JP2009053419A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique capable of performing a satisfactory color shift correction by suppressing variation in a second direction in terms of positions of latent images formed by different light emitting element groups. <P>SOLUTION: The image forming apparatus comprises: an image forming part which forms the latent image on the surface of a latent image carrier moving in the second direction by a line head provided with a plurality of light emitting elements grouped by every light emitting element group, and performs registration mark forming operation that operation to transfer the registration mark obtained by developing the latent image to a transfer medium is performed for a plurality of colors; a correction means which performs the color shift correction based on the registration mark formed by the registration mark forming operation; and a control means which controls the registration mark forming operation by the image forming part. The control means allows the image forming part to perform the registration mark forming operation after adjusting the light emitting timing of the plurality of light emitting elements so that the positions in the second direction of the latent images formed by the respective light emitting element groups may be aligned with each other. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image forming method that enable good color misregistration correction.

  Conventionally, an image forming apparatus that forms a color image by superimposing a plurality of color toner images on a transfer medium is known. In such an image forming apparatus, color misregistration correction is performed in order to satisfactorily perform this color image. For example, the image forming apparatus described in Patent Document 1 forms registration marks (“detection pattern” in the same document) for a plurality of colors. Then, these registration marks are detected, and color misregistration correction is executed based on the detection result.

Japanese Patent No. 2642351

  By the way, in order to realize high-resolution image formation, the surface of the latent image carrier can be exposed by the following line head. The line head includes a plurality of light emitting elements grouped for each light emitting element group. Each light emitting element group can emit a light beam toward the surface of the latent image carrier that moves in the sub-scanning direction, and can expose different areas in the main scanning direction orthogonal to the sub-scanning direction. When forming a registration mark, the light emitting element group exposes the surface of the latent image carrier to form a latent image, and the latent image is developed to form a registration mark. However, the position of the latent image formed by different light emitting element groups may vary in the sub-scanning direction due to fluctuations in the moving speed of the surface of the latent image carrier. As a result, the registration mark edges in the sub-scanning direction may vary, and good color misregistration correction may not be performed.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a technique capable of performing excellent color misregistration correction by suppressing variations in the sub-scanning direction of the positions of latent images formed by different light emitting element groups. And

  In order to achieve the above object, the image forming apparatus according to the present invention is grouped for each light emitting element group and a latent image carrier whose surface moves in a second direction orthogonal to or substantially orthogonal to the first direction. A line head provided with a plurality of light emitting elements, and a latent image is formed by exposing the surface of the latent image carrier with a light beam by the line head and developing the latent image in the second direction. An image forming unit that performs a registration mark forming operation, performing a transfer operation on a transfer medium that moves to a plurality of colors, a correction unit that performs color misregistration correction based on the registration mark formed by the registration mark forming operation, and an image Each light emitting element group emits a light beam toward the surface of the latent image carrier and emits a light beam toward the surface of the latent image carrier. Different regions in the first direction can be exposed, and the control unit adjusts the light emission timings of the plurality of light emitting elements so that the positions of the latent images formed by the light emitting element groups in the second direction coincide with each other. Later, the image forming unit is caused to execute a registration mark forming operation.

  In order to achieve the above object, the image forming method according to the present invention uses a line head provided with a plurality of light emitting elements grouped for each light emitting element group, and uses a line head that is orthogonal to or substantially orthogonal to the first direction. The operation of forming a latent image by exposing the surface of the latent image carrier moving in the direction to form a latent image and transferring the registration mark obtained by developing the latent image to the transfer medium moving in the second direction for a plurality of colors A registration mark forming step to be performed, a correction step for correcting color misregistration based on the registration mark formed in the registration mark formation step, and a timing adjustment step for adjusting the light emission timings of the plurality of light emitting elements prior to the registration mark formation step Each light emitting element group emits a light beam toward the surface of the latent image carrier and emits light in the first direction by causing the light emitting elements to emit light. Different areas can be exposed, and in the timing adjustment step, the light emission timings of the plurality of light emitting elements are adjusted so that the positions of the latent images formed by the light emitting element groups in the second direction coincide with each other. It is said.

  In the invention configured as described above (image forming apparatus and image forming method), the positions of the latent images formed by the light emitting element groups in the second direction coincide with each other prior to forming the registration marks for a plurality of colors. Thus, the light emission timings of the plurality of light emitting elements are adjusted. Thereby, the dispersion | variation in the 2nd direction of the position of the latent image formed by a different light emitting element group is suppressed. Therefore, variation in the edge of the registration mark in the second direction is suppressed, and good color misregistration correction is possible.

  In addition, the detection unit includes a mark detection unit that detects a registration mark in the detection region, the correction unit performs color misregistration correction based on the detection result of the mark detection unit, and the detection region includes all light emitting elements included in one light emitting element group. You may comprise so that it may be wider in a 1st direction than the width | variety of the latent image formed. With such a configuration, it is possible to detect the registration mark satisfactorily and to perform better color misregistration correction.

  The control means may be configured to adjust the light emission timings of the plurality of light emitting elements based on the moving speed of the surface of the latent image carrier. With this configuration, it is possible to suppress the above-described positional shift of the latent image due to fluctuations in the moving speed of the surface of the latent image carrier with high accuracy. As a result, better color misregistration correction can be performed.

  At this time, speed detection means for detecting the moving speed of the surface of the latent image carrier may be provided, and the control means may be configured to adjust the light emission timings of the plurality of light emitting elements based on the detection result of the speed detection means. . By providing the speed detecting means in this way, it is possible to reliably detect fluctuations in the moving speed of the latent image carrier surface.

  The control unit may be configured to control the image forming unit to form the variation detection mark on the transfer medium and adjust the light emission timings of the plurality of light emitting elements based on the detection result of the variation detection mark. good. With this configuration, it is possible to suppress the above-described displacement of the latent image with high accuracy. As a result, better color misregistration correction can be performed.

I. FIG. 1 is a diagram showing an embodiment of an image forming apparatus to which the present invention can be applied. FIG. 2 is a diagram showing an electrical configuration of the image forming apparatus of FIG. This apparatus uses a color mode in which four color toners of black (K), cyan (C), magenta (M), and yellow (Y) are superimposed to form a color image, and only black (K) toner. Thus, the image forming apparatus can selectively execute a monochrome mode for forming a monochrome image. FIG. 1 is a diagram corresponding to the execution of the color mode. In this image forming apparatus, when an image forming command is given from an external device such as a host computer to a main controller MC having a CPU, a memory, etc., the main controller MC gives a control signal to the engine controller EC and also outputs an image forming command. Corresponding video data VD is supplied to the head controller HC. The head controller HC controls the line head 29 for each color based on the video data VD from the main controller MC, the vertical synchronization signal Vsync from the engine controller EC, and parameter values. As a result, the engine unit EG executes a predetermined image forming operation, and forms an image corresponding to the image forming command on a sheet such as copy paper, transfer paper, paper, and an OHP transparent sheet.

  In the housing main body 3 of the image forming apparatus, an electrical component box 5 containing a power circuit board, a main controller MC, an engine controller EC, and a head controller HC is provided. An image forming unit 7, a transfer belt unit 8, and a paper feed unit 11 are also disposed in the housing body 3. In FIG. 1, a secondary transfer unit 12, a fixing unit 13, and a sheet guide member 15 are disposed on the right side in the housing body 3. The paper feeding unit 11 is configured to be detachable from the apparatus main body 1. The paper feed unit 11 and the transfer belt unit 8 can be removed and repaired or exchanged.

  The image forming unit 7 includes four image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) that form a plurality of images of different colors. Each of the image forming stations Y, M, C, and K is provided with a cylindrical photosensitive drum 21 having a surface with a predetermined length in the main scanning direction MD. Each of the image forming stations Y, M, C, and K forms a corresponding color toner image on the surface of the photosensitive drum 21. The photosensitive drum is arranged so that the axial direction is substantially parallel to the main scanning direction MD. Each photosensitive drum 21 is connected to a dedicated drive motor and is driven to rotate at a predetermined speed in the direction of arrow D21 in the figure. As a result, the surface of the photosensitive drum 21 is conveyed in the sub-scanning direction SD orthogonal to or substantially orthogonal to the main scanning direction MD. A charging unit 23, a line head 29, a developing unit 25, and a photoconductor cleaner 27 are disposed around the photoconductive drum 21 along the rotation direction. Then, a charging operation, a latent image forming operation, and a toner developing operation are executed by these functional units. Therefore, when the color mode is executed, the toner images formed at all the image forming stations Y, M, C, and K are superimposed on the transfer belt 81 of the transfer belt unit 8 to form a color image, and the monochrome mode is executed. In some cases, a monochrome image is formed using only the toner image formed at the image forming station K. In FIG. 1, since the image forming stations of the image forming unit 7 have the same configuration, only a part of the image forming stations is given a sign for convenience of illustration, and the sign is omitted for the other image forming stations. .

  The charging unit 23 includes a charging roller whose surface is made of elastic rubber. The charging roller is configured to rotate in contact with the surface of the photosensitive drum 21 at the charging position, and at a peripheral speed in the driven direction with respect to the photosensitive drum 21 as the photosensitive drum 21 rotates. Followed rotation. The charging roller is connected to a charging bias generator (not shown). The charging roller is supplied with the charging bias from the charging bias generator and is charged at the charging position where the charging unit 23 and the photosensitive drum 21 come into contact with each other. The surface of 21 is charged.

  The line head 29 is arranged with respect to the photosensitive drum 21 such that its longitudinal direction corresponds to the main scanning direction MD and its width direction corresponds to the sub-scanning direction SD. Accordingly, the longitudinal direction of the line head 29 is substantially parallel to the main scanning direction MD. The line head includes a plurality of light emitting elements arranged side by side in the longitudinal direction, and is spaced from the photosensitive drum 21. From these light emitting elements, the surface of the photosensitive drum 21 charged by the charging unit 23 is irradiated with light (that is, exposed) to form a latent image on the surface. In addition, a head controller HC is provided to control the line heads 29 of each color, and the line heads 29 are controlled based on video data VD from the main controller MC and signals from the engine controller EC. That is, the image data included in the image formation command is input to the image processing unit 51 of the main controller MC. Various image processing is performed on the image data to create video data VD of each color, and the video data VD is given to the head controller HC via the main-side communication module 52. In the head controller HC, the video data VD is given to the head control module 54 via the head side communication module 53. As described above, the head controller module 54 is supplied with the signal indicating the parameter value related to the latent image formation and the vertical synchronization signal Vsync from the engine controller EC. Based on these signals, video data VD, and the like, the head controller HC creates signals for controlling the element driving for the line heads 29 of the respective colors and outputs the signals to the line heads 29. Thus, the operation of the light emitting elements is appropriately controlled in each line head 29, and a latent image corresponding to the image formation command is formed.

  The photosensitive drum 21, the charging unit 23, the developing unit 25, and the photosensitive cleaner 27 of each of the image forming stations Y, M, C, and K are unitized as a photosensitive cartridge. Each photoconductor cartridge is provided with a nonvolatile memory for storing information related to the photoconductor cartridge. Then, wireless communication is performed between the engine controller EC and each photoconductor cartridge. In this way, information on each photoconductor cartridge is transmitted to the engine controller EC, and information in each memory is updated and stored.

  The developing unit 25 has a developing roller 251 on which toner is carried. The charged toner is developed at a developing position where the developing roller 251 and the photosensitive drum 21 come into contact with each other by a developing bias applied to the developing roller 251 from a developing bias generator (not shown) electrically connected to the developing roller 251. Is moved from the developing roller 251 to the photosensitive drum 21, and the electrostatic latent image formed by the line head 29 becomes obvious.

  The toner image that has been made visible at the development position in this way is conveyed in the rotational direction D21 of the photosensitive drum 21, and then the primary transfer position TR1 where the transfer belt 81 described later and each photosensitive drum 21 come into contact with each other. 1 is primarily transferred to the transfer belt 81.

  A photoreceptor cleaner 27 is provided in contact with the surface of the photoreceptor drum 21 on the downstream side of the primary transfer position TR1 in the rotation direction D21 of the photoreceptor drum 21 and on the upstream side of the charging unit 23. The photoconductor cleaner 27 abuts on the surface of the photoconductor drum to clean and remove toner remaining on the surface of the photoconductor drum 21 after the primary transfer.

  The transfer belt unit 8 includes a drive roller 82, a driven roller 83 (blade facing roller) disposed on the left side of the drive roller 82 in FIG. 1, and a transfer belt 81 stretched between these rollers. . The surface of the transfer belt 81 is driven to circulate in the direction of the conveyance direction D81 perpendicular to the main scanning direction MD. Further, the transfer belt unit 8 is disposed on the inner side of the transfer belt 81 so as to be opposed to each of the photosensitive drums 21 included in the image forming stations Y, M, C, and K when the photosensitive cartridge is mounted. Four primary transfer rollers 85Y, 85M, 85C, and 85K are provided. Each of these primary transfer rollers 85 is electrically connected to a primary transfer bias generator (not shown). As will be described in detail later, when the color mode is executed, as shown in FIG. 1, all the primary transfer rollers 85Y, 85M, 85C, and 85K are positioned on the image forming stations Y, M, C, and K side. Then, the transfer belt 81 is pushed and brought into contact with the photosensitive drums 21 included in the image forming stations Y, M, C, and K, so that the primary transfer position TR1 is set between each photosensitive drum 21 and the transfer belt 81. Form. Then, by applying a primary transfer bias from the primary transfer bias generator to the primary transfer roller 85 at an appropriate timing, the toner images formed on the surfaces of the photosensitive drums 21 correspond respectively. A color image is formed by transferring to the surface of the transfer belt 81 at the primary transfer position TR1.

  On the other hand, when the monochrome mode is executed, among the four primary transfer rollers 85, the color primary transfer rollers 85Y, 85M, and 85C are separated from the image forming stations Y, M, and C facing each other, and the monochrome primary transfer is performed. By bringing only the roller 85K into contact with the image forming station K, only the monochrome image forming station K is brought into contact with the transfer belt 81. As a result, the primary transfer position TR1 is formed only between the monochrome primary transfer roller 85K and the image forming station K. Then, by applying a primary transfer bias from the primary transfer bias generator to the monochrome primary transfer roller 85K at an appropriate timing, the toner image formed on the surface of each photosensitive drum 21 is subjected to primary transfer. A monochrome image is formed by transferring to the surface of the transfer belt 81 at a position TR1.

  Further, the transfer belt unit 8 includes a downstream guide roller 86 disposed on the downstream side of the monochrome primary transfer roller 85K and on the upstream side of the driving roller 82. Further, the downstream guide roller 86 is formed between the primary transfer roller 85K and the photosensitive drum 21 at the primary transfer position TR1 formed by the monochrome primary transfer roller 85K contacting the photosensitive drum 21 of the image forming station K. It is configured to contact the transfer belt 81 on a common inscribed line.

  The driving roller 82 circulates and drives the transfer belt 81 in the direction of the arrow D81 in the figure, and also serves as a backup roller for the secondary transfer roller 121. A rubber layer having a thickness of about 3 mm and a volume resistivity of 1000 kΩ · cm or less is formed on the peripheral surface of the driving roller 82, and secondary transfer is omitted by grounding through a metal shaft. The conductive path of the secondary transfer bias supplied from the bias generation unit via the secondary transfer roller 121 is used. When the rubber layer having high friction and shock absorption is provided on the driving roller 82 in this way, the sheet enters the contact portion (secondary transfer position TR2) between the driving roller 82 and the secondary transfer roller 121. Is difficult to be transmitted to the transfer belt 81, and image quality deterioration can be prevented.

  The sheet feeding unit 11 includes a sheet feeding unit having a sheet feeding cassette 77 capable of stacking and holding sheets and a pickup roller 79 that feeds sheets one by one from the sheet feeding cassette 77. The sheet fed from the sheet feeding unit by the pickup roller 79 is fed to the secondary transfer position TR2 along the sheet guide member 15 after the sheet feeding timing is adjusted by the registration roller pair 80.

  The secondary transfer roller 121 is provided so as to be able to come into contact with and separate from the transfer belt 81 and is driven to come into contact with and separate from a secondary transfer roller drive mechanism (not shown). The fixing unit 13 includes a heating roller 131 that includes a heating element such as a halogen heater and is rotatable, and a pressure unit 132 that presses and biases the heating roller 131. Then, the sheet on which the image is secondarily transferred is guided to a nip portion formed by the heating roller 131 and the pressure belt 1323 of the pressure portion 132 by the sheet guide member 15, and in the nip portion, a predetermined value is provided. The image is heat-fixed at temperature. The pressure unit 132 includes two rollers 1321 and 1322 and a pressure belt 1323 stretched between them. A nip portion formed by the heating roller 131 and the pressure belt 1323 by pressing the belt tension surface stretched by the two rollers 1321 and 1322 against the peripheral surface of the heating roller 131 among the surfaces of the pressure belt 1323. Is configured to be widely taken. Further, the sheet thus subjected to the fixing process is conveyed to a paper discharge tray 4 provided on the upper surface of the housing body 3.

  Further, in this apparatus, a cleaner portion 71 is disposed to face the blade facing roller 83. The cleaner unit 71 includes a cleaner blade 711 and a waste toner box 713. The cleaner blade 711 removes foreign matters such as toner and paper dust remaining on the transfer belt after the secondary transfer by bringing the tip of the cleaner blade 711 into contact with the blade facing roller 83 via the transfer belt 81. The foreign matter removed in this way is collected in a waste toner box 713. Further, the cleaner blade 711 and the waste toner box 713 are integrally formed with the blade facing roller 83. Therefore, when the blade facing roller 83 moves as will be described below, the cleaner blade 711 and the waste toner box 713 also move together with the blade facing roller 83.

II. Configuration of Line Head FIG. 3 is a perspective view showing an outline of the line head. 4 is a cross-sectional view in the width direction of the line head shown in FIG. As described above, the line head 29 is arranged with respect to the photosensitive drum 21 such that the longitudinal direction LGD corresponds to the main scanning direction MD and the width direction LTD corresponds to the sub-scanning direction SD. The longitudinal direction LGD and the width direction LTD are substantially orthogonal to each other. The line head 29 includes a case 291, and positioning pins 2911 and screw insertion holes 2912 are provided at both ends of the case 291 in the longitudinal direction LGD. Then, the positioning pin 2911 covers the photosensitive drum 21 and is fitted into a positioning hole (not shown) formed in a photosensitive cover (not shown) positioned with respect to the photosensitive drum 21, thereby The head 29 is positioned with respect to the photosensitive drum 21. Further, the line head 29 is positioned and fixed with respect to the photosensitive drum 21 by screwing and fixing a fixing screw into a screw hole (not shown) of the photosensitive member cover through the screw insertion hole 2912.

  The case 291 holds the lens array 299 at a position facing the surface of the photosensitive drum 21, and includes a light shielding member 297 and a head substrate 293 in the order close to the lens array 299. The head substrate 293 is formed of a material (for example, glass) that can transmit a light beam. A plurality of light emitting element groups 295 are provided on the back surface of the head substrate 293 (the surface opposite to the lens array 299 among the two surfaces of the head substrate 293). That is, the plurality of light emitting element groups 295 are two-dimensionally arranged on the back surface of the head substrate 293 so as to be separated from each other by a predetermined distance in the longitudinal direction LGD and the width direction LTD. Here, each of the plurality of light emitting element groups 295 is configured by two-dimensionally arranging a plurality of light emitting elements, which will be described later. Further, a bottom emission type organic EL (Electro-Luminescence) element is used as the light emitting element. That is, the organic EL element is arranged as a light emitting element on the back surface of the head substrate 293. Thereby, all the light emitting elements 2951 are arranged on the same plane (the back surface of the head substrate 293). When each light emitting element is driven by a drive circuit formed on the head substrate 293, a light beam is emitted from the light emitting element toward the photosensitive drum 21. This light beam is transmitted from the back surface to the front surface of the head substrate 293 and travels toward the light shielding member 297.

  A plurality of light guide holes 2971 are formed in the light shielding member 297 on a one-to-one basis with respect to the plurality of light emitting element groups 295. Further, the light guide hole 2971 is formed as a substantially cylindrical hole penetrating the light shielding member 297 with a line parallel to the normal line of the head substrate 293 as a central axis. Therefore, among the light beams emitted from the light emitting element group 295, the light beams that are directed to other than the light guide hole 2971 corresponding to the light emitting element group 295 are blocked by the light blocking member 297. Thus, all the light emitted from one light emitting element group 295 is directed to the lens array 299 through the same light guide hole 2971, and interference between light beams emitted from different light emitting element groups 295 is prevented by the light shielding member 297. The Then, the light beam that has passed through the light guide hole 2971 formed in the light shielding member 297 is imaged as a spot on the surface of the photosensitive drum 21 by the lens array 299.

  As shown in FIG. 4, the back cover 2913 is pressed against the case 291 via the head substrate 293 by the fixing device 2914. That is, the fixing device 2914 has an elastic force that presses the back cover 2913 toward the case 291, and presses the back cover with the elastic force, thereby making the inside of the case 291 light-tight (that is, from the inside of the case 291. It is sealed so that light does not leak and so that light does not enter from the outside of the case 291. Note that a plurality of fixing devices 2914 are provided in the longitudinal direction of the case 291. The light emitting element group 295 is covered with a sealing member 294.

  FIG. 5 is a perspective view schematically showing the lens array. FIG. 6 is a cross-sectional view of the lens array in the longitudinal direction LGD. The lens array 299 has a lens substrate 2991. The first surface LSFf of the lens LS is formed on the back surface 2991B of the lens substrate 2991, and the second surface LSFs of the lens LS is formed on the surface 2991A of the lens substrate 2991. The first surface LSFf and the second surface LSFs of the lenses facing each other and the lens substrate 2991 sandwiched between these two surfaces function as one lens LS. The first surface LSFf and the second surface LSFs of the lens LS can be formed of, for example, a resin.

  In the lens array 299, the plurality of lenses LS are arranged such that the optical axes OA are substantially parallel to each other. The lens array 299 is arranged so that the optical axis OA of the lens LS is substantially orthogonal to the back surface of the head substrate 293 (the surface on which the light emitting element 2951 is disposed). At this time, the plurality of lenses LS are arranged one-on-one in the plurality of light emitting element groups 295. That is, the plurality of lenses LS are two-dimensionally arranged at a predetermined interval in the longitudinal direction LGD and the width direction LTD corresponding to the arrangement of the light emitting element groups 295. More specifically, a plurality of lens rows LSR in which a plurality of lenses LS are arranged in the longitudinal direction LGD are arranged in the width direction LTD. Specifically, three lens rows LSR1, LSR2, and LSR3 are arranged in the width direction LTD. Further, the three lens rows LSR1 to LSR3 are arranged so as to be shifted from each other by a predetermined lens pitch Pls in the longitudinal direction.

  FIG. 7 is a diagram showing the arrangement of the light emitting element groups in the line head. FIG. 8 is a diagram showing the arrangement of light emitting elements in each light emitting element group. In each light emitting element group 295, eight light emitting elements 2951 are arranged at a predetermined element pitch Pel in the longitudinal direction LGD. Each light emitting element group 295 includes a light emitting element row 2951R in which four light emitting elements 2951 are arranged at a predetermined interval (interval twice the element pitch Pel) in the longitudinal direction LGD, and the element row pitch Pelr in the width direction LTD. Two rows are arranged at intervals. As a result, in each light emitting element group 295, eight light emitting elements 2951 are arranged in a staggered manner. The plurality of light emitting element groups 295 are arranged as follows.

  That is, the plurality of light emitting element groups 295 are arranged so that a plurality of light emitting element group columns 295C in which the three light emitting element groups 295 are shifted in the width direction LTD and the longitudinal direction LGD are arranged in the longitudinal direction LGD. Yes. The light emitting element groups 295 have different longitudinal positions, and each light emitting element group 295 can expose different areas in the main scanning direction MD, as will be described later. In addition, a plurality of light emitting element groups 295 arranged in the longitudinal direction LGD (in other words, a plurality of light emitting element groups 295 arranged at the same position in the width direction) are particularly defined as light emitting element group rows 295R. Note that in this specification, the geometric center of gravity of the light emitting element 2951 is set as the position of the light emitting element 2951, and the geometric center of gravity of all light emitting element positions belonging to the same light emitting element group 295 is set as the position of the light emitting element group 295. Further, the longitudinal direction position and the width direction position mean the longitudinal direction component and the width direction component at the position of interest, respectively.

  Corresponding to the arrangement of the light emitting element groups 295 described above, a light guide hole 2971 is formed in the light shielding member 297 and a lens LS is arranged. That is, the gravity center position of the light emitting element group 295, the central axis of the light guide hole 2971, and the optical axis OA of the lens LS are configured to substantially coincide with each other. The light beam emitted from the light emitting element 2951 of the light emitting element group 295 enters the lens array 299 through the light guide hole 2971. The lens array 299 images the incident light beam as a spot on the surface of the photosensitive drum 21 (photosensitive member surface). Then, the surface of the photoreceptor is exposed by the light beam thus imaged to form a latent image.

III. Terms Used in Line Head FIGS. 9 and 10 are explanatory diagrams of terms used in this specification. Here, the terms used in this specification will be organized using these drawings. In the present specification, as described above, the conveyance direction of the surface (image surface IP) of the photosensitive drum 21 is defined as a sub-scanning direction SD, and a direction substantially orthogonal to the sub-scanning direction SD is defined as a main scanning direction MD. ing. The line head 29 is arranged with respect to the surface (image surface IP) of the photosensitive drum 21 so that the longitudinal direction LGD corresponds to the main scanning direction MD and the width direction LTD corresponds to the sub-scanning direction SD. Has been.

  A set of a plurality of (eight in FIG. 9 and FIG. 10) light emitting elements 2951 arranged on the head substrate 293 in a one-to-one correspondence with the plurality of lenses LS included in the lens array 299 is defined as a light emitting element group 295. To do. That is, in the head substrate 293, the light emitting element group 295 including the plurality of light emitting elements 2951 is disposed for each of the plurality of lenses LS. Further, the light beam from the light emitting element group 295 is imaged toward the image plane IP by the lens LS corresponding to the light emitting element group 295, whereby a set of a plurality of spots SP formed on the image plane IP is obtained. It is defined as group SG. That is, the plurality of spot groups SG can be formed in one-to-one correspondence with the plurality of light emitting element groups 295. In each spot group SG, the most upstream spot in the main scanning direction MD and the sub-scanning direction SD is particularly defined as the first spot. The light emitting element 2951 corresponding to the first spot is particularly defined as the first light emitting element.

  Further, as shown in the column “on image plane” in FIG. 10, a spot group row SGR and a spot group column SGC are defined. That is, a plurality of spot groups SG arranged in the main scanning direction MD are defined as spot group rows SGR. The plurality of spot group rows SGR are arranged side by side in the sub-scanning direction SD at a predetermined spot group row pitch Psgr. A plurality (three in the figure) of spot groups SG arranged at the spot group row pitch Psgr in the sub-scanning direction SD and at the spot group pitch Psg in the main scanning direction MD are defined as a spot group column SGC. The spot group row pitch Psgr is a distance in the sub-scanning direction SD between the geometric centroids of two spot group rows SGR arranged at the same pitch. The spot group pitch Psg is a distance in the main scanning direction MD between the geometric centroids of two spot groups SG arranged at the same pitch.

  Lens rows LSR and lens columns LSC are defined as shown in the “lens array” column of FIG. That is, a plurality of lenses LS arranged in the longitudinal direction LGD are defined as a lens row LSR. The plurality of lens rows LSR are arranged side by side in the width direction LTD at a predetermined lens row pitch Plsr. A plurality (three in the figure) of lenses LS arranged at the lens row pitch Plsr in the width direction LTD and at the lens pitch Pls in the longitudinal direction LGD are defined as a lens row LSC. The lens row pitch Plsr is a distance in the width direction LTD of the geometric centroids of the two lens rows LSR arranged at the same pitch. The lens pitch Pls is a distance in the longitudinal direction LGD between the geometric centroids of the two lenses LS arranged at the same pitch.

  As shown in the column “Head Substrate” in the drawing, a light emitting element group row 295R and a light emitting element group column 295C are defined. That is, a plurality of light emitting element groups 295 arranged in the longitudinal direction LGD is defined as a light emitting element group row 295R. The plurality of light emitting element group rows 295R are arranged side by side in the width direction LTD at a predetermined light emitting element group row pitch Pegr. In addition, a plurality of (three in the figure) light emitting element groups 295 arranged at the light emitting element group row pitch Pegr in the width direction LTD and at the light emitting element group pitch Peg in the longitudinal direction LGD are defined as a light emitting element group column 295C. The light emitting element group row pitch Pegr is a distance in the width direction LTD between the geometric centroids of two light emitting element group rows 295R arranged at the same pitch. The light emitting element group pitch Peg is the distance in the longitudinal direction LGD of the geometric centroids of the two light emitting element groups 295 arranged at the same pitch.

  As shown in the “light emitting element group” column of FIG. 2, a light emitting element row 2951R and a light emitting element column 2951C are defined. That is, in each light emitting element group 295, a plurality of light emitting elements 2951 arranged in the longitudinal direction LGD is defined as a light emitting element row 2951R. The plurality of light emitting element rows 2951R are arranged side by side in the width direction LTD at a predetermined light emitting element row pitch Pelr. A plurality of (two in the figure) light emitting elements 2951 arranged in the width direction LTD at the light emitting element row pitch Pelr and at the longitudinal direction LGD in the longitudinal direction LGD are defined as a light emitting element row 2951C. The light emitting element row pitch Pelr is the distance in the width direction LTD of the geometric centroids of two light emitting element rows 2951R arranged at the same pitch. The light emitting element pitch Pel is the distance in the longitudinal direction LGD between the geometric centroids of two light emitting elements 2951 arranged at the same pitch.

  As shown in the column “Spot Group” in the figure, a spot row SPR and a spot column SPC are defined. That is, in each spot group SG, a plurality of spots SP arranged in the longitudinal direction LGD are defined as spot rows SPR. The plurality of spot rows SPR are arranged side by side in the width direction LTD at a predetermined spot row pitch Pspr. Further, a plurality of (two in the figure) spots arranged at the spot pitch Pspr in the width direction LTD and at the spot pitch Psp in the longitudinal direction LGD are defined as a spot row SPC. The spot row pitch Pspr is the distance in the sub-scanning direction SD between the geometric centroids of two spot rows SPR arranged at the same pitch. The spot pitch Psp is a distance in the longitudinal direction LGD between the geometric centroids of two spots SP arranged at the same pitch.

IV. Exposure Operation by Line Head FIG. 11 is a perspective view showing the exposure operation by the line head. In the figure, the description of the lens array is omitted. As shown in the figure, each light emitting element group 295 can expose different regions ER (ER1 to ER6). For example, the light emitting element group 295_1 emits a light beam from each light emitting element 2951, thereby forming a spot group SG_1 on the surface of the photosensitive member moving in the sub scanning direction SD (moving direction D21). Thereby, the light emitting element group 295_1 can expose the region ER_1 having a predetermined width in the main scanning direction MD. Similarly, the light emitting element groups 295_2 to 295_6 can expose the regions ER_2 to ER_6.

  Meanwhile, in the line head 29, the three light emitting element groups 295 are shifted from each other in the width direction LTD and the longitudinal direction LGD to form a light emitting element group column 295C. For example, as shown in FIG. 11, the light emitting element groups 295_1 to 295_3 constituting the light emitting element group column 295C are arranged so as to be shifted from each other in the width direction LTD. The three light emitting element groups 295 constituting the light emitting element group column 295C expose three exposure areas ER that are continuous in the main scanning direction MD. As described above, the light emitting element group column 295C is configured by shifting the light emitting element groups 295 that expose the three exposure regions ER continuous in the main scanning direction MD to each other in the width direction LTD. Corresponding to the fact that the light emitting element groups 295 are thus shifted in the width direction LTD, the positions of the spot groups SG formed by the light emitting element groups 295 are also different in the sub-scanning direction SD.

  FIG. 12 is a side view showing the exposure operation by the line head. Hereinafter, the exposure operation by the line head will be described with reference to FIGS. As shown in these drawings, the light emitting element groups 295 belonging to the same light emitting element group row 295R form a spot group SG at substantially the same position in the sub-scanning direction SD (movement direction D21). On the other hand, light emitting element groups belonging to different light emitting element group rows 295R form spot groups SG at different positions in the sub-scanning direction SD (movement direction D21). That is, the first light emitting element group row 295R_1 counted in the width direction LTD forms the spot groups SG_1 and SG_4 on the uppermost stream in the sub scanning direction SD. A second light emitting element group row 295R_2 forms spot groups SG_2 and SG_5 at a position downstream of the spot groups SG_1 and SG_4 by a distance d. Further, the third light emitting element group row 295R_3 forms spot groups SG_3 and SG_6 at a position downstream from the spot groups SG_2 and SG_5 by a distance d.

  As described above, the formation position of the spot group SG in the sub-scanning direction SD differs depending on the light emitting element group 295. Therefore, for example, when forming a latent image extending in the main scanning direction MD, each light emitting element group row 295R emits light at a different timing to form a spot group SG.

  FIG. 13 is a schematic diagram showing an example of a latent image forming operation by the line head. Hereinafter, an example of a latent image forming operation by the line head will be described with reference to FIGS. First, the first light emitting element group row 295R_1 forms the spot group SG for a predetermined time. Thereby, a group latent image GL1 having a predetermined width in the sub-scanning direction SD is formed in the regions ER_1, ER_4,. Here, the group latent image GL is a latent image formed by one light emitting element group 295. Next, at the timing when the group latent image GL1 by the light emitting element group row 295R_1 is conveyed in the sub-scanning direction SD by the distance d, the second light emitting element group row 295R_2 forms the spot group SG for a predetermined time. Thereby, a group latent image GL2 having a predetermined width in the sub-scanning direction SD is formed in the regions ER_2, ER_5,. Further, the third light emitting element group row 295R_3 forms a spot group SG for a predetermined time at the timing when the latent images by the light emitting element group rows 295R_1 and 295R_2 are conveyed in the sub-scanning direction SD by the distance d. Thereby, a group latent image GL3 having a predetermined width in the sub scanning direction SD is formed in the regions ER_3, ER_6,. In this way, the light emitting element group rows 295R emit light at different timings, so that the positions of the group latent images GL formed by the light emitting element groups 295 in the sub-scanning direction SD coincide with each other. Then, group latent images GL whose positions in the sub-scanning direction SD coincide with each other are continuously formed in the main scanning direction MD, and a latent image LI extending in the main scanning direction MD is formed (FIG. 13).

  However, due to the eccentricity of the photosensitive drum, the moving speed of the photosensitive member surface may have a speed fluctuation as shown in FIG. 14, for example. Here, FIG. 14 is a diagram showing the relationship between the speed fluctuation of the moving speed of the photosensitive member surface and time. As a result, the positions of the group latent images GL1 to GL3 formed by the light emitting element groups 295_1 to 295_3 may vary in the sub scanning direction SD. That is, the positions of the three group latent images GL1 to GL3 formed continuously in the main scanning direction MD may vary in the sub-scanning direction SD.

  FIG. 15 is a diagram showing positional variations that can occur in a latent image. As in the case shown in FIG. 13, first, the first light emitting element group row 295R_1 forms the spot group SG for a predetermined time to form the group latent image GL1. Next, the second light emitting element group row 295R_2 forms the spot group SG for a predetermined time to form the group latent image GL2. At this time, the group latent image GL2 is formed to be shifted from the group latent image GL1 by a distance ΔGL12 in the sub-scanning direction SD due to fluctuations in the moving speed of the photosensitive member surface. Further, the third light emitting element group row 295R_3 forms the spot group SG for a predetermined time, and the group latent image GL3 is formed. Also in this case, the group latent image GL3 is formed to be shifted from the group latent image GL2 by the distance ΔGL23 in the sub-scanning direction SD due to fluctuations in the moving speed of the photosensitive member surface. As described above, the position of the three group latent images GL (GL1 to GL3) continuously formed in the main scanning direction MD may vary in the sub-scanning direction SD due to fluctuations in the moving speed of the photoreceptor surface.

  In summary, in the line head 29, each of the N light emitting element groups 295 capable of exposing a continuous area in the main scanning direction MD is shifted from each other in the width direction LTD corresponding to the sub scanning direction SD. 295C is configured. Here, in this specification, N is the number of light emitting element groups 295 constituting one light emitting element group column 295C (in other words, the number of light emitting element group rows 295R). N = 3. As described above, when the latent image is formed by such a line head 29, the positions of the N group latent images GL continuously formed in the main scanning direction MD may vary in the sub scanning direction SD. .

  Incidentally, as will be described later, in order to execute the image forming operation satisfactorily, the image forming apparatus 1 executes the color misregistration correction operation. That is, in the image forming apparatus 1, registration marks are formed for a plurality of colors (yellow (Y), magenta (M), cyan (C), black (K)) (registration mark forming operation), and these resists are registered. A mark is detected. Then, color misregistration correction is executed based on the detection result. However, as described above, when the positions of the N group latent images GL continuously formed in the main scanning direction MD vary in the sub scanning direction SD, the edges of the registration marks in the sub scanning direction SD may vary. there were. As a result, good color misregistration correction may not be performed. Therefore, in the image forming apparatus according to the present invention, the light emission timing of the light emitting element 2951 is adjusted before the registration mark forming operation. This will be described below.

V. Color Misregistration Correction FIG. 16 is a flowchart showing a flow up to color misregistration correction, FIG. 17 is a block diagram showing an electrical configuration for executing the flow shown in FIG. 16, and FIG. It is a figure which shows the structure which performs mark formation operation | movement. Note that FIG. 18 corresponds to the case viewed from vertically below (the lower side in FIG. 1). First, in the present embodiment, the light emission timing adjustment step is executed, and the light emission timings of the light emitting elements 2951 are set so that the positions of the group latent images GL formed by the light emitting element groups 295 in the sub-scanning direction SD coincide with each other. Is adjusted (step S100). Thereby, the positional variation of the N group latent images GL continuously formed in the main scanning direction MD as shown in FIG. 15 is suppressed. Details of the light emission timing adjustment step will be described later.

  Next, a registration mark forming operation is executed in the registration mark forming step (step S200). In this registration mark forming operation, registration marks RM for each toner color are formed. Specifically, each of the image forming stations Y, M, C, and K forms a latent image on the surface of the photosensitive drum 21 included in each of the image forming stations Y, M, C, and K, and develops the latent image with each toner color. ), RM (M), RM (C), RM (K). These registration marks RM are transferred to the surface of the transfer belt 81 along the conveying direction D81 (FIG. 18). The registration mark RM thus formed on the transfer belt 81 is transported in the transport direction D81 and detected by the optical sensor SC (SCa, SCb). These two optical sensors SCa and SCb are arranged at the end portion in the main scanning direction MD while facing the winding portion of the transfer belt 81 around the driving roller 82.

  FIG. 19 is a diagram illustrating an example of an optical sensor. The optical sensor SC includes a light emitting unit Eem that emits the irradiation light Lem toward the surface of the transfer belt 81 and a light receiving unit Erf that receives the reflected light Lrf reflected by the transfer belt 81. Further, the optical sensor SC includes a condensing lens CLem that condenses the irradiation light Lem emitted from the light emitting unit Eem, and a condensing lens CLrf that condenses the reflected light Lrf reflected by the surface of the transfer belt 81. Therefore, the emitted light Lem emitted from the light emitting portion Eem is condensed on the surface of the transfer belt 81 by the condenser lens CLem. Thereby, a sensor spot SS is formed on the surface of the transfer belt 81. Then, the reflected light Lrf reflected by the area of the sensor spot SS is condensed by the condenser lens CLrf and detected by the light receiving unit Erf. In this way, the optical sensor SC detects an object in the sensor spot SS. As the optical sensor SC, various conventionally proposed ones can be used, and a so-called distance limited reflective photoelectric sensor BGS (Back Ground Suppression) or the like may be used. As such BGS, for example, there is a model E3Z-LL61-F80 5M manufactured by OMRON Corporation. The BGS projects a light beam as a sensor spot and detects an object inside the sensor spot.

  FIG. 20 is a diagram illustrating processing executed on the detection result of the optical sensor. As described above, the registration marks RM of the respective colors are formed side by side in the transport direction D81 and are transported in the transport direction D81 and pass through the sensor spot SS. Thus, each color registration mark RM is detected by the optical sensor SC.

  The column “Registration Mark” in FIG. 20 shows registration marks RM formed on the surface of the transfer belt 81. Group toner images GM1 to GM3 shown in the same column are toner images obtained by developing the group latent images GL1 to GL. As described above, the light emission timing adjustment step is performed prior to the registration mark formation step. Accordingly, the positions of the group latent images GL coincide with each other in the sub-scanning direction SD, and as a result, the positions of the group toner images GM also coincide with each other. Therefore, the edges of the registration marks RM in the sub-scanning direction SD are aligned and do not vary.

  As shown in the figure, the sensor spot SS in the main scanning direction MD has a main scanning spot diameter Dsm wider than the unit width Wlm. Here, the unit width Wlm is the width (in other words, the group latent image GL) in the main scanning direction MD when the group latent image GL is formed by all the light emitting elements 2951 included in one light emitting element group 295. The width of the toner image GM in the main scanning direction MD).

  The column “SENSING PROFILE” in FIG. 20 shows the detection result of the optical sensor SC. When the registration marks RM (Y), RM (M), RM (C), and RM (K) pass through the sensor spot SS, the optical sensor SC detects the detection waveforms PR (Y) and PR (M) corresponding to the registration marks. , PR (C), PR (K) are output to the misalignment calculator 551 (FIG. 17). This detected waveform is output as a voltage signal. Note that both the displacement calculator 551 and a later-described light emission timing calculator 552 are provided in the engine controller EC.

In the position shift calculator 551, the detection waveforms PR (Y), PR (M), PR (C), and PR (K) output from the optical sensor SC are binarized by the threshold voltage Vth, and are shown in FIG. Binary signals BS (Y), BS (M), BS (C), BS (K) as shown in the “after binarization” column are obtained. The rising edge of the reference yellow (Y) binary signal BS (Y) and the binary signals BS (M) and BS (C) for each color of magenta (M), cyan (C), and black (K). , BS (K) from the time intervals T1, T2, and T3 with respect to the rising edge, the displacement of the formation position of each color registration mark RM is obtained by calculation. In other words, on behalf of magenta (M),
Dm: Position shift of registration mark RM (M) with respect to registration mark RM (Y) S81: Transfer speed of transfer belt surface T1: Measured value of time interval T1rf: Time interval when there is no position shift for magenta Dm = S81 × (T1-T1rf)
To determine the misalignment Dm for magenta (M). Such calculation is also performed for cyan (C) and black (K), and a positional shift (color shift information) for each toner color is obtained. The color misregistration information obtained in this way is output to the light emission timing calculator 552, and the light emission timing calculator 552 obtains the optimum light emission timing based on the color misregistration information. The light emission of the line head 29 is controlled according to the light emission timing thus obtained, and the color shift is corrected.

  As described above, in the above-described embodiment, the light emitting elements 2951 emit light before forming the registration mark RM for a plurality of colors (yellow (Y), magenta (M), cyan (C), and black (K)). Timing is adjusted. Thereby, the positional variation of the N group latent images GL formed continuously in the main scanning direction MD as shown in FIG. 15 is suppressed. Therefore, variation in the edge of the registration mark RM in the sub-scanning direction SD is suppressed, and good color misregistration correction is possible.

  In the above embodiment, the sensor spot SS has a main scanning spot diameter Dsm wider than the unit width Wlm in the main scanning direction. Therefore, for example, even when the apparatus environment such as the temperature and humidity inside the image forming apparatus changes rapidly, the registration mark RM can be detected stably. The reason for this will be described.

  FIG. 21 is a diagram illustrating a registration mark detection operation in the case where the apparatus environment changes rapidly. Even when the light emission timing adjustment process is performed as described above, the apparatus environment rapidly changes after the process, and the N group latent images GL continuously formed in the main scanning direction MD There may be variations in position. As a result, as shown in the column “Registration Mark” in the figure, the position of the group toner image GM may be shifted in the sub-scanning direction SD, and the edge of the registration mark RM in the sub-scanning direction SD may vary. . Here, let us consider a case where the registration mark RM is detected by the sensor spot SS1 and a case where the registration mark RM is detected by the sensor spot SS2 in a state where such a variation occurs.

  Each registration mark RM (Y), RM (M), RM (C), RM (K) is transported in the transport direction D81 and passes through the sensor spots SS1, SS2. At this time, as shown in the column of “Registration Mark” in the figure, the boundary portion BD of the two group toner images GM adjacent to the main scanning direction MD passes through the sensor spot SS2. On the other hand, the boundary portion BD does not pass through the sensor spot SS1, and a substantially central portion of one group toner image GM passes through. As a result, the detection results obtained by the sensor spots SS1 and SS2 are as shown in the “sensing profile” column of FIG. That is, the detection waveforms PR1 (Y), PR1 (M), PR1 (C), PR1 (K) of each registration mark RM by the sensor spot SS1 have substantially the same shape and are stable. On the other hand, the detection waveforms PR2 (Y), PR2 (M), PR2 (C), and PR2 (K) (broken line) of each registration mark RM by the sensor spot SS2 have different shapes. This is because the detection waveform PR2 by the sensor spot SS2 is distorted due to the influence of the boundary portion BD. Thus, the detection waveform PR2 (detection result) by the sensor spot SS2 is distorted due to the influence of the boundary portion BD and is not stable. As a result, the color misregistration correction operation may not be performed properly.

  On the other hand, in the above embodiment, the sensor spot SS has a main scanning spot diameter Dss wider than the unit width Wlm in the main scanning direction MD. Such a sensor spot SS can reliably detect a flat portion FL extending linearly with a unit width Wlm in the main scanning direction MD. That is, in the optical sensor SC having the sensor spot SS shown in the figure, such a flat portion FL can be sufficiently taken into the detection result, and the influence of the boundary portion BD can be relatively reduced. Therefore, this optical sensor SC can detect the registration mark RM stably and is suitable.

VI-1. First Example of Light Emission Timing Adjustment Step FIG. 22 is a flowchart showing a first example of the light emission timing adjustment step. As shown in FIG. 22, in the light emission timing adjustment step, first, speed variation detection of the photosensitive member is executed (step S101). FIG. 23 is a diagram showing a configuration for executing the speed fluctuation detection of the photosensitive member. As shown in FIG. 23, an encoder ECD and an encoder sensor ES are provided at one end of the photosensitive drum 21 in the main scanning direction MD. The encoder ECD has a plurality of slits SL provided radially with respect to the rotation center of the photosensitive drum 21. The encoder ECD rotates with the photosensitive drum 21, and when the photosensitive drum 21 rotates, the encoder ECD also rotates. The encoder sensor ES detects the slit SL that faces the slit SL and passes through the facing position. The detection result of the encoder sensor ES is output to the engine controller EC (FIG. 17).

  The engine controller EC includes a photoconductor speed calculator 553 and a comparator 554 in addition to the displacement calculator 551 and the light emission timing calculator 552 described above. The detection result of the encoder sensor ES is input to the photoconductor speed calculator 553. The photoconductor speed calculator 553 calculates the speed fluctuation of the surface of the photoconductor drum 21 from the detection result of the encoder sensor ES and outputs the calculation result to the comparator 554. Thus, the encoder ECD, the encoder sensor ES, and the photosensitive member speed calculator 553 function as the “speed detection means” of the present invention.

  The light emission timing of the light emitting elements 2951 is adjusted for each light emitting element group row 295R based on the moving speed of the surface of the photosensitive drum detected by the speed detecting means. That is, the comparator 554 causes the surface speed of the photosensitive drum 21 at the timing of emitting the first light emitting element group row 295R_1 and the photosensitive drum 21 at the timing of emitting the second light emitting element group row 295R_2. Compare the surface speed. Further, the comparator 554 causes the surface speed of the photosensitive drum 21 at the timing of emitting the first light emitting element group row 295R_1, and the photosensitive drum 21 at the timing of emitting the third light emitting element group row 295R_3. Compare the surface speed. The comparison result is output to the light emission timing calculator 552.

  FIG. 24 is a diagram illustrating an example of the operation of the light emission timing calculator. Tr1 to Tr3 in the figure are set values of light emission timings of the light emitting element groups 295_R1 to 295_R3 in the first row, and are stored in the light emission timing calculator 552 in advance. Based on the output result of the comparator 554, the light emission timing calculator 552 is between the light emission timing Tr1 of the light emitting element group 295_R1 in the first row and the light emission timing Tr2 of the light emitting element group 295_R2 in the second row. It is determined whether or not the surface speed of the surface has changed. For example, when it is determined that the surface speed of the photosensitive drum 21 has increased, the light emission timing of the light emitting element group 295_R2 in the second row is reset to a timing Tr2 'that is earlier than the timing Tr2. Further, the light emission timing calculator 552, based on the output result of the comparator 554, between the light emission timing Tr2 of the light emitting element group 295_R1 in the first row and the light emission timing Tr3 of the light emitting element group 295_R3 in the third row. It is determined whether or not the surface speed of the drum 21 has changed. For example, when it is determined that the surface speed of the photosensitive drum 21 has become slow, the light emission timing of the light emitting element group 295_R2 in the third row is reset to a timing Tr3 'that is later than the timing Tr3. In this way, the light emission timings of the respective light emitting elements 2951 are adjusted so that the positions of the group latent images GL formed by the respective light emitting element groups 295 coincide with each other in the sub scanning direction SD. Then, the head controller HC controls the light emission of each light emitting element 2951 based on the light emission timing adjusted in this way.

  Thus, in the first example of the light emission timing adjustment step, the light emission timing of each light emitting element 2951 is adjusted so that the positions of the group latent images GL formed by each light emitting element group 295 in the sub-scanning direction SD coincide with each other. Is done. Therefore, variation in the sub-scanning direction SD of the position of the group latent image GL formed by the different light emitting element groups 295 is suppressed. Then, after the light emission timing adjustment step (step S100), the registration mark forming operation (step S200) and the color misregistration correction (step S300) described above are executed, so that better color misregistration correction can be performed (FIG. 16).

  In the first example of the light emission timing adjustment step, the light emission timing of the light emitting element 2951 is adjusted based on the moving speed of the surface of the photosensitive drum. Therefore, it is possible to suppress the positional deviation in the sub-scanning direction SD of the group latent image GL due to the fluctuation of the moving speed of the photosensitive drum surface with high accuracy.

  In the first example of the light emission timing adjustment process, the encoder ECD, the encoder sensor ES, and the photosensitive member speed calculator 553 constitute a “speed detecting unit”, and the speed detecting unit detects the moving speed of the photosensitive member surface. is doing. The engine controller EC adjusts the light emission timing of the light emitting element 2951 based on the detection result of the speed detection means. By providing the speed detecting means in this way, it is possible to reliably detect fluctuations in the moving speed of the photosensitive drum surface, and the group latent image GL caused by fluctuations in the moving speed of the photosensitive drum surface can be detected. The positional deviation in the sub scanning direction SD can be more reliably suppressed.

VI-2. Second Example of Light Emission Timing Adjustment Step As described with reference to FIG. 15, when a latent image is formed by the above-described line head 29, the positions of N group latent images GL formed continuously in the main scanning direction MD. However, in some cases, the sub-scanning direction SD varies. Therefore, in a second example of the light emission timing adjustment process described below, a variation detection mark is formed in order to detect variations in N group latent images GL that are actually formed. Then, the variation detection mark is detected by the optical sensor, and the light emission timing of the light emitting element 2951 is adjusted based on the detection result.

  FIG. 25 is a flowchart showing a second example of the light emission timing adjustment step, and FIG. 26 is a diagram showing the detection operation of the variation detection mark. As described above, in the second example of the light emission timing adjustment process, a variation detection mark is formed (step S111). Specifically, first, a test latent image TLI is formed. As shown in FIG. 26, the test latent image TLI is composed of N or more group latent images GL continuously in the main scanning direction MD (N = 3 in the figure). Each of these group latent images GL is formed by all the light emitting elements 2951 included in one light emitting element group 295. In the main scanning direction MD, the test latent image TLI has a width not less than N times the unit width Wlm. Have More specifically, in FIG. 26, the test latent image TLI is formed by adjoining eight group latent images GL continuous in the main scanning direction MD, and is eight times the unit width Wlm in the main scanning direction MD. Have a width of

  The group latent images GL1 to GL3 constituting the test latent image TLI are developed to form group toner images GM1 to GM3. In this way, the test latent image TLI is developed to form the variation detection mark Dm. The variation detection mark Dm is transferred to the transfer belt 81 and conveyed in the conveyance direction D81, and is detected at the sensor spot SS of the optical sensor SC (step S112).

  In the example shown in the figure, the sensor spot SS has a main scanning spot diameter Dsm wider than (N−1) times the unit width Wlm in the main scanning direction MD. Specifically, the main scanning spot diameter Dsm of the sensor spot SS is wider than twice the unit width Wlm. Thus, in the first example shown in the figure, the main scanning spot diameter Dsm of the sensor spot SS is wider than (N−1) times the unit width Wlm. Accordingly, any of the N group toner images GM1 to GM3 passes through the sensor spot SS. As a result, the detection result of the variation detection mark DM reflects the positional variation of the N group latent images GL continuous in the main scanning direction MD. In the example shown in the figure, the main scanning spot diameter Dsm is set to be shorter than N times the unit width Wlm in the main scanning direction MD. And the following processes are performed with respect to the detection result of such an optical sensor SC.

  FIG. 27 is a diagram showing processing executed on the detection result of the optical sensor, and FIG. 28 is a diagram showing an electrical configuration for executing processing on the detection result of the optical sensor. The column of “variation detection mark” in FIG. 27 shows the actually formed variation detection mark DM. The column “reference mark” in FIG. 27 shows an ideal mark having no positional variation of the group toner image GM in the sub-scanning direction SD, that is, the reference mark DMr. In the “sensing profile” column of FIG. 27, the solid line waveform is a reference waveform PR (DMr) corresponding to the detection waveform when the reference mark DMr is detected by the sensor spot SS, and the broken line waveform is a variation due to the sensor spot SS. It is a detection waveform PR (DM) of the detection mark DM.

  The optical sensor SC outputs a detection waveform PR (DM) of the variation detection mark DM to the engine controller EC. The engine controller EC includes a time shift calculator 555, a reference time storage unit 556, a position shift calculator 557, and a light emission timing calculator 552. The detected waveform PR (DM) is input to the time shift calculator 555. The time shift calculator 555 obtains a time interval Td from when the rising edge of the detection waveform PR (DM) passes the low threshold voltage Vlow to the passage of the high threshold voltage Vhig. Then, the time shift calculator 555 calculates a difference ΔT = Td−Tdr between the time interval Td and the reference time interval Tdr stored in the reference time storage unit 556. The reference time interval Tdr is a time interval from when the rising edge of the reference waveform PR (DMr) passes through the low threshold voltage Vlow to the passage through the high threshold voltage Vhigh, and is stored in the reference time storage unit 556 in advance. Yes.

  The time shift calculator 555 calculates the position variation ΔDgm of the group toner image GM from the difference ΔT and the peripheral speed S21 of the photosensitive drum 21, and outputs the position variation ΔDgm to the light emission timing calculator 552. The light emission timing calculator 552 obtains the light emission timing of the line head 29 based on the positional variation ΔDgm (step S113). Specifically, the light emission timing is calculated so that the positional variation ΔDgm decreases. In this way, the light emission timings of the respective light emitting elements 2951 are adjusted so that the positions of the group latent images GL formed by the respective light emitting element groups 295 in the sub scanning direction SD coincide with each other. Based on the adjusted light emission timing, the head controller 29 controls light emission of each light emitting element 2951.

  Thus, in the second example of the light emission timing adjustment step, the light emission timing of each light emitting element 2951 is adjusted so that the positions of the group latent images GL formed by each light emitting element group 295 in the sub-scanning direction SD coincide with each other. Is done. Therefore, variation in the sub-scanning direction SD of the position of the group latent image GL formed by the different light emitting element groups 295 is suppressed. Then, after the light emission timing adjustment step (step S100), the registration mark forming operation (step S200) and the color misregistration correction (step S300) described above are executed, so that better color misregistration correction can be performed (FIG. 16).

  In the second example of the light emission timing adjustment step, the variation detection mark DM is formed on the transfer belt 81, and the light emission timing of the light emitting element 2951 is adjusted based on the detection result of the variation detection mark DM. That is, the actual variation of the group latent image GL is detected, and the light emission timing of the light emitting element 2951 is adjusted based on the detection result. Therefore, it is possible to suppress the positional deviation of the group latent image GL described above with high accuracy.

  Thus, in the above embodiment, the main scanning direction MD corresponds to the “first direction” of the present invention, and the sub-scanning direction SD corresponds to the “second direction” of the present invention. In the above embodiment, the image forming stations Y, N, C, and K correspond to the “image forming portion” of the present invention, the photosensitive drum 21 corresponds to the “latent image carrier” of the present invention, and the light emitting element group. The column 295C corresponds to the “group column” of the present invention, the optical sensor SC corresponds to the “mark detection means” of the present invention, and the sensor spot SS corresponds to the “detection region” of the present invention. Further, the engine controller EC and the head controller HC function as “correction means” and “control means” of the present invention.

VII. Others The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in “VI-2. Second example of light emission timing adjustment step”, the main scanning spot diameter Dsm is set to be shorter than N times the unit width Wlm in the main scanning direction MD. However, as shown in FIG. 29, the variation mark detection operation may be executed by a sensor spot SS having a main scanning spot diameter Dsm wider than N times the unit width Wlm.

  FIG. 29 is a diagram showing another example of the detection operation of the variation detection mark, and corresponds to the case where N = 3. In the example shown in the figure, the sensor spot SS has a main scanning spot diameter Dsm wider than N times the unit width Wlm in the main scanning direction MD. Accordingly, it is possible to sufficiently reflect the positional variation of the N group latent images GL (GL1 to GL3) continuous in the main scanning direction MD in the detection result of the variation detection mark DM. The reason for this will be described with reference to FIGS.

  In FIG. 27, all the N group toner images GM1 to GM3 formed continuously in the main scanning direction MD have a width Wlm in the main scanning direction MD. However, in the example shown in FIG. 27, all the width Wlm in the main scanning direction MD passes through the sensor spot SS for the group toner image GM2, but the width Wlm in the main scanning direction MD for the group toner images GM1 and GM3. Only part of the sensor spot SS passes. On the other hand, in another example shown in FIG. 29, the entire width Wlm in the main scanning direction MD passes through the sensor spot SS for all of the group toner images GM1 to GM3. Therefore, another example shown in FIG. 29 can sufficiently reflect the positional variation of the N group latent images GL (GL1 to GL3) continuous in the main scanning direction MD in the detection result of the variation detection mark DM. It is suitable.

  Further, in “VI-2. Second Example of Light Emission Timing Adjustment Step”, the test latent image TLI (variation detection mark DM) is formed wider than N times the unit width Wlm in the main scanning direction MD. However, as shown in FIG. 29, the detection operation of the variation detection mark may be executed by the test latent image TLI (variation detection mark DM) having a width N times the unit width Wlm in the main scanning direction MD.

  FIG. 30 is a diagram showing still another example of the detection operation of the variation detection mark, and corresponds to the case where N = 3. In yet another example shown in FIG. 30, the test latent image TLI has a width N times the unit width Wlm in the main scanning direction MD. The test latent image TLI includes N group latent images GL1 to GL3 continuous in the main scanning direction MD, and each of the N group latent images GL1 to GL3 has one light emitting element group 295. All the light emitting elements 2951 are formed. In other words, in FIG. 30, the test latent image TLI is formed with N group latent images GL1 to GL3 each having a unit width Wlm aligned in the main scanning direction MD. The test latent image TLI is developed to form a variation detection mark DM, and the variation detection mark DM is detected by the sensor spot SS.

  In this way, in yet another example shown in FIG. 30, the widths of the N group latent images GL1 to GL3 in the main scanning direction MD are all equal to the unit width Wlm. Therefore, the influence of the group latent images GL1 to GL3 on the detection result of the optical sensor SC can be made substantially equal among the N group latent images GL1 to GL3. Therefore, another example shown in FIG. 30 can appropriately reflect the positional variation of the N group latent images GL (GL1 to GL3) continuous in the main scanning direction MD in the detection result of the variation detection mark DM. It is suitable.

  Further, the above embodiment corresponds to a case where one light emitting element group column 295C is configured by three light emitting element groups 295, that is, corresponds to a case where N = 3. However, the number of the light emitting element groups 295 constituting one light emitting element group column 295C is not limited to 3, and may be 2 or more (that is, N may be an integer of 2 or more).

  In the above embodiment, the light emitting element group 295 has eight light emitting elements 2951. However, the number of light emitting elements 2951 included in the light emitting element group 295 is not limited to this, and may be two or more.

In the above embodiment, an organic EL element is used as the light emitting element 2951. However, an element that can be used as the light emitting element 2951 is not limited to an organic EL element.
Emitting Diode) can also be used as the light emitting element 2951.

  In the above embodiment, the case where the present invention is applied to a so-called tandem type image forming apparatus has been described. However, the image forming apparatus to which the present invention is applicable is not limited to the tandem type. For example, Japanese Patent Laid-Open No. 2002-132007 discloses a so-called rotary type in which a photosensitive member and an exposure unit are provided one by one, and latent images corresponding to the respective colors are sequentially formed on the surface of the photosensitive member by using the exposure unit. An image forming apparatus is described. It is also possible to apply the present invention to such a rotary image forming apparatus.

1 is a diagram illustrating an embodiment of an image forming apparatus to which the present invention is applicable. FIG. 2 is a diagram illustrating an electrical configuration of the image forming apparatus in FIG. 1. The perspective view which shows the outline of a line head. FIG. 4 is a cross-sectional view in the width direction of the line head shown in FIG. 3. The perspective view which shows the outline of a lens array. Sectional drawing of the longitudinal direction LGD of a lens array. The figure which shows arrangement | positioning of the light emitting element group in a line head. The figure which shows arrangement | positioning of the light emitting element in each light emitting element group. Explanatory drawing of the term used by this specification. Explanatory drawing of the term used by this specification. The perspective view which shows the exposure operation | movement by a line head. The side view which shows the exposure operation | movement by a line head. FIG. 5 is a schematic diagram illustrating an example of a latent image forming operation by a line head. The figure which shows the relationship between the speed fluctuation | variation of the moving speed of the photoreceptor surface, and time. The figure which shows the position variation which may generate | occur | produce in a latent image. 6 is a flowchart showing a flow up to color misregistration correction. FIG. 17 is a block diagram showing an electrical configuration for executing the flow shown in FIG. 16. The figure which shows the structure which performs registration mark formation operation. The figure which shows an example of an optical sensor. The figure which shows the process performed with respect to the detection result of an optical sensor. The figure which shows the registration mark detection operation | movement when an apparatus environment fluctuates. The flowchart which shows the 1st example of a light emission timing adjustment process. FIG. 3 is a diagram illustrating a configuration for executing speed variation detection of a photoreceptor. The figure which shows an example of operation | movement of a light emission timing calculator. The flowchart which shows the 2nd example of a light emission timing adjustment process. The figure which shows the detection operation of a dispersion | variation detection mark. The figure which shows the process performed with respect to the detection result of an optical sensor. The figure which shows the electrical structure which performs a process with respect to the detection result of an optical sensor. The figure which shows another example of the detection operation | movement of a variation detection mark. The figure which shows another example of the detection operation of a variation detection mark.

Explanation of symbols

  Y, M, C, K ... Image forming station (image forming unit), 21Y, 21K ... Photosensitive drum (latent image carrier), 29 ... Line head, 295 ... Light emitting element group, 295C ... Light emitting element group column (group) Column), 295R ... light emitting element group row, 2951 ... light emitting element, 293 ... head substrate, 299 ... lens array, MD ... main scanning direction (first direction), SD ... sub-scanning direction (second direction), LGD ... longitudinal Direction, LTD ... Width direction, SC ... Optical sensor (mark detection means), SS ... Sensor spot (detection area), TLI ... Test latent image, RM (Y), RM (M), RM (C), RM (K ) ... Registration mark, DM ... Variation detection mark, ECD ... Encoder (speed detection means), ES ... Encoder sensor (speed detection means), 553 ... Photosensitive Body speed calculator (speed detection means)

Claims (6)

A latent image bearing member whose surface moves in a second direction orthogonal to or substantially orthogonal to the first direction, and a line head provided with a plurality of light emitting elements grouped for each light emitting element group, the latent image An operation is performed for a plurality of colors in which a latent image is formed by exposing a light beam to the surface of the carrier with the line head and a registration mark obtained by developing the latent image is transferred to a transfer medium moving in the second direction. An image forming unit that performs a registration mark forming operation;
Correction means for performing color misregistration correction based on the registration mark formed by the registration mark forming operation;
Control means for controlling the registration mark forming operation by the image forming unit,
Each of the light emitting element groups emits the light emitting element, emits a light beam toward the surface of the latent image carrier, and can expose different areas in the first direction.
The control means adjusts the light emission timings of the plurality of light emitting elements so that the positions in the second direction of the latent images formed by the light emitting element groups coincide with each other, and then registers the registration marks on the image forming unit. An image forming apparatus that executes a forming operation.   Mark detection means for detecting the registration mark in a detection area, the correction means performs color misregistration correction based on a detection result of the mark detection means, and the detection area includes all of the light emitting element groups. The image forming apparatus according to claim 1, wherein a width of the latent image formed by the light emitting element is wider in the first direction.   The image forming apparatus according to claim 1, wherein the control unit adjusts light emission timings of the plurality of light emitting elements based on a moving speed of the surface of the latent image carrier.   The image according to claim 3, further comprising speed detection means for detecting a moving speed of the surface of the latent image carrier, wherein the control means adjusts light emission timings of the plurality of light emitting elements based on a detection result of the speed detection means. Forming equipment.   The control unit controls the image forming unit to form a variation detection mark on the transfer medium, and adjusts the light emission timing of the plurality of light emitting elements based on the detection result of the variation detection mark. 2. The image forming apparatus according to 2. Using a line head provided with a plurality of light emitting elements grouped for each light emitting element group, a light beam is exposed to the surface of the latent image carrier moving in a second direction orthogonal to or substantially orthogonal to the first direction. A resist mark forming step of performing an operation of transferring a registration mark obtained by developing the latent image and developing the latent image on a transfer medium moving in the second direction for a plurality of colors;
A correction step of performing color misregistration correction based on the registration mark formed in the registration mark formation step;
Prior to the registration mark formation step, comprising a timing adjustment step of adjusting the light emission timing of the plurality of light emitting elements,
Each of the light emitting element groups emits the light emitting element, thereby emitting a light beam toward the surface of the latent image carrier and exposing different areas in the first direction,
In the timing adjustment step, the light emission timings of the plurality of light emitting elements are adjusted so that the positions in the second direction of the latent images formed by the light emitting element groups are aligned with each other. .

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