JP2009150963A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which prevents accuracy in density correction control from being lowered if a belt type image carrier is vibrated when performing density correction control by measuring a test pattern formed on the belt type image carrier by an optical sensor. <P>SOLUTION: The image forming apparatus provides the image density correction control to perform correction of image density by correcting a density correction amount by using a correction coefficient α determined on the basis of random vibration information on a belt ground of the belt type image carrier 130 obtained by reading the belt ground thereof by a density detection means 150L. The correction coefficient α is a coefficient to make the density correction amount small when a vibration component caused by the vibration of the belt type image carrier 130 is large in density information. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention provides an image forming apparatus such as a copying machine, a printer, a recorded image display apparatus, and a facsimile machine that forms a toner image on an image carrier by an electrophotographic method or an electrostatic recording method and transfers the toner image to a transfer material. It relates to the device. In particular, the present invention relates to an image forming apparatus that detects the density of a test pattern image formed on the surface of an image carrier and corrects the image gradation.

  In recent years, various image forming apparatuses using an electrophotographic system or the like are known. In the image forming apparatus, particularly in the color image forming apparatus, small space, low cost, and further, high speed, high resolution, and high image quality are problems, and new technologies are being developed.

  In terms of speeding up the color image forming apparatus, speeding up is realized by implementing a tandem system that can shorten the transfer material transport path. A tandem image forming apparatus is an image forming apparatus in which a plurality of image forming units (image forming units) including a photosensitive drum, a developing device, a drum cleaner, and a charger are arranged to form an image forming station. In this tandem image forming apparatus, each color image formed in the image forming station is transferred onto a belt-like intermediate transfer member (that is, an intermediate transfer belt) as an image carrier, and transferred to a transfer material in a lump. To do.

  In such an image forming apparatus, the misalignment of the three axes of the driving roller, the tension roller, and the driven roller that suspends the intermediate transfer belt due to the distortion of the apparatus main body and the like occurs in the intermediate transfer belt. It is known that a shift occurs.

Accordingly, the following method is employed as means for correcting the deviation of the intermediate transfer belt.
(1) The intermediate transfer belt is properly detected by electrically detecting the deviation of the intermediate transfer belt and moving one end of one of the plurality of roller members holding the intermediate transfer belt up and down with a solenoid or a stepping motor. The method of moving back to the position.
(2) A method in which the diameter of at least one roller member holding the intermediate transfer belt is set to be larger than both end portions in the central portion in the axial direction so that the roller member exhibits a crown shape as a whole.
(3) A method of providing a belt deviation regulating rib on the intermediate transfer belt.

  However, in the method (1), since the vertical movement of the roller member necessary to cause the artificial transfer of the intermediate transfer belt is performed using electrical components such as a solenoid or a stepping motor, the overall size of the apparatus is increased. And cost increase.

  In the method (2), the roller member is crowned to positively distort the intermediate transfer belt, thereby generating an internal stress difference, thereby suppressing the deviation of the intermediate transfer belt. Therefore, it is necessary to use an intermediate transfer belt made of a material having sufficient elasticity. In addition, so-called permanent deformation of the intermediate transfer belt due to creep distortion is prevented while utilizing distortion generated in the intermediate transfer belt. Therefore, for example, when a rubber belt is used as the intermediate transfer belt, it is necessary to use a material having low rubber hardness and high elasticity, and also to set the thickness to satisfy the mechanical strength. There is a restriction that there is.

  In view of the above, and with the improvement of component accuracy and material performance, in recent years, the method of providing the belt deviation regulating rib of (3) is often used.

  Further, the gradation of the density of the color image obtained by the image forming apparatus and the density stability have a great influence on the image forming characteristics of the color image forming apparatus.

  However, in general, it is known that in a color image forming apparatus, the density of an image to be formed fluctuates due to environmental changes or long-term use. In particular, in the case of an electrophotographic color image forming apparatus, even if there is a slight change in density, the color balance of the formed image may be lost, so it is necessary to always maintain a constant gradation-density characteristic.

  Therefore, such a color image forming apparatus employs the following means.

  That is, each color toner is provided with gradation correction means such as several kinds of exposure amounts according to absolute temperature and humidity, process conditions such as development bias, and a look-up table (LUT) for correcting image data. . Then, based on the absolute humidity measured by the temperature / humidity sensor, the optimum process condition and optimum value for gradation correction in the environment are selected.

  Also, a density detection patch image (test pattern) is formed on the intermediate transfer belt using toner of each color so that a constant gradation-density characteristic can be obtained even if the characteristics of each part of the apparatus fluctuate. . Then, the density of the unfixed toner image is optically detected by a density detection sensor, and density correction control is performed by changing process conditions such as exposure amount and development bias based on the detection result. A stable image is obtained by this configuration or by performing image data correction (see Patent Document 1).

  When the characteristic A of the image forming apparatus is obtained as shown in FIG. 10, in order to obtain a desired gradation characteristic T, a characteristic B obtained by inversely converting the characteristic A with respect to the characteristic T is held as a look-up table. The gradation characteristic T can be obtained by modulating the signal.

  In order to obtain the characteristic A, it is desirable that the number of gradations of the density detection patch is large. However, increasing the number of patches causes problems such as an increase in downtime and an increase in toner consumption. Therefore, the first control for obtaining the characteristic A with higher accuracy by increasing the number of gradations, and obtaining the characteristic T by obtaining the difference from the characteristic A by reducing the number of gradations and increasing the frequency of execution although the accuracy is slightly reduced. By using the second control, stable gradation characteristics can be obtained over a long period of time.

  As shown in FIG. 11, when the characteristic of the image forming apparatus is shifted from the characteristic GA to the characteristic GB, the gradation characteristic is shifted from the characteristic T to the characteristic TC as shown in FIG.

  In order to correct this, the characteristic T can be obtained by correcting the lookup table with the correction amount C-AB obtained by inversely converting the difference amount D-AB of FIG. 13 obtained from the difference between the characteristic GA and the characteristic GB. .

  Further, in the case of having a plurality of image stations as described above, when the toner images developed at each station are sequentially transferred onto the intermediate transfer belt, it is necessary to align the images at each station at a desired position. Therefore, an optical sensor for registration is provided at a position facing the intermediate transfer belt and downstream of the most downstream station.

The optical sensors used for the registration control and density correction control described above are:
(1) A type in which one light emitting element 150a irradiates the intermediate transfer belt 130 at a predetermined incident angle, and a light receiving element (first light receiving element) 150b is provided in the regular reflection direction (FIG. 5).
(2) In addition to the above (1), there is a second light receiving element 150c that also receives the diffusion component of the irradiated light (FIG. 6).
There are two types.

  Since the registration control mainly needs to be able to identify the position of the formed image, the regular reflection light type sensor of (1) is used to execute control by forming a plurality of high-density line-shaped images. .

  On the other hand, in the density correction control, since the information on the diffused light component is used to detect the amount of toner on the intermediate transfer belt, the type (2) is used.

  In the registration control, the toner image formed at each image forming station is arranged at a desired position on the intermediate transfer belt, and the image position deviation between the image forming stations is corrected in both the sub-scanning direction and the main scanning direction. It is an object. Therefore, as shown in FIG. 7, it is necessary to dispose optical sensors 150L and 150R as density detecting means in the vicinity of both ends of the image area L in the main scanning direction.

  On the other hand, regarding density correction control, there are longitudinal density unevenness due to various factors such as sensitivity unevenness of the photosensitive drum and longitudinal unevenness of the development characteristics. Therefore, it is only necessary that the control can be performed in a relatively stable region determined by various configurations of the image forming apparatus such as the manufacturing method and type of the photosensitive drum and the developer supply and transport configuration.

  In general, even when a density difference occurs at both ends of the longitudinal direction, it is desirable to dispose the sensor in the center because the density difference becomes relatively small. However, since it is necessary to arrange sensors at both ends in the registration control, a total of three sensors are arranged, resulting in a complicated apparatus and an increase in cost.

  In view of the above requirements, in the conventional models equipped with registration control and density correction control, the sensor of type (2) capable of detecting specularly reflected light and diffused light is arranged at one end in the main scanning direction. . The sensor of the type (1) that can detect only specularly reflected light is arranged on the other side.

  However, in the apparatus having the sensor arrangement configuration, variations in density correction control accuracy have occurred, and there have been cases where color variation is large.

This is because the reading accuracy of the patch detection sensor depends on the direction of the intermediate transfer belt, and the detection accuracy of the sensor may be remarkably reduced because the influence of the belt substrate is particularly large at the highlight density.
JP 2003-84532 A

  The present invention has been made in view of such problems.

  It is an object of the present invention to measure the accuracy of density correction control when the belt-shaped image carrier vibrates when the test pattern formed on the belt-like image carrier is measured by density detection means and density correction control is performed. It is an object of the present invention to provide an image forming apparatus that does not deteriorate.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides a belt-shaped image carrier,
A plurality of image forming units capable of forming a toner image on the surface of the belt-shaped image carrier;
Density detecting means for optically reading the toner image formed on the surface of the belt-shaped image carrier,
In an image forming apparatus that forms a test pattern on the surface of the belt-shaped image carrier and performs image density correction based on a density correction amount determined based on density information read by the density detection unit.
An image density correction that corrects the image density by correcting the density correction amount using a correction coefficient determined based on random vibration information of the background read by the density detection unit of the belt background of the belt-shaped image carrier. Have control,
The image forming apparatus according to claim 1, wherein the correction coefficient is a coefficient for reducing a density correction amount when a vibration component due to vibration of the belt-shaped image carrier is large in the density information.

  According to the present invention, even when the background of the belt-shaped image carrier is vibrating, good density stability can be obtained by correcting the coefficient used for density correction control according to the vibration state of the background.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
FIG. 1 shows a schematic configuration of a tandem color copier which is an embodiment of an image forming apparatus to which the present invention is applied.

(Overall configuration of image forming apparatus)
According to this embodiment, an image forming apparatus 100 that is a tandem color copier has four image forming units (image forming units) provided in the apparatus, and the first, second, third, A fourth image forming station P (Pa, Pb, Pc, Pd) is formed. In each image forming station P, a toner image of a different color can be formed by an image forming unit through processes of electrostatic latent image formation, development, and transfer. The image is transferred to the intermediate transfer body 130.

  More specifically, in this embodiment, the image forming station P (Pa, Pb, Pc, Pd) is a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 3 (3a, 3b, 3c, 3d). It has. Around each photosensitive drum 3 (3a, 3b, 3c, 3d), there are a drum charger 2 (2a, 2b, 2c, 2d) as a charging unit, and a developing unit 1 (1a, 1b) as a developing unit. 1c, 1d) are arranged. In the present embodiment, an exposure device 9 that is a laser beam scanner device as an exposure unit is installed below the image forming device 100, and image exposure is performed on the photosensitive drum 3 of each image forming station P in accordance with an image information signal. An electrostatic latent image is formed. Further, a transfer device 4 as a transfer unit is disposed above the device.

  The transfer device 4 includes a belt-like image carrier (intermediate transfer member), that is, an intermediate transfer belt 130 installed adjacent to the photosensitive drums 3a to 3d. The intermediate transfer belt 130 is moved toward the photosensitive drum 3 by a roller-shaped transfer member serving as a primary transfer unit disposed opposite to each photosensitive drum 3, that is, by a primary transfer roller 41 (41a, 41b, 41c, 41d). It is pressed. Thus, the photosensitive drum 3 and the intermediate transfer belt 130 form a first transfer portion (nip) T1 (T1a, T1b, T1c, T1d). Details of the transfer device 4 will be described later.

  With this configuration, the image forming stations P (Pa, Pb, Pc, Pd) sequentially form yellow, magenta, cyan, and black toner images on the respective photosensitive drums 3.

  The toner images of the respective colors formed on the photosensitive drums 3a, 3b, 3c, and 3d are transferred onto the intermediate transfer belt 130 at the first transfer portion T1 (T1a, T1b, T1c, T1d).

  The transfer residual toner on the photosensitive drum 3 is removed by a photoreceptor cleaner 5 (5a, 5b, 5c, 5d) as a cleaning unit.

  On the other hand, the toner images of the respective colors superimposed on the intermediate transfer belt 130 are recorded on the conveyance path 140 by the second transfer unit T2 where the secondary transfer roller 7 as a secondary transfer unit is disposed. Is transferred onto the transfer material S.

  The transfer material S on which the toner images of the respective colors have been transferred is sent to the fixing device 8 where the toner images are fixed by heating and pressurizing, and then discharged out of the apparatus as recorded images.

  In this example, image formation was performed at a process speed of 150 mm / sec at 35 sheets / min, the maximum image width was 310 mm, and the image was based on the longitudinal center.

(Transfer device)
Next, the transfer device 4 will be further described.

  The intermediate transfer belt 130, which is a belt-shaped image carrier in this embodiment, was a polyimide seamless belt having a circumferential length of 700 mm and a width of 350 mm and subjected to conductive treatment.

  Inside the intermediate transfer belt 130, a belt support member 135 molded from resin is disposed, and each member is disposed on the belt support member 135.

  That is, the driving roller 131, the tension roller 132, and the driven roller 133 are disposed on the belt support member 135, and the intermediate transfer belt 130 is suspended so as to move in the arrow direction. Further, the primary transfer rollers 41a, 41b, 41c, and 41d are rotatably supported on the belt support member 135.

  The tension roller 132 is applied with outward pressure from the inner surface of the belt by a pressure spring (not shown) to apply tension to the intermediate transfer belt 130. Further, the secondary transfer counter roller 131 on which the secondary transfer roller 7 is disposed so as to oppose the transfer material S sandwiches and conveys the transfer material S to form an optimal secondary transfer portion (nip) T2, and, as described above, the belt 130. It also functions as a drive roller that rotationally drives the belt 130 with the frictional force.

  The primary transfer rollers 41 (41a, 41b, 41c, 41d) are arranged to face the photosensitive drums 3 (3a, 3b, 3c, 3d) at the respective image forming stations P (Pa, Pb, Pc, Pd). ing. The primary transfer roller 41 is formed by adhering an EPDM sponge to an aluminum core, and has an outer diameter of 14 mm, a hardness of about 14 ° (Asker C 500 g load), and press contact with the intermediate transfer belt 130 with a pressure of 0.8 N. It is rotating following.

  The primary transfer roller 41 can be applied with a predetermined high voltage from the end of the cored bar through a conductive bearing.

  The secondary transfer counter roller 131 which is also a driving roller is a hollow aluminum roller, and is a roller whose outer periphery is coated with NBR. Although not shown, the secondary transfer counter roller 131 has its shaft passed through a bearing held by the belt support member 135, and a gear is disposed at the end of the shaft. The secondary transfer counter roller 131 receives a driving force supplied from a motor (not shown) of the apparatus main body, rotates at a predetermined speed, and is driven by the friction force of the surface NBR and the inner surface of the intermediate transfer belt 130. Then, the intermediate transfer belt 130 is rotated. The shaft of the secondary transfer counter roller 131 is grounded, and is disposed opposite to the intermediate transfer belt 130 to form a bias path with the secondary transfer roller 7 to which a secondary transfer bias is applied.

  The driven roller (idler roller) 133 is an aluminum hollow roller, and although not shown, the shaft is passed through a bearing held by the belt support member 135 and rotated. The idler roller 133 is arranged so that the traveling direction of the intermediate transfer belt 130 sandwiched between the primary transfer rollers 41d facing the photosensitive drum 3d of the most downstream image forming station Pd is the optimum direction for primary transfer. Yes. Further, the intermediate transfer belt 130 provided between the idler roller 133 and the secondary transfer roller 7 is also optimally arranged for secondary transfer.

  The photosensitive drums 3a, 3b, 3c, and 3d of the image forming stations Pa, Pb, Pc, and Pd are in contact with the outer periphery of the intermediate transfer belt 130. Further, the secondary transfer roller 7 that transfers the toner image on the intermediate transfer belt 130 onto the transfer material, and the belt cleaning member 134 that collects the transfer residual toner after the secondary transfer are in contact with the outer periphery of the intermediate transfer belt 130. .

  The secondary transfer roller 7 is made of a conductive sponge, has an outer diameter of 24 mm, and is in pressure contact with the secondary transfer counter roller 131 at 2.5 N on one side. The secondary transfer roller 7 has a gear (not shown) arranged at the end of the shaft, and is rotated with a speed difference with respect to the belt peripheral speed by receiving a drive from the apparatus main body. A bias can be supplied to the metal core, and the bias is applied during transfer.

  In the belt cleaning 134, a plate-shaped cleaning blade made of urethane rubber is disposed at a position facing the tension roller 132 of the intermediate transfer belt 130 in a counter direction with respect to the rotation direction of the belt, and is brought into contact with a pressure of 1.0 N. Yes.

  As described above, the intermediate transfer belt 130 is suspended by the three axes of the secondary transfer counter roller 131, the tension roller 132, and the driven roller 133 that face the secondary transfer roller 7 in FIG. It can be rotated in the direction of arrow b.

  Further, as shown in FIG. 8, a tapered regulating member 141 is provided at the end of the tension roller 132 provided with the rotating shaft 142. Therefore, when the intermediate transfer belt 130 moves in the direction of the arrow a, the end regulating rib 140 provided on the belt 130 abuts against the regulating member 141, thereby suppressing the movement of the intermediate transfer belt 130. The other end of the tension roller 132 has the same configuration.

  As shown in FIG. 7, the detection sensors 150L and 150R are positioned at LL = LR = 135 mm from the longitudinal image region center CL on the intermediate transfer belt 130 toward both ends. The detection sensor 150L is the type shown in FIG. 6 which is a sensor capable of detecting the toner density, and the detection sensor 150R is the type shown in FIG. 5 which is a sensor capable of detecting only regular reflection light.

  In FIG. 6, the distance between the reference plane (not shown) of the sensor and the intermediate transfer belt 130 is 10 mm, the light emitting portion 150a is an infrared light emitting LED having a peak wavelength of 940 nm, and the incident angle α is 15 °. . On the other hand, the light receiving portions 150b and 150c are Si phototransistors, the peak sensitivity wavelength is 850 nm, the regular reflection component light receiving portion 150b has an incident angle β of 15 °, and the diffused light component light receiving portion 150c has an incident angle γ of 45 °. Is arranged.

  Here, referring to FIG. 9, a patch image (test pattern) 113 is formed for each color in the image forming unit 102 which is an image unit constituting the image forming station P in FIG. 1, and the intermediate transfer belt. 130 is transferred.

  In the present embodiment, since the image density control has a characteristic portion, the patch image 113 detected by the detection sensor 150L will be described. The patch image detected by the detection sensor 150R for registration control can be a patch pattern known to those skilled in the art according to the prior art, and description thereof is omitted.

  In the present embodiment, as shown in FIG. 7, the patch image 113 detected by the detection sensor 150L is patch image 113 (113a, 113b, 113c, 113) of each color, that is, yellow, magenta, cyan, and black. 113d) formed. For example, taking the yellow patch image 113a as an example, the patch image 113a is a gradation pattern composed of a plurality of patterns 113a1, 113a2,. In this embodiment, the number of patch images is 16 (n = 16) for each color.

  In this embodiment, the length of the patch image 113 in the main scanning direction is 15 mm and the length in the sub-scanning direction is 33 mm, and the sampling in the patch detection 114 is performed at 10 points at a pitch of 20 msec from 3 mm from the tip of one patch. The sampled detection value is sent to the detection value density conversion 115.

  For the sampled detection values sent to the detection value density conversion 115, the maximum value and the minimum value are removed from the 10 points, and the remaining 8 points are averaged to obtain detection values of the patch image.

  The patch detection 114 detects the specularly reflected light component and the diffused light component, and the detected value density conversion 115 calculates density information from these signals.

  The regular reflection component has the highest light reception level when the substantially mirror-finished belt surface is detected. Since the portion to which the toner is attached has an uneven shape, the light receiving level of the specular reflection component decreases according to the amount of toner attached.

  When the diffused light component is incident on the mirror surface portion, it becomes almost zero level. As the toner adheres, irregular reflection occurs on the toner surface, so that the light receiving level of the diffuse light component increases.

  However, with black toner, the toner itself absorbs incident light, so that no irregular reflection component is generated, and the received light level of diffused light hardly changes.

  Since the roughness of the surface of the intermediate transfer belt gradually changes with long-term use, the output level of the sensor varies. Moreover, the level also fluctuates due to dirt on the sensor surface.

  Therefore, the patch density data is interpolated using the signal value of the background portion corresponding to each patch data.

Since the background level directly under the patch cannot be detected at the same time, after the belt drive is stabilized, the belt background is detected for one round and stored in memory together with the phase information. Information is read from the memory, and the concentration is obtained using the following equation (1). If the post-conversion density is Dpatch, the background read value is Sbase, and the patch read value is Spatch, the converted density D is obtained by the following equation (1).
Dpatch = Spatch / Sbase (1)

  The secondary transfer counter roller 131, the tension roller 132, and the driven roller 133 that suspend the intermediate transfer belt 130 are arranged in parallel. However, component tolerance and alignment deviation occur. Further, if there is a difference in circumferential length between both ends of the intermediate transfer belt 130, the intermediate transfer belt 130 moves to one side in the sub-scanning direction.

  As described above, the position of the intermediate transfer belt 130 is regulated in the vicinity of the regulating member 141 provided at the end of the tension roller 132 by the regulating rib 140 disposed on the inner surface of the belt to regulate the movement in the sub-scanning direction. The As described above, the direction of this shift depends on the relationship between the three axes and the characteristics of the single belt, so that it is incorporated in the apparatus main body and is relatively stable if the combination of the components does not change.

  As described above, when the intermediate transfer belt 130 is shifted to one side, the surface of the intermediate transfer belt 130 is slightly bent.

  When the deflection occurs at the opposing portion of the patch sensor, the signal on the ground greatly fluctuates due to the vibration of the belt.

  Sensors must be placed close to each other to obtain a sufficient amount of received light, but specularly reflected light is output significantly when the belt, which is the object to be measured, moves in the depth direction from the focal position, or when the reflective surface is tilted from parallel. Decrease. That is, if the belt vibrates up or down due to the deflection of the intermediate transfer belt or a wave is generated, the output of the ground fluctuates irregularly.

  In a patch image of a highlight portion (low density portion) with a small amount of toner, an error in density information obtained by belt vibration becomes large.

  In other words, in an image forming apparatus that has been conventionally employed and has a density sensor only on one side, the accuracy of density correction control has been reduced depending on the direction of the belt.

  In this embodiment, in order to solve the above-described problem, the density correction control is performed in which the characteristics of the background signal obtained by the density sensor are detected and the correction coefficient of the density correction control is changed. That is, according to the image density correction control of this embodiment, the density correction amount is corrected by using the correction coefficient determined based on the random vibration information of the background read from the belt background of the intermediate transfer belt by the density sensor.

  First, the background detection operation will be described with reference to FIG.

  When the background detection mode is called at a predetermined timing (S1), the motor driving of the main unit is started and the driving of the intermediate transfer belt 130 is started (S2). After about 1000 msec necessary for the intermediate transfer belt to rotate normally, sampling of the surface of the intermediate transfer belt 130 by the sensor 150L is started (S3). Sampling was performed at 20 msec intervals as in the case of normal image density correction control. The detection of the background signal can be performed by using either regular reflection light or diffused light. However, since the belt fluttering has a great influence on the regular reflection component, it is only necessary to sample regular reflection light in the sensor selection mode. When the rotation for one rotation of the belt is detected (S4), the sampling by the sensor 150L is finished (S5, S6), and then the signal value is calculated (S7).

  The vibration state of the regular reflection component changes depending on the state of the intermediate transfer belt. FIG. 3 shows the vibration state of the belt corresponding to the two states. The vertical axis represents the specular reflection component output value after AD conversion of the sampled patch signal, and the horizontal axis represents time. Thus, it can be seen that irregular vibration may be observed in the signal of the regular reflection component on the belt surface.

  Since the vibration of the intermediate transfer belt is detected as the variation of the background signal as described above, the dispersion value S is obtained from the sampled signal value (S7). The dispersion value S was obtained from the average value Save of the I-th sampling signals Si and Si as follows.

  A correction coefficient αn is determined from the obtained dispersion value according to Table 1 (S8). Here, n is a sampling number.

  As shown in Table 1, when the variance value S increases, the correction coefficient is set to zero and no correction is performed.

  Next, density correction control will be described with reference to FIG.

  An image density correction process (image density correction control) is activated by the density correction execution command (S21).

  Here, the image density correction processing will be described with reference to FIG. 9 which is a block diagram showing the configuration of the image forming apparatus 100 of the present embodiment. Desired patch image data 110 is generated in the image processing unit 101 of the image forming apparatus 100.

  The patch image data 110 is subjected to a screen process by a halftone process 111 to generate a patch gradation image signal 112.

  The patch tone image signal 112 is transferred to the image forming unit 102, and a patch image 113 is formed on the intermediate transfer belt 130 by the above-described image forming process (S22). The diffused light component is sampled (S23). The measured value is converted into density information by the detected value density conversion block 115 (S25), and transferred to the image processing unit 101 as patch density information 116. The image processing unit 101 collates the patch density information and creates a correction table.

  As in the flow of FIG. 4, in the correction table creation block 117 of FIG. 9, the difference value with the target density is calculated (S26), the difference value and the correction value are calculated (S27), and the correction table is created. Performed (S28).

In this embodiment, a correction amount F obtained by multiplying the correction amount C-AB obtained from the gradation patch described with reference to FIG. 13 by a value based on the amount of noise obtained as a result of detecting the belt background is used. . The correction amount F is obtained as follows.
Correction amount F = (correction amount C−AB) × α

  Here, a problem that occurs when the correction coefficient α based on the background signal is not used and an effect of this embodiment will be described.

  As shown in FIG. 14, when the gradation characteristic of the image forming apparatus changes from GA to GB, the characteristic GB is originally obtained by patch control. That is, in the conventional patch control, the test pattern 113 as shown in FIG. 7 is formed on the surface of the intermediate transfer belt. Then, the density correction amount is determined based on the density information read from the test pattern by the detection sensor, and the image density correction based on the density correction amount, that is, the density correction control for changing the process conditions such as the exposure amount and the developing bias. Is going.

  However, when the intermediate transfer belt substrate vibrates due to the vibration of the intermediate transfer belt, noise (vibration component) is added to the substrate information or highlight patch information, and a characteristic GC with low accuracy is detected as a result of the calculation. There was a case.

  At this time, the density difference becomes D-AC in FIG. 15 and is inverted from the ideal density difference value D-AB in the highlight portion. When modulation is performed based on this result, the gradation characteristic becomes the T-AC characteristic of FIG. The characteristic T-AC is not preferable as a reproduced image because the gradation is lost in the highlight portion.

  On the other hand, the case where the correction coefficient α is obtained from the dispersion value S based on the vibration of the base used in the present embodiment will be described.

  In this embodiment, since the background signal variance S shown in FIG. 3 is 30.3, the correction coefficient α is set to 0.30.

  FIG. 17 shows density difference characteristics when the correction coefficient α is used.

  The above-described D-AC is multiplied by a coefficient α = 0.30, and the correction amount is suppressed. The gradation characteristic obtained by this correction is T-αAC in FIG. Gradation loss in the highlight area caused by T-AC was eliminated, smooth gradation characteristics were obtained over the entire density, and the ideal gradation T could be approached compared to TB without correction. .

  As described above, in the state where the background of the intermediate transfer belt vibrates, the density detection accuracy in the highlight portion is lowered, so that sufficient gradation stability cannot be obtained. However, by reading the background vibration as in this embodiment and correcting the coefficient used for density correction control according to the dispersion value, density stability can be obtained even when the background vibration exists. .

  Regarding the detection timing of the background signal, for example, when the apparatus main body is installed or when the unit of the intermediate transfer belt is replaced, it is preferable to automatically start the background signal.

  Further, the background detection may be performed prior to the gradation correction control.

  Alternatively, other control may be performed at the non-image formation timing, and the background information may be obtained in synchronization with the timing when the toner image does not exist in the sensor area.

  Alternatively, when an image is formed only in a region inside the sensor region at both ends, the background of the patch portion may be detected in parallel with the image formation.

  In addition, regarding the background detection, an example in which sampling is performed for one rotation of the belt has been shown. However, the present invention is not limited to this, and depending on the sampling cycle and the belt driving method, the number of samplings can accurately capture the background vibration. Of course it is good.

  In this embodiment, an example is described in which a sensor is disposed at the end. However, when the density sensor disposed at the center also vibrates the intermediate transfer belt and a noise component is detected. It is clear that the present invention can be applied and the same effect can be obtained.

  In this embodiment, an image forming apparatus that performs density correction by forming a patch on the intermediate transfer belt is described as an example. However, the present invention is not particularly limited thereto. For example, the present invention can also be applied to an image forming apparatus that uses a belt-shaped photosensitive member (that is, a photosensitive belt) as an image carrier on which a toner image is formed, and forms a patch on the photosensitive belt.

  In this embodiment, an example using a patch pattern over a plurality of gradation signals as a gradation correction patch has been described. However, the present invention is not limited to this, and correction is performed using a single level patch. The same effect can be expected for control.

  As can be understood from the above description, according to this embodiment, the belt-shaped image carrier, that is, the patch with low accuracy even in the state where the ground of the intermediate transfer belt (or the photosensitive belt) is vibrating. Good gradation stability can be obtained without causing a problem of gradation due to density information.

  Further, in addition to the above, the same effect can be obtained without increasing the number of sensors by applying the density detecting means to the image forming apparatus arranged in the vicinity of the end of the image forming area in the main scanning direction. .

Example 2
In this embodiment, a correction coefficient β according to a correction table is used instead of the correction coefficient α.
This utilizes the fact that the influence of the background vibration is large for the density patch in the vicinity of the highlight portion (low density portion) but small for the high density portion.

  A correction coefficient β is obtained from the correction table of Table 2. The patch number column in Table 2 is patch number n = 1 to 16, and the signal level is the signal level of the patch corresponding to the patch number on a one-to-one basis. In this embodiment, the signal level of 256 gradations from 0 to 255 is shown. Of these, patches were formed every 16 levels.

  The base dispersion value S is a classification based on the dispersion value obtained by the same method as the dispersion value obtained in the first embodiment. The boundaries of the variance values were divided into areas of 4, 10, 25, 50 or less and greater than 50. The value of each cell is a correction value βn applied from the variance value and the patch number. Here, n indicates a patch number.

  For example, when the dispersion value S is 8, 4 <8 ≦ 10, so the column of the background dispersion value 10 in FIG. 18 is applied, and 0.84 is applied to the density difference data at the patch number 5, that is, the signal level 80. Double correction.

  When the correction of the present embodiment is used when the detection accuracy of the low density portion patch near the highlight portion is reduced as in the characteristic GC described with reference to FIG. 14, the dispersion value S = 30.3, so the background dispersion in Table 2 A column of values 50 was applied, and the difference data of each patch number was multiplied by a coefficient.

  FIG. 18 shows a difference characteristic D-AC including a lot of errors, and a correction error characteristic D-βAC when the correction value β is used in this embodiment. Thus, since D-βAC has a small correction amount in the highlight portion (low density portion), the inversion amount with low accuracy is small. Further, in the high density portion, it almost matches the ideal correction characteristic D-AB. The result of using this D-βAC is the gradation characteristic of FIG. The gradation loss in the highlight portion that has occurred in T-AC has been eliminated, and almost ideal correction has been made from the middle to the vicinity of the high density portion, so there are many regions that match the target gradation characteristic T. .

  As described above, in this embodiment, the correction table is changed in accordance with the vibration level of the background using the fact that the background vibration has a large influence on the highlight patch. Further, by optimizing the correction amount for the highlight portion and the correction amount for the high density portion, an image with more excellent gradation can be obtained.

  That is, according to the present embodiment, in addition to the operational effects obtained in the first embodiment, the correction coefficient is further increased in the high density portion than in the low density portion. As a result, it is possible to obtain more excellent gradation characteristics in the high density portion that is not easily affected by the background while causing no loss of gradation in the low density portion.

1 is a schematic configuration diagram of an embodiment of an image forming apparatus according to the present invention. It is a flowchart explaining one Example of density correction control according to this invention. It is a figure which shows an example of the measurement result of the background signal in density control. It is a flowchart explaining one Example of density correction control according to this invention. It is a schematic block diagram explaining an example of a patch sensor. It is a schematic block diagram explaining the other example of a patch sensor. FIG. 4 is a diagram illustrating a positional relationship between an intermediate transfer belt and a patch sensor. FIG. 3 is a schematic configuration diagram of an end portion of an intermediate transfer belt for explaining an intermediate transfer belt shift regulating mechanism. It is a block diagram for explaining density correction control in the image forming apparatus. It is a figure showing a density characteristic and a correction characteristic. 6 is a diagram illustrating a change in density characteristics of the image forming apparatus. FIG. It is a figure showing the change of a gradation characteristic. It is a figure showing the density difference and correction characteristic in gradation correction control. FIG. 6 is a diagram illustrating a change in density characteristics of an image forming apparatus detected when accuracy is low. It is a figure showing a density difference in case accuracy is low. It is a figure showing the gradation characteristic after amendment when accuracy is low. It is a figure showing the density | concentration difference correct | amended by 1st Example of this invention. It is a figure showing the density | concentration difference correct | amended by 2nd Example of this invention. It is a figure showing the gradation characteristic after correction by the 2nd example of the present invention.

Explanation of symbols

1 (1a, 1b, 1c, 1d) Developer (developing means)
2 (2a, 2b, 2c, 2d) Charger (charging means)
3 (3a, 3b, 3c, 3d) Photosensitive drum 4 Transfer device (transfer means)
7 Secondary transfer roller (secondary transfer means)
9 Exposure equipment (exposure means)
41 (41a, 41b, 41c, 41d) Primary transfer roller (primary transfer means)
DESCRIPTION OF SYMBOLS 100 Image forming apparatus 101 Image processing part 102 Image forming part 113 Patch image (test pattern)
130 Intermediate transfer belt (belt-shaped image carrier)
150L, 150R Concentration detection sensor (concentration detection means)
P (Pa, Pb, Pc, Pd) Image forming station (image forming unit)

Claims (6)

A belt-shaped image carrier;
A plurality of image forming units capable of forming a toner image on the surface of the belt-shaped image carrier;
Density detecting means for optically reading the toner image formed on the surface of the belt-shaped image carrier,
In an image forming apparatus that forms a test pattern on the surface of the belt-shaped image carrier and performs image density correction based on a density correction amount determined based on density information read by the density detection unit.
An image density correction that corrects the image density by correcting the density correction amount using a correction coefficient determined based on random vibration information of the background read by the density detection unit of the belt background of the belt-shaped image carrier. Have control,
The image forming apparatus according to claim 1, wherein the correction coefficient is a coefficient for reducing a density correction amount when a vibration component due to vibration of the belt-shaped image carrier is large in the density information.   The image forming apparatus according to claim 1, wherein the density detecting unit is disposed in the vicinity of an end portion in the main scanning direction of the image forming area.   3. The image formation according to claim 1, wherein the test pattern is a gradation pattern including a plurality of patterns having different densities, and the correction coefficient uses a different value depending on the density of the gradation pattern. apparatus   The image forming apparatus according to claim 3, wherein the correction coefficient increases a correction amount in a high density portion compared to a low density portion.   The belt-like image bearing member is a belt-like intermediate transfer member capable of forming a toner image, and the toner image is transferred to the intermediate transfer member from a photosensitive member on which the toner image is formed by a transfer unit. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   The belt-like image bearing member is a belt-like photosensitive member capable of forming a toner image, and an electrostatic latent image corresponding to an image information signal is formed on the photosensitive member, and the electrostatic latent image is developed by a developing unit. The image forming apparatus according to claim 1, wherein the image is developed with a developer to form a toner image.

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