JP2011257767A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color image forming apparatus which is inexpensive and compact, and gives high image quality with color shift in the sub-scanning direction smaller than or equal to (n-1)/n pixel.SOLUTION: An image forming apparatus comprises detection means which detects the deviation amount in a prescribed direction in an image carrier and controlling means which changes correspondence between a mirror surface deflecting a light beam for forming a reference toner image and a mirror surface deflecting a light beam for forming other toner images by controlling emission timing of the light beam so that the deviation amount for the reference toner image becomes 1/2 pixel or less, and which changes correspondence between the mirror surface deflecting the light beam for forming the reference toner image and a mirror surface deflecting at least one light beam by controlling emission timing of the light beam so that the deviation amount of toner images of two colors mostly remote from each other, among toner images transferred from n photoreceptors becomes (n-1)/n pixel or less.

Description

  The present invention relates to an electrophotographic image forming technique.

  A traditional tandem type color image forming apparatus forms a toner image on a photoreceptor provided for each color of yellow, magenta, cyan, and black, and then superimposes each toner image on an intermediate transfer belt. Later, batch transfer onto the recording medium is performed. Also, in such an image forming apparatus, a laser and a rotating polygon mirror (polygon mirror) are provided for each photoconductor. For example, when four toners having different colors are used, four photoreceptors and four rotating polygon mirrors are prepared.

  By the way, each toner image is caused by an irradiation position shift for each color due to a mechanical attachment error between each photoconductor and the scanning optical device or a mechanical attachment error of each optical component, and driving unevenness of the photoconductor or intermediate transfer belt. Color misregistration may occur when images are superimposed.

  Conventionally, in a plurality of scanning optical devices provided according to a plurality of photosensitive members, by accurately adjusting the rotational phase of each rotary polygon mirror according to the phase difference of the beam detection signal of each scanning optical device, An apparatus that reduces color misregistration in the sub-scanning direction has been proposed (Patent Document 1).

JP 2000-141750 A

  By the way, a conventional image forming apparatus provided with the same number of scanning optical devices as a plurality of photosensitive members is suddenly expensive and large. In order to further reduce the cost and size, a scanning optical device that shares a single rotating polygonal mirror with a plurality of light sources is advantageous. In this scanning optical apparatus, laser light from a plurality of light sources is simultaneously deflected and scanned by a single rotating polygon mirror, and exposed to a plurality of photoconductors.

  However, in this optical scanning device, the technique described in Patent Document 1 cannot be applied for color misregistration adjustment. This is because, since one rotating polygon mirror is shared, it is impossible in principle to adjust the rotation phase of the rotating polygon mirror for each color. Therefore, color misregistration adjustment must be performed by changing the mirror surface of the rotary polygon mirror for each color beam. However, this method has a problem that a color shift of one pixel (one scan line interval) at the maximum occurs in the sub-scanning direction.

  Therefore, an object of the present invention is to solve at least one of such problems and other problems. Other issues can be understood throughout the specification.

  According to the present invention, an image forming apparatus is provided corresponding to each of n photoconductors and the n photoconductors, and the n photoconductors are formed on the n photoconductors to form a toner image. N light sources for emitting light beams for exposing the photoconductors, and a plurality of mirror surfaces, so that the light beams emitted from the n light sources scan the n photoconductors. A rotary polygon mirror that deflects the light beams emitted from the n light sources by the plurality of mirror surfaces, and an electrostatic latent image formed on the n photoconductors by scanning the light beams. A plurality of developing means for developing the toner images using toners of different colors, an image carrier that moves in a predetermined direction and onto which the toner images formed on the n photoconductors are transferred, and The image bearing of toner images transferred from n photoconductors Detecting means for detecting a deviation amount in the predetermined direction on the body; and a toner image formed on one of the n photoconductors as a reference toner image, and the reference toner The light beam emission timing for forming the reference toner image is adjusted so that the deviation amount of the toner image other than the reference toner image that should overlap the reference toner image with respect to the image is within ½ pixel. By controlling the light beam emission timing for forming a toner image other than the reference toner image, a mirror surface for deflecting the light beam for forming the reference toner image and a toner image other than the reference toner image are formed. The correspondence relationship with the mirror surface that deflects the light beam to be changed is changed, and the deviation amount of the toner image other than the reference toner image with respect to the reference toner image is ½ pixel Of the two toner images transferred from the n photoconductors, and the amount of deviation between the two most distant toner images is larger than (n−1) / n pixels, the two most distant colors The light for forming a toner image other than the reference toner image with respect to the emission timing of the light beam for forming the reference toner image so that the deviation amount of the toner image is within (n-1) / n pixels. By controlling the emission timing of at least one light beam, the correspondence between the mirror surface that deflects the light beam for forming the reference toner image and the mirror surface that deflects the at least one light beam is changed. And a control means.

  According to the present invention, there is provided a low-cost, compact and high-quality color image forming apparatus in which the color shift in the sub-scanning direction is at most (n-1) / n pixels or less.

1 is a schematic cross-sectional view of a tandem type color printer according to an embodiment. It is a schematic sectional drawing which shows the scanning optical apparatus and image formation part which concern on embodiment. It is a perspective view which shows the whole structure of a scanning optical apparatus. It is sectional drawing of a laser holder part. It is sectional drawing of a laser holder part. 1 is a block diagram of a tandem color printer according to an embodiment. 6 is an exemplary flowchart of a color misregistration correction process according to the embodiment. It is a figure which shows the example of a memory | storage of the color shift amount which concerns on embodiment. It is a figure which shows the example of a memory | storage about the color shift amount after correction | amendment. It is a flowchart which shows an example of the registration correction | amendment which concerns on embodiment. 10 is an exemplary flowchart of another color misregistration correction process according to the embodiment.

[First Embodiment]
A first embodiment in which the present invention is applied to a tandem color image forming apparatus (printer) will be described below.

  FIG. 1 is a schematic cross-sectional view of a tandem type color printer according to an embodiment. The color printer 100 includes an image forming unit 101Bk that forms a black image, an image forming unit 101C that forms a cyan image, an image forming unit 101M that forms a magenta image, and a yellow image. Four image forming units (image forming units) of the image forming unit 101Y to be formed are provided. These four image forming units 101Bk, 101C, 101M, and 101Y are arranged in a line at regular intervals. The scanning optical device 102 is a unit for exposing the photosensitive member of the image forming unit. The intermediate transfer belt 103 is a transfer body on which toner images for the respective colors are superimposed. The transfer roller 104 is a roller that transfers a toner image formed on the intermediate transfer belt 103 onto a recording material. The transfer rollers 105Bk, 105C, 105M, and 105Y are rollers for transferring the toner image formed on the photoconductor onto the intermediate transfer belt 103. A registration detection sensor (hereinafter referred to as a registration sensor) 106 detects the amount of color misregistration, detects registration correction patterns for each color formed on the intermediate transfer belt 103, and uses the reference color in the present embodiment. Detects the color misregistration amount for a certain yellow. Here, yellow is used as the reference color because the image forming unit 101Y is farthest from the fixing unit 107, and therefore, the influence of the component being thermally expanded due to the heat of the fixing unit is small. .

  FIG. 2 is a schematic cross-sectional view illustrating the scanning optical device and the image forming unit according to the embodiment. In each of the image forming units 101Bk, 101C, 101M, and 101Y, drum-type photoconductors (hereinafter referred to as photosensitive drums) 201Bk, 201C, 201M, and 201Y are installed. Around the photosensitive drums 201Bk, 201C, 201M, and 201Y, primary chargers 202Bk, 202C, 202M, and 202Y, developing devices 203Bk, 203C, 203M, and 203Y are disposed, respectively.

  The developing devices 203Bk, 203C, 203M, and 203Y contain black toner, cyan toner, magenta toner, and yellow toner, respectively. Each of the photosensitive drums 201Bk, 201C, 201M, and 201Y is a negatively charged OPC photosensitive member having a photoconductive layer on an aluminum drum base, and is driven in a direction indicated by an arrow (clockwise in FIG. 1) by a driving device (not shown). Direction) at a predetermined process speed. The primary chargers 202Bk, 202C, 202M, 202Y as primary charging means uniformly apply the surface of each photosensitive drum 201Bk, 201C, 201M, 201Y to a predetermined negative potential by a charging bias applied from a charging bias power source (not shown). Is charged.

  The developing devices 203Bk, 203C, 203M, and 203Y develop toner images by attaching toners of respective colors to the electrostatic latent images formed on the photosensitive drums 201Bk, 201C, 201M, and 201Y, respectively (visualization). To do.

  The transfer rollers 105Bk, 105C, 105M, and 105Y shown in FIG. 1 are in contact with the photosensitive drums 201Bk, 201C, 201M, and 201Y through the intermediate transfer belt 103.

  An optical case 240 stores each optical component of the scanning optical device. Reference numeral 241 denotes an upper lid, which is attached to the optical case 240 to seal the scanning optical device 102 and prevent entry of dust or toner into the scanning optical device 102. The upper lid 241 is provided with slit-like openings at positions corresponding to the photosensitive drums 201Bk, 201C, 201M, and 201Y, and dust-proof glasses 243Bk, 243M, 243M, and 243Y, which are transparent members, are attached. . For this reason, it is possible to expose the scanning light to each of the photosensitive drums 201Bk, 201C, 201M, and 201Y through the dust-proof glass 243Bk, 243M, 243M, and 243Y, but dust, toner, and the like enter the scanning optical device 102. Can be prevented.

  Inside the optical case 240 is a polygon mirror 210 as a rotating polygon mirror. The polygon mirror 210 deflects and scans the light beam emitted from the semiconductor laser (lasers 402 and 403 in FIG. 4) by rotating a motor (not shown) at a constant speed. A first imaging lens 221 is an fθ lens that, together with the second imaging lenses 222 and 223, scans the laser beam at a constant speed and forms a spot image on the photosensitive drum. Note that the first imaging lens 221 is configured by a cylinder lens because light beams emitted from a plurality of semiconductor lasers are incident at different angles. In the sub-scanning direction, the light beam of the first semiconductor laser (laser 402 in FIG. 4) is imaged by the second imaging lens 222, and the second semiconductor laser (laser 403 in FIG. 4) The light beam is imaged by the second imaging lens 223.

  Reference numerals 224 to 226 denote folding mirrors that reflect the light beam in a predetermined direction. In particular, reference numeral 224 denotes a final folding mirror disposed with respect to the light beam of the first semiconductor laser. Reference numeral 225 denotes a separation folding mirror disposed with respect to the light beam of the second semiconductor laser. Reference numeral 226 denotes a final folding mirror arranged with respect to the light beam of the second semiconductor laser. As described above, the separation folding mirror 225 and the final folding mirror 226 reflect the light flux of the second semiconductor laser a plurality of times, thereby effectively utilizing the small space and substantially the same as the light flux of the first semiconductor laser. It is possible to have the same optical path length.

  On the other side of the polygon mirror 210, on the other hand, a first imaging lens 231 corresponding to the third and fourth semiconductor lasers (lasers 512 and 513 in FIG. 5), second imaging lenses 232 and 233, and A final folding mirror 234 arranged for the light flux of the fourth semiconductor laser, a separation folding mirror 235 arranged for the light flux of the third semiconductor laser, and the final folding mirror 36 are arranged. As described above, the separation folding mirror 235 and the final folding mirror 236 reflect the light flux of the third semiconductor laser a plurality of times, thereby effectively using the small space and substantially the same as the light flux of the fourth semiconductor laser. The optical path length can be For this reason, it is possible to make the scanning optical device 102 compact.

  FIG. 3 is a perspective view showing the overall configuration of the scanning optical device. 4 and 5 are cross-sectional views of the laser holder portion. In the figure, reference numeral 301 denotes a laser holder that holds semiconductor lasers (single beam lasers) 402 and 403 as light sources by press-fitting them into lens barrel holding portions 301a and 301b. The lens barrel holders 301a and 301b are configured with their optical axes inclined so that the optical paths of the semiconductor lasers 402 and 403 intersect each other with a predetermined angle θ in the sub-scanning direction. Moreover, a part of the outer shape of these lens barrels is integrated. For this reason, it is possible to keep the distance between the semiconductor lasers 402 and 403 close to each other. Aperture portions 401c and 401d corresponding to the respective semiconductor lasers 402 and 403 are provided on the front end side of the lens barrel holding portions 301a and 301b, and the light beams emitted from the semiconductor lasers 402 and 403 are formed into a desired optimum beam shape. is doing. Adhesive portions 401e and 401f of collimator lenses 406 and 407 are provided at two positions in the main scanning direction at the further ends of the lens barrel holding portions 301a and 301b. Here, the collimator lenses 406 and 407 convert the light beams that have passed through the aperture portions 401c and 401d into substantially parallel light beams. The collimator lenses 406 and 407 adjust the irradiation position and focus while detecting the optical characteristics of the laser beam. When the position is determined, the ultraviolet light is applied to the bonding portions 401e and 401f by irradiating an ultraviolet curable adhesive. The

  The side wall of the optical case 240 is provided with a fitting hole portion and an elongated hole portion for positioning the laser holder 301 in the sub-scanning direction, and is provided on the outer shape portions of the lens barrel holding portions 301 a and 301 b of the laser holder 301. The fitting part is fitted so that it can be attached. In this way, the laser holder 301 is attached to the optical case 240 by fitting the fitting portions provided in the outer portions of the lens barrel holding portions 301a and 301b that hold the semiconductor lasers 402 and 403 to form an optical path. The positional relationship between the semiconductor lasers 402 and 403 and each optical component stored in the optical case 240 can be accurately guaranteed.

  The laser holder 311 is the same component as the laser holder 301, and holds the semiconductor lasers 512 and 513 by press-fitting them into the lens barrel holding portions 511a and 311b. The electric circuit board 314 is electrically connected to the semiconductor lasers 402, 403, 512, and 513 and includes a laser driving circuit. Reference numerals 305 and 315 denote BD (Beam Detect) sensors provided on the electric circuit board 314. The BD sensors 305 and 315 detect the light beam reflected by the polygon mirror 210 and output a synchronization signal in the main scanning direction. Thereby, the timing of the scanning start position of the image edge can be adjusted. Here, the lens barrel holding portions 511a and 311b are provided with the optical axes inclined so that the optical paths of the semiconductor lasers 512 and 513 intersect each other with a predetermined angle θ in the sub-scanning direction. Is partly integrated. Aperture portions 511c and 511d corresponding to the respective semiconductor lasers 512 and 513 are provided on the front end side of the lens barrel holding portions 511a and 311b, and the light beams emitted from the semiconductor lasers 512 and 513 are formed into a desired optimum beam shape. ing. Adhesive portions 511e and 511f of collimator lenses 516 and 517 are provided at two positions in the main scanning direction at the further ends of the lens barrel holding portions 511a and 311b. Here, like the collimator lenses 406 and 407, the collimator lenses 516 and 517 adjust the irradiation position and focus, and are bonded and fixed to the bonding portions 511e and 511f.

  Here, since the BD slit portion 315g of the laser holder 311 is disposed on the opposite side to the laser holder 301, and the BD slit portion 305g of the laser holder 301 is disposed on the opposite side of the laser holder 311, the laser holder 311 With the holder 301 rotated 180 degrees, the semiconductor laser 512 is placed on the same surface of the optical case 240 so that the position of the semiconductor laser 512 is next to the semiconductor laser 403 and the position of the semiconductor laser 513 is next to the semiconductor laser 402. Yes. For this reason, it is possible to arrange the intervals between the semiconductor lasers 402 and 403 and the semiconductor lasers 512 and 513 in the main scanning direction so as to be short according to the reflecting surface of the polygon mirror 210 facing the scanning optical system.

  As described above, even when the semiconductor lasers 402, 403, 512, and 513 are arranged in the same direction, the angle that can be scanned by the polygon mirror 210 can be widened, so that the optical path length of the scanning optical system can be shortened. In addition, since there is no incident system component on the opposite side of the surface on which the laser holders 301 and 311 between the polygon mirror 210 are arranged, the scanning optical device can be made compact by bringing it closer to the polygon mirror 210. .

  The laser holder 311 is positioned with respect to the optical case 240 in the same manner as the laser holder 301. Therefore, the positional relationship between the semiconductor lasers 512 and 513 and each optical component stored in the optical case 240 can be accurately guaranteed.

  Reference numeral 308 denotes a cylindrical lens having a predetermined refractive power only in the sub-scanning direction. Lens portions 308a and 308b corresponding to the light beams emitted from the semiconductor lasers 402 and 403 are integrally formed. Reference numeral 309 denotes a BD lens that forms an image of the light beam reflected by the polygon mirror 210 on the light receiving surface of the BD sensor 305 described above. Here, immediately before the BD sensor 305, a BD slit portion 305g provided in the laser holder 301 is disposed. Since the BD slit portion 305g is an opening that is narrow in the main scanning direction and long in the sub-scanning direction, the light beam received by the BD sensor 305 can be detected in the main scanning direction with high accuracy by restricting the light beam received in the main scanning direction. it can. In the present embodiment, the BD sensor 305 and the BD slit portion 305g are provided at positions corresponding to the semiconductor laser 402, but the BD sensor corresponding to the semiconductor laser 403 is not provided. This is because the semiconductor lasers 402 and 403 are provided in one laser holder 301 in the sub-scanning direction, so that the timing of the scanning start position of the image edge by the semiconductor laser 403 can be the same as that of the semiconductor laser 402. Because.

Reference numeral 318 denotes a cylindrical lens having a predetermined refractive power only in the sub-scanning direction, and lens portions 318a and 318b corresponding to light beams emitted from the semiconductor lasers 512 and 513 are integrally formed. Reference numeral 319 denotes a BD lens that forms an image of a light beam reflected by the polygon mirror 210 and then reflected again by the reflection mirror 230 on the light receiving surface of the BD sensor 315 described above. Here, immediately before the BD sensor 315, there is a BD slit portion 315g provided in the laser holder 311. Since the BD slit portion 315g is an opening that is narrow in the main scanning direction and long in the sub-scanning direction, the light beam received by the BD sensor 15 can be detected in the main scanning direction with high accuracy by restricting the light beam received in the main scanning direction. it can. In the present embodiment, the BD sensor 315 and the BD slit portion 315g are provided at positions corresponding to the semiconductor laser 512, but the BD sensor corresponding to the semiconductor laser 513 is not provided. This is because the semiconductor lasers 512 and 513 are provided in one laser holder 311 in the sub-scanning direction, so that the timing of the scanning start position of the image edge by the semiconductor laser 513 can be the same as that of the semiconductor laser 512. Because
Next, the flow until the light beams emitted from the semiconductor lasers 402, 3, 12, and 13 are exposed to the photosensitive drums 201Bk, 201C, 201M, and 201Y as the scanning lights E1, E2, E3, and E4 will be described.

  The light beams emitted from the semiconductor lasers 402 and 403 are limited in size by the diaphragm portions 401 c and 401 d of the laser holder 301, converted into substantially parallel light beams by the collimator lenses 406 and 407, and the lens of the cylindrical lens 308. The light enters the portions 308a and 308b. The light beam incident on the cylindrical lens 308 is transmitted as it is in the main scanning section, and on the other hand, it converges in the sub-scanning section and forms an almost linear image on the same surface of the polygon mirror 210. At this time, the light is incident obliquely with an angle θ in the sub-scanning direction. Then, the polygon mirror 210 rotates and is emitted with an angle θ in the sub-scanning direction while performing deflection scanning. Of the two light beams emitted from the polygon mirror 210, the light beam emitted from the semiconductor laser 402 passes through the BD slit portion 305 g provided in the laser holder 301 and is received by the BD sensor 305. At this time, by restricting the light beam in the main scanning direction, the light beam can be detected with high accuracy in the main scanning direction. The BD sensor 305 detects the light beam emitted from the semiconductor laser 402 and outputs a synchronization signal, and adjusts the timing of the scanning start position of the image edge portion by the semiconductor lasers 402 and 403.

  Here, since the semiconductor lasers 402 and 403 are provided in one laser holder 1 in the sub-scanning direction, the timing of the scanning start position of the image edge by the semiconductor laser 403 is substantially the same as that of the semiconductor laser 402. can do. The light beams emitted from the semiconductor lasers 402 and 403 after the timing adjustment are transmitted through the first imaging lens 221.

  Thereafter, the light beam emitted from the semiconductor laser 402 is transmitted through the second imaging lens 222, reflected by the final folding mirror 224, transmitted through the dust-proof glass 243Bk, and exposed to the photosensitive drum 201Bk as scanning light E1. On the other hand, the light beam emitted from the semiconductor laser 403 is reflected downward by the separation folding mirror 225, then passes through the second imaging lens 223, is reflected by the final folding mirror 226, and passes through the dust-proof glass 243M. The photosensitive drum 201C is exposed as scanning light E2.

  Further, the light beams emitted from the semiconductor lasers 512 and 513 are limited in size by the diaphragm portions 511 c and 511 d of the laser holder 311, converted into substantially parallel light beams by the collimator lenses 516 and 517, and the cylindrical lens 318. The light enters the lens portions 318a and 318b. The light beam incident on the cylindrical lens 318 is transmitted as it is in the main scanning section, converges in the sub-scanning section, and forms an almost linear image on the same surface of the polygon mirror 210. At this time, the light is incident obliquely with an angle θ in the sub-scanning direction. Then, the polygon mirror 210 rotates and is emitted with an angle θ in the sub-scanning direction while performing deflection scanning. Of the two light beams emitted from the polygon mirror 210, the light beam emitted from the semiconductor laser 512 and reflected by the polygon mirror 210 is reflected once again by the reflection mirror 230, and then BD slit 315 g provided in the laser holder 311. And is received by the BD sensor 315. At this time, by restricting the light beam in the main scanning direction, the light beam can be detected with high accuracy in the main scanning direction. The BD sensor 315 detects the light beam emitted from the semiconductor laser 512 and outputs a synchronization signal, and adjusts the timing of the scanning start position of the image edge portion by the semiconductor lasers 512 and 513.

  Here, since the semiconductor lasers 512 and 513 are provided in one laser holder 311 in the sub-scanning direction, the timing of the scanning start position of the image edge by the semiconductor laser 513 is substantially the same as that of the semiconductor laser 512. can do. The light beams emitted from the semiconductor lasers 512 and 513 after timing adjustment are transmitted through the first imaging lens 231.

  Thereafter, the light beam emitted from the semiconductor laser 512 is reflected downward by the separation folding mirror 235, then passes through the second imaging lens 233, is reflected by the final folding mirror 236, and passes through the dust-proof glass 243M. The photosensitive drum 201M is exposed as scanning light E3. On the other hand, the light beam emitted from the semiconductor laser 513 passes through the second imaging lens 232, is reflected by the final folding mirror 234, passes through the dust-proof glass 243Y, and is exposed to the photosensitive drum 201Y as scanning light E4.

  FIG. 6 is a block diagram of the tandem color printer according to the embodiment. The system control unit 601 includes a CPU, a ROM, a RAM, an ASIC, and the like, and controls image forming processing (including registration correction pattern forming processing) of the color printer 100, transfer sheet conveyance processing, and the like. For example, when a registration correction pattern is formed, the system control unit 601 instructs the laser driving circuit included in the electric circuit board 314 to emit laser light, whereby the registration correction pattern for each color is assigned to each photosensitive drum 201Bk, 201C. , 201M, 201Y, and electrostatic latent images are formed on the photosensitive drums 201Bk, 201C, 201M, 201Y charged by the primary chargers 202Bk, 202C, 202M, 202Y. After that, the toner of each color that has been frictionally charged in the developing devices 203Bk, 203C, 203M, and 203Y is attached to the electrostatic latent image, thereby the registration correction pattern toner for each color on each photosensitive drum 201Bk, 201C, 201M, and 201Y. An image is formed, and the toner image is transferred from the photosensitive drums 201Bk, 201C, 201M, and 201Y onto the intermediate transfer belt 103 at each primary transfer nip portion. A memory 602 provided in the system control unit 601 stores a color shift amount with respect to a reference color (yellow) detected by the registration sensor 106. During image formation, the system control unit 601 can perform registration correction by selecting a mirror surface of the polygon mirror 210 corresponding to each color and performing exposure control.

Next, in a color image forming apparatus in which n number of photosensitive drums are polarized and scanned by one polygon mirror, registration correction is performed to reduce the amount of color misregistration to a maximum of (n−1) / n pixels or less. A correction amount determination method (that is, a method for selecting a mirror surface of the polygon mirror 210) will be described.

  FIG. 7 is an exemplary flowchart of color misregistration correction processing according to the embodiment. The mirror surface selection process of the polygon mirror 210 is preferably performed, for example, when the color printer 100 is turned on, after printing a predetermined number of transfer sheets, or after a predetermined time has elapsed. Also good.

In this embodiment, since there are four photosensitive drums 201Bk, 201C, 201M, and 201Y, the registration correction amount is set so that the color misregistration amount is a maximum (4-1) /4=0.75 pixel or less. A mirror surface of the polygon mirror 210 is selected.

In step S <b> 701, the system control unit 601 detects the color misregistration amount of each color with respect to the reference color (yellow) by causing the registration sensor 106 to detect the registration correction pattern of each color formed on the intermediate transfer belt 103. The color misregistration amount is recognized as a deviation in the toner image formation position.

  In step S702, the system control unit 601 determines that the color misregistration amount of each color is A, B, C in descending order when the color misregistration amount of each of the other colors with respect to the reference color (yellow) is within a range of ± 0.5 pixels. Is stored in the memory 602.

  FIG. 8 is a diagram illustrating a storage example of the color misregistration amount according to the embodiment. In the example of FIG. 8, the color misregistration amount of the black toner Bk is “+0.45”, which is the largest and is stored in the memory 602 as the maximum value A. The color misregistration amount of the magenta toner M is “−0.35”, which is the smallest and is stored as the minimum value C. The color misregistration amount of cyan toner C is “+0.40”, which is neither the maximum value nor the minimum value, and is stored as an intermediate value B. Note that the color misregistration amount of the yellow toner Y is apparently 0 because of its nature. Therefore, it is not necessary to store the color misregistration in the memory 602. However, the color misregistration of the other color toners based on the yellow toner Y is low. It is shown for easy understanding that the amount is calculated.

  In step S703, the system control unit 601 determines whether or not the difference between the maximum value (A) and the minimum value (C) of the detected color misregistration amount exceeds 0.75. Here, if it is determined that it does not exceed 0.75, the amount of color misregistration is already within the allowable range (0.75 pixels), so the mirror surface of the polygon mirror 210 and the light beam corresponding to each color The combination does not require modification. Therefore, the process of this flowchart is complete | finished.

On the other hand, when it is determined that the value exceeds 0.75, the combination of the mirror surfaces of the polygon mirror 210 at the present time is inappropriate. Incidentally, in this case, the maximum value A of the color misregistration amount is on the plus side of yellow, and
A ≧ 0.25 pixel is satisfied (because the smallest value that C can take is −0.5). Further, the minimum value C of the color misregistration amount is on the minus side of yellow, and
C ≦ −0.25 pixel (because the largest possible value of A is +0.5).

  In step S704, the system control unit 601 determines whether the detected color misregistration amount B that is neither the maximum value A nor the minimum value C is a positive value (that is, B> 0?). If it is a positive value, since there are A and B on the plus side of yellow, the process proceeds to step S705. Otherwise, the process proceeds to step S708.

  In step S705, the system control unit 601 determines which of B and C has a smaller color misregistration amount than yellow. For example, it is determined whether or not the difference between the absolute values of B and C is positive (that is, B− | C |> 0?). If this difference is a positive value, the process proceeds to step S706; otherwise, the process proceeds to step S707.

  In step S706, the system control unit 601 subtracts 1 from each of the values A and B and writes the result in the memory 602 in order to reduce the amount of color misregistration. As a result, for the toner whose color misregistration amount detected by the registration sensor 106 is the maximum (A) and the toner whose color misregistration amount is an intermediate value (B), the mirror surface of the polygon mirror 210 is moved back by one. Change to the minus side of. The reason why 1 is subtracted is that one mirror surface of the polygon mirror corresponds to one dot (line) in the sub-scanning direction. For example, if a certain line in the sub-scanning direction is formed by a certain mirror surface, the adjacent line is formed by the adjacent mirror surface. In addition, when the polygon mirror is rotated at a constant speed, one reason is that, in principle, the color misregistration in the sub-scanning direction can be corrected only in units of one dot.

This is because if the difference between the absolute values of B and C is positive, C is less in color shift than yellow than B. That is,
B> 0.25, and as a result,
This is because | C + 1 |> | B-1 |. The amounts of color misregistration of colors other than yellow are all negative, and the minimum value after correction is B−1 (the B toner is most deviated from yellow).

B-1> -0.75
Because
Maximum value after correction (this value is 0 for yellow) —Minimum value after correction (B−1) <0.75 pixel is satisfied. Therefore, since the amount of color misregistration between the four color toners is within 0.75 pixel, this flowchart is ended.

On the other hand, if NO in step S705, the color misregistration amount of B for yellow is equal to or less than C,
| C + 1 | ≦ | B-1 |
It becomes. Therefore, in step S707, the system control unit 601 adds 1 to C so as to reduce the color misregistration amount, and stores the result in the memory 602. This causes the mirror surface of the polygon mirror 210 to be changed to the previous plus side for the toner whose color misregistration detected by the registration sensor 106 is the minimum (C). The maximum color misregistration amount is now C + 1,
C + 1 ≦ 0.75
Because
Maximum value of color misregistration amount (C + 1) −minimum value (this is 0 for yellow) ≦ 0.75 pixels, and this flowchart is ended.

  By the way, if it is No in S704, B and C exist on the minus side of yellow. Therefore, in step S708, the system control unit 601 determines which of A and B has a smaller color shift amount with respect to yellow. For example, it is determined whether or not the difference between the detected maximum value A of the color misregistration amount A and the absolute value of the intermediate value B is 0 or more (that is, A− | B | ≧ 0?). If the difference between the detected maximum value A of color misregistration amount A and the absolute value of the intermediate value B is 0 or more, the process proceeds to step S709.

  In step S709, the system control unit 601 subtracts 1 from the value of A and stores the result in the memory 602. As a result, the mirror surface of the polygon mirror 210 is changed to the next minus side for the toner having the maximum color misregistration detected by the registration sensor 106 (A).

That is, if A− | B | ≧ 0, the color misregistration amount of B toner with respect to yellow is equal to or less than A, and
| A-1 | ≦ | B + 1 |
It becomes. Therefore, for the toner of A, the mirror surface of the polygon mirror 210 is changed to the next minus side. As for the amount of color misregistration of the four toners, yellow 0 has the maximum value, A-1 has the minimum value, and
A-1 ≧ −0.75
It is. Therefore, since the maximum value of color misregistration amount (0 of yellow) −minimum value ≦ 0.75 pixels, the correction of the color misregistration amount ends.

If NO in step S708, the amount of color shift with respect to yellow is smaller for the A toner than for the B toner. Also,
B <−0.25,
| A-1 |> | B + 1 |
It becomes. Therefore, the process proceeds to step S710, and the system control unit 601 stores the color misregistration amount in the memory 602 as B + 1 and C + 1 so that the color misregistration amount is reduced. As a result, the toner whose color misregistration amount detected by the registration sensor 106 is the minimum (C) and the toner whose color misregistration amount is an intermediate value (B) are moved to the previous mirror surface of the polygon mirror 210. Change to the plus side. The corrected maximum value is now C + 1, the corrected minimum value is 0 for yellow,
C + 1 <0.75
Because
Maximum value after correction−Minimum value after correction <0.75 pixels. Therefore, the color misregistration correction process is terminated.

  In this way, the mirror surface of the polygon mirror 210 is selected so that the color misregistration between the toners is within 0.75 pixels at the maximum.

  FIG. 9 is a diagram illustrating a storage example of the corrected color misregistration amount. This correction amount is when the color misregistration amount illustrated in FIG. 8 is detected. That is, the maximum value (+0.45) −the minimum value (−0.35) = + 0.80, which satisfies the condition of step S703. Subsequently, since the value of B (+0.40) is positive, the condition of step S704 is satisfied. Since B− | C | is + 0.40− | −0.35 | = + 0.75, the condition of step S705 is also satisfied. Therefore, in step S706, the color misregistration amount A for Bk toner and the color misregistration amount B for C toner are each subtracted by 1.00.

  FIG. 10 is a flowchart illustrating an example of registration correction according to the embodiment. This flowchart is executed for colors other than the reference color (yellow in the above example).

  When a print start signal is input, the system control unit 601 reads the corrected color misregistration amount from the memory 602 in step S1001.

  In step S1002, in step S1001, the system control unit 601 determines whether the read color misregistration amount is less than −0.5. If so, the process proceeds to step S1003.

  In step S1003, the system control unit 601 selects a mirror surface that is one after the mirror surface of the reference color (for example, yellow) as the mirror surface for the color. That is, the system control unit 601 controls the laser emission timing so that the light beam strikes the next mirror surface.

  On the other hand, if the color misregistration amount is −0.5 or more, in step S1004, the system control unit 601 determines whether the color misregistration amount exceeds 0.5. If it does not exceed, there is no need to select a new mirror surface, so this subroutine is terminated. On the other hand, if exceeded, in step S1005, the system control unit 601 selects the mirror surface immediately before the mirror surface of the reference color (eg, yellow) as the mirror surface for the color. That is, the system control unit 601 controls the laser emission timing so that the light beam strikes the previous mirror surface.

As described above, in the present embodiment, a suitable mirror surface of the polygon mirror 210 can be selected for each color from the amount of color misregistration with respect to yellow, which is the reference color stored in the memory 602. Thereafter, the system control unit 601 instructs the electric circuit board 14 to control information on the selected mirror surface (light emission timing). Next, the laser drive circuit of the electric circuit board 14 emits a laser beam from the semiconductor lasers 402, 403, 512 and 513 in accordance with the control surface from the system control unit 601 in accordance with the mirror surface of the selected polygon mirror 210. . In this way, by selecting and exposing the mirror surface of the polygon mirror 210, it is possible to form an image with registration correction. In this case, the amount of color misregistration is (4-1) /4=0.75 pixels or less and can be set to a substantially minimum value.

  As described above, the color image forming apparatus according to the present embodiment simultaneously deflects and scans the laser beams emitted from the plurality of semiconductor lasers 402, 403, 512, and 513 by using one polygon mirror 210, thereby performing a plurality of photosensitive operations. The drums 201Bk, 201C, 201M, and 201Y are respectively irradiated and exposed. As a result, the number of parts is reduced, so that the scanning optical device 102 can be reduced in cost and size. In addition, the separation folding mirror 225 and the final folding mirror 226 reflect the light beam of the semiconductor laser 403 a plurality of times, while the separation folding mirror 235 and the final folding mirror 236 cause the light beam of the semiconductor laser 512 to be reflected a plurality of times. By reflecting, it is possible to effectively use a small space and make the optical path length substantially the same as other light beams. Therefore, the scanning optical device can be further reduced in size and size. Therefore, the color printer 100 as a whole can be reduced in cost and size.

In addition, registration correction can be suitably performed by selecting the mirror surface of the polygon mirror 210 for each toner so that the amount of color misregistration is small. That is, the amount of color misregistration with respect to the reference color (for example, yellow) detected by the registration sensor 106 is stored in the memory 602, and at the time of image formation, the system control unit 601 selects the mirror surface of the polygon mirror 210 corresponding to each color. By controlling the exposure, the amount of color misregistration in the sub-scanning direction can be set to (n-1) / n pixels or less (n is a natural number of 2 or more). For this reason, the color printer 100 can be improved in image quality as compared with the conventional color image forming apparatus in which the color misregistration of one pixel occurs at the maximum.

  In addition, by setting the reference color to yellow, which is the color of the toner held in the developing device 203Y disposed at a position where it is most unlikely to be affected by the heat from the fixing device 107, the components are heated by the heat of the fixing device. Suppresses the negative effects of expansion.

[Second Embodiment]
The second embodiment relates to another example of the mirror surface selection process of the polygon mirror 210 in the first embodiment. It should be noted that the same reference numerals are assigned to the portions that have already been described, for the sake of brevity.

  FIG. 11 is an exemplary flowchart of another color misregistration correction process according to the embodiment. As described above, the system control unit 601 executes steps S701 to S703. As a result, when the difference between the maximum value and the minimum value of the color misregistration amount exceeds 0.75 in step S703, the process proceeds to the following step S1104.

In step S1104, the system control unit 601 determines whether the value of the intermediate color misregistration amount C is positive. If it is positive, the process proceeds to step S1105, where the system control unit 601 adds 1 to C and stores it in the memory 602. That is, for the color having the intermediate color misregistration amount C, the mirror surface of the polygon mirror 210 is changed to the previous plus side. Thus, the corrected maximum value is C + 1, and the corrected minimum value is 0 for yellow. Also,
C + 1 ≦ 0.75
Therefore, since the maximum value−the minimum value ≦ 0.75 pixels of the color misregistration amount, this processing is terminated.

On the other hand, if the value of B is not positive in step S1104, the process proceeds to step S1106, and the system control unit 601 stores a value obtained by subtracting one from the value of A in the memory 602. That is, the mirror surface for the color corresponding to the color misregistration amount A is changed to the next minus side. This is because the colors corresponding to B and C are shifted to the minus side from yellow. The corrected maximum value is 0 for yellow, the minimum value is A-1, and
A-1 ≧ −0.75
Therefore, the maximum value of the color misregistration amount−the minimum value ≦ 0.75 pixels. Therefore, this process ends.

Thus, also in the second embodiment, the mirror surface of the polygon mirror 210 can be selected so that the amount of color misregistration is at most (4-1) /4=0.75 pixels or less. Therefore, the color printer 100 can have higher image quality than before.

[Other Embodiments]
In the scanning optical apparatus according to the first embodiment and the second embodiment, a method has been described in which two laser beams are incident on both sides of one polygon mirror and four photosensitive drums are exposed. Needless to say, the present invention can be applied to a color image forming apparatus in which four laser beams are incident on one side of a single polygon mirror and four photosensitive drums are exposed.

  In the above-described embodiment, one rotating polygon mirror is applied to the four photoconductors. However, the present invention is not limited to this. For n (n is a natural number of 2 or more) photoreceptors, m (m is a natural number of 1 or more and less than n) rotating polygon mirrors. It is applicable to the image forming apparatus to be used.

Claims (4)

n photoconductors;
N light sources provided corresponding to the n photoconductors and emitting light beams for exposing the n photoconductors to form toner images on the n photoconductors,
The light beams emitted from the n light sources by the plurality of mirror surfaces so that the light beams emitted from the n light sources scan the n photoconductors. A rotating polygon mirror to deflect
A plurality of developing means for developing the electrostatic latent images formed on the n photoconductors by scanning the light beams as the toner images using toners of different colors;
An image carrier that moves in a predetermined direction and onto which toner images formed on the n photoconductors are transferred;
Detecting means for detecting a deviation amount of the toner image transferred from the n photoconductors in the predetermined direction on the image carrier;
Of the toner images formed on the n photoconductors, a toner image formed on one of the photoconductors is used as a reference toner image, and the reference toner image other than the reference toner image to be superimposed on the reference toner image is used. Light beam emission timing for forming a toner image other than the reference toner image with respect to the light beam emission timing for forming the reference toner image so that the deviation amount of the toner image is within 1/2 pixel. By changing the correspondence between the mirror surface that deflects the light beam for forming the reference toner image and the mirror surface that deflects the light beam for forming a toner image other than the reference toner image. In addition, each toner transferred from the n photoconductors, wherein the deviation amount of the toner image other than the reference toner image with respect to the reference toner image is within 1/2 pixel. When the deviation amount of the two most distant toner images is larger than (n-1) / n pixels, the deviation amount of the two most distant toner images is within (n-1) / n pixels. As described above, the reference timing is controlled by controlling the emission timing of at least one light beam of the light beam for forming a toner image other than the reference toner image with respect to the emission timing of the light beam for forming the reference toner image. An image forming apparatus comprising: control means for changing a correspondence relationship between a mirror surface for deflecting a light beam for forming a toner image and a mirror surface for deflecting the at least one light beam. The detecting means detects a deviation amount of a toner image other than the reference toner image with respect to the reference toner image;
The control unit determines a shift amount of toner images of two colors that are farthest from each of the toner images transferred from the n photoconductors based on a shift amount detected by the detection unit. The image forming apparatus according to claim 1. The detecting means detects a deviation amount of the toner image transferred to a position preceding the reference toner image in the predetermined direction as a positive deviation amount, and a deviation of the toner image transferred to the subsequent position. The amount is detected as a negative deviation amount,
The control means sets the deviation amount of the toner image other than the reference toner image with respect to the reference toner image so that the deviation amount of the two most distant toner images is within (n−1) / n pixels. 3. The image forming apparatus according to claim 1, wherein the emission timing of a light beam other than the light beam for forming a toner image having the smallest deviation amount is controlled.   In the case of n = 4, the control means sets one of the predetermined toner images in the predetermined direction with respect to the reference toner image in order to make the amount of deviation of the two most distant toner images within 3/4 pixels. Of the two color toner images other than the reference toner image located on the side and the one color toner image other than the reference toner image located on the other side, the light for forming the one color toner image The image forming apparatus according to claim 1, wherein the emission timing of the beam is controlled.

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