, and 5K, the bias voltage applied to each primary transfer means is as follows: Are switched as follows.

  When the image area in the moving direction of the photosensitive member passes through the primary transfer portion, a bias voltage under normal conditions is applied so that the toner image on the photosensitive member is normally transferred to the intermediate transfer member.

  On the other hand, when an inter-image area between the image areas in the moving direction of the photosensitive member passes, the bias voltage is switched to a condition that does not transfer the toner on the photosensitive member to the intermediate transfer member 70. However, when a patch image according to the present invention is formed in the inter image area of the photoconductor, a bias voltage under normal conditions is applied even when the image passes through the inter image area.

  A recording material P such as paper as a recording medium accommodated in the paper feeding cassette 20 is fed by a paper feeding means 21, passes through a plurality of intermediate rollers 22 A, 22 B, 22 C, 22 D, and registration rollers 23, and is subjected to secondary transfer. It is conveyed to the means 5A. Then, the color image is collectively transferred onto the recording material P.

  The secondary transfer means 5A is a secondary transfer roller in which a core metal is coated with semiconductive solid rubber. The belt-like intermediate transfer member 70 and the recording material P are sandwiched between a pair of rollers of a secondary transfer roller and a backup roller 74. Similarly to the secondary transfer roller, the backup roller 74 has a metal core coated with semiconductive solid rubber.

  The secondary transfer power source is connected to the core of the secondary transfer roller and applies a bias voltage to the secondary transfer unit 5A. This is a constant current power supply that mainly controls the output current. One core of the backup roller 74 is grounded.

  The color image on the intermediate transfer member 70 is transferred to the recording material P by the output of the bias voltage. The recording material P is separated from the intermediate transfer member 70 by the curvature of the backup roller 74 after the transfer. Then, after fixing processing by the fixing device 24, the paper is discharged onto a paper discharge tray 26 by a paper discharge roller 25.

  Residual toner remaining on the intermediate transfer member 70 is removed from the intermediate transfer member 70 by the cleaning means 6A. An optical density detection unit 8 is disposed between the secondary transfer unit 5A and the cleaning unit 6A so as to face the intermediate transfer member 70.

  FIG. 3 is a schematic diagram showing the optical density detection means 8 for detecting the optical density of the intermediate transfer body 70 having the patch images of the respective colors transferred from the photoreceptors 1Y, 1M, 1C, and 1K. The optical density detector 8 includes a light emitting element 81 that emits light and a light receiving element 82 that receives reflected light from the intermediate transfer body 70 side. The light emitting element 81 emits light at 45 ° with respect to the intermediate transfer body 70, and the light receiving element 82 receives light reflected from the intermediate transfer body 70 while facing the intermediate transfer body 70 in parallel. It is not limited to. As shown in the figure, NPA indicates an area on the intermediate transfer body 70 where an image is not formed, and PA indicates an area on the intermediate transfer body 70 having toner on which an image is formed.

  Returning to FIG. The secondary transfer unit 5A is in pressure contact with the intermediate transfer body 70 during the period in which the image is transferred to the recording material P, but is described later so that it is separated from at least the intermediate transfer body 70 when the patch images of the respective colors pass. It is controlled by the control unit. Therefore, the patch image transferred to the intermediate transfer body 70 is detected by the optical density detection means 8 provided on the downstream side of the secondary transfer means 5A without being disturbed by the secondary transfer means 5A. Thereafter, the patch image is removed from the intermediate transfer member 70 by the cleaning means 6A.

  The above charging, exposure, development, and transfer (primary transfer, secondary transfer) cycles are repeated to form a color image on the recording material P. The recording material P on which the color image has been formed is fixed by the fixing device 24, is sandwiched between the paper discharge rollers 25, and is placed on the paper discharge tray 26 outside the apparatus.

[Control Configuration of Image Forming Apparatus 100]
FIG. 4 is a block diagram showing a control unit of the image forming apparatus 100 according to the present invention.

  The printer unit 101, the control unit 102, the scanner unit 103, the image processing unit 104, the operation display unit 105, the image pattern creation unit 106, the storage unit 107, the transmission / reception unit 108, the print controller unit 109, and the like. Each unit is connected by a bus 110.

  The control unit 102 includes a CPU, ROM, RAM, and the like. The CPU of the control unit 102 reads the system program and various control programs stored in the ROM by operating the operation display unit 105 and develops them in the RAM, and performs centralized control of the operation of each part of the image forming apparatus 100 according to the developed programs. To do.

  The operation display unit 105 has an LCD (Liquid Crystal Display), and displays various operation buttons, the status of the apparatus, the operation status of each function, and the like on the display screen in accordance with an input instruction from the control unit 102.

  The scanner unit 103 includes a scanner below a contact glass on which a document is placed, and reads an image of the document. The scanner includes a light source, a CCD (Charge Coupled Device), an imaging optical system, an A / D converter, and the like. Illumination light from the light source scans the document, and reflected light from the document surface forms an image on the CCD. The original image is photoelectrically converted by the CCD and read as R, G, and B signals. The read image is converted from an analog signal to a digital signal by an A / D converter and output to the image processing unit 104. Here, the image is not limited to image data such as graphics and photographs, but also includes images obtained by converting text data such as characters and symbols into image data by the print controller unit.

  The image processing unit 104 converts R, G, and B image data input from the scanner unit 103 into Y, M, C, and K color image data that can be processed by the printer unit 101, and further outputs characteristics of the printer unit 101. Γ correction processing or binarization processing such as a mis-diffusion method is performed to generate print data of Y, M, C, and K colors and output to the printer 101.

  On the other hand, a print job is received by the transmission / reception unit 108 from a personal computer on the network. The received print job is transferred to the print controller unit 109. The print job includes information regarding print processing and print data (file).

  The print controller unit 109 converts text data such as characters and symbols into image data based on the contents of the print job, generates print data as image data of Y, M, C, and K colors, and generates the printer unit 101. Output to.

  The image pattern creation unit 106 creates a raw electronic image pattern of the patch image formed on the intermediate transfer member according to the present invention. Also, an electronic image pattern for resist for aligning each color image is created.

  The storage unit 107 includes a data rewritable non-volatile memory unit that does not lose data even when the HDD or the like is turned off, and a DRAM unit that is used for image processing in which data is lost. The correspondence relationship (characteristic table) between the output of the optical density detection unit according to the present invention and the toner amount of the patch image is stored in the storage unit 107 in advance.

  As described above, the Y, M, C, and K color print data generated by the image processing unit 104 and the print controller unit 109 are stored on the bitmap memory in the DRAM unit of the storage unit 107 in accordance with an instruction from the control unit 102. At the predetermined timing, the images are sequentially read out from the bitmap memory and output to the printer unit 101 as image signals (video signals) of respective colors.

  The control unit 102 comprehensively controls the operation of the image forming apparatus 100 and the like, and executes a smooth printing operation while performing image stabilization control at a predetermined timing. For example, gradation (density) adjustment for optimizing the gradation of the reproduced image is performed as appropriate.

  FIG. 5 is a control block diagram related to image stabilization control of each color image forming unit. There are a plurality of units in each of the image forming units 10Y, 10M, 10C, and 10K indicated by broken lines, and each unit is connected to a bus.

  Each charging power source 2Y1, 2M1, 2C1, 2K1 controls the charging voltage of each photoconductor 1Y, 1M, 1C, 1K. Based on an instruction from the control unit 102, the charging units 2Y, 2M, 2C, and 2K output scorotron voltages to charge the photoreceptors 1Y, 1M, 1C, and 1K to a desired surface potential.

  Each of the image exposure means 3Y, 3M, 3C, and 3K is a means for scanning laser light in the main scanning direction perpendicular to the moving direction on the photoconductor, and the laser according to the video signal output based on the instruction of the control unit 102. The light output (ON / OFF) of the light source is modulated to sequentially form latent images on the color photoconductors 1Y, 1M, 1C, and 1K.

  The developing bias power supplies 4Y2, 4M2, 4C2, and 4K2 change the output of the developing bias voltage applied to the developer carriers 4Y1, 4M1, 4C1, and 4K1 based on instructions from the control unit 102.

  The primary transfer power supplies 5Y1, 5M1, 5C1, and 5K1 output a predetermined output current at a predetermined timing in accordance with a command from the control unit 102, and sequentially transfer each color image formed on each color photoconductor onto the intermediate transfer body 70. doing.

  The secondary transfer power supply 5 </ b> A outputs a predetermined output current at a predetermined timing in accordance with a command from the control unit 102, and collectively transfers the image formed on the intermediate transfer body 70.

  FIG. 6 is a block diagram showing a control unit related to image density detection control, which will be described below with reference to FIG.

  The density detection control unit 201 is provided in the control unit 102 and controls the operation of the optical density detection unit 8 and also determines the toner amount of the patch image formed on the intermediate transfer body 70 based on the output value from the optical density detection unit 8. Image density detection control for detecting with high accuracy is executed.

  The density detection control unit 201 controls the operation of the optical density detection unit 8 and controls the image density detection control for detecting the toner amount of the patch image from the detection value of the optical density detection unit 8, and various types used for the image density detection control. And a non-volatile memory 203 for storing information.

  The optical density detection unit 8 detects the reflection density of the patch image formed on the surface of the intermediate transfer body 70 for image stabilization control, and outputs the detection result (analog) to the density detection control unit 201. To do.

  The optical density detecting means 8 includes a light emitting element 81, a light receiving element 82, a transistor 83, resistance elements 84, 85, 86, 87 and a power input terminal 88. The light emitting element 81 is made of an LED and is connected to the emitter terminal of the transistor 83. The collector terminal of the transistor 83 is connected to the power supply input terminal 87 through the resistance element 85. The power input terminal 87 is connected to a DC 8 volt power source. The base terminal of the transistor 83 is connected to the D / A port 202 a of the CPU 202 via the resistance element 84. The light receiving element 82 includes a phototransistor and is connected to the A / D port 202b of the CPU 202. The A / D port 202b is connected to the power input terminal 87 via the resistance element 86.

  The CPU 202 reads the reference light emission setting value Ir (digital value) stored in the light emission quantity information storage unit 203a in the nonvolatile memory 203, and outputs it as the light emission setting value I to the D / A port 202b. The light emission set value I is D / A converted by the D / A port 202b and outputs a light amount control signal (analog voltage) for controlling the light emission amount. A current ie according to the magnitude of the light quantity control signal flows to the base of the transistor 83, a current ie flows between the collector and the emitter of the light emitting element 81, and the light emitting element 81 emits light with an intensity corresponding to the current ie. The light Pe emitted from the light emitting element 81 is reflected by the surface of the intermediate transfer body 70 on which the patch image is formed and is emitted as secondary light Pr. A part of the secondary light Pr reaches the light receiving element 82 and is received.

  In the light receiving element 82, a current ir having a magnitude corresponding to the intensity of the secondary light Pr flows toward the ground. As a result, a voltage (analog) obtained by multiplying the current ir flowing through the light receiving element by the resistance value of the resistance element 86 at the terminal 89 of the A / D port 202b is a signal corresponding to the intensity of the secondary light Pr. Input to port 202b.

  The A / D port 202b performs A / D conversion on the input voltage (analog) and outputs a detection value Vm to the CPU 202.

The CPU 202 takes in the detection value Vm detected when the patch image of each color passes through the optical density detection means 8 as the patch detection value Vs that is the detection value of the patch image, and the patch detection value Vs is stored in the nonvolatile memory 203. The patch density T (mass per unit area g / m ² ) as the toner amount of the K-color patch image is referred to the characteristic table Tbj as the predetermined characteristic information stored in the characteristic information storage unit 203b. The patch density T calculated through the bus 111 is output to the control unit 102 in FIG.

  The control unit 102 executes image density adjustment control for adjusting image forming conditions of the image forming units 10Y, 10M, 10C, and 10K based on the patch density T of each color patch image including the K color received from the density detection control unit 201. . Then, by operating each color image forming unit under the image forming conditions adjusted by the image density adjustment control, it is possible to stably output a high-quality printed matter.

[Various information in the non-volatile memory 203]
The nonvolatile memory 203 includes a light emission amount information storage unit 203a, a characteristic information storage unit 203b, an offset voltage storage unit 203c, and a correction table storage unit 203d.

  The light emission quantity special information storage unit 203a stores a reference light emission setting value Ir output from the CPU 202 to the D / A port 202a in order to light the light emitting element 81 under a predetermined light emission condition when executing image density detection control. It is a storage area. The reference light emission setting value Ir is a light emission setting value I for lighting the light emitting element 81 under a predetermined light emission condition. The reference light emission set value Ir is acquired in advance and stored in the light emission quantity special information storage unit 203a by the light emission quantity setting control executed prior to the image density detection control.

  The characteristic information storage unit 203b is a storage area for storing a characteristic table Tbj as a plurality of characteristic information for calculating the patch density T from the patch detection value Vs of the optical density detection means 8 for the patch image. Each characteristic table is indicated by Tbj (j = 1, 2.3,...), And is associated with a plurality of dark current ranges.

  The offset voltage storage unit 203 c is a storage area for storing the offset voltage Voff of the optical density detection unit 8. The offset voltage Voff is generated at the terminal 89 between the light receiving element 82 and the resistance element 86 when the current flowing through the light emitting element 81 is zero, that is, when the amount of light emitted from the light emitting element 81 is zero. The detected value Vm corresponding to the voltage to be detected. The offset voltage Voff is caused by the dark current generated in the light receiving element 82 according to the present invention, and varies depending on the temperature change of the light receiving element 82 and the difference between the light receiving elements 82.

  The correction table storage unit 203d is a storage area for storing the correction table CTb according to the present invention. The correction table CTb associates the detected offset voltage Voff with a characteristic table Tbj as characteristic information used in the case of the offset voltage Voff.

[Offset voltage measurement control]
The dark current Id is a current that flows through the light receiving element 82 independently of the light emission amount of the light emitting element 81.

  That is, the offset voltage Voff is a detection value Vm detected by the A / D port 202b when the light emission of the light emitting element 81 is stopped, and is a value obtained by multiplying the dark current Id by the resistance value of the resistance element 86. The patch detection value Vs is also included as a dark current component.

  FIG. 7 is a control flow diagram showing Voff measurement control for measuring the offset voltage Voff, which is executed prior to the image density detection control according to the present invention.

  S201 is a step of obtaining a light emission set value I for making the current ie driving the light emitting element 81 zero. Note that the light emission set value I is normalized so that the current ie becomes zero when I = 0.

  S202 is a step of obtaining the detection value Vm by operating the optical density detection means 8 under the condition of I = 0 (that is, light emission of the light emitting element 81 is stopped).

  S203 is a step of determining the detected value Vm obtained in S202 as the offset voltage Voff.

  S 204 is a step of storing the determined offset voltage Voff in the offset voltage storage unit 203 c in the nonvolatile memory 203.

  As described above, in the Voff measurement control, the CPU 202 acquires the light emission setting value I commanded to the D / A port 202a, and acquires the detection value Vm detected by the A / D port 202b. The acquired detection value Vm is preset in the offset voltage storage unit 203c of the nonvolatile memory 203 as the offset voltage Voff.

[Light emission setting control]
The density detection control unit 201 according to the present invention precedes the execution of the toner amount detection control of the patch image in order to compensate for the change in reflectance of the intermediate transfer body 70 itself due to repeated use of the intermediate transfer body 70. Light emission amount setting control is executed to set in advance an operation condition (predetermined light emission condition) for turning on the light emitting element 81 when executing image density detection control.

  In the light emission amount setting control, a light emission set value I is obtained in which the detection value Vm detected by the optical density detection means 8 when the region NPA of the intermediate transfer body 70 is detected matches the predetermined value Va. Control is performed to store the light emission setting value I as the reference light emission setting value Ir in the light emission amount special information storage unit 203a. This will be described in detail below.

  FIG. 8 is a control flowchart showing the light emission amount setting control according to the first embodiment of the present invention, which is executed prior to the above-described image density detection control.

  S301 is a step of reading the reference light emission setting value Ir already stored in the light emission amount information storage unit 203a in the nonvolatile memory 203. This reference light emission set value Ir is used in the execution of the previous image density detection control.

  S302 is a step of substituting the reference light emission setting value Ir read in S301 for the light emission setting value I.

  S310 is a step of operating the lighting of the light emitting element 81 with the light emission setting value I, and at the same time, a step of detecting a detection value Vm input to the CPU 202 from the A / D port 202b.

  S311 is a step of determining whether or not the detected value Vm detected in step S310 is within the calculated usage range of the predetermined value Va. The detected value Vm is outside the calculated usage range, that is, exceeds Va, or (Va If it is less than -α), the process proceeds to step S312. Α is a constant that defines the calculation usage range of Va.

  S312 is a step of calculating a difference (Va−Vm) by subtracting the detection value Vm from the predetermined value Va and further calculating a command value change ΔI based on the difference (Va−Vm). For example, the calculation is performed using the following equation.

ΔI = β × (Va−Vm)
β (constant)> 0
S313 is a step of adding the change ΔI of the command value obtained in step S312 to the light emission command value I, and returns to step S310 after the arithmetic processing.

  If the detected value Vm is within the calculated use range in step S311, that is, if Va is equal to or less than Va and exceeds (Va−α), the process proceeds to step S314 and thereafter, and the light emission setting value I is changed to a new reference light emission setting value. The reference light emission setting value Ir is stored in the light emission amount information storage unit 203a in the nonvolatile memory 203 (step S315).

  As described above, in the light emission amount setting control, the reference light emission setting value Ir for turning on the light emitting element 81 when the image density detection control is executed can be set in advance before the image density detection control is executed.

[Image Density Detection Control According to First Embodiment]
FIG. 9 is a control flowchart showing image density detection control according to the first embodiment of the present invention, which is executed by the density detection control unit 201. This will be described below with reference to FIG.

  When the detection is received from the control unit 102 (step S101; Yes), the Voff measurement control of the dark current influence component shown in FIG. 7 is executed (step S102).

  S103 is a step of selecting a characteristic table Tbj that matches the Voff voltage detected in step S102.

  Hereinafter, main contents related to the characteristic table Tbj will be described in detail.

A plurality of characteristic tables Tbj are stored in the characteristic information storage unit 203b in the non-volatile memory 203. Here, ten tables such as j = 1, 2, 3,... 10 are stored. . FIG. 10 is a characteristic curve graphed as an example of the characteristic table Tbj. Accordingly, as shown in FIG. 10, each characteristic table Tdj includes a patch detection value Vs when the patch image is detected by the optical density detection unit 8 for each of a large number of patch images having different densities, and the patch image density ( g / m ² ). A characteristic curve KC shown in FIG. 10 is a characteristic curve that connects a large number of elements Ai (consisting of Ti and Vsi, respectively) of the characteristic table Tbj. The characteristic curve KC is developed between two broken lines indicating Vs = Voff and Vs = Va. SR indicates the difference between Va and Voffs, that is, the detection sensitivity range of the optical density detection means 8.

  FIG. 11 is an example of a correction table CTb that associates each Voff range divided into 10 sections with a characteristic table Tbj (Tb1, Tb2,... TbA) that matches the range. The storage location illustrated in FIG. 11 indicates an area 203bj in the characteristic information storage unit 203b in which each characteristic table Tbj is stored. In FIG. 11, the dark current range is divided into 10 sections, but a more accurate characteristic table can be provided by further dividing the range. The number of divisions is appropriately determined from the balance between the increase in the storage capacity of the characteristic data storage unit 203b and the practically required detection accuracy.

  The CPU 202 selects the characteristic table Tbj that matches the offset voltage Voff detected in step S102 with reference to the correction table CTb in FIG. 11 in step S103.

  FIG. 12 shows the characteristic tables Tb1, Tb5, and TbA selected in step S103 when the offset voltage Voff is Voff = a1, Voff = a5, or Voff = aA. KC11, KC15, and KC1A are shown as reference characteristic curves corresponding to the respective characteristic tables.

  Next, in step S104, the CPU 202 executes the light emission amount setting control shown in FIG. At this stage, the density detection control unit 201 outputs a patch image formation start command for forming a patch image on the intermediate transfer member to the control unit 102.

  S105 is a step of determining the presence or absence of patch image formation based on a signal indicating that execution of patch image formation is received from the control unit 102. If the CPU 202 determines that patch image formation is present, the process proceeds to step S106.

  S106 is a step of executing image density detection control. The light emission reference value Ir is read from the light emission amount information storage unit 203a in the nonvolatile memory 203 and substituted into the light emission set value I, and the light emitting element 81 is turned on under the light emission set value I. Thus, the detection value Vm corresponding to the patch image, that is, the patch detection value Vs is detected.

  In step S107, the density T of the patch image is acquired based on the patch detection value Vs detected in step S106 with reference to the characteristic table Tbj selected in step S103.

  S 108 is a step of transferring the density T of the patch image obtained in S 107 to the control unit 102 via the bus 111.

  As described above, the image density detection control according to the first embodiment acquires appropriate characteristic information for calculating the toner amount from the output of the optical density detection unit corresponding to the offset voltage. An error in patch image density detection caused by dark current can be prevented.

[Image Density Detection Control of Second Embodiment]
In the image density detection control according to the second embodiment, the density T of the patch image is acquired according to the series of image density detection control flow of FIG. 9 as in the first embodiment. The difference from the first embodiment is that the light emission amount setting control as shown in FIG. 13 is executed.

  FIG. 13 is a control flowchart relating to the light emission amount setting control according to the second embodiment.

  In the control flowchart of the light emission amount setting control, the difference from the first embodiment is that an S303 step and an S304 step are added between the S302 step and the S310 step.

  S303 is a step of reading the offset voltage Voff from the offset voltage storage unit 203c in the nonvolatile memory 203.

  S304 is a step of adding the offset voltage Voff read in step S303 to the base value Var and substituting it into the predetermined value Va. That is, the following calculation is performed.

Va = Var + Voff
The base value Var is detected when the optical density detector 8 detects the region NPA of the intermediate transfer body 70 when the offset voltage Voff is zero, and is input from the A / D port 202b to the CPU 202. It means a value corresponding to the value Vm.

  As in the first embodiment, in steps S310 to S315, the detection value input from the A / D port 202b to the CPU 202 when the optical density detection unit 8 detects the area NPA of the intermediate transfer body 70. The light emission set value Ir is determined so that Vm matches the predetermined value Va. The determined light emission setting value Ir is stored in the light emission amount information storage unit 203a.

  As in the first embodiment, the series of image density detection control shown in FIG. 9 is executed, the offset voltage Voff is detected in step S102, and the characteristic table Tbj suitable for the offset voltage is selected in step S103.

  Next, in step S104, a light emission set value Ir corresponding to the offset voltage Voff is determined.

  Next, when the patch image is created, the control flow from step S106 to step S108 shown in FIG. 9 is executed, and the patch detection value Vs corresponding to the patch image is referred to the characteristic table Tbj selected in step S103. Thus, the density T of the patch image is acquired. Then, the density T acquired in S107 is transferred to the control unit 102 via the bus 111.

  The plurality of characteristic tables Tbj selected in S103 have a different combination from the plurality of characteristic tables Tbj in the first embodiment because the light emission set value Ir changes according to the offset voltage Voff. As shown in FIG. 15, which will be described later, each characteristic curve corresponding to a plurality of characteristic tables is translated in parallel with the Vs axis.

  FIG. 14 shows an example of a correction table CTb used for image density detection control according to the second embodiment. As shown in the figure, the characteristic table Tb1 is applied when the offset voltage Voff is Voff ≦ a1, and the characteristic table Tb5 is applied when the offset voltage Voff is a4 <Voff ≦ a5. When the offset voltage Voff is a9 <Voff ≦ a1, the characteristic table TbA is applied.

  FIG. 15 shows characteristic curves KC1, KC5, and KCA that graph the correspondence relationship between the characteristic tables Tb1, Tb5, and TbA. As shown in the figure, each characteristic curve moves in parallel with the Vs axis. Therefore, the difference between Va and Voff, that is, the detection sensitivity range SR of the optical density detection means 8 is kept constant regardless of the change in the offset voltage Voff, and the same high detection accuracy even if the offset voltage Voff increases. And has the advantage of superior detection accuracy compared to the image density detection control of the first embodiment.

  In place of the correction table TC as shown in FIG. 13, a single characteristic table Tb is stored in advance in the characteristic information storage unit 203b as the original characteristic table Tbr, and an arithmetic process is executed to adapt to the offset voltage Voff. The characteristic table Tbj to be calculated may be calculated.

  For example, when the characteristic table Tb1 when the offset voltage Voff is a1 is used as the original characteristic table Tbr, when the offset voltage Voff is detected as aj in step S102, each element Ai (Vsi, Ti) may be subjected to the following arithmetic processing to acquire each element Ai (Vsi, Ti) of the new characteristic table Tbj.

Arithmetic processing 1: DVoff = aj−a1
DVoff is the amount of fluctuation of the offset voltage.

Arithmetic processing 2: Vsi = Vsi + DVoff
The new characteristic table Tbj acquired by the above arithmetic processing is stored in the characteristic information storage unit 203b as a characteristic table that conforms to the offset voltage Voff = aj.

  Then, the density of the patch image is acquired with reference to the characteristic table Tbj read from the characteristic information storage unit 203b in step S107 of the image density detection control (FIG. 9).

  As described above, in the image density detection control of the second embodiment, appropriate characteristic information for calculating the toner amount is acquired from the output of the optical density detection unit corresponding to the offset voltage, and the dark current of the light receiving element is obtained. It is possible to prevent an error in patch image density detection caused by this. Furthermore, the sensitivity characteristic of patch image density detection is kept constant with respect to changes in the offset voltage, and image density detection control superior to the image density detection control of the first embodiment is enabled.

[Image Density Stabilization Control of Third Embodiment]
In the image density stabilization control according to the present invention, a plurality of patch images are actually formed on the intermediate transfer body before image formation is performed so that images of all gradations can always be output at a constant density. The toner amount (patch density T) of the patch image is detected, and the image forming conditions of each color image forming unit are appropriately adjusted so that the patch density T matches the target value. This will be described in detail below.

  In the case of a K color patch image, in the characteristic curve shown in FIG. 10, when the toner amount is high, there is a region where the sensitivity of the optical density detection means is low or insensitive, and the patch density T belonging to this region. As for (toner amount of patch image), the toner amount on the intermediate transfer member cannot be obtained directly from the detection value of the optical density detection means. Here, the above region is tentatively referred to as non-detection range NMR.

  In the image density stabilization control of the third embodiment according to the present invention, a patch image having a gradation density Dp corresponding to a toner amount region lower than the non-detection range NMR is formed, and the first embodiment or the second embodiment is formed. The image density detection control is executed to detect the patch density T (the toner amount of the patch image), and the toner quantity (virtual patch density Tv) of the virtual patch image belonging to the non-detection range NMR is predicted based on the patch density T. Virtual patch density detection control is executed.

  Furthermore, image forming condition adjustment control is performed to adjust the image forming condition of the K color image forming unit 10K so that the virtual patch density Tv obtained by the virtual patch density detection control matches the target value.

  Note that the patch image is preferably a plurality of patch images having different gradation densities Dp. By predicting the virtual patch density T based on a plurality of toner amounts, the image forming condition of the K color image forming unit 10K can be set to an appropriate value with high accuracy. Can be adjusted.

  An outline of an example of image density stabilization control for forming K color patch images having two different gradation densities will be described below.

  FIG. 17 is a schematic diagram showing the first patch image Pa1 and the second patch image Pa2 having different gradation densities formed on the intermediate transfer body 70. FIG. The gradation density Dp1 of the first patch image Pa1 is lower than the gradation density Dp2 of the second patch image Pa2. Note that the gradation density Dp is defined by the duty ratio (%) of turning on the laser for each pixel constituting the patch image.

  Control for switching the DUTY ratio for turning on laser light is called pulse width modulation control (PWM control) for turning on laser light, and is generally used as a means for forming a gradation in an image voltage on a photoconductor. When the PWM is 100%, the maximum contrast voltage is formed, and the toner amount of the patch image formed on the intermediate transfer body 70 is maximized.

  The contrast voltage is the difference between the image voltage on the photoconductor (image surface potential) and the development DC bias (DC component of the development bias). The toner amount of the patch image depends on the contrast voltage, and the contrast in a wide toner amount region. It has a linear relationship with voltage.

  FIG. 18 is a characteristic diagram showing the virtual patch density detection control for predicting the toner amount (virtual patch density) Tv of the virtual patch belonging to the non-detection range NMR according to the present invention. The relationship between the tone density Dp of the patch image, the toner amount T of the patch image on the intermediate transfer member 70, and the patch detection value Vs is shown. The horizontal axis indicates the toner amount T of the patch image. The vertical axis of the lower quadrant indicates the tone density Dp of the patch image, and the vertical axis of the upper quadrant indicates the patch detection value Vs.

  The characteristic curve KC of the optical density detection means 8 for the K color patch image is shown in the upper quadrant. An image forming characteristic DC of the K color image forming unit 10K is shown in the lower quadrant. Although the image forming characteristic DC is shown in a linear relationship, the actual image forming characteristic DC has a curved portion.

  When the development DC bias voltage Vdc is changed, the image forming characteristic DC moves in parallel as indicated by an arrow f. For example, when Vdc is increased, patch density T (amount of toner in the patch image) is increased, and when Vdc is decreased, patch density T is decreased.

  When an image forming condition (development DC bias) corresponding to the image forming characteristic DC2 is set in advance, the relationship between the gradation density Dp of the patch image and the patch detection value Vs is determined by the image forming characteristic DC2 and the characteristic curve KC. Determined. Therefore, the first patch detection value Vs1 is detected for the first gradation density Dp1 by the relationship shown by the arrow c. Further, the second patch detection value Vs2 is detected for the second gradation density Dp2 by the relationship shown by the arrow d.

  The controller of the present invention executes the image density detection control of the first embodiment or the second embodiment, and uses the upper quadrant characteristic curve KC to obtain the first patch density T1 from the first patch detection value Vs1. The second patch density T2 is obtained from the second patch detection value Vs2.

  18 shows a toner amount range in which the optical density detector 8 cannot directly detect the patch density (the toner amount of the patch image).

  Next, the control unit of the present invention calculates the toner amount (virtual patch density) Tv of the virtual patch image belonging to the non-detection range NMR based on Equation 2. The meaning of Equation 2 will be described below.

P1 (T1, Dp1), P2 (T2, Dp2) and P3 (Tv, 100) shown in the lower quadrant of FIG. 18 exist on the image forming characteristic DC2, and Dp1, Dp2 and 100 of gradation density values are constants. Therefore, the relationship shown in the following formula 1 is established. The control unit 102 calculates the virtual patch density Tv using Equation 1.
Tv = Ax (T2-T1) + T1 Formula 1
A: Proportional constant Further, the control unit of the present invention constructs an appropriate image forming characteristic by adjusting an appropriate image forming condition so that the virtual patch density Tv obtained by the above calculation matches the target value Tg. ing. Here, an appropriate image forming characteristic indicated by DC1 is constructed by adjusting the development DC bias voltage Vdc.

  AR shown in FIG. 18 is defined by an upper limit value UL and a lower limit value LL, and is a calculation usage range for accommodating the first patch detection value Vs1 and the second patch detection value Vs2.

  The control unit according to the present invention controls the first patch detection value Vs1 and the second patch detection value Vs2 so as to fall within the above calculation usage range AR, and uses the correspondence table shown in Table 1 to calculate based on the offset voltage Voff. Control is performed to change (set) the use range AR (upper limit UL, lower limit LL).

  FIG. 19 shows the characteristic curve KC of the optical density detector 8 when the offset voltage Voff changes, and the calculated usage range AR that changes in accordance with the change in the offset voltage Voff.

  KC1 represents a characteristic curve when the offset voltage Voff is Voff1, and KC2 represents a characteristic curve when Voff2. AR1 indicates the calculated usage range when Voff1, and AR2 indicates the calculated usage range when Voff2.

  The upper limit value UL is a boundary value related to the image forming characteristics of the image forming unit, which prevents a detection error due to the unevenness of the patch density T that occurs in the low toner amount region. The lower limit value LL prevents detection errors caused by the sensitivity of the optical density detection means 8 being too low.

  As described above, the control unit according to the present invention can always set an appropriate calculation use range AR with respect to a change in the offset voltage Voff, and can control a high gradation density belonging to the non-detection range NMR from a plurality of patch densities T. The toner amount (virtual patch density) Tv of the virtual patch image can always be calculated with high accuracy.

  Further, the control unit according to the present invention adjusts an image forming condition (for example, a development DC bias condition) in which the virtual patch density (toner amount of the virtual patch image) Tv obtained by the virtual density detection control matches the target value Tg. The image forming condition adjustment control is performed to construct an optimum image forming characteristic DC. In the example shown in FIG. 18, DC1 corresponds to the optimum image forming characteristics.

  FIG. 16 is a control flowchart showing image density detection control and image density stabilization control of the third embodiment according to the present invention.

  S801 is a step of determining the presence / absence of an execution command for image stabilization control. If yes, the process proceeds to step S802.

  S802 is a step of executing offset voltage measurement control.

  S803 is a step of setting a calculated usage range AR (upper limit value UL and lower limit value LL) suitable for the offset voltage Voff obtained in step S802 with reference to the correspondence table shown in Table 2, and storing it in the nonvolatile memory. is there.

  In step S804, the development DC bias Vdc previously stored in the nonvolatile memory is set as an initial value.

  In step S805, a patch image is formed under other image forming conditions including Vdc. As the patch image, a first patch image and a second patch image having a higher gradation density than the first patch image are formed.

  S806 is a step of determining whether or not the first patch detection value Vs1 is less than the upper limit value UL. If it is less, the process proceeds to step S807. In the above case, the process proceeds to step S808.

  S809 is a step of determining whether or not the second patch detection value Vs2 exceeds the lower limit value LL. If so, the process proceeds to step S810, and if the following, the process proceeds to step S809.

  In step S808, Vdc is added by a predetermined value d, and the process returns to step S805. In step S809, Vdc is subtracted by a predetermined value d, and the process returns to step S805. Then, the process returns to step S805 and image formation is performed again under the new Vdc. The steps from S805 to S809 are repeated, and when the first patch detection value Vs1 and the second patch detection value Vs2 fall within the calculated usage range AR set in S802, the process proceeds to step S810.

  S810 is a step of executing the image density detection control shown in the first embodiment and the second embodiment, and detects the first patch density T1 and the second patch density T2.

  S811 is a step of calculating the virtual patch density Tv based on the first patch density T1 and the second patch density T2 using the arithmetic expression shown above.

  S812 is a step of adjusting the development DC bias Vdc or the development AC bias voltage Vac so that the virtual patch density Tv matches the target value Tg.

  By using the above image density stabilization control, the image forming conditions are adjusted as necessary, so that an optimal image forming characteristic DC is always constructed. In addition, it is possible to provide an image forming apparatus in which image formation is always performed in accordance with optimum image forming characteristics DC, and a high-quality image having a stable color tone and density can be always printed.

  Note that the characteristic table Tbj as characteristic information may be replaced with a functional expression as function information. Table 2 shows an example of a functional expression stored in the characteristic information storage unit 203b of the nonvolatile memory 203 in place of the plurality of characteristic tables Tbj in FIG.

  In addition, f (x) shown in the column of the function formula in Table 2 is a quadratic polynomial shown in the following formula. The variable y corresponds to the patch detection value Vs of the patch image, corresponds to the toner amount (patch density T) of the patch image, and a, b, and c are constants.

y = f (x) = ax ² + bx + c
Higher accuracy can be obtained by setting f (x) to a polynomial having a degree of 3 or higher, but the calculation processing load increases, and it is appropriately selected in consideration of the required accuracy.

  In the above embodiment, the detection control unit 201 measures the offset voltage Voff of the optical density detection unit 8 immediately before execution of the image density detection control, and the measured offset voltage Voff is stored in the offset voltage storage unit of the nonvolatile memory 203. Although the offset voltage measurement control is rewritten to 203c, it may be executed periodically or when the environmental condition or the machine condition changes.

  The plurality of characteristic tables Tbj are registered in advance in the characteristic information storage unit 203b, but are not limited thereto. For example, a reference characteristic table Tbr may be registered in advance, and a characteristic table Tbj that matches the offset voltage Voff may be calculated based on the offset voltage Voff that is appropriately measured.

  Note that the above-described image density stabilization control is executed periodically when the environment or the like changes, or immediately after a measure such as maintenance is performed.

  In the above embodiment, the optical density detecting means 8 is disposed facing the intermediate transfer body 70. However, the optical density detecting means is disposed facing the photoconductor as an image carrier. In this embodiment, the toner amount determination and image density stabilization control technique of the present invention can be applied.

  Further, although the above embodiment is an image forming apparatus using an intermediate transfer member, the present invention can also be applied to an image forming apparatus that directly transfers an image on a photosensitive member onto a sheet. . In that case, the photoconductor or paper becomes an image carrier, and the density of the patch image on the photoconductor or paper is optically detected by the optical density detection means.

1Y, 1M, 1C, 1K photoconductor 10Y, 10M, 10C, 10K Image forming unit 70 Intermediate transfer member (image carrier)
DESCRIPTION OF SYMBOLS 8 Optical density detection means 81 Light emitting element 82 Light receiving element 102 Control part 201 Density detection control part 202 CPU
203 Non-volatile memory (storage unit)
203a Light emission amount information storage unit 203b Characteristic information storage unit 203c Offset voltage storage unit AR calculation usage range Dp Patch tone value (tone value of patch image)
Dp1 First patch tone value Dp2 Second patch tone value I Light emission setting value Ir Reference light emission setting value Pa1 First patch image Pa2 Second patch image T Patch density (toner amount of patch image)
Tv Virtual patch density (virtual patch image toner amount)
Tg Target value Tbj Characteristic table (characteristic information)
Voff offset voltage Vm detection value Vs patch detection value (detection value of patch image)

Claims (8)

An image forming unit that forms an image on the image carrier;
An optical density detector that has a light emitting element and a light receiving element disposed opposite to the image carrier and detects the reflection density of the image carrier surface by lighting the light emitter at a preset reference light emission setting value. Means,
A storage unit that stores predetermined characteristic information that associates a detection value of the optical density detection unit with a toner amount;
The image forming unit is controlled to form a patch image on the image carrier, the optical density detection unit is controlled to detect a detection value of the patch image, and the patch characteristic image is detected using the predetermined characteristic information. A control unit that acquires a toner amount of the patch image based on a detection value;
Prior to the formation of a patch image, the control unit measures an offset voltage generated by a dark current of the light receiving element, acquires characteristic information matching the offset voltage, and sets the characteristic information as the predetermined characteristic information An image forming apparatus.   The control unit controls the optical density detecting unit to detect a non-image surface on the image carrier prior to formation of a patch image, and the detection value of the optical density detecting unit matches a predetermined value. The image forming apparatus according to claim 1, wherein a light emission setting value of a light emitting element is acquired and the light emission setting value is set as the reference light emission setting value.   The image forming apparatus according to claim 2, wherein the control unit changes the reference light emission value in accordance with a change in the offset voltage.   4. The image formation according to claim 1, wherein the control unit adjusts an image forming condition of the image forming unit so that a toner amount of the patch image becomes a target value. 5. apparatus. An image forming unit that forms an image on the image carrier;
An optical density detecting means having a light emitting element and a light receiving element disposed to face the image carrier and lighting the light emitting element under a predetermined light emission condition set in advance to detect a reflection density on the surface of the image carrier. When,
A storage unit that stores predetermined characteristic information that associates the detection value of the patch image by the optical density detection unit with the toner amount of the patch image;
The image forming unit is controlled to form a patch image on the image carrier, and when the detected value of the patch image detected by the optical density detecting means is within the calculated use range, the detected value is set to the predetermined characteristic. When the toner amount of the patch image is calculated with reference to the information, and the image forming condition of the image forming unit is adjusted based on the toner amount of the patch image, while the detection value of the patch image is outside the calculated use range A control unit that changes an image forming condition of the image forming unit so that a detection value of the patch image is within the calculated use range,
The control unit measures an offset voltage generated by a dark current of the light receiving element prior to formation of a patch image, and changes the calculated use range according to the offset voltage. The patch image is a plurality of patch images showing different gradation values;
The control unit calculates a toner amount of a virtual patch image showing a gradation value having a higher density than the plurality of patch images based on a toner amount of the patch image, and the toner amount of the virtual patch image matches a target value. 6. The image forming apparatus according to claim 5, wherein an image forming condition of the image forming unit is adjusted.   The image forming apparatus according to claim 5, wherein the gradation value of the virtual patch image is a maximum density that can be instructed to the image forming unit.   The calculation use range is an upper limit that is a boundary between regions where unevenness of an unacceptable level occurs in the toner amount of the patch image, and a boundary between regions where the detection sensitivity of the optical density detection unit decreases to an unacceptable level. 8. The image forming apparatus according to claim 5, wherein the image forming apparatus is a detection range of the optical density detection unit defined by a lower limit.

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