JP2018083363A - Image formation apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation apparatus which can improve the image quality of a formed image more than the conventional technique even when the curvature occurs on a deflection surface of a polygon mirror.SOLUTION: An image formation apparatus drives a light source to emit a laser beam on the basis of image data of a reference pattern every time one deflection surface of a rotating polygon mirror comes to the irradiation position of the laser beam from the light source, forms a latent image of the reference pattern on a photoreceptor drum by deflecting the laser beam only by the deflection surface, develops the latent image with toner, transfers the developed toner image of the reference pattern to a sheet, detects the toner image of the reference pattern on the sheet by a sensor, obtains a graph 192 of the density distribution in the main-scanning direction, stores the value of the graph 192a indicating the reverse characteristics of the obtained graph 192 of the density distribution as a correction coefficient, applies a correction coefficient to the image data used for emitting the laser beam from the light source toward the deflection surface out of the pieces of image data of the images formed subsequently, and corrects the image data.SELECTED DRAWING: Figure 14

Description

  The present invention deflects a light beam emitted from a light source by each deflecting surface of a polygon mirror, forms an electrostatic latent image on a charged photoconductor by each deflected light beam, and then forms an electrostatic latent image. The present invention relates to an image forming apparatus that develops a latent image with toner and transfers the developed toner image onto a sheet.

For example, in an electrophotographic printer or copying machine, a charged photosensitive member peripheral surface is exposed and scanned by a light beam scanning device to form an electrostatic latent image, and the electrostatic latent image is developed with toner. The upper toner image is further transferred to a recording sheet to form an image.
The light beam scanning device makes a light beam emitted from a light source such as a semiconductor laser incident on each deflecting surface of a polygon mirror that rotates at a constant speed and deflects it within a range of a predetermined scanning angle. In general, the scanning lens collects light on the circumferential surface of the photoconductor and performs exposure scanning in the main scanning direction.

Japanese Patent Laid-Open No. 9-174917 JP 2006-25125 A

When using polygon mirrors, it is ideal that each deflection surface is a flat surface. However, due to manufacturing processing accuracy and the manufacturing process in which the polygon mirror is press-fitted into the rotation axis of the polygon motor, what is actually a very small deflection surface? No, curvature may occur.
FIG. 24A is a schematic plan view showing a state in which the polygon mirror 901 attached to the rotation shaft 902 of the polygon motor is viewed from above in the vertical direction along the rotation shaft 902.

As shown in the figure, the polygon mirror 901 has a square shape in plan view, and four side surfaces are deflection surfaces 911, 912, 913, and 914, respectively. In the example shown in the figure, among the four deflection surfaces, the deflection surfaces 911 and 914 are flat surfaces, but the deflection surfaces 912 and 913 are not flat surfaces (broken lines) but curved. In the figure, the curved shape is exaggerated.
When the deflection surfaces 912 and 913 are curved, the deflection angle of the deflection surfaces is shifted by the amount curved with respect to the plane, and the irradiation position on the photosensitive drum is in the main scanning direction. Deviation occurs.

  FIG. 24B is a diagram schematically showing the state of deviation of the irradiation position of the light beam in the main scanning direction on the photosensitive drum 921. In the figure, a broken line 931 parallel to the main scanning direction indicates a main scanning line that is exposed and scanned by the light beam deflected by the deflecting surface 911. Similarly, broken lines 932, 933, and 934 parallel to the main scanning direction indicate main scanning lines that are exposed and scanned by the light beams deflected by the deflection surfaces 912, 913, and 914, respectively.

  On the main scanning line 931, black circles shown at a position A on one end side and a position B on the other end side in the main scanning direction indicate an exposure area (dot) corresponding to one pixel by the light beam. With respect to the main scanning lines 931 and 934, dots are formed at the positions A and B, respectively, but with respect to the main scanning lines 932 and 933, the dot formation positions are in the main scanning direction with respect to the positions A and B, respectively. Is slightly off.

FIG. 24C is an enlarged view of the vicinity of the position A on the main scanning lines 931 to 933, and shows a state in which a plurality of dots are arranged in a line in the main scanning direction.
For the main scanning line 931, the pitch (interval) between two adjacent dots 951 in the main scanning direction is α (reference), but for the main scanning line 932, the interval between the dots 952 is β (<α). In the main scanning line 933, it can be seen that the interval between the dots 953 is γ (> α). This also applies to the vicinity of position B.

  In the main scanning line 932, since the interval β between the adjacent dots 952 is narrower than the reference α at each of both ends in the main scanning direction, the distribution density of the dots 952 in the main scanning direction is increased and reproduced by the difference. It seems that the toner density of the image is higher than that of the main scanning line 931. On the contrary, in the main scanning line 933, the interval γ between the adjacent dots 953 is wider than the reference α, so the distribution density of each dot 953 in the main scanning direction is lowered by the difference, and the toner density is higher than that of the main scanning line 931. It seems to have faded.

As described above, when a density difference due to the density of the intervals between the dots occurs for each main scanning line on the photosensitive drum 921, further improvement in the image quality of the formed image is prevented.
In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of improving the image quality of a formed image as compared with the related art even when the deflection surface of a polygon mirror is curved.

  In order to solve the above problems, an image forming apparatus according to the present invention drives a light source based on image data, a charging unit that charges the photoconductor, and rotates a light beam emitted from the light source. Exposure in which a plurality of deflecting surfaces of the polygon mirror to be deflected are sequentially deflected, and the charged photoconductor is exposed and scanned in the main scanning direction by each light beam deflected by each deflecting surface to form an electrostatic latent image. A developing unit that develops the electrostatic latent image on the photosensitive member with toner, a transfer unit that transfers the developed toner image from the photosensitive member to the sheet via a sheet or an intermediate transfer member, and the charging unit. The light source is driven based on the image data of the reference pattern along the main scanning direction by controlling the exposure unit, the development unit, and the transfer unit, and the light beam from the light source is directed to one deflection surface of the polygon mirror Emitted and charged photosensitive An electrostatic latent image of a reference pattern is formed on the surface, and after developing the electrostatic latent image of the reference pattern, a series of processes for transferring the developed toner image to the sheet via a sheet or an intermediate transfer member is performed. A pattern formation control unit; a detection unit that detects a density distribution in a main scanning direction of a toner image of a reference pattern after being transferred to the sheet; and after the detection by the detection unit, the original image data of an image to be formed And a correction unit that uses the density distribution detected by the detection unit and performs correction in synchronization with the position of the density distribution in the main scanning direction.

  Further, the correction unit is configured to suppress density variation in the main scanning direction of an image formed depending on non-planarity existing on the one deflection surface based on the density distribution detected by the detection unit. Calculating means for obtaining a correction coefficient; and storage means for storing the correction coefficient obtained by the calculating means, and applying the correction coefficient stored in the storage means to the original image data to perform the correction. You can do it.

Here, a notification means for notifying that an abnormality has occurred in the exposure unit when the correction coefficient is not within a predetermined range may be provided.
Further, the image data of the reference pattern is composed of a plurality of dot data arranged in a line in the main scanning direction, and each of the dot data includes those in which the photoconductor is exposed and not exposed by a light beam. It may be image data that includes one or more dot data that are included and exposed and one or more dot data that are not exposed alternately.

Furthermore, it may be provided with accepting means for accepting an execution instruction for forming the reference pattern, and the pattern formation control means may execute the formation of the reference pattern when the accepting means accepts the execution instruction.
Here, a determination unit that determines that the exposure unit has been replaced may be provided, and the reception unit may receive the determination of the determination unit as the execution instruction.

  A driving unit that rotates the photosensitive member; and a control unit that controls the driving unit so that a peripheral speed of the photosensitive member becomes a predetermined system speed during normal image formation. The pattern formation control means rotates the polygon mirror at the rotation speed at the time of normal image formation with respect to the exposure unit at the time of forming the reference pattern, and controls the driving means to control the peripheral speed of the photoconductor. It is also possible to perform a speed control that decelerates the system from the system speed.

Here, when the number of deflection surfaces of the polygon mirror is M, the pattern formation control means may reduce the peripheral speed of the photosensitive member to a speed of (1 / M) with respect to the system speed. .
Further, the pattern formation control means executes the reference pattern formation with the rotational speed of the polygon mirror and the peripheral speed of the photosensitive member being the same as those during normal image formation, and then the density detected by the detection means. When only a value lower than the lower limit value of the predetermined density range can be detected, the reference pattern may be formed again by switching to the deceleration control.

  Further, the pattern formation control unit forms a reference pattern corresponding to each deflection surface on a one-to-one basis for each deflection surface of the polygon mirror, and the detection unit performs the density distribution for each of the reference patterns. The correction means corresponds to the deflection surface with respect to the original image data of the image used to emit a light beam from the light source toward the deflection surface for each of the deflection surfaces. The correction may be performed using the density distribution detected by the reference pattern.

Further, the correction means converts the data indicating the density of each pixel of the original image data with respect to the original value based on the density value at the position of the pixel having the same positional relationship as the position in the main scanning direction of the density distribution. It may be corrected to a value that increases or decreases the density.
The correction unit includes a light source driving unit that modulates image data based on a pixel clock and outputs the modulated image data to the light source, and outputs the pixel data of the original image data from the light source driving unit. The period of the pixel clock may be corrected to a value larger or smaller than a reference value based on the density value at the position for pixels having the same positional relationship as the position of the density distribution in the main scanning direction.

Further, the correction unit performs the correction on the pixels having the same positional relationship as the position of the density distribution in the main scanning direction based on the difference between the density value at the position and a predetermined reference value. Also good.
The correction unit obtains the average density value of the reference pattern from the detection result of the detection unit, and for the pixels having the same positional relationship as the main scanning direction position of the density distribution, the density value at the position and the average density value The correction may be performed based on the magnitude of the difference.

  With the above configuration, even if the deflection surface of the polygon mirror is curved, the image data is corrected based on the detection result of the density distribution in the main scanning direction of the reference pattern on the sheet. The image quality of the formed image can be improved as compared with the conventional case in which no image is performed.

FIG. 2 is a schematic diagram for explaining a configuration of a printer according to the first embodiment. (A) is a top view for demonstrating the structure of the principal part of an optical scanning part, (b) is a front view of (a). It is a figure which shows the correspondence of the range of the rotation angle of the rotating shaft of a polygon mirror, and the deflection surface to which a laser beam is irradiated. It is a block diagram which shows the structure of a whole control part. It is a block diagram which shows the structure of LD control part. It is a block which shows the structure of a K color LD control part. It is a schematic diagram which shows a mode that the gradation value of each pixel of K color image data equivalent to one page is stored in the storage area of the image data storage unit in units of pixels. 6 is a timing chart showing a writing operation of a first scan line and a second scan line. It is a figure which shows the correspondence of the number of a scanning line, and the number of the deflection surface of a polygon mirror. (A) is a top view which shows the example of the shape of the deflection surface of a polygon mirror, (b), (c), (d) is each the expansion schematic diagram of three deflection surfaces. It is a figure which shows the structural example of the reference | standard pattern formed on the recording sheet. It is a schematic diagram which shows a mode that the gradation value of each pixel of the image data for forming a reference | standard pattern is stored in the storage area of the pattern data storage part per pixel. (A)-(d) is a timing chart of the formation operation of a reference pattern. (A)-(d) is a figure which shows the graph of the density distribution of the main scanning direction of the reference | standard pattern detected by the pattern detection sensor. It is a figure which shows a mode that the correction coefficient of the image data corresponding to each deflection | deviation surface is stored in the storage area of the correction coefficient memory | storage part. It is a figure which shows typically the waveform which shows the magnitude of the light quantity of the laser beam radiate | emitted from a light source part based on the image data after correction | amendment by a correction coefficient for each deflection surface. (A) is a schematic diagram which shows a mode that the gradation value of each pixel of the image data before correction | amendment is stored per pixel in the storage area of an image data storage part, (b) is an image after correction | amendment. It is a schematic diagram which shows a mode that the gradation value of each pixel of data is stored. 6 is a flowchart illustrating details of image data correction coefficient calculation and image data correction processing during print job execution. 5 is a block diagram illustrating a configuration of a K color LD control unit according to a second embodiment. It is a schematic diagram which shows the example of the waveform after correct | amending the pixel clock period which concerns on Embodiment 2 with respect to the reference value for each of each deflection | deviation surface. FIG. 4 is an enlarged schematic diagram illustrating a configuration example of a reference pattern formed on a recording sheet. 14 is a flowchart illustrating details of image data correction processing and image data correction processing when a print job is executed according to the third embodiment. It is a flowchart which shows the content of the display process of the warning message integrated between step S6 and S7 shown in FIG. (A) is a schematic plan view of a polygon mirror, (b) is a diagram schematically showing the state of deviation of the irradiation position of the light beam on the photosensitive drum in the main scanning direction, and (c). FIG. 5 is a schematic diagram showing a state in which a plurality of dots are arranged in a line in the main scanning direction.

Hereinafter, an example in which the embodiment of the image forming apparatus according to the present invention is applied to a tandem color printer (hereinafter referred to as “printer”) will be described.
[Embodiment 1]
<Schematic configuration of image forming apparatus>
FIG. 1 is a schematic diagram for explaining the configuration of the printer 10.

As shown in FIG. 1, the printer 10 is an electrophotographic image forming apparatus, and includes an image processing unit 11, a paper feeding unit 12, a fixing unit 13, and an overall control unit 14.
When the printer 10 is connected to a network (for example, a LAN) and receives an instruction to execute a print job from an external terminal device (not shown), the toner image includes yellow, magenta, cyan, and black colors based on the instruction. And the toner images of these colors are transferred in multiple to form a full color image. Hereinafter, the reproduction colors of yellow, magenta, cyan, and black are represented as Y, M, C, and K, and Y, M, C, and K are added as subscripts to the numbers of the components related to the reproduction colors.

The image processing unit 11 includes image forming units 30Y, 30M, 30C, and 30K corresponding to each of Y to K colors, an optical scanning unit 9 (exposure unit), an intermediate transfer belt 16, and the like.
The image forming unit 30 </ b> Y includes a photosensitive drum 31, a charging unit 32, a developing unit 33, a transfer roller 34 (transfer unit), a cleaner 35, and the like disposed around the photosensitive drum 31. Create a color toner image. The other image forming units 30M to 30K have the same configuration as that of the image forming unit 30Y, and the reference numerals are omitted in FIG.

The intermediate transfer belt 16 is an endless belt, is stretched around a driving roller 17 and a driven roller 18, and is rotationally driven in a direction indicated by an arrow A.
As will be described later, the optical scanning unit 9 includes a laser diode as a light source for each of the colors Y to K. A laser beam LY for forming an image of Y to K colors according to a drive instruction from the overall control unit 14. The LM, LC, and LK are emitted, and the surface of the photosensitive drum 31 of the corresponding image forming unit 30Y to 30K is exposed and scanned in the main scanning direction (normal direction of the paper surface).

By this exposure scanning, an electrostatic latent image is formed on the photosensitive drum 31 charged by the charging unit 32 for each of the image forming units 30Y to 30K, and the formed electrostatic latent image is developed with toner by the developing unit 33. Thus, a corresponding color toner image is formed on the photosensitive drum 31.
The image forming operation in each of the image forming units 30Y to 30K is executed while being shifted in timing by a predetermined time. The toner image developed on the photosensitive drum 31 for each of the image forming units 30Y to 30K is generated by the primary transfer roller 34. The toner images are sequentially transferred to the same position on the intermediate transfer belt 16 by electrostatic force to form a full-color toner image. Note that residual toner remaining on the photosensitive drum 31 without being transferred to the intermediate transfer belt 16 is cleaned by a cleaner 35.

On the other hand, the paper feeding unit 12 includes a paper feeding cassette 41 that stores the recording sheets S, a feeding roller 42 that feeds the recording sheets S in the paper feeding cassette 41 one by one onto the conveyance path 43, and a fed recording sheet S. Is provided with a timing roller pair 44 and the like for taking the timing of feeding the toner to the secondary transfer position 46.
The paper feeding unit 12 conveys the recording sheet S toward the secondary transfer position 46 in accordance with the movement timing of the toner image on the intermediate transfer belt 16, and the full color on the intermediate transfer belt 16 is operated by the secondary transfer roller 45. The toner image is secondarily transferred onto the recording sheet S.

The recording sheet S after passing through the secondary transfer position 46 is conveyed to the fixing unit 13. The toner image (unfixed image) on the recording sheet S is fixed to the recording sheet S by heating and pressurization by the fixing roller 13a and the pressure roller 13a provided in the fixing unit 13, and then the recording sheet S is discharged. The paper is discharged onto a discharge tray 48 through a roller pair 47.
The rollers such as the photosensitive drum 31, the intermediate transfer belt 16, the timing roller pair 44, and the fixing roller 13 a of the image forming units 30 </ b> Y to 30 </ b> K are rotationally driven by the driving motor 8 at a predetermined speed. As a result, the peripheral speed of the photosensitive drum 31 and the intermediate transfer belt 16 and the transport speed of the recording sheet S by each roller (sheet transport means) such as the timing roller pair 44 are maintained at the same constant speed (system speed).

  In the vicinity of the conveyance path 43 downstream of the fixing unit 13 in the sheet conveyance direction and upstream of the discharge roller pair 47 in the sheet conveyance direction, a density distribution in the main scanning direction of a reference pattern (described later) formed on the recording sheet S. A pattern detection sensor 7 is arranged as detection means for detecting. As the pattern detection sensor 7, a CCD (Charge Coupled Device) sensor is used. If the density distribution of the reference pattern in the main scanning direction can be optically detected, another image sensor such as a CMOS sensor may be used. You can also.

An operation unit 15 is disposed on the upper surface of the printer 10 at a position that is easy for the user to see. In addition to keys (not shown) for receiving various inputs such as print instructions and density settings by the user and instructions for executing calculation of correction coefficients (described later) of image data, the operation unit 15 provides warning messages (described later) to the user. Is displayed.
<Configuration of optical scanning unit>
FIG. 2A is a plan view for explaining the configuration of the main part of the optical scanning unit 9, and FIG. 2B is a front view of FIG. In order to make the internal structure easy to understand, the top surface or the side surface of the housing 50 is removed and shown in a perspective view. In FIG. 2B, the light source 51 shown in FIG. 2A is omitted. The X-axis direction shown in FIG. 2A is referred to as the main scanning direction, the Y-axis direction is referred to as the left-right direction, and the Z-axis direction shown in FIG.

As shown in FIG. 2A, the optical scanning unit 9 includes a housing 50, a light source unit 51, a light deflection unit 52 having a polygon mirror 55, a scanning lens group 53, a reflection mirror group 54, and an index sensor. 54Y, 54M, 54C, 54K are provided.
The light source unit 51 includes four semiconductor lasers (light sources) 81Y, 81M, 81C, 81K, four mirrors 82 to 85, and a cylindrical lens 86.

The semiconductor lasers 81Y, 81M, and 81K have the same laser beam (light beam) emission direction, and are arranged at equal intervals in a direction perpendicular to the direction. On the other hand, the semiconductor laser 81C is arranged so that the emission direction of the laser beam is orthogonal to the emission directions of the other semiconductor lasers 81Y, 81M, 81K.
Although not shown in FIG. 2, the height in the vertical direction of the laser beam exit (the position in the Z-axis direction in FIG. 2B) differs between the semiconductor lasers 81 </ b> Y to 81 </ b> K. The optical path height differs between the laser beams LY, LM, LC, and LK.

The mirrors 82, 83, and 84 are arranged so that only one laser beam LY, LM, or LK from each of the exit holes of the semiconductor lasers 81 </ b> Y, 81 </ b> M, and 81 </ b> K is irradiated.
The mirrors 82, 83, and 84 reflect and deflect the emitted light LY, LM, and LK of the semiconductor lasers 81Y, 81M, and 81K by 90 °. The mirror 85 is installed so as to reflect the reflected lights LY to LK of the other three mirrors 82 to 84 and the emitted light LC of the semiconductor laser 81C in the same direction. The height in the vertical direction of the mirrors 84, 83, and 82 gradually becomes shorter and shorter so that the reflected light LY, LM, LK and the outgoing light LC do not interfere with other mirrors before entering the mirror 85. Yes. The emitted lights LY to LK after being reflected by the mirror 85 are collectively referred to as laser light LL.

  The cylindrical lens 86 causes the laser light LL after being reflected by the mirror 85 to pass through the emission port 87 of the light source unit 51 and to be directed to the light deflecting unit 52. Specifically, the cylindrical lens 86 forms an image of the laser beam LL on the reflecting surface of the polygon mirror 55 in the axial direction of the rotation axis 56 of the polygon mirror 55 (the Z-axis direction shown in FIG. 2B). In the direction orthogonal to both the direction and the traveling direction of the laser beam LL (the direction parallel to the paper surface and perpendicular to the laser beam LL in FIG. 2A), the light is converted into parallel light.

The light deflection unit 52 includes a polygon mirror 55 and a polygon motor 57.
The polygon mirror 55 is a regular polygonal column (regular tetragonal column in the example of FIG. 2A), and the four side surfaces 1, 2, 3, and 4 are reflection mirror surfaces (deflection surfaces), respectively.
The polygon motor 57 applies a rotational driving force to the polygon mirror 55 to rotate the polygon mirror 55 around the rotation shaft 56 at the center in the direction indicated by the arrow E in FIG. In this embodiment, the rotation axis of the polygon motor 57 is used as it is as the rotation axis of the polygon mirror 55, and the polygon mirror 55 rotates once when the rotation axis of the polygon motor 57 rotates once. Hereinafter, the rotation axis of the polygon mirror 55 is referred to as a rotation axis 56.

  The polygon motor 57 outputs a reference position signal indicating that every time the rotation shaft 56 of the polygon mirror 55 makes one rotation, and when the rotation angle of the rotation shaft 56 during one rotation outputs the reference position signal. A detector (not shown) such as an encoder for outputting a rotation angle signal indicating which rotation angle (rotation position) is within a range from a rotation reference position (absolute position: 0 ° here) to 360 °. Has been placed.

By constantly monitoring the reference position signal and the rotation angle signal detected by this detector while the polygon mirror 55 is rotating, the polygon mirror 55 is rotated once and the rotation angle of the rotating shaft 56 within that rotation is grasped. can do.
In the present embodiment, as shown in FIG. 3, the irradiation of the laser beam LL emitted from the light source unit 51 when the rotation angle of the rotation shaft 56 is within a range from 0 ° (= rotation reference position) to 90 °. When the deflection surface 1 is positioned (see FIG. 2A) and the rotation angle is within the range of 90 ° to 180 °, the deflection surface 2 is located, and the rotation angle is within the range of 180 ° to 270 °. The polygon mirror 55 is fixed to the rotation shaft of the polygon motor 57 so that the deflection surface 3 is located in some cases and the deflection surface 4 is located when the rotation angle is in the range of 270 ° to 360 °.

  Further, since the time required from the detection of the rotation angle to the emission of the laser beam LL of the light source unit 51 is sufficiently short with respect to the time required for the rotation shaft 56 to rotate by 90 °, for example, a reference position signal is received. If the laser beam LL is emitted from the laser beam LL, the laser beam LL can be deflected by the deflecting surface 1. With respect to the deflection surface 2, if the laser beam LL is emitted after the rotation angle 90 ° of the rotation shaft 56 is detected, the laser beam LL can be deflected by the deflection surface 2. The same applies to the other deflecting surfaces 3 and 4, and if the laser beam LL is emitted after the rotation angle 180 ° (or 270 °) of the rotating shaft 56 is detected, the laser beam LL is deflected to the deflecting surface 3 (or 4). Can be deflected.

Returning to FIG. 2, during the rotation of the polygon mirror 55, the laser light emitted from the light source unit 51 while passing through the position facing the emission port 87 of the light source unit 51 for each of the deflection surfaces 1 to 4. The LL is reflected and deflected, and the angle formed by the traveling direction of the laser light LL and the reflected light RL by rotation, that is, the deflection angle of the laser light LL is changed.
The reflected light RL after being deflected by the polygon mirror 55, that is, the laser light LY, LM, LC, and LK, passes through the highest position in the vertical direction as shown in FIG. In the order of the laser beams LK, LM, and LY, the position in the vertical direction through which the laser beam passes is gradually lowered. This is because the incident angles of the laser beams LY, LM, LC, and LK on the deflection surface parallel to the rotation axis 56 of the polygon mirror 55 are different.

The laser beams LY to LK after being deflected by the polygon mirror 55 pass through scanning lenses 91, 92 and 93 included in the scanning lens group 53. The scanning lens 91 is a toroidal lens, the scanning lenses 92 and 93 are each aspherical lenses, and these two aspherical lenses constitute an fθ lens.
The scanning lenses 91 to 93 have power in each of the main scanning direction and the sub-scanning direction, and couple the passing laser beams LY to LK on the surface of the corresponding photosensitive drum 31 via the reflection mirror group 54. Let me image. Thereby, the imaging point on the surface is exposed.

  The reflection mirror group 54 includes first folding mirrors 101, 102, 103, and 104 that primarily reflect the laser beams LY, LM, LC, and LK after passing through the scanning lenses 91 to 93, and the first folding mirrors 101 and 102, respectively. The second folding mirrors 105, 106, and 107 secondarily reflect the laser beams LY, LM, and LC primarily reflected by the laser beams 103 and 103 toward the corresponding photosensitive drums 31 (FIG. 1). Only the first folding mirror 104 reflects the incident laser beam LK directly toward the photosensitive drum 31.

  As described above, the laser beams LY to LK emitted from the semiconductor lasers 81Y to 81K are deflected by the polygon mirror 55, transmitted through the scanning lenses 91 to 93, and then charged in the image forming units 30Y to 30K via the reflection mirror group 54. Irradiated to the photoconductive drum 31 in a state of being damaged. As a result, the photosensitive drum 31 is exposed and scanned by the laser beam along the main scanning direction which is the axial direction for each of the image forming units 30Y to 30K.

  A region (a substantially fan-shaped range in plan view) Da through which the reflected light RL deflected by the polygon mirror 55 shown in FIG. 2A passes is the main scan of the effective scanning region (image forming region) on the photosensitive drum 31. This corresponds to the scanning angle range of the reflected light RL when the exposure scanning is performed once from the one end to the other end in the direction. A latent image formed on the photosensitive drum 31 by one exposure scanning becomes one scanning line.

The index sensor 54Y is located outside the scanning angle range Da (non-image forming region), and is disposed at a position for receiving only the reflected light RL immediately before entering the scanning angle range Da, and deflects the polygon mirror 55 during rotation. Each time the laser beam LY deflected by each of the surfaces 1 to 4 is received, the detection result (INDEX signal) is sequentially sent to the overall control unit 14.
Whenever the INDEX signal is received from the index sensor 54Y for the Y color, the overall control unit 14 sets the scanning start timing for the effective scanning area on the photosensitive drum 31 by the laser beam LY for each scanning line based on the detection timing. Set to.

  Specifically, every time the laser beam LY after being deflected by each of the deflection surfaces 1 to 4 of the polygon mirror 55 is detected by the index sensor 54Y, an image of one scanning line is detected when a predetermined time has elapsed from the detection timing. Based on the data, the semiconductor laser 81Y is driven to start modulation of the laser light LY. That is, writing of one scanning line to the photosensitive drum 31 is started.

  The other index sensors 54M, 54C, 54K are also outside the scanning angle range Da like the index sensor 54Y, and are arranged at positions that receive only the laser beams LM, LC, LK immediately before entering the scanning angle range Da. . Similarly to the Y color, the M, C, and K colors have scanning start timings for the effective scanning areas by the laser beams LM, LC, and LK based on the detection timings of the laser beams LM, LC, and LK by the index sensors 54M, 54C, and 54K. Set for each scan line.

<Configuration of overall control unit>
FIG. 4 is a block diagram illustrating a configuration of the overall control unit 14.
As shown in the figure, the overall control unit 14 includes a communication interface (I / F) unit 61, a CPU 62, a ROM 63, a RAM 64, an image processing unit 65, an LD control unit 66, and a pattern formation control unit 67. And a pattern data storage unit 68.

The I / F unit 61 is an interface for connecting to a network such as a LAN board.
The ROM 63 stores programs necessary for controlling the image processing unit 11, the paper feeding unit 12, the fixing unit 13, the operation unit 15, and the like. The program stored in the ROM 63 is read and executed by the CPU 62.

The RAM 64 is used as a work area for the CPU 62 during program execution.
The image processing unit 65 converts print job data received from an external terminal via the I / F unit 61 into Y to K color image data, and performs image processing such as γ correction on the converted image data. .
The LD control unit 66 corrects the density variation in the main scanning direction of an image formed depending on the non-planarity existing on the deflecting surface when the deflecting surfaces 1 to 4 of the polygon mirror 55 are not flat. The image data is corrected using the coefficient, and the semiconductor lasers (LD) 81Y to 81K are driven based on the corrected image data to control the emission of the laser beams LY to LK.

The pattern formation control unit 67 executes a pattern formation operation for forming a reference pattern (FIG. 11) used for calculating the correction coefficient on the recording sheet S. This pattern forming operation will be described later.
The pattern data storage unit 68 stores in advance pattern data (image data) used for forming the reference pattern. The pattern data is read when the pattern formation control unit 67 executes the pattern formation operation.

<Configuration of LD control unit>
FIG. 5 is a block diagram illustrating a configuration of the LD control unit 66.
As shown in the figure, the LD control unit 66 includes a Y color LD control unit 66Y, an M color LD control unit 66M, a C color LD control unit 66C, and a K color LD control unit 66K for each of Y to K colors. Since these LD control units are basically the same in configuration, the configuration of the K color LD control unit 66K will be described here, and the description of the other LD control units 66Y to 66C will be omitted.

<Configuration of K color LD control unit>
FIG. 6 is a block diagram illustrating a configuration of the K color LD control unit 66K.
As shown in the figure, the K color LD control unit 66K includes an image data storage unit 661, a correction unit 662, an LD drive unit 663, and a pixel clock oscillator 664.
The image data storage unit 661 stores K color image data after image processing.

FIG. 7 is a schematic diagram showing a state in which the gradation value (density value) Dij of each pixel of K color image data corresponding to one page is stored in the storage area 665 of the image data storage unit 661 in units of pixels. The storage area 665 is two-dimensionally expanded in the main scanning direction and the sub-scanning direction.
One cell corresponds to one pixel, and the gradation value of one pixel here indicates a plurality of values, for example, any one of 256 levels from 0 to 255. Note that, here, one pixel is represented by one dot, but when one pixel is expressed by a plurality of dots, it can be regarded that one dot corresponds to one cell. it can.

The gradation value Dij of each pixel indicates a numerical value 1, 2, 3,... Sequentially assigned in the sub-scanning direction, and j indicates a numerical value 1, 2, 3,. Indicates.
In the figure, a pixel row along the main scanning direction corresponds to image data for forming a latent image of one scanning line, i = 1st scanning line 601 is a first scanning line, and i = 2nd scanning. The line 602 is referred to as a second scanning line, the i = 3rd scanning line 601 is referred to as a third scanning line, and the i = nth scanning line 601 is referred to as an nth scanning line.

Returning to FIG. 6, the correction unit 662 includes an image data correction unit 621, a correction coefficient calculation unit 622, and a correction coefficient storage unit 623.
The correction coefficient calculation unit 622 calculates a correction coefficient used to correct the image data based on the density detection result of the reference pattern by the pattern detection sensor 7. This calculation method will be described later.

The correction coefficient storage unit 623 is a non-volatile storage unit, and stores correction coefficient data calculated by the correction coefficient calculation unit 622. The stored data is read out to correct the image data at the time of printing executed after the calculation of the correction coefficient.
The image data correction unit 621 reads the correction coefficient from the correction coefficient storage unit 623 and corrects the gradation value Dij of each pixel of the K color image data stored in the image data storage unit 661 based on the read correction coefficient. .

The pixel clock oscillator 664 is an oscillator having a crystal resonator or the like, and outputs a clock pulse signal in which H level and L level are alternately repeated at a constant cycle (frequency) to the LD driving unit 663 as a pixel clock. One period of the pixel clock corresponds to one pixel of the formed image.
Each time the LD driving unit 663 receives an INDEX signal from the index sensor 54K by the laser light LK deflected by each of the deflecting surfaces 1 to 4 while the polygon mirror 55 is rotating, the corrected K for each scanning line. The semiconductor laser 81K is controlled to emit light based on the image data K obtained by modulating the color image data based on the pixel clock from the pixel clock oscillator 664, and laser light LK is emitted in units of pixels, and writing to the photosensitive drum 31 is executed. In this sense, the LD driving unit 663 and the pixel clock oscillator 664 can be said to be a light source driving unit that modulates image data based on the pixel clock and outputs the modulated image data to control light emission of the light source.

Hereinafter, writing of the first scanning line 601 and the second scanning line 602 shown in FIG. 7 will be specifically described with reference to the timing chart shown in FIG.
<First scanning line>
When the LD driving unit 663 receives a reference position signal (a signal indicating a rotation angle of 0 °) from the rotating polygon motor 57 (time point ta in FIG. 8), the image data for writing timing setting indicating a predetermined gradation value is obtained. Is output based on the pixel clock. Of the pulse waveform of the image data 551 in FIG. 8, the H level portion indicates light emission, and the L level portion indicates light extinction. Based on the image data 551, a laser beam LK is emitted from the semiconductor laser 81K. The emission of the laser beam LK is for setting the writing start timing before the writing of the first scanning line 601 to the photosensitive drum 31 is started. This is called pre-luminescence.

When the laser light LK emitted from the semiconductor laser 81K by the pre-emission is deflected by the deflecting surface 1 of the rotating polygon mirror 55 and then detected by the index sensor 54K (time point tm), an INDEX signal is output from the index sensor 54K. Is done.
Upon receiving the INDEX signal from the index sensor 54K, the LD driving unit 663 temporarily stops driving the semiconductor laser 81K and turns it off (end of the previous light emission), and when the fixed time has elapsed (time tn), the first scanning line Image data 601 (gradation values D ₁₁ , D ₁₂ , D ₁₃ ... Of each pixel) modulated based on the pixel clock is output to the semiconductor laser 81K as the image data 551a of the first scanning line 601 and the semiconductor laser 81K is driven.

  Similarly to the above, the H level portion of the pulse waveform of the image data 551a indicates light emission, and the L level portion indicates light extinction. One period of the pixel clock corresponds to one pixel, and the photosensitive drum is subjected to pulse amplitude modulation in which the light emission amount of the semiconductor laser 81K changes depending on the height (amplitude) of the H level of the pulse waveform of the image data 551a. The irradiation amount (exposure amount) of one pixel on 31 is changed. In the figure, an example in which the amplitude is constant is shown. For example, the larger the gradation value of the pixel, the larger the amplitude, that is, the amount of exposure of the photosensitive drum 31 increases, and the density of the formed image increases accordingly. Density setting is performed. Note that the density may be changed by variably controlling the exposure time by pulse width modulation in which the light emission amount of the semiconductor laser 81K is constant for each pixel and the light emission time is changed by the gradation value.

The resumption of driving of the semiconductor laser 81K (time tn) is the start of writing of the first scanning line 601 to the photosensitive drum 31.
The semiconductor laser 81K emits a laser beam LK having a light amount corresponding to the gradation values D ₁₁ , D ₁₂ , D ₁₃ ... In units of pixels based on the image data 551a, and the first scanning line 601 to the photosensitive drum 31 is emitted. Writing is executed. When this writing is completed (time tp), the driving of the semiconductor laser 81K is stopped.
<Second scanning line>
When the rotation angle signal indicating the rotation angle of 90 ° is received from the polygon motor 57 after the writing of the first scanning line 601 to the photosensitive drum 31 is finished (time point td), the previous light emission similar to the above is emitted to the image data 552. Based on.

When the laser beam LK emitted from the semiconductor laser 81K is deflected by the deflecting surface 2 of the rotating polygon mirror 55, is detected by the index sensor 54K, and receives the INDEX signal from the index sensor 54K (time point tq), the LD is driven. The unit 663 finishes the previous light emission and outputs the image data (tone values D ₂₁ , D ₂₂ , D ₂₃ ... Of each pixel) of the second scanning line 601 shown in FIG. The data modulated based on the pixel clock is output as image data 552a of the second scanning line 602, and the semiconductor laser 81K is driven.

Resumption of driving of the semiconductor laser 81K (time tr) starts writing of the second scanning line 602 to the photosensitive drum 31.
The semiconductor laser 81K emits a laser beam LK having a light amount corresponding to the gradation values D ₂₁ , D ₂₂ , D ₂₃ ... On the pixel basis based on the image data 552a, and the second scanning line 602 to the photosensitive drum 31 is emitted. Writing is executed. When this writing is completed (time ts), the driving of the semiconductor laser 81K is stopped.

For the third scanning line 603 and the fourth scanning line 604, the pre-emission and writing are repeatedly executed by the same operation as described above.
That is, when a rotation angle signal indicating a rotation angle of 180 ° (or 270 °) is received from the polygon motor 57, the pre-light emission is performed. The laser beam LK emitted from the semiconductor laser 81K is deflected by the deflecting surface 3 (or 4) of the polygon mirror 55 and then detected by the index sensor 54K. When the INDEX signal from the index sensor 54K is received, the pre-emission is finished, and the gradation values D ₃₁ , D ₃₂ , D ₃₃ ... () Of each pixel of the third scanning line 603 (fourth scanning line 604) when a predetermined time elapses. Alternatively, D ₄₁ , D ₄₂ , D ₄₃ ...) Modulated based on the pixel clock are output as image data of the third scanning line 603 (or fourth scanning line 604), and the semiconductor laser 81K is driven.

  The writing operation is basically the same as that described above for each scanning line after the fifth scanning line 605. However, at the time of writing the fifth scanning line 605, the polygon mirror 55 has rotated exactly one rotation from the start of writing of the first scanning line 601. Thus, the laser beam LK emitted from the semiconductor laser 81K is deflected by the deflection surface 1 of the polygon mirror 55. At the time of writing the next sixth scan line 605, the laser light LK emitted from the semiconductor laser 81K is deflected by the deflection surface 2.

  FIG. 9 is a diagram showing the relationship between the scanning line number and the deflection surface number of the polygon mirror 55. The scanning surface number is 1, 5, 9,. The deflection surface 2 is associated with the line numbers 2, 6, 10,..., The deflection surface 3 is associated with the scanning line numbers 3, 7, 11,. The deflection surface 4 is associated with 4, 8, 12,. That is, it is determined in advance which deflection surface is used for the nth scanning line.

After the first scanning line 601, when the writing to the last scanning line is finished, the writing operation to the photosensitive drum 31 corresponding to one page is finished, and the image forming operation is finished or writing for the second and subsequent pages is started. To do.
<About the flatness of the deflection surface of the polygon mirror>
In the configuration in which writing of one scanning line is sequentially performed for each of the deflection surfaces 1 to 4 as described above, it is ideal that all of the deflection surfaces 1 to 4 are perfect planes. In some cases, the deflection surface is microscopically curved.

FIG. 10A is a plan view showing an example in which the deflection surface 1 of the polygon mirror 55 is a flat surface and the deflection surfaces 2 to 4 are curved.
Specifically, the deflection surface 2 shows an example of a shape that is curved in a convex chevron outward with respect to a plane (broken line) as it goes from one end 2a to the other end 2b in the rotation direction (arrow E direction). . Moreover, the deflection surface 3 shows an example of a shape that is curved in a convex mountain shape inward with respect to a plane (broken line) as it goes from one end to the other end in the rotation direction. The deflection surface 4 has a shape in which a portion curved in a convex chevron shape inward and a portion curved in a convex chevron shape outward are continuous with respect to a plane (broken line) from one end to the other end in the rotation direction. An example is shown.

Here, FIG. 10B shows an enlarged schematic diagram of the deflecting surface 2, FIG. 10C shows an enlarged schematic diagram of the deflecting surface 3, and FIG. 10D shows an enlarged schematic diagram of the deflecting surface 4. Is shown. In each figure, the state of the curved or uneven surface of the reflecting surface is exaggerated and is actually extremely small.
When the deflecting surface of the polygon mirror 55 has a curved shape, the image quality of the formed image may be deteriorated as described in the section “Problems to be solved by the invention” above. The following reference pattern formation processing, density distribution detection processing, correction coefficient calculation processing, and image data correction processing are executed in this order, thereby suppressing image quality deterioration of an image formed during printing.

<Reference pattern formation control processing>
FIG. 11 is a diagram illustrating a configuration example of the K-color reference patterns 111 to 114 formed on the recording sheet S.
As shown in the figure, the reference patterns 111 to 114 are long and narrow along the main scanning direction, and are formed on the recording sheet S at regular intervals in the sub-scanning direction (corresponding to the sheet conveying direction). Has been.

  The reference pattern 111 is based on the image data of the reference pattern 111 in synchronization with the timing only when the deflecting surface 1 of the rotating polygon mirror 55 comes to the irradiation position of the laser light LK from the light source unit 51. Is formed by a plurality of scanning lines written on the charged photosensitive drum 31 by the deflected laser light LK after the emitted laser light LK is deflected by the deflecting surface 1. Pattern.

  The reference pattern 112 is based on the image data of the reference pattern 112 in synchronization with the timing only when the deflection surface 2 of the rotating polygon mirror 55 comes to the irradiation position of the laser light LK from the light source unit 51. Is formed by a plurality of scanning lines written on the charged photosensitive drum 31 by the deflected laser light LK after the emitted laser light LK is deflected by the deflecting surface 2. Pattern.

  Similarly, the reference pattern 113 (or 114) is synchronized with the timing only when the deflection surface 3 (or 4) of the rotating polygon mirror 55 comes to the irradiation position of the laser light LK from the light source unit 51. Based on the image data of the reference pattern 113 (or 114), the laser light LK is emitted from the light source unit 51. After the emitted laser light LK is deflected by the deflecting surface 3 (or 4), the deflected laser light LK This is a pattern formed by a plurality of scanning lines written on the charged photosensitive drum 31.

The amount of laser light LK emitted from the light source unit 51 is the same for each of the reference patterns 111 to 114.
Looking at the example in the figure, for the reference pattern 111, the density distribution along the main scanning direction is uniform, but for the reference patterns 112-114, the density is not uniform along the main scanning direction. You can see that it has occurred.

Specifically, the reference pattern 112 is darker at both ends in the main scanning direction than at the central portion in the main scanning direction. This is because the distribution shape of the dots 952 is increased at both ends in the main scanning direction as the scanning line 932 shown in FIG.
On the other hand, the reference pattern 113 is lighter at both ends in the main scanning direction than in the central portion in the main scanning direction. This is because the distribution density of the dots 953 is lowered at both ends in the main scanning direction as shown by a scanning line 933 shown in FIG. Further, the reference pattern 114 has the same density distribution as the reference pattern 113 on the left side of the center in the main scanning direction due to the curved shape of the deflection surface 4, and the density distribution on the right side like the reference pattern 112. Yes.

When the deflection surface is curved in this way, even in normal printing, the density distribution in the main scanning direction of the image formed on the recording sheet S varies depending on the degree of curvature of the deflection surface. This causes a reduction in the image quality of the formed image due to the curved shape of the deflection surface.
For each deflection surface of the polygon mirror 55, how much density variation in the main scanning direction occurs in the formed image of one scanning line by the curved deflection surface is determined in the main scanning direction of the reference pattern formed on the recording sheet S. This can be determined in advance by detecting the concentration distribution.

Therefore, for each of the deflection surfaces of the polygon mirror 55, a reference pattern is formed using the deflection surface, and the density distribution in the main scanning direction of the formed reference pattern is detected. Among the image data used at the time of printing, the image data used to emit laser light from the light source unit 51 toward the deflection surface is corrected.
Thereby, even if the deflection surface of the polygon mirror 55 is curved, the density distribution in the main scanning direction of the image of the scanning line actually written on the photosensitive drum 31 can be made uniform.

  FIG. 12 is a schematic diagram showing a state in which the gradation value of each pixel of the image data for forming the reference patterns 111 to 114 is stored in the storage area 680 of the pattern data storage unit 68 in units of pixels. A state where the region 680 is two-dimensionally expanded in the main scanning direction and the sub-scanning direction is shown, and an example in which the gradation values are 0 and 10 is shown. A gradation value of 0 indicates that the light amount is 0, that is, no light emission, and a gradation value of 10 indicates that light is emitted with a light amount corresponding to the gradation value of 10.

As shown in the figure, the gradation value is the same for each scanning line, the gradation value is 10 for the odd numbered pixels and the gradation value is 0 for the even numbered pixels in the main scanning direction. ing.
Since the gradation values of the pixels arranged in the main scanning direction are 10, 0, 10, 0, 10,... For each scanning line, the image data of the reference patterns 111 to 114 is stored for each scanning line. It can be called pattern data in which dot (hereinafter referred to as “black pixel”) data exposed in the main scanning direction and dot data (hereinafter referred to as “white pixel”) data not exposed are arranged alternately. Thereby, one scanning line constituting the reference pattern becomes an image in which black pixels and white pixels are alternately arranged in the main scanning direction as shown in the enlarged view of the balloon in FIG.

This is because a curved surface is formed by interposing a white pixel between two adjacent black pixels, rather than continuously arranging a plurality of black pixels in the main scanning direction. This is because it becomes easy to determine the density difference between images.
Specifically, when the degree of curvature is large, for example, the interval β shown in FIG. 24C is further narrowed and a plurality of black pixels 952 overlap each other to form a black solid image, the interval β is set. Even if there is a difference, the detected density value does not change (the detection sensitivity is almost absent).

  On the other hand, if a white pixel is interposed between two black pixels, a solid image often does not reach a solid image even if the interval β is reduced to some extent. If the interval β changes, the detected density value also changes. This is because it becomes easier to calculate a correction coefficient with higher accuracy according to the degree of curvature. Note that the configuration is not limited to a configuration in which black pixels and white pixels are alternately arranged one by one in the main scanning direction, but two or more consecutive black pixel columns and two or more consecutive white pixel columns are alternately arranged. It can also be configured. That is, one or more black pixels and one or more white pixels can be arranged alternately.

The reference pattern 111 is a pattern formed by the laser light LK deflected only on the deflection surface 1 of the polygon mirror 55 as described above, and the scanning line number corresponding to the deflection surface 1 among the deflection surfaces 1 to 4 is From the correspondence shown in FIG.
Accordingly, the reference pattern 111 is synchronized with the timing of the first scanning line 681 and the fifth scanning line 681 every time the deflecting surface 1 of the rotating polygon mirror 55 comes to the irradiation position of the laser light LK from the light source unit 51. It can be said that the pattern is formed by emitting the laser light LK from the light source unit 51 based on the image data of the scanning lines in the order of the scanning line 685 and the ninth scanning line 689. This is the same as in normal printing.

  Specifically, as shown in the timing chart of FIG. 13A, when the rotation angle of the polygon mirror 55 reaches 0 ° (time point ta), the preceding light emission is started, and the INDEX signal is received with a delay of time Δt. Then, the pre-emission is finished, and the image data of the first scanning line 681 (the gradation value of each pixel 10, 0, 10, 0...) Is modulated based on the pixel clock from the pixel clock oscillator 664 when a predetermined time elapses. The driving of the semiconductor laser 81K is started by the image data thus obtained (time point t1).

  The laser beam LK emitted from the semiconductor laser 81K is deflected by the deflecting surface 1, and the photosensitive drum 31 charged during rotation is exposed and scanned by the deflected laser beam LK, and a latent image on the first scanning line 681 is obtained. Are formed (first exposure scan). Time points t1 to t2 correspond to the time required for exposure scanning of one scanning line by the laser beam LK with respect to the effective scanning region on the photosensitive drum 31, and the time point t1 starts the exposure scanning and the time point t2 ends the exposure scanning.

After completion of the first exposure scan, the semiconductor laser 81K remains off until the time tb when the rotation angle of the polygon mirror 55 reaches 0 ° again. That is, the deflection surfaces 2 to 4 are not irradiated with the laser beam LK and are not affected by the deflection surfaces 2 to 4.
When the rotation angle of the polygon mirror 55 reaches 0 ° again (time tb), the same control as the first exposure scan is repeatedly executed. That is, when a predetermined time elapses after the start of the previous light emission, the reception of the INDEX signal, and the end of the previous light emission, the image data of the fifth scanning line 685 (the gradation value of each pixel is 10, 0, 10, 0...) The driving of the semiconductor laser 81K is started by the image data modulated based on the image data. The laser beam LK emitted from the semiconductor laser 81K is deflected by the deflecting surface 1, and a latent image of the fifth scanning line 685 is formed on the photosensitive drum 31 rotating in a charged state by the deflected laser beam LK. (Second exposure scanning).

Similarly to the period from the end of the first exposure scan to the start of the second exposure scan, after the end of the second exposure scan, again until the time tc when the rotation angle of the polygon mirror 55 reaches 0 °, The semiconductor laser 81K remains off.
When the rotation angle of the polygon mirror 55 reaches 0 ° again (time tc), the same as in the first and second exposure scans, the start of the previous light emission, the reception of the INDEX signal, and the end of the pre-light emission when the predetermined time has elapsed. The driving of the semiconductor laser 81K is started by image data obtained by modulating the image data of the nine scanning lines 689 (the gradation values of each pixel 10, 0, 10, 0...) Based on the pixel clock. The laser beam LK emitted from the semiconductor laser 81K is deflected by the deflecting surface 1, and a latent image of the ninth scanning line 689 is formed on the photosensitive drum 31 rotating in a charged state by the deflected laser beam LK. (Third exposure scan). Thereafter, similarly to the above, the exposure scanning operation is repeatedly executed in the order of the 13th scanning line, the 17th scanning line,..., And an electrostatic latent image of the reference pattern 111 is formed on the photosensitive drum 31.

  Since the laser beam LK is deflected only on the deflecting surface 1 while the polygon mirror 55 makes one rotation to form one scanning line, the reference pattern 111 is adjacent to the first scanning line 681 that is adjacent in the sub-scanning direction. The distance between the fifth scanning line 685 in the sub-scanning direction, the distance between the fifth scanning line 685 and the ninth scanning line 689 in the sub-scanning direction,... This is a region where no image is formed (see an enlarged view of a balloon in FIG. 11). The same applies to the other reference patterns 112 to 114.

  Returning to FIG. 12, since the reference pattern 112 is a pattern formed by the laser light LK deflected only on the deflection surface 2 of the polygon mirror 55, the deflection surface 2 of the rotating polygon mirror 55 is moved from the light source unit 51. The laser beam LK is emitted from the light source unit 51 in accordance with the image data of the scanning line in the order of the second scanning line 682, the sixth scanning line 686,. It is formed by being emitted.

  Specifically, as shown in the timing chart of FIG. 13B, when the rotation angle of the polygon mirror 55 reaches 90 ° (time td), the pre-light emission is started, and then the pre-light emission is received when the INDEX signal is received. After the predetermined time has elapsed, the image data of the second scanning line 682 (the gradation values of each pixel, 10, 0, 10, 0...) Is modulated based on the pixel data from the pixel clock oscillator 664. The driving of the semiconductor laser 81K is started.

The laser beam LK emitted from the semiconductor laser 81K is deflected by the deflecting surface 2, and the deflected laser beam LK is exposed and scanned on the rotating photosensitive drum 31 in a charged state, and a latent image on the second scanning line 682 is obtained. Are formed (first exposure scan).
After completion of the first exposure scan, the semiconductor laser 81K remains off until the time te when the rotation angle of the polygon mirror 55 reaches 90 °.

  When the rotation angle of the polygon mirror 55 reaches 90 ° again (time te), the same control as the first exposure scan is repeatedly executed. That is, the pixel clock of the image data of the sixth scanning line 686 (the gradation value of each pixel is 10, 0, 10, 0...) When a predetermined time has elapsed since the start of the previous light emission, the reception of the INDEX signal, and the end of the previous light emission The driving of the semiconductor laser 81K is started by the image data modulated based on the above. The laser beam LK emitted from the semiconductor laser 81K is deflected by the deflecting surface 2, and a latent image of the sixth scanning line 686 is formed on the photosensitive drum 31 rotating in a charged state by the deflected laser beam LK. (Second exposure scanning).

After completion of the second exposure scan, the semiconductor laser 81K remains off until the rotation angle of the polygon mirror 55 reaches 90 ° again. When the rotation angle of the polygon mirror 55 reaches 90 ° again, the third exposure scan is executed in the same manner as the first and second exposure scans. Thereafter, each control described above is repeatedly executed.
The same applies to the other reference patterns 113 and 114.

  That is, the reference pattern 113 (or 114) is a pattern formed by the laser light LK deflected only by the deflection surface 3 (or 4) of the polygon mirror 55, and the deflection surface 3 (or the rotation surface of the polygon mirror 55 being rotated). Each time 4) comes to the irradiation position of the laser beam LK from the light source unit 51, the third scanning line 683, the seventh scanning line 686... (Or the fourth scanning line 684, the eighth scan are synchronized with the timing. The laser light LK is emitted from the light source unit 51 based on the image data of the scanning lines in the order of the lines 688.

  As shown in the timing charts of FIGS. 13C and 13D, when the rotation angle of the polygon mirror 55 reaches 180 ° (or 270 °) (time tf or tg), pre-emission is started, and then the INDEX signal is When the light is received, the pre-emission is terminated, and the image data (the gradation value of each pixel is 10, 0, 10, 0,...) Of the third scanning line 683 (or the fourth scanning line 684) when a predetermined time elapses. The driving of the semiconductor laser 81K is started by the image data modulated based on the clock.

The laser beam LK emitted from the semiconductor laser 81K is deflected by the deflecting surface 3 (or 4), and the deflected laser beam LK is exposed and scanned on the photosensitive drum 31 that is rotating in a charged state. A latent image of 683 (or the fourth scanning line 684) is formed (first exposure scanning).
After completion of the first exposure scan, the semiconductor laser 81K remains off until the time th (or ti) when the rotation angle of the polygon mirror 55 reaches 180 ° (or 270 °) again.

  When the rotation angle of the polygon mirror 55 reaches 180 ° (or 270 °) again, the same control as in the first exposure scan is repeatedly performed at the time (th or ti). That is, when a predetermined time has elapsed since the start of the previous light emission, the reception of the INDEX signal, and the end of the previous light emission, the image data of the seventh scan line 687 (or the eighth scan line 688) (the gradation values 10, 0, 10,. The driving of the semiconductor laser 81K is started by image data obtained by modulating 0 ..) based on the pixel clock. The laser light LK emitted from the semiconductor laser 81K is deflected by the deflecting surface 3 (or 4), and the seventh scanning line 687 (or the first scanning line) is formed on the photosensitive drum 31 rotating in a charged state by the deflected laser light LK. A latent image of 8 scanning lines 688) is formed (second exposure scanning). Each control is repeatedly executed in the same manner as described above.

The operation of forming the K-color reference patterns 111 to 114 is executed separately in the order of the reference patterns 111, 112, 113, and 114 after a certain period of time. As a result, the latent images of the reference patterns 111 to 114 are separately formed in this order on the rotating photosensitive drum 31.
The latent images of the K-color reference patterns 111 to 114 formed on the rotating photosensitive drum 31 are developed with the K-color toner in the developing unit 33 in the order of formation, and the developed reference patterns 111 to 114 are developed. The K-color toner image is primarily transferred onto the intermediate transfer belt 16 by the primary transfer roller 34, and then secondarily transferred onto the recording sheet S by the secondary transfer roller 45 at the secondary transfer position 46 and fixed. The image is fixed on the recording sheet S by the section 13. FIG. 11 shows an example of the K reference patterns 111 to 114 formed on the recording sheet S after fixing. In this way, a series of processes of charging, exposure, development, and transfer is executed for each reference pattern.

While the recording sheet S that has passed through the fixing unit 13 passes the detection position of the pattern detection sensor 7, the K reference patterns 111 to 114 on the recording sheet S are detected by the pattern detection sensor 7 in this order.
<Regarding the density distribution of the reference pattern in the main scanning direction>
FIG. 14 is a diagram illustrating a density distribution graph of the K-color reference pattern detected by the pattern detection sensor 7 in the main scanning direction. FIG. 14A illustrates a density distribution graph 191 of the reference pattern 111. ) Shows the density distribution graph 192 of the reference pattern 112, (c) shows the density distribution graph 193 of the reference pattern 113, and (d) shows the density distribution graph 194 of the reference pattern 114. In each figure, the horizontal axis indicates the position in the main scanning direction, and the vertical axis indicates the density.

The pattern detection sensor 7 includes a light emitting unit and a large number of photoelectric conversion elements arranged in the main scanning direction, and uniformly irradiates the reference pattern on the recording sheet S with a certain amount of light from the light emitting unit and reflects the light. Each photoelectric conversion element receives light, and each photoelectric conversion element outputs an electric signal (for example, a voltage value) corresponding to the amount of light received.
The higher the distribution density of the black pixels constituting the reference pattern in the main scanning direction, the smaller the amount of reflected light, and the lower the distribution density, the greater the amount of reflected light. Therefore, the voltage value output from each photoelectric conversion element The size represents the density of the density of the reference pattern. Therefore, the density distribution in the main scanning direction of the reference pattern can be acquired by detecting the magnitude of the output voltage value of each photoelectric conversion element.

In the present embodiment, based on the density detection result of the pattern detection sensor 7, the area from one end to the other end in the main scanning direction of the effective scanning area on the photosensitive drum 31 is divided by the width of one pixel. For each of a plurality of different predetermined positions, a position distribution corresponding to the position and the density value corresponding to the position is detected as a density distribution.
As shown in FIG. 14A, the density of the reference pattern 111 is substantially constant at the reference value R from the position A at one end to the position B at the other end in the main scanning direction.

  As described above, the reference pattern 111 is a pattern formed by using only the deflection surface 1 of the polygon mirror 55, and the first scanning line which is image data for forming the reference pattern 111 as shown in FIG. 681, the fifth scanning line 685,... Have the same gradation value (= 10) for each black pixel. For this reason, with respect to the deflecting surface 1 which is a flat surface, the exposure position of each pixel of the first, fifth,... Scanning lines on the photosensitive drum 31 is essentially the same as the scanning line 931 shown in FIG. Since the amount of the laser light LK for each black pixel is the same, there is no difference in density between one end and the other end in the main scanning direction for each scanning line.

  On the other hand, when viewing the density distribution graph 192 of the reference pattern 112 shown in FIG. 14B, the density increases as it goes from the center in the main scanning direction to the position A on one end and the position B on the other end. It has become. As described above, the gradation value of each pixel of the second scanning line 682, the sixth scanning line 686, etc., which is image data for forming the reference pattern 112, is an image for forming the reference pattern 111. It is the same as the gradation value of each pixel of the first scanning line 681 that is data, and the execution condition of the reference pattern formation is different only in which deflection surface is used.

  For this reason, the density distribution in the main scanning direction of the reference pattern 111 formed using the flat deflection surface 1 is uniform, whereas the density in the main scanning direction of the reference pattern 112 formed using the deflection surface 2 is uniform. Is not uniform, it can be determined that the flatness of the deflecting surface 2 is not secured and the curved surface is formed. The same applies to the density distribution graph 193 of the reference pattern 113 shown in FIG. 14C and the density distribution graph 194 of the reference pattern 114 shown in FIG. 14D.

  Since both the formation of the reference pattern and the image formation of the normal print job use the deflection surfaces 1 to 4 of the polygon mirror 55, the semiconductor laser is driven based on the image data of the pixels having the same gradation value in the normal print job. Even when the image is formed, an image having a non-uniform density distribution in the main scanning direction is formed on the scanning lines formed using the deflection surfaces 2 to 4 like the reference patterns 112 to 114. Become.

In order to solve this problem, in the present embodiment, for each deflection surface, a function indicating the inverse characteristic of the density distribution characteristic (function representing the graph) in the main scanning direction of the reference pattern obtained from the detection result by the pattern detection sensor 7 is provided. It is obtained as a correction coefficient (correction information) for correcting image data, and control is performed to correct image data of a print job to be executed thereafter by applying the correction coefficient obtained in advance. The correction coefficient is calculated by the correction coefficient calculation unit 622 and stored in the correction coefficient storage unit 623. In the present embodiment, the correction coefficient is shared for each of the Y to K colors, and the image data is corrected for each of the Y to K colors by applying the common correction coefficient.
<Correction coefficient of image data>
A correction coefficient of the image data, that is, a function of an inverse characteristic with respect to the density distribution in the main scanning direction of the reference pattern is represented by a one-dot chain line graph 192a shown in FIG. It is represented by a one-dot chain line graph 193a shown in FIG. 14C, and the deflection surface 4 is represented by a one-dot chain line graph 194a shown in FIG.

Specifically, the correction coefficient of the image data is obtained as follows.
For example, the deflection surface 2 will be described. The position where the density is a predetermined reference value R as shown by the graph (function) 192a (the center in the main scanning direction in FIG. 14B) is the reference position. At each position in the main scanning direction other than the reference position, a value Q obtained by subtracting the density value R at the reference position (for example, Qa at position B) from the reference position is converted into a gradation value, and the converted value is A correction coefficient. In the example of the graph 192a, the correction coefficient is negative, and in the example of the graph 193a, the correction coefficient is positive.

The calculation method of the correction coefficient is the same for the other deflection surfaces 3 and 4. The obtained correction coefficient is stored in the correction coefficient storage unit 623. Instead of the configuration in which the reference value R is determined in advance, for example, an average value can be calculated from the density value at each position, and the calculated average value can be used as the reference value R.
FIG. 15 is a diagram illustrating a state in which the correction coefficients of the image data corresponding to the deflection surfaces 1 to 4 are stored in the storage area 700 of the correction coefficient storage unit 623, and the correction coefficients a ₁₁ and a aligned in the main scanning direction. The column 701 of ₁₂ ... Indicates the correction coefficient for the deflection surface 1, the column 702 of the correction coefficients a ₂₁ , a ₂₂ ... Indicates the correction coefficient for the deflection surface 2, the correction coefficients a ₃₁ , a _32. column 703 indicates a correction coefficient for deflection surface 3, the correction coefficient a _14, a ₄₂ ··· column 704 indicates a correction coefficient for deflection surface 4.

One correction coefficient corresponds to one pixel, and numbers 1, 2, 3,... Attached in the order of alignment along the main scanning direction are the numbers 1 of pixels formed at different positions in the main scanning direction. 2, 3... Correspond one to one.
Therefore, for example, the correction coefficient a ₁₁ corresponding to the deflection surface 1 is the gradation value D ₁₁ of the first pixel of the first scanning line 601 and the first pixel level of the fifth scanning line 605 shown in FIG. It becomes a common correction coefficient applied to each of the tone values D ₅₁ . Similarly, the correction coefficient a ₁₂ is applied to each of the gradation value D ₁₂ of the second pixel of the first scanning line 601 and the gradation value D ₅₂ of the second pixel of the fifth scanning line 605. Common correction coefficient.

Further, for example, the correction coefficient a ₂₂ corresponding to the deflection surface 2 is the gradation value D ₂₂ of the second pixel of the second scanning line 602 and the second pixel of the sixth scanning line 606 shown in FIG. It becomes a common correction coefficient applied to each of the gradation values D ₆₂ .
The application of the correction coefficient to the gradation value of each pixel of the original image data (image data before correction) is performed here by adding the correction coefficient to the original gradation value. For example, in the example of the gradation value D ₂₁ of the first pixel in the second scanning line 602 of the original image data, it becomes (D ₂₁ + a ₂₁ ), but since a ₂₁ is negative as described above, it is corrected. Is corrected to a value lower than the original gradation value D ₂₁ .

  In this way, the correction coefficient is calculated for each different position (pixel unit position) in the main scanning direction of the density distribution graph, and has the same positional relationship as the main scanning direction position of the density distribution for each main scanning direction position. For a certain pixel, the gradation value (data indicating the density) of the pixel is set to a value at which the density becomes higher or lower than the original value based on the density value at the position (for example, Qa in FIG. 14B). It is corrected. If the relationship between the pixels in the same position as the detected density value at each position of the density distribution in this manner is regarded as the synchronization of the position in the main scanning direction, the correction by the correction coefficient is as follows. It can be said that correction is performed using the detected density distribution on the original image data of the image to be formed in synchronization with the position of the density distribution in the main scanning direction.

  FIG. 16 is a diagram schematically showing a waveform indicating the magnitude (light quantity value) of the laser light emitted from the light source unit 51 based on the image data corrected by the correction coefficient for each of the deflection surfaces 1 to 4. is there. In the figure, all the pixels of the image data before correction have the same gradation value, and an example in which the light emission amount becomes the reference value R when there is no correction is shown. Further, the magnitude of the light amount value increases as it goes to the H side (upper side) with respect to the reference value R, and becomes smaller as it goes to the L side (lower side) with respect to the reference value R. ing.

As shown in the figure, when the waveform 711 of the light amount value of the laser beam with respect to the deflecting surface 1 is viewed, the light amount value is the reference value R from the time point t1 (exposure scanning start) to the time point t2 (exposure scanning end). It is almost constant.
On the other hand, when the waveform 712 of the light quantity value of the laser beam with respect to the deflecting surface 2 is seen, it has an upward convex chevron shape, and the light quantity value is equal to the reference value R between the time points t1 and t2, and At t2, it is lower than the reference value R. Further, the waveform 713 of the laser light quantity value with respect to the deflection surface 3 has a downwardly convex mountain shape, and the waveform 714 of the laser light quantity value with respect to the deflection surface 4 is a waveform obtained by connecting the waveforms 713 and 712 together. It has become.

The waveform 712 is similar to the waveform of the correction coefficient graph 192a shown in FIG. 14B, the waveform 713 is similar to the waveform of the correction coefficient graph 193a shown in FIG. It is similar to the waveform of the correction coefficient graph 194a shown in FIG. From this, it can be said that for each deflection surface of the polygon mirror 55, the waveform of the light amount value of the laser light based on the corrected image data is a waveform having a shape similar to the graph of the correction coefficient.
<Image data correction method>
FIG. 17 is a schematic diagram showing how the gradation value of each pixel of the image data stored in the storage area 665 of the image data storage unit 661 changes before and after correction, and (a) shows the state before correction. (B) shows after correction.

  As shown in FIG. 17A, before correction, the gradation value of all the pixels is the same value, which is 10 here, and after correction, the deflection surface of the polygon mirror 55 is corrected as shown in FIG. For the first scanning line 601 and the fifth scanning line 605... Corresponding to 1, the gradation value is 10, which is the same as before the correction. As a result, as shown in a graph 711 in FIG. 16, the light amount value of the laser light is also constant at the reference value R for each pixel.

  On the other hand, for the second scanning line 602, the sixth scanning line 606,... Corresponding to the deflection surface 2 of the polygon mirror 55, the gradation value is 10 near the center in the main scanning direction, which is the same as before the correction. In the vicinity of positions A and B at both ends in the scanning direction, the gradation value is 8, which is lower than before correction. As a result, the light quantity value of the laser light applied to the deflecting surface 2 is lower at both ends with respect to the central portion in the main scanning direction as shown in a graph 712 shown in FIG.

  Accordingly, due to the curved shape of the deflecting surface 2 as in the second scanning line 932 shown in FIG. 24C, the pitch of the black pixels in the main scanning direction at the both ends in the main scanning direction on the photosensitive drum 31 is originally higher. Even if it becomes narrow, the density of the black pixel also decreases as the amount of light of the laser beam decreases. As a result, it is possible to prevent the user's eyes from appearing at a higher density than the center in the main scanning direction as in the conventional configuration in which the amount of laser light is not corrected.

  Further, for the third scanning line 603, the seventh scanning line 607,... Corresponding to the deflection surface 3 of the polygon mirror 55, the gradation value is 10 near the center in the main scanning direction and is not changed from that before the correction. In the vicinity of positions A and B at both ends in the scanning direction, the gradation value is 12, which is higher than before correction. As a result, the light quantity value of the laser light applied to the deflecting surface 3 is higher at both ends with respect to the central portion in the main scanning direction as shown by a graph 713 in FIG.

  Therefore, due to the curved shape of the deflecting surface 3 as in the third scanning line 933 shown in FIG. 24C, the pitch of the black pixels in the main scanning direction at the both ends in the main scanning direction on the photosensitive drum 31 is originally higher. Even if it becomes wider, the density of the black pixel increases as the amount of light of the laser beam increases. As a result, it is possible to prevent the user's eyes from appearing at a lower density than the center in the main scanning direction as in the conventional configuration in which the amount of laser light is not corrected.

  Similarly, with respect to the fourth scanning line 604 corresponding to the deflection surface 4 of the polygon mirror 55, the eighth scanning line 608,..., The correction value whose gradation value corresponds to the deflection surface 4 (graph 194a in FIG. 14D). Accordingly, the waveform of the light amount value of the laser beam irradiated on the deflecting surface 4 changes as shown in a graph 714 shown in FIG. This suppresses the user's eyes from seeing density unevenness in the main scanning direction as compared with the conventional configuration in which the amount of laser light is not corrected.

<Calculation of correction coefficient of image data and control of correction of image data>
FIG. 18 is a flowchart showing the contents of image data correction coefficient calculation and image data correction processing at the time of print job execution. When the printer 10 is turned on, the entire control is performed every time a predetermined time elapses. It is repeatedly executed by the unit 14.
As shown in the figure, it is determined whether an instruction to execute correction coefficient calculation has been accepted (step S1). This determination is made when a correction coefficient calculation instruction key (not shown) provided on the operation unit 15 is pressed by a user or a service person. The user or service person can instruct the execution of the calculation of the correction coefficient of the image data by the correction coefficient calculation instruction key when the new printer 10 is first delivered to the office or at any time after the delivery. .

  If it is determined that an instruction to execute correction coefficient calculation has not been received (“No” in step S1), it is determined whether a print instruction has been received (step S8). If it is determined that a print instruction has not been received (“No” in step S8), the control is terminated. When it is determined that the execution instruction has been accepted when the control is started again after a lapse of a certain time from this end ("Yes" in step S1), the polygon motor 57 starts to rotate (step S1). S2) When the rotation speed of the polygon motor 57 is stabilized (“Yes” in step S3), a pattern forming operation for printing the K reference patterns 111 to 114 on the recording sheet S is performed (step S4).

The pattern formation operation is controlled by the pattern formation control unit 67 for each of the deflection surfaces 1 to 4, that is, the above-described charging, exposure, development, transfer, and fixing units, that is, the K image forming unit 30 </ b> K and the optical scanning unit. 9, the intermediate transfer belt 16, the paper feeding unit 12, the secondary transfer roller 45, the fixing unit 13 and the like are controlled.
The reference patterns 111 to 114 are formed in a certain order in the order of the reference pattern 111 using only the deflection surface 1, the reference pattern 112 using only the deflection surface 2, the reference pattern 113 using only the deflection surface 3, and the reference pattern 114 using only the deflection surface 4. Executed. As a result, as shown in FIG. 11, the toner images of the K-color reference patterns 111, 112, 113, and 114 are spaced apart from each other in the sheet conveying direction in this order from the leading end side to the trailing end side of the recording sheet S. In this state, the recording sheet S is formed.

  When the recording sheet S that has passed through the fixing unit 13 passes through the detection position of the pattern detection sensor 7, the toner images of the reference patterns 111 to 114 formed on the recording sheet S are transferred to the reference patterns 111, 112, 113 by the pattern detection sensor 7. , 114, the detection results are acquired (step S5). Here, the density distribution in the main scanning direction of the reference pattern is detected as described above.

Then, based on the detection result of the density distribution in the main scanning direction by the pattern detection sensor 7, the correction coefficients (graphs (functions) 191a to 194a shown in FIG. 14) of the image data are calculated for each of the reference patterns 111 to 114 (see FIG. 14). Step S6).
The calculated correction coefficient is associated with each of the deflection surfaces 1 to 4 and stored in the storage area 700 (FIG. 15) of the correction coefficient storage unit 623 (step S7), and the control is terminated.

When it is determined that a print instruction has been accepted when a certain time has elapsed from this end and the control is started again ("Yes" in step S8), the polygon motor 57 is started to rotate (step S9). Then, the correction coefficient of the image data stored in the storage area 700 of the correction coefficient storage unit 623 is read (step S10).
Then, based on the read correction coefficient, the Y to K image data (FIG. 17A) used in the printing is corrected (step S11). This correction is performed for each image data of Y to K colors, for each image data of the first scanning line, the fifth scanning line,... Corresponding to the deflection surface 1 of the polygon mirror 55 as described above. Is performed by applying one correction coefficient corresponding to the pixel among the correction coefficients a ₁₁ , a ₁₂ , a ₁₃ ... For the deflection surface 1. The same applies to each of the deflection surfaces 2 to 4 (FIG. 17B).

When the rotation speed of the polygon motor 57 is stabilized (“Yes” in step S12), image formation is executed using the corrected Y to K image data (step S13), and the control is terminated.
When it is determined that the execution instruction for calculating the correction coefficient has been accepted again when the control is started after a lapse of a certain time from this end (“Yes” in step S1), the processing in steps S2 to S7 is performed. Run. At this time, in step S7, the newly calculated correction coefficient is overwritten and saved.

In the above description, a configuration example in which it is determined that a correction coefficient calculation execution instruction has been received by pressing a key on the operation unit 15 has been described. However, the present invention is not limited to this. For example, when the optical scanning unit 9 is replaced with a new one, it may be configured that an instruction to execute correction coefficient calculation is accepted when the replacement is determined. This exchange determination can be made as follows.
That is, the INDEX signal is output four times from the index sensor 54K while the polygon mirror 55 makes one rotation in the previous light emission. Time from the first reception of the INDEX signal to the second reception (time points tm to tq in FIG. 8) Time from the second reception to the third reception From the third reception to the fourth reception The time from the fourth reception to the first reception of the next second rotation is measured. Each time is small but different for each optical scanning unit 9. Therefore, the four measurement times measured at a certain time point are stored, and the four measurement times measured at the subsequent time points are compared with the past four measurement times stored. If the two coincide completely, the optical scanning unit 9 is not exchanged. If even one measurement time does not coincide, it can be determined that the optical scanning unit 9 has been exchanged for a new one.

As described above, in the present embodiment, each of the deflection surfaces 1 to 4 of the polygon mirror 55 is actually used under the same conditions as in printing, that is, exposure scanning, development, and transfer processes. The reference patterns 111 to 114 are formed on the recording sheet S, and the correction coefficient of the image data is calculated for each deflection surface based on the detection result of the density distribution in the main scanning direction of the formed reference patterns 111 to 114. Since the image data is corrected with the correction coefficient during the subsequent image formation, even if the deflection surfaces 1 to 4 are curved with respect to the regular plane, the density difference (unevenness) in the main scanning direction of the formed image is suppressed. can do.
[Embodiment 2]
In the first embodiment, the configuration example in which the gradation value of the image data is corrected based on the correction coefficient of the image data, that is, the light emission amount from the semiconductor laser of the light source unit 51 is corrected in units of pixels has been described. 2, the pixel clock cycle is corrected in units of pixels instead of correcting the gradation values of the image data, and this is different from the first embodiment. Hereinafter, in order to avoid duplication of description, the description of the same contents as those of Embodiment 1 is omitted, and the same components are denoted by the same reference numerals.

FIG. 19 is a block diagram illustrating a configuration of the K-color LD control unit 66K according to the second embodiment.
As shown in the figure, the configuration of the K-color LD control unit 66K is basically the same as that of the first embodiment (FIG. 6). However, the pixel clock oscillator 664 of the second embodiment uses a pixel clock for each pixel. An oscillator capable of frequency modulation is used, and the correction unit 662 does not correct the image data stored in the image data storage unit 661 but causes the pixel clock oscillator 664 to correct the cycle of the pixel clock. It is different in that it is.

The LD drive unit 663 uses the original image data (before modulation) stored in the image data storage unit 661 as the pixel clock from the pixel clock oscillator 664 (instructed to be corrected by the image data correction unit 621). The modulated image data K is output to the semiconductor laser 81K.
FIG. 20 is a schematic diagram illustrating an example of waveforms 721 to 724 after the pixel clock period is corrected with respect to the reference value R for each of the deflection surfaces 1 to 4. The reference value R corresponds to the above-described fixed period.

  The pixel clock cycle indicates that the cycle becomes longer as the positive value with respect to the reference value R becomes larger, and the cycle becomes shorter as the negative value becomes larger. Further, the amount of increase / decrease in the period with respect to the reference value R is indicated by a percentage (%), and a value obtained by adding / subtracting the increase / decrease amount to / from the reference value R indicates a corrected pixel clock period, and this increase / decrease amount is the above-described image data It corresponds to the correction coefficient. In the present embodiment, the pixel clock cycle is corrected, the gradation value of the image data for each pixel is not corrected, and the semiconductor laser 81K emits light with the light emission amount corresponding to the original gradation value. Be controlled.

Looking at the waveform 721 for the deflection surface 1, the pixel clock cycle is substantially constant at the reference value R from the exposure scanning start time t1 to the exposure scanning end time t2, and the correction coefficient is substantially zero. It shows that there is. This is the same as the light quantity value in Embodiment 1 being substantially constant.
Looking at the waveform 722 for the deflecting surface 2, it has a downwardly convex chevron shape, and the corrected pixel clock cycle is equal to the reference value R in the middle of the time points t1 and t2, and from the reference value R at the time points t1 and t2. Is also getting longer.

  The longer pixel clock cycle means that the emission interval of the laser light emitted from the light source unit 71 in units of pixels is longer than when the reference value R is set, and the main scanning of each pixel that is exposed and scanned on the photosensitive drum 31 accordingly. This means that the direction pitch (interval) is wider than the reference value R. Accordingly, for example, as shown in FIG. 24C, β of each pixel 952 of the scanning line 932 in the configuration in which no correction is performed can be expanded to the reference value α, and the density difference from the central portion in the main scanning direction can be suppressed. .

As in the first embodiment, the correction coefficient for correcting the pixel clock cycle is based on the detection result of the density distribution in the main scanning direction of the reference pattern, and the main scanning of the density distribution so that the density unevenness of the formed image is suppressed. It is determined in units of pixels according to the magnitude of the density value at the directional position, and is stored in the storage area 700 of the correction coefficient storage unit 623 in units of pixels for each of the deflection surfaces 1 to 4.
The correction unit 662 reads out the correction coefficient (the increase / decrease amount (%) of the period with respect to the reference value R) stored in the correction coefficient storage unit 623 in units of pixels, and corrects the period of the pixel clock for the pixel for each pixel. The pixel clock oscillator 664 is instructed to execute processing for correcting the reference value R to a value larger or smaller than the reference value R based on the coefficient.

The pixel clock oscillator 664 generates and outputs a pixel clock in which the period of the pixel clock is corrected based on the correction coefficient in units of pixels based on an instruction from the correction unit 662.
The LD driving unit 663 outputs image data K modulated based on the corrected pixel clock output from the pixel clock oscillator 664 to the semiconductor laser 81K.
In the example of the waveform 722 for the deflection surface 2 shown in FIG. 20, the correction coefficient is the largest at + 0.5% at both ends (near the time points t1 and t2) where the density difference is the largest with respect to the center in the main scanning direction. The correction coefficient (function) has such characteristics that the correction coefficient becomes smaller as it approaches the center, and the correction coefficient becomes 0 at the center in the main scanning direction.

  Looking at the waveform 723 with respect to the deflection surface 3, it has an upwardly convex chevron shape, and the pixel clock period is equal to the reference value R in the middle of the time points t1 and t2, and with respect to the reference value R at the time points t1 and t2. -0.5% shorter. That the pixel clock cycle is short means that the emission interval of the laser light emitted from the light source unit 71 in units of pixels is shorter than that at the reference value R, and the main scanning direction of each pixel exposed and scanned on the photosensitive drum 31. This means that the pitch (interval) is narrower than the reference value R. Accordingly, for example, as shown in FIG. 24C, the pitch γ of each pixel 953 of the scanning line 933 in the configuration in which no correction is performed can be narrowed to the reference value α, and the density difference from the central portion in the main scanning direction is suppressed. it can.

Looking at the waveform 724 for the deflection surface 4, the upward convex waveform and the downward convex waveform are continuous, and the pitch of each pixel is set to the reference value α as in the deflection surfaces 2 and 3. Accordingly, density unevenness in the main scanning direction can be suppressed.
Correction of the pixel clock period can be performed by using an oscillator capable of frequency modulation, but a configuration different from this can also be used. For example, an oscillator that outputs a clock pulse signal having the same period as the reference value R and a plurality of oscillators that output clock pulse signals having different periods different from the reference value R are arranged, and the period from time t1 to t2 is set. Is divided into a plurality of time blocks, and for each time block, an oscillator that can output a clock pulse signal that is the same as or approximate to the pixel clock cycle to be output is selected from the plurality of oscillators. From the start to the end, the oscillator to be driven can be switched so that the clock pulse signal is output only from the selected oscillator.

Specifically, as shown in FIG. 8, a first oscillator that outputs a clock pulse signal of a first period (> reference value R) for a time block from time tr to time tu for the second scan line is used. For example, a second oscillator that outputs a clock pulse signal of the second period (<first period) is used for the time block from the time point tu to the time point tv.
The pixel clock cycle after correction may not completely match the pixel clock cycle to be output, but the density in the main scanning direction of the formed image due to the curved shape of the deflection surface compared to the conventional case where correction is not performed. Unevenness can be suppressed.
[Embodiment 3]
In the first and second embodiments, the rotation speed (rpm) of the polygon mirror 55 at the time of forming the reference pattern, the rotation speed of the photosensitive drum 31 and the intermediate transfer belt 16, and the conveyance speed of the recording sheet S are set to normal printing (images). In the third embodiment, the number of rotations and the system speed are the same as those at the time of formation. However, in the third embodiment, the rotation speed of the polygon mirror 55 is the same as that during normal printing. Is configured to decelerate more than normal printing, which is different from the first and second embodiments. In the third embodiment, a drive motor 8 (drive means) having a variable rotation speed is used, and the system speed is reduced by reducing the rotation speed of the drive motor 8 from the normal time.

When the rotation speed of the polygon mirror 55 is the same as that at the time of normal printing and the system speed is, for example, half that of normal printing (half speed), the following occurs.
That is, since the rotation speed of the polygon mirror 55 is the same as that in the normal state, the time required for exposure scanning on the photosensitive drum 31 for one scanning line from one end to the other end in the main scanning direction (main scanning). The period) is the same as normal.

On the other hand, if the system speed is set to half the normal speed, the movement distance in the rotational direction of the circumferential surface of the photosensitive drum 31 per unit time is half that of the normal speed, so that the reference pattern 111 is formed on the photosensitive drum 31. The line spacing in the sub-scanning direction between the first scanning line 681 and the fifth scanning line 685.
FIG. 21 is an enlarged schematic diagram illustrating a configuration example of the reference pattern 111 formed on the recording sheet S. As shown in the figure, the line spacing in the sub-scanning direction of the first scanning line 681, the fifth scanning line 685, the ninth scanning line 689,... Constituting the reference pattern 111 is ΔA when the system speed is normal. Become. In the first and second embodiments, since the system speed at the time of forming the reference pattern is the same as that at the normal time, the line interval is ΔA as in the figure.

On the other hand, when the rotation speed of the polygon mirror 55 is the same as that at the normal time and the system speed is half that at the normal time, the formation position of the fifth scan line 685 becomes the position indicated by the broken line 685a with respect to the first scan line 681. The interval becomes ΔB which is half of ΔA at normal time. For the subsequent ninth scan lines 689..., The line interval is similarly ΔB.
This is because the number of scanning lines included per unit length in the sheet conveyance direction (corresponding to the sub-scanning direction) on the recording sheet S increases, and the recording sheet S is more decelerated than usual. Since the number of black pixels per unit area increases, that is, the number of white pixels decreases, the density of the reference pattern increases accordingly. Thereby, for example, even when the pattern detection sensor 7 having a low detection sensitivity for a light density is used, the reference pattern 111 can be detected with high accuracy. Although the reference pattern 111 has been described above, the same applies to the other reference patterns 112 to 114.

FIG. 22 is a flowchart showing the content of the correction processing of the image data and the correction processing of the image data when executing the print job according to the third embodiment.
The flowchart shown in the figure is almost the same as the flowchart shown in FIG. 18, but steps S21, S41, and S91 are executed instead of steps S2, S4, and S9, and this point is different.

In steps S21 and S91, the polygon motor 57 is started to rotate at the same rotational speed as in normal printing.
In step S41, a pattern forming operation for printing the K reference patterns 111 to 114 on the recording sheet S is performed in a state in which the system speed is reduced to half the speed during normal printing. The half speed of the system speed is performed by reducing the rotational speed of the drive motor 8 to half of that during normal printing, and this deceleration control is performed by the overall control unit 14.

In this way, the density distribution in the main scanning direction is formed by forming the reference pattern in a state in which the polygon motor 57 is rotationally driven at the same rotational speed as in normal printing and the system speed is reduced to half that in normal printing. Can be detected with high accuracy.
In the above description, the example of the deceleration control in which the system speed is reduced to half of the normal printing speed has been described. However, the control is not limited to half, and may be reduced.

  For example, when M is an integer, the speed can be set to (1 / M) with respect to the normal printing speed. The integer M can also be the number of deflection surfaces of the polygon mirror 55 (4 in the above example). In this case, the line interval is substantially zero. As the line interval becomes narrower, the number of scanning lines to be detected at the time of one sampling increases, and an inexpensive pattern detection sensor 7 having a low detection sensitivity without reducing the detection accuracy of the density distribution in the main scanning direction. Can be used.

Further, for example, the reference pattern is formed at the system speed during normal printing. However, since the detection accuracy of the pattern detection sensor 7 is low, only a value lower than the lower limit value of the predetermined density range to be detected can be detected. In such a case, it is also possible to switch to deceleration control and perform control for executing the formation of the reference pattern again.
The present invention is not limited to an image forming apparatus, and may be a method for correcting image data, for example. Furthermore, the method may be a program executed by a computer. The program according to the present invention includes, for example, a magnetic disk such as a magnetic tape and a flexible disk, an optical recording medium such as a DVD-ROM, DVD-RAM, CD-ROM, CD-R, MO, and PD, and a flash memory recording medium. It can be recorded on various computer-readable recording media, and may be produced, transferred, etc. in the form of the recording medium, wired and wireless various networks including the Internet in the form of programs, In some cases, the data is transmitted and supplied via broadcasting, telecommunication lines, satellite communications, or the like.
[Modification]
As described above, the image forming apparatus according to the present invention has been described based on the embodiment. However, the content of the present invention is not limited to the above-described embodiment, and for example, the following modifications may be considered. It is done.

  (1) In the above embodiment, a configuration example in which correction coefficients common to Y to K colors are calculated by forming the K reference patterns 111 to 114 is described, but the present invention is not limited to this. The positions at which the laser beams LY to LK emitted from the semiconductor lasers 81Y to 81K are applied to one deflection surface of the polygon mirror 55 are different in the rotation axis direction (height direction) of the polygon mirror 55 on the deflection surface. (Deviated). Therefore, strictly speaking, the degree of deviation of the deflection angle due to the curved shape of the deflection surface differs for each of the laser beams LY to LK.

  If the difference between the colors becomes too large, a configuration using a common correction coefficient can suppress a decrease in image quality for a formed image of one color, but may not be able to suppress a decrease in image quality for a formed image of another color. obtain. In such a case, a reference pattern is separately formed for each of the colors Y to K, and a correction coefficient is obtained for each color, thereby suppressing deterioration in the image quality of the formed image during printing for each color. Can do.

Further, in the configuration in which the exposure unit 9 is individually arranged for each of the image forming units 30Y to 30K, it is possible to take a configuration in which a reference pattern is formed for each color and a correction coefficient is obtained for each color.
(2) In the above-described embodiment, the example in which the polygon mirror 55 has a regular quadrangular prism shape having four deflection surfaces has been described. However, the present invention is not limited to this, and for example, a positive mirror having six deflection surfaces is used. A hexagonal prism shape or a regular octagonal prism shape having eight deflection surfaces can also be used. For each of the plurality of deflection surfaces, formation of the reference pattern, detection of the density distribution, calculation of the correction coefficient, and application of the calculated correction coefficient to the image data are the same as described above.

(3) In the above embodiment, while the pattern detection sensor 7 is disposed at an intermediate position between the fixing unit 13 and the discharge roller pair 47, the recording sheet S being conveyed passes through the detection position of the pattern detection sensor 7. In addition, the configuration example for detecting the reference pattern formed on the recording sheet S has been described, but the configuration is not limited thereto.
For example, in a device configuration such as a copying machine having both a scanner function and a print function, a scanner capable of detecting the density distribution in the main scanning direction of the reference pattern formed on the recording sheet S without providing the pattern detection sensor 7. It is also possible to adopt a configuration for detecting by the above. Specifically, when the recording sheet S on which the reference pattern is formed is accommodated in the discharge tray 48 outside the apparatus, the user manually removes the recording sheet S from the discharge tray 48 and removes the removed recording sheet S from the scanner. After setting to the reading position, the scanner performs a reference pattern reading operation on the recording sheet S.

  (4) In the first embodiment, the configuration example in which the correction coefficient of the image data is calculated from the detection result of the density distribution in the main scanning direction of the reference pattern formed on the recording sheet S has been described. If the correction coefficient is not within the predetermined range, the degree of curvature of the deflection surface exceeds the assumed value, and it is assumed that the image data correction cannot suppress the deterioration of the image quality, and the polygon mirror 55 (exposure unit 9) has an abnormality. A configuration may be adopted in which a warning message to the effect is displayed on the operation unit 15 to notify the user or service person.

FIG. 23 is a flowchart showing the contents of the warning message display process incorporated between steps S6 and S7 shown in FIG.
When the correction coefficients are calculated as shown in FIG. 23 (step S6), it is determined whether or not all of the calculated correction coefficients are within a predetermined range (step S61). The predetermined range is a range of correction coefficients that are assumed to ensure a minimum image quality by correcting the image data even when the deflection surface is curved, and is determined in advance by experiments or the like.

If it is determined that it is within the predetermined range (“Yes” in step S61), the process proceeds to step S7. This case is the same as the first embodiment. On the other hand, if it is determined that it is not within the predetermined range (“No” in step S61), the above warning message is displayed on the display unit 15a of the operation unit 15 (step S62), and the control is terminated.
In this case, after the user or serviceman who has seen the warning message repairs or replaces the exposure unit 9 with a new one, the correction coefficient is within a predetermined range when the correction coefficient is calculated again. If determined, the warning message is turned off. Even if a warning message is displayed, the execution of printing may be permitted as it is. Alternatively, the execution of printing is prohibited by displaying a warning message, and if the warning message is turned off thereafter, the prohibition is prohibited. It is also possible to cancel and permit execution of printing.

The notification method to the user or the like is not limited to the message display, and may be, for example, a warning voice output or a warning sent to the service person's terminal device via the network.
(5) In the above embodiment, the correction coefficient corresponding to each deflection surface is calculated for all the deflection surfaces 1 to 4 of the polygon mirror 55. However, the present invention is not limited to this. For example, the deflection coefficient is calculated for at least one deflection surface. Even when the correction coefficient corresponding to the surface is calculated, it is possible to prevent the image quality of the formed image from being lowered as compared with the conventional case.

  (6) In the above embodiment, the configuration example in the case where the image forming apparatus is applied to a tandem color printer has been described. However, the present invention is not limited to this. An image forming apparatus having a configuration in which a light beam emitted from a light source is deflected by a rotating polygon mirror, and the surface of a photosensitive member such as a photosensitive drum or a photosensitive belt is exposed and scanned by the deflected light beam. For example, the present invention can be applied to a copying machine, a facsimile machine, an MFP (Multiple Function Peripheral), and the like.

Further, the configuration example in which the toner image after development on the photosensitive member is transferred to the recording sheet S via the intermediate transfer member such as the intermediate transfer belt 16 has been described, but the present invention is not limited to this. The present invention can also be applied to a configuration in which the toner image on the photosensitive member is directly transferred to the recording sheet S without using an intermediate transfer member.
Further, the configurations of the above embodiment and the above modification may be combined as much as possible.

  The present invention can be widely applied to an image forming apparatus in which a light beam emitted from a light source is deflected by a polygon mirror, and the photosensitive member is exposed and scanned by the deflected light beam.

1, 2, 3, 4 Polygon mirror deflection surface 7 Pattern detection sensor 8 Drive motor 9 Optical scanning unit (exposure unit)
DESCRIPTION OF SYMBOLS 10 Printer 15 Operation part 31 Photosensitive drum 33 Developing part 34, 45 Transfer roller 55 Polygon mirror 57 Polygon motor 66 LD control part 67 Pattern formation part 68 Pattern data storage part 81Y, 81M, 81C, 81K Semiconductor laser 111, 112, 113 114 Reference pattern 191a, 192a, 193a, 194a Correction coefficient graph 621 Image data correction unit 622 Correction coefficient calculation unit 623 Correction coefficient storage unit 661 Image data storage unit 662 Correction unit 663 LD drive unit 664 Pixel clock oscillators 701, 702, 703, 704 A column indicating a correction coefficient

Claims (14)

A photoreceptor,
A charging unit for charging the photoreceptor;
The light source is driven based on the image data, and the light beam emitted from the light source is sequentially deflected by each of the plurality of deflection surfaces of the rotating polygon mirror, and is charged by each light beam deflected by each deflection surface. An exposure unit that exposes and scans the photoconductor in the main scanning direction to form an electrostatic latent image;
A developing unit for developing the electrostatic latent image on the photoreceptor with toner;
A transfer unit that transfers the developed toner image from the photosensitive member to a sheet via a sheet or an intermediate transfer member;
The charging unit, the exposure unit, the developing unit, and the transfer unit are controlled to drive the light source based on the image data of the reference pattern along the main scanning direction, and the light beam from the light source is one deflection surface of the polygon mirror The electrostatic latent image of the reference pattern is formed on the charged photosensitive member, and the electrostatic latent image of the reference pattern is developed. Then, the developed toner image is transferred to a sheet or an intermediate transfer member. Pattern formation control means for performing a series of processes to be transferred to the sheet via,
Detecting means for detecting a density distribution in the main scanning direction of the toner image of the reference pattern after being transferred to the sheet;
After completion of detection by the detection means, correction means for correcting the original image data of the image to be formed using the density distribution detected by the detection means and synchronizing with the position in the main scanning direction of the density distribution When,
An image forming apparatus comprising: The correction means includes
Calculation means for obtaining a correction coefficient for suppressing density variation in the main scanning direction of an image formed depending on non-planarity existing on the one deflection surface based on the density distribution detected by the detection means; ,
Storage means for storing the correction coefficient obtained by the calculation means,
The image forming apparatus according to claim 1, wherein the correction is performed by applying a correction coefficient stored in the storage unit to the original image data.   The image forming apparatus according to claim 2, further comprising a notification unit that notifies that an abnormality has occurred in the exposure unit when the correction coefficient is not within a predetermined range. The image data of the reference pattern is
Consists of a plurality of dot data arranged in a line in the main scanning direction,
The dot data includes image data in which the photoconductor is exposed and non-exposed with a light beam, and image data in which one or more exposed dot data and one or more unexposed dot data are alternately repeated. The image forming apparatus according to claim 1, wherein the image forming apparatus is provided. Receiving means for receiving an execution instruction for forming the reference pattern;
The pattern formation control means includes
The image forming apparatus according to claim 1, wherein when the receiving unit receives the execution instruction, the reference pattern is formed. A determination means for determining that the exposure unit has been replaced;
The image forming apparatus according to claim 5, wherein the receiving unit receives the determination of the determination unit as the execution instruction. Drive means for rotationally driving the photoreceptor;
Control means for controlling the drive means so that the peripheral speed of the photoconductor becomes a predetermined system speed during normal image formation;
With
The pattern formation control means includes
At the time of forming the reference pattern, the polygon mirror is rotated with respect to the exposure unit at a rotation speed at the time of normal image formation, and the driving unit is controlled so that the peripheral speed of the photoconductor is higher than the system speed. The image forming apparatus according to claim 1, wherein speed control for decelerating is performed. When the number of deflection surfaces of the polygon mirror is M,
The pattern formation control means includes
The image forming apparatus according to claim 7, wherein the peripheral speed of the photosensitive member is reduced to a speed of (1 / M) with respect to the system speed. The pattern formation control means includes
The density value detected by the detecting means is a lower limit value of a predetermined density range after the reference pattern is formed under the same conditions as those for normal image formation with the rotational speed of the polygon mirror and the peripheral speed of the photoconductor. 9. The image forming apparatus according to claim 7, wherein when only a lower value can be detected, the reference pattern is formed again by switching to the deceleration control. The pattern formation control means includes
For each of the polygon mirror deflection surfaces, a one-to-one reference pattern is formed on each deflection surface;
The detection means includes
Detecting the density distribution for each of the reference patterns;
The correction means includes
For each of the deflection surfaces, a density distribution detected by a reference pattern corresponding to the deflection surface is obtained with respect to original image data of an image used to emit a light beam from the light source toward the deflection surface. The image forming apparatus according to claim 1, wherein the correction is performed by using the image forming apparatus. The correction means includes
Data indicating the density of each pixel of the original image data is a value that increases or decreases with respect to the original value based on the density value at the position of the pixel having the same positional relationship as the position in the main scanning direction of the density distribution. The image forming apparatus according to claim 1, wherein the image forming apparatus corrects the value to the following value. The correction means includes
A light source driving unit that modulates image data based on a pixel clock and outputs the modulated image data to the light source,
The period of the pixel clock output from the light source driving unit for each pixel of the original image data is set to a reference value based on the density value at the position of the pixel having the same positional relationship as the position in the main scanning direction of the density distribution. The image forming apparatus according to claim 1, wherein the image forming apparatus corrects the value to a large value or a small value. The correction means includes
12. The correction is performed on a pixel having the same positional relationship as a main scanning direction position of the density distribution based on a magnitude of a difference between a density value at the position and a predetermined reference value. Or the image forming apparatus of 12. The correction means includes
Obtain the average density value of the reference pattern from the detection result of the detection means,
13. The correction is performed on a pixel having the same positional relationship as a main scanning direction position of the density distribution based on a difference between a density value at the position and the average density value. The image forming apparatus described in 1.