JP2009037161A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can use a digital-analog converter having a plurality of channels integrated in a single element as a digital-analog converter used for correcting the beam quantity of a laser beam source. <P>SOLUTION: The image forming apparatus includes: the laser beam source 6 that adjusts the beam quantity of the laser beam in accordance with an analog voltage from the DA converter 7; a correction data storage part 605 that prestores correction data for each of the laser beam sources 6 corresponding to respective colors for each unit time (ts) obtained by dividing a single scanning period (tc) into the integer pieces of periods; a rotation angle control part 602 that adjusts a rotation angle difference among respective polygon mirrors 2 so that a timing difference between timings at which laser beams are reflected from respective polygon mirrors 2 to respective synchronization sensors 9 may become integer multiple of the unit time (ts); and a laser beam correction/control part 603 for successively setting respective correction data in the DA converter 7 having the plurality of channels integrated in the single element for each unit time (ts) after when the laser beam is detected by each synchronization sensor 9. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus including a plurality of image forming units that multiplex-transfer toner images of different colors onto recording paper.

  In recent years, in an image forming apparatus such as a full-color printer or a full-color copying machine, four image forming units including a photosensitive drum for individually forming four color toner images of yellow, magenta, cyan, and black are provided on a transfer belt. A so-called tandem system arranged along the conveyance direction of the recording paper is widely used. In the tandem method, a color image is formed by superimposing and transferring toner images of respective colors formed on a photosensitive drum onto a recording sheet conveyed by a transfer belt.

  Such a tandem image forming apparatus includes a laser scanner unit (LSU) that performs laser scanning of the photosensitive drum independently for each color. A polygon mirror motor for driving the polygon mirror for sweeping the laser light needs to be arranged and controlled for each color. When the polygon mirror motors for each color are independently and asynchronously rotated, a color shift of less than one scanning width (42.3 μm at 600 dpi) occurs in the sub-scanning direction due to the tandem configuration. Furthermore, if the multi-beam method is used to cope with high-speed printing, one scanning width increases by the number of beams, and thus color misregistration also increases.

  Therefore, by detecting the angle of the mirror surface of the reference polygon mirror and controlling the drive of each polygon mirror motor so as to adjust the angle of the mirror surface of the other polygon mirror based on this polygon mirror, A technique for reducing color misregistration is known (for example, see Patent Document 1).

On the other hand, a technique is known in which the power of a laser diode is controlled during printing in order to correct the sensitivity unevenness of the photoconductor in the main scanning direction and the light amount unevenness of the optical system. In this case, it is necessary to change the power of the laser diode during one scan. Therefore, correction data indicating the power of the laser diode for correcting the above-described unevenness of sensitivity and light amount is prepared for one scan in the main scanning direction, and this correction data is supplied to the digital / analog converter at a constant cycle timing. By setting and converting to an analog voltage, a control voltage corresponding to the correction data is applied to the laser diode to correct the amount of laser light.
JP-A 64-73369

  However, in the case where the light quantity of the laser diode is corrected using the digital / analog converter as described above, there is a disadvantage in that a digital / analog converter corresponding to the number of colors is required, resulting in an increase in cost.

  On the other hand, in recent years, inexpensive digital-analog converters in which a plurality of channels are integrated into one element are in the market. If such a digital analog converter having a plurality of channels with one element is used instead of a plurality of digital analog converters corresponding to the number of colors, it is considered that the cost can be reduced.

  When setting digital values to a digital-analog converter with multiple channels with one element, there is only one interface circuit for setting digital values, so it is not possible to set digital values for multiple channels at the same time. Can not. Further, in order to set a digital value for one channel, a certain access time is required. Therefore, even if an attempt is made to change the digital value of another channel while accessing a certain channel, the access conflicts and the digital value cannot be changed.

  On the other hand, as described above, when the color misregistration is reduced by controlling the driving of each polygon mirror motor, the timing of the laser beam reflected by each polygon mirror is different, and one scan of each color laser diode is performed. Each start timing is different. Therefore, the timing at which the light amount of the laser diode should be changed, that is, the timing at which the correction data is set in the digital-analog converter is also different for each color. In addition, the time interval between the set timings is not constant because it is determined depending on the color misregistration correction amount.

  Then, if a digital-analog converter having a single element and having multiple channels is used in place of a plurality of digital-analog converters corresponding to the number of colors, the setting timing of digital values corresponding to the respective colors will compete. There is a fear. Therefore, a digital analog converter having a plurality of channels with one element cannot be used in place of a plurality of digital analog converters corresponding to the number of colors.

  The present invention has been made in view of such circumstances, and corrects the light quantity of a laser light source in an image forming apparatus including a plurality of image forming units that multiplex-transfer toner images of different colors onto a recording sheet. An object of the present invention is to provide an image forming apparatus capable of using a digital-analog converter having a single element and a plurality of channels as a digital-analog converter used for this purpose.

  An image forming apparatus according to the present invention includes an image forming unit for forming a toner image for each of a plurality of colors, arranged in a predetermined direction, transports a recording sheet in the arrangement direction, and toner images of each color on the recording sheet. A multi-transfer image forming unit, a control unit for controlling the operation of the image forming unit for each color, and a digital / analog converter in which a plurality of channels for converting a digital value into an analog voltage are integrated in one element. The image forming unit for each color is output from the photosensitive drum, a laser light source that outputs a laser beam whose light amount is adjusted according to an analog voltage output from the control unit, and the laser light source that rotates. The reflected laser beam is reflected by the polygon mirror that scans the photosensitive drum in the main scanning direction, and is reflected by the polygon mirror at the reference position when the scanning is started. A beam detector for detecting laser light, and the controller for each laser light source corresponding to each color for each unit time obtained by dividing the period into an integral number within one scanning period in the main scanning direction. A difference between timings at which the laser light is reflected from the polygon mirror for each color to the beam detectors is stored in the unit time. The correction data storage unit stores each correction data for correcting the intensity of the laser beam in advance. A rotation angle control unit that adjusts a difference in rotation angle between the polygon mirrors so that the laser beam is detected by the beam detection unit for each color. A laser light correction control unit that sequentially reads out correction data of a color corresponding to light from the correction data storage unit every unit time and sets the correction data in the digital-analog converter; For example, the digital-to-analog converter, the access time for setting the correction data as a digital value is shorter than the unit time, the from the plurality of channels to the laser light source of each color, the analog voltage is output.

  According to this configuration, in the image forming unit for each color that multiplex-transfers the toner image, the polygon mirror rotates, and the photosensitive drum is scanned in the main scanning direction by the laser light output from the laser light source. Further, the beam detector detects the laser beam reflected by the polygon mirror at the reference position when scanning is started. Also, the difference in timing at which the laser beam is reflected from the polygon mirror for each color to each beam detector by the rotation angle control unit is an integral multiple of a unit time obtained by dividing one scanning period in the main scanning direction into an integral number. Thus, the difference in rotation angle between the polygon mirrors is adjusted. Then, from when the laser beam is detected by the beam detector for each color, color correction data corresponding to the detected laser beam is sequentially transferred to the digital-analog converter every unit time by the laser beam correction controller. Set with a digital value. Further, the digital value is converted into an analog voltage by the digital-analog converter and output to the laser light source. If it does so, the light quantity of the laser beam output from a laser light source will be adjusted according to an analog voltage.

  In this case, the rotation angle difference between the polygon mirrors is such that the difference in timing at which the laser beam is reflected from the polygon mirror for each color to each beam detector by the rotation angle control unit is an integral multiple of unit time. Therefore, the time difference in timing when the laser beam is detected by the beam detector for each color is also an integral multiple of the unit time. Since the laser beam is detected by the beam detector for each color, the color correction data corresponding to the detected laser beam is sequentially set as a digital value in the digital-analog converter every unit time. The timing at which the correction data for each color is set in the digital-analog converter is synchronized. Then, since it is known that the setting timings of the correction data for each color are synchronized in advance, a digital-analog converter in which a plurality of channels are integrated into one element is avoided to avoid a contention of access. Instead of the digital-analog converter, it can be used as a digital-analog converter used to correct the light quantity of the laser light source. The correction data storage unit stores correction data for each laser light source corresponding to each color for each unit time obtained by dividing one scanning period into an integral number, while each beam from the polygon mirror for each color is stored. The rotation angle difference between the polygon mirrors is adjusted so that the difference in the timing at which the laser beam is reflected to the detector is an integral multiple of the unit time, so that the correction data for each color is converted from digital to analog every unit time. By being set in the converter, it becomes easy to set the correction data in the digital-analog converter at a correct timing in one scanning period.

  The unit time is preferably set to be equal to or longer than a time obtained by multiplying the access time by the number of colors of the plurality of colors. According to this configuration, correction data for all colors can be set in the digital-analog converter within a unit time.

  The digital-analog converter accepts the setting of the correction data for each color by a serial signal, and the access time transfers the correction data for each color to the digital-analog converter by the serial signal. Time is preferred.

  According to this configuration, since the correction data is transferred from the laser light correction control unit to the digital-analog converter by a serial signal, the number of signal terminals between the laser light correction control unit and the digital-analog converter is reduced. As a result, the cost can be easily reduced.

  The image forming unit further includes a color misregistration detecting unit that detects a color misregistration of the toner images of the respective colors that are multiplex-transferred by the image forming unit, and the rotation angle control unit performs a laser from the polygon mirror for each color to the beam detecting unit Adjusting a difference in rotation angle between the polygon mirrors so as to correct a color shift detected by the color shift detector while making a difference in timing at which light is reflected an integral multiple of the unit time. preferable.

  According to this configuration, the color shift of the toner image formed by the image forming unit of each color is detected by the color shift detection unit. The rotation angle control unit corrects the color misregistration detected by the color misregistration detection unit while the difference in timing at which the laser beam is reflected from the polygon mirror for each color to each beam detection unit is made an integral multiple of unit time. In this way, the difference in rotation angle between the polygon mirrors is adjusted to reduce color misregistration. In this case, even if the time difference of the timing at which the laser beam is detected by the beam detector for each color is changed by adjusting the difference in rotation angle, the time difference is an integral multiple of the unit time. Therefore, as described above, a digital-to-analog converter in which a plurality of channels are integrated in one element can be used in place of a plurality of digital-to-analog converters according to the background art while avoiding access contention.

  In the image forming apparatus having such a configuration, each polygon is set so that the difference in timing at which the laser beam is reflected from the polygon mirror for each color to each beam detection unit is an integral multiple of unit time by the rotation angle control unit. Since the difference in the rotation angle between the mirrors is adjusted, the time difference of the timing at which the laser beam is detected by the beam detector for each color is also an integral multiple of the unit time. Since the laser beam is detected by the beam detector for each color, color correction data corresponding to the detected laser beam is sequentially set as a digital value in the digital-analog converter every unit time. The timing at which the correction data for each color is set in the digital-analog converter is synchronized. Then, since it is known that the setting timing of the correction data for each color is synchronized in advance, a digital-to-analog converter in which a plurality of channels are integrated in one element is avoided by avoiding access conflict. Instead of the digital-analog converter, it can be used as a digital-analog converter used for correcting the light quantity of the laser light source.

  Embodiments according to the present invention will be described below with reference to the drawings. In addition, the structure which attached | subjected the same code | symbol in each figure shows that it is the same structure, The description is abbreviate | omitted. FIG. 1 is a schematic diagram showing a schematic configuration of a tandem type color printer 1 which is an example of an image forming apparatus according to the present invention. A color printer 1 shown in FIG. 1 includes a paper feed unit 12 having a paper feed cassette 120 that can be inserted into and removed from the apparatus main body 11 to store recording paper P, and a paper feed cassette that is driven by the paper feed unit 12. A secondary transfer roller 137 that transfers the toner image onto the recording paper P while conveying the recording paper P fed out from 120, and a toner image on the recording paper P onto which the toner image has been transferred by the secondary transfer roller 137. And a paper discharge unit 15 for discharging the recording paper P subjected to the fixing process by the fixing device 14 is provided on the upper surface of the apparatus main body 11. Further, a control unit 60 that controls the operation of each unit in the color printer 1 is provided.

  The image forming apparatus according to the present invention is not limited to a color printer, and may be, for example, a copying machine, a facsimile, or a complex machine that combines these.

  The paper feeding unit 12 is provided with a pickup roller 121 at the upper left position of the paper feeding cassette 120 shown in FIG. 1, and the recording paper P stored in the paper feeding cassette 120 by the drive of the pickup roller 121 is one sheet at a time. The pickup is picked up and sent out to the secondary transfer roller 137.

  The image forming unit 13 includes image forming units 131K, 131C, 131Y, and 131M, a transfer belt 136 onto which the toner image is transferred by the image forming units 131K, 131C, 131Y, and 131M, and the transfer belt 136. A secondary transfer roller 137 for further transferring the toner image onto the recording paper P fed from the paper feed cassette 120 is provided.

  The image forming units 131K, 131C, 131Y, and 131M are sequentially arranged in the order of the image forming units 131K, 131C, 131Y, and 131M from the upstream side (the right side in FIG. 1) to the downstream side. Each of the image forming units 131K, 131C, 131Y, and 131M is positioned and attached to each device in the device main body 11 with a predetermined relative positional relationship.

  Each of the image forming units 131K, 131C, 131Y, and 131M is provided with a photosensitive drum 132 that is an image carrier, and a charger 134 is provided to face each of the photosensitive drums 132. An exposure device 135 (exposure unit) is provided downstream of the charger 134 in the rotation direction of the photosensitive drum 132 (the arrow direction in FIG. 1). The exposure device 135 irradiates the peripheral surface of the photosensitive drum 132 uniformly charged by the charger 134 with laser light based on image data received by the communication I / F unit 200 described later. As a result, an electrostatic latent image is formed on the peripheral surface of each photosensitive drum 132. A developing device 133 is provided further downstream of the exposure device 135 in the rotation direction of the photosensitive drum 132. By supplying toner from the toner container of the developing device 133 to the electrostatic latent image, a toner image is formed on the peripheral surface of the photosensitive drum 132.

  Further, each photosensitive drum 132 is provided with a cleaning device 20 that removes residual toner on the circumferential surface of the photosensitive drum 132 and cleans it. The peripheral surface of the photosensitive drum 132 cleaned by the cleaning device 20 is directed to the charger 134 for a new charging process.

  The transfer belt 136 is stretched between the driving roller 136a and the driven roller 136b so that the surface thereof is in contact with the peripheral surface of the photosensitive drum 132 at a position immediately above the image forming units 131K, 131C, 131Y, and 131M. . A primary transfer roller 136 c is provided directly above each photosensitive drum 132 via a transfer belt 136.

  Each toner image formed on the photosensitive drum 132 of each of the image forming units 131K, 131C, 131Y, and 131M in accordance with the traveling speed of the transfer belt 136 driven by the driving roller 136a is transferred to the primary transfer roller 136c. As a result, they are superimposed on each other and transferred onto the transfer belt 136, and a color toner image is formed on the surface of the transfer belt 136.

  A registration roller 145 that conveys the recording paper P to the nip portion is provided in the recording paper conveyance path from the paper feeding cassette 120 to the nip portion of the secondary transfer roller 137 and the driving roller 136a. The registration roller 145 adjusts the recording paper conveyance timing to the nip portion of the secondary transfer roller 137 and the driving roller 136a, and then conveys the recording paper to the nip portion. In this nip portion, the color toner image on the transfer belt 136 is transferred to the recording paper P by the secondary transfer roller 137, and a color toner image is formed on the recording paper P.

  A transfer belt cleaning device 160 is provided on the right side of the driven roller 136b in FIG. 1, and toner remaining on the surface of the transfer belt 136 after the transfer of the toner image onto the recording paper P is transferred to the transfer belt. The transfer belt 136 removed by the cleaning device 160 and cleaned thereby is supplied to the photosensitive drum 132.

  The fixing device 14 performs a fixing process on the color toner image transferred to the recording paper P by the image forming unit 13. The fixing device 14 includes a heat roller 141 that is heated by an energized heating element, and a pressure roller 142 that is disposed so as to face the heat roller 141 and whose peripheral surface is pressed against the peripheral surface of the heat roller 141. . The color toner image transferred to the recording paper P by the secondary transfer roller 137 in the image forming unit 13 is fixed by heating when the recording paper P passes between the heat roller 141 and the pressure roller 142. Then, the paper is discharged to the paper discharge unit 15.

  Further, the transfer belt 136 is located at a position downstream of the image forming unit 131M at the most downstream position in the running direction and upstream of the nip portion between the drive roller 136a and the secondary transfer roller 137. A detection sensor 245 (beam detection unit) is provided to face the surface of the belt 136. The detection sensor 245 includes a reflective photosensor and the like, and includes a light emitting unit that emits light to the toner pattern on the transfer belt 136 and a light receiving unit that receives reflected light of the light emitted from the light emitting unit. The presence or absence of a toner pattern on the transfer belt 136 is indicated by the amount of light received by the portion. The detection sensor 245 detects a toner pattern formed on the transfer belt 136 by a predetermined image forming unit among the image forming units 131K, 131C, 131Y, and 131M.

  FIG. 2 is a schematic structural diagram showing an example of the internal configuration of the exposure apparatus 135 shown in FIG. The exposure device 135 includes a polygon mirror 2, a polygon mirror motor 3, an fθ lens 4, a collimator lens 5, a laser light source 6, a mirror 8, and a synchronization sensor 9 (beam detection unit) housed in a casing. .

  The laser light source 6 is configured using a laser oscillator such as a laser diode. Then, the laser light source 6 outputs laser light whose light amount is adjusted according to the control analog voltage output from the control unit 60. The collimator lens 5 is disposed in the vicinity of the laser light source 6 and adjusts the beam diameter of the laser light output from the laser light source 6.

  The polygon mirror 2 rotates at a predetermined speed and deflects the laser light so that the laser light is scanned in the longitudinal direction (main scanning direction) of the photosensitive drum 132. In the present embodiment, since the polygon mirror 2 rotates clockwise, the laser beam is scanned from left to right.

  The polygon mirror motor 3 rotates the polygon mirror 2 at a predetermined speed. For example, the polygon mirror motor 3 rotates the polygon mirror in synchronization with a clock signal output from the control unit 60. Accordingly, the polygon mirror motor 3 can adjust the rotation angle of the polygon mirror by rotating the polygon mirror by a desired angle depending on the presence or absence of the clock signal.

  The fθ lens 4 guides the laser beam to the mirror 8 so that the laser beam is scanned at a constant speed in the longitudinal direction (main scanning direction) of the photosensitive drum 132. The mirror 8 reflects the laser light output from the fθ lens 4 toward an opening (not shown) provided in the housing and outputs the reflected light to the photosensitive drum 132.

  The synchronous sensor 9 detects the laser beam immediately before the laser beam starts exposure for one line (one scan). The synchronization sensor 9 is composed of an optical sensor such as a photodiode, and outputs a laser beam detection signal BD to the control unit 60. For example, when the synchronization sensor 9 receives the laser beam reflected by the polygon mirror 2, the beam detection signal BD is changed to a low level in a pulsed manner for the time width during which the laser beam passes the light receiving surface of the synchronization sensor 9. It has become.

  FIG. 3 is a block diagram illustrating an example of an electrical configuration of the image forming apparatus illustrated in FIG. 1. The communication I / F unit 200 is an interface circuit such as Ethernet (registered trademark) or USB (Universal Serial Bus). The communication I / F unit 200 receives image data from a terminal device such as a personal computer connected to the network 201 such as Ethernet (registered trademark) or USB, and outputs the image data to the control unit 60.

  The digital analog (DA) converter 7 is a digital analog converter in which four channels are integrated into one element. The digital-analog converter 7 receives correction data SDk, SDc, SDy, SDm for correcting the light amounts of the four laser light sources 6 respectively provided in the image forming units 131K, 131C, 131Y, 131M. 60 is received as a serial signal. Then, the digital-analog converter 7 converts the correction data SDk, SDc, SDy, SDm for the four channels received from the control unit 60 into analog voltages Vk, Vc, Vy, Vm. It outputs to the four laser light sources 6 provided one by one.

  The transfer time required to transfer the correction data for one channel from the control unit 60 to the digital-analog converter 7 using a serial signal is an access time ta. The access time ta is set to 500 nsec, for example.

  The control unit 60 includes, for example, a CPU (Central Processing Unit) that executes predetermined arithmetic processing, a nonvolatile ROM (Read Only Memory) that stores a predetermined control program, and a RAM (Random) that temporarily stores data. Access Memory) and these peripheral circuits and the like. The control unit 60 functions as an image formation processing unit 601, a rotation angle control unit 602, a laser beam correction control unit 603, and a color misregistration correction unit 604 by executing a control program stored in the ROM. Further, for example, correction data SDk, SDc, SDy, SDm are stored in advance in a nonvolatile ROM. In this case, the ROM functions as the correction data storage unit 605.

  The correction data storage unit 605 stores a laser for each laser light source corresponding to each color for each unit time ts obtained by dividing one scan time tc into an integral number within one scan time tc that is one scan time in the main scan direction. Each correction data for correcting the light intensity is stored in advance as correction data SDk, SDc, SDy, SDm.

  The image forming processing unit 601 controls the operation of each unit in the color printer 1 such as the image forming unit 13 and the fixing device 14, and forms an image on recording paper based on the image data received by the communication I / F unit 200. To do.

  The rotation angle control unit 602 determines that the difference in timing at which laser light is reflected from each polygon mirror 2 to each beam detection unit 9 in the image forming units 131K, 131C, 131Y, and 131M is an integral multiple of a predetermined unit time ts. Thus, the rotation of each polygon mirror motor 3 is controlled to adjust the difference in rotation angle between each polygon mirror 2.

  The laser light correction control unit 603 performs color correction data SDk, corresponding to the detected laser light from when the beam detection unit 9 detects the laser light in the image forming units 131K, 131C, 131Y, and 131M. SDc, SDy, and SDm are sequentially read from the correction data storage unit 605 every unit time ts, converted into a serial signal, and output to the digital / analog converter 7.

  The color misregistration correction unit 604 performs color misregistration correction processing in the sub-scanning direction every time the color printer 1 is powered on and started up, for example. Specifically, for example, the color misregistration correction unit 604 uses, for example, the image forming units 131K, 131C, 131Y, and 131M for the respective colors to place black, cyan, and black on the transfer belt 136 with a predetermined interval W, for example. Yellow and magenta toner patterns are formed. Then, based on the intervals of the black, cyan, yellow, and magenta toner patterns detected by the detection sensor 245 and the interval W, the color misregistration amount in the sub-scanning direction is detected.

  For example, when the interval between the black and cyan toner patterns is smaller than the interval W and the interval between the cyan and yellow toner patterns is larger than the interval W, the color misregistration correction unit 604 scans the cyan laser light in the image forming unit 131C. Since the rotation angle control unit 602 delays the rotation angle of the polygon mirror motor 3 of the image forming unit 131C, the color misregistration is corrected. In this case, the rotation angle control unit 602 changes the rotation angle of the polygon mirror motor 3 so that the scanning timing changes in units of unit time ts.

  Next, a characteristic operation of the color printer 1 shown in FIG. 1 will be described. FIG. 4 is an explanatory diagram for explaining an example of operations of the rotation angle control unit 602 and the laser beam correction control unit 603 shown in FIG. FIG. 4A shows an example of the detection signal BDk of the synchronous sensor 9 in the black image forming unit 131K and the analog voltage Vk output from the digital-analog converter 7 to the laser light source 6 of the image forming unit 131K. Yes. FIG. 4B is an explanatory diagram for explaining a difference from the background art. FIG. 4C shows an example of the detection signal BDc of the synchronization sensor 9 in the cyan image forming unit 131C and the analog voltage Vc output from the digital-analog converter 7 to the laser light source 6 of the image forming unit 131C. Yes.

  In the example shown in FIG. 4, the unit time ts is set to a time during which the laser light passes through one surface of the polygon mirror 2, that is, a time obtained by dividing one scanning time tc in the main scanning direction into seven. That is, the unit time ts is a time obtained by dividing one scanning time tc into an integer number. Then, at every scanning time tc, the detection signals BDk and BDc of the synchronous sensor 9 change to a low level in a pulse shape to indicate the detection of the laser beam.

  First, when the detection signal BDk from the synchronization sensor 9 in the black image forming unit 131K at the most upstream position in the running direction of the transfer belt 136 falls (timing T1), the laser beam correction control unit 603 causes the correction data storage unit. The correction data SDk stored in 605 is read out, converted into a serial signal, and output to the digital / analog converter 7. Then, the correction data SDk is converted into the analog voltage Vk by the digital / analog converter 7 and output to the laser light source 6 in the image forming unit 131K.

  Thereafter, the correction data SDk stored in the correction data storage unit 605 is sequentially read out by the laser beam correction control unit 603 every unit time ts, converted into a serial signal, and output to the digital / analog converter 7. . Then, the digital / analog converter 7 converts the correction data SDk into an analog voltage Vk and outputs it to the laser light source 6 in the image forming unit 131K.

  Thereby, the light quantity of the laser light source 6 of the image forming unit 131K is corrected for one scanning time tc. In this case, the correction data SDk is set to the digital-analog converter 7 by the laser light correction control unit 603 every unit time ts from the timing T1.

  On the other hand, in an image forming unit other than the image forming unit 131K, for example, the image forming unit 131C, the rotation angle of the polygon mirror 2 is adjusted to be delayed from the image forming unit 131K in order to correct color misregistration. At this time, if the laser beam is reflected from the polygon mirror 2 to the beam detector 9 in the image forming unit 131K, and the laser beam is reflected from the polygon mirror 2 to the beam detector 9 in the image forming unit 131C. If the rotation of the polygon mirror motor 3 is not controlled so that the difference between the two and the unit time ts is an integral multiple of the unit time ts, for example, as shown in FIG. 4B, the laser is detected by the synchronous sensor 9 in the image forming unit 131C. The timing at which the detection signal BDc falls when the light is detected (timing T2) is a timing at which an integral multiple of the unit time ts has elapsed from the timing T1 (by the laser light correction control unit 603, the correction data SDk to the digital-analog converter 7 is This occurs regardless of the setting timing.

  Then, after the timing at which an integral multiple of the unit time ts has elapsed from the timing T1, when the detection signal BDc falls during the access time ta, the digital-analog converter 7 competes for access and the laser light correction control unit 603 Setting of the correction data SDc to the digital-analog converter 7 cannot be executed.

  In order to avoid such access competition, for example, the setting timing of the correction data SDc to the digital-analog converter 7 is synchronized with the setting timing of the correction data SDk, and an integral multiple of the unit time ts has elapsed from the timing T1. It can be done at the timing. However, since the correction data for each unit time ts in one scanning time tc is stored in the correction data storage unit 605, the correction data SDc is sequentially set in the digital-analog converter 7 every unit time ts from the timing T2. Otherwise, the analog voltage Vc output from the digital-analog converter 7 changes at a timing different from the timing at which the light amount correction should be originally performed in the laser light source 6 of the image forming unit 131C, and the light amount of the laser light source 6 changes.

  Accordingly, the rotation of the polygon mirror motor 3 is not controlled so that the difference from the timing at which the laser beam is reflected from the polygon mirror 2 to the beam detector 9 in the image forming unit 131C is an integral multiple of the unit time ts. However, the contention of access in the digital-analog converter 7 cannot be avoided by synchronizing the setting timing of the correction data SDc to the digital-analog converter 7 with the setting timing of the correction data SDk.

  On the other hand, the rotation angle control unit 602 shown in FIG. 3 has a predetermined timing difference at which the laser light is reflected from each polygon mirror 2 to each beam detection unit 9 in the image forming units 131K, 131C, 131Y, and 131M. The rotation of each polygon mirror motor 3 is controlled so as to be an integral multiple of the unit time ts to adjust the difference in rotation angle between the polygon mirrors 2.

  Therefore, in the color printer 1 shown in FIG. 1, even when color misregistration correction is performed by the color misregistration correction unit 604, as shown in FIG. 4C, the most upstream in the running direction of the transfer belt 136. The timing at which the detection signal BDk from the synchronization sensor 9 falls in the black image forming unit 131K at the position (timing T1), and the timing at which the detection signal BDc falls by detecting the laser light at the synchronization sensor 9 in the image forming unit 131C. The time interval with (timing T3) is always an integral multiple of the unit time ts, and the timings T1 and T3 are synchronized. In FIG. 4, an example of the detection signal BDk and the detection signal BDc is described. Similarly, the falling timings of the yellow detection signal BDy and the magenta detection signal BDm are synchronized.

  Then, the laser beam correction control unit 603 can set the correction data SDk, SDc, SDy, and SDm in the digital-analog converter 7 at timings synchronized with each other, so that access competition is avoided. The timing at which the correction data SDk, SDc, SDy, SDm are set in the digital-analog converter 7, that is, the timing at which the analog voltages Vk, Vc, Vy, Vm change is the rise of the detection signals BDk, BDc, BDy, BDm. Since the unit time ts elapses from the downstream, the light amount of the laser light source 6 can be corrected correctly at the timing when the light amount should be corrected.

  Since the laser light correction control unit 603 needs to set new correction data SDk, SDc, SDy, SDm in the digital-analog converter 7 every unit time ts, the unit time ts has a plurality of colors in the access time ta. It is set to be longer than the time multiplied by the number of colors (4) of (black, cyan, yellow, magenta).

  For example, if the access time ta is about 500 nsec, the unit time ts may be 500 nsec × 4 = 2 μsec or more. Then, since the light amount correction of the laser light source 6 by the laser light correction control unit 603 is executed every unit time ts, the light amount of the laser light source 6 can be corrected every 2 μsec. When the image resolution is 600 dpi (dot per inch) and the scanning time tc is 320 μsec, the color misregistration is conspicuous if the rotation angle of the polygon mirror is corrected with an accuracy equal to or less than the angle corresponding to the scanning time of 100 μsec. Absent. Further, if the correction is performed with an accuracy equal to or less than an angle corresponding to a scanning time of 10 μsec, the color shift is not recognized by human eyes. Therefore, if the unit time ts is 100 μsec or less, particularly 10 μsec or less, sufficient accuracy can be obtained as the correction accuracy of the rotation angle of the polygon mirror. As described above, since the unit time ts may be 2 μsec or more, it is easy to correct the color shift and improve the image quality of the color image.

  Further, in each of the image forming units 131K, 131C, 131Y, and 131M, a difference in timing at which the laser light is reflected from each polygon mirror 2 to each beam detection unit 9 is an integral multiple of a predetermined unit time ts. By setting the difference in the rotation angle between the polygon mirrors 2, it becomes easy to reduce the possibility that the image processing start timing of the sub-scanning will fluctuate due to the jitter of the polygon mirror motor 3 or the like.

  As described above, according to the color printer 1 shown in FIG. 1, the tandem type image forming apparatus including the plurality of image forming units 131K, 131C, 131Y, and 131M that multiplex-transfer toner images of different colors onto the recording paper P. The digital-analog converter 7 having a plurality of channels with one element can be used as the digital-analog converter 7 used for correcting the light quantity of the laser light source 6 of each color. .

1 is a schematic diagram illustrating a schematic configuration of a tandem type color printer which is an example of an image forming apparatus according to the present invention. FIG. 2 is a schematic structural diagram showing an example of an internal configuration of the exposure apparatus shown in FIG. 1. FIG. 2 is a block diagram illustrating an example of an electrical configuration of the image forming apparatus illustrated in FIG. 1. It is explanatory drawing for demonstrating an example of operation | movement with the rotation angle control part shown in FIG. 3, and a laser beam correction control part. (A) shows an example of the detection signal of the synchronous sensor in the black image forming unit and the analog voltage output from the digital-analog converter to the laser light source of the image forming unit. (B) is explanatory drawing for demonstrating the difference with background art. (C) shows an example of the detection signal of the synchronization sensor in the cyan image forming unit and the analog voltage output from the digital-analog converter to the laser light source of the image forming unit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Color printer 2 Polygon mirror 3 Polygon mirror motor 4 f (theta) lens 5 Collimator lens 6 Laser light source 7 Digital-analog converter 8 Mirror 9 Beam detection part 13 Image formation part 14 Fixing device 60 Control part 131,131K, 131C, 131Y, 131M Image Formation unit 132 Photosensitive drum 133 Developing device 134 Charger 135 Exposure device 136 Transfer belt 245 Detection sensor 601 Image formation processing unit 602 Rotation angle control unit 603 Laser light correction control unit 604 Correction unit 605 Correction data storage unit BD, BDk, BDc , BDy, BDm Detection signal SDk, SDc, SDy, SDm Correction data Vk, Vc, Vy, Vm Analog voltage ta Access time tc Scan time ts Unit time

Claims (4)

An image forming unit for forming a toner image for each of a plurality of colors, arranging the image forming units in a predetermined direction, conveying the recording paper in the arrangement direction, and transferring the toner images of each color onto the recording paper;
A control unit for controlling the operation of the image forming unit for each color;
A digital-analog converter in which a plurality of channels for converting a digital value into an analog voltage are integrated in one element;
The image forming unit for each color is
A photosensitive drum;
A laser light source that outputs a laser beam whose light amount is adjusted according to an analog voltage output from the control unit;
A polygon mirror that reflects the laser light output from the laser light source while rotating and scans the photosensitive drum in a main scanning direction;
A beam detection unit for detecting laser light reflected by the polygon mirror at a reference position when starting the scanning;
The controller is
Correction that stores in advance each correction data for correcting the intensity of the laser beam for each laser light source corresponding to each color for each unit time obtained by dividing the period into an integral number within one scanning period in the main scanning direction. A data storage unit;
Rotation that adjusts the difference in rotation angle between the polygon mirrors so that the difference in timing at which the laser light is reflected from the polygon mirror for each color to the beam detector is an integral multiple of the unit time. An angle control unit;
When the laser beam is detected by the beam detection unit for each color, color correction data corresponding to the detected laser beam is sequentially read from the correction data storage unit every unit time, and the digital analog A laser beam correction control unit set in the converter,
The digital-to-analog converter is
The access time for setting the correction data as a digital value is shorter than the unit time,
The image forming apparatus, wherein the analog voltage is output from the plurality of channels to the laser light source for each color. The image forming apparatus according to claim 1, wherein the unit time is set to be equal to or longer than a time obtained by multiplying the access time by the number of colors of the plurality of colors. The digital-analog converter receives a setting of correction data of each color by a serial signal,
The image forming apparatus according to claim 1, wherein the access time is a time for transferring the correction data of each color to the digital-analog converter by the serial signal. A color misregistration detecting unit for detecting color misregistration of the toner images of the respective colors that are multiple-transferred by the image forming unit;
The rotation angle control unit is configured to detect the color detected by the color misregistration detection unit while setting a difference in timing at which laser light is reflected from the polygon mirror for each color to the beam detection unit as an integral multiple of the unit time. The image forming apparatus according to claim 1, wherein a difference in rotation angle between the polygon mirrors is adjusted so as to correct a shift.

Images