JP2016212266A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that accurately determines the amount of color shift to perform color shift correction irrespective of the density of a detection pattern.SOLUTION: An image forming apparatus comprises: forming means that forms a color shift amount detection pattern on an image carrier; detection means that detects the detection pattern formed on the image carrier; and correction means that determines the amount of color shift to perform color shift correction on the basis of a result of detection from the detection means. The correction means corrects a result of detection from the detection means according to the density of the detection pattern formed on the image carrier by the forming means, and determines the amount of color shift on the basis of a result of detection after the correction.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image forming apparatus such as a color printer or a color copying machine having a plurality of image forming units.

  In an image forming apparatus that forms multicolor images by superimposing different color developer images formed by a plurality of image forming units, the relative positions of the developer images formed by the image forming units are shifted. Color misregistration can occur. For this reason, Patent Document 1 has a configuration in which a color misregistration detection pattern is formed on an intermediate transfer belt, which is detected by an optical sensor to acquire a color misregistration amount, and color misregistration correction is performed based on the acquired color misregistration amount. Disclosure.

JP 2007-86641 A

  The developer is consumed by forming the detection pattern on the intermediate transfer belt. However, it is required to reduce the amount of developer consumed by the detection pattern. However, if the density of the detection pattern is lowered to suppress the developer consumption, the accuracy of color misregistration correction may be affected.

  The present invention provides an image forming apparatus that performs color misregistration correction by accurately obtaining a color misregistration amount regardless of the density of a detection pattern.

  According to one aspect of the present invention, an image forming apparatus includes: a forming unit that forms a detection pattern of a color misregistration amount on an image carrier; a detection unit that detects a detection pattern formed on the image carrier; Correction means for obtaining a color misregistration amount based on a detection result and performing color misregistration correction, the correction means according to the density of a detection pattern formed on the image carrier by the forming means. The detection result is corrected, and the color misregistration amount is obtained based on the corrected detection result.

  According to the present invention, color misregistration correction can be performed by accurately obtaining the color misregistration amount regardless of the density of the detection pattern.

1 is a configuration diagram of an image forming apparatus according to an embodiment. The block diagram of the sensor by one Embodiment. The figure which shows the detection pattern by one Embodiment. 1 is a control configuration diagram of an image forming apparatus according to an embodiment. FIG. The figure which shows the developing bias by one Embodiment. Explanatory drawing of the shift | offset | difference of the detection timing by the density | concentration of the detection pattern by one Embodiment. Explanatory drawing of the correction process of the detection timing of the detection pattern by one Embodiment. 1 is a control configuration diagram of an image forming apparatus according to an embodiment. FIG. 10 is a flowchart of detection pattern density determination processing according to an embodiment; Explanatory drawing of the shift | offset | difference of the detection timing by the density | concentration of the detection pattern by one Embodiment. Explanatory drawing of the correction process of the detection timing of the detection pattern by one Embodiment.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. In addition, the following embodiment is an illustration and does not limit this invention to the content of embodiment. In the following drawings, components that are not necessary for the description of the embodiments are omitted from the drawings.

<First embodiment>
FIG. 1 is a schematic configuration diagram of the image forming apparatus according to the present embodiment. In FIG. 1, alphabets a, b, c, and d at the end of the reference numerals indicate the colors of the developer images involved in the formation of the members indicated by the reference numerals, respectively yellow (Y), magenta (M), It shows that it is cyan (C) and black (K). In the following description, when it is not necessary to distinguish colors, reference numerals excluding the last alphabet are used. The photoconductor 1 is an image carrier and is rotationally driven in the direction of the arrow in the figure at the time of image formation. The charging roller 2 outputs a charging bias to charge the surface of the photoreceptor 1 to a predetermined potential. The exposure unit 11 irradiates the photoreceptor 1 with laser light 12 modulated based on the image signal, and forms an electrostatic latent image on the photoreceptor 1. The developing unit 8 has a corresponding color developer. The developing roller 4 of the developing unit 8 outputs a developing bias, thereby causing the developer to adhere to the electrostatic latent image on the photoreceptor 1 to form a developer image. The primary transfer roller 81 outputs a primary transfer bias, and transfers the developer formed on the photoreceptor 1 to an intermediate transfer belt 80 that is an image carrier. Note that a multicolor developer image is formed by superimposing the developer images of the respective photoreceptors 1 and transferring them onto the intermediate transfer belt 80. The cleaning unit 3 removes the developer that is not transferred to the intermediate transfer belt 80 and remains on the photosensitive member 1. The photosensitive member 1, the charging roller 2, the cleaning unit 3, and the developing unit 8 are an integrated process cartridge 9 that is detachable from the image forming apparatus. In FIG. 1, members having the same alphabet at the end constitute an image forming unit that forms a corresponding color developer image on the intermediate transfer belt 80.

  The intermediate transfer belt 80 is stretched by three rollers, and is rotationally driven in the direction of the arrow in the figure at the time of image formation. Accordingly, the developer image transferred to the intermediate transfer belt 80 is conveyed to a position facing the secondary transfer roller 82 by the rotation of the intermediate transfer belt 80. The secondary transfer roller 82 transfers the developer image on the intermediate transfer belt 80 to a recording material conveyed through the conveyance path 87. Thereafter, the recording material is conveyed to a fixing unit (not shown), and the developer image is fixed. A sensor 60 that detects a detection pattern for color misregistration correction is provided at a position facing the intermediate transfer belt 80.

  FIG. 2 is a configuration diagram of the sensor 60 according to the present embodiment. 2 indicates the moving direction of the intermediate transfer belt 80. The sensor 60 includes detection units 300 and 301 at two different positions in a direction orthogonal to the conveyance direction of the intermediate transfer belt 80. In the present embodiment, two detection units are used, but two or more detection units may be used. The configurations of the detection units 300 and 301 are the same, and include a light emitting element 302 and light receiving elements 303 and 304, respectively. The light emitting element 302 irradiates light toward the intermediate transfer belt 80. As an example, the light emitting element 302 is configured to emit light at an angle of 15 ° with respect to the normal direction of the surface of the intermediate transfer belt 80. The light receiving element 303 is configured to detect the light irradiated by the light emitting element 302 and diffusely reflected by the intermediate transfer belt 80. As an example, the light receiving element 303 is configured to receive light traveling at an angle of 45 ° with respect to the normal direction of the surface of the intermediate transfer belt 80. On the other hand, the light receiving element 304 is configured to mainly receive light irradiated by the light emitting element 302 and regularly reflected by the intermediate transfer belt 80. As an example, the light receiving element 304 is configured to receive light traveling at an angle of 15 ° with respect to the normal direction of the surface of the intermediate transfer belt 80. When the color misregistration detection pattern 200 enters the irradiation region of the light emitting element 302, the amount of light received by the light receiving elements 303 and 304 changes to a value different from that when the detection pattern is not in the irradiation region of the light emitting element 302. Therefore, it is possible to detect the position of each color patch included in the detection pattern from the change in the amount of light received by the light receiving elements 303 and 304, thereby determining the relative position of each color and measuring the color shift amount. it can.

  FIG. 3 shows a detection pattern 200 used for color misregistration correction according to the present embodiment. The detection pattern includes patches of each color. Note that the letters Y, M, C, and K next to each patch in FIG. 2 indicate that the colors of the patches are yellow, magenta, cyan, and black, respectively. The detection pattern 200 of the present embodiment includes a patch in which a black patch is superimposed on a yellow patch, and magenta and cyan patches. In the present embodiment, the position of a patch of another color is detected using a black patch as a reference, and the color misregistration amount is measured by comparing with the reference value of each color. For example, the difference between the value obtained by multiplying the time from detection of a black patch to the detection of the magenta patch by the moving speed of the intermediate transfer belt 80 and the reference value of magenta is the moving direction of the intermediate transfer belt 80. This is the amount of magenta color misregistration in the (sub-scanning direction). Since the moving speed of the intermediate transfer belt 80 is common to each color, the color misregistration amount can also be expressed by a difference in detection timing of each color patch. When color misregistration occurs in a direction (main scanning direction) orthogonal to the moving direction of the intermediate transfer belt 80, the difference between detection times of two patches of the same color changes. By comparing this difference between colors other than black with a reference black difference, the color misregistration amount of the color in the main scanning direction can be detected.

  FIG. 4 is a control configuration diagram of the image forming apparatus according to the present embodiment. When the user operates the host computer 400 to perform printing, the host computer 400 transmits image data to be printed to the controller 401 of the image forming apparatus. The controller 401 communicates with the engine control unit 402 to send and receive a predetermined command, and outputs a video signal corresponding to an image to be formed to the engine control unit 402 at a predetermined timing. The image control unit 414 controls the forming unit 61 based on a video signal received via the video interface unit 410 to form an image. 2 means the entire member that forms an image including the configuration shown in FIG. The image control unit 414 controls the density of an image to be formed and the operation timing of each member based on the image forming conditions at that time. For example, the charging bias output from the charging roller 2, the exposure timing by the exposure unit 11, the developing bias output from the developing roller 4 and the like are controlled based on the image forming conditions. Note that the image forming conditions are determined by the CPU 411 based on the results of color misregistration correction control and density correction control. The memory unit 418 includes a volatile memory and a non-volatile memory, and is a holding unit that holds programs executed by the CPU 411 and various data used by the engine control unit 402 in its processing.

  When executing the color misregistration correction control, the engine control unit 402 causes the forming unit 61 to form the detection pattern 200, and the sensor 60 detects the detection pattern to obtain the color misregistration amount. Then, image forming conditions are set so as to reduce the color misregistration amount, for example, the timing of image formation is determined. In the present embodiment, the user can set the density of the detection pattern 200 using, for example, the host computer 400 in order to suppress the developer consumption by the detection pattern 200. That is, the controller 401 functions as an input unit that allows the host computer 400 to set the density of the detection pattern 200. In this embodiment, the density is changed by the developing bias. However, the density may be changed by changing other image forming conditions such as exposure intensity by the exposure unit 11. FIG. 5 shows an example of density and developing bias for each color. In the present embodiment, the detection pattern 200 formed at a density of 100% is used as a reference, and hereinafter referred to as a reference detection pattern. In addition, the detection pattern 200 formed with a density of less than 100% is hereinafter referred to as a low density detection pattern.

  FIG. 6 is a diagram illustrating a change in patch detection timing of the detection pattern 200 due to a difference in density. In the present embodiment, the output of the light receiving element 303 that receives diffusely reflected light is used for color misregistration correction control. The CPU 411 detects the edge of each patch by comparing a detection voltage corresponding to the amount of light received output from the light receiving element 303 with a threshold value. Here, the light emitted to the chromatic color, in this example, the yellow, cyan, and magenta patches is mainly diffusely reflected. On the other hand, the light irradiated on the surface of the intermediate transfer belt 80 is mainly regularly reflected. In the black patch, both diffuse reflection light and regular reflection light are small. Therefore, when the yellow, cyan, and magenta patches of the detection pattern 200 in FIG. 3 enter the irradiation area of the light emitting element 302, the light receiving element 303 is more than when there is a black patch in the irradiation area or when no patch is formed. The light received by increases. FIG. 6 shows, as an example, a change with time of the detection voltage output from the light receiving element 303 when the magenta patch passes through the irradiation region of the light emitting element 302. 6A shows the case where the detection pattern 200 is formed with a density of 100%, and FIG. 6B shows the case where the detection pattern 200 is formed with a density less than 100%.

  The CPU 411 compares the detection voltage with a threshold value, and when the detection voltage exceeds the threshold value, determines that the front edge of magenta has been detected, and then detects the rear edge of magenta when the detection voltage falls below the threshold value. Is determined. Here, as the patch density decreases, the amount of diffusely reflected light also decreases accordingly, and thus the maximum value of the detection voltage decreases. Therefore, the timing at which the detection voltage exceeds the threshold and the timing at which the detection voltage falls below the threshold vary depending on the density of the detection pattern 200. Therefore, it is necessary to perform correction so that the edge detection timing is constant regardless of the density of the detection pattern 200.

FIG. 7 is an explanatory diagram of detection timing correction. Reference numeral 700 is an approximation of a rising portion of a detection voltage by a linear function when a standard detection pattern, that is, a chromatic patch of the detection pattern 200 formed at a density of 100% is detected. Similarly, reference numeral 701 approximates a rising portion of the detection voltage by a linear function when a low-density detection pattern, that is, a chromatic patch of the detection pattern 200 formed with a density of less than 100% is detected. . Line 700 and line 701 are the following linear functions, respectively.
Line 700 = At + B
Line 701 = Ct + D
As shown in FIG. 7, the line 700 reaches the threshold at time tm1, and the line 701 reaches the threshold at time tm2. Therefore, when the detection timing of the edge of the reference detection pattern is used as a reference, an error of Δtm = tm2−tm1 occurs in the detection timing of the edge of the low density detection pattern. If the threshold value is V, tm2 = (V−D) / C and tm1 = (V−B) / A, so Δtm = ((V−D) / C) − ((V -B) / A).

  In the present embodiment, for each density used in the detection pattern 200, a coefficient of a function that approximates a temporal change in the amount of light detected by the light receiving element 303 is obtained in advance and held in the memory unit 418. When the low density detection pattern is formed, the engine control unit 402 corrects the edge detection timing based on the corresponding density approximation function and the reference density detection pattern approximation function. Specifically, in the above example, the edge detection timing of the low density detection pattern is tm2, but the amount of color misregistration is obtained with the time that is advanced by Δtm as the edge detection timing. In the above example, the detection voltage rises, but in the case of the fall, the edge detection timing is set to the time when the edge detection timing of the low density detection pattern is delayed by Δtm. In the above example, the rising and falling edges of the detection voltage are approximated by a linear function, but a configuration approximating by a quadratic or higher function may be used.

  With the above configuration, color misregistration correction can be performed with high accuracy regardless of the density of the detection pattern to be formed. Further, by causing the user to select the density of the detection pattern, it is possible to suppress the amount of developer consumed by the detection pattern.

<Second embodiment>
Next, the second embodiment will be described focusing on the differences from the first embodiment. In the first embodiment, the color shift amount is detected by the light receiving element 303. In the present embodiment, the detection pattern 200 is detected based on the amount of light received by the light receiving element 304 that mainly receives regular reflection light. Furthermore, in the present embodiment, the density of the detection pattern is determined based on the remaining amount of developer for each color. FIG. 8 shows a control configuration in the present embodiment. Each process cartridge 9 of the forming unit 61 includes a nonvolatile memory 801. The non-volatile memory 801 stores the remaining amount of the developer when the process cartridge 9 is shipped. The remaining amount detection unit 800 determines the developer image formed on each photoconductor 1 based on the video signal output from the controller 401, thereby determining the consumption amount of the developer of each color, and determining the remaining amount of the developer. Ask. The remaining amount detection unit 800 stores the remaining amount of developer for each color in the nonvolatile memory 801 of the process cartridge 9 for the corresponding color.

  FIG. 9 is a flowchart of the density determination process for the detection pattern 200 to be formed, which is performed during the color misregistration correction control. In S10, the engine control unit 402 reads the remaining amount of developer stored in the nonvolatile memory 801 of each process cartridge 9, and determines whether there is a developer whose remaining amount is equal to or less than the threshold value. If there is no developer whose remaining amount is equal to or less than the threshold value, the engine control unit 402 determines in S12 that the reference detection pattern, that is, the detection pattern 200 with a density of 100% is used for color misregistration correction. On the other hand, when there is a developer whose remaining amount is equal to or less than the threshold value, the engine control unit 402 determines in S11 that the low density detection pattern, that is, the detection pattern 200 having a density of less than 100% is used for color misregistration correction. In S11, the density can be determined according to the remaining amount. For example, the remaining amount can be divided into a plurality of stages, and the density of the detection pattern can be determined according to the stage of the developer with the smallest color. In this case, the lower the remaining amount, the lower the density of the detection pattern. In addition, the threshold used in S10 and the remaining amount of developer are stored in the memory unit 418 in advance. In addition, it can also be set as the structure which a user can set this threshold value.

  As described above, in the color misregistration correction according to the present embodiment, the output of the light receiving element 304 that receives regular reflection light is mainly used. The light emitted to the chromatic color, in this example, the yellow, cyan, and magenta patches is mainly diffusely reflected, and the light applied to the surface of the intermediate transfer belt 80 is mainly regularly reflected. In the black patch, both diffuse reflection light and regular reflection light are small. Therefore, when the chromatic color patch enters the irradiation region of the light emitting element 302, the light received by the light receiving element 304 is reduced as compared with the case where there is a black patch in the irradiation region or when no patch is formed. FIG. 10 shows, as an example, a change with time of the detection voltage output from the light receiving element 304 when the magenta patch passes through the irradiation region of the light emitting element 302. 10A shows a case where a reference detection pattern is formed, and FIG. 10B shows a case where a low density detection pattern is formed.

  The CPU 411 compares the detection voltage with a threshold value, and determines that the front edge of magenta has been detected when the detection voltage falls below the threshold value, and then detects the rear edge of magenta when the detection voltage exceeds the threshold value. Is determined. Here, when the density of the patch decreases, the amount of specular reflection increases accordingly, and the minimum value of the detection voltage increases accordingly. Therefore, as in the first embodiment, the edge detection timing changes depending on the density of the detection pattern 200, and this needs to be corrected.

FIG. 11 is an explanatory diagram of detection timing correction. Reference numeral 1100 is obtained by approximating a falling portion of the detection voltage with a linear function when a chromatic patch of the standard detection pattern is detected. Similarly, reference numeral 1101 is obtained by approximating a falling portion of the detection voltage with a linear function when a chromatic color patch of the low density detection pattern is detected. Note that the lines 1100 and 1101 are the following linear functions, respectively.
Line 1100 = At + B
Line 1101 = Ct + D
As shown in FIG. 11, the line 1100 reaches the threshold at time tm1, and the line 1102 reaches the threshold at time tm2. Therefore, when the detection timing of the edge of the reference detection pattern is used as a reference, an error of Δtm = tm2−tm1 occurs in the detection timing of the edge of the low density detection pattern. If the threshold value is V, tm2 = (V−D) / C and tm1 = (V−B) / A, so Δtm = ((V−D) / C) − ((V -B) / A).

  Also in this embodiment, as in the first embodiment, for each density used in the detection pattern 200, a coefficient of a function that approximates a temporal change in the amount of light detected by the light receiving element 304 is obtained in advance, and the memory unit 418 is obtained. To keep. When the low density detection pattern is formed, the engine control unit 402 corrects the edge detection timing based on the corresponding density approximation function and the reference density detection pattern approximation function. Specifically, in the above example, the edge detection timing of the low density detection pattern is tm2, but the amount of color misregistration is obtained with the time that is advanced by Δtm as the edge detection timing. In the above example, the detection voltage falls, but in the case of the rise, the edge detection timing is set to the time when the edge detection timing of the low density detection pattern is delayed by Δtm. In the above example, the rise and fall of the detection voltage are approximated by a linear function, but a configuration approximating by a quadratic or higher function may be used.

  As described above, the color misregistration correction can be performed with high accuracy regardless of the density of the detection pattern formed in the present embodiment. The engine control unit 402 determines the density of the detection pattern in the color misregistration correction control based on the remaining amount of developer. Thereby, it is possible to suppress the consumption amount of the developer due to the color misregistration correction according to the remaining amount of the developer without causing the user to set the density.

<Summary>
As described above, in the present embodiment, the color misregistration detection pattern 200 is formed on the intermediate transfer belt 80 that is an image carrier, and the detection pattern 200 is detected by the sensor 60 that is a detection unit. The engine control unit 402 obtains the color misregistration amount based on the detection result of the detection pattern 200 by the sensor 60. More specifically, the detection pattern 200 includes a plurality of color patches used for image formation, and the engine control unit 402 determines the relative positions of the other color patches with respect to the reference patch. Detect and obtain the amount of color misregistration. The position of each patch is obtained by comparing the detection result of the detection pattern with a threshold value. For example, the position of each patch can be detected from the timing when the detection result reaches the threshold value. Note that the position of each patch can be detected based on the timing at which the front edge of the patch is detected or the timing at which the rear edge is detected in the moving direction of the intermediate transfer belt 80. Alternatively, the patch position may be detected based on both the timing at which the front edge of the patch is detected and the timing at which the rear edge is detected.

  Then, the engine control unit 402 performs color misregistration correction based on the obtained color misregistration amount. That is, the engine control unit 402 also functions and operates as a color misregistration correction unit. At this time, the engine control unit 402 corrects the detection result of the detection pattern by the sensor 60 based on the density of the detection pattern, and obtains the color shift amount based on the detection result after correction. For example, the reference density is determined in advance. The engine control unit 402 corrects the detection result of the actually formed detection pattern so as to approach the detection result when the detection pattern of the reference density is formed. For this reason, the memory unit 418 of the engine control unit 402 holds correction information indicating the correction amount of the detection result. That is, the memory unit 418 functions and operates as a correction information holding unit.

  The correction information is provided for each density applied to the detection pattern. In the color misregistration correction control, the engine control unit 402 corrects the detection result based on the correction information corresponding to the density of the detection pattern formed on the intermediate transfer belt 80 to obtain the color misregistration amount. More specifically, the engine control unit 402 corrects the timing at which the detection result of the detection pattern by the sensor 60 becomes a predetermined threshold value based on correction information corresponding to the density of the detection pattern.

  Here, the correction information can be, for example, information indicating a temporal change in the detection result when the detection pattern is detected by the sensor 60. Information indicating this time change is provided for each concentration applied to the detection pattern. The detection result when the detection pattern is detected by the sensor 60 is, for example, the output signal of the sensor 60 before and after the detection pattern passes through the detection region of the sensor 60, more specifically, the temporal change in the amount of light received by the sensor 60. It is information which shows. In this case, the engine control unit 402 obtains the timing at which the received light amount of the sensor 60 becomes a predetermined value based on the information indicating the time change for each of the reference density and the density of the formed detection pattern, and further, the difference between the two timings. Ask for. Then, the timing at which the detection result when the detection pattern is detected by the sensor 60 becomes the predetermined value is corrected. Specifically, the timing actually detected by the sensor 60 is corrected so as to approach the timing when the reference density detection pattern is formed.

  The information indicating the time change can be approximate information of the time change. Specifically, the output signal of the sensor 60 can be approximated by a function, and the coefficient of this function can be used as information indicating a time change. Note that the correction information is not information indicating a change in time but may be, for example, a timing difference to be corrected. Specifically, the light reception amount of the sensor 60 when the detection pattern of the reference density is formed becomes a predetermined value, and the light reception amount of the sensor 60 when the detection pattern of each density other than the reference density is formed is a predetermined value. The difference from the timing becomes the correction information. In addition, when there is a possibility that the threshold is changed due to other conditions, information indicating a time change is used as the correction information.

  The engine control unit 402 may be configured to allow the user to input the density of the detection pattern formed in the color misregistration correction. Further, the engine control unit 402 can also determine the density of the detection pattern formed in the color misregistration correction based on the remaining amount of the developer of each color. In this case, the engine control unit 402 also functions and operates as a detection pattern density determination unit. For example, the density of the detection pattern can be determined such that the lower the developer remaining amount, the lower the overall density. For example, the engine control unit 402 determines to form the detection pattern with the first density when the remaining amount of developer is the first remaining amount, and when the remaining amount of developer is the second remaining amount. It is assumed that the detection pattern is determined to be formed at a second density lower than the first density. In this case, for example, the second remaining amount is smaller than the first remaining amount.

  In the above embodiment, the detection pattern formed at a density of 100% is used as a reference, and the detection result of the detection pattern formed at a density other than that is corrected so as to approach the detection result of the detection pattern at a density of 100%. did. However, the reference concentration may be a concentration other than 100%. Further, in the first embodiment, diffuse reflection light is used, and in the second embodiment, the position of the detection pattern is detected using regular reflection light. Therefore, in the first embodiment, the light receiving element 304 can be omitted from the detection unit used only for color misregistration correction. For example, if the detection unit 300 is used only for color misregistration correction and the detection unit 301 is used for color misregistration correction and density correction, the light receiving element 304 of the detection unit 300 can be omitted. Similarly, in the second embodiment, the light receiving element 303 can be omitted from the detection unit used only for color misregistration correction.

[Other Embodiments]
The present invention supplies a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

  80: intermediate transfer belt, 61: forming unit, 60: sensor, 402: engine control unit

Claims (9)

Forming means for forming a color misregistration amount detection pattern on the image carrier;
Detection means for detecting a detection pattern formed on the image carrier;
Correction means for obtaining a color misregistration amount based on the detection result of the detection means and performing color misregistration correction;
With
The correction unit corrects a detection result of the detection unit according to a density of a detection pattern formed on the image carrier by the forming unit, and obtains a color shift amount based on the detection result after correction. Image forming apparatus. For each density of the detection pattern, further comprising holding means for holding correction information indicating a correction amount of the detection result;
The image forming apparatus according to claim 1, wherein the correction unit corrects a detection result of the detection unit based on correction information corresponding to a density of a detection pattern formed on the image carrier by the forming unit. .   The image forming apparatus according to claim 2, wherein the correction unit corrects a timing at which the detection result becomes a predetermined value based on the correction information. The correction information includes the timing at which the detection result by the detection unit when the reference density detection pattern is formed becomes the predetermined value, and the detection by the detection unit when the detection pattern having a density different from the reference density is formed. It is information indicating the difference from the timing when the result becomes the predetermined value,
The correction means corrects the timing at which the detection result of the detection pattern formed on the image carrier by the forming means becomes the predetermined value with the difference corresponding to the density of the detection pattern. The image forming apparatus according to claim 3.   5. The image forming apparatus according to claim 2, wherein the correction information is information indicating a temporal change in a detection result of the detection unit with respect to the density of the detection pattern. 6.   The image forming apparatus according to claim 5, wherein the correction information is a coefficient when the temporal change is approximated by a function.   The image forming apparatus according to claim 1, further comprising an input unit that allows a user to input a density of a detection pattern formed on the image carrier by the forming unit. A remaining amount detecting means for detecting the remaining amount of developer;
A determining unit that determines a density of a detection pattern formed on the image carrier by the forming unit according to a remaining amount of the developer;
The image forming apparatus according to claim 1, further comprising: The determining means determines that the detection pattern is formed with the first density when the remaining amount of the developer is the first remaining amount, and when the remaining amount of the developer is the second remaining amount, It is configured to determine that the detection pattern is formed at a second density lower than the first density,
The image forming apparatus according to claim 8, wherein the second remaining amount is less than the first remaining amount.

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