JP2006159502A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct a scanning magnification error in a main scanning between respective beams which occurs when a multi-beam scanning optical system is used. <P>SOLUTION: Patterns 301 for correction are formed on an intermediate transfer belt 31 by respective lasers, a main scanning length is acquired from a signal which is sensed by photo-sensors 60 and 61, and the main scanning magnifications for respective lasers are corrected. When the patterns for correction are formed, by increasing a laser intensity more than a normal intensity, the patterns for correction having a wider line width are formed, and accurate patterns can be formed without placing a gap even by writing with one laser. <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. (For example, refer to Patent Document 1).
JP 2001-013430 A

  FIG. 1 shows a known laser scanning unit. Laser light 209 emitted from a laser diode (not shown) scans on the photosensitive drum 11 via a folding mirror 16 to a polygon mirror 204 that is driven to rotate at a predetermined rotational speed by a polygon motor 203. FIG. 2 is a view of FIG. 1 as viewed from above. The BD sensors 206 and 208 are arranged in the optical path of the laser beam scanned by the polygon mirror being rotated in the direction of the arrow. The BD sensor 206 is disposed at the head in the laser scanning direction, and the BD sensor 208 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, the scanning incident angle to the photosensitive drum 11 may be 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, and the optical path length is changed. 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 11 is different. As a result, there is a problem that vertical line fluctuations occur and affect image degradation.

  The present invention has been made paying attention to the above-described problems, and an image forming apparatus capable of correcting a scanning magnification error of main scanning between beams that occurs when a multi-beam scanning optical system is used. The purpose is to provide.

  In order to achieve the above-described object, the present invention configures an image forming apparatus as follows.

A plurality of image forming units arranged side by side, each image forming unit developing a first image carrier and an electrostatic latent image formed on the first image carrier to form a visible image A visible image on the first image carrier formed in each of the image forming units carried on the second image carrier or on the second image carrier. In an image forming apparatus that is transferred superimposed on a transfer material,
The plurality of image forming units have a multi-beam scanning optical system that scans a plurality of lines with a plurality of beams, and each beam corresponds to a clock generated by a frequency variable clock generating means capable of adjusting a writing modulation speed. And a correction pattern for detecting the main scanning scan length formed on the second image carrier by each of the multi-beam scanning optical systems. Forming means, and the correction pattern is formed for each beam, and at the time of pattern formation, a light-emitting element and a light-receiving element that change the light amount of the beam, and a pattern reading means that reads the correction pattern at a predetermined position;
The frequency of the frequency variable clock is corrected based on the result read by the pattern reading means.

  In a multi-beam scanning device and an image forming apparatus equipped with the multi-beam scanning device, the length of the scanning line of each beam (writing) on the surface to be scanned during one scanning between the beams using the registration correction pattern Width) is detected and corrected to reduce the difference in scanning position among a plurality of beams scanned on the scanned surface, or the length of the scanning line of each beam on the scanned surface during one scan By eliminating the difference in thickness (writing width), it is possible to suppress image deterioration due to the difference in writing width.

  The best mode for carrying out the present invention will be described below based on examples.

(First embodiment)
Hereinafter, the present invention will be described based on embodiments shown in the drawings.

  FIG. 4 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, 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, charges of a uniform charge amount are given to the surfaces of the photosensitive drums 11a to 11d. Next, the optical systems 13a to 13d expose light beams such as laser beams modulated according to the recording image signal onto the photosensitive drums 11a to 11d via the folding mirrors 16a to 16d, 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 feeding unit 20 includes cassettes 21a and 21b and a manual feed tray 27 for storing the recording material P, pickup rollers 22a and b and 26 for feeding the recording material P one by one in the cassette or from the manual feed tray, and each pickup roller. A pair of paper feed rollers 23 and a paper feed guide 24 for conveying the recording material P sent out from the recording roller to the registration rollers, and for feeding the recording material P to the secondary transfer region Te in accordance with the image forming timing of the image forming unit. It consists of registration rollers 25a and 25b.

  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 having a heat source such as a halogen heater inside, a pressure roller 41b to which the roller is pressed (this roller may also have a heat source), and a transfer material to the nip portion of the roller pair. Guide 43 for guiding P, fixing heat insulating covers 46 and 47 for confining the heat of the fixing unit, and inner discharge roller for further guiding the transfer material P discharged from the roller pair to the outside of the apparatus 44, an outer discharge roller 45, a discharge tray 48 on which the transfer material P is stacked, and the like.

  The control unit 70 includes a CPU (not shown) and a motor drive board (not shown) for controlling the mechanism in each unit.

  Next, a description will be added in accordance with the operation of the apparatus. When an image forming operation start signal is issued, first, the transfer material P is sent out one by one from the cassette 21a by the pickup roller 22a. 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 25a and 25b. 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, an operation of forming an electrostatic latent image on the photoconductor 11 in the image forming unit 10 will be described. In this embodiment, the image forming unit 10 has four stations, all of which have the same configuration, and one of them will be described. FIG. 5 represents the configuration of one of the four stations. In FIG. 5, an image signal sent from the image output unit 1 </ b> R or an external device such as a computer (not shown) is sent to the image writing timing control circuit 201. The image writing timing control circuit 201 generates a laser lighting signal according to the image signal. The laser drive control circuit 202 modulates and drives the laser diode 207 according to the laser lighting signal. In this embodiment, two laser chips (207-1, 207-2) are arranged at a predetermined interval in the diode 207 so that dots are formed at a predetermined pitch in the sub-scanning direction. The laser light is reflected by the polygon mirror 204 that rotates in the direction of the arrow when driven by the polygon motor 203, corrected by fθ by the f−θ lens 205, reflected by the folding mirror 16, and scanned on the photosensitive drum 11. Thus, an electrostatic latent image corresponding to the image signal is formed on the photosensitive drum 11. The BD sensor 206 is provided in the vicinity of the scanning start position of one line of laser light, detects line scanning of the laser light, and is input to the image writing timing circuit 201 as a BD signal.

  Next, a laser beam light quantity control circuit will be described. FIG. 6 is a block diagram showing the configuration of the laser control circuit. As shown in FIG. 6, in this embodiment, a laser chip 207 composed of two lasers (207-1, 207-2) and one photodiode (hereinafter referred to as PD) sensor 207-3 is used. Then, a total of four current sources, that is, two types of bias current sources (71-1, 72-2) and pulse current sources (72-1, 72-2) of the laser drive control circuit 202 are applied to the laser chip 207. Two lasers (207-1, 207-2) are caused to emit light by using a signal from the write timing control circuit 201. Further, in order to stabilize the light emission of the two lasers (207-1, 207-2), feedback is provided to the bias current sources (71-1, 71-2) using the output signal from the PD sensor 207-3. The amount of applied bias current is controlled. That is, the output signal from the PD sensor 207-3 is input to the current-voltage converter 74, then amplified by the amplifier 75, and input to a well-known APC (Auto Power Control) circuit (76-1, 76-2). The APC circuit (76-1, 76-2) is supplied as a control signal to the bias current source (71-1, 71-2). The amount of laser beam can be changed by controlling the APC circuits (76-1, 76-2) from a sequence controller 73 comprising a CPU (not shown). A registration correction operation between the colors necessary for the image forming apparatus of the multiple transfer system that transfers the images of the respective colors onto the transfer belt and the paper transport belt will be described.

FIG. 7 (a perspective view of the main part of FIG. 4) is a schematic diagram in the vicinity of pattern image detecting means (including a CCD sensor) 60 and 61 for detecting a registration correction pattern image of the image forming apparatus. The intermediate transfer belt 31 is made of an elastic material such as rubber or elastomer as a raw material, and has a circumferential Young's modulus of 1 × 10 ⁷ Pa or more. The thickness of the intermediate transfer belt 31 is preferably 0.3 mm to 3 mm from the viewpoint of ensuring thickness accuracy and strength and realizing flexible rotational driving. Further, the intermediate transfer belt 31 is adjusted to a desired resistance value (preferably the volume resistance value is 1 × 10 ¹¹ Ωcm or less) by adding a conductive agent such as metal powder (carbon or the like). Further, it is wound around the drive roller 32 to receive drive and is rotated in the direction of arrow B. On the other hand, the pattern image detection means 60 and 61 which are photosensors composed of light emitting and receiving elements are located between the photosensitive drum 11a located at the most downstream side in the belt traveling direction and the driving roller 32 among the plurality of photosensitive drums, A registration correction pattern image formed on the intermediate transfer belt 31 is read. Further, as shown in FIG. 7, they are arranged at both ends of the intermediate transfer belt 31. Then, before performing the copying operation, a registration correction pattern image 301 is formed on the intermediate transfer belt at a predetermined timing, read by the pattern image detecting means 60 and 61, and the registration shift on the photosensitive drum corresponding to each color. Is detected, the image signal to be recorded is electrically corrected, and / or the folding mirror 16a provided in the laser beam optical path is driven to correct the optical path length change or the optical path change.

  There are various patterns in this registration correction pattern image, and the first line segment arranged with a predetermined angle with the process direction which is the moving direction of the transfer belt as shown in FIG. A pattern composed of second line segments arranged symmetrically across an imaginary line perpendicular to the direction, and a process direction that is a moving direction of the transfer belt as shown in FIG. For example, there is a pattern including a first line segment and a second line segment parallel to a direction (main scanning direction) orthogonal to the process direction.

  Such a registration correction pattern image is read by a photosensor including an LED, a light emitting element such as a phototransistor, and a light receiving element. Two photosensors are arranged at a predetermined distance in a direction orthogonal to the process direction (main scanning direction), and a registration correction pattern image is also formed so as to pass over the photosensor. FIG. 9 shows how the photosensors 60 and 61 detect the registration correction pattern 301 on the transfer belt 31. The transfer belt 31 is made of a material having a higher reflectance of light (for example, infrared light) emitted from the LEDs 161 in the photosensors 60 and 61 than the pattern 301. Due to the difference in reflectance, Pattern detection is possible.

  FIG. 10 shows how the phototransistor 162 receives the light emitted by the LED 161 and the reflected light reflected by the pattern 301 or the transfer belt 31, and FIG. 11 shows the light receiving circuit that receives the reflected light and converts it into an electrical signal. Indicates. First, when the transfer belt 31 is detected, the amount of reflected light is large, so that a large amount of photocurrent flows through the phototransistor 162, current-voltage is converted by the resistor 407, and amplified by the resistors 402 to 404 and the operational amplifier 401. When the pattern 301 is detected, the amount of reflected light is small, so that a smaller photocurrent flows in the phototransistor 162 than in the transfer belt portion. Similarly, current-voltage conversion is performed by the resistor 407 and amplified by the resistors 402 to 404 and the operational amplifier 401. . A state in which the light receiving circuit detects reflected light in the order of the transfer belt portion → the pattern portion → the transfer belt portion is shown in FIG. The threshold level is set by the variable resistor 406 between the transfer belt detection level and the pattern detection level, that is, the pattern detection output 602 is generated by comparing the threshold value with the current-voltage converted value. Can do.

  The pattern detection output 602 that is sent sequentially is read, registration deviation is detected from the pattern interval, etc., electrical correction is applied to the image signal to be recorded, and / or folding is provided in the laser beam optical path. The mirror is driven to correct the optical path length change or optical path change.

Next, changes in potential on the drum when the amount of laser beam light is increased or decreased will be described. Predetermined potential places the laser beam to the photosensitive drum 11 is irradiated to the laser beam when irradiated charged to V _D decreases the potential of the photosensitive drum as shown by the curve 501 in FIG. 12. At this time, an area having a threshold level V _th 503 or less determined by the process conditions becomes an electrostatic latent image and is visualized by the developing device. This potential drop increases as the amount of laser beam increases, and the potential on the photosensitive drum 11 due to the laser beam having a large amount of light becomes a curve 502. The width on the surface of the photosensitive drum where the curve 501 and the curve 502 are equal to or lower than the threshold level V _th 503 is the width W1 and the width W2, respectively. A thick pattern is formed.

  In the present invention, this registration correction pattern is used to detect and correct the difference in the scanning line length (writing width) of each laser on the surface to be scanned during one scanning between the lasers. is there. In this embodiment, a scanning device using two beams will be described.

  First, a method for detecting a difference in scanning line length of each beam will be described. The transfer belt 31 is driven to rotate, a scanning length detection pattern for main scanning is written by the laser 207-1, and this is detected by the pattern image detection means 60 and 61. Subsequently, the main scanning scanning length detection pattern is written by the laser 207-2, and this is detected by the pattern image detection means 60 and 61. At the time of writing, by increasing the amount of light of each of the lasers 207-1 and 207-2, an electrostatic latent image having a large line width is formed on the photosensitive drum 11, and main scanning having a large line width is performed on the transfer belt 31. An inspection length detection pattern is formed. As a result, as shown in FIG. 16, the laser beam light quantity is increased as shown in FIG. 16B compared to FIG. 16A, which is a correction pattern based on a normal laser light quantity. And accurate pattern formation becomes possible.

  As a pattern for detecting a deviation in the main scanning direction, there are FIG. 8A or 8B, and the like, and this is formed on the front side and the back side in the main scanning direction and detected to scan in the main scanning direction. Differences in length can be detected.

  For example, when the pattern shown in FIG. 8A is formed and detected, the time from when the first line segment 1001 is detected until the second line segment 1002 is detected is T1 as shown in FIG. The distance from the intersection of the first line segment 1001 and the second line segment 1002 to the position where the sensor passes in the main scanning direction is proportional to T1, and the angle between the first line segment 1001 and the second line segment 1002 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 difference in main scanning line length between beams will be described. 901 in FIG. 14 indicates a pattern written by the laser 207-1 and read by the sensor, and 902 indicates a pattern written by the laser 207-2 and read by the sensor. In the figure, T1, T2, T3, and T4 represent the passage times between the lines of the pattern. By comparing T1 / 2 and T3 / 2, the difference between the writing positions of the respective beams can be calculated as T2 / 2 and T4 / 2. By comparing, it is possible to calculate the difference in the writing end position of each beam. That is, (T1-T3) / 2 + (T2-T4) / 2 is the scanning length difference between the beams.

  Next, a method for correcting the difference in scanning line length of each laser will be described. FIG. 13 is a detailed diagram of the laser controller and the image writing timing controller in FIG. In FIG. 13, the LD 207 includes two lasers 207-1 and 207-2 and a photodiode 207-3. The laser drive control circuit 202 that modulates and drives each laser receives each laser beam by the photodiode 207-3 and controls the laser drive current according to the result. The laser drive control circuits 202-1 and 202-2 control the lasers 207-1 and 207-2, respectively.

  Details of the image writing timing control circuit 201 will be described. First-in first-out (FIFO) 803-1 and 803-2 comprising line memories receive an image signal from the outside and generate an image based on a timing signal from a read signal generation circuit 805 generated based on a BD signal. Start reading signals. Further, the readout clock of each pixel is output from CLK generation circuits 804-1 and 804-2 which are configured by a known frequency synthesizer circuit for fine adjustment. The timing signal from the timing read timing generation circuit 805 is also input to the CLK generation circuits 804-1 and 804-2, and a clock synchronized with this timing is output. Further, since the image size (write width) of main scanning can be changed according to the frequency of CLK output from the CLK generation circuit 804, this frequency can be finely adjusted according to the difference in scanning line length of each laser. Correction can be performed. Image data read from the FIFOs 803-1 and 803-2 are input to the pulse width modulation units 802-1 and 802-2, modulated to a predetermined width according to the image data, and a laser lighting signal for determining the laser lighting time. Is input to the laser drive control circuit.

  As described above, the main scanning scan length detection pattern is used to detect the difference in the scanning length of each laser on the surface to be scanned during one scanning between the lasers, and set a predetermined frequency in the CLK generation circuit. By doing so, the scanning length difference can be corrected.

(Second embodiment)
FIG. 17 shows another embodiment of the image forming apparatus of the present invention. In the color image forming apparatus described in the first embodiment, the second image carrier 31 is an intermediate transfer belt. However, in the color image forming apparatus of this embodiment, the second image carrier 31 is a transfer material. A transfer material conveying belt that carries and conveys P, and is otherwise configured in the same manner as the color image forming apparatus of the first embodiment. Therefore, members having the same functions and actions are denoted by the same reference numerals, and further description is omitted.

  Also in the present invention, a registration correction pattern is used to detect and correct the difference in the scanning line length (writing width) of each beam on the surface to be scanned during one scanning. is there. Also in the present embodiment, the same effect as that of the first embodiment can be achieved.

The figure which shows the laser scanning unit in which the laser is inputted perpendicularly to the drum The figure which looked at the laser scanning unit of Drawing 1 from the upper part The figure which shows the laser scanning unit in which the laser is obliquely incident on the drum 1 is a schematic sectional view of an image forming apparatus according to a first embodiment. Configuration diagram of scanning optical system of the present invention Block diagram of laser beam control circuit The figure showing how to detect the registration pattern formed on the intermediate transfer belt The figure showing the example of a pattern for detecting main scanning magnification error Diagram showing how the photo sensor reads the pattern on the transfer belt Diagram showing output when photo sensor reads pattern Circuit diagram of an embodiment of a light receiving circuit that receives the output of a photosensor Diagram showing changes in potential on photosensitive drum due to laser irradiation Block diagram of laser control unit and image writing timing control unit of the present invention Diagram showing how the main scanning length error between lasers is detected by pattern Diagram showing the principle of calculating main scanning magnification by pattern The figure which shows the detail of the pattern for a correction | amendment when increasing laser light quantity Schematic configuration sectional view of an image forming apparatus in the second embodiment

Explanation of symbols

11 Photosensitive drum 16 Folding mirror 31 Intermediate transfer belt 60, 61 Photo sensor 201 Image writing timing control circuit 202 Laser drive control circuit 207 Laser diode 203 Polygon motor 204 Polygon mirror 205 f-θ lens 206 BD sensor 301 Correction pattern

Claims (3)

A plurality of image forming units arranged side by side, each image forming unit developing a first image carrier and an electrostatic latent image formed on the first image carrier to form a visible image A visible image on the first image carrier formed in each of the image forming units carried on the second image carrier or on the second image carrier. In an image forming apparatus that is transferred superimposed on a transfer material,
The plurality of image forming units have a multi-beam scanning optical system that scans a plurality of lines with a plurality of beams, and each beam corresponds to a clock generated by a frequency variable clock generating means capable of adjusting a writing modulation speed. Modulation driven,
A light amount changing means for changing the light amount of each beam;
Correction pattern forming means for detecting the scanning length of the main scanning formed on the second image carrier in each of the multi-beam scanning optical systems;
The correction pattern is formed for each beam,
Change the light intensity of the beam when forming the pattern.
A pattern reading means comprising a light emitting element and a light receiving element, and reading the correction pattern at a predetermined position;
An image forming apparatus, wherein the frequency of the frequency variable clock is corrected based on a result read by the pattern reading unit.   The image forming apparatus according to claim 1, wherein the second image carrier is an endless belt.   2. The image forming apparatus according to claim 1, wherein the amount of light is increased during pattern formation compared to normal image formation.

Images