JP2010049027A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for preventing or reducing both of a color shift and density unevenness in a sub-scanning direction. <P>SOLUTION: An image forming apparatus includes: a position detecting sensor part 35 and a reference position detecting part 101 which detect a position to be detected predetermined in the circumference of each photoreceptor drum 3, at a predetermined position; a speed command value storage part 102 which stores a speed command value common in each photoreceptor drum 3; a correction value storage part 103 which previously stores a correction value related to the speed command value for making passage speeds when each of a plurality of positions on the circumference of the photoreceptor drum 3 passes through a transfer position where a toner image formed on each photoreceptor drum 3 is transferred to an intermediate transfer belt 10 the same in the plurality of positions; and a rotation control part 104 which sets the detection timing of the position to be detected of each photoreceptor drum to a predetermined time difference and rotation controls the photoreceptor drum 3 by using the speed command value stored in the speed command value storage part 102 and the correction value stored in the correction value storage part 103. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a color printer or a color copying machine provided with a plurality of photosensitive drums that multiplexly transfer toner images of different colors onto a recording sheet.

  In an image forming apparatus such as a full-color printer or a full-color copying machine, a plurality of photosensitive drums that form yellow, magenta, cyan, and black toner images for each color are arranged along the conveyance belt and conveyed by the conveyance belt. A tandem system that forms a color image by superimposing and transferring (multiple transfer) toner images of respective colors on a sheet of paper is widely known.

  In this type of image forming apparatus, in order to obtain a high-quality color image, when transferring each color toner image onto a sheet, the transfer position of each color toner image does not cause a positional shift and is exactly the same. It is required to transfer to the position.

  However, the position of the rotating shaft of the photosensitive drum may deviate from the design position due to manufacturing errors or mounting errors of parts related to the photosensitive drum. Further, the position of the rotation shaft of the photosensitive drum may be decentered due to a manufacturing error of the photosensitive drum itself. Further, the friction coefficient of the peripheral surface of the photosensitive drum becomes nonuniform at each position on the peripheral surface, and there are individual differences in the mode of distribution of the friction coefficient. Due to these factors, the transfer position of each color toner image may be displaced, or the transfer belt that is in sliding contact with the photosensitive drum may be stretched or contracted depending on the sliding contact state, resulting in a color different from the desired color. There is a problem that so-called color misregistration and density unevenness occur in the sub-scanning direction.

Japanese Patent Application Laid-Open Publication No. 2003-228561 discloses an output means for outputting a pattern indicating a speed change in one rotation of the photosensitive drum of each image forming unit, and a speed in one rotation of the photosensitive drum determined by the output of the output means. Setting means for setting a correction value based on the phase difference of change, and one rotation of each photosensitive drum by accelerating or decelerating the rotational speed of the photosensitive drum based on the correction value set by the setting means. And a correcting means for adjusting the phase of the speed change for each ([Claim 1]) ”.
JP 2002-244395 A

  However, since the technique of Patent Document 1 only matches the phase of the speed change for each rotation of each photosensitive drum, the above-described density unevenness in the sub-scanning direction caused by the expansion / contraction of the transfer belt is avoided. I can't.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a technique capable of preventing or reducing the occurrence of color misregistration and density unevenness in the sub-scanning direction.

  In the first aspect of the present invention, cylindrical first and second photosensitive drums each having a peripheral surface on which visible images of different colors are formed are arranged, and the first and second photosensitive drums are arranged. An image forming apparatus for forming an image by multiple transfer of visible images respectively formed on the peripheral surfaces of the drum onto a transfer target that travels in an arrangement direction of the first and second photosensitive drums, A position detection unit configured to detect a predetermined position of each of the circumferential surfaces of the first and second photosensitive drums as a detected position, and detect the detected position at a predetermined position; A first storage unit that stores a speed command value common to the second photoconductor drum, and a plurality of positions arranged in the circumferential direction of the peripheral surface of the first and second photoconductor drums, respectively. 2 Transfer position at which a visible image is transferred from the photosensitive drum to the transfer target Is set in the first photosensitive drum and a second storage unit that stores in advance a correction value related to the speed command value for making the passage speed when passing through each of the plurality of positions the same. The detection timing of the detected position and the detection timing of the detected position set on the second photosensitive drum are set to a predetermined time difference, and the speed command value stored in the first storage unit and A rotation control unit that controls a rotation operation of the first and second photosensitive drums based on the correction value stored in the second storage unit;

  According to this invention, the detection timing of the detected position set on the first photosensitive drum and the detection timing of the detected position set on the second photosensitive drum are set to a predetermined time difference. The phase of the visible image formed on the peripheral surface of the first photoconductive drum while taking into account the distance between the first and second photoconductive drums and the traveling speed of the transfer target, and The phase of the visible image formed on the peripheral surface of the second photosensitive drum can be matched. The phase refers to a position in the circumferential direction of the circumferential surface of the photosensitive drum.

  Further, the rotational operation of the first and second photosensitive drums is controlled based on the speed command value stored in the first storage unit and the correction value stored in the second storage unit. The plurality of positions can have the same passing speed when each of the plurality of positions arranged in the circumferential direction of the peripheral surfaces of the first and second photosensitive drums passes through the transfer position.

  As a result, both so-called color misregistration and density unevenness in the sub-scanning direction can be prevented or reduced.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the first and second storage units are nonvolatile memories.

  According to the present invention, the speed command value and the correction value can be constantly held in the image forming apparatus (even if the main power of the image forming apparatus is turned off).

  According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the transfer member is a transfer belt that is in sliding contact with the photosensitive drum.

  According to the present invention, when the photosensitive drum and the transfer belt are in sliding contact with each other, the friction coefficient of the peripheral surface of the photosensitive drum is not uniform at each position on the peripheral surface. The invention according to claim 1 or 2, particularly in a configuration in which the photosensitive drum and the transfer belt are slidably contacted with each other because the transfer belt may expand and contract as a result of the sliding contact state with the transfer belt changing in the circumferential direction. Becomes effective.

  On the other hand, as the correction value, when the first and second photosensitive drums are rotated only by the speed command value stored in the first storage unit, as in the invention described in claim 4, It is good to set it as the value which eliminates the difference of the above-mentioned passage speed generated in a plurality of positions.

  Further, the predetermined time difference may be a value obtained by dividing the distance between the first and second photosensitive drums by the traveling speed of the transferred body, as in the invention described in claim 5.

  According to the present invention, it is possible to prevent or reduce the occurrence of so-called color shift and density unevenness in the sub-scanning direction.

  A printer as an example of an image forming apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of a printer according to an embodiment of the present invention. As shown in FIG. 1, the printer 1 includes image forming units 2M, 2C, 2Y, and 2K arranged in parallel for each color of magenta (M), cyan (C), yellow (Y), and black (K) in the printer body. Has been.

  The image forming units 2M, 2C, 2Y, and 2K (collectively referred to as the image forming unit 2) perform color image formation (printing) on paper. For example, the photosensitive drum 3 made of amorphous silicon, A charging unit 4, an exposure unit 5, a developing unit 6, and a photoconductor cleaning unit 7 are provided around the body drum 3.

  The charging unit 4 uniformly charges the surface of the photosensitive drum 3 to a predetermined potential. The exposure unit 5 irradiates the surface of the photosensitive drum 3 with a laser beam (or LED light) generated based on image data transmitted from an image data storage unit 40 (see FIG. 2), which will be described later, and the drum. An electrostatic latent image is formed on the surface. The developing unit 6 makes the electrostatic latent image appear as a toner image by attaching the toner supplied from the toner supply unit 61 to the electrostatic latent image formed on the photosensitive drum 3. The photoconductor cleaning unit 7 removes toner on the surface of the photoconductor drum 3 after the primary transfer of a toner image to an intermediate transfer belt 10 described later.

  Below the image forming units 2M, 2C, 2Y, and 2K, an intermediate transfer roller 9 (primary transfer roller) and an intermediate transfer belt for performing intermediate transfer (primary transfer) of the toner image that has become apparent on the surface of the photosensitive drum 3. 10 is disposed. The intermediate transfer belt 10 is formed of a predetermined belt body, and is endlessly rotated by the driving rollers 11 to 13 while being pressed against the photosensitive drum 3 by the intermediate transfer roller 9 disposed to face each of the photosensitive drums 3. It is configured. The toner images of the respective colors formed on the respective photosensitive drums 3 are transferred and superimposed on the intermediate transfer belt 10 that is rotated endlessly in the order of magenta, cyan, yellow, and black at the same timing. As a result, a color image composed of four colors Y, M, C, and K is formed on the intermediate transfer belt 10. In each of the image forming units 2M to 2K, positions where the toner image is transferred from the photosensitive drum 3 to the intermediate transfer belt 10 are referred to as transfer positions P1 to P4 (see FIG. 1).

  A secondary transfer roller 14 is provided through the intermediate transfer belt 10 at a position facing the drive roller 13. The secondary transfer roller 14 transfers a color image on the intermediate transfer belt 10 to a sheet by a transfer bias from a control unit 100 (see FIG. 2) described later.

  The printer 1 also includes a paper feed unit 15 that feeds paper toward the image forming units 2Y to 2K. The paper feed unit 15 includes a paper feed cassette 151 that stores paper of each size, a transport path 152 that is a path through which the paper is transported, a transport roller 153 that transports the paper in the transport path 152, and the like. The sheets taken out from the cassette 151 one by one are conveyed toward the image forming units 2Y to 2K, that is, the position of the secondary transfer roller 14. The paper feeding unit 15 conveys the paper subjected to the secondary transfer process to the fixing unit 16 and discharges the paper subjected to the fixing process to the paper discharge tray 17 at the upper part of the printer main body.

  A fixing unit 16 is provided at an appropriate position downstream of the secondary transfer roller 14 in the conveyance path 152. The fixing unit 16 fixes the toner image transferred to the paper. The fixing unit 16 includes a heat roller 161 and a pressure roller 162. The heat on the paper is melted by the heat of the heat roller 161, and the pressure is applied by the pressure roller 162 to fix the toner on the paper.

  In addition, the printer 1 includes a static elimination cleaning 18. The neutralization cleaning 18 removes (collects) toner (residual toner) on the intermediate transfer belt 10. The neutralization cleaning 18 includes a cleaning electrode (not shown) and a cleaning brush (rotating brush). A cleaning bias having a polarity opposite to the charged charge of the toner is applied to the cleaning brush by the cleaning electrode. The toner is removed by moving the toner to the cleaning brush.

  FIG. 2 is a block configuration diagram illustrating an example of a schematic configuration of the printer 1. As shown in FIG. 2, the printer 1 includes a network I / F (interface) unit 30, an image data storage unit 40, a user interface unit 50, a recording unit 60, and a control unit 100.

  The network I / F unit 30 controls transmission / reception of various data to / from an information processing apparatus (external device) such as a PC connected via a network such as a LAN. The image data storage unit 40 temporarily stores image data transmitted from a PC or the like via the network I / F unit 30. The user interface unit 50 is provided, for example, on the front unit of the printer 1 and functions as an input key for inputting various instructions by the user or functions as a display unit for displaying predetermined information.

  The recording unit 60 includes the image forming unit 2, the transfer unit 80, the fixing unit 16, and the paper feeding unit 9, and performs image printing on a sheet based on image data stored in the image data storage unit 40. is there. The transfer unit 80 includes the above-described intermediate transfer belt 10, drive rollers 11 to 13, secondary transfer roller 14, and the like, and transfers the toner image on the photosensitive drum 3 to a sheet via the intermediate transfer belt 10. is there. The cleaning unit 70 includes the static elimination cleaning 18 (including the photosensitive member cleaning unit 7).

  The control unit 100 reads a ROM (Read Only Memory) storing various control programs, a RAM (Random Access Memory) having a function of temporarily storing data and a function as a work area, and the control program from the ROM. CPU to be executed, an ASIC that operates based on an instruction from the CPU, and the like.

  In the printer 1 of this embodiment, the position of the rotation axis of each photoconductive drum 3 in each image forming unit 2M, 2C, 2Y, 2K is shifted from the design position due to manufacturing errors or mounting errors of parts related to the photoconductive drum 3. In some cases, the rotational axis of the photosensitive drum may be decentered due to manufacturing errors of the photosensitive drum itself. Furthermore, the coefficient of friction of the peripheral surface of the photosensitive drum 3 becomes nonuniform at each position on the peripheral surface, and there is an individual difference in the mode of distribution of the friction coefficient. Due to these various factors, the transfer positions of the toner images of the respective colors are displaced, or the intermediate transfer belt 10 that is in sliding contact with the photosensitive drum 3 expands and contracts depending on the sliding contact state. There is a problem that color misregistration in which an image of a color different from the color is formed and density unevenness occur in the sub-scanning direction.

  In order to prevent or reduce such color misregistration and density unevenness in the sub-scanning direction at the same time, the printer 1 of this embodiment includes a reference position sensor unit 35 as shown in FIG. A reference position detection unit 101, a speed command value storage unit 102, a correction value storage unit 103, and a rotation control unit 104 are provided.

  The reference position sensor unit 35 uses a predetermined position on the peripheral surface of each photosensitive drum 3 in each of the image forming units 2M, 2C, 2Y, and 2K as a detected position, and the detected position is determined in advance. The position is detected and includes a reference position sensor 350 installed on each photosensitive drum 3.

The reference position sensor 350 has substantially the same configuration in each of the image forming units 2M, 2C, 2Y, and 2K, and as shown in FIG. 3, a light shielding body 351, a pair of light projecting and receiving devices 352 and 353, It is configured with. As shown in FIG. 4A, a plurality of positions L ₀ , L ₁ ,..., L _n−1 , L _n arranged in the circumferential direction are arranged on the circumferential surface of the photosensitive drum 3 at the transfer position P 1. -P4 is set as an adjustment target position of the passing speed, and one of these positions is set as a reference position. Here, it is assumed that the position L ₀ is the reference position, and the light shield 351 is installed at the reference position L ₀ on the side surface of the photosensitive drum 3.

  On the other hand, the light projecting and receiving devices 352 and 353 perform light projecting and receiving operations through a certain gap. As shown in FIG. 5, the light receiving device 353 receives the light output from the light projecting device 352. In this case, an L (low) signal is output, while an H (high) signal is output as a detection signal when the light output from the projector 352 is not received.

  In the reference position sensor 350 having such a configuration, when viewed from the side of the photosensitive drum 3, the light shielding body 351 moves on the circumference along with the rotation of the photosensitive drum 3 and is moving. At a certain timing, the light passes through the gap between the light emitters / receivers 352 and 353. At this time, the light output from the projector 352 is blocked by the light shield 351, and an H (high) detection signal is output from the light receiver 353.

  The reference position detection unit 101 constitutes the position detection unit together with the reference position sensor unit 35. The reference position detection unit 101 is set for each photoconductor drum 3 using a detection signal output from the reference position sensor unit 35. Each of the reference positions is detected.

The speed command value storage unit 102 stores a speed command value common to the respective photosensitive drums 3. The speed command value common to the respective photosensitive drums 3 is a command value indicating the same angular velocity (rotation angle per unit time) between the photosensitive drums 3. If the photosensitive drum 3 is manufactured and attached as designed, when the photosensitive drum 3 is rotated based on this speed command value, as shown in FIG. wherein each position _L 0 of the peripheral surface of _3, L _{1, ···,} passing speed _{of L n-1, L n} passes through the transfer position P1~P4 is the same velocity _{V 1.}

The correction value storage unit 103 stores a correction value Δv related to the rotational speed of each photosensitive drum 3. As shown in FIG. 4B, when the rotation of each photosensitive drum 3 is controlled only with a common speed command value, the positions L ₀ , L ₁ ,..., L _n−1 , L _n are transferred to the transfer drum. Ideally, the passing speed through the position is the same. However, the speeds at which the positions L ₀ , L ₁ ,..., L _n−1 , L _n pass through the transfer position are different from each other due to the above-described errors and the like. Each position _{L 0,} L 1, · · _·, when expressed the rate of passage of the _{L n-1, L n} in the graph, as shown in FIG. 4 (c), shown in FIG. 4 (b) waveform (waveform indicating the velocity V ₁₎ is often a sinusoidal waveform with the amplitude center.

The correction value is for canceling the difference between the passing speeds of the positions L ₀ , L ₁ ,..., L _n−1 , L _n . That is, the respective positions _{L 0,} L 1, · · _·, if the graph of the rate of passage of _{L n-1, L n} is a sine waveform as shown in FIG. 4 (c), the position _{L 0} , L ₁ ,..., L _n−1 , L _n , as shown in FIG. 4D, the waveform shown in FIG. a value corresponding to a point on the waveform upside down relative to the (waveform indicating the velocity V _1). This correction value is obtained in advance by actual measurement.

As shown in FIG. 6, the correction value storage unit 103 stores the positions L ₀ , L ₁ ,..., L _n−1 , L _n corresponding to the points on the waveform and the correction values Δv ₀ , Δv. ₁ ,..., Δv _n−1 , Δv _n are stored in a table format.

  The rotation control unit 104 controls the operation of a motor M (see FIG. 8) that rotates the photosensitive drums 3. FIG. 7 is a flowchart showing processing of the rotation control unit 104.

  As shown in FIG. 7, when the main power supply of the printer 1 is turned on (YES in step # 1), the rotation control unit 104 matches the phase of each photosensitive drum 3 (step # 2). Here, for example, the black photosensitive drum 3 is used as a reference, and the phases of the photosensitive drums 3 of other colors are matched with the phases of the photosensitive drums 3 corresponding to black. The reference photosensitive drum 3 is referred to as a reference drum, and the photosensitive drum whose phase is matched with the reference drum phase is referred to as a correction target drum.

  Matching the phases of the photosensitive drums 3 means detecting timing of the reference position set in the reference drum 3 by the reference position sensor unit 35 and the reference position sensor of the reference position set in the correction target drum 3. The detection timing by the unit 35 is to have a time difference obtained by dividing the distance between the reference drum 3 and the correction target drum 3 by the traveling speed of the intermediate transfer belt 10.

  Next, the rotation control unit 104 corrects the speed command value stored in the speed command value storage unit 102 and the correction set in the correction table for the reference drum 3 and the correction target drum 3 stored in the correction value storage unit 103. The rotation of the reference drum 3 and the correction target drum 3 (the operation of the motor M that rotationally drives the reference drum 3 and the correction target drum 3) is controlled using the value (step # 3).

That is, for example, when the table shown in FIG. 6 is for the photosensitive drum 3 corresponding to magenta, the rotation control unit 104, for example, the position L ₁ (correction) on the circumferential surface of the photosensitive drum 3 corresponding to magenta. Value Δv ₁ ) passes through the transfer position P 1, the speed command value stored in the speed command value storage unit 102, and the correction table for the photosensitive drum 3 stored in the correction value storage unit 103. Then, based on the correction value Δv ₁ set for the position L ₁ , the operation of the motor M that rotationally drives the photosensitive drum 3 is controlled.

In the case where a stepping motor is employed as the motor M for rotationally driving each photosensitive drum 3, the number of drive pulses output to the stepping motor is proportional to the drive amount, and therefore the number of drive pulses is increased. based on, it is possible to detect the driving amount from the reference position L _0, thus, it is possible to detect the position on the circumferential surface of the photosensitive drum 3 passing through the current transfer position.

By the above operation, the passing speeds at which the positions L ₀ , L ₁ ,..., L _n−1 , L _{n on the} photosensitive drums 3 pass through the transfer position can be made the same. As a result, the position of the rotation axis of the photosensitive drum is shifted from the design position due to manufacturing errors or mounting errors of parts related to the photosensitive drum, or the position of the rotation axis of the photosensitive drum due to manufacturing errors of the photosensitive drum itself. To prevent or reduce both color misregistration and density unevenness in the sub-scanning direction even if the image is eccentric or the coefficient of friction of the surface of the photosensitive drum is uneven at each position on the surface. Can do.

  In this case, the following modifications may be employed instead of or in addition to the embodiment.

  [1] In the above-described embodiment, the control using the speed command value and the correction value is performed after the phases of the photosensitive drums 3 are matched with each other. For example, the configuration shown in FIG. Control may be performed so that the speed at which each position on the peripheral surface of each photoconductive drum passes through the transfer position is the same at each position while the phases of the respective photoconductive drums 3 are matched.

  As shown in FIG. 8, the control unit 100 ′ in the printer 1 of this embodiment includes an adder 201, 209, 211, a subtracter 205, an integrator 202, a multiplier 203, 204, 206, 211, It functions as a differentiator 207. In FIG. 8, the reference drum is represented as the reference drum 3a, the correction target drum is represented as the correction target drum 3b, and only one correction target drum 3b is illustrated. 8 is for detecting the speed of the gear G that transmits the driving force of the motor M to the correction target drum 3b.

  The adder 201 adds the speed command value stored in the speed command value storage unit 102 and the speed detected by the encoder E. The integrator 202 integrates the output of the adder 201. The multiplier 203 multiplies the output of the integrator 202 by a weight coefficient ki1. The multiplier 204 multiplies the output of the adder 201 by a weight coefficient kp. The subtracter 205 subtracts the output of the position detection sensor 35a in the reference drum 3a from the output of the position detection sensor 35b in the correction target drum 3b.

  The multiplier 206 multiplies the output of the subtracter 205 by a weight coefficient ki2. The differentiator 207 differentiates the speed information output from the encoder E. The multiplier 208 multiplies the output of the differentiator 207 by a weight coefficient kd. The adder 209 adds outputs from the multipliers 203, 204, 206, and 208. The amplifier 210 amplifies the output of the adder 209. The adder 211 adds the output of the amplifier 210 and the correction value for the correction target drum 3b.

  [2] The application target of this case is not limited to a printer, but also includes image forming apparatuses such as multifunction peripherals and facsimiles.

1 is a side view schematically showing an internal configuration of a printer as an example of an image forming apparatus according to an embodiment of the present disclosure. FIG. 2 is a block diagram illustrating an electrical configuration of a printer. It is a figure which shows the structure of a position detection sensor. (A) is a diagram showing a circumferential position on the circumferential surface of the photosensitive drum, and (b) is based on this speed command value when the photosensitive drum is manufactured and attached as designed. When the photosensitive drum 3 is rotated, the positions L ₀ , L ₁ ,..., L _n−1 , L _{n on} the peripheral surface of the photosensitive drum pass through the transfer positions _{P 1} to _{P 4.} (C) is a graph showing the speed when there is a difference in speed passing through the transfer position between the positions L ₀ , L ₁ ,..., L _n−1 , L _{n due to} various errors and the like. Graph (d) is a graph showing a correction value for canceling the error shown in (c). It is a figure which shows the output signal of a position detection sensor. It is a figure which shows the table of a correction value. It is a flowchart which shows the process of a rotation control part. It is a figure which shows a deformation | transformation form.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Printer 2 (2M, 2C, 2Y, and 2K) Image formation part 3 Photosensitive drum 10 Intermediate belt 80 Transfer part 35 Reference position sensor part 350 Reference position sensor 100 Control part 101 Reference position detection part 102 Speed command value storage part 103 Correction Value storage unit 104 Rotation control unit

Claims (5)

Cylindrical first and second photosensitive drums each having a peripheral surface on which different colors of visible images are formed are arranged and formed on the peripheral surfaces of the first and second photosensitive drums, respectively. An image forming apparatus that forms an image by multiple transfer of the visualized image to a transfer target that travels in the arrangement direction of the first and second photosensitive drums,
A position detection unit configured to detect a predetermined position of each peripheral surface of each of the first and second photosensitive drums as a detected position and detect each detected position at a predetermined position;
A first storage for storing a speed command value common to the first and second photosensitive drums;
Each of the plurality of positions arranged in the circumferential direction of the peripheral surfaces of the first and second photosensitive drums is a transfer position at which a visible image is transferred from the first and second photosensitive drums to the transfer target. A second storage unit that stores in advance a correction value related to the speed command value for making the passing speed when passing the same at the plurality of positions;
The detection timing of the detected position set on the first photosensitive drum and the detection timing of the detected position set on the second photosensitive drum are set to a predetermined time difference, and the first An image forming apparatus comprising: a rotation control unit that controls a rotation operation of the first and second photosensitive drums based on a speed command value stored in one storage unit and a correction value stored in the second storage unit. .   The image forming apparatus according to claim 1, wherein the first and second storage units are nonvolatile memories.   The image forming apparatus according to claim 1, wherein the transfer target is a transfer belt that is in sliding contact with the photosensitive drum.   The correction value eliminates the difference between the passing speeds generated at the plurality of positions when the first and second photosensitive drums are rotated only by the speed command value stored in the first storage unit. The image forming apparatus according to any one of claims 1 to 3, wherein the image forming apparatus has a value to be determined.   The image forming apparatus according to claim 1, wherein the predetermined time difference is a value obtained by dividing a distance between the first and second photosensitive drums by a traveling speed of the transfer target. .

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