JP2009160917A - Image forming device and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology capable of preventing the misalignment of a position at which a latent image is formed caused by the variation in the moving velocity of the surface of a latent image carrier from occurring, thereby achieving preferable latent image formation. <P>SOLUTION: An image forming device includes: an exposure head having a plurality of light emitting elements arranged in a first direction, a first imaging optical system adapted to image light emitted from the light emitting elements, and a second imaging optical system disposed in a second direction with respect to the first imaging optical system; a latent image carrier adapted to move in the second direction; a detection section adapted to detect a moving time the latent image carrier takes to move from a first position to a second position in the second direction; and a control section adapted to control the time from emission of a first part of the light emitting elements adapted to emit light to be imaged by the first imaging optical system to emission of a second part of the light emitting elements adapted to emit light to be imaged by the imaging optical system based on the detection result of the detection section, thereby aligning a latent image formed on the latent image carrier by the first imaging optical system and a latent image formed on the latent image carrier by the second imaging optical system in the first direction. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image forming method using an exposure head that forms an image of a light beam emitted from a light emitting element by an imaging optical system.

  As such an exposure head, there is known a line head that forms an image of light beams emitted from a plurality of light emitting elements as spots. For example, in the line head described in Patent Document 1 (exposure head in the same document), a number of light emitting elements corresponding to one line are arranged in a direction corresponding to the main scanning direction, and light emitted from each light emitting element. The beam is imaged as a spot by a gradient index lens. Then, a latent image for each line is sequentially formed on the surface of the latent image carrier that moves in the sub-scanning direction, so that a two-dimensional latent image corresponding to a desired image is formed.

JP 2004-66758 A

  By the way, a line head having a light emitting element group obtained by grouping a plurality of light emitting elements can be used to realize formation of a higher resolution latent image. That is, in this line head, a plurality of light emitting element groups are arranged at different positions in the direction corresponding to the sub-scanning direction (width direction) to form a light emitting element group row, and this light emitting element group row is a main scan. A plurality of lines are arranged in a direction corresponding to the direction (longitudinal direction). However, in such a line head in which a plurality of light emitting element groups are arranged at different positions in the width direction, when the moving speed of the surface of the latent image carrier fluctuates, the latent image formation position shifts in the sub-scanning direction. As a result, there are cases where good latent image formation cannot be performed.

  The present invention has been made in view of the above problems, and is a technique that enables the formation of a good latent image by suppressing the occurrence of displacement of the latent image forming position due to the fluctuation of the moving speed of the surface of the latent image carrier. The purpose is to provide.

  In order to achieve the above object, an image forming apparatus according to the present invention includes: a light emitting element disposed in a first direction; a first imaging optical system that forms an image of light emitted from the light emitting element; An exposure head having a second imaging optical system disposed in a second direction with respect to the one imaging optical system, a latent image carrier that moves in the second direction, and a second image in the second direction. A detector that detects a moving time of the latent image carrier that moves from the first position to the second position, and a first light emitting element that emits light imaged by the first imaging optical system emits light. Until the second light emitting element that emits light imaged by the imaging optical system emits light, based on the detection result of the detection unit, and the latent image is generated by the first imaging optical system. A controller that arranges the latent image formed on the carrier and the latent image formed on the latent image carrier by the second imaging optical system in the first direction. It is characterized in that was.

  Further, in order to achieve the above object, the image forming method according to the present invention includes a detection step of detecting a time for moving the latent image carrier from the first position to the second position, and the first imaging optics. Detects the time from when the first light emitting element emitting light imaged by the system emits light until the second light emitting element emitting light imaged by the second imaging optical system emits light And a control step of controlling based on the detection result of the step.

  The invention (image forming apparatus and image forming method) configured as described above detects the time for the latent image carrier to move from the first position to the second position. Then, after the first light emitting element that emits light imaged by the first imaging optical system emits light, the second light emitting element that emits light imaged by the second imaging optical system The time until light emission is controlled based on the detection result. Accordingly, the light emitted from the light emitting element that emits the light imaged on the first imaging optical system is imaged on the second imaging optical system arranged in the second direction of the first imaging optical system. Even when the speed of the latent image carrier fluctuates before the light emitting element emitting light emits light, it is possible to form a latent image satisfactorily.

  The control unit emits the light imaged by the first imaging optical system after the first light emitting element emitting the light imaged by the first imaging optical system emits light. The time until the third light emitting element arranged on the second direction side of the first light emitting element emits light is controlled based on the detection result of the detection unit, and the first light emitting element is used for the latent image carrier. The latent image to be formed and the latent image formed on the latent image carrier by the third light emitting element may be arranged in the first direction. In such a configuration, the light emitted from the first light emitting element that emits light imaged by the first imaging optical system emits the light imaged by the first imaging optical system and the first light emitting element. A latent image can be satisfactorily formed even when the speed of the latent image carrier fluctuates before the third light emitting element disposed on the second direction side of the light emitting element emits light. It is possible.

  The present invention is preferably applied to an image forming apparatus in which the latent image carrier is a photosensitive drum that rotates in the second direction. In particular, it is preferable to apply the present invention to an image forming apparatus having a driving source and a gear that transmits a driving force from the driving source to the photosensitive drum. That is, in such a configuration, the rotational speed may vary. Therefore, by applying the present invention, it becomes possible to form a good latent image regardless of fluctuations in the rotational speed of the photosensitive drum.

  At this time, the second of the first imaging optical system and the second imaging optical system is larger than the distance of the photosensitive drum obtained by multiplying the period of the speed fluctuation of the photosensitive drum by the average speed of the photosensitive drum. It is preferable that the distance in the direction is increased.

  The speed fluctuation cycle can be easily obtained from the reciprocal of the value obtained by multiplying the number of rotations of the photosensitive drum per unit time by the number of gear teeth.

  The present invention can be applied to an image forming apparatus in which a gear is connected to a photosensitive drum via a coupling, or an image forming apparatus in which the gear and the photosensitive drum are integrally connected. In any of these configurations, the rotational speed of the photosensitive drum may fluctuate. Therefore, according to the present invention, it is preferable to realize good latent image formation regardless of fluctuations in the rotational speed of the photosensitive drum.

  Further, the detection unit may be configured to include an encoder disk having slits arranged radially from the rotation center of the photosensitive drum, and an optical sensor for detecting the slits. Such a detection unit can obtain the position of the photosensitive drum with high accuracy, and is advantageous in realizing good latent image formation.

  Further, the detection unit may be configured to include two optical sensors arranged in the radial direction of the photosensitive drum with the rotation center of the photosensitive drum interposed therebetween. In this way, by arranging two optical sensors in the radial direction of the photosensitive drum across the rotation center of the photosensitive drum, the influence of the eccentricity of the two optical sensors with respect to the rotation center of the photosensitive drum is suppressed. The position of the photosensitive drum can be obtained with high accuracy.

  Further, the distance in the second direction of the latent image carrier between the light imaged by the first imaging optical system and the light imaged by the second imaging optical system is the second direction of the pixel. You may comprise so that it may be an integral multiple of this inter-pixel distance. With this configuration, it is possible to simplify the control of the light emission timing of the light emitting element.

  The inter-pixel distance in the second direction may be configured to be shorter than the inter-pixel distance in the first direction in the pixel. With this configuration, a high-resolution latent image can be formed in the second direction. At this time, the control means can relatively easily realize such high-resolution latent image formation by controlling light emission of the light emitting element by PWM control.

  Also, a latent image carrier, an exposure head including a light emitting element and an imaging optical system that forms an image of light from the light emitting element on the latent image carrier, a detection unit that detects the position of the latent image carrier, and a detection unit The image forming apparatus may be configured to include a control unit that controls light emission of the light emitting element based on the detection result to form a latent image on the latent image carrier. The image forming apparatus configured as described above detects the position of the latent image carrier. And based on this detection result, light emission of the light emitting element is controlled to form a latent image on the latent image carrier. Therefore, it is possible to form a latent image satisfactorily even when the speed of the latent image carrier varies.

  In order to achieve the above object, an image forming apparatus according to the present invention has a latent image carrier whose surface moves in a second direction orthogonal or substantially orthogonal to the first direction, and a light emitting element in which a plurality of light emitting elements are grouped. A head substrate provided with a plurality of groups, and a lens array provided with an imaging optical system for each light emitting element group that forms an image of the light beam emitted from the light emitting elements of the light emitting element group to form spots on the surface of the latent image carrier. A line head, a control means for causing the light emitting element to emit light at a timing according to the movement of the surface of the latent image carrier, and a detection means for detecting the moving speed of the surface of the latent image carrier. A plurality of light emitting element group rows in which a plurality of light emitting element groups are arranged at different positions in the second direction are provided in the first direction, and each light emitting element of the light emitting element group emits light simultaneously. When a plurality of spots are defined as spot groups, the light emitting element groups in the light emitting element group row form spot groups at different positions in the second direction, and the control means detects the latent image carrier surface detected by the detecting means. The light emission timing of the light emitting element is adjusted according to the moving speed of the light emitting element.

  In order to achieve the above object, the line head control method according to the present invention includes a head substrate on which a plurality of light emitting element groups in which a plurality of light emitting elements are grouped, and light emitted from the light emitting elements of the light emitting element group. Each light-emitting element of a line head having a lens array provided with an image-forming optical system that forms a beam and forms a spot on the surface of the latent image carrier for each light-emitting element group emits light at a predetermined light emission timing. An exposure process for exposing the surface of the latent image carrier that moves in a second direction that is orthogonal to or substantially orthogonal to one direction is provided. On the head substrate of the line head, a plurality of light emitting element groups are arranged at different positions in the second direction. A plurality of light emitting element group rows are provided in the first direction, and a plurality of spots formed by simultaneous light emission of each light emitting element of the light emitting element group is defined as a spot group. In the light emitting element group row, each light emitting element group forms a spot group at a different position in the second direction, and in the exposure step, the moving speed of the surface of the latent image carrier is detected, and light is emitted based on the detection result. It is characterized by adjusting the light emission timing of the element.

  In the invention thus configured (image forming apparatus, line head control method), the moving speed of the surface of the latent image carrier is detected, and the light emission timing of the light emitting element is adjusted based on the detection result. Accordingly, it is possible to suppress the occurrence of the shift of the latent image forming position due to the fluctuation of the moving speed of the surface of the latent image carrier and to form a good latent image.

  Further, the control means may be configured to adjust the light emission timing of the light emitting element according to the difference between the moving speed of the surface of the latent image carrier and the reference speed. In such a configuration, since the light emission timing of the light emitting element is adjusted according to the deviation of the moving speed of the surface of the latent image carrier from the reference speed, it is possible to effectively suppress the deviation of the latent image forming position. Thus, a good latent image can be formed.

  The reference speed may be an average value of the moving speed of the surface of the latent image carrier. In such a configuration, since the light emission timing of the light emitting element is adjusted according to the deviation from the average moving speed of the latent image carrier surface, the deviation of the latent image forming position can be effectively suppressed. Thus, a good latent image can be formed.

  The latent image carrier is a photosensitive drum that rotates about a rotation axis that is orthogonal or substantially orthogonal to the second direction, and image formation in which the peripheral surface of the photosensitive drum moves in the second direction as the surface of the latent image carrier. It is particularly preferable to apply the present invention to an apparatus. This is because such a photosensitive drum may have uneven rotation speed, and this uneven rotation speed causes fluctuations in the moving speed of the photosensitive drum peripheral surface as the surface of the latent image carrier. is there. Therefore, it is preferable to apply the present invention to such an image forming apparatus to suppress the occurrence of the shift of the latent image forming position due to the fluctuation of the moving speed of the surface of the latent image carrier.

  Further, the detection means includes an encoder disk having a plurality of slits arranged radially around the rotation axis of the photosensitive drum, and an optical sensor for detecting the slit, and the photosensitive drum is based on the detection result of the optical sensor. You may comprise so that the moving speed of a surrounding surface may be detected. In such a configuration, the detection of the moving speed of the circumferential surface of the photosensitive drum is executed based on a plurality of slits arranged radially around the rotation axis of the photosensitive drum. Therefore, the movement speed of the photosensitive drum peripheral surface can be detected with high accuracy.

  Further, the detection means may be configured to have two optical sensors arranged so as to sandwich the rotating shaft of the photosensitive drum. In such a configuration, two optical sensors are provided. These two optical sensors are arranged so as to sandwich the rotating shaft of the photosensitive drum. Therefore, as will be described later, even when two optical sensors are arranged to be biased with respect to the rotation axis, it is possible to detect the moving speed satisfactorily while suppressing the influence of such a bias. It has become.

  The control means causes the light emitting element to emit light at a timing corresponding to the movement of the surface of the latent image carrier, thereby forming spots on the pixels on the surface of the latent image carrier, and a plurality of light emitting element group rows. The pitch of the spot groups in the second direction may be an integer multiple of the pixel pitch in the second direction. This is because, as will be described later, this configuration makes it possible to simplify the light emission timing control of the light emitting element.

A. Explanation of Terms Before explaining embodiments of the present invention, terms used in this specification will be explained.

  1 and 2 are explanatory diagrams of terms used in this specification. Here, the terms used in this specification will be organized using these drawings. In this specification, the transport direction of the surface (image surface IP) of the photosensitive drum 21 is defined as a sub-scanning direction SD, and a direction orthogonal or substantially orthogonal to the sub-scanning direction SD is defined as a main scanning direction MD. . 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. 1 and FIG. 2) 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. 2, 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 adjacent to each other in the sub-scanning direction SD. The spot group pitch Psg is the distance in the main scanning direction MD of the geometric centroids of two spot groups SG adjacent to each other in the main scanning direction MD.

  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 two lens rows LSR adjacent to each other in the width direction LTD. The lens pitch Pls is a distance in the longitudinal direction LGD between the geometric centroids of the two lenses LS adjacent to each other in the longitudinal direction LGD.

  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 adjacent to each other in the width direction LTD. The light emitting element group pitch Peg is the distance in the longitudinal direction LGD between the geometric centers of gravity of two light emitting element groups 295 adjacent to each other in the longitudinal direction LGD.

  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 a distance in the width direction LTD of the geometric centroids of two light emitting element rows 2951R adjacent to each other in the width direction LTD. The light emitting element pitch Pel is a distance in the longitudinal direction LGD between the geometric centroids of two light emitting elements 2951 adjacent to each other in the longitudinal direction LGD.

  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 a distance in the sub-scanning direction SD between the geometric centroids of two spot rows SPR adjacent to each other in the sub-scanning direction SD. The spot pitch Psp is a distance in the longitudinal direction LGD between the geometric centroids of two spots SP adjacent to each other in the main scanning direction MD.

B. First Embodiment FIG. 3 is a diagram showing an example of an image forming apparatus to which the present invention is applicable. FIG. 4 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. 3 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.

  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 in the housing main body 3 of the image forming apparatus. 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. 3, 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 (circumferential surface) having 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 that is orthogonal 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. 3, the image forming stations of the image forming unit 7 have the same configuration, and therefore, for convenience of illustration, only some image forming stations are denoted by reference numerals, and the other image forming stations are omitted. .

  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 disposed with respect to the photosensitive drum 21 such that the longitudinal direction thereof corresponds to the main scanning direction MD and the width direction thereof corresponds to the sub-scanning direction SD. Is substantially parallel to the main scanning direction MD. The line head 29 includes a plurality of light emitting elements arranged side by side in the longitudinal direction, and is spaced apart from the photosensitive drum 21. Then, light is emitted from these light emitting elements to the surface of the photosensitive drum 21 charged by the charging unit 23, and an electrostatic latent image is formed on the surface (exposure process).

  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 developing position in this way is conveyed in the rotational direction D21 of the photosensitive drum 21, and then a primary transfer position TR1 at which each of the photosensitive drums 21 comes into contact with the transfer belt 81, which will be described in detail later. 1 is primarily transferred to the transfer belt 81.

  In this embodiment, the photosensitive drum cleaner 27 is provided in contact with the surface of the photosensitive drum 21 on the downstream side of the primary transfer position TR1 in the rotational direction D21 of the photosensitive drum 21 and on the upstream side of the charging unit 23. It has been. 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 driving roller 82, a driven roller 83 (blade facing roller) disposed on the left side of the driving roller 82 in FIG. 3, and a stretched direction of these rollers in the direction of the arrow D81 (conveying direction). And a transfer belt 81 that is driven to circulate. 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. 3, all the primary transfer rollers 85Y, 85M, 85C, 85K are positioned on the image forming stations Y, M, C, 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.

  FIG. 5 is a perspective view showing an outline of the line head in the first embodiment. FIG. 6 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 bottom emission organic EL (Electro-Luminescence) elements are arranged as light emitting elements 2951 on the back surface of the head substrate 293 (the surface opposite to the lens array 299 of the two surfaces of the head substrate 293). ing. The plurality of light emitting elements 2951 are arranged in groups for each light emitting element group 295, as will be described later. The light beams emitted from the respective light emitting element groups 295 are transmitted from the back surface to the front surface of the head substrate 293 and directed to 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. 6, 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. 7 is a perspective view schematically showing the lens array. FIG. 8 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, a plurality of lenses LS are arranged so that their 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). The lenses LS are provided one-on-one with respect to the light emitting element group 295, and a plurality of lenses LS are two-dimensionally arranged corresponding to the arrangement of the light emitting element groups 295 described later. That is, a plurality of lens rows LSC in which three lenses LS are arranged at different positions in the width direction LTD are arranged along the longitudinal direction LGD.

  FIG. 9 is a diagram showing the configuration of the back surface of the head substrate, which corresponds to the case where the back surface is viewed from the front surface of the head substrate. FIG. 10 is a diagram showing the arrangement of light emitting elements in each light emitting element group. In FIG. 9, the lens LS is indicated by a two-dot chain line. This is for indicating that the light emitting element groups 295 are provided in one-to-one relationship with the lens LS. It does not indicate that it is arranged on the back surface of the head substrate. As shown in FIG. 9, a plurality of light emitting element group columns 295C in which three light emitting element groups 295 are arranged at different positions in the width direction LTD are arranged in the longitudinal direction LGD. In other words, three light emitting element group rows 295R in which a plurality of light emitting element groups 295 are arranged along the longitudinal direction LGD are arranged in the width direction LTD. At this time, the light emitting element group rows 295R are shifted from each other in the longitudinal direction LGD so that the light emitting element groups 295 do not overlap with each other in the longitudinal direction LGD. Here, reference numerals 295R_A, 295R_B, and 295R_C are attached to the three light emitting element group rows in order from the upstream side in the width direction LGD.

  In each light emitting element group 295, two light emitting element rows 2951R in which four light emitting elements 2951 are arranged along the longitudinal direction LGD are arranged in the width direction LTD (FIG. 10). At this time, the light emitting element rows 2951R are shifted from each other in the longitudinal direction LGD so that the light emitting elements 2951 do not overlap with each other in the longitudinal direction LGD. As a result, eight light emitting elements 2951 are arranged in a staggered manner. Also, as shown in FIG. 10, each light emitting element group 295 is arranged symmetrically with respect to the optical axis OA of the corresponding lens LS. That is, the eight light emitting elements 2951 constituting the light emitting element group 295 are arranged symmetrically with respect to the optical axis OA. Therefore, a light beam from the light emitting element 2951 relatively far from the optical axis OA can be imaged with little aberration.

  Corresponding to each light emitting element group row 295R_A, 295R_B, 295R_C, drive circuits DC_A (for each light emitting element group row 295R_A), DC_B (for each light emitting element group row 295R_B), DC_C (for each light emitting element group row 295R_C) These drive circuits DC_A and the like are configured by TFTs (Thin Film Transistors), for example (FIG. 9). Each drive circuit DC_A and the like is arranged on one side in the width direction LTD of the corresponding light emitting element group row 295R_A and the like, and is connected to the light emitting elements 2951 such as the light emitting element group row 295R_A by the wiring WL. When the drive circuit DC_A or the like gives a drive signal to each light emitting element 2951, each light emitting element 2951 emits light beams having the same wavelength. The light emitting surface of the light emitting element 2951 is a so-called perfect diffusion surface light source, and the light beam emitted from the light emitting surface follows Lambert's cosine law.

  The driving operation of the driving circuit DC is controlled based on the video data VD. That is, when the main controller MC receives the vertical request signal VREQ from the head controller HC, it generates video data VD for one page (FIG. 4). Each time the main controller MC receives the horizontal request signal HREQ from the head controller HC, the video data VD is transmitted to the head controller HC line by line. Then, the head controller HC controls the drive circuit DC based on the received video data VD. Next, a specific configuration for realizing these control operations will be described.

  In the first embodiment, the above-described signals, that is, the request signals VREQ and HREQ sent from the head controller HC to the main controller MC and the video data VD sent from the main controller MC to the head controller HC correspond to each color of YMCK. There are 4 pairs. In the following, the colors are distinguished by attaching a hyphen and a code representing the color to each signal as necessary. For example, the vertical request signal, horizontal request signal, and video data for yellow are represented as VREQ-Y, HREQ-Y, and VD-Y, respectively.

  FIG. 11 is a block diagram showing the configuration of the main controller. The main controller MC includes an image processing unit 51 that performs signal processing necessary for image data included in an image formation command given from an external device, and a main-side communication module 52. The image processing unit 51 is provided with a color conversion processing block 511 that develops RGB image data into CMYK image data corresponding to each toner color. Further, the image processing unit 51 is provided with image processing blocks 512Y (for yellow), 512M (for magenta), 512C (for cyan), and 512K (for black) corresponding to each toner color. Is processed as follows. That is, in the image processing block 512Y and the like, the image data is bitmap-developed in accordance with the resolution of the line head 29, and screen processing, gamma correction, etc. are performed on the data after the bitmap development, and the video data VD-Y and the like are generated. In this way, the image data is converted into information having a pixel as a minimum unit. Here, the pixel is a minimum unit constituting an image formed by the line head 29. This series of signal processing is executed for one page of image for each input of the vertical request signal VREQ-Y, and the generated video data VD-Y for each line is sequentially output to the main communication module 52.

  In the main communication module 52, the four colors of video data VD-Y, VD-M, VD-C, and VD-K output from the image processing unit 51 are time-division multiplexed and multiplexed video data VD is serially transmitted to the head controller HC via the differential output terminals TX + and TX−. On the other hand, the vertical request signals VREQ-Y, VREQ-M, VREQ-C, VREQ-K and horizontal request signals HREQ-Y, HREQ time-division multiplexed from the head controller HC via the differential input terminals RX +, RX-. -M, HREQ-C, and HREQ-K are input. These request signals VREQ and HREQ are developed in parallel, and vertical request signals VREQ (VREQ-Y, etc.) for each color are input to the corresponding color image processing block 512 (512Y, etc.).

  FIG. 12 is a block diagram showing the configuration of the head controller. The head controller HC includes a head side communication module 53 and a head control module 54. In the head side communication module 53, request signals of four colors output from the head control module 54, that is, vertical request signals VREQ-Y, VREQ-M, VREQ-C, VREQ-K and horizontal request signals HREQ-Y, HREQ. -M, HREQ-C, and HREQ-K are time-division multiplexed. The time-division multiplexed request signal is serially transmitted to the main controller MC via the differential output terminals TX + and TX−. On the other hand, video data VD-Y, VD-M, VD-C, and VD-K that are time-division multiplexed are input from the main controller MC via the differential input terminals RX + and RX-. These video data VD-Y and the like are developed in parallel and input to the corresponding color head control block 541Y and the like.

  In the head control module 54, four sets of head control blocks 541Y (for yellow), 514M (for magenta), 514C (for cyan), and 514K (for black) are provided corresponding to each color. The head control block 541Y etc. outputs each request signal VREQ-Y, HREQ-Y etc. to request the video data VD-Y etc., while the corresponding color line head based on the received video data VD-Y etc. 29 exposure operations are controlled.

  FIG. 13 is a block diagram showing the configuration of the head control block and the like in the first embodiment. Here, the Y head control block 541Y for yellow will be described, but the other blocks 541M, 541C, and 541K have the same structure. The Y head control block 541Y is provided with a request signal generator 542 that generates request signals VREQ-Y and HREQ-Y based on a synchronization signal Vsync supplied from the engine controller EC. The request signal generation unit 542 starts counting an internal timer when receiving the synchronization signal Vsync, and outputs a vertical request signal VREQ-Y indicating the head of the page when a predetermined waiting time has elapsed. Following the output of the vertical request signal VREQ-Y, the request signal generation unit 542 repeatedly outputs the horizontal request signals HREQ-Y corresponding to the number of lines constituting one page image at a predetermined interval. These request signals VREQ-Y and HREQ-Y are sent to the head-side communication module 53, and are time-division multiplexed together with request signals of other colors and transmitted to the main controller MC.

  The horizontal request signal HREQ-Y is also input to the divided HREQ signal generation unit 543, and the divided HREQ signal generation unit 543 generates the divided HREQ signal by multiplying the input request signal HREQ-Y by 16 times, for example. To do. The divided HREQ signal is input to the light emission order control unit 544, and the light emission order control unit 544 rearranges the video data VD-Y based on the divided HREQ signal. That is, as described later, each light emitting element group row 295R forms a spot group SG at a position shifted from each other in the sub-scanning direction SD by the sub-scanning spot group pitch Psgs (FIGS. 14, 15, etc.). Therefore, in order to form the spot latent images for one line side by side in the main scanning direction MD, the video data VD-Y is transferred to each drive circuit DC_A and the like in consideration of the difference in the formation position of the spot group SG. Need to send. Therefore, in the light emission order control unit 544, the video data VD-Y is distinguished into video data VD-Y_A, VD-Y_B, and VD-Y_C corresponding to the light emitting element group rows 295R_A, 295R_B, and 295R_C. The video data VD-Y_A, VD-Y_B, and VD-Y_C are rearranged in accordance with the difference in the formation position of the spot group SG in the group rows 295R_A, 295R_B, and 295R_C. In this way, the video data VD-Y received line by line from the top of the page is rearranged in the order of transmission to the drive circuit DC_A and the like. The spot latent image is a latent image formed on the surface of the photosensitive drum by the spot SP.

  The output buffer 545 supplies the video data VD-Y_A, VD-Y_B, and VD-Y_C to the driving circuits DC_A, DC_B, and DC_C through the data transfer lines. The output buffer 545 is constituted by a shift register, for example. Then, the drive circuit DC_A and the like drive and emit light from the light emitting element 2951 based on the video data VD-Y_A and the like supplied from the output buffer 545. At this time, drive light emission of the drive circuit DC_A and the like is performed in synchronization with a light emission timing Tu supplied from a light emission timing signal generation unit 546 described below.

  The divided HREQ signal is also input to the light emission timing signal generation unit 546, and the light emission timing signal generation unit 546 generates the light emission timing Tu based on the divided HREQ signal. The light emission timing signal generation unit 546 is connected to each drive circuit DC_A and the like via a light emission timing control line LTu, and each of the light emission timing control lines LTu is shared between the drive circuits. The light emission timing signal generation unit 546 gives the light emission timing Tu to each drive circuit DC_A and the like via the light emission timing control line LTu. Then, the drive circuit DC_A and the like drive and emit the light emitting elements 2951 such as the corresponding light emitting element group row 295R_A based on the video data VD-Y_A and the like supplied in advance at the light emission timing Tu. In this way, by controlling the driving light emission of the light emitting element 2951 at each light emission timing Tu, each spot SP can be formed on the pixel PX on the surface of the photosensitive drum. Therefore, the spot forming operation will be described below.

  FIG. 14 is a perspective view for explaining the spot forming operation, and FIG. 15 is a diagram showing spot groups formed on the surface of the photosensitive drum at the light emission timing Tu in the first embodiment. In FIG. 14, the description of the lens array 299 is omitted. Here, first, the relationship between the spot group SG and the pixel PX will be described, and then the spot formation at the light emission timing Tu will be described.

  As shown in FIG. 14, each light emitting element group 295 can form spot groups SG in different exposure regions ER in the main scanning direction MD. Here, the spot group SG is a set of a plurality of spots SP formed by the light emitting elements 2951 of the light emitting element group 295 emitting light simultaneously. In the first embodiment, the three light emitting element groups 295 capable of forming the spot group SG in the exposure region ER continuous in the main scanning direction MD are arranged so as to be shifted from each other in the width direction LTD. That is, for example, the three light emitting element groups 295_1, 295_2, 295_3 capable of forming the spot groups SG_1, SG2, SG3 in the exposure regions ER_1, ER_2, ER3 continuous in the main scanning direction MD are mutually connected in the width direction LTD. They are staggered. These three light emitting element groups 295 constitute a light emitting element group column 295C, and a plurality of light emitting element group columns 295C are arranged along the longitudinal direction LGD. As a result, as described in the description of FIG. 9, the three light emitting element group rows 295R_A, 295R_B, and 295R_C are arranged in the width direction LTD, and the light emitting element group rows 295R_A and the like are mutually connected in the sub scanning direction SD. Spot groups SG are formed at different positions.

  As shown by a broken line in FIG. 15, a plurality of pixels PX are virtually provided on the surface of the photosensitive drum 21, and one line of pixels PX arranged in the main scanning direction MD is sub-scanning. A plurality of lines are arranged in the direction SD. The pitch between adjacent pixels in the main scanning direction MD is the main scanning pixel pitch Rmd, and the pitch between adjacent pixels in the sub scanning direction SD is the sub scanning pixel pitch Rsd. In these figures, the main scanning resolution and the sub scanning resolution are both 600 dpi (dot per inch), and the main scanning pixel pitch Rmd and the sub scanning pixel pitch Rsd are equal to each other. Here, the resolution is the pixel density and represents the number of pixels per inch.

  By the way, the pixel pitch on the surface of the photosensitive drum can be obtained from, for example, the pixel pitch of the image formed on the paper. However, in the sub-scanning direction SD, the moving speed of the surface of the photosensitive drum and the conveyance speed of the paper may be slightly different. In this case, the sub-scanning pixel pitch is different between the surface of the photosensitive drum and the paper. Therefore, when obtaining the sub-scanning pixel pitch on the surface of the photosensitive drum from the image formed on the paper, the sub-scan obtained from the image on the paper is obtained by calculating the speed ratio between the moving speed of the surface of the photosensitive drum and the conveyance speed of the paper The pixel pitch may be multiplied. As the speed ratio, for example, a value described in the specification of an image forming apparatus such as a printer can be used.

  As shown in FIGS. 14 and 15, in each spot group SG, two spot rows SPRa and SPRb are arranged at a spot row pitch Pspr in the sub-scanning direction SD. In the first embodiment, the spot row pitch Pspr is set to an integral multiple (1 times) of the sub-scanning pixel pitch Rsd (FIG. 15). Further, the spot groups SG formed by the different light emitting element group rows 295R are at different positions in the sub-scanning direction SD, and the pitch in the sub-scanning direction SD between these spot groups SG is the sub-scanning spot group pitch. Psgs. The sub-scanning spot group pitch Psgs is set to an integral multiple (160 times) of the sub-scanning pixel pitch Rsd. In the first embodiment, the pitch Pegr between the light emitting element groups in the longitudinal direction LTD and the pitch Plsr between the lenses LS in the longitudinal direction LTD are equal to each other and are an integral multiple (160 times) of the sub-scanning pixel pitch. Thus, the line head 29 is configured. That is, by configuring the line head 29 in this way, the sub-scanning spot group pitch Psgs can be easily set to an integral multiple of the sub-scanning pixel pitch Rsd.

  As described above, in the line head of the first embodiment, since the sub-scanning spot group pitch Psgs is set to an integral multiple of the sub-scanning pixel pitch Rsd, all the spots SP are simultaneously applied to the pixels PX at the light emission timing Tu. It is possible to form. Therefore, in the first embodiment, the light emission timing Tu is shared among all the light emitting element groups 295. Therefore, the light emission of the light emitting elements 2951 forming each spot SP is switched at the timing Tu at which each spot SP comes to the position corresponding to the pixel PX (FIG. 15). Here, the switching of light emission means switching from non-light emission to light emission and switching from light emission to non-light emission. In the first embodiment, the light emission of the light emitting element 2951 is switched at such a light emission timing Tu, so that a spot SP is formed for each pixel PX, and a spot latent image can be formed on each pixel PX. It has become.

  FIG. 16 is a diagram showing an example of the spot latent image forming operation. This figure shows a state of latent image formation by one light emitting element group 295. As shown in the column of “first light emission timing Tu” in the figure, when the light emitting element 2951 is driven to emit light at the first light emission timing Tu and each spot SP of the spot rows SPRa and SPRb is formed, Spot latent images Lspa and Lspb are formed for the pixel PX. Here, the spot latent image Lspa is a latent image formed by the spot row SPRa, and the spot latent image Lspb is a latent image formed by the spot row SPRb. Next, at the second light emission timing Tu when the surface of the photosensitive drum 21 is moved by a distance corresponding to the sub-scanning pixel pitch Rsd, the light emitting element 2951 is driven to emit light, and the spots SP of the spot rows SPRa and SPRb are formed. The In this way, as shown in the column of “second light emission timing Tu”, a new spot latent image Lspa is formed downstream of each spot latent image formed at the first light emission timing Tu in the sub scanning direction SD by one pixel. , Lspb is formed. As described above, each light emitting element 2951 emits light in synchronization with the light emission timing Tu generated at a predetermined interval, whereby a spot latent image is formed on each pixel PX. Further, as can be seen from the same column, by executing this latent image forming operation, a line latent image LI composed of spot latent images Lspa and Lspb is formed. The line latent image LI has a width of 1 pixel in the sub-scanning direction SD and a length of 8 pixels in the main scanning direction MD.

  Incidentally, as described above, in the image forming apparatus of the present embodiment, the surface of the photosensitive drum 21 moves in the sub-scanning direction SD. The light emitting element group rows 295R_A, 295R_B, and 295R_C are exposed to different positions in the sub-scanning direction SD at a timing according to the movement of the surface of the photosensitive drum, thereby forming a latent image. However, the speed of the surface of the photosensitive drum may fluctuate due to an environmental change or the like inside the apparatus, and as a result, a latent image may not be formed satisfactorily.

  FIG. 17 is a diagram illustrating an example of a latent image formation defect that may occur in the image forming apparatus of the present embodiment. In the column of “head substrate 293” in the same figure, the light emitting element groups 295 arranged on the head substrate 293 are shown. Further, the column “surface of the photosensitive drum 21” in the figure shows the state of the latent image formed when the moving speed of the surface of the photosensitive drum varies. In the latent image forming operation shown in the example of FIG. 6, first, a plurality of line latent images LI_A are formed by the most upstream light emitting element group row 295R_A in the width direction LTD. Next, a plurality of line latent images LI_B are formed by the light emitting element group row 295R_B located downstream of the light emitting element group row 295R_A in the width direction LTD. Further, a plurality of line latent images LI_C are formed by the light emitting element group row 295R_C located downstream of the light emitting element group row 295R_C in the width direction LTD. By the way, in the example of the figure, the formation positions of the line latent images LI_A, LI_B, and LI_C are shifted in the sub-scanning direction SD (the column “surface of the photosensitive drum 21”). Such a formation position shift occurs between the formation of the line latent image LI_A and the formation of the line latent image LI_B, or between the formation of the line latent image LI_B and the formation of the line latent image LI_C. This occurs due to fluctuations in the moving speed of the body drum surface. Therefore, in the first embodiment, an encoder that detects the moving speed of the surface of the photosensitive drum is provided in order to suppress the shift of the latent image forming position.

  FIG. 18 is a perspective view showing the configuration of the encoder. FIG. 19 is a front view showing the configuration of the encoder, which corresponds to the case of viewing from the length direction of the rotation axis AR21 of the photosensitive drum 21. The rotation axis AR21 extends in a direction orthogonal or substantially orthogonal to the sub-scanning direction SD, and is parallel or substantially parallel to the main scanning direction MD. As described above, the line head 29 is disposed to face the surface SF21 of the photosensitive drum 21, and each light emitting element group row 295R of the line head 29 exposes different positions in the sub scanning direction SD. In FIG. 19, the exposure position of each light emitting element group row 295R is represented by positions PS_A, PS_B, PS_C, and the positions PS_A, PS_B, PS_C are exposed by the light emitting element group rows 295R_A, 295R_B, 295R_C, respectively. The

  The encoder ECD has a substantially disc-shaped encoder disk ED and a transmissive photosensor SC. The central portion of the encoder disk ED is attached to the rotation axis AR21 of the photosensitive drum 21, and the encoder disk ED rotates as the photosensitive drum 21 rotates. The encoder disk ED is provided with a plurality of slits SL radially about the rotation axis AX. The slit SL is detected by the photosensor SC. That is, the photosensor SC has a light emitting unit SCe and a light receiving unit SCr, and light is emitted from the light emitting unit SCe toward the light receiving unit SCr. The photosensor SC is arranged so that the encoder disk ED is sandwiched between the light emitting element SCe and the light receiving part SCr. Therefore, the light that has passed through the slit SL out of the light emitted from the light emitting unit SCe enters the light receiving unit SCr. On the other hand, the light receiving unit SCr that has detected the incident light outputs an encoder signal Senc. Therefore, the moving speed of the photosensitive drum surface SF21 can be detected by measuring the interval of the encoder signal Senc. The encoder signal Senc output from the light receiving unit SCr is supplied to the head controller HC via the engine controller EC (FIG. 13). The light emission timing signal generation unit 546 of the head controller HC adjusts the light emission timing Tu of the light emitting element 2951 based on the encoder signal Senc.

FIG. 20 is a diagram illustrating a light emission timing adjustment operation executed by the head controller in the first embodiment. In the figure, each of the encoder signal Senc and the light emission timing Tu is represented as a low level pulse. In the first embodiment, the light emission timing is adjusted based on the difference between the moving speed of the photosensitive drum surface SF21 and the ideal speed (reference speed). Here, the ideal speed is the moving speed of the photosensitive drum surface SF21 when there is no speed fluctuation. Specifically, the reference signal interval Ts (ref), which is the interval of the encoder signal Senc when the photosensitive drum surface SF21 is moving at the ideal speed, and the signal interval Ts (n) of the actually measured encoder signal Senc. The light emission timing Tu is adjusted based on the difference from (). In the figure, the signal interval Ts (1) between the first encoder signal Senc (1) and the second encoder signal Senc (2) is equal to the reference signal interval Ts (ref). Therefore, between the first encoder signal Senc (1) and the second encoder signal Senc (2), the interval of the emission timing Tu (emission interval Tt (1)) is set to the reference emission interval Tt (ref). The Next, when the signal interval Ts (2) between the second encoder signal Senc (2) and the third encoder signal Senc (3) is short, it is determined that the photosensitive drum surface speed has shifted to the higher side, and the light emission is performed. The interval Tt (2) is set short. Specifically, the following expression Tt (2) = (Ts (2) / Ts (ref)) × Tt (ref)
Based on the above, the light emission interval Tt (2) is set. That is, when generalized, the light emission interval Tt (n + 1) is expressed by the following equation Tt (n) with respect to the interval Ts (n) between the nth encoder signal Senc (n) and the (n + 1) th encoder signal Senc (n + 1). n + 1) = (Ts (n) / Ts (ref)) × Tt (ref)
Is set based on The light emission timing Tu is adjusted so that the light emission timing Tu has the light emission interval Tt (n + 1).

  Thus, in the first embodiment, the encoder ECD detects the moving speed of the surface SF21 (circumferential surface) of the photosensitive drum 21, and the light emission timing Tu of the light emitting element 2951 is adjusted based on the detection result. Therefore, the occurrence of the shift of the latent image forming position due to the fluctuation of the moving speed of the photosensitive drum surface SF21 is suppressed, and a good latent image can be formed.

  In the first embodiment, the head controller HC (control means) adjusts the light emission timing Tu of the light emitting element 2951 according to the difference between the moving speed of the photosensitive drum surface SF21 and the ideal speed (reference speed). . Therefore, since the light emission timing Tu of the light emitting element 2951 is adjusted according to the deviation of the moving speed of the photosensitive drum surface SF21 from the ideal speed, it is possible to effectively suppress the deviation of the latent image forming position. Good latent image formation is possible.

  In the image forming apparatus 1 according to the first embodiment, the photosensitive drum 21 that rotates about the rotation axis AR21 is used as the latent image carrier. It is particularly preferable to apply the present invention. This is because such a photosensitive drum 21 may have non-uniformity in the rotational speed about the rotation axis AR21. The non-uniformity in the rotational speed is caused by fluctuations in the moving speed of the photosensitive drum surface SF21 (circumferential surface). It is a cause. Therefore, it is preferable to apply the present invention to the image forming apparatus 1 so as to suppress the occurrence of the shift of the latent image forming position due to the fluctuation of the moving speed of the photosensitive drum surface SF21.

  In the first embodiment, an encoder ECD having an encoder disk ED provided with a plurality of slits SL arranged radially about the rotation axis AR21 of the photosensitive drum 21 and a photosensor SC for detecting the slits SL, The moving speed of the photosensitive drum surface SF21 is detected. That is, the detection of the moving speed of the photosensitive drum surface SF21 is performed based on the plurality of slits SL arranged radially around the rotation axis AR21 of the photosensitive drum 21. Therefore, it is possible to detect the moving speed of the photosensitive drum surface SF21 with high accuracy.

  In the first embodiment, since the sub-scanning spot group pitch Psgs is set to an integral multiple of the sub-scanning pixel pitch Rsd, the light emission of each light emitting element 2951 of each light emitting element group 295 is performed at a common light emission timing Tu. It is possible to control. Therefore, the light emission timing control of the light emitting element 2951 is simplified, and the image forming apparatus 1 of the first embodiment is suitable.

C. Second Embodiment In the first embodiment, the sub-scanning spot group pitch Psgs is set to an integral multiple of the sub-scanning pixel pitch Rsd, and each light emitting element group row 295R emits light at the same light emission timing Tu. However, the sub-scanning spot group pitch Psgs can be set to a non-integer multiple of the sub-scanning pixel pitch Rsd. In this case, the light emission timings Tu_A, Tu_B, and Tu_C are given to the respective light emitting element group rows 295R_A, 295R_B, and 295R_C. Such a case will be described below.

  FIG. 21 is a block diagram illustrating a configuration of a head control block and the like in the second embodiment. In the following, differences from the first embodiment will be mainly described, and common portions will be denoted by corresponding reference numerals and description thereof will be omitted. In the second embodiment, as shown in the figure, the light emission timing signal generator 546 has a plurality of light emission timings corresponding to the light emitting element group rows 295R_A, 295R_B, and 295R_C, that is, the light emission timing Tu_A (each light emitting element group row 295R_A). ), Light emission timing Tu_B (for each light emitting element group row 295R_B), and light emission timing Tu_C (for each light emitting element group row 295R_C). Each light emission timing Tu_A, Tu_B, Tu_C generates a light emission timing signal via a corresponding light emission timing control line LTu_A (for each light emission timing Tu_A), LTu_B (for each light emission timing Tu_B), LTu_C (for each light emission timing Tu_C). The signal is supplied from the unit 546 to each of the drive circuits DC_A, DC_B, and DC_C. Also in the second embodiment, the light emission timing signal generation unit 546 of the head controller HC uses each light emission timing Tu_A, based on the speed of the photosensitive drum surface SF21 in order to suppress the occurrence of the shift of the latent image forming position. Tu_B and Tu_C are adjusted.

  FIG. 22 is a diagram illustrating the light emission timing adjustment operation executed by the head controller in the second embodiment. In the figure, each of the encoder signal Senc and the light emission timing Tu is represented as a low level pulse. In the second embodiment, each of the light emission timings Tu_A, Tu_B, and Tu_C is adjusted based on the difference between the moving speed of the photosensitive drum surface SF21 and the ideal speed (reference speed). Since the adjustment operation of each light emission timing Tu_A, Tu_B, Tu_C is basically the same, the adjustment operation of the light emission timing Tu_A will be mainly described below, and the adjustment operation of other light emission timings Tu_B, Tu_C The description will be given the same reference numerals and the description will be omitted as appropriate.

As shown in the figure, the reference signal interval Ts (ref), which is the interval of the encoder signal Senc when the photosensitive drum surface SF21 is moving at the ideal speed, and the signal interval Ts of the actually measured encoder signal Senc. The light emission timing Tu_A is adjusted based on the difference from (n). In the figure, the signal interval Ts (1) between the first encoder signal Senc (1) and the second encoder signal Senc (2) is equal to the reference signal interval Ts (ref). Accordingly, between the first encoder signal Senc (1) and the second encoder signal Senc (2), the interval of the emission timing Tu_A (emission interval Tt_A (1)) is set to the reference emission interval Tt_A (ref). The Next, when the signal interval Ts (2) between the second encoder signal Senc (2) and the third encoder signal Senc (3) is short, it is determined that the photosensitive drum surface speed has shifted to the higher side, and the light emission is performed. The interval Tt_A (2) is set short. Specifically, the following formula: Tt_A (2) = (Ts (2) / Ts (ref)) × Tt_A (ref)
Based on the above, the light emission interval Tt (2) is set. That is, when generalized, the light emission interval Tt_A (n + 1) is expressed by the following equation Tt_A () with respect to the interval Ts (n) between the nth encoder signal Senc (n) and the (n + 1) th encoder signal Senc (n + 1). n + 1) = (Ts (n) / Ts (ref)) × Tt_A (ref)
Is set based on The light emission timing Tu_A is adjusted so that the light emission timing Tu_A is equal to the light emission interval Tt_A (n + 1).

Further, for the light emission intervals Tt_B (n + 1) and Tt_C (n + 1) of other light emission timings Tu_B and Tu_C, the same expression Tt_B (n + 1) = (Ts (n) / Ts (ref)) × Tt_B (ref)
Tt_C (n + 1) = (Ts (n) / Ts (ref)) × Tt_C (ref)
Is set based on

  Thus, in the second embodiment, the moving speed of the surface SF21 (circumferential surface) of the photosensitive drum 21 is detected by the encoder ECD, and the light emission timings Tu_A, Tu_B, Tu_C of the light emitting element 2951 are determined based on the detection result. Adjusted. Therefore, the occurrence of the shift of the latent image forming position due to the fluctuation of the moving speed of the photosensitive drum surface SF21 is suppressed, and a good latent image can be formed.

D. Third Embodiment In the first and second embodiments, the video data VD is exchanged between the main controller MC and the head controller HC by asynchronous serial communication. However, the communication mode of the video data VD is not limited to this.

  FIG. 23 is a diagram illustrating configurations of a main controller, a head controller, and a line head in the third embodiment. In the third embodiment, the head controller HC outputs a horizontal synchronization signal Hsync to the main controller MC to request the video data VD. On the other hand, the main controller MC outputs the video data VD to the head controller HC in synchronization with the horizontal synchronization signal Hsync. In the head controller HC that has received the video data VD, the video data VD_A (for the light emitting element group row 295R_A) and the video data VD_B (light emitting element group) corresponding to the light emitting element group rows 295R_A, 295R_B, and 295R_C of the line head 29, respectively. Row 295R_B) and video data VD_C (light emitting element group row 295R_C) are distinguished, and each video data VD_A and the like are output toward the line head 29.

  On the back surface of the head substrate 293 of the line head 29, a plurality of light emitting element group rows 295R in which a plurality of light emitting element groups 295 are arranged in the longitudinal direction LGD are formed side by side in three rows (295R_A, 295R_B, 295R_C). Further, on the back surface of the head substrate 293, three drive circuits DC_A (for the light emitting element group row 295R_A), DC_B (for the light emitting element group row 295R_B), DC_C are provided for each light emitting element group row 295R_A, 295R_B, 295R_C. (For the light emitting element group row 295R_C) is provided.

  In each light emitting element group 295, two light emitting element rows 2951R in which eight light emitting elements are arranged in the longitudinal direction LGD are arranged in the width direction LTD. Further, the light emitting elements 2951 in the respective light emitting element group rows 295R_A, 295R_B, and 295R_C are connected to the corresponding driving circuits DC_A, DC_B, and DC_C by the wiring WL. Therefore, each of the drive circuits DC_A, DC_B, and DC_C can drive the corresponding light emitting element group rows 295R_A, 295R_B, and 295R_C through the wiring WL. Thus, the light emitting element group 295 driven by the drive circuit DC_A or the like emits light, and a spot group SG is formed on the surface SF21 of the photosensitive drum 21.

  The light emitting operation of the light emitting element group 295 is controlled as follows. First, the video data VD_A, VD_B, and VD_C transferred from the head controller HC are temporarily held in the interface circuit I / F. When receiving the horizontal synchronization signal Hsync from the head controller HC, the interface circuit I / F outputs each video data VD_A and the like to the drive circuit DC_A and the like (FIG. 23). Then, the drive circuit VD_A and the like drive the light emitting element group 295 based on the received video data VD_A and the like to form the spot group SG. Thus, each light emitting element 2951 emits light in synchronization with the horizontal synchronization signal Hsync.

  In the third embodiment, similarly to the first embodiment, the sub-scanning spot group pitch Psgs is set to an integral multiple of the sub-scanning pixel pitch Rsd, and each light emitting element group row 295R emits light at the same timing. Thus, a spot latent image can be formed on each pixel PX. Therefore, the light emitting element group rows 295R_A, 295R_B, and 295R_C can emit light in synchronization with the common horizontal synchronization signal Hsync, thereby simplifying the control.

  By the way, also in the third embodiment, the light emitting element group rows 295R_A, 295R_B, and 295R_C are exposed at different positions in the sub-scanning direction SD at the timing according to the movement of the photosensitive drum surface SF21 to form a latent image. Is done. Therefore, the latent image may not be formed satisfactorily due to the speed fluctuation of the photosensitive drum surface SF21. Therefore, in the third embodiment, the speed of the photosensitive drum surface SF21 is detected by the encoder ECD, and the horizontal synchronization signal Hsync is adjusted based on the detection result.

FIG. 24 is a diagram illustrating a horizontal synchronization signal adjustment operation executed by the head controller in the third embodiment. In the figure, each of the encoder signal Senc and the horizontal synchronization signal Hsync is represented as a low level pulse. In the third embodiment, the horizontal synchronization signal is adjusted based on the difference between the moving speed of the photosensitive drum surface SF21 and the ideal speed (reference speed). That is, the reference signal interval Ts (ref), which is the interval of the encoder signal Senc when the photosensitive drum surface SF21 is moving at the ideal speed, and the signal interval Ts (n) of the encoder signal Senc that is actually measured. Based on the difference, the horizontal synchronization signal Hsync is adjusted. In the figure, the signal interval Ts (1) between the first encoder signal Senc (1) and the second encoder signal Senc (2) is equal to the reference signal interval Ts (ref). Therefore, between the first encoder signal Senc (1) and the second encoder signal Senc (2), the interval (synchronization interval Th (1)) of the horizontal synchronization signal Hsync is set to the reference synchronization interval Th (ref). Is done. Next, when the signal interval Ts (2) between the second encoder signal Senc (2) and the third encoder signal Senc (3) is short, it is determined that the photosensitive drum surface speed has shifted to the higher one, and synchronization is performed. The interval Th (2) is set short. Specifically, the following formula Th (2) = (Ts (2) / Ts (ref)) × Th (ref)
Based on the above, the synchronization interval Th (2) is set. That is, generally speaking, with respect to the interval Ts (n) between the nth encoder signal Senc (n) and the (n + 1) th encoder signal Senc (n + 1), the synchronization interval Th (n + 1) is expressed by the following equation Th ( n + 1) = (Ts (n) / Ts (ref)) × Th (ref)
Is set based on Then, the horizontal synchronization signal Hsync is adjusted so that the interval of the horizontal synchronization signal Hsync becomes the synchronization interval Th (n + 1). Thus, in the third embodiment, the light emission timing of the light emitting element 2951 is adjusted by adjusting the horizontal synchronization signal Hsync.

  As described above, in the third embodiment, the moving speed of the surface SF21 (circumferential surface) of the photosensitive drum 21 is detected by the encoder ECD, and the light emission timing of the light emitting element 2951 is adjusted based on the detection result. Therefore, the occurrence of the shift of the latent image forming position due to the fluctuation of the moving speed of the photosensitive drum surface SF21 is suppressed, and a good latent image can be formed.

E. Fourth Embodiment In the third embodiment, the sub-scanning spot group pitch Psgs is set to an integral multiple of the sub-scanning pixel pitch Rsd, and each light emitting element group row 295R emits light at the same light emission timing Tu. . However, the sub-scanning spot group pitch Psgs can be set to a non-integer multiple of the sub-scanning pixel pitch Rsd. In this case, horizontal synchronization signals Hsync_A, Hsync_B, and Hsync_C are given to the respective light emitting element group rows 295R_A, 295R_B, and 295R_C. Such a case will be described below.

  FIG. 25 is a diagram illustrating configurations of a main controller, a head controller, and a line head in the fourth embodiment. In the following description, differences from the third embodiment will be mainly described, and common portions are denoted by corresponding reference numerals and description thereof is omitted. In the fourth embodiment, the head controller HC requests three types of horizontal synchronization signals Hsync_A (for the light emitting element group row 295R_A), Hsync_B (light emitting element) to request video data corresponding to each light emitting element group row 295R_A, 295R_B, 295R_C. The element group row 295R_B) and Hsync_C (for the light emitting element group row 295R_C) are output to the main controller MC. In the main controller MC, when the horizontal synchronization signal Hsync_A is received, the video data VD_A is output to the head controller HC. When the horizontal synchronization signal Hsync_B is received, the video data VD_B is output to the head controller HC, and the horizontal synchronization signal Hsync_C is When received, the video data VD_C is output to the head controller HC. The head controller HC outputs each video data VD_A, VD_B, VD_C received from the main controller MC to the line head 29.

  In the line head 29, the video data VD_A, VD_B, and VD_C transferred from the head controller HC are temporarily held in the interface circuit I / F. The three types of horizontal synchronization signals Hsync_A, Hsync_B, and Hsync_C are input to the interface circuit I / F. In the interface circuit I / F, when the horizontal synchronization signal Hsync_A is received, the video data VD_A is output to the drive circuit DC_A, and when the horizontal synchronization signal Hsync_B is received, the video data VD_B is output to the drive circuit DC_B. When the synchronization signal Hsync_C is received, the video data VD_C is output to the drive circuit DC_C (FIG. 25). Thus, each light emitting element group row 295R_A and the like emit light in synchronization with the corresponding horizontal synchronization signal Hsync_A and the like.

  In the fourth embodiment, the light emitting element group rows 295R_A, 295R_B, and 295R_C are exposed at different positions in the sub-scanning direction SD at the timing according to the movement of the photosensitive drum surface SF21, thereby forming a latent image. Is done. Therefore, the latent image may not be formed satisfactorily due to the speed fluctuation of the photosensitive drum surface SF21. Therefore, also in the fourth embodiment, the speed of the photosensitive drum surface SF21 is detected by the encoder ECD, and the horizontal synchronization signal Hsync is adjusted based on the detection result.

  FIG. 26 is a diagram illustrating a horizontal synchronization signal adjustment operation performed by the head controller in the fourth embodiment. In the figure, each of the encoder signal Senc and the horizontal synchronization signals Hsync_A, Hsync_B, and Hsync_C is represented as a low level pulse. In the fourth embodiment, the horizontal synchronization signals Hsync_A, Hsync_B, and Hsync_C are adjusted based on the difference between the moving speed of the photosensitive drum surface SF21 and the ideal speed (reference speed). Since the adjustment operations of the horizontal synchronization signals Hsync_A, Hsync_B, and Hsync_C are basically the same, the adjustment operation of the horizontal synchronization signal Hsync_A will be mainly described below, and adjustment of the other horizontal synchronization signals Hsync_B and Hsync_C will be described. The explanation of the operation will be given the corresponding reference numerals and the explanation will be omitted as appropriate.

As shown in the figure, the reference signal interval Ts (ref), which is the interval of the encoder signal Senc when the photosensitive drum surface SF21 is moving at the ideal speed, and the signal interval Ts of the actually measured encoder signal Senc. Based on the difference from (n), the horizontal synchronization signal Hsync_A is adjusted. In the figure, the signal interval Ts (1) between the first encoder signal Senc (1) and the second encoder signal Senc (2) is equal to the reference signal interval Ts (ref). Therefore, between the first encoder signal Senc (1) and the second encoder signal Senc (2), the interval of the horizontal synchronization signal Hsync_A (synchronization interval Th_A (1)) is set to the reference synchronization interval Th_A (ref). Is done. Next, when the signal interval Ts (2) between the second encoder signal Senc (2) and the third encoder signal Senc (3) is short, it is determined that the speed of the photosensitive drum surface SF21 has shifted to the higher one, The synchronization interval Th_A (2) is set short. Specifically, the following formula Th_A (2) = (Ts (2) / Ts (ref)) × Th_A (ref)
Based on the above, the synchronization interval Th (2) is set. That is, generally speaking, with respect to the interval Ts (n) between the n-th encoder signal Senc (n) and the (n + 1) -th encoder signal Senc (n + 1), the synchronization interval Th_A (n + 1) is expressed by the following equation Th_A ( n + 1) = (Ts (n) / Ts (ref)) × Th_A (ref)
Is set based on Then, the horizontal synchronization signal Hsync_A is adjusted so that the interval of the horizontal synchronization signal Hsync_A becomes the synchronization interval Th_A (n + 1).

The same formula Th_B (n + 1) = (Ts (n) / Ts (ref)) is also applied to the synchronization intervals Th_B (n + 1) and Th_C (n + 1) of the other horizontal synchronization signals Hsync_B and Hsync_C. × Th_B (ref)
Th_C (n + 1) = (Ts (n) / Ts (ref)) × Th_C (ref)
Is set based on Thus, in the fourth embodiment, the light emission timing of the light emitting element 2951 is adjusted by adjusting the horizontal synchronization signals Hsync_A, Hsync_B, and Hsync_C.

  As described above, in the fourth embodiment, the moving speed of the surface SF21 (circumferential surface) of the photosensitive drum 21 is detected by the encoder ECD, and the light emission timing of the light emitting element 2951 is adjusted based on the detection result. Therefore, the occurrence of the shift of the latent image forming position due to the fluctuation of the moving speed of the photosensitive drum surface SF21 is suppressed, and a good latent image can be formed.

F. Fifth Embodiment By the way, the image forming apparatus sequentially forms one line latent image having a length of one page in the main scanning direction MD in the sub-scanning direction SD, thereby generating a two-dimensional latent image for one page. Can be formed. Therefore, in the fifth embodiment, this latent image forming operation will be described. In the following, in order to facilitate understanding of the latent image forming operation, it is formed at a predetermined position (position indicated by an arrow FL in FIGS. 27, 28, and 29) on the circumferential surface of the photosensitive drum 21. The description will be made by paying attention to one line latent image.

  27, 28, and 29 are explanatory diagrams of the light emission timing control of the light emitting element in the latent image forming operation, and correspond to the case where the latent image is sequentially formed by the light emitting element when viewed in plan. In the figure, a white circle surrounded by a dotted line represents a light emitting element 2951 that is turned off, a white circle surrounded by a solid line represents a light emitting element 2951 that is turned on, and a hatched circle represents a spot latent area. Represents the image Lspa. In the light emitting element group 295, a reference numeral 2951Ra is assigned to a light emitting element row on the upstream side in the sub scanning direction SD, and a reference numeral 2951Rb is assigned to the light emitting element row on the downstream side in the sub scanning direction SD.

  In this embodiment, the light emitting element row pitch (light emitting element row distance) Pelr = 0.1 mm, the lens row pitch (lens row distance) Plsr = 1 mm, and the photosensitive member peripheral speed = 100 mm / s. The spot group pitch (spot group distance) Psgs is an integral multiple of the sub-scanning pixel pitch (sub-scanning pixel distance) Rsd, while the spot row pitch (spot-line distance) Pspr is the sub-scanning pixel pitch (sub-scanning pixel). (Distance) Rsd is a non-integer multiple (not shown). In order to facilitate understanding of the light emission timing control, in this embodiment, the lens LS1 is erect and has an imaging magnification of 1 ×.

  As the peripheral surface of the photosensitive drum 21 (the surface of the latent image carrier) moves, the predetermined position FL where the line latent image is formed moves in the sub-scanning direction SD. As will be described below, each light emitting element 2951 emits light at a timing when the predetermined position FL comes to a position where each light emitting element 2951 can form the spot SP. Thereby, a line latent image can be formed at a predetermined position. Specifically, it is as follows.

  As shown in FIG. 27, at time T1 (= 0 ms), the light emitting element row 2951Ra that emits the light imaged by the lens LS1 at the uppermost stream in the sub-scanning direction SD emits light and performs main scanning at a predetermined position FL. A plurality of spot latent images Lspa arranged in a straight line in the direction MD are formed. Further, at time T1 + ΔT (= 1 ms) after the time ΔT (= 1 ms) has elapsed, the light emitting element row 2951Rb that emits the light imaged by the lens LS1 emits light, and is aligned in the main scanning direction MD at the predetermined position FL. A spot latent image Lspb is formed. Thus, by shifting the light emission of the light emitting element rows 2951Ra and 2951Rb by time ΔT, a plurality of spot latent images Lspb formed by the light emitting element rows 2951Ra and a plurality of spot latent images Lspb formed by the 2951Rb are predetermined. Lined up linearly in the main scanning direction MD at the position FL.

  Next, as shown in FIG. 28, after the light emitting element row 2951Ra emitting the light imaged by the lens LS2 emits light at time T2 (= 10 ms), the lens LS2 at time T2 + ΔT (= 11 ms). The light emitting element row 2951Rb that emits the light to be imaged emits light. Thus, a plurality of spot latent images Lspa and Lspb arranged linearly in the main scanning direction MD at the predetermined position FL are formed by the lens LS2. In addition, the time difference between the light emission of the light emitting element row 2951Ra (2951Rb) that emits the light imaged by the lens LS1 and the light emission of the light emitting element row 2951Ra (2951Rb) that emits the light imaged by the lens LS2. T2-T1). Accordingly, the plurality of spot latent images Lspa and Lspb formed by the lens LS1 and the plurality of spot latent images Lspa and Lspb formed by the lens LS2 are linearly arranged in the main scanning direction MD at the predetermined position FL.

  Further, as shown in FIG. 29, after the light emitting element row 2951Ra emitting the light imaged by the lens LS3 emits light at time T3 (= 20 ms), the light is connected by the lens LS3 at time T3 + ΔT (= 21 ms). The light emitting element row 2951Rb emitting the imaged light emits light. Thus, a plurality of spot latent images Lspa and Lspb arranged in a straight line in the main scanning direction MD are formed by the lens LS3. Further, the time difference between the light emission of the light emitting element row 2951Ra (2951Rb) that emits the light imaged by the lens LS2 and the light emission of light emitting element row 2951Ra (2951Rb) that emits the light imaged by the lens LS3 ( T3-T2). Accordingly, the plurality of spot latent images Lspa and Lspb formed by the lenses LS1 and LS2 and the plurality of spot latent images Lspa and Lspb formed by the lens LS3 are linearly arranged in the main scanning direction MD. Thus, one line latent image (one page length line latent image) having a length of one page in the main scanning direction MD is formed.

  As shown in FIG. 28, the spot group pitch Psgs is an integral multiple of the sub-scanning pixel pitch Rsd. The element row 2951Ra (2951Rb) also emits light. The light emitting element row 2951Ra (2951Rb) corresponding to the lens LS1 forms a latent image other than the predetermined position FL. Similarly in FIG. 29, the light emitting element rows 2951Ra (2951Rb) corresponding to the lenses LS1, LS2, and LS3 emit light (2951Rb) simultaneously. On the other hand, since the spot row pitch Pspr is a non-integer multiple of the sub-scanning pixel pitch Rsd, the light emitting element row 2951Ra and the light emitting element row 2951Rb do not emit light simultaneously.

  By the way, fluctuations in the peripheral speed of the photosensitive drum 21 may cause rattling of the one-page length line latent image. For example, the speed of the photosensitive drum 21 fluctuates between time 0 ms and time 10 ms, and as a result, the predetermined position FL may pass or not reach the spot formation position of the lens LS2 at time 10 ms. (FIGS. 27 and 28). For this reason, the spot latent images Lspa and Lspb formed by the lens LS1 and the spot latent images Lspa and Lspb formed by the lens LS2 may shift in the sub-scanning direction SD.

  Therefore, in this embodiment, the encoder ECD as shown in FIGS. 18 and 19 detects the position of the circumferential surface of the photosensitive drum 21 (detection step), and the head control block 541Y and the like are based on the detection result of the encoder ECD. The light emission timing of the light emitting element 2951 is adjusted (latent image forming step). Specifically, the position FL on the surface of the photosensitive drum 21 is a position (first position) where the light emitting element 2951 that emits light imaged by the lens LS1 (first imaging optical system) forms the spot SP. ) Until the light emitting element 2951 that emits the light imaged by the lens LS2 (second imaging optical system) moves to the position (second position) where the spot SP is formed (time difference (T2−T1)). Is equivalent). Based on this detection result, the time from when the light emitting element 2951 that emits the light imaged by the lens LS1 emits light until when the light emitting element 2951 that emits the light imaged by the lens LS2 emits light is emitted. Control. Accordingly, the plurality of spot latent images Lspa and Lspb formed by the lens LS1 and the plurality of spot latent images Lspa and Lspb formed by the lens LS2 are linearly arranged in the main scanning direction MD at the predetermined position FL. Similarly, the position FL on the surface of the photosensitive drum 21 is a position (first position) where the light emitting element 2951 that emits light imaged by the lens LS2 (first imaging optical system) forms a spot SP. ) To the position (second position) where the light emitting element 2951 that emits the light imaged by the lens LS3 (second imaging optical system) moves to the position (second position) where the spot SP is formed (time difference (T3−T2)) Is equivalent). Based on this detection result, the time from when the light emitting element 2951 that emits the light imaged by the lens LS2 emits light until when the light emitting element 2951 that emits the light imaged by the lens LS3 emits light is emitted. Control. As a result, the plurality of spot latent images Lspa and Lspb formed by the lens LS2 and the plurality of spot latent images Lspa and Lspb formed by the lens LS3 are linearly arranged in the main scanning direction MD at the predetermined position FL. In this way, it is possible to form a one-page length line latent image without shakiness regardless of the peripheral speed fluctuation of the photosensitive drum 21, and a good latent image forming operation can be executed.

  In the fifth embodiment, since the time difference ΔT between the light emission of the light emitting element row 2951Ra and the light emission of the light emitting element row 2951Rb belonging to the same light emitting element group 295 is sufficiently short, the speed of the photoconductive drum 21 between the time differences ΔT. Assuming that the fluctuation does not significantly affect the latent image forming position, the time difference ΔT is not adjusted based on the position of the photosensitive drum 21. However, when the speed fluctuation of the photosensitive drum 21 during the time difference ΔT has a non-negligible influence on the latent image formation position, the time difference ΔT may be adjusted based on the position of the photosensitive drum 21. In other words, the time from the light emission of the light emitting element 2951Ra (first light emitting element) in the light emitting element row 2951Ra to the light emission of the light emitting element 2951 (third light emitting element) in the light emitting element row 2951Rb may be adjusted. Thereby, the latent image by the light emitting element row 2951Ra and the latent image by the light emitting element row 2951Rb are linearly arranged in the main scanning direction MD at the predetermined position FL.

G. Sixth Embodiment FIG. 30 is a view of a photosensitive drum and a line head in a sixth embodiment as viewed from the longitudinal direction (main scanning direction), and the line head 29 is partially shown in cross section. FIG. 31 is a diagram of the photosensitive drum and the line head in the sixth embodiment as viewed from the width direction (sub-scanning direction). In the present embodiment, the photoconductor drum 21 is provided with a gear GR. The gear GR has a substantially disk shape, and a plurality of teeth GRt are arranged at a predetermined pitch on the peripheral surface. The center of the gear GR is attached to the rotation shaft of the photosensitive drum 21, and a plurality of teeth GRt are arranged radially with respect to the rotation center of the photosensitive drum 21. Further, as shown in FIG. 31, the gear GR is integrally connected to the photosensitive drum 21 without using a coupling or the like. Then, the engine unit EG (drive source) gives a driving force to the gear GR. Specifically, this driving force is transmitted via another gear (not shown) that engages with the tooth GRt. Thus, the photosensitive drum 21 rotates. As described above, the photosensitive drum 21 is rotated by the external force acting on the gear GR, and the circumferential surface of the photosensitive drum 21 moves in the sub-scanning direction SD.

  In such a configuration in which the photosensitive drum 21 is driven by the gear GR, the peripheral speed of the photosensitive drum 21 may fluctuate due to variations in the pitch of the teeth GRt. Therefore, for such a configuration, the light emission of the light emitting element 2951 that irradiates the light imaged on the first lens LS (for example, the lens LS1) based on the result of detecting the position of the photosensitive drum 21. Time difference (for example, time difference (T2−T1)) from the light emitting element 2951 that irradiates the light imaged on the second lens LS (for example, the lens LS2) to the peripheral surface of the photosensitive drum 21. It is preferable to adjust based on the position.

H. Seventh Embodiment By the way, there is periodicity in the speed fluctuation of the circumferential surface of the photosensitive drum 21 generated by the configuration using the gear GR as described above. This speed fluctuation period is easily obtained from the reciprocal of the value obtained by multiplying the number of rotations of the photosensitive drum 21 per unit time by the number of teeth GRt of the gear GR (the number of teeth). Fluctuation period [s] = 60 / (CT21 × NT)
Given in. Here, the value CT21 is the number of rotations [rpm] per unit time of the photosensitive drum 21, and NT is the number of teeth of the gear GR. The lens row pitch Plsr, that is, the distance (inter-lens distance) Plsr in the width direction LTD between the lenses LS1 and LS2, or the distance Plsr in the width direction LTD between the lenses LS2 and LS3 is the photosensitive drum during the speed variation period. It is preferable that the value is longer than the value obtained by multiplying the average value of the peripheral speeds (the distance of the photosensitive drum 21). Specifically, when CT21 = 60 [rpm] and the number of teeth = 40, the speed fluctuation period = 0.025 [s]. Accordingly, when the average of the photosensitive drum peripheral speed is 94.2 [mm / s], a value obtained by multiplying the speed fluctuation period by the average of the photosensitive drum peripheral speed is 2.355 [mm]. Therefore, the inter-lens distance Plsr is preferably 2.36 [mm] or more. In particular, the inter-lens distance Plsr is preferably set to 25 to 100 times the value obtained by multiplying the speed fluctuation cycle by the average of the photosensitive drum peripheral speed.

I. Others As described above, in the above embodiment, the main scanning direction MD and the longitudinal direction LGD correspond to the “first direction” of the present invention, and the sub-scanning direction SD and the width direction LTD correspond to the “second direction” of the present invention. The lens LS corresponds to the “imaging optical system” of the present invention, the photosensitive drum 21 corresponds to the “latent image carrier” of the present invention, and the surface SF21 (circumferential surface) of the photosensitive drum 21 is the book. This corresponds to the “surface of the latent image carrier” of the invention. The head controller HC corresponds to the “control unit” of the present invention, the encoder ECD corresponds to the “detection unit” of the present invention, and the photosensor SC corresponds to the “optical sensor” of the present invention. The relationship between the lens LS1 and the lens LS2 (or the lens LS2 and the lens LS3) corresponds to the relationship between the “first imaging optical system” and the “second imaging optical system” of the present invention. . The line head 29 corresponds to the “exposure head” of the present invention.

  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 the encoder ECD of the above embodiment, one photosensor SC is provided (FIG. 19). However, the number of photosensors SC is not limited to this, and may be two or three or more. Therefore, for example, an encoder ECD can be configured as shown below.

  FIG. 32 is a diagram illustrating another configuration of the encoder. In the following description, differences from the encoder shown in FIG. 19 will be mainly described, and common parts will be denoted by corresponding reference numerals and description thereof will be omitted. In the encoder ECD shown in the figure, two photosensors SC1 and SC2 are provided. These two photosensors SC1 and SC2 are arranged so as to sandwich the rotation axis AR21 of the photosensitive drum 21 in the width direction LTD. In other words, the two photosensors SC1 and SC2 are arranged so as to be approximately two-fold symmetric about the rotation axis AR21. And the light-receiving part (illustration omitted) of each photosensor SC1, SC2 detects the light which passed slit SL, and the detection signal of each photosensor SC1, SC2 is obtained. The sum of the detection signals of the photosensors SC1 and SC2 thus obtained is output as an encoder signal Senc to the engine controller EC.

  As described above, in the encoder ECD shown in FIG. 32, the two photosensors SC1 and SC2 are disposed so as to sandwich the rotation axis AR21 of the photosensitive drum 21 in the width direction LTD (in the radial direction of the photosensitive drum 21). The sum of the detection signals of the photosensors SC1, SC2 is output to the engine controller EC as an encoder signal Senc. Therefore, for example, even when the encoder ECD is eccentrically attached to the rotation shaft AR21 of the photosensitive drum 21, the eccentric component can be canceled by taking the sum of the detection signals of the photosensors SC1 and SC2. Is possible. Therefore, the encoder signal Senc can be stably obtained, and the speed of the photosensitive drum surface can be detected with high accuracy.

  In the above embodiment, the light emission timing of the light emitting element is adjusted based on the difference between the moving speed of the photosensitive drum surface and the ideal speed. However, the light emission timing of the light emitting element may be adjusted based on the difference between the moving speed of the surface of the photosensitive drum and the average value of the moving speed. Specifically, for example, the adjustment operation as shown in FIG. 20 is performed by setting the interval of the encoder signal Senc when the photosensitive drum surface SF21 is moving at the average speed as the reference signal interval Ts (ref). it can. In such a configuration, the light emission timing of the light emitting element 2951 is adjusted according to the deviation from the average moving speed of the surface of the photosensitive drum, so that the deviation of the latent image forming position can be effectively suppressed. Thus, a good latent image can be formed.

  In the above embodiment, the main scanning resolution and the sub-scanning resolution are both 600 dpi, but each resolution is not limited to 600 dpi. In particular, regarding the sub-scanning resolution, a resolution of 600 dpi or more can be realized relatively easily by subdividing the light emission time of the light emitting element 2951 by so-called pulse width modulation control called PWM (Pulse Width Modulation) control. Therefore, for example, the sub-scanning resolution may be increased by setting the sub-scanning resolution to 2400 dpi while the main-scanning resolution is set to 600 dpi. At this time, since the sub-scanning resolution is four times the main scanning resolution, the sub-scanning pixel pitch Rsd is a quarter of the main scanning pixel pitch Rmd.

  In the first to fourth embodiments, the light emitting element group column 295C is configured by the three light emitting element groups 295, but the number of the light emitting element groups 295 constituting the light emitting element group column 295C is not limited thereto. Two or more is sufficient.

  In the first to fourth embodiments, the spot group SG is composed of two spot rows SPR, but the number of spot rows SPR constituting the spot group SG is not limited to this, and may be one. Or three or more lines.

  In the first and second embodiments, the spot row SPR is composed of four spots SP, but the number of spots SP constituting the spot row SPR is not limited to this.

  In the first to fourth embodiments, the light emitting element group 295 is composed of the two light emitting element rows 2951R. However, the number of the light emitting element rows 2951R constituting the light emitting element group 295 is not limited thereto. .

  In the sixth embodiment, the gear GR is integrally connected to the photosensitive drum 21. However, it can also be configured as shown in FIG. FIG. 33 is a diagram showing another connection mode between the gear and the photosensitive drum. In the figure, the gear GR is connected to the photosensitive drum 21 via a coupling CPL.

  Next, examples of the present invention will be shown. However, the present invention is not limited by the following examples as a matter of course, and it is of course possible to implement the present invention with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. They are all included in the technical scope of the present invention.

  FIG. 34 is a diagram showing an example of the present invention and showing the effectiveness of the present invention. First, the column of “Latent image carrier speed unevenness” in the upper part of FIG. The graph described in the same column shows the speed unevenness of the moving speed of the surface of the latent image carrier. That is, in this graph, the horizontal axis represents time [s], and the vertical axis represents the speed unevenness on the surface of the latent image carrier. Note that this speed unevenness is expressed as a deviation from the average value of the moving speed. As can be seen from this graph, in this example, the moving speed of the surface of the latent image carrier has a speed variation of ± 2% or more.

  Next, the column of “latent image forming position (before correction control)” in the middle of FIG. 34 will be described. The graph described in the same column shows the latent image formation position when the latent image is formed without executing the correction control in the state where the speed unevenness as described above occurs. Here, the correction control is control for adjusting the light emission timing as described in the first to fourth embodiments. That is, in this graph, the horizontal axis represents the image feed direction position [mm] (corresponding to the position in the sub-scanning direction SD), and the vertical axis represents the position error [μm]. Here, the position error corresponds to a shift between the position where the latent image should be originally formed and the position where the latent image was actually formed. As can be seen from this graph, a position error of about 30 [μm] at maximum occurs in the state without correction control.

  Next, the column of “latent image forming position (after correction control)” in the lower part of FIG. 34 will be described. The graph described in the same column shows the latent image formation position when the latent image is formed while executing the correction control in the state where the speed unevenness as described above occurs. That is, in this graph, the horizontal axis represents the image feed direction position [mm], and the vertical axis represents the position error [μm]. As can be seen from this graph, the position error is suppressed to a few [μm] by executing the correction control. As described above, in this embodiment, it is found that the latent image can be formed satisfactorily by suppressing the influence of the speed unevenness on the surface of the latent image carrier by forming the latent image while executing the correction control. It was.

Explanatory drawing of the term used by this specification. Explanatory drawing of the term used by this specification. 1 is a diagram illustrating an example of an image forming apparatus according to the present invention. FIG. 4 is a diagram illustrating an electrical configuration of the image forming apparatus in FIG. 3. The perspective view which shows the outline of the line head concerning this invention. FIG. 6 is a cross-sectional view in the width direction of the line head shown in FIG. 5. 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 the structure of the back surface of a head board | substrate. The figure which shows arrangement | positioning of the light emitting element in each light emitting element group. The block diagram which shows the structure of a main controller. The block diagram which shows the structure of a head controller. FIG. 2 is a block diagram showing a configuration of a head control block and the like in the first embodiment. The perspective view for demonstrating spot formation operation | movement. FIG. 3 is a diagram showing spot groups formed on the surface of the photosensitive drum in the first embodiment. The figure which shows an example of a spot latent image formation operation | movement. FIG. 3 is a diagram illustrating an example of a latent image formation defect that may occur in the image forming apparatus according to the present embodiment. The perspective view which shows the structure of an encoder. The front view which shows the structure of an encoder. The figure which shows the adjustment operation of the light emission timing in 1st Embodiment. FIG. 6 is a block diagram showing a configuration of a head control block and the like in the second embodiment. The figure which shows the adjustment operation of the light emission timing in 2nd Embodiment. The figure which shows structures, such as a main controller, in 3rd Embodiment. The figure which shows the adjustment operation of the horizontal synchronizing signal in 3rd Embodiment. The figure which shows the structures of the main controller etc. in 4th Embodiment. The figure which shows the adjustment operation of the horizontal synchronizing signal in 4th Embodiment. Explanatory drawing of the light emission timing control of the light emitting element in latent image formation operation | movement. Explanatory drawing of the light emission timing control of the light emitting element in latent image formation operation | movement. Explanatory drawing of the light emission timing control of the light emitting element in latent image formation operation | movement. The figure which shows the photosensitive drum and line head in 6th Embodiment. The figure which shows the photosensitive drum and line head in 6th Embodiment. The figure which shows the other structure of an encoder. The figure which shows another connection aspect of a gearwheel and a photoreceptor drum. The figure which shows the Example of this invention.

Explanation of symbols

  21, 21Y, 21K ... photosensitive drum (latent image carrier), SF21 ... photosensitive drum surface (circumferential surface), AR21 ... rotating shaft, 29 ... line head, 293 ... head substrate, 295 ... light emitting element group, 295C ... Light emitting element group row, 2951 ... Light emitting element, 299 ... Lens array, LS ... Lens (imaging optical system), SP ... Spot, SG ... Spot group, MD ... Main scanning direction (first direction), SD ... Sub scanning Direction (second direction), LGD ... longitudinal direction (first direction), LTD ... width direction (second direction), ECD ... encoder (detection unit), ED ... encoder disk, SL ... slit, SC ... photo Sensor (optical sensor), MC ... main controller, HC ... head controller (control unit)

Claims (15)

A light emitting element disposed in a first direction, a first imaging optical system that forms an image of light emitted from the light emitting element, and a second direction with respect to the first imaging optical system An exposure head having a second imaging optical system,
A latent image carrier that moves in the second direction;
A detection unit for detecting a moving time of the latent image carrier that moves from the first position in the second direction to the second position;
From the first light emitting element that emits the light imaged by the first imaging optical system until the second light emitting element that emits the light imaged by the imaging optical system emits light Is controlled based on the detection result of the detection unit, and the latent image formed on the latent image carrier by the first imaging optical system and the latent image by the second imaging optical system. A control unit that arranges a latent image formed on the carrier in the first direction;
An image forming apparatus comprising:   The controller emits light imaged by the first imaging optical system after the first light emitting element emitting light imaged by the first imaging optical system emits light. And the time until the third light emitting element arranged on the second direction side of the first light emitting element emits light is controlled based on the detection result of the detection unit, and the first light emitting element is controlled. The image forming apparatus according to claim 1, wherein a latent image formed on the latent image carrier by the third light emitting element and a latent image formed on the latent image carrier by the third light emitting element are arranged in the first direction. .   The image forming apparatus according to claim 1, wherein the latent image carrier is a photosensitive drum that rotates in the second direction.   The image forming apparatus according to claim 3, further comprising: a driving source; and a gear that transmits a driving force from the driving source to the photosensitive drum.   The distance between the first imaging optical system and the second imaging optical system is greater than the distance between the photosensitive drums, which is obtained by multiplying the period of speed fluctuation of the photosensitive drums by the average speed of the photosensitive drums. The image forming apparatus according to claim 4, wherein the distance in the second direction is long.   The image forming apparatus according to claim 5, wherein the period of the speed fluctuation is obtained from an inverse number of a value obtained by multiplying the number of rotations of the photosensitive drum per unit time by the number of teeth of the gear.   The image forming apparatus according to claim 4, wherein the gear is connected to the photosensitive drum via a coupling.   The image forming apparatus according to claim 4, wherein the gear and the photosensitive drum are integrally connected.   9. The image formation according to claim 3, wherein the detection unit includes an encoder disk having slits arranged radially from a rotation center of the photosensitive drum, and an optical sensor for detecting the slit. apparatus.   The image forming apparatus according to claim 9, wherein the detection unit includes two optical sensors arranged in a radial direction of the photosensitive drum sandwiching a rotation center of the photosensitive drum.   The distance in the second direction of the latent image carrier between the light imaged by the first image forming optical system and the light imaged by the second image forming optical system is the first distance in the pixel. The image forming apparatus according to claim 1, which is an integral multiple of the distance between the pixels in two directions.   The image forming apparatus according to claim 11, wherein a distance between pixels in the second direction is shorter than a distance between pixels in the first direction in the pixels.   The image forming apparatus according to claim 12, wherein the control unit controls light emission of the light emitting element by PWM control.   A latent image carrier, an exposure head including a light emitting element and an imaging optical system that forms an image of light from the light emitting element on the latent image carrier, a detection unit that detects a position of the latent image carrier, An image forming apparatus comprising: a control unit configured to control light emission of the light emitting element based on a detection result of the detection unit to form a latent image on the latent image carrier. A detection step of detecting a time for moving the latent image carrier from the first position to the second position;
After the first light emitting element that emits the light imaged by the first imaging optical system emits light, the second light emitting element that emits the light imaged by the second imaging optical system emits light. A control step for controlling the time until the detection based on the detection result of the detection step;
An image forming method comprising:

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