Control device and image forming device

Abstract

To prevent occurrence of phase shift (color shift) of density unevenness between images on image carriers.SOLUTION: A control unit 37 of an image forming apparatus 100 is provided, which transfers images formed on a plurality of rotationally-driven image carriers 2Y, 2M, 2C, 2K onto a transfer target material 1 to be overlapped with each other, and the control unit has correction means that corrects the relative rotational positions between the plurality of image carriers so that the phases of image carrier rotational period density unevenness of the images formed on the plurality of image carriers match each other on the transfer target body. The correction means corrects the relative rotational positions by changing the rotational speed of at least one image carrier of the plurality of image carriers in a non-image forming operation period.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a control device and an image forming apparatus. [Background technology]

[0002] 2. Description of the Related Art Conventionally, there is known an image forming apparatus that transfers images formed on a plurality of rotationally driven image carriers onto a transfer material so that the images are superimposed on one another.

[0003] For example, Patent Document 1 discloses an image forming apparatus that transfers images formed on four photosensitive drums (image carriers) onto a sheet of paper (transfer receiving material) carried and transported on a paper transport belt so that the images are superimposed on each other. In this image forming apparatus, density unevenness occurs in each image transferred from each photosensitive drum onto the paper due to factors such as eccentricity of each photosensitive drum. This density unevenness has a rotational period corresponding to the rotational period of the photosensitive drum (image carrier rotation period density unevenness). If the phase of this density unevenness does not match on the paper, a phase shift (color shift) in the density unevenness occurs between the images on each photosensitive drum. Therefore, in this image forming apparatus, the timing at which each photosensitive drum stops rotating is controlled so that the phase of the density unevenness of the image on the reference photosensitive drum matches the phase of the density unevenness of the image on the other photosensitive drums. Summary of the Invention [Problem to be solved by the invention]

[0004] However, in conventional image forming apparatuses, there is still room for improvement in suppressing the occurrence of phase shift (color shift) in density unevenness due to the rotation period of the image carrier. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention provides a control device for an image forming apparatus that transfers images formed on a plurality of rotationally driven image carriers onto a transfer receiving material so that the images are superimposed on one another, the control device comprising: a correction unit that corrects the relative rotational positions of the plurality of image carriers so that the phases of image carrier rotation period density unevenness of the images formed on the plurality of image carriers coincide on the transfer receiving material; and a contact / separation unit that contacts and separates at least one image carrier of the plurality of image carriers from the transfer receiving material, wherein the correction unit, during a non-image forming operation period, while the contact / separation unit separates the at least one image carrier from the transfer receiving material, without changing the rotation speed of a reference image carrier selected from the plurality of image carriers, the at least one image carrier The remaining image carriers included in correcting the relative rotational position by changing the rotation speed of the the correction means calculates a correction amount for correcting the relative rotational position by increasing the rotational speed of an image carrier for which an upper limit drive speed of a drive means that rotationally drives the plurality of image carriers is different from the rotational speed before the change than a lower limit drive speed of the drive means, and calculates a correction amount for correcting the relative rotational position by decreasing the rotational speed of an image carrier for which a lower limit drive speed of the drive means is different from the rotational speed before the change than the upper limit drive speed of the drive means, and selects the reference image carrier from the plurality of image carriers so that the maximum value of each correction amount calculated for the remaining image carriers is smallest; The contact / separation means brings the at least one image carrier and the transfer material into contact with each other after the correction by the correction means is completed. [Effects of the Invention]

[0006] According to the present invention, it is possible to suppress the occurrence of phase shift (color shift) in density unevenness between images on each image carrier. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a diagram illustrating an example of the overall configuration of an image forming apparatus according to an embodiment; [Figure 2] FIG. 2 is a block diagram illustrating the configuration of the image forming apparatus. [Figure 3] 4 is an explanatory diagram showing uneven density of each color (Y, M, C) before correction by a correction unit of the image forming apparatus. FIG. [Figure 4] 4 is an explanatory diagram showing color unevenness when the color toner images having the density unevenness shown in FIG. 3 are transferred onto a recording sheet. [Figure 5] 4A and 4B are explanatory diagrams showing uneven density of each color (Y, M, C) after correction by the correction means. [Figure 6] 6 is an explanatory diagram showing color unevenness when the color toner images having the density unevenness shown in FIG. 5 are transferred onto a recording sheet. [Figure 7]10 is a diagram illustrating a state (state before correction) at the start of phase alignment control by the image forming apparatus. FIG. [Figure 8] 10 is a diagram illustrating a state (post-correction state) at the end of phase alignment control by the image forming apparatus. FIG. [Figure 9] 4A and 4B are diagrams illustrating density unevenness on a photosensitive drum occurring in the image forming apparatus; [Figure 10] 4 is a flowchart illustrating an image forming operation of the image forming apparatus. [Figure 11] 4 is a timing chart illustrating an example of the operation of the image forming apparatus relating to alignment control according to the embodiment. [Figure 12] 6 is a flowchart illustrating an operation for detecting the rotational phase of each photosensitive drum in the embodiment. [Figure 13] 4A to 4C are explanatory diagrams showing the phases of density unevenness on each photosensitive drum. [Figure 14] 10 is a flowchart showing the flow of a calculation process of a phase correction value by a control device during a calculation period. [Figure 15] FIG. 4 is an explanatory diagram showing an example of a target rotation speed of a photosensitive drum. DETAILED DESCRIPTION OF THE INVENTION

[0008] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an image forming apparatus according to the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram illustrating an example of the overall configuration of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic image forming apparatus and a full-color machine with a quadruple tandem intermediate transfer system. However, this embodiment can also be applied to other image forming apparatuses, such as a full-color machine with a quadruple tandem direct transfer system, a full-color machine with a single-drum intermediate transfer system, or a monochrome machine with a single-drum direct transfer system.

[0009] As shown in FIG. 1, the image forming apparatus 100 includes an intermediate transfer belt 1 as a transfer material. The image forming apparatus 100 also includes photosensitive drums 2Y, 2M, 2C, and 2K as image carriers arranged in a row along the direction of movement of the surface of the intermediate transfer belt 1. The subscripts Y, M, C, and K added to the reference numerals indicate the colors yellow, magenta, cyan, and black, respectively. The image forming apparatus 100 includes a contact / separation unit that moves three of the photosensitive drums 2Y, 2M, 2C, and 2K toward and away from the intermediate transfer belt 1.

[0010] Taking the yellow imaging station (imaging unit) as an example, the image forming apparatus 100 has the following components arranged around the photosensitive drum 2Y in the order of its rotation. When the photosensitive drums 2Y, 2M, 2C, and 2K are not particularly distinguished from one another, they will be collectively referred to as the photosensitive drum 2, with the subscripts removed. Similarly, when the components provided for each color, such as the toner image detection sensor, main charger, optical writing unit, and developing unit described below, are not distinguished from one another, they will be collectively referred to with the subscripts removed.

[0011] The image forming apparatus 100 includes a main charger 3Y and a photointerrupter 18Y as a rotational position detection unit that detects the rotational position of the photosensitive drum 2Y. The image forming apparatus 100 further includes an optical writing unit 4Y that exposes the photosensitive drum 2Y to light and writes an electrostatic latent image onto the photosensitive drum 2Y.

[0012] The image forming apparatus 100 further includes a developing unit 5Y, a primary transfer roller 6Y, a photosensitive drum cleaning unit 7Y including a blade, a brush, etc., and a static elimination unit 8Y.

[0013] The main charger 3Y, the optical writing unit 4Y, and the developing unit 5Y constitute a forming unit that forms a toner image (image) on the photosensitive drum 2Y. The image forming apparatus 100 can transfer the toner image formed on the photosensitive drum 2Y by the forming unit onto the intermediate transfer belt 1 by the primary transfer roller 6Y. The same applies to the other colors.

[0014] The intermediate transfer belt 1 is rotatably supported by rollers 11, 12, and 13. The image forming apparatus 100 is provided with a belt cleaning unit 15 equipped with a blade, a brush, etc., at a position opposite roller 12. The intermediate transfer belt 1, rollers 11, 12, and 13, and belt cleaning unit 15 constitute an intermediate transfer unit 33. The image forming apparatus 100 is provided with a secondary transfer roller 16 at a position opposite roller 13, for secondarily transferring the toner image formed on the intermediate transfer belt 1 onto recording paper 20.

[0015] The image forming apparatus 100 includes a scanner unit 9 as an image reading means, an ADF (Auto Document Feeder) 10 as an automatic document feeding means, and the like, located vertically above the optical writing units 4Y, 4C, 4M, and 4K.

[0016] The image forming apparatus 100 is provided with paper feed trays 17 at the bottom of the apparatus main body 99. The image forming apparatus 100 feeds recording paper 20 as recording material stored in each paper feed tray 17 using a pickup roller 21 and a paper feed roller 22, and transports the recording paper 20 using a transport roller pair 23. The image forming apparatus 100 sends the recording paper 20 to a nip portion N2, which is a secondary transfer portion where the intermediate transfer belt 1 and the secondary transfer roller 16 face each other, at a predetermined timing using a registration roller pair 24.

[0017] The image forming apparatus 100 is provided with a fixing unit 25 as a fixing means downstream of the nip portion N2 in the paper transport direction. In Fig. 1, reference numeral 26 denotes a paper discharge tray, reference numeral 27 denotes a pair of switchback rollers, and reference numeral 37 denotes a control device as a control means.

[0018] The developing units 5Y, 5M, 5C, and 5K are equipped with developing rollers 5Ya, 5Ma, 5Ca, and 5Ka as developer carriers, which are arranged opposite the photosensitive drums 2Y, 2M, 2C, and 2K with a predetermined development gap therebetween. The developing rollers 5Ya, 5Ma, 5Ca, and 5Ka carry two-component developer containing toner and carrier contained in the developing units 5Y, 5M, 5C, and 5K. The developing rollers 5Ya, 5Ma, 5Ca, and 5Ka deposit the toner in the carried two-component developer onto the photosensitive drums 2Y, 2M, 2C, and 2K in a development region facing the photosensitive drums 2Y, 2M, 2C, and 2K, forming toner images on the photosensitive drums 2Y, 2M, 2C, and 2K.

[0019] In this embodiment, the image forming apparatus 100 includes photointerrupters 18Y, 18M, 18C, and 18K as rotational position detection means for detecting the rotational positions of the photosensitive drums 2Y, 2M, 2C, and 2K. However, the present invention is not limited to this, and the image forming apparatus 100 may also include a configuration for detecting the rotational positions, such as a rotary encoder.

[0020] The optical writing units 4Y, 4C, 4M, and 4K have four semiconductor lasers driven by a laser control unit based on image information. The optical writing units 4Y, 4C, 4M, and 4K emit four writing beams that irradiate the photoconductor drums 2Y, 2M, 2C, and 2K, which have been uniformly charged in the dark by the main chargers 3Y, 3M, 3C, and 3K. The optical writing units 4Y, 4C, 4M, and 4K scan the photoconductor drums 2Y, 2M, 2C, and 2K in the dark with the writing beams to write electrostatic latent images for Y, C, M, and K on the surfaces of the photoconductor drums 2Y, 2M, 2C, and 2K.

[0021] In this embodiment, the optical writing units 4Y, 4C, 4M, and 4K perform optical scanning using laser light emitted from a semiconductor laser as follows: The optical writing units 4Y, 4C, 4M, and 4K perform optical scanning by deflecting the laser light with a polygon mirror, reflecting it with a reflecting mirror, and passing it through an optical lens. The optical writing units 4Y, 4C, 4M, and 4K can also be provided with a unit that performs optical writing using an LED array instead of a semiconductor laser.

[0022] Next, the image forming operation of the image forming apparatus 100 will be explained. When a print start command is input, the rollers around the photosensitive drums 2Y, 2M, 2C, and 2K, around the intermediate transfer belt 1, and on the paper feed transport path, etc., begin to rotate at predetermined timing, and the feeding of recording paper from the paper feed tray 17 begins.

[0023] Meanwhile, the surfaces of the photosensitive drums 2Y, 2M, 2C, and 2K are charged to a uniform potential by the charging units 3Y, 3M, 3C, and 3K, and then exposed to writing light from the optical writing units 4Y, 4C, 4M, and 4K in accordance with image data. The potential pattern after exposure is called an electrostatic latent image. The electrostatic latent images carried on the photosensitive drums 2Y, 2M, 2C, and 2K are developed into the desired color by supplying toner from the developing units 5Y, 5M, 5C, and 5K to the surfaces of the photosensitive drums 2Y, 2M, 2C, and 2K carrying the electrostatic latent images.

[0024] In the configuration of Figure 1, there are photosensitive drums 2Y, 2M, 2C, and 2K for four colors, so toner images of yellow, magenta, cyan, and black (the color order varies depending on the system) are developed on each of the photosensitive drums 2Y, 2M, 2C, and 2K.

[0025] The toner images developed on the photosensitive drums 2Y, 2M, 2C, and 2K are transferred to the intermediate transfer belt 1 at nip portions N1, which serve as primary transfer portions and are contact points between the photosensitive drums 2Y, 2M, 2C, and 2K and the intermediate transfer belt 1. That is, the toner images are transferred onto the intermediate transfer belt 1 by a primary transfer bias and pressure applied to primary transfer rollers 6Y, 6M, 6C, and 6K, which are disposed opposite the photosensitive drums 2Y, 2M, 2C, and 2K. The image forming apparatus 100 repeats this primary transfer operation for each of the four colors while synchronizing the timing of each operation, thereby forming a full-color toner image on the intermediate transfer belt 1, with the toner images of each of the four colors overlapping each other.

[0026] The image forming apparatus 100 transfers the full-color toner image formed on the intermediate transfer belt 1 to the recording paper 20, which is conveyed in time by a pair of registration rollers 24, at the nip portion N2. At this time, the secondary transfer is performed by a secondary transfer bias and pressure applied to the secondary transfer roller 16. The image forming apparatus 100 heats and fixes the toner image carried on the surface of the recording paper 20 by passing the recording paper 20, onto which the full-color toner image has been transferred, through a fixing unit 25.

[0027] In the case of single-sided printing, the image forming apparatus 100 conveys the recording paper 20 straight to the paper output tray 26, but in the case of double-sided printing, the conveying direction is changed downward and the paper is conveyed to the paper inversion section. The image forming apparatus 100 reverses the conveying direction of the recording paper 20 that has reached the paper inversion section using the switchback roller pair 27, and discharges the paper from the paper inversion section so that the trailing edge of the paper becomes the leading edge. This is called the switchback operation, and this operation turns the recording paper 20 over. The inverted recording paper 20 does not return to the fixing unit 25, but passes through the paper re-feeding transport path and merges with the original paper feed path. The image forming apparatus 100 then transfers a toner image to the recording paper in the same way as when printing on the front side, passes it through the fixing unit 25, and discharges it. This is the double-sided printing operation.

[0028] To fully explain the operation of each part, the photosensitive drums 2Y, 2M, 2C, and 2K that have passed through the nip N1 carry primary transfer residual toner on their surfaces. The image forming apparatus 100 removes primary transfer residual toner using photosensitive drum cleaning units 7Y, 7M, 7C, and 7K. The image forming apparatus 100 then uniformly discharges the surfaces using charge removal units 8Y, 8M, 8C, and 8K to prepare for charging for the next image formation. The intermediate transfer belt 1 that has passed through the nip N2 also carries secondary transfer residual toner on its surface. The image forming apparatus 100 removes secondary transfer residual toner using the belt cleaning unit 15 to prepare for the transfer of the next toner image. The image forming apparatus 100 repeats these operations to perform single-sided or double-sided printing.

[0029] The image forming apparatus 100 is equipped with a toner image detection sensor 30 serving as pattern image detection means, which is an optical sensor unit including an optical sensor and detects the density of the toner image formed on the outer peripheral surface of the intermediate transfer belt 1. The toner image detection sensor 30 functions as a toner adhesion amount detection sensor that detects the density of the toner image, which is the image on the intermediate transfer belt 1, in order to detect uneven density of the image by detecting the amount of toner adhesion on the intermediate transfer belt 1. The image forming apparatus 100 uses the toner image detection sensor 30 to detect a correction control image pattern (phase detection pattern image) formed on the surface of the intermediate transfer belt 1 to be used for correcting phase shift (color misregistration) of uneven density of each color image.

[0030] In the configuration example shown in FIG. 1, image forming apparatus 100 is provided with toner image detection sensor 30 at position P1 before secondary transfer, which is a position facing the portion of intermediate transfer belt 1 wrapped around roller 11. Image forming apparatus 100 may also be provided with toner image detection sensor 30 at position P2 after secondary transfer, which is a position downstream of nip portion N2. When toner image detection sensor 30 is located downstream of nip portion N2, such as position P2, it is preferable to configure it as follows. It is preferable to provide roller 14 inside intermediate transfer belt 1 to prevent intermediate transfer belt 1 from vibrating, and to provide toner image detection sensor 30 facing roller 14.

[0031] Of the two positions for the toner image detection sensor 30, the position P1 before secondary transfer is a position where the toner pattern on the intermediate transfer belt 1 is detected before the secondary transfer process. This configuration is often adopted if there are no constraints on the machine layout. Since the toner image of the image pattern for correction control is formed and detected immediately, there is little waiting time, and since there is no need for the toner image of the image pattern to pass through the nip portion N2, no special measures are required.

[0032] However, in many models, the secondary transfer position is located immediately after the imaging station for the fourth color (black in the example of FIG. 1), such as nip N2, and in such cases, it is difficult to install a sensor at position P1 due to space limitations. In such cases, toner image detection sensor 30 is installed at position P2, which is the position after secondary transfer, and the toner image of the image pattern formed on intermediate transfer belt 1 is passed through nip N2, after which the density of the toner image is detected by toner image detection sensor 30. Possible methods for passing through nip N2 include separating secondary transfer roller 16 from intermediate transfer belt 1 and applying a reverse bias to secondary transfer roller 16, but this is not particularly limited here.

[0033] FIG. 2 is a block diagram illustrating the configuration of the image forming apparatus 100. As shown in FIG. As shown in FIG. 2, the image forming apparatus 100 includes a transfer device 200, a photointerrupter 18, a toner image detection sensor 30, a forming unit 220, a motor M, a photosensitive drum 2, and a rotary encoder 230. The photointerrupter 18 includes four photointerrupters 18K, 18C, 18M, and 18Y, one for each photosensitive drum. The toner image detection sensor 30 is not dedicated to each color but is shared by all colors. The forming unit 220 includes forming units 220Y, 220M, 220C, and 220K, each for a different color. The motor M includes motors MK, MC, MM, and MY, each for a different color. The rotary encoder 230 is also provided for each color and includes rotary encoders 230Y, 230M, 230C, and 230K, respectively. The transfer device 200 includes a detection device 210 and a control device 37.

[0034] The detection device 210 detects density unevenness of the toner image that occurs in accordance with the rotation cycle of the photosensitive drum 2. Specifically, the detection device 210 detects the density unevenness based on the rotation position (rotation phase) of each photosensitive drum detected by the photointerrupter 18 and the detection result of the image pattern for correction control of each photosensitive drum 2 detected by the toner image detection sensor 30. In this embodiment, the detection device 210 starts the operation of detecting the rotation position (rotation phase) of each of the four photosensitive drums 2 when the difference between the rotation speed of at least two photosensitive drums 2 and a predetermined rotation speed falls within a predetermined range.

[0035] The detection device 210 also includes a calculation unit 211. In this embodiment, the calculation unit 211 starts calculation to calculate the rotational positions when the operation of detecting the rotational positions (rotational phases) of at least two of the four photosensitive drums 2 is completed.

[0036] The control device 37 controls the rotation speed of the photosensitive drums 2 by controlling the rotation of the motor M based on the detection result by the detection device 210 and the rotation speed of each of the four photosensitive drums 2 detected by the rotary encoder 230. In this embodiment, the control device 37 controls the rotation speed of at least two of the four photosensitive drums 2 so that they rotate at a predetermined speed when the four photosensitive drums 2 start to rotate.

[0037] The control device 37 also includes a correction unit 371. In this embodiment, the correction unit 371 starts correcting density unevenness when calculations for calculating correction values ​​for the rotational positions (rotational phases) of at least two of the four photosensitive drums 2 are completed.

[0038] The detection device 210 and the control device 37 include a CPU (Central Processing Unit) and both a non-volatile memory and a volatile memory. The detection device 210 and the control device 37 can realize their respective functions by the CPU using the volatile memory as an operating area and executing programs stored in the non-volatile memory. The detection device 210 and the control device 37 may realize their respective functions using electronic circuits such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array) instead of a CPU.

[0039] The detection device 210 and the control device 37 may each include a CPU, ASIC, FPGA, or the like, or the detection device 210 and the control device 37 may each realize their respective functions using a common CPU, ASIC, FPGA, or the like.

[0040] Next, the control contents in the image forming apparatus 100 will be described with reference to FIGS. FIG. 3 is an explanatory diagram showing density unevenness of each color (Y, M, C) before correction by the correcting means 371. FIG. 4 is an explanatory diagram showing color unevenness when the color toner images (images) having the density unevenness shown in FIG. 3 are transferred onto a recording sheet. FIG. 5 is an explanatory diagram showing the density unevenness of each color (Y, M, C) after correction by the correcting means 371. FIG. 6 is an explanatory diagram showing color unevenness when the color toner images (images) having the density unevenness shown in FIG. 5 are transferred onto a recording sheet.

[0041] The horizontal axes in Figures 3 to 6 represent the position on the intermediate transfer belt 1 (on the recording paper 20) in the sub-scanning direction (the direction in which the recording paper is transported). On the other hand, the vertical axes in Figures 3 and 5 represent the image density (toner density), and the vertical axes in Figures 4 and 6 represent the color evaluation indices L*, a*, and b*.

[0042] As shown in Figures 3 and 4, if the photosensitive drums 2 for each color are driven independently when density unevenness (fluctuations in toner adhesion amount) occurs in the toner images of each color according to the rotation period of each photosensitive drum 2, color shifts will occur when toner images of two or more colors are superimposed. For example, when a high-density toner image portion and a low-density toner image portion overlap, the ratio of toner adhesion amount between these colors will deviate significantly from the original ratio, resulting in a change in color. As a result, when density unevenness occurs in the toner images of each color (Y, M, C) as shown in Figure 3, the a* and b* components will fluctuate significantly periodically, resulting in periodic color fluctuations, as shown in Figure 4.

[0043] In this embodiment, as shown in FIGS. 5 and 6, phase alignment control is performed to correct the relative rotational positions of the photosensitive drums 2 so that the phases of density variations in the toner images of each color corresponding to the rotation cycle of each photosensitive drum 2 coincide with each other on the intermediate transfer belt 1 (on the recording paper 20). This phase alignment control causes high-density toner image portions and low-density toner image portions to overlap on the intermediate transfer belt 1, as shown in FIG. 5. As a result, deviations in the ratio of toner adhesion amounts between these colors from the original ratio can be prevented, thereby suppressing color variations. As a result, even in a situation where density variations occur in the toner images of each color (Y, M, C) as shown in FIG. 5, the fluctuation amplitudes of the a* and b* components can be kept small, as shown in FIG. 6, thereby suppressing color variations.

[0044] FIG. 7 is a diagram illustrating a state (state before correction) at the start of phase alignment control by the image forming apparatus 100. In FIG. FIG. 8 is a diagram illustrating a state (post-correction state) at the end of phase alignment control by the image forming apparatus 100. In FIG. FIG. 9 is a diagram illustrating density unevenness occurring on the photosensitive drum in the image forming apparatus 100. In FIG.

[0045] The phase alignment control of this embodiment is performed with the photosensitive drum 2 spaced apart from the intermediate transfer belt 1. The phase alignment control of this embodiment adjusts the phases φt (rotational positions of each photosensitive drum 2) of density unevenness, based on the peak positions of the density unevenness occurring on the three photosensitive drums 2Y, 2M, and 2C, to match each other on the intermediate transfer belt 1 (on the recording paper 20). Specifically, the relative rotational positions of the three photosensitive drums 2Y, 2M, and 2C are corrected so that the phases φt of the density unevenness of each of the three photosensitive drums 2Y, 2M, and 2C match each other on the intermediate transfer belt 1 (on the recording paper 20). The phases φt of the density unevenness of each photosensitive drum 2 are represented by the rotational angle difference B from the reference rotational position (home position: rotation origin) HP of each photosensitive drum 2. The initial phases φt (rotational angle difference B) of the density unevenness of each photosensitive drum 2 are previously acquired through a separate adjustment operation.

[0046] In this embodiment, for density unevenness that has a rotation period of the photosensitive drum 2 along the sub-scanning direction, the phase φt of density unevenness for each color is detected at a predetermined density unevenness detection timing. Then, the relative rotational positions of the photosensitive drums 2 are corrected as shown in FIG. 8 so that the detected phases φt are aligned on the recording paper 20. This makes it possible to suppress color misalignment (color variations) in images that are superimposed with two or more colors.

[0047] FIG. 10 is a flowchart illustrating an image forming operation of the image forming apparatus 100. As shown in FIG. 10, an example will be described in which an image is formed using four colors, Y, M, C, and K, and phase alignment control is performed for the photosensitive drums 2Y, 2M, and 2C of three of the four colors. Although this embodiment performs phase alignment control for the three colors Y, M, and C, which have the greatest effect on image quality, it can also be applied to cases in which K color is included, or colors other than Y, M, C, and K (special colors such as white and transparent colors) are included.

[0048] Image forming apparatus 100 starts an image forming operation in response to an instruction to start image formation from an external device such as a PC (Personal Computer), or an operation input to start image formation to an operation unit provided in image forming apparatus 100.

[0049] First, the image forming apparatus 100 causes the control device 37 to start the rotation of the photosensitive drums 2Y, 2M, 2C, and 2K (S61). At this time, the three photosensitive drums 2Y, 2M, and 2C are spaced apart from the intermediate transfer belt 1. Next, the image forming apparatus 100 causes the control device 37 to determine whether the difference between the rotational speed of the photosensitive drums 2Y, 2M, and 2C and a predetermined rotational speed is within a predetermined range (S62). The predetermined rotational speed is a predetermined target rotational speed, etc.

[0050] If it is determined in step S62 that the rotational phases are within the predetermined range (Yes in S62), the image forming apparatus 100 detects the rotational phases φ of the three photosensitive drums 2Y, 2M, and 2C by the detection device 210. HPThe image forming apparatus 100 starts detecting the rotation phases φ of all three photosensitive drums 2Y, 2M, and 2C by the detection device 210 (S63). HP When it is determined that the detection has been completed (Yes in S64), the calculation means 211 calculates a phase correction value for each of the photosensitive drums 2Y, 2M, and 2C (S65). The phase correction value is a correction value for correcting the rotational position of each of the photosensitive drums 2Y, 2M, and 2C so that the phases φt of the density irregularities of the toner images of the three colors Y, M, and C, which have the photosensitive drum rotation period, match each other on the intermediate transfer belt 1 (on the recording paper 20).

[0051] After the calculation by the calculation means 211 is completed, the image forming apparatus 100 starts correcting the rotational positions of the photosensitive drums 2Y, 2M, and 2C using the correction means 371 based on the calculated phase correction value (S66). After that, when the image forming apparatus 100 has finished correcting the rotational positions of the photosensitive drums 2Y, 2M, and 2C (Yes in S67), the control device 37 controls the contact / separation means to bring the photosensitive drums 2Y, 2M, and 2C into contact with the intermediate transfer belt 1 (S68). Then, the image forming apparatus 100 starts forming an image on the recording paper 20 (S69), and when image formation is completed, stops the photosensitive drum 2 (S70), thereby ending operation.

[0052] In this way, after performing the phase alignment control, the image forming apparatus 100 can continue to perform the image forming operation without stopping the rotational driving of the photosensitive drums 2Y, 2M, and 2C, and form an image on the recording paper 20.

[0053] Note that there is no particular limit to the number of colors used in image formation as long as it is two or more, and the order in which detection or correction is performed on each photosensitive drum 2 can also be changed as appropriate. In addition, in the explanation of Figure 10, explanation of control of components other than the photosensitive drum 2 has been omitted.

[0054] FIG. 11 is a timing chart illustrating the operation of the image forming apparatus 100 relating to the alignment control of this embodiment. As shown in FIG. 11, the alignment control of this embodiment transitions in the order of a start-up period, a phase detection period, a calculation period, a phase correction period, and a speed control period.

[0055] The start-up period is the period from when the photosensitive drum 2 of each color starts to rotate until the rotation speed stabilizes at the target rotation speed (for example, it is determined to be stable when it is continuously within ±3% of the target rotation speed for 50 ms.) During the start-up period, the control device 37 starts up the motors MY, MM, and MC of the photosensitive drums 2Y, 2M, and 2C for which phase alignment control is performed, that is, performs an operation to converge the photosensitive drums 2Y, 2M, and 2C to a predetermined rotation speed.

[0056] In the phase detection period, after the start-up period is over, the rotation phase (rotation origin HP) φ of each photosensitive drum 2 is detected by the photointerrupter 18 attached to each photosensitive drum 2. HP During the phase detection period, the detection device 210 detects the rotation phase φ of the photosensitive drum 2 based on the rising edge of the detection signal of the rotation origin HP output by the photointerrupter 18 of each of the photosensitive drums 2Y, 2M, and 2C for which phase alignment control is performed. HP Detect.

[0057] In the calculation period, after the phase detection period of the photosensitive drums 2Y, 2M, and 2C ends, the rotation phase φ of each detected photosensitive drum is HP Based on this, the phases φty, φtm, φtc of the density unevenness occurring in each of the photosensitive drums 2Y, 2M, 2C with respect to the rotation cycle of the photosensitive drum are calculated. Then, a phase correction value is calculated to correct the rotation position of each photosensitive drum so that the phases φty, φtm, φtc of the density unevenness occurring in each of the photosensitive drums 2Y, 2M, 2C with respect to the rotation cycle of the photosensitive drum are aligned with each other on the intermediate transfer belt 1 (on the recording paper 20).

[0058] The phase correction period is a period after the calculation period ends in which the calculated phase correction values ​​for each photosensitive drum 2Y, 2M, and 2C are used to accelerate or decelerate the rotational speed of each photosensitive drum, thereby correcting the rotational positions of each photosensitive drum 2Y, 2M, and 2C. During the phase correction period, the correction unit 371 outputs drive commands (acceleration or deceleration) corresponding to each phase correction value calculated during the calculation period to the motors MY, MM, and MC of each photosensitive drum 2Y, 2M, and 2C, thereby correcting the relative rotational positions of each photosensitive drum.

[0059] The speed control period is a period after the phase correction period ends in which the rotation speeds of the photosensitive drums 2Y, 2M, and 2C are returned to their original predetermined rotation speeds (target rotation speeds). During the phase correction period, the rotation speeds of the photosensitive drums 2Y, 2M, and 2C are different from one another. Therefore, the photosensitive drums 2Y, 2M, and 2C do not contact the intermediate transfer belt 1 until the phase correction period ends and the speed control period begins, after the rotation speeds of the photosensitive drums 2Y, 2M, and 2C have stabilized at the predetermined rotation speeds.

[0060] FIG. 12 shows the rotation phases φ of the photosensitive drums 2Y, 2M, and 2C in this embodiment. HP 10 is a flowchart illustrating a detection operation. During the phase detection period, the image forming apparatus 100 calculates the rotation phase φ of each photosensitive drum 2Y, 2M, 2C based on the first interruption of the signal that the photointerrupter 18 detects the rotation origin HP of each photosensitive drum 2Y, 2M, 2C. HP The detection operation starts.

[0061] Specifically, when the image forming apparatus 100 enters the phase detection period and there is an interruption of the detection signal of the rotation origin HP of any of the photosensitive drums, it starts a process of storing, for example, the number of output pulses of the rotary encoder 230Y of the photosensitive drum 2Y in the control device 37 (S81). Then, the number of output pulses of the rotary encoder 230Y when there is an interruption of the detection signal of the rotation origin HP of each of the photosensitive drums 2Y, 2M, 2C is acquired as the rotational positions θy, θm, θc of the rotation origin HP of each of the photosensitive drums 2Y, 2M, 2C. The rotational phases φ of each of the photosensitive drums 2Y, 2M, 2C in this embodiment are HP are expressed as the rotational positions θy, θm, and θc of the photosensitive drums according to the number of output pulses of the rotary encoder 230Y.

[0062] 13(a) to 13(c) are explanatory diagrams showing the phases φty, φty, and φtc of density unevenness of the photosensitive drums 2Y, 2M, and 2C, respectively. During the calculation period, the rotation phases φ of the photosensitive drums 2Y, 2M, and 2C detected during the phase detection period are HP (=θy, θm, θc) and the rotational position φ of the rotation origin HP of each photosensitive drum 2Y, 2M, 2C acquired in advance. HP and the peak positions (phases of density unevenness) φty, φtm, φtc of density unevenness (θyoffset, θmoffset, θcoffset), the phase correction values ​​of the photosensitive drums 2Y, 2M, 2C are calculated.

[0063] In detail, the rotation phase φ of each of the detected photosensitive drums 2Y, 2M, and 2C is HP The current peak positions (phases of density unevenness) φty, φtm, and φtc of each density unevenness are calculated based on (=θy, θm, θc).Then, the rotational positions of each photosensitive drum 2 at which the calculated phases φty, φtm, and φtc of density unevenness coincide with each other on the intermediate transfer belt 1 (on the recording paper 20) are calculated as phase correction values.

[0064] In this embodiment, a reference photosensitive drum is selected from the three photosensitive drums 2Y, 2M, and 2C. The rotational position of the selected reference photosensitive drum is not corrected, but the rotational positions of the remaining two photosensitive drums are corrected to correct the relative rotational positions of these three photosensitive drums 2Y, 2M, and 2C. This reduces the number of photosensitive drums whose rotational positions are corrected, simplifying and speeding up phase alignment control. Of course, the relative rotational positions of these three photosensitive drums 2Y, 2M, and 2C can also be corrected by correcting the rotational positions of all three photosensitive drums.

[0065] As a method for selecting the reference photosensitive drum, for example, the reference photosensitive drum is selected from the three photosensitive drums 2Y, 2M, and 2C so that the maximum value of each correction amount for the remaining two photosensitive drums (photosensitive drums for which the rotational positions are corrected) is the smallest. By using such a selection method, it becomes possible to correct the relative rotational positions of the three photosensitive drums 2Y, 2M, and 2C more quickly, thereby shortening the processing time.

[0066] FIG. 14 is a flowchart showing the flow of the calculation process of the phase correction value by the control device 37 during the calculation period. First, the control device 37 determines whether the current target rotation speed V of the photosensitive drum 2 (the rotation speed of the photosensitive drum 2 after the start-up period) is closer to the upper limit rotation speed Vmax of the photosensitive drum 2 than to the lower limit rotation speed Vmin of the photosensitive drum 2 (S1). Specifically, the control device 37 determines whether (Vmax-V)≦(V-Vmin). The lower limit rotation speed Vmin corresponds to the lower limit drive speed of the motor M of the photosensitive drum 2, and the upper limit rotation speed Vmax corresponds to the upper limit drive speed of the motor M of the photosensitive drum 2.

[0067] 15, for example, if the target rotation speed V of the photosensitive drum 2 is closer to the upper limit rotation speed Vmax than to the lower limit rotation speed Vmin of the photosensitive drum 2 (Yes in S1), processing step S2 is executed. Conversely, if the rotation speed V of the photosensitive drum 2 is closer to the lower limit rotation speed Vmin than to the upper limit rotation speed Vmax of the photosensitive drum 2 (No in S1), processing step S3 is executed.

[0068] In processing step S2, when each of the three photosensitive drums 2Y, 2M, and 2C is used as a reference, the difference in target rotational position (target rotational phase difference) between the reference photosensitive drum and each of the remaining two photosensitive drums is calculated.

[0069] Specifically, the rotation phases θy, θm, and θc of the photosensitive drums 2Y, 2M, and 2C detected during the phase detection period and the rotation position φ of the rotation origin HP of each photosensitive drum that has been acquired in advance are used. HP and the phases φty, φtm, φtc of the density unevenness are used to calculate the phases φty, φtm, φtc of the density unevenness at the current time for each photosensitive drum 2. That is, the phases φty, φtm, φtc of the density unevenness at the current time for each photosensitive drum 2 are calculated from φty=(θy+θyoffset), φtm=(θm+θmoffset), φtc=(θc+θcoffset), respectively.

[0070] Therefore, for example, when the Y-color photosensitive drum 2Y is used as a reference, the target rotational phase difference eYM between the photosensitive drum 2Y and the photosensitive drum 2M is calculated by the following formula (1). Also, the target rotational phase difference eYC between the photosensitive drum 2Y and the photosensitive drum 2C is calculated by the following formula (2). eYM = φty-φtm = (θy+θyoffset)-(θm+θmoffset) ···(1) eYC = φty-φtc = (θy+θyoffset)-(θc+θcoffset) ···(2)

[0071] Similarly, when the M-color photosensitive drum 2M is used as a reference, the target rotational phase difference eMY between the photosensitive drum 2M and the photosensitive drum 2Y is calculated using the following formula (3). Also, the target rotational phase difference eMC between the photosensitive drum 2M and the photosensitive drum 2C is calculated using the following formula (4). eMY = φtm-φty = (θm+θmoffset)-(θy+θyoffset) ···(3) eMC = φtm-φtc = (θm+θmoffset)-(θc+θcoffset) ···(4)

[0072] Similarly, when the C-color photosensitive drum 2C is used as a reference, the target rotational phase difference eCY between the photosensitive drum 2C and the photosensitive drum 2Y is calculated using the following formula (5). Also, the target rotational phase difference eCM between the photosensitive drum 2C and the photosensitive drum 2M is calculated using the following formula (6). eCY = φtc-φty = (θc+θcoffset)-(θy+θyoffset) ···(5) eCM = φtc-φtm = (θc+θcoffset)-(θm+θmoffset) ···(6)

[0073] Meanwhile, in process step S3, similarly to process step S2, when each of the three photosensitive drums 2Y, 2M, and 2C is used as a reference, the difference in target rotational position (target rotational phase difference) between that photosensitive drum and the remaining two photosensitive drums is calculated, using the following equations (7) to (12).

[0074] eYM = φtm-φty = (θm+θmoffset)-(θy+θyoffset) ···(7) eYC = φtc-φty = (θc+θcoffset)-(θy+θyoffset) ···(8) eMY = φty-φtm = (θy+θyoffset)-(θm+θmoffset) ···(9) eMC = φtc-φtm = (θc+θcoffset)-(θm+θmoffset) ···(10) eCY = φty-φtc = (θy+θyoffset)-(θc+θcoffset) ···(11) eCM = φtm-φtc = (θm+θmoffset)-(θc+θcoffset) ···(12)

[0075] Next, the target rotational phase differences eYM, eYC, eMY, eMC, eCY, and eCM calculated as above are converted into values ​​between 0 and 360°. Specifically, if the calculated target rotational phase difference value is less than zero (negative value) (Yes in S4), 360° is added to that value and converted into a value (S5). On the other hand, if the value is greater than zero (positive value) (Yes in S6), 360° is subtracted from that value and converted into a value (S7).

[0076] Thereafter, using the values ​​(converted values) of the target rotational phase differences eYM, eYC, eMY, eMC, eCY, and eCM described above, a reference photosensitive drum is selected that minimizes the maximum correction amount (S8). Specifically, for example, when each of the photosensitive drums 2Y, 2M, and 2C is used as a reference, the larger of the target rotational phase differences (converted values) of the remaining two photosensitive drums is selected. Then, the photosensitive drum that minimizes the value of the selected target rotational phase differences is identified. The photosensitive drum identified in this manner is selected as the reference photosensitive drum.

[0077] After the reference photosensitive drum is selected, the rotational positions of each photosensitive drum, i.e., phase correction values, are determined so that the phases φty, φtm, and φtc of the density variations of the photosensitive drums 2Y, 2M, and 2C will coincide with each other on the intermediate transfer belt 1 (on the recording paper 20) (S9). Specifically, the rotational position of the reference photosensitive drum is not corrected, so its phase correction value is zero. For the remaining two photosensitive drums, their phase correction values ​​are determined using the target rotational phase difference (converted value) from the reference photosensitive drum.

[0078] Specifically, when correcting the rotational position of the photosensitive drum 2 by changing the rotational speed of the photosensitive drum 2, as in this embodiment, the larger the change in rotational speed, the faster the rotational position of the photosensitive drum 2 can be corrected to the target rotational position. Therefore, for example, as shown in FIG. 15 , when the target rotational speed V of the photosensitive drum 2 is closer to the upper rotational speed Vmax than to the lower rotational speed Vmin of the photosensitive drum 2 (Yes in S1), it is better to reduce the rotational speed of the photosensitive drum 2 for correction. Therefore, in this case, in processing step S9, the target rotational phase difference (converted value) is converted to a negative value and determined as the phase correction value. This reduces the rotational speed of the remaining two photosensitive drums other than the reference photosensitive drum to correct their rotational positions, allowing for more rapid correction.

[0079] On the other hand, if the target rotation speed V of the photosensitive drum 2 is closer to the lower limit rotation speed Vmin than to the upper limit rotation speed Vmax of the photosensitive drum 2 (No in S1), the target rotation phase difference (value after conversion) is determined as a positive value as the phase correction value. As a result, the rotation speeds of the remaining two photosensitive drums other than the reference photosensitive drum are increased to correct the rotation positions, and the correction can be completed more quickly.

[0080] Here, when θy=200°, θm=200°, θc=90°, θyoffset=200°, θmoffset=180°, θcoffset=180°, Vmax=420, Vmin=200, and V=400, the calculation process for the phase correction value is as follows:

[0081] First, in processing step S1, Vmax-V=20 and V-Vmin=200, so (Vmax-V)≦(V-Vmin) (Yes in S1), and processing proceeds to processing step S2. Then, the values ​​of the target rotational phase differences eYM, eYC, eMY, eMC, eCY, and eCM calculated in processing step S2 are eYM=-20, eYC=-130, eMY=-20, eMC=110, eCY=-130, and eCM=-110.

[0082] The values ​​of these target rotational phase differences eYM, eYC, eMY, eMC, eCY, and eCM are converted to eYM=340, eYC=230, eMY=340, eMC=110, eCY=230, and eCM=250, respectively, in process steps S4 to S7. Thereafter, in process step S8, the larger of the target rotational phase differences (converted values) when each of photoconductor drums 2Y, 2M, and 2C is used as the reference is selected, resulting in eYM=340, eMY=340, and eCM=250. The smallest value among these is eCM=250, and therefore photoconductor drum 2C corresponding to this is selected as the reference photoconductor drum.

[0083] Thereafter, the phase correction value for the reference photosensitive drum 2C is set to zero because the rotational position is not corrected. For the remaining two photosensitive drums 2Y and 2M, the phase correction values ​​are determined using the target rotational phase differences eCY (=230) and eCM (=250) from the reference photosensitive drum 2C. Specifically, in this example, the target rotational speed V of the photosensitive drum 2 is closer to the upper limit rotational speed Vmax of the photosensitive drum 2 than to the lower limit rotational speed Vmin of the photosensitive drum 2 (Yes in S1). Therefore, in processing step S9, the target rotational phase difference (converted value) is converted to a negative value and determined as the phase correction value. Therefore, the phase correction value (target rotational position) for the photosensitive drum 2Y is set to -230, and the phase correction value (target rotational position) for the photosensitive drum 2M is set to -250.

[0084] The above description is merely an example, and each of the following aspects provides unique effects. [First aspect] The first aspect is a control device 37 of an image forming apparatus 100 that transfers each image (e.g., toner image) formed on a plurality of rotationally driven image carriers (e.g., photosensitive drums 2Y, 2M, 2C, 2K) onto a transfer material (e.g., intermediate transfer belt 1) so that they overlap each other, and is characterized by having a correction means 371 that corrects the relative rotational positions between the plurality of image carriers so that the phases φty, φtm, φtc of the image carrier rotation period density unevenness of each image formed on the plurality of image carriers match on the transfer material, and the correction means corrects the relative rotational positions by changing the rotational speed of at least one of the plurality of image carriers during a non-image forming operation period. In conventional image forming apparatuses, the rotational positions of multiple image carriers are controlled when the image carriers are stopped so that the phases of density unevenness due to the rotation period of the image carriers match with each other on the transfer material. In this case, if a relative shift occurs in the rotational operation (for example, the drive start timing of each image carrier, the rise speed profile up to the target speed, etc.) when the rotational drive of the multiple image carriers is resumed, a phase shift (color shift) in density unevenness occurs between the images on each image carrier. According to this aspect, by changing the rotational speed of at least one of the image carriers during a non-image forming operation period, the relative rotational positions of the multiple image carriers are corrected so that the phases of density unevenness due to the rotation period of the image carriers coincide with each other on the transfer material. This allows the relative rotational positions of the multiple image carriers to be corrected while the multiple image carriers are being rotated. Therefore, even if a relative misalignment in the rotational movements of the multiple image carriers occurs when the rotational movements of the multiple image carriers start, the relative rotational positions of the multiple image carriers can be corrected during the non-image forming operation period until the start of the image forming operation, and the image forming operation can be started in this state, thereby suppressing the occurrence of phase shifts (color shifts) in density unevenness between images on each image carrier.

[0085] [Second mode] The second aspect is characterized in that, in the first aspect, the correction means corrects the relative rotational positions by not changing the rotational speed of a reference image carrier (e.g., C-color photosensitive drum 2C) selected from the plurality of image carriers, but by changing the rotational speeds of the remaining image carriers (e.g., Y-color and M-color photosensitive drums 2Y, 2M). This reduces the number of image carriers whose rotation speeds are changed, thereby simplifying and speeding up the control of correction of the relative rotational positions.

[0086] [Third aspect] A third aspect is characterized in that, in the second aspect, the correction means selects the reference image carrier from the plurality of image carriers so that the maximum value of each correction amount for the rotational position of the remaining image carriers is smallest. This allows the relative rotational position to be corrected more quickly.

[0087] [Fourth aspect] A fourth aspect is a feature of any of the first to third aspects, wherein the correction means corrects the relative rotational position by increasing the rotational speed of at least one image carrier when an upper limit drive speed (e.g., Vmax) of a drive means (e.g., motor M) that rotationally drives the multiple image carriers is farther from the pre-change rotational speed (e.g., target rotational speed V) of the multiple image carriers than a lower limit drive speed (e.g., Vmin) of the drive means, and corrects the relative rotational position by decreasing the rotational speed of at least one image carrier when the lower limit drive speed of the drive means is farther from the pre-change rotational speed than the upper limit drive speed of the drive means. When correcting the relative rotational positions of multiple image carriers by changing the rotational speed of at least one image carrier, the greater the change in the rotational speed of the image carrier, the faster the relative rotational positions of the multiple image carriers can be corrected. According to this aspect, when increasing the rotational speed of the image carrier results in a greater change in the rotational speed of the image carrier, the rotational speed of the image carrier is increased for correction, and in the opposite case, the rotational speed of the image carrier is decreased for correction. Therefore, the correction of the relative rotational positions can be made more quickly.

[0088] [Fifth mode] The fifth aspect is an image forming apparatus 100 that transfers each image (e.g., toner image) formed on a plurality of rotationally driven image carriers (e.g., photosensitive drums 2Y, 2M, 2C, 2K) onto a transfer material (e.g., intermediate transfer belt 1) so that the images overlap each other, and is characterized in that the above-mentioned control device is used as a control device 37 that controls a driving means (e.g., motor M) that rotationally drives the plurality of image carriers. According to this aspect, no color shift occurs due to relative misalignment of rotational movements when the rotational driving of multiple image carriers is resumed, and the occurrence of phase shift (color shift) in density unevenness between images on each image carrier can be suppressed.

[0089] [Sixth aspect] A sixth aspect is the fifth aspect, wherein the rotational positions of the plurality of image carriers (for example, the rotational phase φ of each photosensitive drum) are HP=θy, θm, θc), pattern image detection means (e.g., toner image detection sensor 30) that detects pattern images for phase detection formed on the plurality of image carriers, and phase detection means (e.g., detection device 210) that detects the phase of density unevenness during the rotation period of the image carrier for each image formed on the plurality of image carriers based on the detection results of the rotation position detection means and the pattern image detection means. This makes it possible to suppress the occurrence of phase shifts (color shifts) in density unevenness between images on each image carrier even if the phases of density unevenness due to the rotation period of the image carriers are shifted among the images formed on the image carriers. [Explanation of symbols]

[0090] 1: Intermediate transfer belt 2: Photosensitive drum 3: Charger 4: Optical writing unit 5: Development unit 6: Primary transfer roller 7: Photosensitive drum cleaning unit 8: Static neutralization unit 9: Scanner section 11~14: Laura 15: Belt cleaning unit 16: Secondary transfer roller 17: Paper tray 18: Photointerrupter 20: Recording paper 21: Pickup roller 22: Paper feed roller 23: Pair of conveying rollers 24: Registration roller pair 25: Fuser unit 26: Paper output tray 30: Toner image detection sensor 33: Intermediate transfer unit 37: Control device 100: Image forming device 200: Transfer device 210:Detection device 211: Arithmetic means 220: Forming means 230: Rotary encoder 371: Correction means M: Motor [Prior art documents] [Patent documents]

[0091] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-81171

Claims

1. A control device for an image forming apparatus that transfers images formed on a plurality of rotationally driven image carriers onto a transfer material so that the images are superimposed on each other, comprising: a correcting means for correcting relative rotational positions of the plurality of image carriers so that phases of image carrier rotation period density unevenness of the images formed on the plurality of image carriers coincide with each other on the transfer material; a contact / separation unit for contacting and separating at least one of the plurality of image carriers with the transfer material, the correction means corrects the relative rotational positions by not changing the rotational speed of a reference image carrier selected from the plurality of image carriers, and by changing the rotational speeds of the remaining image carriers included in the at least one image carrier, while the contacting and separating means separates the at least one image carrier from the transfer material during a non-image forming operation period; the correction means calculates a correction amount for correcting the relative rotational position by increasing the rotational speed of an image carrier for which an upper limit drive speed of a drive means that rotationally drives the plurality of image carriers is different from the rotational speed before the change than a lower limit drive speed of the drive means, and calculates a correction amount for correcting the relative rotational position by decreasing the rotational speed of an image carrier for which a lower limit drive speed of the drive means is different from the rotational speed before the change than the upper limit drive speed of the drive means, and selects the reference image carrier from the plurality of image carriers so that the maximum value of each correction amount calculated for the remaining image carriers is smallest; The control device is characterized in that the contact / separation means brings the at least one image carrier and the transfer material into contact with each other after the correction by the correction means is completed.

2. An image forming apparatus that transfers images formed on a plurality of rotationally driven image carriers onto a transfer material so that the images overlap each other, 10. An image forming apparatus, comprising: a control device for controlling a driving means for driving the plurality of image carriers to rotate;

3. 3. The image forming apparatus according to claim 2, a rotational position detecting means for detecting the rotational position of each of the plurality of image carriers; a pattern image detecting means for detecting a pattern image for phase detection formed on the plurality of image carriers; and a phase detection means for detecting a phase of density unevenness due to the rotation period of the image carriers of each image formed on the plurality of image carriers based on the detection results of the rotation position detection means and the pattern image detection means.

Images