Image formation apparatus

Abstract

To reduce a positional deviation of writing start in a main scan direction.SOLUTION: An image formation apparatus comprises: a photoreceptor which is driven to rotate; generation means which generates image signals on the basis of image data; scanning means which forms an electrostatic latent image on the photoreceptor by scanning the photoreceptor in a main scan direction with a scan beam based on the image signals from the generation means; and control means which outputs synchronous signals indicating second timing to determine first timing at which the generation means outputs the image signals to the scanning means. The control means has correction information indicating correction values, and determines the second timing on the basis of third timing notified from the scanning means and the correction values.SELECTED DRAWING: Figure 11

Description

[Technical field]

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

[0002] In an electrophotographic image forming apparatus, a photoconductor is rotated and repeatedly scanned and exposed by scanning light emitted based on image data to form an electrostatic latent image on the photoconductor. The electrostatic latent image is developed with a developer such as toner, and a toner image (toner image) is formed on the photoconductor. The image forming apparatus has an optical scanning device equipped with a scanning lens for concentrating the scanning light on the photoconductor and a rotating polygon mirror for moving the scanning light in the main scanning direction. The main scanning direction is parallel to the rotation axis of the photoconductor. The trajectory of the scanning light on the photoconductor is referred to as a scanning line, and the moving speed of the scanning light on the photoconductor is referred to as a scanning speed. In a color image forming apparatus, a photoconductor corresponding to each of a plurality of colors used for image formation is provided.

[0003] In an image forming apparatus, a horizontal synchronization signal is used to determine the timing to start scanning of a photoconductor (hereinafter, the scanning start timing). Hereinafter, the position in the main scanning direction of the photoconductor where the scanning light is scanning at the scanning start timing is referred to as the scanning start position. The scanning start position is the position where the formation (writing) of an electrostatic latent image starts in the scanning of the photoconductor, and can also be referred to as the formation start position. If the actual scanning start position differs from the target position, it causes color shift. Patent Document 1 discloses a configuration in which the period from the generation timing of the horizontal synchronization signal to the scanning start timing is measured and stored in advance for each image forming apparatus. Patent Document 1 discloses a technology in which, in order to correct the scanning start position, the writing start timing of the laser beam in the optical scanning device is measured in advance based on the horizontal synchronization signal, and the measurement result is stored in the image forming apparatus as correction information. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-300604 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, when correcting the scanning start position based on the horizontal synchronization signal, if the timing of the horizontal synchronization signal deviates from the ideal due to variations in the assembly of the optical scanning device or errors in mounting the sensor, this can cause image defects.

[0006] The present invention provides a technique for reducing the writing start position deviation in the main scanning direction. [Means for solving the problem]

[0007] According to one aspect of the present invention, an image forming apparatus includes a photosensitive member that is rotated, a generating means for generating an image signal based on image data, a scanning means for forming an electrostatic latent image on the photosensitive member by scanning the photosensitive member in a main scanning direction with a scanning light based on the image signal from the generating means, and a control means for outputting to the generating means a synchronization signal indicating a second timing for determining a first timing at which the generating means outputs the image signal to the scanning means, wherein the control means has correction information indicating a correction value and determines the second timing based on a third timing notified by the scanning means and the correction value. Effect of the Invention

[0008] According to the present invention, it is possible to reduce the writing start position deviation in the main scanning direction. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram illustrating the configuration of an image forming apparatus according to an embodiment. [Diagram 2] FIG. 1 is a configuration diagram of an optical scanning device according to an embodiment. [Diagram 3] 5A and 5B are diagrams illustrating a relationship between image height and partial magnification according to an embodiment. [Figure 4] FIG. [Diagram 5] FIG. 2 is a block diagram of a modulation section according to one embodiment. [Figure 6] FIG. 4 is an explanatory diagram of halftone processing according to an embodiment. [Figure 7] 11 is a timing chart of partial magnification correction and brightness correction according to an embodiment. [Figure 8] FIG. 1 is a diagram illustrating the timing shift of the BDI signal caused by a placement error of the BD sensor. [Figure 9] 1 is a diagram illustrating how the timing of a horizontal synchronization signal is shifted due to a positional error in the BD sensor. [Figure 10] 6 is a diagram illustrating the effect of deviation in the timing of a horizontal synchronization signal. [Figure 11] FIG. 2 illustrates an exposure control configuration according to an embodiment. [Figure 12] An explanatory diagram showing how the timing of the horizontal synchronization signal does not shift even if the timing of the BDI signal shifts. [Figure 13] 6 is a timing chart of partial magnification correction and density correction according to an embodiment. [Figure 14] FIG. 4 is a diagram illustrating an example of frequency correction information according to an embodiment. [Figure 15] FIG. 2 is a block diagram of a modulation section according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0011] FIG. 1 is a schematic diagram of an image forming apparatus according to the present embodiment. In the following drawings, components that are not necessary for understanding the embodiment are omitted for the sake of simplicity. The image forming units 500Y, 500M, 500C, and 500K form yellow, magenta, cyan, and black toner images on the intermediate transfer body 504, respectively. The image forming units 500Y, 500M, 500C, and 500K form toner images on the intermediate transfer body 504 in a superimposed manner, thereby reproducing colors different from yellow, magenta, cyan, and black. The image forming units 500Y, 500M, 500C, and 500K have the same configuration, and each includes a photoconductor 4, a charging roller 502, an optical scanning device 400, a developing roller 501, and a primary transfer roller 503. In the following description, the image forming units 500Y, 500M, 500C, and 500K are also collectively referred to as image forming units 500.

[0012] During image formation, the photoconductor 4 is rotated counterclockwise in the figure. The charging roller 502 charges the surface of the rotating photoconductor 4 to a uniform potential. The optical scanning device 400 repeatedly scans the rotating photoconductor 4 in the main scanning direction with the scanning light 208 based on image data, thereby forming an electrostatic latent image on the photoconductor 4. The main scanning direction is parallel to the rotation axis of the photoconductor 4 and is the direction in which the scanning light 208 moves. The direction perpendicular to the main scanning direction and in which the scanning lines are formed in sequence is referred to as the sub-scanning direction. In the photoconductor 4, the direction opposite to the rotation direction of the photoconductor 4 corresponds to the sub-scanning direction. The developing roller 501 develops the electrostatic latent image on the photoconductor 4 with toner to form a toner image on the photoconductor 4. The primary transfer roller 503 transfers the toner image on the photoconductor 4 to the intermediate transfer body 504. During image formation, the intermediate transfer body 504 is rotated clockwise in the figure. Therefore, the toner image on intermediate transfer body 504 is transported to a position facing secondary transfer roller 506. Secondary transfer roller 506 transfers the toner image on intermediate transfer body 504 onto a sheet transported along transport path 505. Thereafter, the sheet is transported to a fixing unit (not shown) where the toner image is fixed. After the toner image is fixed, the sheet is discharged outside the image forming apparatus.

[0013] FIG. 2 is a configuration diagram of an optical scanning device 400 according to this embodiment. In FIG. 2(A), the direction from top to bottom corresponds to the main scanning direction, and in FIG. 2(B), the direction perpendicular to the paper surface corresponds to the main scanning direction. The scanning light (light beam) 208 emitted by the light source 401 is shaped into an elliptical shape by the aperture stop 402 and enters the coupling lens 403. The scanning light 208 that passes through the coupling lens 403 is converted into a substantially parallel light and enters the anamorphic lens 404. The anamorphic lens 404 has a positive refractive power in the main scanning cross section, and converts the incident scanning light 208 into a convergent light in the main scanning cross section. In addition, the anamorphic lens 404 focuses the light beam near the reflecting surface 405a of the deflector 405 in the sub-scanning cross section, forming a long line image in the main scanning direction.

[0014] The scanning light 208 that passes through the anamorphic lens 404 is reflected by a reflecting surface 405a of a deflector (rotating polygon mirror) 405. The scanning light 208 reflected by the reflecting surface 405a passes through an imaging lens 406 and forms a light spot on the surface (scanned surface 407) of the photoconductor 4. By rotating the deflector 405 in the direction of arrow A at a constant angular velocity by a driving unit (not shown), the light spot moves in the main scanning direction on the scanned surface 407 of the photoconductor 4, and the scanned surface 407 is scanned.

[0015] A beam detection (hereinafter, abbreviated as BD) sensor 409 and a BD lens 408 constitute a synchronization optical system that determines the timing of writing an electrostatic latent image on a scanned surface 407. The BD sensor 409 detects the scanning light 208 reflected in a predetermined direction by the deflector 405. The BD sensor 409 outputs a BDI signal indicating the detection timing of the scanning light 208 to the control unit 1 (FIG. 4). The control unit 1 outputs a BDO signal to the image signal generating unit 100 (FIG. 4) based on the BDI signal. The BDO signal is a horizontal synchronization signal, that is, a reference timing for determining the "scanning start timing". Although the BD lens 408 is used in this embodiment, a configuration that does not use the BD lens 408 is also possible.

[0016] The imaging lens 406 in this embodiment does not have so-called fθ characteristics. Therefore, even if the deflector 405 is rotated at a constant angular velocity, the scanning speed is not constant. That is, in the optical scanning device 400 of this embodiment, the scanning speed changes depending on the position (image height) of the photoconductor 4 in the main scanning direction. More specifically, in the optical scanning device 400 of this embodiment, the scanning speed at the end portion is faster than that at the center portion in the main scanning direction of the photoconductor 4. By using the imaging lens 406 having the fθ characteristics, it is possible to reduce the distance D1 between the imaging lens 406 and the deflector 405. In addition, the imaging lens 406 without the fθ characteristics can reduce the length LW in the main scanning direction and the thickness LT in the optical axis direction compared to the imaging lens with the fθ characteristics. That is, by using the imaging lens 406 without the fθ characteristics, the optical scanning device 400 can be made smaller.

[0017] FIG. 3 shows an example of the relationship between the image height of the scanning light 208 and the partial magnification according to this embodiment. The partial magnification is the ratio of the scanning speed at each image height to the scanning speed when the image height is 0. The image height of 0 means that the light spot is on the optical axis of the imaging lens 406, and is also referred to as the on-axis image height below. The image height other than the on-axis image height is referred to as the off-axis image height. Furthermore, the image height corresponding to the end of the scanning line is referred to as the most off-axis image height. In the example of FIG. 3, the scanning speed at the on-axis image height is the lowest, and the scanning speed increases as the absolute value of the image height increases. Therefore, if the pixel width in the main scanning direction is determined at a constant time interval, the pixel width will differ between the on-axis image height and the off-axis image height. In order to suppress the variation in pixel width due to the image height, in this embodiment, a partial magnification correction (first correction control) to be described later is performed.

[0018] Furthermore, in the optical scanning device 400 of this embodiment, the scanning speed increases as the image height approaches the most off-axis image height. Therefore, if the emission luminance of the light source 401 is constant, the total exposure amount per unit length decreases as the image height approaches the most off-axis image height. In order to suppress the difference in the total exposure amount per unit length due to the image height, in this embodiment, in addition to the partial magnification correction, an exposure amount correction (second correction control) to be described later is performed.

[0019] FIG. 4 shows an exposure control configuration in an image forming apparatus. A modulation section 101 of an image signal generating section 100 receives image data from a host computer (not shown) and generates a VDO signal, which is an image signal. The VDO signal is, for example, a pulse width modulation (PWM) signal. A control section 1 controls the entire image forming apparatus. The control section 1 also controls an optical scanning device 400 to control the emission brightness (emission intensity) of a light source 401. The optical scanning device 400 controls the on / off of the emission of light from the light source 401 based on the VDO signal.

[0020] After receiving a notification from the image signal generating unit 100 by serial communication that the generation of the VDO signal has been completed, the control unit 1 transmits a TOP signal, which is a vertical synchronization signal, and a BDO signal, which is a horizontal synchronization signal, to the image signal generating unit 100. The TOP signal is a synchronization signal in the sub-scanning direction, and the BDO signal is a synchronization signal in the main scanning direction. The control unit 1 generates the BDO signal based on the BDI signal received from the optical scanning device 400. When the scanning start timing occurs after a predetermined period (first period) after receiving the BDO signal, the image signal generating unit 100 outputs the VDO signal to the optical scanning device 400. The first period is set so that the scanning position in the main scanning direction by the scanning light 208 at the scanning start timing after the first period after receiving the BDO signal becomes the target position to start scanning. Details of the configurations of the image signal generating unit 100, the control unit 1, and the optical scanning device 400 shown in FIG. 4 will be described later.

[0021] 5 is a block diagram of the modulation unit 101 of the image signal generation unit 100. The density correction processing unit 121 performs density correction processing on image data received from a host computer (not shown). Specifically, the density correction processing unit 121 holds a density correction table. The density correction table is information indicating the relationship between an input pixel value and an output pixel value. The density correction processing unit 121 uses the pixel value of each pixel indicated by the received image data as an input pixel value, and outputs the corresponding output pixel value to the halftone processing unit 122 by referring to the density correction table. The correction using the density correction table is so-called gamma correction.

[0022] The halftone processing unit 122 performs halftone (screen) processing of the input image data. FIG. 6(A) is an explanatory diagram of an example of halftone processing performed by the halftone processing unit 122. In the example of FIG. 6(A), density is expressed by a matrix 153 of 200 lines with 3 pixels in each of the main scanning direction and the sub-scanning direction. Reference numeral 157 indicates one pixel. The white part in the figure is a non-exposed area that is not an object of exposure, and the black part is an exposed area that is an object of exposure. FIG. 6(A) shows that the exposed area increases with an increase in gradation. As shown in FIG. 6(B), one pixel 157 is divided into a plurality of pixel pieces in the main scanning direction. In the example of FIG. 6(B), one pixel is divided into 16 pixel pieces. In this embodiment, an exposed area or a non-exposed area is set with the pixel piece as a unit. The parallel-serial (PS) conversion unit 123 converts the parallel signal input from the halftone processing unit 122 into a serial signal. The serial signal output by the PS conversion unit 123 is a PWM signal, and one pulse corresponds to one pixel piece. For example, when the pulse is at a high level, the corresponding pixel piece is exposed, and when the pulse is at a low level, the corresponding pixel piece is not exposed.

[0023] A phase-locked loop (PLL) 127 generates an image clock 126 based on a clock (VCLK) 125 and outputs it to a PS conversion unit 123, a (fast-in-fast-out) FIFO 124, and a pixel piece insertion / extraction unit 128. Note that VCLK 125 is also input to the density correction processing unit 121, the halftone processing unit 122, and the PS conversion unit 123.

[0024] In this embodiment, the partial magnification correction is performed by inserting and removing the above-mentioned pixel pieces according to the position of the pixel in the main scanning direction. For this reason, in this embodiment, a pixel piece insertion and removal unit 128 is provided. The pixel piece insertion and removal unit 128 inserts and removes pixel pieces (pulses of a PWM signal) based on the partial magnification correction information 314 (FIG. 7), thereby suppressing the change in the length of each pixel in the main scanning direction even if the scanning speed changes depending on the image height. For the partial magnification correction, the pixel piece insertion and removal unit 128 controls a write enable (WE) signal 131 and a read enable (RE) signal 132 output to the FIFO 124. The FIFO 124 takes in a serial signal from the PS conversion unit 123 only when the WE signal 131 is at a "high level". When extracting a pixel piece for the partial magnification correction, the pixel piece insertion and removal unit 128 makes the WE signal 131 at a "low level". The FIFO 124 accumulates the data taken in from the PS conversion unit 123 in a buffer. FIFO 124 reads out the accumulated data in synchronization with image clock 126 only when RE signal 132 is at "high level" and outputs it as a VDO signal. When inserting a pixel piece for partial magnification correction, pixel piece insertion / extraction unit 128 sets RE signal 132 to "low level". As a result, FIFO 124 does not update the data it is outputting, and continues to output data corresponding to the previous pixel piece. In other words, the pixel piece to be inserted is the same as the pixel piece immediately upstream in the main scanning direction. Note that, since FIFO 124 reads out the accumulated data in synchronization with image clock 126, the frequency of the VDO signal, which is an image signal, matches the frequency of image clock 126.

[0025] In this embodiment, the exposure amount correction is performed by controlling the light emission luminance (intensity) of the light source 401. Therefore, in the following description, the term "luminance correction" is used instead of "exposure amount correction." The control configuration of the light emission luminance of the light source 401 will be described below with reference to FIG. 4. The control unit 1 includes a CPU 2, an 8-bit digital-to-analog converter (DAC) 21, and an IC 3 incorporating a regulator (REG) 22. The optical scanning device 400 includes a memory 304, a VI conversion circuit 306 that converts a voltage into a current, a driver IC 9, the light source 401, and a dummy resistor 10.

[0026] The memory 304 stores partial magnification information 317 (FIG. 7) and brightness correction information 315 (FIG. 7) indicating a correction value of the current supplied to the light source 401. IC3 sets the voltage that REG22 outputs to DAC21. This voltage becomes the reference voltage of DAC21. Next, IC3 sets input data for DAC21 based on the brightness correction information 315 stored in the memory 304, and causes DAC21 to output a brightness correction voltage 312 in synchronization with the BDO signal. The VI conversion circuit 306 converts this brightness correction voltage 312 into a brightness correction current Id and outputs it to the driver IC9.

[0027] Based on the VDO signal, the driver IC 9 controls the switch 14 to switch whether the current IL flows to the light emitting unit 11 of the light source 401 or to the dummy resistor 10, thereby controlling the on / off of the light emission of the light source 401. The current value of the current IL is obtained by subtracting the current value of the luminance correction current Id output by the VI conversion circuit 306 from the current value of the current Ia flowing to the constant current circuit 15. The current value of the current Ia flowing to the constant current circuit 15 is feedback-controlled based on the value detected by the photodetector 12 provided in the light source 401 so that the light emission luminance of the light source 401 becomes a predetermined value. This feedback control is performed, for example, when the current value of the luminance correction current Id is 0.

[0028] In this way, the current value of the current IL can be changed by controlling the value of the luminance correction current Id by the input data to the DAC 21. The emission luminance of the light source 401 can be adjusted by controlling the current value of the current IL flowing through the light emitting unit 11 of the light source 401. The luminance correction is performed by adjusting the emission luminance according to the image height.

[0029] Fig. 7 is a timing chart for explaining the partial magnification correction and brightness correction according to the present embodiment. As described above and shown in Fig. 7, the VDO signal is output after a first period from the BDO signal. In Fig. 8, the scanning period is a period during which the photoconductor 4 is scanned only once with the scanning light, and the left end of the scanning period in the figure corresponds to the scanning start timing.

[0030] The partial magnification information 317 indicates the rate of change of the scanning speed at each image height relative to a reference scanning speed. In the example of Fig. 8, the reference scanning speed is the scanning speed at the on-axis image height. In this embodiment, the main scanning direction is divided into a plurality of sections, and the partial magnification information indicates the partial magnification of each section. The partial magnification of the section near the on-axis image height is 0. The partial magnification increases toward the end of the scanning line, and the partial magnification at the most off-axis image height is 35%.

[0031] The CPU 2 of the control unit 1 reads out the partial magnification information 317 from the memory 304 via serial communication, generates partial magnification correction information 314 based on the partial magnification information 317, and notifies the pixel piece insertion / extraction unit 128 of the modulation unit 101. Note that the partial magnification correction information 314 may also be configured to be stored in advance in the memory 304. Furthermore, the partial magnification information 317 or the partial magnification correction information 314 may also be configured to be stored in advance in the image signal generation unit 100.

[0032] The partial magnification correction information 314 in Fig. 8 indicates the number of pixel pieces to be inserted or extracted for every 100 pixel pieces. Note that a negative value indicates that a pixel piece is to be extracted, and a positive value indicates that a pixel piece is to be inserted. The partial magnification correction information 314 in Fig. 8 indicates that 17 pixel pieces are inserted for every 100 pixel pieces in the section near the on-axis image height, and 18 pixel pieces are extracted for every 100 pixel pieces in the section near the most off-axis image height.

[0033] Further, the CPU 2 reads out the luminance correction information 315 from the memory 304. The luminance correction information 315, like the partial magnification information 317, is provided for each section in the main scanning direction, and is information for determining a value to be set to the input of the DAC 21 when the section is scanned with the scanning light 208. The DAC 21 outputs the luminance correction voltage 312 according to the input data, and the VI conversion circuit 306 outputs the luminance correction current Id having a current value according to the voltage value of the luminance correction voltage 312 to the driver IC 9. As described above, the luminance correction current Id changes, and the current IL changes, and the emission luminance of the light source 401 also changes. In the example of FIG. 7, the luminance correction information 315 is set so that the luminance correction voltage 312 is smallest at the most off-axis image height and largest near the axial image height. Therefore, the current IL is largest at the most off-axis image height and smallest near the axial image height. In FIG. 7, Papc1 is the emission luminance of the light source 401 when the luminance correction current Id is 0. 7, the emission luminance of the light source 401 at the on-axis image height is 0.74 times that at the most off-axis image height. This is because the scanning speed at the on-axis image height is 1 / 1.35≒0.74 times that at the most off-axis image height.

[0034] In this manner, in this embodiment, partial magnification correction (first correction control) and luminance correction (second correction control) are performed. Partial magnification correction is to correct the scanning time for forming one pixel according to the position in the main scanning direction so that the width of the pixel formed on the photoconductor 4 by the scanning light 208 becomes a predetermined value. Moreover, luminance correction is to correct the exposure amount of the pixel formed by the scanning light 208 according to the position in the main scanning direction so that the density of the pixel formed on the photoconductor 4 by the scanning light 208 becomes a density corresponding to the pixel value of the pixel.

[0035] In addition, partial magnification correction uses partial magnification correction information 314, and luminance correction uses luminance correction information 315. Partial magnification correction information 314 is information that indicates the relationship between a position or section in the main scanning direction and the amount of insertion or removal of pixel pieces. Luminance correction information 315 is information that indicates the relationship between a position or section in the main scanning direction and the correction amount of emission luminance of light source 401. With this configuration, exposure can be performed with suppressed image defects even when scanning is performed without using a scanning lens having fθ characteristics.

[0036] In the partial magnification correction information 314 shown in Fig. 7, pixel pieces are inserted near the on-axis image height, and pixel pieces are extracted near the most off-axis image height. However, it is also possible to set the amount of pixel piece insertion / removal at the on-axis image height to 0, and to increase the amount of pixel piece extraction as the image approaches the most off-axis image height. Conversely, it is also possible to set the amount of pixel piece insertion / removal at the most off-axis image height to 0, and to increase the amount of pixel piece insertion as the image approaches the on-axis image height. It is also possible to improve image quality by setting the maximum absolute value of the amount of pixel piece insertion / removal.

[0037] Next, an example of a cause of an error in the BDI signal output by the BD sensor 409 will be described. The BD sensor 409b in FIG. 8 indicates the BD sensor 409 actually disposed in the optical scanning device 400. The BD sensor 409a indicates the BD sensor 409 disposed in an ideal state. In FIG. 8, reference symbol La1 indicates the scanning light 208 incident on the BD sensor 409a, and reference symbol Lb1 indicates the scanning light 208 incident on the BD sensor 409b. As shown in FIG. 8, the timing at which the scanning light 208 is incident on the BD sensor 409b is later than the timing at which the scanning light 208 is incident on the BD sensor 409a disposed in an ideal state. That is, in FIG. 8, the timing at which the BD sensor 409b outputs the BDI signal is later than the ideal timing.

[0038] Since the BDI signal is delayed from the ideal timing, the BDO signal is also delayed from the ideal timing, and therefore the actual scanning start position is shifted downstream in the scanning direction from the target position. The intersection of the light beam La2 and the scanned surface 407 of the photoconductor 4 in FIG. 9 is the target position for scanning start, and the intersection of the light beam Lb2 and the scanned surface 407 is the actual scanning start position. Note that the deviation of the BDI signal is caused not only by the installation error of the BD sensor 409, but also by the installation error of the BD lens 408. Also, a slit is sometimes provided on the upstream side of the BD sensor 409, and if the slit position is shifted, an error occurs in the BDI signal.

[0039] 9 shows the relationship between the BDI signal, BDO signal, and VDO signal based on the BD sensor 409a and the BDI signal, BDO signal, and VDO signal based on the BD sensor 409b. The BDO signal is output after a predetermined period Ta has elapsed from the timing of the BDI signal, and the VDO signal is output after a predetermined period Tb has elapsed from the timing of the BDO signal. In other words, the timing when the period Tb has elapsed from the timing of the BDO signal is set as the scanning start timing. According to FIG. 9, the output timing of the BDI signal by the BD sensor 409b is delayed by a period Tr from the output timing of the ideal BDI signal output by the BD sensor 409a. Therefore, in the case of the BD sensor 409b, the output timing of the BDO signal and the VDO signal is also delayed by a period Tr from the ideal timing.

[0040] FIG. 9 also shows the change in the scanning speed in one scan of the photoconductor 4 based on the partial magnification characteristic. As described above, in partial magnification correction, the number of pixel pieces inserted and removed is changed according to the scanning speed. Here, when the output timing of the BDO signal is ideal, the VDO signal is output at the ideal scanning start timing, and therefore, the scanning position of the light beam 208 at that time is the target position. Therefore, partial magnification correction can be performed with high accuracy based on the partial magnification correction information 314. On the other hand, when the output timing of the BDO signal is delayed from the ideal timing, the actual scanning start position is downstream of the target position. In this case, the modulation unit 101 performs partial magnification correction by assuming that the actual scanning start position is the target position, even though it is downstream of the target position.

[0041] FIG. 10(A) shows this state. In FIG. 10, the solid line indicates the actual partial magnification, and the dotted line indicates the partial magnification used for correction when the output timing of the BDO signal is delayed from the ideal timing. As shown in FIG. 10(A), the partial magnification used for correction is higher than the actual partial magnification on the upstream side of the scanning line. As a result, pixel pieces are excessively extracted on the upstream side of the scanning line, and the pixel width becomes shorter than the target width. On the other hand, the partial magnification used for correction is lower than the actual partial magnification on the downstream side of the scanning line. As a result, pixel pieces are not sufficiently extracted on the downstream side of the scanning line, and the pixel width becomes longer than the target width. FIG. 10(B) shows the difference between the target value and the width of the pixel formed when the output timing of the BDO signal is delayed from the ideal timing for each image height. The same applies to the exposure correction (brightness correction), and the exposure correction according to the scanning speed is not performed due to the shift in the output timing of the BDO signal.

[0042] 11 shows an exposure control configuration according to the present embodiment for correcting the timing deviation of the BDO signal. In addition to partial magnification information 317 and brightness correction information 315, the memory 304 of the optical scanning device 400 stores timing correction information indicating the deviation amount (correction value) from the ideal timing of the BDI signal.

[0043] For example, in the example of FIG. 9, the correction value in the case of the BD sensor 409b is Tr. The timing correction information is measured for each individual optical scanning device 400 before shipment from the factory and stored in the memory 304. The control unit 1 acquires the correction value indicated by the timing correction information from the memory 304. Then, the control unit 1 corrects the reference period Ta (FIG. 9) from the reception of the BDI signal to the output of the BDO signal with the correction value to obtain the correction period. For example, when the correction value is Tr, the correction period is (Ta-Tr). The control unit 1 outputs the BDO signal at a timing that is the correction period (Ta-Tr) after the reception timing of the BDI signal. Therefore, as shown in FIG. 12, even if the timing of the BDI signal is different from the ideal timing, the output timing of the BDO signal can be made closer to the ideal timing. Therefore, the scanning start timing (output timing of the VDO signal) can also be made closer to the ideal timing. Therefore, the error between the scanning start position and the target position can be reduced, and therefore partial magnification correction and brightness correction can be made with high accuracy.

[0044] <Other> In this embodiment, partial magnification correction is performed by inserting and removing pixel pieces, and exposure amount correction is performed by brightness correction. Here, instead of the above-mentioned brightness correction, the following density correction can be used as the exposure amount correction. Furthermore, partial magnification correction can also be performed by correcting the frequency of the image clock 126 (hereinafter referred to as frequency correction) instead of the above-mentioned insertion and removal of pixel pieces.

[0045] First, the density correction will be described. The density correction is to correct the pixel value of each pixel according to the image height in order to correct the exposure amount of the pixel. When the density correction is performed, the luminance correction is not performed, so the luminance correction information 315 is not stored in the memory 304 of the optical scanning device 400. In addition, the current value of the current IL flowing through the light source 401 is controlled to be constant during one scan. Therefore, electrical circuits such as DAC21, REG22, and VI conversion circuit 306 for performing the luminance correction are not required.

[0046] When performing density correction, density correction information 319 (FIG. 13) is stored in the memory 304 of the optical scanning device 400. The density correction information 319 is information indicating the relationship between a position or section in the main scanning direction and the correction amount of the pixel value. The correction amount of the pixel value is set so that the amount of decrease increases as the scanning speed decreases. The CPU 2 of the control unit 1 reads out the density correction information 319 stored in the memory 304 and outputs it to the modulation unit 101 of the image signal generation unit 100. The density correction processing unit 121 of the modulation unit 101 performs density correction processing based on the density correction information 319. Specifically, the pixel value (tone value) of the pixel is changed based on the density correction information.

[0047] FIG. 13 is a timing chart of partial magnification correction and density correction. The part related to partial magnification correction is the same as FIG. 7. The density correction information 319 indicates the position of a pixel in the main scanning direction and the amount of decrease in the pixel value of the pixel. Specifically, when the correction value is "07h" in FIG. 13, the density correction processing unit 121 outputs a value obtained by subtracting 7 from the pixel value before density correction, and when the correction value is "0Fh", the density correction processing unit 121 outputs a value obtained by subtracting 15 from the pixel value before density correction. Note that the "pixel value before density correction" means the output pixel value indicated by the density correction table. That is, the density correction processing unit 121 converts the input pixel value to the output pixel value based on the density correction table, and corrects the output pixel value based on the density correction information 319. As shown in FIG. 13, when the pixel value before density correction is 255 (FFh), the pixel value of the pixel near the axial image height output by the density correction processing unit 121 is 240 (F0h). Therefore, the amount of decrease in density is 15 / 255 ≒ 5.8%. 13, the density is not reduced at the most off-axis image height, and is reduced by about 5.8% at the on-axis image height. If density correction is not performed when the scanning speed at the most off-axis image height is 135% of that at the on-axis image height, the image density at the on-axis image height will not be 135% of the image density at the most off-axis image height. This is because the relationship between the total exposure amount per unit area of ​​the photoconductor 4 and the toner density of the final image formed is not linear due to the exposure sensitivity characteristics of the photoconductor 4 and the development characteristics of the toner. The density correction information 319 is set taking these factors into consideration.

[0048] Next, frequency correction will be described. FIG. 14 shows an example of frequency correction information used for frequency correction. In FIG. 14, the frequency at each image height is shown as a ratio to the frequency at the axial image height. In the example of FIG. 14, the main scanning direction is divided into nine sections. The letters "a" to "k" in FIG. 14 indicate the positions in the main scanning direction that are the ends of each section. For the positions in the main scanning direction that are the ends of each section, an ideal frequency is set for correcting the partial magnification at that position. In this way, the frequency correction information is information indicating the end positions of each section in the main scanning direction and the frequencies at the end positions, and is stored in advance in the memory 304.

[0049] FIG. 15 is a configuration diagram of the modulation unit 101 when performing partial magnification correction by frequency correction. When the scanning light 208 scans the end positions of each section, the PLL 127 generates the image clock 126 based on the VCLK 125 so that the frequency is the frequency indicated by the frequency correction information. When scanning a position other than the end positions of the section, the modulation unit 101 sequentially changes the parameters given to the PLL 127 so that the frequency of the image clock 126 becomes a frequency obtained by linearly interpolating the frequencies at the two end positions of the section. The pulse width modulation unit 129 outputs a VDO signal according to the image clock 126 based on the image data output by the halftone processing unit 122. By controlling the frequency of the VDO signal (image signal) according to the partial magnification, that is, the scanning speed, it is possible to suppress the variation in pixel width caused by the variation in the scanning speed.

[0050] Although the embodiment has been described using a color image forming apparatus, the above embodiment can also be applied to a monochrome image forming apparatus. As described above, when the scanning speed changes according to the image height, partial magnification correction and exposure amount correction are performed, but when the timing of the BDO signal (horizontal synchronization signal) deviates from the ideal timing, the accuracy of the partial magnification correction and exposure amount correction deteriorates. The above embodiment suppresses the deterioration of the accuracy of the partial magnification correction and exposure amount correction by bringing the timing of the BDO signal closer to the ideal timing based on the timing correction information. Therefore, even in a monochrome image forming apparatus, when the scanning speed changes according to the image height, the above embodiment can suppress the deterioration of the accuracy of the partial magnification correction and exposure amount correction.

[0051] [Other embodiments] The present invention can also be realized by a process in which a program for implementing one or more of the functions of the above-described embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. The present invention can also be realized by a circuit (e.g., ASIC) that implements one or more of the functions.

[0052] The disclosure of this embodiment includes the following configuration. (Configuration 1) A photoconductor that is rotated; A generating means for generating an image signal based on image data; a scanning means for scanning the photoconductor in a main scanning direction with a scanning light based on the image signal from the generating means to form an electrostatic latent image on the photoconductor; a control means for outputting to the generating means a synchronization signal indicating a second timing for determining a first timing at which the generating means outputs the image signal to the scanning means; Equipped with The control unit has correction information indicating a correction value, and determines the second timing based on a third timing notified by the scanning unit and the correction value. (Configuration 2) 2. The image forming apparatus according to claim 1, wherein the control unit obtains a correction period by correcting a first predetermined period with the correction value, and determines a timing that is the correction period after the third timing as the second timing. (Configuration 3) 3. The image forming apparatus according to claim 1, wherein the generating unit determines that a timing that is a second predetermined period after the second timing is the first timing. (Configuration 4) The scanning means is A light source that emits the scanning light; a rotating polygon mirror for scanning the photoconductor in the main scanning direction with the scanning light emitted from the light source; a detection means for detecting the scanning light reflected by the rotating polygon mirror; Equipped with 4. The image forming apparatus according to any one of configurations 1 to 3, wherein the third timing is a timing at which the detection unit detects the scanning light. (Configuration 5) 5. The image forming apparatus according to configuration 4, wherein the correction value indicates a difference between a timing at which the detection unit detects the scanning light and a reference timing at which the detection unit should detect the scanning light. (Configuration 6) the scanning means has a storage means for storing the correction information, 6. The image forming apparatus according to any one of configurations 1 to 5, wherein the control unit acquires the correction information from the storage unit. (Configuration 7) 7. The image forming apparatus according to any one of configurations 1 to 6, wherein the scanning means scans the photoconductor with the scanning light whose scanning speed changes depending on the position in the main scanning direction. (Configuration 8) The image forming apparatus of configuration 7, wherein the control means or the generation means executes a first correction control that corrects a change in pixel width due to a change in the scanning speed, and a second correction control that corrects a change in exposure amount due to a change in the scanning speed, based on the second timing. (Configuration 9) The image data is data indicating whether or not to expose each of the pixel pieces obtained by dividing one pixel, The image forming apparatus of configuration 8, wherein the first correction control is a control for extracting data of the pixel pieces indicated by the image data or inserting data of the pixel pieces into the image data in accordance with the scanning speed in order to correct a change in the pixel width due to a change in the scanning speed. (Configuration 10) the generating means outputs the image signal to the scanning means in accordance with an image clock; 9. The image forming apparatus according to configuration 8, wherein the first correction control is a control for changing a frequency of the image clock in response to the scanning speed in order to correct a change in the pixel width caused by a change in the scanning speed. (Configuration 11) The image forming apparatus according to any one of configurations 8 to 10, wherein the second correction control is a control for changing an emission luminance of a light source of the scanning means according to a scanning speed in order to correct a change in the exposure amount due to a change in the scanning speed. (Configuration 12) The image forming apparatus according to any one of configurations 8 to 10, wherein the second correction control is a control that corrects pixel values ​​indicated by the image data according to the scanning speed in order to correct a change in the exposure amount due to a change in the scanning speed.

[0053] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0054] 100: image signal generating unit, 1: control unit, 400: optical scanning device

Claims

1. A rotating photoreceptor, A generation means for generating an image signal based on image data, A scanning means that forms an electrostatic latent image on the photoreceptor by scanning the photoreceptor in the main scanning direction with scanning light based on the image signal from the generating means, Control means that outputs a synchronization signal to the generation means indicating a second timing for determining a first timing for the generation means to output the image signal to the scanning means, Equipped with, The control means has correction information indicating a correction value, and determines the second timing based on the third timing notified by the scanning means and the correction value, in an image forming apparatus.

2. The image forming apparatus according to claim 1, wherein the control means determines a correction period by correcting a first predetermined period with the correction value, and determines the timing that is after the correction period from the third timing as the second timing.

3. The image forming apparatus according to claim 2, wherein the generating means determines the first timing to be a second predetermined period later than the second timing.

4. The scanning means is A light source that emits the aforementioned scanning light, A rotating polyhedron mirror for scanning the photoreceptor in the main scanning direction with the scanning light emitted from the light source, A detection means for detecting the scanning light reflected by the rotating polyhedron mirror, Equipped with, The image forming apparatus according to claim 1, wherein the third timing is the timing at which the detection means detects the scanning light.

5. The image forming apparatus according to claim 4, wherein the correction value indicates the difference between the timing at which the detection means detects the scanning light and the reference timing at which the detection means should detect the scanning light.

6. The scanning means includes a storage means for storing the correction information, The image forming apparatus according to claim 5, wherein the control means acquires the correction information from the storage means.

7. The image forming apparatus according to any one of claims 1 to 6, wherein the scanning means scans the photoreceptor with scanning light whose scanning speed changes according to the position in the main scanning direction.

8. The image forming apparatus according to claim 7, wherein the control means or the generation means performs, with reference to the second timing, a first correction control for correcting the change in pixel width due to the change in scanning speed, and a second correction control for correcting the change in exposure amount due to the change in scanning speed.

9. The aforementioned image data is data indicating whether or not to expose each pixel segment obtained by dividing a single pixel, The image forming apparatus according to claim 8, wherein the first correction control is a control that, in order to correct the change in pixel width due to the change in scanning speed, extracts the data of the pixel piece indicated by the image data or inserts the data of the pixel piece into the image data according to the scanning speed.

10. The generation means outputs the image signal to the scanning means according to the image clock, The image forming apparatus according to claim 8, wherein the first correction control is a control that changes the frequency of the image clock in accordance with the scanning speed in order to correct the change in the pixel width due to the change in the scanning speed.

11. The image forming apparatus according to claim 8, wherein the second correction control is a control that changes the luminescence brightness of the light source of the scanning means according to the scanning speed in order to correct the change in exposure amount due to the change in scanning speed.

12. The image forming apparatus according to claim 8, wherein the second correction control is a control that corrects the pixel values ​​indicated by the image data according to the scanning speed in order to correct the change in exposure amount due to the change in scanning speed.

Images