Image forming apparatus

Abstract

To prevent a shift in image position between images on a transfer target material even if the operating speed of an optical deflector of optical writing means is changed.SOLUTION: An image forming apparatus repeatedly scans surfaces of a plurality of photoreceptors 504 along a main scanning direction by optical writing means that deflects writing light according to image information by an optical deflector 202, writes latent images according to the image information on the surfaces of the plurality of photoreceptors, and transfers images obtained by visualizing the latent images on the surfaces of the plurality of photoreceptors so that the images overlap each other on a transfer target material. The image forming apparatus has optical deflector control means 350 that controls the operating speed of the optical deflector, and transfer timing changing means 350 that changes the timing to start transfer of the image on at least one photoreceptor of the plurality of photoreceptors to the transfer target material from one at a reference operating speed so as to reduce a shift in image position on the transfer target material between the images on the plurality of photoreceptors occurring due to a change in the operating speed of the optical deflector from the reference operating speed.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to an image forming apparatus.

Background Art

[0002] Conventionally, an image forming apparatus that visualizes a latent image on a surface of a photoreceptor that moves on a surface by repeatedly scanning along a main scanning direction with a light writing unit that deflects writing light by a light deflector and transfers the image to a transfer material is known.

[0003] For example, Patent Document 1 discloses an image forming apparatus provided with writing drive control means (light deflector control means) for controlling the rotation speed (operating speed) of a polygon mirror (light deflector) in order to finely adjust the image magnification in the sub-scanning direction (the surface movement direction of the photoreceptor). In this image forming apparatus, in order to cancel the change in the image magnification in the main scanning direction caused by this fine adjustment, the writing drive control means also controls the writing clock frequency. Specifically, the writing drive control means uses a magnification correction table that describes the correspondence between the sub-scanning direction magnification adjustment value and the main scanning direction magnification adjustment value for canceling the change in the image magnification in the main scanning direction due to each sub-scanning direction magnification adjustment value. Then, the writing drive control means controls the rotation speed of the polygon mirror according to the sub-scanning direction magnification adjustment value and controls the writing clock frequency according to the corresponding main scanning direction magnification adjustment value.

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in a conventional image forming apparatus, there is a problem that color misregistration that occurs when writing on the surfaces of a plurality of photoreceptors by a light writing unit and transferring each image obtained by visualizing the latent images on these photoreceptor surfaces onto a transfer material so as to overlap each other cannot be suppressed.

Means for Solving the Problems

[0005] To solve the above-mentioned problems, the present invention provides an image forming apparatus that uses an optical writing means to deflect writing light corresponding to image information with an optical deflector to repeatedly scan the surfaces of a plurality of surface-moving photoreceptors along the main scanning direction, to write latent images corresponding to the image information to the surfaces of the plurality of photoreceptors, and to transfer the images, which are made visible images of the latent images on the surfaces of the plurality of photoreceptors, onto a transfer material so that they overlap each other. To reduce the sub-scanning magnification error of the image of at least one of the plurality of photoreceptors, when writing a latent image to the at least one photoreceptor: The operating speed of the optical deflector Change control Light that performs Deflection device control means, By the aforementioned optical deflector control means Operating speed of the optical deflector change To reduce the image positional shift on the transfer material between each image of the plurality of photoreceptors that occurs as a result of the modification, The aforementioned small At least one photoreceptor's image transfer start timing to the transfer material change It is characterized by having means for changing the transfer timing. [Effects of the Invention]

[0006] According to the present invention, even if the operating speed of the optical deflector of the optical writing means is changed from the reference operating speed, it is possible to suppress the image position shift (color shift) on the transfer material between each image of multiple image carriers. [Brief explanation of the drawing]

[0007] [Figure 1] A schematic diagram showing the main components of a color printer according to this embodiment. [Figure 2] A schematic diagram showing the layout of the incident optical system of the MY unit in the same color printer. [Figure 3] A schematic diagram showing the layout of the scanning optical system of the MY unit. [Figure 4] An explanatory diagram showing the configuration of the writing control unit in the color printer. [Figure 5] (a) is a diagram showing the original image to be formed. (b) is a diagram showing the image from (a) stretched in the sub-scan direction. (c) is a diagram showing the state where the stretching of the image in the sub-scan direction is canceled (and consequently the image stretched in the main scan direction). [Figure 6] (a) is an explanatory diagram showing the relationship between the write clock and the position on the photoreceptor drum when the image is stretched in the main scanning direction. (b) is an explanatory diagram showing the relationship between the write clock and the position on the photoreceptor drum when the main scanning magnification is adjusted for the image in (a). [Figure 7] A flowchart showing an overview of the control flow of the writing control unit. [Figure 8] This diagram illustrates how image position shifts (color shifts) occur in each main scanning line of the photoreceptor drum on the intermediate transfer belt before and after changing the rotation speed of the optical deflector. [Figure 9] (a) is a timing chart showing the start timing of writing for each photoreceptor before correction, after changing the rotation speed of the optical deflector. (b) is a timing chart showing the start timing of writing for each photoreceptor after correction, after changing the rotation speed of the optical deflector. [Figure 10] A flowchart showing the process for correcting the writing start timing of each photoreceptor drum in the embodiment. [Figure 11] This diagram illustrates an example of a color matching correction pattern for each color formed on the intermediate transfer belt while the optical deflector is operating at a reference rotation speed. [Modes for carrying out the invention]

[0008] The following describes one embodiment in which the present invention is applied to a color printer as an image forming apparatus.

[0009] Figure 1 is a schematic diagram showing the main components of the color printer 500 according to this embodiment. This color printer 500 is a tandem-type multi-color printer that can form a full-color image by superimposing the toner images of four colors: black, cyan, magenta, and yellow. This color printer 500 is equipped with an optical writing device 100 as a means of optical writing, and four photosensitive drums 501, 502, 503, and 504. It is also equipped with four cleaning units 605Y, 605M, 605C, and 605K, and four charging devices 602Y, 602M, 602C, and 602K. Furthermore, it is equipped with four developing devices 604Y, 604M, 604C, and 604K equipped with developing rollers 603Y, 603M, 603C, and 603K. Furthermore, it also includes an intermediate transfer belt 606, a secondary transfer roller 613, a fixing device 610, a paper feed roller 608, a pair of registration rollers 609, a paper discharge roller 612, and a paper discharge tray 611, which are intermediate transfer bodies that serve as the transfer targets.

[0010] The photoconductor drum 501, cleaning unit 605K, charging device 602K, developing roller 603K, and developing device 604K constitute the image station (hereinafter referred to as "K station") that forms a black image. The photoconductor drum 502, cleaning unit 605C, charging device 602C, developing roller 603C, and developing device 604C constitute the image station (hereinafter referred to as "C station") that forms a cyan image. The photoconductor drum 503, cleaning unit 605M, charging device 602M, developing roller 603M, and developing device 604M constitute the image station (hereinafter referred to as "M station") that forms a magenta image. The photoconductor drum 504, cleaning unit 605Y, charging device 602Y, developing roller 603Y, and developing device 604Y constitute the image station (hereinafter referred to as "Y station") that forms a yellow image.

[0011] Each of the photoreceptor drums 501, 502, 503, and 504 has a photosensitive layer on its circumferential surface and is rotated in the direction of the arrow in Figure 1 by a rotation mechanism. Each of the charging devices 602Y, 602M, 602C, and 602K uniformly charges the surface of the corresponding photoreceptor drum 501, 502, 503, and 504.

[0012] The optical writing device 100 is composed of a K-C unit 100A that exposes and scans the black photosensitive drum 501 and the cyan photosensitive drum 502, and an M-Y unit 100B that exposes and scans the magenta photosensitive drum 503 and the yellow photosensitive drum 504. The optical writing device 100 irradiates writing light (scanning light) by lighting control based on image data with the surface of each corresponding photosensitive drum as the scanned surface, and forms an electrostatic latent image on the surface of the photosensitive drum. The electrostatic latent image formed here is conveyed to a developing area facing the developing roller of the developing devices 604Y, 604M, 604C, 604K as the photosensitive drums 501, 502, 503, 504 rotate.

[0013] Each of the developing devices 604Y, 604M, 604C, 604K is equipped with a developing roller that carries charged toner. A predetermined developing bias is applied to the developing roller, and due to the action of the developing electric field formed thereby, the toner on the developing roller adheres to the electrostatic latent image on the photosensitive drum. Thereby, images with toner attached (hereinafter referred to as "toner images") are formed on the photosensitive drums 501, 502, 503, 504.

[0014] The tonner images formed in this way are conveyed to a primary transfer area facing the intermediate transfer belt 606 as the photosensitive drums 501, 502, 503, 504 rotate. Then, the yellow, magenta, cyan, and black tonner images on each of the photosensitive drums 501, 502, 503, 504 are sequentially primary transferred onto the intermediate transfer belt 606 at a timing when they overlap each other. Thereby, a multicolor color image is formed on the intermediate transfer belt 606. Each cleaning unit 605Y, 605M, 605C, 605K removes the transfer residual toner remaining on the surface of the corresponding photosensitive drums 501, 502, 503, 504 without being transferred.

[0015] On one hand, the recording paper 510, which is the recording material, is conveyed one by one to the resist roller pair 609 by the paper feed roller 608. The resist roller pair 609 sends the recording paper 510 to the secondary transfer area where the intermediate transfer belt 606 and the secondary transfer roller 613 face each other at a predetermined timing. In this secondary transfer area, the multicolor toner image on the intermediate transfer belt 606 is secondarily transferred onto the recording paper 510. The recording paper 510 onto which the multicolor toner image has been transferred is then sent to the fixing device 610. The fixing device 610 fixes the toner image on the recording paper 510 to the recording paper by heat and pressure. The recording paper 510 after fixing is discharged onto the discharge tray 611 via the discharge roller 612.

[0016] Next, the configuration and operation of the optical writing device 一百 will be described. Since the basic configurations of the K-C unit 100A and the M-Y unit 100B that constitute the optical writing device 100 are the same, in the following description, the configuration and operation of the optical writing device 100 will be described using the M-Y unit 100B. In the following description, the color separation codes Y, M, C, and K will be omitted as appropriate.

[0017] FIG. 2 is a schematic diagram showing the layout of the incident optical system of the M-Y unit 100B. The light source unit 101 includes a light source 102 composed of a surface-emitting laser or the like that emits laser light with linear polarization, and a quarter-wave plate 105 that converts the laser light emitted from the light source 102 into circular polarization. It also has a collimating lens 106 that makes the laser light converted into circular polarization by the quarter-wave plate 105 into parallel light, and an aperture 107 that cuts off the laser light parallelized by the collimating lens 106. These optical components 102, 105, 106, and 107 are positioned at predetermined positions with respect to the light source holder and assembled integrally. The laser light emitted from the light source unit 101 enters the light deflector 202 as the light scanning means through the incident optical system.

[0018] The incident optical system includes a beam deflection splitter (PBS) 203 that splits the laser light emitted from the light source unit 101 into two beams in the sub-scanning direction (front-to-back direction in Figure 2). It also includes a quarter-wave plate 204 that converts the polarization characteristics of the two split laser beams L1 and L2 from linearly polarized to circularly polarized. Furthermore, it includes a cylindrical lens 205 that images each of the circularly polarized laser beams L1 and L2 onto the mirror surfaces of two rotating polyhedron mirrors (polygon mirrors) 202a and 202b mounted on the optical deflector 202. The cylindrical lens 205 has a focusing function only in the sub-scanning direction for the circularly polarized laser light.

[0019] The laser beams L1 and L2, formed into a predetermined laser profile by this incident optical system, are imaged onto the mirror surfaces of the rotating polyhedron mirrors 202a and 202b of the optical deflector 202. The optical deflector 202 stably drives the rotating polyhedron mirrors 202a and 202b together at a predetermined rotational speed (operating speed) around a rotation axis parallel to the sub-scanning direction. As the laser beams L1 and L2 are incident on the mirror surfaces of the rotating polyhedron mirrors 202a and 202b, the laser beams L1 and L2 are scanned in the main scanning direction, as shown in Figure 2.

[0020] In this embodiment, an tip synchronization sensor 311 is provided on the upstream side of each photoreceptor drum 501, 502, 503, and 504 in the main scanning direction. When the tip synchronization sensor 311 detects laser beams L1 and L2, a tip synchronization signal is output from the tip synchronization sensor 311. The write control unit, described later, starts lighting control based on image data, using the timing of the output of this tip synchronization signal as a reference, and synchronizes the start timing of latent image writing and the start timing of writing for each scan line in the main scanning direction.

[0021] Figure 3 is a schematic diagram showing the layout of the scanning optical system of the MY unit 100B. One of the laser beams scanned by the optical deflector 202, laser beam L1 (the laser beam scanned on the mirror surface of the upper rotating polyhedron mirror 202a), passes through the scanning lens 301 and the long lens 302, and then through the dustproof glass 305. It is then scanned at a constant speed on the surface of the photoreceptor drum 504. Mirrors 303a, 303b, and 303c are installed in this optical path to reflect the laser beam L1. The other laser beam L2 (the laser beam scanned on the mirror surface of the lower rotating polyhedron mirror 202b) passes through the scanning lens 301 and the long lens 302, and then through the dustproof glass 305, and is scanned at a constant speed on the surface of the photoreceptor drum 503. Mirror 304 is installed in this optical path to reflect the laser beam L2.

[0022] The aforementioned incident optical system, optical deflector 202, and scanning optical system are all integrally mounted on the optical housing 400, which serves as a holding member as shown in Figure 3.

[0023] Next, we will explain how to adjust the magnification in the sub-scanning direction of the image (hereinafter referred to as "sub-scanning magnification"). Figure 4 is an explanatory diagram showing the configuration of the write control unit 350. When adjusting the sub-scan magnification of an image, it is also possible to adjust the sub-scan magnification by processing the image data used to generate the write light. However, if the sub-scan magnification is changed significantly, the memory capacity for storing the image data will increase, and the processing load on the data processing unit 351, such as the ASIC that performs the data processing, will also increase, which can lead to an increase in the cost of the write control unit 350.

[0024] Therefore, the write control unit 350 employs an inexpensive data processing unit 351 that does not have a function to adjust the sub-scan magnification by data processing, while the sub-scan magnification is adjusted by adjusting the rotation speed (operating speed) of the optical deflector 202. For example, to decrease the sub-scan magnification, the rotation speed of the optical deflector 202 is increased to narrow the spacing between the scan lines of the written light on the photoreceptor drum 504, and to increase the sub-scan magnification, the rotation speed of the optical deflector 202 is decreased to widen the spacing between the scan lines of the written light on the photoreceptor drum 504. It is also possible to use a combination of adjusting the sub-scan magnification by the data processing unit 351 and adjusting the sub-scan magnification by adjusting the rotation speed of the optical deflector 202.

[0025] To give a specific example, when forming an image based on the image data of the image shown in Figure 5(a), if the image is stretched in the sub-scanning direction as shown in Figure 5(b) (B2>B1), the rotation speed of the optical deflector 202 is increased to reduce the sub-scanning magnification. In this case, the spacing between the scan lines of the writing light on the photoreceptor drum 504 becomes narrower, so the sub-operation magnification decreases. As a result, the stretching of the image in the sub-scanning direction can be canceled out, as shown in Figure 5(c).

[0026] When adjusting the sub-scan magnification by adjusting the rotation speed of the optical deflector 202, the scanning speed in the main scanning direction will also change. In this case, as shown in Figure 4, the spacing of one dot of the electrostatic latent image written with each clock pulse of the writing light, which is a repeating pulse light, will change, and the image magnification in the main scanning direction (hereinafter referred to as "main scanning magnification") will also change. For example, if the rotation speed of the optical deflector 202 is increased to decrease the sub-scan magnification, the scanning speed of the writing light will increase, so the main scanning magnification will increase. Conversely, if the rotation speed of the optical deflector 202 is decreased to increase the sub-scan magnification, the scanning speed of the writing light will decrease, so the main scanning magnification will decrease.

[0027] In the specific examples shown in Figures 5(a) to 5(c), if the rotation speed of the optical deflector 202 is increased to cancel the image stretching in the sub-scanning direction, the scanning speed of the writing light increases, so the spacing between the one-dot electrostatic latent image written with each clock pulse of the writing light widens. As a result, the main scan magnification increases, and the image stretches in the main scan direction (A2>A1), as shown in Figure 5(c).

[0028] Thus, when adjusting the sub-scan magnification by adjusting the rotation speed of the optical deflector 202, the main scan magnification changes in conjunction with this, so it is necessary to cancel out the change in the main scan magnification. For this reason, for example, when the write control unit 350 adjusts the sub-scan magnification by adjusting the rotation speed of the optical deflector 202, it adjusts the write clock frequency to adjust the main scan magnification and cancel out the change in the main scan magnification.

[0029] Figures 6(a) and 6(b) illustrate how to adjust the main scan multiplier by adjusting the write clock frequency. As shown in the specific examples in Figures 5(a) to 5(c), when the rotation speed of the optical deflector 202 is increased to cancel the image stretching in the sub-scanning direction, the spacing between the one-dot electrostatic latent image written with each clock pulse of the writing light widens. As a result, as shown in Figure 6(a), the image length in the main scanning direction becomes A2, which is longer than the original A1, and the main scanning magnification increases.

[0030] To cancel out this change in the main scan magnification, the write clock frequency of the writing light should be increased. As a result, even if the scanning speed of the writing light increases due to an increase in the rotation speed of the optical deflector 202, the write clock frequency of the writing light will increase, and as shown in Figure 6(b), the spacing between the one-dot electrostatic latent image written with each clock pulse of the writing light will narrow. Consequently, the image length in the main scan direction returns to the original A1, and the change in the main scan magnification is canceled out.

[0031] Figure 7 is a flowchart illustrating the control flow of the write control unit 350. If the rotation speed (operating speed) of the optical deflector 202 is changed during the writing of an electrostatic latent image (latent image writing period), the sub-scan magnification and main scan magnification will change within a single image, causing image distortion. Therefore, generally, the rotation speed of the optical deflector 202 to adjust the sub-scan magnification is changed during the non-latent image writing period (between pages). Specifically, the control unit 352 of the writing control unit 350 monitors the paper gate signal (S1), and at the timing of negate (Yes in S1), changes the rotation speed of the optical deflector 202 to adjust the sub-scan magnification (S2). At this time, the control unit 352 controls the motor 202c of the optical deflector 202 so that the rotation speed of the optical deflector 202 corresponds to the sub-scan magnification indicated by the input sub-scan magnification information.

[0032] Furthermore, the control unit 352 also adjusts the main scan magnification to cancel out the change in main scan magnification that occurs when the rotation speed of the optical deflector 202 is changed (S3). At this time, the control unit 352 controls the light source 102 so that the write clock frequency becomes the frequency corresponding to the changed rotation speed of the optical deflector 202.

[0033] As mentioned above, if the rotation speed (operating speed) of the optical deflector 202 is changed, an image position shift (color shift) may occur on the intermediate transfer belt 606 between the images of the photoreceptor drums 501, 502, 503, and 504. Specifically, if the rotation speed of the optical deflector 202 is changed from the reference rotation speed (reference operating speed), a color shift will occur if the timing of the start of writing latent images to each photoreceptor drum 501, 502, 503, and 504 changes as a result of this change.

[0034] Figure 8 is an explanatory diagram showing the image positions (the positions of each image formed to overlap each other; for example, the leading edge positions of each image) of each main scanning line of the photoreceptor drums 501, 502, 503, and 504 on the intermediate transfer belt 606 before and after changing the rotation speed of the optical deflector 202.

[0035] Before the rotation speed of the optical deflector 202 is changed, that is, when the rotation speed of the optical deflector 202 is at the reference rotation speed (reference operating speed), no color misalignment occurs, as shown in the intermediate transfer belt 606 shown in the lower part of Figure 8. However, in this embodiment, the start timing of writing latent images to each photoreceptor drum 501, 502, 503, and 504 is controlled based on the number of scans by the optical deflector 202 from a predetermined printing start timing. Specifically, the writing control unit 350 counts the number of times the tip synchronization sensor 311 detects laser light, and when this count reaches a predetermined number, it determines that the writing start timing has arrived.

[0036] In this way, to control the writing start timing of each photoreceptor drum 501, 502, 503, and 504, if the rotation speed of the optical deflector 202 is changed, the writing start timing of the latent image to each photoreceptor drum 501, 502, 503, and 504 will change accordingly. As a result, for example, if the rotation speed of the optical deflector 202 is changed to a slower speed, a color shift occurs as shown in the intermediate transfer belt 606 shown in the upper part of Figure 8. This is because the leading edge Ty, Tm, Tc on the intermediate transfer belt 606, transferred from the upstream photoreceptor drums 502, 503, and 504, reaches the transfer positions Pm, Pc, Pk on the downstream photoreceptor drums 501, 502, and 503 before the leading edge Tm, Tc, Tk on the photoreceptor drums 501, 502, and 503.

[0037] Therefore, in this embodiment, the transfer start timing of the image from at least one photoreceptor drum to the intermediate transfer belt 606 is changed (corrected) so that the amount of color shift caused by changing the rotation speed of the optical deflector 202 is reduced. Specifically, for example, the writing start timing of the electrostatic latent image by the optical writing device 100 to each photoreceptor drum 501, 502, 503, and 504 (image formation start timing) is changed (corrected) in relation to the printing start timing.

[0038] Figures 9(a) and (b) show timing charts before and after correcting the write start timing of each photoreceptor drum 501, 502, 503, and 504 after changing the rotation speed of the optical deflector 202. Figure 9(a) is the timing chart before correction of the write start timing, and Figure 9(b) is the timing chart after correction of the write start timing.

[0039] The timing chart shown in Figure 9(a) represents the case before the rotation speed of the optical deflector 202 was changed, i.e., when the rotation speed of the optical deflector 202 was the reference rotation speed. In other words, in this timing chart, the start timing of writing to each photoreceptor drum 501, 502, 503, and 504 relative to the start timing of printing is determined so that no color shift occurs when the rotation speed of the optical deflector 202 is the reference rotation speed.

[0040] In this case, if the rotation speed of the optical deflector 202 is changed from the reference rotation speed, and latent image writing (image formation) of each photoreceptor drum 501, 502, 503, and 504 is started at the writing start timing according to the timing chart in Figure 9(a), a color shift will occur as described above. Therefore, in this embodiment, the writing start timing to the photoreceptor drums 501, 502, 503, and 504 is changed (corrected) so as to reduce the image position shift (color shift) on the intermediate transfer belt 606 between each image of the photoreceptor drums 501, 502, 503, and 504 caused by this change.

[0041] For example, if the rotation speed of the light deflector 202 is changed to be slower, the start timing of writing to each photoreceptor drum 501, 502, 503, and 504 relative to the start timing of printing is changed (corrected) to that of the timing chart in Figure 9(b). As a result, the start timing of the transfer of each image from the downstream M, C, and K photoreceptor drums 501, 502, and 503 to the intermediate transfer belt 606 is advanced, and the image positional shift (color shift) on the intermediate transfer belt 606 between the images of the photoreceptor drums 501, 502, 503, and 504 is reduced or eliminated.

[0042] In this embodiment, the start timing for writing to each photoreceptor drum 501, 502, 503, and 504 is determined based on the print start timing signal. Specifically, the start timing for writing to the K-colored photoreceptor drum 504 is determined based on the print start timing signal, and then the start timing for writing to the other photoreceptor drums 502, 503, and 504 is determined based on the determined start timing for writing to the K-colored photoreceptor drum 501. The start timing for writing to the other photoreceptor drums 502, 503, and 504 is determined from the distance between the photoreceptor drums (distance between the primary transfer positions of both photoreceptor drums) Lky, Lkm, and Lkc between the K-colored photoreceptor drum 501 and the other photoreceptor drums 502, 503, and 504, and the process speed (surface movement speed of the photoreceptor drums).

[0043] Furthermore, if the rotation speed of the optical deflector 202 is changed from the reference rotation speed, the write start timing of each photoreceptor drum 501, 502, 503, and 504 (the write start timing before correction) is multiplied by a correction coefficient corresponding to the changed rotation speed to correct the write start timing of each photoreceptor drum. That is, the write start timing of each photoreceptor drum 501, 502, 503, and 504 is corrected according to the following equation (1). Corrected write start timing = Uncorrected write start timing × Correction coefficient ... (1)

[0044] The correction coefficient (adjustment parameter) can be determined, for example, from the ratio of the rotational speed of the optical deflector 202 before modification (reference rotational speed) to the rotational speed after modification, as shown in equation (2) below. Correction factor = Changed rotation speed ÷ Reference rotation speed ... (2)

[0045] Figure 10 is a flowchart showing the flow of the correction process for the write start timing of each photoreceptor drum 501, 502, 503, and 504 in this embodiment. When an image formation instruction (print job) is input, the control unit 352 drives the photoreceptor drums 501, 502, 503, and 504 at a predetermined surface movement speed, and controls the drive motor so that the intermediate transfer belt 606 moves across the surface at a reference speed, thereby starting the image formation operation (S10). At this time, the start timing of writing for each photoreceptor drum 501, 502, 503, and 504 is determined using a reference write start timing value (timing determination parameter) stored in the memory unit of the control unit 352.

[0046] Initially, this reference write start timing value is calculated from an ideal design value to prevent color misalignment. However, over time, it becomes the value of the write start timing corrected by the most recent reference write start timing correction process described later.

[0047] When the conditions for executing the reference write start timing correction process are met, the control unit 352 operates the optical deflector 202 at a reference rotation speed and executes the reference write start timing correction process. In the reference write start timing correction process, first, a pattern image for correcting the reference write start timing (hereinafter referred to as the "color matching correction pattern") is formed on the intermediate transfer belt 606 (S12). Then, the color matching correction pattern formed on the intermediate transfer belt 606 is detected by the pattern detection sensor 26, which is a detection means (S13).

[0048] Figure 11 is an explanatory diagram showing an example of the color matching correction patterns TPy, TPm, TPc, and TPk for each color formed on the intermediate transfer belt 606 when the optical deflector 202 is operated at a reference rotation speed. The reference K-color color matching correction pattern TPk is formed by writing at a predetermined reference writing start timing (fixed timing). On the other hand, the other color matching correction patterns TPy, TPm, and TPc are formed on the intermediate transfer belt 606 at predetermined intervals in the sub-scanning direction, according to the reference writing start timing value stored in the memory unit of the control unit 352.

[0049] The start timing of the transfer of color matching correction patterns TPy, TPm, and TPc of other colored photoreceptor drums 502, 503, and 504 to the intermediate transfer belt 606 (sub-scanning direction position on the intermediate transfer belt 606) relative to the color matching correction pattern TPk of the reference K-colored photoreceptor drum 501 may change over time. For example, mounting position errors of each photoreceptor drum 501, 502, 503, and 504 (errors in the distance between photoreceptor drums), and errors due to changes in component dimensions caused by temperature changes or wear may occur, which can change the transfer start timing. The above-described reference writing start timing correction process is performed to correct such changes over time.

[0050] The control unit 352 corrects the reference write start timing values ​​of the non-K color photoreceptor drums 502, 503, and 504 based on the detection results of the color matching correction patterns TPy, TPm, TPc, and TPk formed on the intermediate transfer belt 606 by the pattern detection sensor 26 (S14). Specifically, the control unit 352 calculates pattern correction values ​​(reference alignment parameters) that allow the interval between the color matching correction pattern TPk of the K color photoreceptor drum 501 and the color matching correction patterns TPy, TPm, and TPc of the photoreceptor drums 502, 503, and 504 to be set to a predetermined interval. Then, it adds the respective pattern correction values ​​to the reference write start timing values ​​of the non-K color photoreceptor drums 502, 503, and 504 to correct the reference write start timing values ​​of the non-K color photoreceptor drums 502, 503, and 504. As a result, even if color shifts occur due to time-dependent changes such as errors in the distance between photoreceptor drums, temperature changes, or wear and tear on components, the execution of the reference writing start timing correction process suppresses the occurrence of color shifts.

[0051] Here, the control unit 352 changes the rotation speed of the optical deflector 202 from the reference rotation speed when a predetermined optical deflector rotation speed change condition is met (Yes in S15). This predetermined optical deflector rotation speed change condition includes a condition for performing a process to correct a sub-scanning magnification error, in which the sub-scanning direction length of the formed image becomes longer or shorter than the original image. In this case, by changing the rotation speed of the optical deflector 202 to a rotation speed corresponding to the magnitude of the sub-scanning magnification error, the scan line spacing on each photoreceptor drum 501, 502, 503, 504 (the spacing of scan lines in the sub-scanning direction) changes, and the stretching or shrinking of the image is corrected.

[0052] However, when the rotation speed of the optical deflector 202 is changed, the timing at which the laser light is detected by the tip synchronization sensor 311 also changes, so the time interval at which the tip synchronization sensor 311 detects the laser light also changes. As a result, the time it takes for the count value of the number of times the tip synchronization sensor 311 detects the laser light changes, and the timing at which writing begins for each photoreceptor drum 501, 502, 503, and 504 begins changes. Consequently, the timing of latent image writing for each photoreceptor drum 501, 502, 503, and 504 changes, and as described above, image positional shifts (color shifts) occur on the intermediate transfer belt 606 between the images of each photoreceptor drum 501, 502, 503, and 504. To suppress this color shift, in this embodiment, a correction coefficient is calculated to correct the writing start timing of each photoreceptor drum 501, 502, 503, and 504 according to the rotation speed of the optical deflector 202 after the change, as described above (S16).

[0053] Subsequently, the control unit 352 corrects the reference write start timing using the calculated correction coefficient (S17). Then, the control unit 352 starts the image forming operation. At this time, the corrected write start timing corrected in processing step S17 is used for the write start timing of each photoreceptor drum 501, 502, 503, and 504.

[0054] In this embodiment, the corrected write start timing does not require any special correction for the reference K color because the write start timing coincides with the print start timing. On the other hand, the corrected write start timing for other colors that require correction is as shown in equations (3-1) to (3-3) below. Corrected write start timing (C color) = (Start timing of writing reference for color C + pattern correction value) × correction coefficient ... (3-1) Corrected write start timing (M color) = (M color reference writing start timing + pattern correction value) × correction coefficient ... (3-2) Corrected write start timing (Y color) = (Y color reference writing start timing + pattern correction value) × correction coefficient ... (3-3)

[0055] In this embodiment, the corrected write start timing is calculated using a correction coefficient for all photoreceptor drums 502, 503, and 504 that require correction. However, the corrected write start timing may be calculated only for the photoreceptor drum used for image formation. In this case, the processing load on the control unit 352 can be reduced.

[0056] Furthermore, although this embodiment describes a tandem-type image forming apparatus having four stations that perform image formation by superimposing toner images of four colors—black, cyan, magenta, and yellow—the apparatus is not limited to this. For example, it may be a tandem-type image forming apparatus having five stations, including one station that uses toners of colors other than these (hereinafter referred to as "special color toners"). Examples of special color toners include transparent toner, white toner, and gold toner.

[0057] The above is just one example; each of the following embodiments produces its own unique effects. [First aspect] The first embodiment is an image forming apparatus (e.g., a color printer 500) which uses an optical writing means (e.g., an optical writing device 100) that deflects writing light corresponding to image information with an optical deflector 202 to repeatedly scan the surfaces of a plurality of moving photoreceptors 501, 502, 503, 504 along the main scanning direction to write latent images corresponding to the image information to the surfaces of the plurality of photoreceptors, and transfers each of the images, which are made visible images of the latent images on the surfaces of the plurality of photoreceptors, onto a transfer material (e.g., an intermediate transfer belt 606) so that they overlap each other, wherein the optical deflector The optical deflector is characterized by having an optical deflector control means (e.g., a write control unit 350) that controls the operating speed (e.g., rotation speed) of the deflector, and a transfer timing changing means (e.g., a write control unit 350) that changes the transfer start timing of the image of at least one of the multiple photoreceptors to the transfer material from the one at the reference operating speed, so as to reduce the image position shift on the transfer material between each image of the multiple photoreceptors caused by changing the operating speed of the optical deflector from a reference operating speed. Conventionally, in an image forming apparatus that visualizes a latent image written by an optical writing means and transfers the image formed on a photoreceptor to a transfer material, the operating speed of the optical deflector of the optical writing means may be changed from the reference operating speed. For example, in such an image forming apparatus, due to various factors, an image may be formed that is stretched or compressed in the direction of surface movement of the photoreceptor or transfer material (sub-scanning direction) compared to the original image. Such stretching or compression of an image is called sub-scanning magnification error. When such sub-scanning magnification error occurs, by changing the operating speed of the optical deflector of the optical writing means from the reference operating speed, the latent image formed on the photoreceptor can be stretched or compressed in the sub-scanning direction during writing, thereby canceling out the sub-scanning magnification error. However, some image forming apparatuses perform image formation by making visible latent images written on the surfaces of multiple photoreceptors by an optical writing means, and then transferring these images onto a transfer material so that they overlap each other. In this type of image forming apparatus, if the operating speed of the optical deflector of the optical writing means is changed from the standard operating speed, and the timing of the start of writing the latent image to each photoreceptor changes accordingly, then a misalignment of the image positions (color misalignment) occurs on the transfer material between the images of the multiple photoreceptors. For example, if the timing of the start of writing the latent image to each photoreceptor is controlled based on the number of scans by the optical deflector from a predetermined printing start timing, then changing the operating speed of the optical deflector will change the timing of the start of writing the latent image to each photoreceptor. In this case, for example, if the operating speed of the optical deflector is changed to be slower than the reference operating speed (operating speed before the change), the leading edge of the image transferred from the photoreceptor upstream in the direction of movement of the material surface will reach the transfer position of the photoreceptor downstream in the direction of movement of the material surface before the leading edge of the image on the photoreceptor downstream in the direction of movement of the material surface. As a result, a misalignment of image positions (color misalignment) occurs on the material surface between the images of multiple photoreceptors. Therefore, in this embodiment, in order to reduce the image positional shift on the transfer material between each image of the multiple photoreceptors caused by changing the operating speed of such an optical deflector, the timing of the start of image transfer of at least one of the multiple photoreceptors to the transfer material is changed from that at the reference operating speed. As a result, even if the operating speed of the optical deflector is changed from the reference operating speed, the image positional shift (color shift) on the transfer material between each image of the multiple photoreceptors can be suppressed.

[0058] [Second aspect] The second embodiment is characterized in that, in the first embodiment, the transfer timing changing means changes the start timing of the transfer of the image to the transfer material by changing the start timing of the writing of the latent image onto the at least one photoreceptor. According to this, it is possible to change the transcription start timing with a simple configuration.

[0059] [Third aspect] The third embodiment is characterized in that, in the first or second embodiment, the transfer timing changing means determines the transfer start timing of the image of at least one photoreceptor to the transfer material using a reference alignment parameter (e.g., a reference writing start timing) for aligning the image positions on the transfer material between each image of the plurality of photoreceptors when the operating speed is the reference operating speed, and determines the transfer start timing of the image of at least one photoreceptor to the transfer material using an adjustment parameter (e.g., a correction coefficient) corresponding to the changed operating speed when the light deflector control means changes the operating speed from the reference operating speed. According to this, the timing of transcription initiation can be changed through simple control.

[0060] [Fourth aspect] The fourth aspect is characterized in that, in the third aspect, the reference alignment parameter is an added value (e.g., pattern correction value) that is added to a timing determination parameter (reference writing start timing before correction) that determines the timing of the start of the transfer of the image of the at least one photoreceptor to the transfer material, and the adjustment parameter is a correction coefficient that is multiplied by the reference alignment parameter. According to this, the timing of transcription initiation can be changed with simpler control.

[0061] [Fifth aspect] The fifth embodiment is characterized in that, in the third or fourth embodiment, the transfer timing changing means determines the start timing of the transfer of the image of at least one photoreceptor to the transfer material using different adjustment parameters for each photoreceptor. According to this, by changing the adjustment parameters for each photoreceptor, it is possible to compensate for deviations over time. [Explanation of symbols]

[0062] 26: Pattern detection sensor 100: Optical writing device 100A: KC Unit 100B: MY Unit 101: Light source unit 102 :Light source 202: Optical deflector 202a: Upper rotating polyhedron 202b: Lower rotating polyhedron 202c: Motor 204 :4 wave plate 205: Cylindrical lens 301: Scanning lens 302: Long lens 303a~303c,304: Mirror 305: Dustproof glass 311: Advanced Synchronized Sensor 350: Write control unit 351: Data Processing Unit 352: Control Unit 400: Optical Housing 500: Color printer 501-504: Photoconductor drum 510: Recording paper 602: Charging device 603: Developing Roller 604: Developing equipment 605: Cleaning Unit 606: Intermediate transfer belt 608: Paper feed roller 609: Resist Roller vs. 610: Fixing device 611: Paper output tray 612: Paper output roller 613: Secondary transfer roller L1, L2: Laser light [Prior art documents] [Patent Documents]

[0063] [Patent Document 1] Japanese Patent Publication No. 2005-059602

Claims

1. An image forming apparatus that uses an optical writing means to deflect writing light corresponding to image information using an optical deflector to repeatedly scan the surfaces of multiple surface-moving photoreceptors along the main scanning direction, writing latent images corresponding to the image information to each of the multiple photoreceptors, and transferring the images, which are made visible images of the latent images on the surfaces of the multiple photoreceptors, onto a transfer material so that they overlap each other, Optical deflector control means that controls the operation speed of the optical deflector when writing a latent image to at least one of the plurality of photoreceptors in order to reduce the sub-scanning magnification error of the image of at least one of the photoreceptors, An image forming apparatus characterized by having a transfer timing changing means for changing the start timing of the transfer of an image of at least one photoreceptor to the transfer material, such that the image position shift on the transfer material between each image of the plurality of photoreceptors caused by changing the operating speed of the light deflector by the light deflector control means is reduced.

2. In the image forming apparatus according to claim 1, The image forming apparatus is characterized in that the transfer timing changing means changes the start timing of the transfer of the image to the transfer material by changing the start timing of the writing of the latent image onto the at least one photoreceptor.

3. In the image forming apparatus according to claim 1 or 2, The transfer timing changing means determines the start timing of the transfer of the image of at least one photoreceptor to the transfer material using a reference alignment parameter for aligning the image positions on the transfer material between each image of the plurality of photoreceptors when the operating speed is the reference operating speed before it is changed by the optical deflector control means, and determines the start timing of the transfer of the image of at least one photoreceptor to the transfer material using an adjustment parameter corresponding to the changed operating speed when the operating speed is changed by the optical deflector control means.

4. In the image forming apparatus according to claim 3, The aforementioned reference alignment parameter is an added value that is added to a timing determination parameter that determines the timing of the start of transferring the image of the at least one photoreceptor to the transfer material. The image forming apparatus is characterized in that the adjustment parameter is a correction coefficient multiplied by the reference alignment parameter.

5. In the image forming apparatus according to claim 3 or 4, The transfer timing changing means is characterized by determining the start timing of the transfer of the image of at least one photoreceptor to the transfer material using different adjustment parameters for each photoreceptor.

Images