JP2017056630A - Image formation device and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress degradation of an image in a region where a frequency is switched, relating to an image formation device having an optical configuration in which a scanning speed by which laser beam moves along main scanning direction on a surface of a photoreceptor is not constant.SOLUTION: In PWM process, an image signal is inputted using a second clock, and the inputted image signal is processed by pulse width modulation (PWM), which is outputted using a first clock. At this time, the first clock is decided according to a partial magnification characteristic information and a gradation number of the image signal. The second clock is generated from the first clock at a different frequency according to an image height so that each pixel of the image signal makes latent image at a constant interval along the main scanning direction on a surface of a photoreceptor.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a technique for performing optical writing using a laser beam in an image forming apparatus using an electrophotographic system.

  An electrophotographic image forming apparatus has an optical scanning unit for exposing a photosensitive member. The optical scanning unit emits a laser beam based on the image data, reflects the emitted laser beam with a rotary polygon mirror (deflector), and transmits the scanning lens to irradiate and expose the photosensitive member. Then, a latent image is formed on the photosensitive member by performing scanning by rotating the rotating polygonal mirror to move the laser beam spot formed on the surface of the photosensitive member by rotating the rotating polygonal mirror.

  Usually, a lens having a so-called fθ characteristic is used as a scanning lens. The fθ characteristic means that the laser beam is imaged on the surface of the photoconductor so that the spot of the laser beam on the surface of the photoconductor moves on the surface of the photoconductor at a constant speed when the rotary polygon mirror rotates at a constant angular velocity. It is an optical characteristic to be caused. By using a scanning lens having such fθ characteristics, appropriate exposure can be performed. A scanning lens having such an fθ characteristic has a larger volume and a higher cost than a scanning lens that does not have an fθ characteristic. Therefore, for the purpose of downsizing and cost reduction of the image forming apparatus, it is considered to use a scanning lens that does not use the scanning lens itself or does not have the fθ characteristic. When a scanning lens having no fθ characteristic is used, there arises a problem that the laser beam spot on the surface of the photoconductor does not move on the surface of the photoconductor at a constant speed. In this case, a partial magnification correction technique is generally known in which electrical correction is performed with respect to a change in the scanning speed of the laser beam spot so that appropriate exposure can be obtained.

  Partial magnification correction is a process of reducing the image data against the increase in spot diameter due to a partial increase in scanning speed, and conversely against the reduction in spot diameter due to a partial decrease in scanning speed. This is a technology for performing enlargement processing. Reduction and enlargement are relative processes, and only one of the processes may be performed. For example, image data reduction processing may be performed only for the increase in spot diameter due to partial increase in scanning speed. As a method for correcting partial magnification, Patent Document 1 discloses a method of switching the transfer frequency (image clock) of an image signal for each region during one scanning for irradiating and non-irradiating a laser beam to the photosensitive member. Has been.

  In addition, in an electrophotographic image forming apparatus that performs image exposure with laser light, a PWM signal that is pulse-width modulated (PWM) according to multivalued image data to form an image having gradation properties on transfer paper. Thus, a laser diode used for image exposure is driven. For example, conventional pulse width modulation (PWM) has gradation by controlling to output a PWM signal with a period of a PWM clock (PWM clock) obtained by dividing one cycle of an image clock into a predetermined number. The formed image is formed.

JP 58-1225064 A

  The frequency of the PWM clock that generates the PWM signal is faster than the frequency of the image clock, and is a frequency that is faster by the number of divisions of the image clock corresponding to the resolution of pulse width modulation (the number of divisions of one cycle of the image clock). In order to generate such a high-speed frequency, a PLL (Phase Locked Loop) or the like is necessary. When the frequency is changed for each region as in Patent Document 1, in order to generate a PWM clock from an image clock, A different PLL is required for each region.

  The PLL is a circuit that generates a clock synchronized with the phase of the input clock. Therefore, in different PLLs, the phases of clocks generated from different input clocks (image clocks) corresponding to different frequency regions are aligned for each region during one scan even if they coincide by chance. It is difficult to switch by state. For this reason, when the frequency of the image clock is changed for each region, the boundary between the regions having different frequencies cannot satisfy the cycle of one cycle of the image clock and the PWM clock. When the phase of the image clock is switched at the boundary of the region, the width of one pixel at the boundary (one cycle of the clock) is different from the original width, so that the image is deteriorated. Furthermore, since the PWM clock of one pixel at the boundary is different from the original clock, it is output at a gradation different from the original number of gradations, and the image deteriorates.

  An image forming apparatus according to the present invention is an image forming apparatus having an optical configuration in which a scanning speed at which a laser beam moves in a main scanning direction on a surface of a photoreceptor is not constant, and the scanning speed at an arbitrary image height of the photoreceptor. An image signal is input using an acquisition means for acquiring partial magnification characteristic information indicating the amount of deviation of the scanning speed of each image height and a second clock, and the input image signal is subjected to pulse width modulation (PWM) processing. PWM processing means for outputting a signal using a first clock, the first clock is determined according to the partial magnification characteristic information and the number of gradations of the image signal, and on the surface of the photoconductor Frequency control means for generating the second clock from the first clock at a different frequency according to the image height so that each pixel of the image signal can be latently imaged at equal intervals in the main scanning direction. Characterized in that it.

  According to the present invention, it is possible to perform exposure while suppressing image deterioration in an image forming apparatus having an optical configuration in which the scanning speed at which the laser light moves in the main scanning direction on the surface of the photoreceptor is not constant.

1 is a schematic configuration diagram of an image forming apparatus. It is sectional drawing of an optical scanning device, (a) is a main scanning sectional view, (b) is a figure which shows a sub-scanning sectional view. It is a figure which shows the relationship between image height and partial magnification at the time of fitting the scanning position on a to-be-scanned surface with the characteristic of Y = K (theta). It is a block diagram which shows the structure of an image signal generation part. It is a figure which shows the time chart of a synchronizing signal and an image signal, and the dot image on a to-be-scanned surface. It is a block diagram which shows an image modulation part. It is a block diagram which shows the detail of a frequency control part and a PWM process part. It is a figure which shows an example of a PWM table. It is a figure which shows an example of VCLK production | generation. It is a figure which shows an example of the magnification correction by frequency control. It is a figure which shows the setting method of each partial magnification correction setting value by frequency control and PWM conversion. It is a flowchart which shows the control parameter calculation of frequency control and a PWM table. It is a figure which shows the example of VCLK which can be produced | generated by dividing from PWMCLK. It is a figure which shows the example of the area | region of frequency switching.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments do not limit the present invention according to the claims, and all combinations of features described in the present embodiments are essential to the solution means of the present invention. Not exclusively.

[Example 1]
<Image forming apparatus>
FIG. 1 is a schematic configuration diagram of an image forming apparatus according to the present embodiment. The laser driving unit 300 in the optical scanning device 400 serving as an optical scanning unit generates scanning light (laser light) 410 based on the image signal output from the image signal generation unit 100 and the control signal output from the control unit 200. Emitted toward the photosensitive drum (photoconductor) 500. Then, the photosensitive drum (photosensitive member) 500 charged by a charging unit (not shown) is scanned with a laser beam 410 to form a latent image on the surface of the photosensitive drum 500. A toner is attached to the latent image formed in this manner by developing means (not shown) to form a toner image corresponding to the latent image. The toner image is transferred to a recording medium such as paper fed from the paper feeding unit 900 and transported to a position where the roller 600 contacts the photosensitive drum 500. The toner image transferred to the recording medium is thermally fixed to the recording medium by the fixing device 700 and is discharged out of the apparatus through the paper discharge roller 800.

<Optical scanning device>
2A and 2B are cross-sectional views of the optical scanning device 400 according to the present embodiment. FIG. 2A shows a main scanning cross section, and FIG. 2B shows a sub-scanning cross section.

  In this embodiment, laser light (light beam) 410 emitted from the light source 401 is shaped into an elliptical shape by the aperture stop 402 and enters the coupling lens 403. The light beam that has passed through the coupling lens 403 is converted into substantially parallel light and enters the anamorphic lens 404. The substantially parallel light includes weakly convergent light and weakly divergent light. The anamorphic lens 404 has a positive refractive power in the main scanning section, and converts an incident light beam into convergent light in the main scanning section. The anamorphic lens 404 condenses the light beam in the vicinity of the deflecting surface 405a of the deflector 405 in the sub-scan section, and forms a long line image in the main scanning direction.

  The light beam that has passed through the anamorphic lens 404 is reflected by a deflecting surface (reflecting surface) 405a of a deflector (polygon mirror) 405. The light beam reflected by the reflecting surface 405 a passes through the imaging lens 406 as scanning light 410 (see FIG. 1) and enters the surface of the photosensitive drum 500. The imaging lens 406 is an imaging optical element. In the present embodiment, the imaging optical system is constituted by only a single imaging optical element (imaging lens 406). The surface of the photosensitive drum 500 on which the light beam that has passed (transmitted) through the imaging lens 406 is incident is a surface to be scanned 407 that is scanned by the light beam.

  The imaging lens 406 forms an image of a light beam on the surface to be scanned 407 to form a predetermined spot-like image (spot). When the deflector 405 is driven to rotate at a constant angular velocity in the direction of arrow A by a driving unit (not shown), the spot moves on the scanned surface 407 in the main scanning direction, and electrostatically acts on the scanned surface 407 on the photosensitive member. A latent image is formed. The main scanning direction is a direction parallel to the surface of the photosensitive drum 500 and orthogonal to the moving direction of the surface of the photosensitive drum 500. The sub-scanning direction is a direction orthogonal to the main scanning direction and the optical axis of the light beam.

  A beam detect (hereinafter referred to as BD) sensor and a BD lens are synchronization optical systems that determine the timing for writing an electrostatic latent image on the scanning surface 407. The light beam that has passed through the BD lens is incident on and detected by a BD sensor including a photodiode. The writing timing is controlled based on the timing at which the light beam is detected by the BD sensor.

  The light source 401 is a semiconductor laser chip. The light source 401 of this embodiment has a configuration including one light emitting unit. However, the light source 401 may include a plurality of light emitting units that can independently control light emission. Even when a plurality of light emitting units are provided, a plurality of light beams generated therefrom reach the scanned surface 407 via the coupling lens 403, the anamorphic lens 404, the deflector 405, and the imaging lens 406, respectively. On the surface to be scanned 407, spots corresponding to the respective light beams are formed at positions shifted in the sub-scanning direction.

  Note that various optical members such as the light source 401, the coupling lens 403, the anamorphic lens 404, the imaging lens 406, and the deflector 405 described above are housed in a housing (optical box).

<Imaging lens>
As shown in FIG. 2, the imaging lens 406 has two optical surfaces (lens surfaces), an incident surface (first surface) 406a and an exit surface (second surface) 406b. The imaging lens 406 has a configuration in which the light beam deflected by the deflection surface 405a scans the scanned surface 407 with desired scanning characteristics in the main scanning section. Further, the imaging lens 406 has a configuration in which the spot of the laser beam 410 on the scanned surface 407 has a desired shape. Further, the imaging lens 406 has a conjugate relationship between the vicinity of the deflection surface 405a and the vicinity of the surface to be scanned 407 in the sub-scan section. Thus, the surface tilt is compensated (scanning position deviation in the sub-scanning direction on the surface to be scanned 407 when the deflection surface 405a tilts is reduced).

  The imaging lens 406 according to the present embodiment is a plastic mold lens formed by injection molding, but a glass mold lens may be adopted as the imaging lens 406. Since the molded lens can be easily molded into an aspherical shape and is suitable for mass production, the productivity and optical performance can be improved by adopting the molded lens as the imaging lens 406.

  The imaging lens 406 does not have a so-called fθ characteristic. That is, when the deflector 405 is rotating at an equal angular velocity, it does not have a scanning characteristic that moves the spot of the light beam passing through the imaging lens 406 at a constant velocity on the scanned surface 407. As described above, by using the imaging lens 406 having no fθ characteristic, the imaging lens 406 can be disposed close to the deflector 405 (at a position where the distance D1 is small). Further, the imaging lens 406 not having the fθ characteristic can be made smaller in the main scanning direction (width LW) and the optical axis direction (thickness LT) than the imaging lens having the fθ characteristic. Therefore, the housing of the optical scanning device 400 can be reduced in size.

  In addition, in the case of a lens having an fθ characteristic, there may be a sharp change in the shape of the entrance surface and exit surface of the lens when viewed in the main scanning section. You may not get On the other hand, the imaging lens 406 having no fθ characteristics can obtain good imaging performance because there is little abrupt change in the shape of the entrance surface and exit surface of the lens when viewed in the main scanning section. Can do.

  The scanning characteristic of the imaging lens 406 that does not have such an fθ characteristic is expressed by the following equation (1).

  In Expression (1), the scanning angle (scanning field angle) by the deflector 405 is θ, the light beam condensing position (image height) on the scanned surface 407 is Y [mm], and the axial image height. The image formation coefficient at is K [mm], and the coefficient (scanning characteristic coefficient) for determining the scanning characteristic of the imaging lens 406 is B. In this embodiment, the on-axis image height indicates the image height on the optical axis (Y = 0 = Ymin), and corresponds to the scanning angle θ = 0. The off-axis image height indicates the image height (Y ≠ 0) outside the central optical axis (when the scanning angle θ = 0), and corresponds to the scanning angle θ ≠ 0. Further, the most off-axis image height refers to the image height (Y = + Ymax, −Ymax) when the scanning angle θ is maximum (maximum scanning field angle). The scanning width W, which is the width in the main scanning direction of a predetermined region (scanning region) where a latent image can be formed on the scanned surface 407, is expressed as W = | + Ymax | + | −Ymax |. The center of the predetermined area is the on-axis image height and the end is the most off-axis image height.

  Here, the imaging coefficient K is a coefficient corresponding to f at the scanning characteristic (fθ characteristic) Y = fθ when parallel light enters the imaging lens 406. In other words, the imaging coefficient K is a coefficient for making the condensing position Y and the scanning angle θ proportional to each other in the same manner as the fθ characteristic when a light beam other than parallel light enters the imaging lens 406.

  Supplementing the scanning characteristic coefficient, since equation (1) when B = 0 is Y = Kθ, it corresponds to the scanning characteristic Y = fθ of the imaging lens having the fθ characteristic used in the conventional optical scanning device. . In addition, since the equation (1) when B = 1 is Y = K tan θ, it corresponds to the projection characteristic Y = f tan θ of a lens used in an imaging device (camera) or the like. That is, by setting the scanning characteristic coefficient B in the range of 0 ≦ B ≦ 1 in the expression (1), a scanning characteristic between the projection characteristic Y = ftan θ and the fθ characteristic Y = fθ can be obtained.

  Here, when the equation (1) is differentiated by the scanning angle θ, the scanning speed of the light beam on the scanned surface 407 with respect to the scanning angle θ is obtained as shown in the following equation (2).

  Further, when the equation (2) is divided by the velocity dY / dθ = K at the on-axis image height, the following equation (3) is obtained.

  Expression (3) expresses a deviation amount (partial magnification) of the scanning speed of each off-axis image height with respect to the scanning speed of the on-axis image height. In the optical scanning device 400 according to the present embodiment, the scanning speed of the light beam is different between the on-axis image height and the off-axis image height except when B = 0.

  FIG. 3 shows the relationship between the image height and the partial magnification when the scanning position on the scanned surface 407 according to the present embodiment is fitted with the characteristic of Y = Kθ. In the present embodiment, the scanning characteristic shown in Expression (1) is given to the imaging lens 406, and as shown in FIG. 3, the partial magnification increases from the on-axis image height to the off-axis image height. It has become. This is because the scanning speed gradually increases from the on-axis image height to the off-axis image height. The partial magnification of 30% means that the irradiation length in the main scanning direction on the surface to be scanned 407 is 1.3 times when light is irradiated for a unit time. Therefore, if the pixel width in the main scanning direction is determined at a fixed time interval determined by the cycle of the image clock, the pixel density differs between the on-axis image height and the off-axis image height.

  As described above, in the case of an optical configuration that employs an imaging lens that does not have fθ characteristics, variation in partial magnification in the main scanning direction may not be appropriate in order to maintain good image quality. Therefore, in order to maintain a good image quality even in an optical configuration employing an imaging lens that does not have fθ characteristics, partial magnification correction as described in the present embodiment is performed.

  Note that, as the optical path length from the deflector 405 to the photosensitive drum 500 becomes shorter, the angle of view becomes larger, so that the difference in scanning speed between the above-described on-axis image height and the most off-axis image height increases. In general, an optical configuration in which the change rate of the scanning speed is 20% or more such that the scanning speed at the most off-axis image height is 120% or more of the scanning speed at the on-axis image height is conceivable. In the case of such an optical configuration, it is difficult to maintain good image quality due to the influence of partial magnification in the main scanning direction.

  The change rate C (%) of the scanning speed is a value represented by C = ((Vmax−Vmin) / Vmin) * 100, where Vmin is the slowest scanning speed and Vmax is the fastest scanning speed. In the optical configuration of this embodiment, the slowest scanning speed is obtained at the on-axis image height (center portion of the scanning region), and the fastest scanning speed is obtained at the most off-axis image height (end portion of the scanning region).

  It is known that the change rate of the scanning speed is 35% or more in the case of an optical configuration with an angle of view of 52 ° or more. The conditions for the angle of view to be 52 ° or more are as follows. For example, in the case of an optical configuration that forms a latent image having a short side width of an A4 sheet in the main scanning direction, the optical path from the deflection surface 405a to the scanned surface 407 when the scanning width W = 214 mm and the scanning field angle is 0 °. Length D2 (see FIG. 2) = 125 mm or less. In the case of an optical configuration that forms a latent image having a short side width of the A3 sheet in the main scanning direction, the optical path length D2 from the deflection surface 405a to the scanned surface 407 when the scanning width W = 300 mm and the scanning field angle is 0 °. (See FIG. 2) = 247 mm or less.

  In an image forming apparatus having such an optical configuration, it is possible to obtain good image quality even when an imaging lens having no fθ characteristic is used by using the configuration of the present embodiment described below. It becomes.

  3 is stored as partial magnification characteristic information 317 in the memory 304 shown in FIG. 4. The information indicating the partial magnification characteristics for each image height of the optical scanning device 400 as shown in FIG. The partial magnification characteristic information 317 may be measured and stored in individual apparatuses after the optical scanning apparatus 400 is assembled. If there is little variation between the individual apparatuses, representative characteristics are not measured separately. You may remember. Further, considering that the partial magnification characteristic information 317 of the optical scanning device 400 changes with the passage of time, it may be periodically measured again in the device. In addition, when accuracy is more important, remeasurement may be performed for each job.

<Image signal generator>
FIG. 4 is an electric block diagram illustrating a configuration of an image signal generation unit related to exposure control in the image forming apparatus. The image signal generation unit 100 includes an image modulation unit 101, a CPU 102, and a bus 103. The image signal generation unit 100 receives print information from a host computer (not shown), and generates a VDO signal (image signal) 110 corresponding to the image data. The image signal generation unit 100 also has a function as a pixel width correction unit. The control unit 200 performs control of the image forming apparatus and light amount control of the light source 401. The laser engine driving unit 300 causes the light source 401 to emit light by supplying current to the light source 401 (not shown) of the laser driving unit 301 based on the VDO signal 110.

  When the image signal generation unit 100 is ready to output an image signal for image formation, it instructs the engine control unit 303 to start printing through the serial communication 113. When preparation for printing is completed, the engine control unit 303 generates a TOP signal 112 that is a sub-scanning synchronization signal and a BD signal 111 that is a main-scanning synchronization signal from the synchronization signal generation unit 302, and sends them to the image signal generation unit 100. Send. When receiving the synchronization signal, the image signal generation unit 100 outputs the VDO signal 110 that is an image signal to the laser engine driving unit 300 at a predetermined timing.

  Main constituent blocks of the image modulation unit 101 will be described later with reference to FIG.

  FIG. 5A is a timing chart of various synchronization signals and image signals when the image signal generation unit 100 performs an image forming operation corresponding to one page of the recording medium. Time elapses from left to right in the figure. “HIGH” in the TOP signal 112 is a signal indicating that the leading edge of the recording medium has reached a predetermined position. When the image signal generation unit 100 receives “HIGH” of the TOP signal 112, the image signal generation unit 100 transmits a VDO signal 110, which is an image signal, to the laser driving unit 301 in synchronization with the BD signal 111, which is a main scanning synchronization signal. The laser driving unit 301 emits the light source 401 based on the VDO signal 110 and a latent image is formed on the photosensitive drum 500.

  In FIG. 5A, for simplicity of illustration, the VDO signal 110 is described as being continuously output across a plurality of BD signals 111. However, actually, the VDO signal 110 is output in a predetermined period of time from when the BD signal 111 is output until the next BD signal 111 is output.

<Partial magnification correction method>
Next, a partial magnification correction method for correcting the partial magnification described above will be described. Prior to the description, the factors of the partial magnification and the correction principle will be described with reference to FIG. FIG. 5B is a diagram showing the timing of the BD signal 111 and the VDO signal 110 and the dot image formed by the latent image on the surface to be scanned 407. Time elapses from left to right in the figure.

  When the image signal generator 100 receives the rising edge of the BD signal 111, it outputs a VDO signal 110 after a predetermined timing so that a latent image can be formed at a desired distance from the left end of the photosensitive drum 500. The light source 401 emits light based on the VDO signal 110, and a latent image corresponding to the VDO signal 110 is formed on the scanned surface 407.

  First, a case where a dot-shaped latent image is formed by causing the light source 401 to emit light for the same period at the on-axis image height and the most off-axis image height based on the VDO signal 110 will be described. The size of this dot corresponds to one dot of 600 dpi (width of 42.3 μm in the main scanning direction). As described above, the optical scanning device 400 according to the present embodiment has an optical configuration in which the scanning speed at the end portion (the most off-axis image height) is faster than the central portion (on-axis image height) on the scanned surface 407. is there. As shown in the latent image A, the latent image dot1 having the most off-axis image height is enlarged in the main scanning direction as compared with the latent image dot2 having the on-axis image height. As described above, since the scanning speed of the end portion (the most off-axis image height) is high, when the light source 401 emits light during the same period, the end portion (the most off-axis image height) is more in the main scanning direction. This is because the exposed area is enlarged. Therefore, in this embodiment, as the partial magnification correction, the image width of the VDO signal 110 is corrected according to the position in the main scanning direction. That is, by correcting the partial magnification, the light emission time interval of the most off-axis image height is made shorter than the light emission time interval of the on-axis image height, and the latent image dot3 of the most off-axis image height and the on-axis as shown in the latent image B The latent image dot4 at the image height is made the same size. By such correction, dot-shaped latent images corresponding to the respective pixels can be formed at substantially equal intervals in the main scanning direction.

  FIG. 6 is a block diagram illustrating an example of the image modulation unit 101. The image modulation unit 101 includes a halftone processing unit 122, a PWM processing unit 123, and a frequency control unit 127. The image modulation unit 101 may include other configurations, but a characteristic configuration is shown here.

  The halftone processing unit 122 performs a conversion process for screen (dithering) the input multi-level parallel 8-bit image signal and expressing the density in the image forming apparatus. The halftone processing unit 122 outputs the multilevel 4-bit image signal after the halftone processing to the PWM processing unit 123. The PWM processing unit 123 stores a PWM table for performing PWM processing on the multi-value parallel 4-bit image signal after halftone processing input from the halftone processing unit 122. The PWM processing unit 123 stores a plurality of types of PWM tables corresponding to the number of divisions of one pixel as PWM processing, and the PWM table used by the PWM table selection signal (LUTSEL) 128 from the frequency control unit 127 is stored. select. In this embodiment, the PWM processing is assumed to have a configuration in which one pixel represented by 4 bits is divided into 16 divisions or more in order to form an image having gradation properties on transfer paper. However, the number of divisions may be other than the number of divisions. When an image signal is output from the halftone processing unit 122 to the PWM processing unit 123, the image signal is transferred according to an image clock (VCLK) generated by a frequency control unit 127 described later. Although details will be described later, the frequency of the image clock (VCLK) is modulated in accordance with the position (image height) in the main scanning direction in consideration of the partial magnification. Specifically, the image clock used for transferring the image signal corresponding to the end portion in the main scanning direction (most off-axis image height) has the image signal corresponding to the central portion in the main scanning direction (on-axis image height). The frequency is higher than the image clock used for transfer. As a result, as shown by a latent image B in FIG. 5B, a latent image having the same pixel width is exposed at the end portion in the main scanning direction (most off-axis image height) and in the central portion in the main scanning direction (on-axis image height). It can be formed on the scanned surface of the body. In the present embodiment, a description will be given mainly of processing when generating (modulating) the image clock (VCLK).

  The PWM processing unit 123 performs PWM processing to convert the image signal into information corresponding to ON / OFF of a laser for printing by the image forming apparatus, and outputs the information as a VDO signal 110 to the engine drive unit 300 at the subsequent stage. .

  The frequency control unit 127 generates, based on the reference clock (BASECLK) 121, a PWM processing clock (PWMCLK) 126 and an image clock (VCLK) 125 in which one cycle of the clock corresponds to one pixel. The PWMCLK 126 is generated based on the partial magnification characteristic information 317. In this embodiment, the generated PWMCLK 126 has a fixed frequency. The generated VCLK 125 has a frequency corresponding to the frequency division ratio of PWMCLK 126. Further, the generated VCLK 125 has a frequency different depending on the main scanning position based on frequency magnification correction information (details will be described later) indicating the frequency at each main scanning position, which is calculated based on the partial magnification characteristic information 317. Become. A method for calculating the frequency of PWMCLK and VCLK generated by the frequency control unit 127 will be described later with reference to FIG. The PWMCLK 126 is supplied to the PWM processing unit 123. The VCLK 125 is output to the halftone processing unit 122 and the PWM processing unit 123.

  FIG. 7A is a block diagram illustrating an example of the configuration of the frequency control unit 127. The frequency control unit 127 includes a PLL (Phase Locked Loop) 700, a main scanning position counting unit 706, a frequency switching unit 707, and a frequency switching setting holding unit 708. The PLL 700 generates PWMCLK 126 from the reference clock 121 according to the setting of the frequency switching setting holding unit 708 according to the setting from the CPU bus 103. For example, the PLL 700 generates a PWMCLK 126 having a frequency that takes into account the partial magnification with a high scanning speed and the number of gradations of the image signal processed by the PWM processing unit 123. The reference clock 121 may be any clock that can generate the PWMCLK 126 by the PLL 700. In this embodiment, PWMCLK 126 has a common fixed frequency at any of the main scanning positions, and PLL 700 only needs to have a single PLL.

  The main scanning position counting unit 706 counts the number of cycles of VCLK 125 based on the BD signal 111 and the TOP 112 signal, thereby counting the current main scanning position, and notifies the frequency switching unit 707 of the count value.

  The frequency switching setting holding unit 708 notifies the frequency switching unit 707 of the frequency switching main scanning position and the VCLK frequency division ratio based on the frequency magnification correction information 1103 set by the CPU bus 103. The frequency switching main scanning position indicates the main scanning position when the frequency is switched, and is determined based on the partial magnification characteristic information 317 and the frequency of VCLK that can be generated. The VCLK dividing ratio is a dividing ratio of VCLK generated by dividing PWMCLK. The frequency division ratio is determined in consideration of the number of gradations to be expressed. Further, the frequency switching setting holding unit 708 notifies the frequency switching unit 707 of the value of LUTSEL corresponding to the VCLK frequency division ratio based on the frequency magnification correction information. The frequency magnification correction information 1103 will be described later with reference to FIG.

  The frequency switching unit 707 can determine the current main scanning position based on the count value from the main scanning position counting unit 706. Since the frequency switching main scanning position and the VCLK frequency division ratio are notified from the frequency switching setting holding unit 708, the frequency switching unit 707 can determine the frequency of the VCLK 125 to be generated at the current main scanning position. . Further, the frequency switching unit 707 can determine the frequency switching timing of the VCLK 125 based on the current main scanning position. The frequency switching unit 707 generates VCLK 125 from PWMCLK 126 according to the current main scanning position, and outputs it together with LUTSEL 128. The LUTSEL 128 is a signal for designating (selecting) the PWM table used by the PWM processing unit 123 to the PWM processing unit 123 as described above. The VCLK frequency division ratio corresponds to the number of divisions in which the image signal for one pixel is divided into pixel pieces of less than one pixel in the PWM processing unit 123. Since the PWM processing unit 123 holds a conversion table (PWM table) corresponding to the number of divisions, the PWM processing unit 123 selects a PWM table to be used based on the LUTSEL 128. A method of generating VCLK 125 from PWMCLK 126 will be described later with reference to FIG.

  FIG. 7B is a block diagram illustrating an example of the configuration of the PWM processing unit 123. The PWM conversion unit 710 selects a PWM table corresponding to the LUTSEL 128 from the held PWM tables 0, 1, 2, 3 (711, 712, 713, 714) according to the input LUTSEL 128. Then, the PWM conversion unit 710 converts the input multi-level parallel 4-bit image signal into parallel data represented by the number of bits corresponding to the VCLK frequency division ratio according to the selected PWM table. The parallel data after PWM conversion is replaced with PWMCLK 126 using a line buffer (not shown), so that it is converted into a serial signal and output as a VDO signal 110 to the engine drive unit 300 at the subsequent stage.

  FIG. 8 is a diagram illustrating a setting example of the PWM table. FIG. 8A shows an example of the PWM table 0. The PWM table 0 is a PWM table when one pixel represented by one cycle of VCLK 125 is divided into 17 by PWMCLK 126. That is, this is a conversion table when one pixel is divided into 17 pixel pieces. The PWM table is a conversion table in which input image signal values are associated with output signal values. The PWM table shows how the divided pixel pieces are turned on in accordance with the number of gradations of input data (here, all 16 gradations). In FIG. 8, “0” of the output data indicates that the corresponding pixel piece is turned off, and “1” indicates that the corresponding pixel piece is turned on. In this embodiment, the number of divisions for dividing one pixel into pixel pieces corresponds to the VCLK frequency division ratio. Therefore, the PWM table 0 in FIG. 8A corresponds to VCLK generated by dividing PWMCLK by 17. VCLK 125, LUTSEL 128 corresponding to VCLK 125, and PWMCLK 126 are input from frequency switching unit 707 to PWM conversion unit 710. Therefore, the PWM table corresponding to VCLK is selected. Similarly, FIG. 8B is a PWM table when one pixel is divided into 18, FIG. 8C is a PWM table when one pixel is divided into 20, and FIG. 8D is a PWM table when one pixel is divided into 21. The PWM table in the case is shown.

  Next, a state in which the frequency switching unit 707 generates VCLK 125 using PWMCLK 126 will be described with reference to FIG.

  The frequency switching unit 707 counts the number of cycles of the PWMCLK 126 in synchronization with the PWMCLK 126. The frequency switching unit 707 generates a VCLK 125 by comparing the value of the VCLK frequency division ratio notified from the frequency switching setting holding unit 708 with the value of the PWM counter. In this embodiment, since the PWM counter starts counting from 0, a cycle from 0 of the PWM counter to a number smaller by 1 than the value of the VCLK division ratio is one cycle of VCLK. For example, when 17 is notified as the value of the VCLK frequency division ratio, the cycle from 0 to 16 of the PWM counter becomes one cycle of VCLK. In FIG. 9, when the VCLK frequency division ratio is 17, the frequency division of 17 is realized by setting the high period of VCLK to 9 cycles and the low period to 8 cycles. Further, when the VCLK frequency division ratio is 18, the frequency division of 18 is realized by setting the high period and the low period of VCLK to 9 cycles. However, the ratio of the high period and the low period is not limited. The PWM counter is cleared to 0 after generating one cycle of VCLK.

  Thereby, VCLK having a frequency division ratio of PWMCLK can be generated. Since VCLK generated in this way has the same phase, it is possible to change the frequency without breaking the clock cycle even when the VCLK cycle is switched according to the main scanning position when partial magnification correction is performed. . Accordingly, it is possible to prevent the latent image in the region where the VCLK cycle is switched from being deteriorated.

  Next, FIG. 10 shows how the image data is scaled by frequency scaling. FIG. 10A shows an example of the VDO signal 110 and the image data output at that time when the frequency of the VCLK 125 is divided by 17 of the PWMCLK 126, that is, 17 cycles. The image data is expressed in units of pixel pieces obtained by dividing one pixel into 17 by PWM processing. When VCLK125 is PWMCLK126 divided by 17, it is enlarged 106.25% (1.0625 = 17 ÷ 16) as compared with the case of 16 division.

  On the other hand, as shown in FIG. 10B, when the frequency of VCLK 125 is divided by 21 of PWMCLK 126, that is, 21 cycles, 131.25% (1.3125 = 21) with respect to the case of 16 divisions. ÷ 16) Enlarged.

  Thus, the frequency of VCLK 125 is determined by the frequency division ratio of PWMCLK 126. As the frequency division ratio increases, one cycle of VCLK 125 becomes longer, that is, the frequency of VCLK 125 decreases. Conversely, as the frequency division ratio decreases, one cycle of VCLK 125 becomes shorter, that is, the frequency of VCLK 125 increases. Therefore, if the VCLK 125 is generated so that the frequency division ratio decreases from the central portion to the end portion in the main scanning direction in consideration of the partial magnification, as described above, the portion by the frequency modulation of the image clock is performed. Magnification correction can be performed. In this embodiment, the VCLK 125 is generated in a plurality of cycles of the PWMCLK 126 as described above, so that the clock frequency can be switched without breaking the clock cycle at the VCLK frequency switching timing. Therefore, partial magnification correction can be realized by changing the clock frequency according to the main scanning position. Further, since VCLK 125 is composed of 16 or more cycles of PWMCLK 126, it is possible to use partial magnification correction without impairing gradation.

  Next, a method for setting each partial magnification correction setting value by frequency scaling and PWM conversion will be described with reference to FIG. The frequency magnification correction section 1101 is a section for setting a frequency magnification at each main scanning position within one cycle of the BD signal 111. This section is also a section for setting VCLK frequency division information in the frequency switching unit 707. One section corresponds to one cycle of VCLK. The frequency magnification correction section 1101 shown in FIG. 11 has a total of N sections. The frequency magnification correction information 1103 represents the VCLK frequency division ratio and PWM table selection information (LUTSEL) in each section of the frequency magnification correction section 1101.

  In FIG. 11, as an example, VCLK frequency division ratio 17 and LUTSEL0 are set for 100 divisions of frequency magnification correction divisions 0 to 99 and 760 to 859. Similarly, VCLK frequency division ratio 18 and LUTSEL1 are set for 60 sections of frequency magnification correction sections 100 to 159 and 700 to 759. VCLK frequency division ratio 19 and LUTSEL2 are set for 70 sections of frequency magnification correction sections 160 to 229 and 630 to 699. VCLK frequency division ratio 20 and LUTSEL3 are set for 400 sections of frequency magnification correction sections 230 to 629. Such frequency magnification correction information 1103 is held in the frequency switching setting holding unit via the CPU bus 103. A method for generating the frequency magnification correction information 1103 will be described later. As shown in FIG. 11, the frequency division ratio is set so as to decrease from the center to the end in the main scanning direction, that is, the frequency of VCLK increases.

  In this embodiment, as an example, N = 860, and four types of VCLK are assigned to 100 sections, 60 sections, 70 sections, and the remaining 400 sections from both ends of the main scanning. Yes. However, the number of partitions is not limited to 860, and can be any number of partitions. In this embodiment, the frequency magnification correction section 1101 for N sections is prepared, and the method of setting the VCLK frequency division ratio and the setting value of the PWM table selection (LUTSEL) is shown. However, the setting method is limited to this method. It is not something that can be done. As an example, a setting register for a section may be prepared, and the section start position and end position may be set together with the VCLK frequency division ratio and the PWM table selection value as set values.

  Next, a flow for calculating the frequency magnification correction information 1103 will be described with reference to FIG. In the present embodiment, it is assumed that the clock frequency for transferring a pixel having a partial magnification of 0% is 30 MHz in the case of having the characteristics shown in FIG.

  FIG. 12 is a flow for calculating the frequency magnification correction information 1103 and is executed by the CPU 102. The flow shown in FIG. 12 may be executed every time the partial magnification characteristic information shown in FIG. 3 is updated, or may be executed at an arbitrary timing.

  In step S1200, the CPU 102 acquires partial magnification characteristic information 317 from the memory 304 via the serial communication 113.

  In step S1201, the CPU 102 calculates the fastest frequency among the clock frequencies necessary for transferring one pixel from the partial magnification characteristic information 317. In this embodiment, the scanning speed at both ends of the main scanning is fast as shown in FIG. 3, and the scanning speed is 30% faster than that at the center. Accordingly, when considering partial magnification correction, the clock frequency required for transferring one pixel at the main scanning end is 39 MHz which is 30% faster than the clock frequency of 30 MHz for transferring pixels having a partial magnification of 0%.

  In step S1202, the CPU 102 calculates the PWMCLK frequency from the fastest image transfer clock 39 MHz calculated in step S1201. That is, the CPU 102 calculates the PWMCLK frequency based on the partial magnification characteristic information. In this embodiment, one pixel needs to be divided into 16 or more in order to maintain the gradation represented by the 4-bit image signal. Therefore, 624 MHz, which is 16 times the frequency of 39 MHz, becomes PWMCLK. In this example, gradation can be maintained if the number of divisions is 16 or more. For example, the frequency of 17 divisions or the frequency of 20 divisions may be set to PWMCLK. Here, the following will be described assuming that PWMCLK is 624 MHz, which is 16 times the frequency of 39 MHz.

  In step S1203, as shown in FIG. 13, VCLK that can be generated by frequency division is calculated from PWMCLK calculated in step S1202.

  In step S1204, a frequency switching region is calculated from the VCLK frequency calculated in step S1203 and the partial magnification characteristic information 317. Region calculation will be described with reference to FIG. The horizontal axis in FIG. 14 indicates the image height (main scanning position), and the vertical axis indicates the frequency of VCLK. The thick line corresponds to the partial magnification characteristic information 317 shown in FIG. In FIG. 3, the vertical axis indicates the partial magnification, and FIG. 14 indicates the VCLK frequency. For example, when the partial magnification at the center of the image height is 0% and the partial magnification at the left end of the image height is 30%, the image height corresponding to the partial magnification at the left end of the image height of 30% is VCLK of 39 MHz as described above. Become. The frequency of VCLK corresponding to one pixel width at the center of the image height corresponding to the partial magnification of 0% is 30 MHz. FIG. 14 is a diagram in which the partial magnification characteristic information of FIG. 3 is mapped to VCLK that can be generated by frequency division as described above.

  In the figure, what is indicated by a broken line on the horizontal axis is the VCLK frequency that can be generated calculated in step S1203. The intersections (14a, 14b, 14c, 14d) between the VCLK frequency that can be generated and the partial magnification characteristic information 317 indicated by the broken line are calculated. In this embodiment, the intersection 14a is the main scanning positions 51 and 809, the intersection 14b is the main scanning positions 130 and 730, the intersection 14c is the main scanning positions 195 and 665, and the intersection 14d is the main scanning positions 265 and 595.

  Next, frequency switching is performed so that the intersections (14a, 14b, 14c, 14d) are near the center of the frequency switching region. In the present embodiment, the main scanning sections 0 to 99 and 760 to 869 are regions having a frequency division ratio of 17 when VCLK is generated. The main scanning sections 100 to 159 and 700 to 759 are regions with a frequency division ratio of 18 when VCLK is generated. The main scanning sections 160 to 229 and 630 to 699 are areas with a division ratio of 19 when VCLK is generated. The remaining main scanning sections 230 to 629 are regions having a frequency division ratio of 20 when VCLK is generated. LUTSEL values 0, 1, 2, and 3 are assigned to each main scanning section.

  This calculation flow is completed as described above. In the present exemplary embodiment, the example in which the frequency magnification correction information is acquired using the CPU 102 inside the image forming apparatus has been described. However, after having a host computer (not shown) and the like, and calculating the frequency magnification correction information according to this calculation flow and the pre-measured characteristic information, the frequency magnification correction information is held in a nonvolatile memory or disk drive (not shown). It can also take a configuration. Then, the CPU 102 may acquire the frequency magnification correction information that is held.

  As described above, by performing partial magnification correction with the configuration described in the present embodiment, it is possible to perform partial magnification correction in accordance with scanning characteristics. In particular, the frequency of VCLK 125 can be changed according to the main scanning position without breaking the clock cycle of VCLK 125, and since VCLK 125 is an integer cycle of PWMCLK 126, PWM processing can be performed by simple PWM table conversion. .

  As described above, even when the scanning lens having the fθ characteristic is not used, by performing electrical partial magnification correction that suppresses image defects and image quality deterioration, and keeping the laser spot width constant during one scan, Appropriate exposure can be obtained.

  Such partial magnification correction by frequency modulation is performed by inserting or removing pixel pieces at the time of transferring an image signal, so that density unevenness and screen are reduced compared to pixel piece insertion / extraction methods that perform simple image interpolation and thinning scaling. Therefore, it is possible to prevent image quality deterioration due to interference.

<Other examples>
In the embodiment described above, the example in which the deviation amount of the scanning speed at each off-axis image height with respect to the scanning speed at the axial image height is used as the partial magnification characteristic information is not limited to this. For example, it may be processing that uses the amount of deviation of the scanning speed of each image height with respect to the scanning speed at an arbitrary image height that is not the on-axis image height. In any case, any information may be used as long as it can determine how much deviation occurs at the scanning speed of each image height.

  In the above-described embodiment, an example in which the PWM conversion table switching signal LUTSEL is input from the frequency control unit 127 to the PWM processing unit 123 has been described. However, the present invention is not limited to this. For example, the count value of the main scanning position counting unit 706 and the frequency magnification correction information 1103 may be input to the PWM processing unit 123, and the PWM processing unit 123 may switch the table to be used based on these values.

  The present invention supplies a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

Claims (18)

An image forming apparatus having an optical configuration in which a scanning speed at which a laser beam moves in a main scanning direction on a surface of a photoconductor is not constant,
Acquisition means for acquiring partial magnification characteristic information indicating a deviation amount of the scanning speed of each image height with respect to a scanning speed at an arbitrary image height of the photoconductor;
PWM processing means for inputting an image signal using a second clock and outputting a signal obtained by subjecting the input image signal to pulse width modulation (PWM) processing using the first clock;
The first clock is determined according to the partial magnification characteristic information and the number of gradations of the image signal, and the pixels of the image signal are latently spaced on the surface of the photoconductor at regular intervals in the main scanning direction. An image forming apparatus comprising: frequency control means for generating the second clock from the first clock at a different frequency according to an image height so that an image can be obtained.   The frequency control means determines a frequency at the fastest image height based on the partial magnification characteristics, and sets a frequency at which the number of gradations of the image signal can be expressed at the determined frequency at the fastest image height to the first clock. The image forming apparatus according to claim 1, wherein the image forming apparatus is determined as:   The image forming apparatus according to claim 1, wherein the first clock has a frequency common to each image height.   4. The image forming apparatus according to claim 1, wherein a frequency of the second clock changes according to an image height. 5.   The image forming apparatus according to claim 4, wherein the image height at which the frequency of the second clock changes is determined based on the partial magnification characteristic information.   The image forming apparatus according to claim 1, wherein the second clock is a clock corresponding to a frequency division ratio of the first clock.   The PWM processing means is a conversion table in which an input image signal value is associated with an output signal value, and one conversion table is selected from a plurality of conversion tables in which the number of bits of the output signal value is different. The image forming apparatus according to claim 1, wherein PWM processing is used.   The image forming apparatus according to claim 7, wherein the number of bits corresponds to a frequency division ratio of the first clock.   The image forming apparatus according to claim 7, wherein the PWM processing unit selects a conversion table to be used according to an image height.   The image forming apparatus according to claim 7, wherein the PWM processing unit selects a conversion table to be used according to the second clock.   9. The image forming apparatus according to claim 7, wherein the frequency control unit outputs a signal designating a conversion table used by the PWM processing unit to the PWM processing unit in accordance with the second clock. . An image forming apparatus having an optical configuration in which a scanning speed at which a laser beam moves in a main scanning direction on a surface of a photoconductor is not constant,
Acquisition means for acquiring partial magnification characteristic information indicating a deviation amount of the scanning speed of each image height with respect to a scanning speed at an arbitrary image height of the photoconductor;
PWM processing means for inputting an image signal and outputting a signal obtained by pulse width modulation (PWM) of the input image signal using a first clock;
Frequency for controlling the PWM processing means to change the number of cycles of the first clock used for processing to output a signal corresponding to one pixel of the image signal for each image height based on the partial magnification characteristic information Control means.   The PWM processing means is a clock corresponding to the frequency division ratio of the first clock, and for each image height so that each pixel of the image signal can be a latent image at equal intervals in the main scanning direction on the surface of the photoconductor. The image forming apparatus according to claim 12, wherein the image signal is input using a second clock having a different frequency.   The image forming apparatus according to claim 13, wherein the second clock that differs for each image height is determined based on the partial magnification characteristic information.   The image forming apparatus according to claim 13, wherein the number of cycles is a value corresponding to the frequency division ratio. An image forming apparatus having an optical configuration in which a scanning speed at which a laser beam moves in a main scanning direction on a surface of a photoconductor is not constant, and an image signal is input using a second clock, and the input image signal has a pulse width A control method of an image forming apparatus having PWM processing means for outputting a modulated (PWM) processed signal using a first clock,
An acquisition step of acquiring partial magnification characteristic information indicating a deviation amount of the scanning speed of each image height with respect to a scanning speed at an arbitrary image height of the photoconductor;
The first clock is determined according to the partial magnification characteristic information and the number of gradations of the image signal, and the pixels of the image signal are latently spaced on the surface of the photoconductor at regular intervals in the main scanning direction. And a frequency control step of generating the second clock from the first clock at a different frequency according to an image height so that the second clock can be imaged. An image forming apparatus having an optical configuration in which the scanning speed at which laser light moves on the surface of a photoconductor is not constant, and an image signal is input and a signal obtained by pulse width modulation (PWM) of the input image signal is received. A method for controlling an image forming apparatus having PWM processing means for outputting using a first clock,
An acquisition step of acquiring partial magnification characteristic information indicating a deviation amount of the scanning speed of each image height with respect to a scanning speed at an arbitrary image height of the photoconductor;
Frequency for controlling the PWM processing means to change the number of cycles of the first clock used for processing to output a signal corresponding to one pixel of the image signal for each image height based on the partial magnification characteristic information And a control step.   A program for causing a computer to execute the control method according to claim 16 or 17.