JP5806963B2 - Image forming apparatus - Google Patents

Description

  The present invention generates a driving current of a light source that irradiates a light beam for forming an electrostatic latent image based on an analog signal generated by smoothing a periodic pulse signal indicating the light amount of the light beam. Regarding technology.

  Electrophotographic image formation includes a process of forming an electrostatic latent image of an image indicated by image data on a photosensitive drum, a step of supplying toner to the electrostatic latent image to form a toner image, and a toner image A step of transferring to the paper, and a step of fixing the toner image transferred to the paper to the paper.

  In the process of forming the electrostatic latent image, the light beam emitted from the light source is controlled by the light source to be deflected by the polygon mirror, and the main scanning line is drawn on the rotating photosensitive drum. An electrostatic latent image is formed on the photosensitive drum.

  When the main scanning line is drawn on the photosensitive drum while the magnitude of the driving current of the light source is fixed, the amount of light beam (in other words, the intensity) irradiated to the photosensitive drum depends on the position on the photosensitive drum. Different. The cause of this is that the distance between the photosensitive drum and the polygon mirror differs between the central portion and both ends of the photosensitive drum (the distance between the polygon mirror and the central portion of the photosensitive drum is different between the polygon mirror and the photosensitive drum). And the optical characteristics of a condensing lens disposed between the polygon mirror and the photosensitive drum.

  If the light amount of the light beam applied to the photosensitive drum varies depending on the position on the photosensitive drum, unevenness in image density occurs.

  Therefore, the magnitude of the drive current of the light source is adjusted during main scanning so that the amount of light beam irradiated onto the photosensitive drum is constant on the photosensitive drum. For example, the main scanning line is divided into a plurality of regions (blocks), a PWM (Pulse Width Modulation) signal indicating the amount of light beam irradiated to the light source for each region is generated, the PWM signal is smoothed, and the light beam There has been proposed a technique of generating an analog signal having a magnitude corresponding to the amount of light and adjusting the magnitude of the drive current of the light source based on the analog signal (see, for example, Patent Document 1 and Patent Document 2).

JP 2005-262509 A JP 2009-262344 A

  When an analog signal is generated by smoothing the PWM signal, a ripple occurs in the analog signal corresponding to the phase of the PWM signal. Ripple can cause image quality degradation regardless of monochrome or color images.

  When the circuit for smoothing the PWM signal is a low-pass filter made up of a CR filter, ripples can be prevented from occurring in the analog signal by increasing the time constant of the CR filter. However, if the time constant is increased, the response time from when the PWM signal is input to the CR filter until the analog signal is output becomes longer, which delays control of the light amount of the light beam. Therefore, there is a limit to increasing the time constant of the CR filter.

  The object of the present invention is the basis of the driving current of a light source that irradiates a light beam for forming an electrostatic latent image, and even if a ripple occurs in an analog signal generated by smoothing a pulse signal, An object of the present invention is to provide an image forming apparatus capable of preventing deterioration in image quality.

An image forming apparatus according to an aspect of the present invention that achieves the above object includes a photoconductor and a light source that emits a light beam, and scans the light beam emitted by the light source in a main scanning direction, thereby the photoconductor. Generating a main scanning line for each of a plurality of colors used for color expression, and generating a periodic pulse signal indicating a light quantity of the light beam irradiated on the light source. A pulse generator for each color and an analog signal generated by smoothing the pulse signal generated by the pulse generator with a low-pass filter comprising a CR filter for each of the plurality of colors A smoothing unit that performs the generation of the driving current of the light source for each of the plurality of colors based on the analog signal generated by the smoothing unit, and the photoconductor. Ru For each of the plurality of colors, setting a timing at which an effective image period in which the main scanning line is treated as an effective image is started in order to align the writing start position of the main scanning line with the plurality of colors. And a timing setting unit for each of the plurality of colors in order to prevent streak noise extending along the sub-scanning direction from appearing in the image due to the ripple generated in the analog signal. An irradiation control unit that instructs the pulse generation unit to generate the pulse signals whose phases are different from each other by ¼ period in the plurality of colors when the timing set as the reference is used as a reference. And comprising.

  An image forming apparatus according to an aspect of the present invention is generated by smoothing a periodic pulse signal indicating a light amount of a light beam for a driving current of a light source that irradiates a light beam for forming an electrostatic latent image. It is premised on a technique for generating based on an analog signal. According to the present inventor, when a pulse signal having the same frequency and phase is used for drawing each main scanning line due to the ripple generated in the analog signal, streak noise extending along the sub-scanning direction appears in the image. I found that there was a case.

  By using pulse signals having the same frequency and phase, the positions of the ripples in the main scanning direction are aligned in each main scanning line. As a result, the ripples are aligned in the sub scanning direction. When the ripples are aligned along the sub-scanning direction over a relatively long distance in the sub-scanning direction, it is recognized on the image as streak noise extending along the sub-scanning direction.

  When forming a color image, among a plurality of colors used for color expression, the light source used for drawing the main scanning line for that color is controlled to blink so that the main scanning line is drawn on the photoreceptor. . Therefore, the ripple generated in the analog signal used for generating the driving current of the light source affects the image quality. In contrast, for a color that is not used among a plurality of colors, the light source used for drawing the main scanning line for the color is controlled to be turned off. Therefore, the ripple generated in the analog signal used for generating the driving current of the light source does not affect the image quality.

  In forming a color image, one or several of a plurality of colors are combined and a main scanning line for those colors is drawn. Therefore, although it depends on the color image to be formed, it is unlikely that the same color is continuously used over a relatively long distance in the sub-scanning direction. Since the ripple of the analog signal is generated corresponding to the phase of the pulse signal, if the phase of the pulse signal changes, the position of the ripple in the main scanning direction changes. Therefore, in the image forming apparatus according to one aspect of the present invention, the phase of the pulse signal is made different for each color.

  That is, in the image forming apparatus according to one aspect of the present invention, for each of a plurality of colors, the pulse signals having different phases from each other in the plurality of colors are based on the timing set in the timing setting unit. Generate. Thus, according to one aspect of the present invention, in a plurality of colors, the frequencies of the pulse signals are the same, but the phases of the pulse signals are different from each other. Therefore, it is possible to avoid the ripples affecting the image quality from being aligned along the sub-scanning direction over a relatively long distance in the sub-scanning direction. Therefore, it is possible to prevent streak-like noise extending along the sub-scanning direction from appearing in the image due to the influence of the ripple.

  As described above, according to one aspect of the present invention, a ripple is generated in an analog signal generated by smoothing a pulse signal, which is a basis of a driving current of a light source that irradiates a light beam for forming an electrostatic latent image. Even so, it is possible to prevent the image quality from being degraded.

  In the above configuration, the light beam scanned in the main scanning direction is received at a predetermined position and outputs a BD signal, and each of the plurality of colors is synchronized with the BD signal. An effective image period measurement unit that starts measurement of the effective image period, and the timing setting unit changes the period from the generation of the BD signal to the start of the effective image period, thereby changing the main scanning line. The start position of writing is aligned with the plurality of colors.

  In this configuration, one aspect of the present invention is applied to an aspect in which measurement of an effective image period is started in synchronization with a BD signal for each of a plurality of colors. According to this structure, it has the same effect as the one aspect | mode of this invention mentioned above.

  In the above configuration, there are a plurality of the photoreceptors and the exposure units, the plurality of photoreceptors are arranged in tandem, and main scanning lines for colors assigned from the plurality of colors are drawn, The plurality of exposure units draw main scanning lines for colors assigned from the plurality of colors on the photoconductors assigned the same color.

  In this configuration, one aspect of the present invention is applied to a tandem image forming apparatus. In the tandem image forming apparatus, there are a plurality of photoreceptors and exposure units corresponding to a plurality of colors used for color expression (for example, a photoreceptor and exposure unit for yellow, a photoreceptor and exposure unit for cyan, Magenta photoconductor and exposure section, and black photoconductor and exposure section). Since errors such as attachment positions of the photosensitive member and the exposure unit inevitably occur between the respective colors, the correction of aligning the writing start position of the main scanning line with the plurality of colors is performed. By applying one aspect of the present invention to a tandem image forming apparatus, the above-described effects of the one aspect of the present invention are obtained.

  According to the present invention, even if a ripple occurs in an analog signal that is generated by smoothing a pulse signal, it becomes a basis of a driving current of a light source that irradiates a light beam for forming an electrostatic latent image. A reduction in image quality can be prevented.

1 is a diagram illustrating an outline of an internal structure of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating a configuration of the image forming apparatus illustrated in FIG. 1. FIG. 2 is a diagram showing an arrangement relationship of optical components constituting an exposure unit provided in the image forming apparatus shown in FIG. 1. It is a block diagram which shows the structure of the drive current production | generation apparatus which produces | generates the drive current of a light source. In Comparative Example 1, when the start timing of the effective image period is the same for yellow, cyan, magenta, and black, a time chart showing the relationship between the effective image period and the pulse signal and a diagram showing the start position of writing of each color. is there. In Comparative Example 2, when the start timing of the effective image period is adjusted for yellow, cyan, magenta, and black, the time chart showing the relationship between the effective image period and the pulse signal, and the position of the start of writing of each color are shown. . It is a graph showing the relationship between a pulse signal (PWM signal) and an analog voltage. It is an enlarged view of an image in which streak-like noise extending along the sub-scanning direction appears. In the present embodiment, when the start timing of the effective image period is adjusted for each of yellow, cyan, magenta, and black, a time chart showing the relationship between the effective image period and the pulse signal and a diagram showing the start position of writing of each color It is. It is a figure which shows an example of the ripple which generate | occur | produced in the analog voltage when drawing n main scanning lines on the photoconductive drum using this embodiment. FIG. 6 is a time chart showing the relationship between the effective image period and the pulse signal when the start timing of the effective image period is adjusted for each of yellow and cyan on the basis of the BD signal, and a diagram showing the write start position of each color. .

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing an outline of the internal structure of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 can be applied to, for example, a digital multi-function peripheral having a copy, printer, scanner, and facsimile function. The image forming apparatus 1 includes an apparatus body 100, a document reading unit 200, and a document feeding unit 300.

  A document reading unit 200 is disposed on the apparatus main body 100, and a document feeding unit 300 is disposed on the document reading unit 200.

  The document feeder 300 functions as an automatic document feeder, and can continuously send a plurality of documents placed on the document placement unit 301 to the document reading unit 200.

  The document reading unit 200 includes a carriage on which an exposure lamp or the like is mounted, a document table made of a transparent member such as glass, a CCD (Charge Coupled Device) sensor, and a document reading slit (all not shown). The CCD sensor outputs the read original as image data.

  The apparatus main body 100 includes a sheet storage unit 101, an image forming unit 103, and a fixing unit 105. The sheet storage unit 101 is disposed at the lowermost part of the apparatus main body 100 and includes two sheet cassettes 101a and 101b that can store a bundle of sheets.

  Among the paper cassettes 101a and 101b, in the bundle of paper stored in the selected cassette, the uppermost paper is sent out toward the paper conveyance path 107 of the apparatus main body 100 by driving a pickup roller (not shown). . The sheet is conveyed to the image forming unit 103 through the sheet conveyance path 107.

  The sheet conveyance path 107 extends in a substantially vertical direction from the lower side to the upper side along one side surface (right side surface in FIG. 1) of the apparatus main body 100, and the other side surface (left side surface in FIG. 1) above. And extends substantially horizontally along the lower side of the document reading unit 200. A discharge tray 131 is provided at the end.

  The image forming unit 103 forms a toner image on the conveyed paper. The image forming unit 103 includes a yellow image forming unit 111Y, a magenta image forming unit 111M, a cyan image forming unit 111C, and a black image forming unit 111BK, which are arranged in the order in which the toner images are transferred to the transfer belt 117. These units have the same configuration and will be described by taking the yellow image forming unit 111Y as an example.

  The yellow image forming unit 111Y includes a photosensitive drum 113 and an exposure unit 115. Around the photosensitive drum 113, a charging unit 119, a developing unit 121, and a cleaning unit 123 are arranged. The charging unit 119 uniformly charges the peripheral surface of the photosensitive drum 113. The exposure unit 115 generates a modulated light beam corresponding to image data (image data output from the document reading unit 200, image data transmitted from a personal computer, image data received by facsimile, etc.), and uniformly charged. Irradiate to the peripheral surface of the photosensitive drum 113. As a result, an electrostatic latent image corresponding to yellow image data is formed on the peripheral surface of the photosensitive drum 113. In this state, by supplying yellow toner from the developing unit 121 to the peripheral surface of the photosensitive drum 113, a toner image corresponding to yellow image data is formed on the peripheral surface.

  The transfer belt 117 can move counterclockwise while being sandwiched between the photosensitive drum 113 and the primary transfer roller 125. The yellow toner image is transferred from the photosensitive drum 113 to the transfer belt 117. The yellow toner remaining on the peripheral surface of the photosensitive drum 113 is removed by the cleaning unit 123. The above is the description of the yellow image forming unit 111Y.

  Above the yellow image forming unit 111Y, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit 111BK, containers containing corresponding color toners, that is, a yellow toner container 127Y, a magenta toner container 127M, A cyan toner container 127C and a black toner container 127BK are arranged. Toner for each color is supplied with toner from the corresponding container.

  As described above, the yellow toner image is transferred to the transfer belt 117, and the magenta toner image is transferred onto the toner image. Similarly, the cyan toner image and the black toner image are transferred onto the transfer belt 117. As a result, a color toner image is formed on the transfer belt 117. In this way, a toner image of each color pattern is superimposed and transferred onto the transfer belt 117, whereby a color toner image is formed on the transfer belt 117. The color toner image is transferred by the secondary transfer roller 129 onto the sheet conveyed from the sheet storage unit 101 described above.

  The sheet on which the color toner image is transferred is sent to the fixing unit 105. The fixing unit 105 includes a heating roller and a fixing roller. The paper on which the color toner image is transferred is sandwiched between these rollers. As a result, heat and pressure are applied to the color toner image and the paper, and the color toner image is fixed to the paper. The sheet is discharged to a discharge tray 131.

  FIG. 2 is a block diagram showing a configuration of the image forming apparatus 1 shown in FIG. The image forming apparatus 1 has a configuration in which an apparatus main body 100, a document reading unit 200, a document feeding unit 300, an operation unit 400, a control unit 500, and a communication unit 600 are connected to each other by a bus. Since the apparatus main body 100, the document reading unit 200, and the document feeding unit 300 have already been described, description thereof will be omitted.

  The operation unit 400 includes an operation key unit 401 and a display unit 403. The display unit 403 has a touch panel function, and displays a screen including soft keys. The user operates the soft keys while viewing the screen to make settings necessary for executing functions such as copying.

  The operation key unit 401 is provided with operation keys including hard keys. Specifically, a function switch key for switching between a start key, a numeric keypad, a stop key, a reset key, a copy, a printer, a scanner, and a facsimile is provided.

  The start key is a key for starting operations such as copying and facsimile transmission. The numeric keypad is a key for inputting numbers such as the number of copies and a facsimile number. The stop key is a key for stopping the copy operation or the like halfway. The reset key is a key for returning the set contents to the initial setting state.

  The function switching key includes a copy key, a transmission key, and the like, and is a key for switching between a copy function, a transmission function, and the like. When the copy key is operated, an initial copy screen is displayed on the display unit 403. When the transmission key is operated, an initial screen for facsimile transmission and mail transmission is displayed on the display unit 403.

  The control unit 500 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an image memory, and the like. The CPU executes control necessary for operating the image forming apparatus 1 on the above-described components of the image forming apparatus 1 such as the apparatus main body 100. The ROM stores software necessary for controlling the operation of the image forming apparatus 1. The RAM is used for temporary storage of data generated during execution of software, storage of application software, and the like. The image memory temporarily stores image data (image data output from the document reading unit 200, image data transmitted from a personal computer, image data received by facsimile, etc.).

  The communication unit 600 includes a facsimile communication unit 601 and a network I / F unit 603. The facsimile communication unit 601 includes an NCU (Network Control Unit) for controlling connection of a telephone line with a destination facsimile and a modulation / demodulation circuit for modulating / demodulating a signal for facsimile communication. The facsimile communication unit 601 is connected to the telephone line 605.

  A network I / F unit 603 is connected to a LAN (Local Area Network) 607. A network I / F unit 603 is a communication interface circuit for executing communication with a terminal device such as a personal computer connected to the LAN 607.

  The exposure unit 115 will be described in detail. FIG. 3 is a diagram showing an arrangement relationship of optical components that constitute the exposure unit 115. The exposure unit 115 includes a light source 31, a polygon mirror 10, two scanning lenses 33 and 35, and the like. The light source 31 is a laser diode, for example, and irradiates the light beam LB.

  A collimator lens 37 and a cylindrical lens 39 are arranged on the optical path between the light source 31 and the polygon mirror 10. The collimator lens 37 converts the light beam LB emitted from the light source 31 into parallel light. The cylindrical lens 39 condenses the light beam LB that has been converted into parallel light into a linear shape. The light beam LB condensed linearly is incident on the polygon mirror 10.

  On the optical path between the polygon mirror 10 and the photosensitive drum 113, a scanning lens 33 and a scanning lens 35 are disposed. The light beam LB incident on the deflection surface of the polygon mirror 10 is reflected and deflected by the deflection surface and imaged on the photosensitive drum 113 by the scanning lenses 33 and 35. In other words, an electrostatic latent image is formed on the photosensitive drum 113 by scanning the photosensitive drum 113 with the light beam LB.

  The exposure unit 115 further includes a BD lens 41 and a BD sensor 43. The light beam LB scans the photosensitive drum 113 from one side 113a to the other side 113b of the photosensitive drum 113, and the light beam LB exceeding the effective scanning range R is condensed by the BD lens 41. The BD sensor 43 receives the light. When the BD (Beam Detect) sensor 43 receives the light beam LB, the BD (Beam Detect) sensor 43 generates a BD signal as a reference for starting scanning (main scanning) on the photosensitive drum 113.

  As described above, the exposure unit including the exposure unit 115 of the yellow image forming unit 111Y, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit 111BK emits the light beam LB irradiated by the light source 31. Drawing the main scanning line on the photosensitive drum 113 (an example of a photosensitive member) by scanning in the main scanning direction is executed for each of the four colors used for color expression.

  In the present embodiment, a drive current for the light source 31 is generated based on the pulse signal. FIG. 4 is a block diagram showing the configuration of the drive current generator 10 that generates the drive current S3 of the light source 31. As shown in FIG. The drive current generation device 10 includes a yellow light source control unit 21Y, a cyan light source control unit 21C, a magenta light source control unit 21M, a black light source control unit 21BK, a timing setting unit 17, an effective image period measurement unit 18, and an irradiation control unit 19.

  The yellow light source control unit 21Y performs control to irradiate a light beam LB for forming an electrostatic latent image of an image indicated by yellow image data (hereinafter, yellow). The cyan light source control unit 21C performs control to irradiate a light beam LB for forming an electrostatic latent image of an image indicated by cyan image data (hereinafter, cyan). The magenta light source control unit 21M controls to irradiate a light beam LB for forming an electrostatic latent image (hereinafter, magenta) of an image indicated by magenta image data. The black light source control unit 21BK performs control to irradiate a light beam LB for forming an electrostatic latent image of an image indicated by black image data (hereinafter, black).

  Since the yellow light source control unit 21Y, the cyan light source control unit 21C, the magenta light source control unit 21M, and the black light source control unit 21BK have the same configuration, the yellow light source control unit 21Y will be described as an example. The yellow light source control unit 21Y includes a pulse generation unit 11, a smoothing unit 12, and an LD driver circuit 13.

  The pulse generation unit 11 generates a periodic pulse signal S1 indicating the amount of light beam LB irradiated on the light source 31. The pulse generation unit 11 is realized by, for example, an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array). For example, a PWM signal or a PDM (Pulse Density Modulation) signal can be used as the periodic pulse signal S1. The PDM signal is a signal having a variable density (interval) at which pulses having a constant pulse width are output. In the present embodiment, the pulse signal S1 generated by the pulse generator 11 will be described by taking a PWM signal as an example. The light quantity of the light beam LB irradiated on the light source 31 is shown using the duty ratio of the PWM signal.

  The smoothing unit 12 includes a low-pass filter including a CR filter, and smoothes the pulse signal S1 generated by the pulse generation unit 11 to generate an analog voltage S2 (analog signal). The analog voltage S2 indicates the light amount of the light beam LB irradiated on the light source 31.

  The analog voltage S2 is sent to the LD driver circuit 13. An image data signal indicating an image to be printed on paper is input to the LD driver circuit 13. The LD driver circuit 13 executes control for generating the drive current S3 of the light source 31 and lighting control of the light source 31 using the analog voltage S2 and the image data signal.

  The LD driver circuit 13 includes a comparison unit 14 and a drive current generation unit 15. The analog voltage S <b> 2 generated by the smoothing unit 12 is input to one input unit of the comparison unit 14 and sent to the drive current generation unit 15. The drive current generator 15 generates the drive current S3 of the light source 31 using the analog voltage S2.

  The light source 31 is turned on by the driving current S3 and irradiates the light beam LB. In addition to irradiating the photosensitive drum 113, the light beam LB is received by the light receiving unit 16 made of a photodiode. The signal output from the light receiving unit 16 is input to the other input unit of the comparison unit 14.

  The anode of the laser diode that is the light source 31 is connected to the cathode of the photodiode that is the light receiving unit 16. These anodes and cathodes are connected to a power source.

  The comparison unit 14 and the drive current generation unit 15 constitute a light amount control unit 20 (APC unit). The light amount control unit 20 drives the drive current in the APC period so that the light amount of the light beam LB irradiated on the light source 31 in the effective image period matches the light amount of the light beam LB indicated by the pulse signal S1 generated by the pulse generation unit 11. The size of S3 is automatically controlled (APC). The effective image period is a period during which the main scanning line drawn on the photosensitive drum 113 is treated as an effective image.

  The BD sensor 43 receives the light beam LB scanned in the main scanning direction at a predetermined position and outputs a BD signal. The BD signal is a signal serving as a reference for starting main scanning on the photosensitive drum 113.

  The pulse generation unit configured by the pulse generation unit 11 of the yellow light source control unit 21Y, the cyan light source control unit 21C, the magenta light source control unit 21M, and the black light source control unit 21BK has a period indicating the light amount of the light beam LB to be irradiated to the light source 31. The generation of the pulse signal S1 having the same frequency in the four colors is executed for each of the four colors.

  The smoothing unit configured by the smoothing unit 12 of the yellow light source control unit 21Y, the cyan light source control unit 21C, the magenta light source control unit 21M, and the black light source control unit 21BK smoothes the pulse signal S1 generated by the pulse generation unit 11, The generation of the analog voltage S2 is executed for every four colors.

  The drive current generator composed of the drive current generator 15 of the yellow light source controller 21Y, the cyan light source controller 21C, the magenta light source controller 21M, and the black light source controller 21BK is based on the analog voltage S2 generated by the smoother 12. Thus, the generation of the drive current S3 of the light source 31 is executed for each of the four colors.

  The timing setting unit 17, the effective image period measurement unit 18, and the irradiation control unit 19 are functional blocks executed by the control unit 500.

  The timing setting unit 17 sets the timing at which an effective image period in which the main scanning line is treated as an effective image starts in order to align the writing start position of the main scanning line drawn on the photosensitive drum 113 with the four colors. Do this for each of the four colors. Accordingly, the main scanning line drawn on the yellow photosensitive drum 113, the main scanning line drawn on the cyan photosensitive drum 113, the main scanning line drawn on the magenta photosensitive drum 113, and black The writing start positions of the main scanning lines drawn on the photosensitive drum 113 are aligned.

  This setting will be described. FIG. 5 is a time chart showing the relationship between the effective image period and the pulse signal S1 and the start of writing of each color when the start timing T of the effective image period is the same for yellow, cyan, magenta, and black in Comparative Example 1. FIG. FIG. 6 is a time chart showing the relationship between the effective image period and the pulse signal S1 and the start of writing of each color when the start timing T of the effective image period is adjusted for yellow, cyan, magenta, and black in Comparative Example 2. It is a figure which shows a position.

  In Comparative Example 1 and Comparative Example 2, the pulse signal S1 for yellow, the pulse signal S1 for cyan, the pulse signal S1 for magenta, and the pulse signal S1 for black have the same frequency and phase. For these colors, the start timing T of the effective image period is synchronized with the BD signal for each color. Reference symbol SP indicates a spot of the light beam LB irradiated on the photosensitive drum 113.

  As shown in FIG. 5, when the start timing T of the effective image period is made the same for yellow, cyan, magenta, and black, there is a shift in the writing start position of the main scanning line for each color. This is because errors such as attachment positions of the photosensitive drum 113 and the exposure unit 115 inevitably occur between the colors.

  In the case of FIG. 5, the writing start position of the cyan and black main scanning lines is one spot ahead of the spot SP in the main scanning direction compared to the writing start position of the yellow main scanning line. . The writing start position of the magenta main scanning line is two spots before the spot SP in the main scanning direction compared to the writing start position of the yellow main scanning line.

  Therefore, as shown in FIG. 6, the start timing T of the effective image period for cyan and black is compared with the start timing T of the effective image period for yellow by an amount corresponding to one spot SP. To delay. Further, the start timing T of the effective image period for magenta is delayed from the start timing T of the effective image period for yellow by an amount corresponding to the period corresponding to two spots SP.

  As described above, the timing setting unit 17 adjusts the period from the generation of the BD signal to the start of the effective image period, thereby eliminating the deviation of the writing start position of the main scanning line for each color and the main scanning for each color. The line start position is aligned.

  Returning to the description of FIG. The effective image period measurement unit 18 starts measuring the effective image period for each of the four colors in synchronization with the BD signal.

  The irradiation control unit 19 generates, for each of the four colors, a pulse signal S1 in which the phase of the pulse signal S1 with respect to the timing T set in the timing setting unit 17 is different from each other in the four colors. Commands the yellow light source control unit 21Y, the cyan light source control unit 21C, the magenta light source control unit 21M, and the pulse generation unit 11 of the black light source control unit 21BK. This will be described in detail later.

  Here, the relationship between the PWM signal, which is the pulse signal S1 generated by the pulse generation unit 11, and the analog voltage S2 generated by the smoothing unit 12 will be described. FIG. 7 is a graph showing this relationship. The horizontal axis of the graph indicates time, and the vertical axis of the graph indicates the value of the analog voltage S2. The duty ratio of the PWM signal generated by the pulse generator 11 is 50%.

  When the pulse generator 11 starts generating the PWM signal, the analog voltage S2 gradually increases from 0V and reaches 1.0V from the start of generating the PWM signal. The analog voltage S2 has a ripple corresponding to the phase of the PWM signal.

  According to the present inventor, when a pulse signal S1 (PWM signal) having the same frequency and phase is used for drawing each main scanning line due to the influence of a ripple generated in the analog voltage S2, a streak extending along the sub scanning direction is used. I found that the noise may appear in the image. FIG. 8 is an enlarged view of an image in which streak noise extending along the sub-scanning direction appears. The vertical stripe is noise.

  By using the pulse signal S1 having the same frequency and phase, the positions of the ripples in the main scanning direction are aligned in each main scanning line. As a result, the ripples are aligned in the sub scanning direction. When the ripples are aligned along the sub-scanning direction over a relatively long distance in the sub-scanning direction, it is recognized on the image as streak noise extending along the sub-scanning direction.

  When forming a color image, among the four colors used for color expression, the light source 31 used for drawing the main scanning line for the color is controlled to blink so that the main scanning line is formed on the photosensitive drum 113. Drawn. Therefore, the ripple generated in the analog voltage S2 used for generating the drive current S3 of the light source 31 affects the image quality. On the other hand, of the four colors that are not used, the light source 31 used for drawing the main scanning line for that color is controlled to be turned off. Therefore, the ripple generated in the analog voltage S2 used for generating the drive current S3 of the light source 31 does not affect the image quality.

  In forming a color image, one or several of the four colors are combined and a main scanning line for those colors is drawn. Therefore, although it depends on the color image to be formed, it is unlikely that the same color is continuously used over a relatively long distance in the sub-scanning direction. As described with reference to FIG. 7, the ripple of the analog voltage S2 is generated corresponding to the phase of the pulse signal S1, and therefore the position of the ripple in the main scanning direction changes when the phase of the pulse signal S1 changes. Therefore, in this embodiment, the phase of the pulse signal S1 is varied for each color.

  In other words, in the image forming apparatus 1 according to the present embodiment, for each of the four colors, the pulses of the pulse signal S1 when the timing T set in the timing setting unit 17 is used as a reference are different in the four colors. A signal S1 is generated. This will be described with reference to FIG. FIG. 9 is a time chart showing the relationship between the effective image period and the pulse signal S1 and the writing of each color when the start timing T of the effective image period is adjusted for each of yellow, cyan, magenta, and black in this embodiment. It is a figure which shows the position of a start. 9 differs from FIG. 6 in that the phases of the four color pulse signals S1 are different from each other. For example, the irradiation control unit 19 controls the pulse generation unit 11 for each color as follows. Using the pulse signal S1 for yellow as a reference, the pulse signal S1 for cyan is delayed by a quarter cycle, the pulse signal S1 for magenta is delayed by a quarter cycle, and the pulse signal S1 for black is delayed by a quarter cycle. As a result, the phases of the pulse signals S1 for the respective colors are made different from each other.

  FIG. 10 is a diagram illustrating an example of a ripple generated in the analog voltage S2 when n main scanning lines are drawn on the photosensitive drum 113 using this embodiment. Of the n main scanning lines, the k + 19th main scanning line from the kth main scanning line is shown. Ripple that affects image quality is surrounded by a dotted line.

  In the k-th to k + 12-th main scanning lines, the yellow, cyan, and black light sources 31 are controlled to be turned off, and the magenta light source 31 is controlled to blink. The ripple generated in the magenta analog voltage S2 may affect the image quality. On the other hand, the ripple generated in the analog voltage S2 for yellow, cyan, and black does not affect the image quality.

  On the other hand, in the k + 13th to k + 19th main scanning lines, the yellow and cyan light sources 31 are controlled to blink, and the magenta and black light sources 31 are controlled to be turned off. The ripple generated in the analog voltage S2 for yellow and cyan may affect the image quality. On the other hand, the ripple generated in the magenta and black analog voltage S2 does not affect the image quality.

  Thus, according to the present embodiment, in the four colors, the frequency of the pulse signal S1 is the same, but the phase of the pulse signal S1 is different from each other. Therefore, it is possible to avoid the ripples affecting the image quality from being aligned along the sub-scanning direction over a relatively long distance in the sub-scanning direction. Therefore, it is possible to prevent streak-like noise extending along the sub-scanning direction from appearing in the image due to the influence of the ripple.

  As described above, according to the present embodiment, the analog voltage S2 generated by smoothing the pulse signal S1 becomes the basis of the drive current S3 of the light source 31 that irradiates the light beam LB for forming the electrostatic latent image. Even if a ripple occurs, it is possible to prevent the image quality from being degraded.

  The reason why the phase of the pulse signal S1 is different with respect to the writing start position of the main scanning line aligned with the four colors is as follows. For example, if the phase of the pulse signal S1 is made different with respect to the BD signal, when the start timing T of the effective image period is adjusted in order to align the writing start position among the four colors, This is because the phase of the pulse signal S1 may be the same during the effective image period in two or more colors. An example of this is shown in FIG. It can be seen that the phases of the yellow pulse signal S1 and the cyan pulse signal S1 are the same during the effective image period. For two or more colors, if the phase of the pulse signal S1 is the same during the effective image period, ripples affecting the image quality may be aligned along the sub-scanning direction over a relatively long distance in the sub-scanning direction. Therefore, streak-like noise extending along the sub-scanning direction may appear in the image due to the influence of the ripple.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Drive current generation apparatus 31 Light source 113 Photosensitive drum (an example of a photosensitive body)
115 Exposure unit S1 Pulse signal S2 Analog voltage (an example of an analog signal)
S3 Drive current

Claims (3)

A photoreceptor,
Including a light source that emits a light beam, and scanning the light beam emitted by the light source in a main scanning direction to draw a main scanning line on the photosensitive member for each of a plurality of colors used for color expression An exposure unit;
Generating a periodic pulse signal indicating the amount of the light beam irradiated to the light source for each of the plurality of colors; and
Smoothing the pulse signal generated by the pulse generation unit with a low-pass filter including a CR filter to generate an analog signal for each of the plurality of colors;
Based on the analog signal generated by the smoothing unit, generating a driving current for the light source for each of the plurality of colors,
Setting the timing at which an effective image period in which the main scanning line is treated as an effective image starts in order to align the writing start position of the main scanning line drawn on the photoconductor with the plurality of colors. A timing setting unit for each of the plurality of colors;
In order to prevent streak noise extending along the sub-scanning direction from appearing in the image due to the ripple generated in the analog signal, the timing set in the timing setting unit for each of the plurality of colors And an irradiation control unit that instructs the pulse generation unit to generate the pulse signals having phases different from each other by a quarter period in the plurality of colors. apparatus. A BD sensor that receives the light beam scanned in the main scanning direction at a predetermined position and outputs a BD signal;
An effective image period measurement unit that starts measurement of the effective image period in synchronization with the BD signal for each of the plurality of colors;
2. The image according to claim 1, wherein the timing setting unit aligns the writing start position of the main scanning line with the plurality of colors by changing a period from generation of the BD signal to start of the effective image period. Forming equipment. There are a plurality of the photoreceptors and the exposure parts,
The plurality of photoconductors are arranged in tandem, and main scanning lines for colors assigned from the plurality of colors are drawn,
The image forming apparatus according to claim 1, wherein the plurality of exposure units draw a main scanning line for a color assigned from the plurality of colors on the photoconductor to which the same color is assigned.