JP2006150666A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately compensate the main scanning length of each laser beam on a photo-sensitive drum by an inexpensive compensating means in an image forming device having a multiple beam. <P>SOLUTION: In order to sense a main scanning length difference between respective laser beams, a pattern for compensation is formed by a forming speed of 1/(beam quantity), and the pattern for compensation is sensed by a photo-sensor 60. A high frequency clock number forming one pixel at a specified location on a line which is scanned by a corresponding laser beam is changed from the sensing result. Thus, the scanning length on the surface of the photo-sensitive drum 11 is electrically compensated, and the scanning lengths by a plurality of laser beams can be equalized to each other. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a multi-beam image formation in which a plurality of beams are scanned by a plurality of beams in a digital copying machine, a facsimile, a laser printer, and a digital copying machine having a combination of these functions. Relates to the device.

  A conventional image forming apparatus uses a drum-shaped electrophotographic photosensitive member as an image carrier, that is, a laser scanning optical system that irradiates light on a photosensitive drum with a light emitting element such as a laser beam. Has proposed an image forming apparatus for forming an electrostatic latent image on a photosensitive drum.

  In recent years, “high speed image formation” and “improvement of image formation density (resolution)” have been demanded. For this reason, the main scanning direction is realized by increasing the image clock for forming each pixel, and the sub-scanning direction is realized by increasing the rotation speed of the polygon motor. However, due to the limitation of the rotational speed of the polygon motor, the scanning speed by the laser is controlled by 1 / (the number of laser elements) by simultaneously scanning a plurality of laser beams on the photoconductor in parallel with one scanning. A multi-beam scanning optical system for forming an image on a photoreceptor has been proposed.

  In the configuration in which the multi-beam optical system scans each laser beam on the photosensitive member, it is necessary to correct the mismatch in the scanning magnification in the main scanning direction, which is caused by the manufacturing variation in the optical characteristics of each optical element. . Therefore, the laser modulation speed, which is one of the parameters for determining the scanning magnification in the main scanning direction, can be adjusted separately for each laser, and the scanning magnification on the photosensitive member of each beam can be scanned at a constant and equal magnification. Therefore, it must be possible to form a higher quality image.

For this reason, BD sensors at the beginning and rear end in the main scanning direction are arranged, the main scanning magnification of each beam is detected by the BD sensor, and the main scanning magnification is corrected by finely adjusting the image clock frequency of each beam. (See Patent Document 1).
JP 2001-013430 A

  FIG. 1 shows a known laser scanning unit. Laser light 104 emitted from a laser diode (not shown) scans the photosensitive drum 101 via a folding mirror 105 to a polygon mirror 102 that is driven to rotate at a predetermined rotational speed by a polygon motor 103. FIG. 2 is a view of FIG. 1 as viewed from above. The BD sensors 106 and 107 are disposed in the optical path of the laser beam scanned by the polygon mirror being rotated in the direction of the arrow. The BD sensor 106 is disposed at the head in the laser scanning direction, and the BD sensor 107 is disposed at the rear end in the laser scanning direction, and performs synchronization signal output and main scanning magnification detection in the main scanning direction.

  As shown in FIG. 1, it is sufficient that the scanning incident angle to the photosensitive drum 101 is the same for each beam. Usually, however, the return light due to reflection from the photosensitive drum is reduced, or the reason shown in FIG. Thus, the incident angle is different. For this reason, the correct scanning magnification at the BD sensor arrangement position differs on the photosensitive drum. As a result, the scanning length of the laser beam on the photosensitive drum 101 differs as shown in FIG. 4 (laser scanning light on the photosensitive member in the case of four beams LD1 to LD4). As a result, there is a problem that vertical line fluctuations occur and affect image degradation.

  Further, in order to adjust the frequency of the image clock for each beam, the frequency modulation means is provided by the number of beams, and in the case of an image forming apparatus having a plurality of image forming units, the number of beams × the image forming unit. There are as many frequency modulation means as there are numbers. Since the frequency modulation means is relatively expensive, there is a problem that the main scanning adjustment means between the beams becomes expensive.

  The present invention has been made paying attention to the above-described problems, and in a multi-beam image forming apparatus, the main scanning length difference of each laser beam can be accurately detected, and the main scanning magnification of each beam can be detected. It is an object of the present invention to provide an image forming apparatus capable of accurately correcting the above with an inexpensive configuration.

  The present invention is an image forming apparatus having a multi-beam scanning optical system that scans corresponding lines with a plurality of beams, and a pattern for forming a pattern for detecting a difference in scanning length between the plurality of beams. A difference in scanning length between each of the plurality of beams based on a result of the forming means, a detecting means for reducing the forming speed to 1 / (number of beams) at the time of forming the pattern, and detecting the pattern; The scanning length difference detecting means for detecting the difference between the scanning length difference and the scanning length difference detecting means calculates a scanning length difference between a reference beam and another beam, and a main clock composed of a plurality of high frequency clocks having a predetermined frequency. A write clock is generated for each of the plurality of beams, and the main clock is generated according to a write position for each of the plurality of beams. Based on the detection result of the scanning length difference detection means, the clock generation means for variably controlling the number of high-frequency clocks constituting the clock, and the main clock of the corresponding beam other than the reference beam among the plurality of beams Correction means for correcting the scanning length by the corresponding beam by controlling the writing position for changing the number of high-frequency clocks to be configured.

  In the multi-beam image forming apparatus, it is possible to accurately detect the main scanning length difference of each laser beam. Further, the correction of the main scanning magnification of each beam can be accurately performed with an inexpensive configuration.

  Hereinafter, the present invention will be described based on embodiments shown in the drawings.

  FIG. 5 is a cross-sectional view of the main part of the image forming apparatus embodying the present invention. The image forming apparatus of the present embodiment is an electrophotographic system, and is described as a color image output apparatus in which a plurality of image forming units considered to be particularly effective for the present invention are arranged in parallel and an intermediate transfer system is adopted. Go.

  The color image forming apparatus includes an image reading unit 1R and an image output unit 1P. The image reading unit 1R optically reads a document image, converts it into an electrical signal, and sends it to the image output unit 1P, but the detailed description is omitted.

  The image output unit 1P is roughly divided into an image forming unit 10 (four stations a, b, c, and d are arranged in parallel, and the configuration is the same), a paper feeding unit 20, an intermediate transfer unit 30, a fixing unit. The unit 40, the cleaning unit 50, the optical sensor unit 60, and the control unit 70 are configured.

  Further, each unit will be described in detail. The image forming unit 10 is configured as described below. Photosensitive drums 11a, 11b, 11c, and 11d as image carriers are pivotally supported at their centers and are driven to rotate in the direction of the arrow. Opposing to the outer peripheral surfaces of the photosensitive drums 11a to 11d, primary chargers 12a, 12b, 12c, 12d, optical systems 13a, 13b, 13c, 13d, folding mirrors 16a, 16b, 16c, 16d, and developing device 14a in the rotational direction. , 14b, 14c, and 14d are arranged. In the primary chargers 12a to 12d, the surface of the photosensitive drums 11a to 11d is given a uniform charge amount. Next, the optical system 13a to 13d exposes the photosensitive drums 11a to 11d on the photosensitive drums 11a to 11d through the folding mirrors 16a to 16d by exposing light beams such as a laser beam modulated according to the recording image signal, and electrostatic latent images are formed there. Form. Further, the electrostatic latent images are visualized by developing devices 14a to 14d each containing developer of four colors such as yellow, cyan, magenta and black (hereinafter referred to as toner). On the downstream side of the image transfer regions Ta, Tb, Tc, and Td where the visualized visible image is transferred to the intermediate transfer member, the photosensitive drums 11a to 11d are not transferred to the transfer material by the cleaning devices 15a, 15b, 15c, and 15d. The toner remaining on 11d is scraped off to clean the drum surface. By the process described above, image formation with each toner is sequentially performed.

  The paper feed unit 20 is a pair of paper feed rollers for conveying the recording material P fed from the pickup roller 22 for feeding the recording material P one by one from the cassette 21 for storing the recording material P to the registration roller 25. 23, a paper feed guide 24, and a registration roller 25 for feeding the recording material P to the secondary transfer region Te in accordance with the image forming timing of the image forming unit.

  The intermediate transfer unit 30 will be described in detail. The intermediate transfer belt 31 includes a driving roller 32 that transmits driving to the intermediate transfer belt 31, a driven roller 33 that is driven by the rotation of the intermediate transfer belt 31, and a secondary transfer counter roller that faces the secondary transfer region Te across the belt. 34 is wound. Among these, a primary transfer plane A is formed between the driving roller 32 and the driven roller 33. The drive roller 32 is coated with rubber (urethane or chloroprene) having a thickness of several millimeters on the surface of the metal roller to prevent slippage with the belt. The drive roller 32 is rotationally driven by a pulse motor (not shown). Primary transfer chargers 35 a to 35 d are arranged behind the intermediate transfer belt 31 in the primary transfer regions Ta to Td where the photosensitive drums 11 a to 11 d and the intermediate transfer belt 31 face each other. A secondary transfer roller 36 is disposed to face the secondary transfer counter roller 34, and a secondary transfer region Te is formed by a nip with the intermediate transfer belt 31. The secondary transfer roller 36 is pressed with an appropriate pressure against the intermediate transfer member. A cleaning unit 50 (blade 51 and waste toner box 52 for storing waste toner) for cleaning the image forming surface of the intermediate transfer belt 31 is provided on the intermediate transfer belt and downstream of the secondary transfer region Te. ing.

  The fixing unit 40 includes a fixing roller 41a provided with a heat source such as a halogen heater, a pressure roller 41b (which may also be provided with a heat source) pressed against the roller, and a nip portion of the roller pair. A guide 43 for guiding the transfer material P to the fixing unit, fixing heat insulating covers 46 and 47 for confining the heat of the fixing unit, and an inner portion for further guiding the transfer material P discharged from the roller pair to the outside of the apparatus. A sheet discharge roller 44, an outer sheet discharge roller 45, a sheet discharge tray 48 on which the transfer material P is stacked, and the like.

  The registration (color misregistration) detection sensor 60 reads a registration correction pattern image and a density correction pattern image formed on the intermediate transfer belt 31. Based on the result, registration (color misregistration) correction and density / gradation correction are performed to improve image quality.

  The control unit 70 includes a CPU (not shown) for controlling the operation of the mechanism in each unit, a motor driver unit (not shown), and the like.

  Next, a description will be added in accordance with the operation of the apparatus.

  When an image forming operation start signal is issued from the CPU, first, the transfer material P is sent one by one from the cassette 21 by the pickup roller 22. The transfer material P is guided between the paper feed guides 24 by the paper feed roller pair 23 and conveyed to the registration rollers 25. At that time, the registration roller is stopped, and the leading edge of the paper hits the nip portion. Thereafter, the registration roller starts rotating in accordance with the timing at which the image forming unit starts image formation. The rotation timing is set so that the transfer material P and the toner image primarily transferred from the image forming unit onto the intermediate transfer belt exactly coincide with each other in the secondary transfer region Te.

  On the other hand, in the image forming unit, when an image forming operation start signal is issued, a high voltage is applied to the toner image formed on the photosensitive drum 11d that is the most upstream in the rotation direction of the intermediate transfer belt 31 by the process described above. Further, primary transfer is performed on the intermediate transfer belt 31 in the primary transfer region Td by the primary transfer charger 35d. The primarily transferred toner image is conveyed to the next primary transfer region Tc. In this case, image formation is delayed by a time during which the toner image is conveyed between the image forming portions, and the next toner image is transferred with the resist aligned on the previous image. Thereafter, the same process is repeated, and eventually the four color toner images are primarily transferred onto the intermediate transfer belt 31.

  Thereafter, when the recording material P enters the secondary transfer region Te and contacts the intermediate transfer belt 31, a high voltage is applied to the secondary transfer roller 36 in accordance with the passing timing of the recording material P. Then, the four color toner images formed on the intermediate transfer belt by the process described above are transferred onto the surface of the recording material P. Thereafter, the recording material P is accurately guided to the fixing roller nip portion by the conveyance guide 43. The toner image is fixed on the paper surface by the heat of the roller pair 41a and 41b and the pressure of the nip. Thereafter, the paper is transported by the internal and external paper discharge rollers 44 and 45, and the paper is discharged outside the apparatus and stacked on the paper discharge tray 48.

  Next, registration correction will be described.

  FIG. 6 is a diagram showing how the photosensors 60 a and 60 b read the registration correction pattern on the transfer belt 31. The photosensors 60a and 60b include an LED 501 and a PTr (phototransistor) 502. For example, infrared light is irradiated from the LED 501 onto the transfer belt 31, and reflected light is read by the PTr 502 and transferred to a light receiving circuit (not shown). As shown in FIG. 7, two photosensors 60 and 60b are arranged in a direction perpendicular to the transfer belt traveling direction, and among the plurality of photosensitive drums, the photosensitive drum 11a is located on the most downstream side in the belt traveling direction. Located between the drive rollers 32. The registration correction pattern image 601 formed on the intermediate transfer belt 31 is read using the difference in reflectance between the registration correction pattern formed of toner and the transfer belt 31.

  FIG. 8 shows an example of a registration correction pattern. The correction pattern 601 includes a pattern 801 and a pattern 802. The pattern 801 is for detecting the deviation amount between the sub-scanning writing position and the main scanning inclination, and the pattern 802 is for detecting the deviation amount between the main scanning writing position and the main scanning magnification.

  The principle of detecting the shift amount of the main scanning magnification of each color will be described with reference to FIG. As shown in FIG. 9, the time from when the first line segment 901 is detected until the second line segment 902 is detected is T1. The distance from the intersection of the first line segment 901 and the second line segment 902 to the position where the sensor passes in the main scanning direction is proportional to T1, and the angle between the first line segment 901 and the second line segment 902 is If it is 90 degrees, it becomes T1 / 2. If the scanning length of the main scanning is different, the position where the pattern is formed is different, so that this T1 / 2 is different.

A method for calculating the shift amount of the main scanning magnification of each color will be described. The patterns 701a and 701c in FIG. 10 are formed of yellow, and the patterns 701b and 701d are formed of magenta. A sensor 60a disposed on the front side in the main scanning direction detects the patterns 701a and 701b as the intermediate transfer belt 31 is conveyed. Similarly, the patterns 701c and 701d are detected by the sensor 60b. Each sensor detects patterns 701a to 701d, and outputs detection results as shown in FIG. The sensor output has a logic “H” in the pattern area and a logic “L” outside the pattern, that is, the base area. From the center to the center of the logic “H” section of the pattern 701a is T1, and similarly, the patterns 701b to 701d are T2 to T4. T1, T2, T3, and T4 represent the passage time between the lines of the pattern. By comparing T1 / 2 and T2 / 2, the difference in the writing position of each beam is compared with T3 / 2 and T4 / 2. Thus, the difference between the writing end positions of the respective beams can be calculated. That is,
(T1-T2) / 2 + (T3-T4) / 2 (1)
Is the main scanning magnification difference of each color.

  Further, in order to cancel drive unevenness such as a motor for driving the intermediate transfer belt 31 and a motor for driving the photosensitive member, the correction accuracy is improved by reading the registration correction pattern a plurality of times (for example, 10 times). .

  In this embodiment, the registration correction pattern image 601 is formed on the intermediate transfer belt 31 at a predetermined timing before the image forming operation is performed, read by the photosensors 61a and 61b, and on the photosensitive drum corresponding to each color. , And the electric signal is applied to the image signal to be recorded and / or the folding mirror 16a provided in the optical path of the laser beam is driven to change the optical path length or the optical path. Make corrections.

  Next, a means for correcting the scanning length difference on the photosensitive drum between the laser beams will be described. In this embodiment, a pattern for detecting the main scanning length difference between the laser beams is formed, read by a photo sensor, the main scanning length difference is detected, and scanning is performed with the corresponding laser light based on the detection result. The high-frequency clock that forms a specific pixel on the line is changed.

  First, a method for detecting the scanning length difference on the photosensitive member by each laser beam will be described. FIG. 11 shows an example of a pattern for detecting the main scanning length difference between the laser beams, which is basically the same shape as the main scanning magnification correction pattern for each color. This pattern is formed by each laser beam in order to detect the main scanning length difference of each laser beam (four beams in this embodiment: LD1 to LD4). However, since the pattern formed only by LD1 has a resolution different from that of the pattern generated by four beams as shown in FIG. 12, accurate detection cannot be performed. For this reason, at the time of pattern formation, the formation speed in the sub-scanning direction (the rotational speed of the photosensitive drum and the conveyance speed of the intermediate transfer belt) is set to 1 / (number of beams). The resolution will be obtained. In this embodiment, since there are four beams, the formation speed in the sub-scanning direction is ¼. The main scanning length difference after pattern detection can be calculated by the same calculation as the main scanning magnification error between colors.

  Assuming that the main scanning length of each laser beam is LD1 <LD2 <LD3 <LD4 as shown in FIG. 4, in this embodiment, LD1 to LD3 are matched with the scanning length of LD4 having the longest main scanning length. For this reason, each scanning length difference between LD1-LD4, between LD2-LD4, and between LD3-LD4 is detected, and the scanning length of LD1-3 is correct | amended.

  Next, a means for correcting the main scanning length difference between the laser beams will be described. FIG. 13 shows the laser control unit. The optical system 13 includes four laser diodes 1001, a polygon mirror 1002, a polygon motor 1003, a polygon control unit 1004, and an f-θ lens 1010. Laser light emitted from the laser diode 1001 is scanned by a polygon mirror 1002 that rotates in the direction of an arrow in the drawing by a polygon motor 1003 that is driven to rotate, and is well-known fθ corrected by an f-θ lens 1009, and passes through a folding mirror 16. Then, the photosensitive drum 11 is irradiated. The polygon motor control unit 1004 is a control unit for accurately rotating the polygon motor 1003 with a predetermined rotation. The BD sensor 1005 is provided in the vicinity of the scanning start position of one line of laser light, detects line scanning (BD signal) of laser light, and is input to the image signal timing control unit 1006.

  As illustrated in FIG. 14, the image signal timing control unit 1006 includes a selector 1100, a FIFO (First In First Out memory) 1101 to 1104, and a BD delay circuit 1105. An image signal is input to the selector 1100, and the input image signal is switched for each line and input to a FIFO (First In First Out memory) 1101 to 1104. The BD delay circuit 1105 delays the capturing of the output of the BD sensor 36 in accordance with the main scanning writing timing of each laser diode from the BD sensor 1005. This delay amount is determined according to the amount of physical displacement of the four laser beams on the photosensitive drum 11 in the main scanning direction.

  FIFOs 1101 to 1104 are line memories, and output image data corresponding to four laser diodes input from an image signal generation unit (not shown) to the modulation unit 1007 based on a timing signal from the BD delay circuit 1100.

  On the other hand, the pattern detection output from the sensor 60 is input to the correction amount calculation unit 1010, and the correction amount calculation unit 1010 outputs the main scanning length difference correction amount of each laser beam to the modulation unit 1007.

  As illustrated in FIG. 15, the modulation unit 1007 includes a PLL circuit 1201, a frequency dividing circuit 1202, a modulation circuit 1203, an output circuit 1204, and a counter circuit 1205. The PLL circuit 1201 receives a basic clock (basic CLK) and outputs a high-frequency clock that is n times the basic clock. This high frequency clock is input to the frequency dividing circuit 1202 and the output circuit 1204, respectively. The frequency dividing circuit 1202 counts the input high frequency clock once x times, and outputs a clock (main clock) obtained by dividing the input high frequency clock by 1 / x (see FIG. 16). Here, x may be any number as long as it is an integer. Here, for convenience of explanation, it is assumed that the main clock having the same period as the basic clock inputted to the PLL circuit 1201 is output after being divided by 1 / n. The clock output from the frequency dividing circuit 61 is input to the counter circuit 1205.

The modulation circuit 1203 modulates the image signal in synchronization with a clock signal described later. Usually, in order to represent the gradation of the laser, the lighting time within a unit time is controlled by PWM modulation. Therefore, in this embodiment, PWM modulation (particularly digital PWM modulation) is performed. explain. For example, when PWM-modulating an A-bit image signal, the image signal is converted into 2 ^A pulse width data. here,
2 ^A = n (2)
The constants are determined so that The modulation circuit 1203 generates pulse width data from the image signal and outputs the pulse width data to the output circuit 1204 (see FIG. 17).

  The output circuit 1204 outputs a PWM signal synchronized with the high frequency clock output from the PLL circuit 1201 and a clock signal synchronized with the high frequency clock in accordance with the pulse width data output from the modulation circuit 1203. The PWM signal is output to the laser driver 1008, and the clock signal is output to the image processing unit (not shown) and the modulation circuit 1203 (see FIGS. 18A, 18D, and 18E).

  The counter circuit 1205 counts the clock (clock obtained by dividing the high frequency clock by 1 / n) output from the frequency dividing circuit 1202 (see FIG. 18B). When the count value reaches the set value, the counter circuit 1205 outputs a predetermined signal to the output circuit 1204 (see FIGS. 18B and 18C). Here, the value set in the counter circuit 1205 is a value determined according to the value obtained by the above equation (1).

  When the counter circuit 1205 outputs the predetermined signal to the output circuit 1204, the output circuit 1204 performs an operation different from normal. In normal operation, one cycle (PWM signal, clock signal) is generated with n high-frequency clocks, but when the predetermined signal is input, the PWM signal has a cycle different from the cycle. The clock signal is output (see FIG. 18).

  Next, a specific configuration of the output circuit 1204 will be described with reference to FIG. FIG. 19 is a block diagram showing a configuration of the output circuit 1204 of FIG.

  As shown in FIG. 19, the output circuit 1204 includes a modulation control unit 80, nine D-type flip-flops 81a to 81i, nine two-input AND circuits 82a to 82i, and two two-input selector circuits 83 and 84. And a 9-input OR circuit 86 and a 2-input OR circuit 87.

  The modulation circuit 1203 modulates the input image signal into 8-bit pulse width data. Each bit of the pulse width data is input to one of the inputs of the 2-input AND circuits 82a to 82i. Here, the same data is input to the two-input AND circuits 82h and 82i.

  The flip-flops 81a to 81i output the input of the D terminal to the Q terminal at the rising edge of the high frequency clock (CLK). The outputs of the flip-flops 81a to 81i are connected to the other inputs of the two-input AND circuits 82a to 82i. At the same time, the flip-flops 81a to 81i are connected in cascade such that the output of the flip-flop 81a is connected to the input of the flip-flop 81b and the output of the flip-flop 81b is connected to the input of the flip-flop 81c. The output of the flip-flop 81h is also connected to a 2-input selector circuit 83 and a 2-input selector circuit 84. The output of the flip-flop 81 i is also connected to the 2-input selector circuit 83.

  The outputs of the 2-input AND circuits 82a to 82i are each connected to a 9-input OR circuit 86, and the output of the 9-input OR circuit 86 is output as a PWM signal. The 2-input selector circuit 83 selects the outputs of the flip-flops 81 h to 81 i according to the output of the modulation control unit 80 and is connected to one of the inputs of the 2-input OR circuit 87. The other input of the 2-input selector circuit 84 is connected to GND. The 2-input selector circuit 84 controls whether or not the output of the flip-flop 81h is input to the flip-flop 81i according to the output of the modulation control unit 80.

  The modulation control unit 80 switches the selection operation of the two-input selector circuits 83 and 84 according to the output of the counter circuit 64.

  A timing signal is input to the other input of the 2-input OR circuit 87, and an output of the 2-input OR circuit 87 is input to the flip-flop 81a.

  Next, the operation of the output circuit 1204 will be described with reference to FIG. FIG. 20 is a timing chart showing an operation example of the output circuit 1204 of FIG.

  A timing signal is input to the 2-input OR circuit 87 in synchronization with the high-frequency clock (CLK) input to the flip-flops 81a to 81i. This timing signal is a signal having a width corresponding to one clock of the high-frequency clock. As a result, one of the outputs of the shift register of the ring composed of the flip-flops 81a to 81i is always "1". The modulation control unit 80 receives the output of the counter circuit 64 and controls the size of the ring-shaped shift register (that is, the number of flip-flops constituting the ring-shaped shift register) of the two-input selector circuits 83 and 84. Switch operation. When one pixel is composed of eight high-frequency clocks (CLK), the output of the flip-flop 81h is selected by the 2-input selector circuit 83, and GND is selected by the 2-input selector circuit 84. When one pixel is composed of nine high-frequency clocks (CLK), the output of the flip-flop 81i is selected by the 2-input selector circuit 83, and the output of the flip-flop 81h is selected by the 2-input selector circuit 84. With these switching operations, “1” is output once to the 8/9 high-frequency clock (CLK) as the outputs of the flip-flops 81a to 81i.

  Pulse width data is set in the 2-input OR circuits 82a to 82i, and the pulse width data changes for each pixel (= 8/9 CLK). In each of the 2-input AND circuits 82a to 82i, an AND operation of “1” is performed once for the set data and 8/9 high-frequency clocks (CLK). The AND outputs of the 2-input OR circuits 82a to 82i are ORed. As a result of this OR operation, a PWM signal composed of 8/9 high frequency clocks (CLK) is output.

  Although not shown in the figure, the same configuration is used, and an image clock pattern is input to a location corresponding to the image data, or outputs of specific locations (for example, 81a and 81e) of the flip-flops 81a to 81i are used as JK flip-flops. By inputting to the circuit, a clock signal composed of 8/9 high frequency clocks (CLK) can be output in the same manner as the PWM signal.

  As described above, as shown in FIG. 20, the number of components of one pixel is set to nine high-frequency clocks (CLK) at a specific location (writing position) in one cycle (image effective area), and eight high-frequency clocks at other times. By controlling to be the clock (CLK), the scanning length difference of each laser beam on the surface of the photosensitive drum 11 is electrically corrected, and the scanning lengths of the four laser beams can be made equal to each other.

  In the present embodiment, the specific location where the width of one pixel is changed is determined by the counter circuit 64, but may be determined by another timer means, for example.

  The operations of registration correction and main scanning length difference correction between laser beams will be described using the flowchart of FIG. Registration correction and laser beam main scanning length difference correction need to be performed after a predetermined time has elapsed since the power was turned on, after a predetermined number of sheets have been printed, or immediately after the main power is turned on. However, each correction differs in the number of printed sheets and the predetermined time that need to be corrected. Therefore, it is determined in step 1 whether a predetermined time has elapsed after the power is turned on, a predetermined number of sheets have been printed, or immediately after the main power is turned on. If YES, the process returns to step 2; In step 2, the timing at which registration correction is necessary (whether the predetermined number of sheets has elapsed, whether the predetermined time has elapsed, or immediately after the main power supply is turned on) is determined. If YES, the process proceeds to step 3; Since the formation speed in the sub-scanning direction differs between the normal registration correction and the laser beam main scanning length difference correction when forming the correction pattern, in step 3, the conveyance speed in the sub-scanning direction is set to the normal setting. (Specifically, the rotational speed of the photosensitive drum and the conveyance speed of the intermediate transfer belt are set), a correction pattern is formed in step 4, the correction pattern is detected in step 5, and registration is performed based on the detection result in step 6. Correction is performed, and then the process proceeds to step 7. In step 7, it is determined whether it is necessary to correct the main scanning length difference between the laser beams. If YES, the process proceeds to step 8; In step 8, the conveyance speed in the sub-scanning direction is set to 1 / number of beams, in the present embodiment, 1/4, in step 9, a correction pattern is formed, in step 10, the correction pattern is detected, and in step 11 Then, the main scanning length difference between the laser beams is corrected based on the detection result, the process proceeds to the end, and this flow is finished.

  As described above, the main scanning length difference correction pattern of each laser beam is formed at a formation speed of 1 / (number of beams), and the correction pattern is detected to accurately determine the main scanning length difference of each laser beam. Can be detected.

  In addition, an inexpensive correction means is possible by adopting a configuration in which the scanning length is corrected by changing the number of high-frequency clocks forming one pixel at a specific location on the line scanned with the corresponding laser beam based on the detection result. become.

The figure which shows the conventional laser scanning unit. The figure which shows the conventional laser scanning unit. The figure which shows the conventional laser scanning unit. The figure showing the scanning length difference on the photosensitive drum with four laser beams 1 is a diagram illustrating an image forming apparatus in the present embodiment. The figure which shows the registration detection sensor in a present Example. The figure showing a mode that the pattern on the intermediate transfer belt in a present Example is read. The figure which shows the pattern for registration correction | amendment in a present Example. The figure showing the principle which detects the main scanning magnification in a present Example Diagram showing registration correction pattern and sensor output in this embodiment The figure which shows the pattern for main scanning length difference correction between beams in a present Example. The figure which shows a pattern when the pattern for main scanning length difference correction between beams in a present Example is formed with one beam. Block diagram of a laser control unit that performs beam main scanning length difference correction in this embodiment The figure which shows the image signal timing control part in a present Example. The figure which shows the modulation part in a present Example. Timing chart showing an operation example of the frequency dividing circuit in this embodiment Timing chart showing an example of operation of the modulation circuit in this embodiment Timing chart showing an operation example of the counter circuit in this embodiment The figure which shows the output circuit in a present Example. Timing chart showing an operation example of the output circuit in this embodiment Flow chart in this embodiment

Explanation of symbols

1R image reading unit 1P image output unit 10 image forming units 11a to 11d photosensitive drum 60 photosensor

Claims (3)

An image forming apparatus having a multi-beam scanning optical system that scans corresponding lines with a plurality of beams,
Pattern forming means for forming a pattern for detecting a difference in scanning length due to each of the plurality of beams;
When forming the pattern, reduce the formation speed to 1 / (number of beams)
Detecting means for detecting the pattern;
A scanning length difference detecting means for detecting a difference in scanning length of each of the plurality of beams based on a result of the detecting means;
The scanning length difference detecting means calculates a scanning length difference between a reference beam and another beam,
A main clock composed of a plurality of high-frequency clocks having a predetermined frequency is generated as a write clock for each of the plurality of beams, and the main clock is configured according to a write position for each of the plurality of beams. Clock generation means for variably controlling the number of high-frequency clocks;
Based on the detection result of the scanning length difference detection means, the correspondence is achieved by controlling the write position for changing the number of high-frequency clocks constituting the main clock of the corresponding beam other than the reference beam among the plurality of beams. Correcting means for correcting the scanning length by the beam to be
An image forming apparatus comprising:   The image forming apparatus according to claim 1, wherein the reference beam is a beam having the longest scanning length.   The image forming apparatus according to claim 1, further comprising a high-frequency clock generating unit that generates the high-frequency clock from a basic clock, wherein the high-frequency clock is a clock that is an integral multiple of the basic clock.

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