JP2013003198A - Optical scanner and image forming apparatus with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image improved in the accuracy of image magnification correction by correcting image magnification in the direction of main scanning, on the basis of the estimation of a scanning period during the change in the speed of a rotary polygon mirror.SOLUTION: An optical scanner that deflects an optical beam so as to scan the surface of a photoreceptor drum 11 in a predetermined direction comprises: a writing clock generation part 110 configured to generate a clock signal; an LD driver 105 configured to emit a light beam from a laser diode 106 on the basis of the clock signal and image data; a rotation control part 103 configured such that the rotation speed of the rotary polygon mirror 104 for deflecting a light beam is controlled according to a magnification in order to alter a sub-scanning magnification for an electrostatic latent image; a BD sensor 107 configured to output a detection signal by detecting the light beam deflected by the rotary polygon mirror; and a frequency calculating part 109 configured such that on the basis of the amount of change in the period of a detection signal that changes due to the alteration of the rotation speed of the rotary polygon mirror, the clock signal is modulated so that the length of the electrostatic latent image in the direction of main scanning has a predetermined length.

Description

  The present invention relates to an optical scanning device for scanning a light beam deflected by a rotary polygon mirror, and an image forming apparatus including the optical scanning device.

  In an electrophotographic image forming apparatus, a laser beam modulated by an image signal is rotated on a photosensitive drum rotated in a transfer material conveyance direction (sub-scanning direction) by a rotary polygon mirror in a direction orthogonal to the transfer material conveyance direction. An electrostatic latent image is formed by deflection scanning in the (main scanning direction). The electrostatic latent image is developed with toner and visualized. The visualized toner image on the photosensitive drum is transferred to one surface (first surface) on the transfer material, and the toner image is fixed to the transfer material by applying heat and pressure to the transfer material with a fixing device. The image formation is performed on the transfer material by a series of steps. When performing duplex printing, the transfer material that has passed through the fixing device is reversed by the reversing unit, and image formation is performed on the other surface (second surface) by going through the above-described image forming process again.

  In the case of continuous duplex printing, for example, the first surface of the first transfer material, the first surface of the second transfer material, the first surface of the third transfer material, and the first sheet. The image formation is performed in the order of the second surface of the first transfer material, the first surface of the fourth transfer material, the second surface of the second transfer material, and the first surface of the fifth transfer material.

  When performing double-sided printing with the image forming apparatus, when the transfer material passes through the fixing device in the image formation on the first surface of the transfer material, the heat contained in the transfer material evaporates due to the heat of the fixing device. At the same time, the image printed on the first surface is also reduced. As a result, when the first surface and the second surface of the transfer material are printed at the same main scanning magnification (image magnification in the main scanning direction) and sub-scanning magnification (image magnification in the sub-scanning direction), the size of the printed image Changes between the first surface and the second surface. For this reason, when printing an image on the second surface, it is necessary to perform correction to reduce the sub-scanning magnification by increasing the rotation speed of the rotary polygon mirror. On the other hand, there is a transfer material that expands after the fixing process depending on the paper type, and the image printed on the first surface is enlarged as the transfer material expands. In that case, when printing an image on the second surface, it is necessary to make correction to increase the sub-scanning magnification by slowing the rotation speed of the rotary polygon mirror. However, if the speed of the rotary polygon mirror is changed during image formation, the scanning magnification of the rotary polygon mirror changes, and the image magnification in both the main scanning direction and the sub-scanning direction fluctuates. In particular, the image magnification in the main scanning direction is sensitive to speed fluctuations of the rotary polygon mirror, and an unacceptable image may be output. Therefore, the rotation speed of the rotary polygon mirror is changed within the interval between the transfer material where no image is formed and the transfer material, and the image formation is performed after the rotation speed of the rotary polygon mirror becomes constant. Control was performed conventionally.

  However, in recent years, the printing speed of the image forming apparatus has increased, and there is a tendency to increase the number of printed sheets per unit time by shortening the transfer material conveyance interval as much as possible. If the transfer interval of the transfer material becomes shorter, the speed change of the rotary polygon mirror performed for correcting the sub-scan magnification of the first surface and the second surface will not be completed within the transfer material transfer interval time. When correcting the magnification, control is generally performed to increase the transfer material conveyance interval. However, if the transfer material conveyance interval is lengthened, the number of printed sheets per unit time is reduced, and the productivity as an image forming apparatus is reduced.

  In order to solve this problem, in Patent Document 1, for example, when the printing surface of the transfer material is changed from the first surface to the second surface, the following control is performed to change the speed of the rotary polygon mirror. A technique is disclosed in which the main scanning magnification is not changed even when image formation is performed. That is, at the time of correcting the sub-scanning magnification, the scanning cycle by the rotary polygon mirror is measured in a state where the rotation speed of the rotary polygon mirror does not reach the target speed. Then, the laser is adjusted so that the main scanning magnification is based on the difference between the measured scanning cycle value and the scanning cycle value (target value) when the rotational speed of the rotary polygon mirror reaches the target speed. This is a method of adjusting the clock cycle of a write clock for modulating light. In this method, since the clock cycle of the writing clock is sequentially corrected, an image output with a small fluctuation in the main scanning magnification can be obtained even when the rotational speed of the rotary polygon mirror is not stable.

JP 2006-258946 A

  However, in Patent Document 1, the scanning cycle of the rotary polygon mirror used when calculating the main scanning magnification is the latest scanning cycle measured, but is not a prediction of the next scanning cycle. Therefore, since the speed change of the rotating polygon mirror is performed sequentially, if the main scanning magnification is calculated using only the scanning period measured in the past instead of the next predicted scanning period, The calculation is different from the current main scanning magnification by the amount of acceleration (or deceleration). As a result, image formation is performed at a magnification different from the desired main scanning magnification.

  The present invention has been made under such circumstances, and by correcting the image magnification in the main scanning direction based on the prediction of the scanning period when the speed of the rotating polygon mirror fluctuates, the correction accuracy of the image magnification is improved. An object is to provide an improved image.

  In order to solve the above-described problems, the present invention is configured as follows.

  In an optical scanning device that forms an electrostatic latent image on a rotationally driven photoconductor and deflects the light beam so that a light beam emitted from a light source scans the photoconductor in a predetermined direction, a clock signal is generated. Generating means for generating; driving means for emitting the light beam from the light source based on the clock signal and image data; and a rotation direction of the photosensitive member of the electrostatic latent image formed by the light beam. In order to change the magnification, in response to detection of the light beam deflected by the rotating polygon mirror, control means for controlling the rotation speed of the rotating polygon mirror that deflects the light beam to a rotation speed corresponding to the magnification An output means for outputting a detection signal; and the clock signal based on a fluctuation amount of a period of the detection signal which fluctuates when the rotational speed of the rotary polygon mirror is changed by the control means. Optical scanning device and a modulation means the length of the electrostatic latent image in the predetermined direction to modulate the period such that the predetermined length.

  ADVANTAGE OF THE INVENTION According to this invention, the image which improved the correction precision of image magnification can be provided by correcting the image magnification of a main scanning direction based on the scanning period prediction at the time of the speed fluctuation | variation of a rotary polygon mirror.

Sectional drawing which shows the whole structure of the image forming apparatus of an Example. The figure which shows the structure of the exposure control part of an Example. The flowchart which shows the operation | movement sequence at the time of image formation of an Example. Timing chart of period and write clock period change of rotary polygon mirror of embodiment

  Hereinafter, the form for implementing this invention is demonstrated in detail by an Example.

[Overview of image forming apparatus]
FIG. 1 is a cross-sectional view showing the overall configuration of a digital copying machine which is an example of an image forming apparatus provided with the optical scanning device of this embodiment. The basic operation of the digital copying machine will be described with reference to FIG. . In FIG. 1, when a reading operation is performed by the operation unit 2, the document reading device 1 reads an image of a set document. The read image data of the document is temporarily stored in an image memory (not shown) installed in the system control unit 101 (FIG. 2). Next, when a transfer material such as a recording sheet is conveyed from the transfer material stacking unit 14 and the transfer material detection unit 100 detects the conveyed transfer material, the system control unit 101 performs image memory in accordance with the transfer material conveyance timing. The image data stored in is sent to the exposure control unit 10. A latent image is formed on the photosensitive drum 11, which is a rotationally driven photoconductor, by the irradiation light generated according to the image data by the exposure control unit 10, and the latent image is formed as a toner image on the photosensitive drum 11 by the developing device 13. Developed. In the transfer unit 16, the developed toner image is transferred to the first surface (surface) on the transfer material. The transferred toner image is fixed on the transfer material by the fixing unit 17, passes through the paper discharge conveyance path 21, and is discharged to the outside of the image forming apparatus by the paper discharge roller pair 18. The surface of the photosensitive drum 11 after the transfer is cleaned by a cleaner 25, and the cleaned surface of the photosensitive drum 11 is discharged by a pre-exposure lamp 27 and then charged by a primary charger 28. By repeating the image forming process described above, image formation is performed on a plurality of transfer materials.

  In FIG. 1, the sensor 19 detects the trailing edge of the transfer material, and the flapper 20 discharges the printed transfer material as it is or prints the transfer material on the conveying paths 22, 23, 24 in order to print the reverse side of the opposite side. Switch between guiding or not. During double-sided printing, when the rear end of the transfer material on which image formation on the first surface (front surface) has been completed is detected by the sensor 19, the paper discharge roller pair 18 rotates in the reverse direction, and the flapper 20 transfers the transfer material to the transport path 22, It switches to the direction led to 23,24. Then, the transfer material that has passed through the conveyance paths 22, 23, and 24 is reversed, and the second surface (back surface) is image-formed through the same image forming process as that for image formation on the first surface described above. For example, when performing double-sided printing of five transfer materials using the image forming apparatus of this embodiment, image formation is performed in the following order. That is, the first surface of the first transfer material (hereinafter abbreviated as the transfer material), the first surface of the second sheet, the first surface of the third sheet, the second surface of the first sheet, the second surface of the fourth sheet, Printing is performed in the order of the first surface, the second surface of the second sheet, the first surface of the fifth sheet, the second surface of the third sheet, the second surface of the fourth sheet, the second surface of the fifth sheet. Therefore, in the image forming apparatus of this embodiment, first, the first surface of the three transfer materials is continuously printed, and then the first surface and the second surface of the transfer material are alternately printed. The last three transfer materials are continuously printed on the second surface.

[Outline of exposure control unit]
FIG. 2 is a diagram illustrating a configuration of the exposure control unit 10 included in the optical scanning device. In FIG. 2, the system control unit 101 includes a CPU, a ROM, and a RAM (not shown), and controls the entire image forming apparatus including the exposure control unit 10. The ROM stores a control program for controlling the image forming apparatus, the CPU controls the entire image forming apparatus including the exposure control unit 10 based on the program stored in the ROM, and the RAM performs various controls by the CPU. Used as work memory when performing.

  The writing control unit 102 is based on a scanning cycle measurement using a scanning cycle counter, which will be described later, and based on a control instruction from the system control unit 101, a rotation control unit 103, a frequency calculation unit 109, an LD driver 105, and an image signal generation unit. 111 is controlled. The operation unit 2 includes an input unit for inputting various types of information and a display unit for displaying various types of information in addition to switches for instructing image reading. The operator can operate the main scanning magnification (magnification with respect to the original image in the main scanning direction (direction perpendicular to the transfer material conveyance direction)) of the image printed on the first surface and the second surface of the transfer material from the operation unit 2. The scanning magnification (the magnification with respect to the original image in the sub-scanning direction (transfer material conveyance direction)) can be individually input. The input main scanning magnification value and sub-scanning magnification value are stored in a RAM (not shown) of the system control unit 101.

  The rotation control unit 103 controls the rotation speed of the rotating polygon mirror 104 based on the acceleration / deceleration signals output from the writing control unit 102 to the rotating polygon mirror 104. The frequency calculation unit 109 sets the write clock cycle based on the main scanning magnification and the BD signal (beam detection signal) cycle (or the BD signal cycle count value measured by the scan cycle counter) input from the write control unit 102. Calculate and output to the write clock generator 110. The write clock generation unit 110 generates a write clock signal having the write clock cycle output from the frequency calculation unit 109 as a signal cycle and outputs the write clock signal to the image signal generation unit 111. The image signal generation unit 111 modulates the write clock signal output from the write clock generation unit 110 with the image signal output from the write control unit 102, and controls a laser emission pattern that controls the blinking drive of the laser diode 106. A signal is output to the LD driver 105. An LD (laser diode) driver 105 is a laser diode based on a laser control signal for controlling turning on / off of the laser diode 106 output from the writing control unit 102 and a laser emission pattern signal output from the image signal generation unit 111. The lighting control of 106 is performed. The BD sensor 107 is disposed in the vicinity of the scanning start position in the main scanning direction on the side of the photosensitive drum 11, and detects a light beam emitted from the laser diode 106 that is a light source and reflected by the reflecting surface of the rotary polygon mirror 104. The BD signal is output to the write control unit 102.

  In this embodiment, the transfer material on which the toner image is transferred to the first surface passes through the fixing device 17, whereby moisture contained in the transfer material evaporates, and the transfer material and the transferred image are in the main scanning direction and the sub image. It is assumed that the transfer material and the image after the toner image fixing on the second surface are contracted by 1% in the scanning direction and are not contracted. Further, considering that the image on the first surface of the transfer material shrinks by 1% by passing through the fixing device 17, the image printed on the first surface is enlarged by 1% with respect to the original image. There is a need. Therefore, it is assumed that the image magnification of the first surface is set to 101% for both the main scanning magnification and the sub-scanning magnification via the input unit of the operation unit 2. On the other hand, since the transfer material for the second surface does not shrink even after passing through the fixing device 17, the image magnification of the second surface is set to 100 for both the main scanning magnification and the sub scanning magnification. % Is set.

  The sub-scan magnification that is the image magnification in the rotation direction of the photosensitive drum 11 depends on the BD signal cycle. Therefore, if the BD signal cycle is increased, the sub-scan magnification is increased. Conversely, if the BD signal cycle is decreased, the sub-scan magnification is increased. Get smaller. Similarly, the main scanning magnification which is the image magnification in the direction orthogonal to the rotation direction of the photosensitive drum 11 depends on the writing clock cycle. Therefore, when the writing clock cycle becomes longer, the main scanning magnification becomes larger. The main scanning magnification decreases when the embedded clock cycle is short. Further, since the BD signal is a signal indicating the start of scanning in the main scanning direction on the photosensitive drum 11, the BD signal period which is the scanning period of the rotary polygon mirror 104 is obtained by measuring the time between the BD signals. In this embodiment, the BD signal cycle is measured by a scanning cycle counter of the writing control unit 102. That is, when the writing control unit 102 detects the BD signal from the BD sensor 107, the writing control unit 102 resets the scanning cycle counter, inputs the clock signal, and counts the number of input clocks until the next BD signal is received. To measure time (measurement of the BD signal period). In this embodiment, the count value of the scanning cycle counter that measures the BD signal cycle when the sub-scan magnification is 100% is “10000”, and the count value of the scanning cycle counter that measures the BD signal cycle when the sub-scan magnification is 101%. Is “10100”. This count value is used as a set value of the BD signal cycle when the second surface and the first surface of the transfer material are printed. In the following, it is assumed that the BD signal cycle is expressed not by time but by the count value of the scanning cycle counter. Similarly, the main scanning magnification and the sub-scanning magnification are also expressed by the count value of the scanning cycle counter.

[Operation sequence of double-sided printing of image forming apparatus]
Next, an operation sequence of the image forming apparatus when performing double-sided printing on a plurality of transfer materials in the present embodiment will be described. FIG. 3 is a flowchart illustrating an operation sequence during image formation according to the present exemplary embodiment. This procedure is executed by the CPU of the system control unit 101, the writing control unit 102 of the exposure control unit 10 controlled by the CPU, and the like based on a control program for controlling the image forming apparatus stored in the ROM.

  First, in step 1 of FIG. 3 (hereinafter referred to as S1), the scanner motor that rotates the rotary polygon mirror 104 is activated. The system control unit 101 uses a scanner motor start instruction signal and a BD signal cycle for setting the sub-scan magnification of the first surface of the transfer material to 101% (the count value is 10100. Hereinafter, the numerical values in parentheses are scanning cycle counters). Is output to the write control unit 102. Then, the writing control unit 102 outputs an acceleration signal to the rotation control unit 103 in order to activate the scanner motor, and the rotation control unit 103 activates the scanner motor based on the input acceleration signal. Thereafter, the writing control unit 102 outputs a laser control signal to the LD driver 105, and the LD driver 105 turns on / off the laser diode 106 based on the laser control signal. The laser light emitted from the laser diode 106 is reflected by the mirror surface of the rotary polygon mirror 104 rotated by the scanner motor, passes through various mirrors and lenses (not shown), and is deflected and scanned onto the photosensitive drum 11. On the photosensitive drum 11, the BD sensor 107 disposed upstream of the laser scanning line sends a BD signal to the writing control unit 102 when detecting the laser beam. Then, as described above, the writing control unit 102 measures the period of the BD signal by the scanning period counter based on the input BD signal. When the measured count value of the BD signal period is equal to or greater than the value of the BD signal period (10100) of the first surface, the writing control unit 102 increases the rotation speed because the rotation speed of the rotary polygon mirror 104 is low. An acceleration signal is output to the rotation control unit 103, and if not, a deceleration signal is output to the rotation control unit 103. Based on the acceleration signal / deceleration signal from the writing control unit 102, the rotation control unit 103 controls the scanning cycle of the rotary polygon mirror 104 to be equal to the BD signal cycle (10100) of the first surface. When the measurement result by the scanning cycle counter is stabilized within a predetermined error range, the writing control unit 102 turns on the laser diode 106 for the LD driver 105 only during the time period when the laser beam scans around the BD sensor 107. A laser control signal is output. Then, the writing control unit 102 outputs a print ready signal to the system control unit 101.

Next, in S2, image forming conditions for the first surface of the first transfer material are set. When the system control unit 101 receives the print ready signal, the main scanning magnification (count value is 10100. The numerical values in parentheses below are the values of the scanning cycle counter in order to set the main scanning magnification of the first surface of the transfer material to 101%. Is output to the write control unit 102. The writing control unit 102 outputs the main scanning magnification (10100) of the first surface and the BD signal cycle (10100) of the first surface input from the system control unit 101 in S1 to the frequency calculation unit 109. In the frequency calculation unit 109 (corresponding to the clock cycle calculation means), the write clock signal used for image formation on the first surface from the main scanning magnification (10100) on the first surface and the BD signal cycle (10100) on the first surface. the period M _n calculated by using equation (1) below. In addition, the meaning of the symbols A, B, C, D, and E in the formula (1) and their values are as follows.

M _n = A × B / C × D / E (1)
M _n : Write clock period A: Write clock period when main scanning magnification is 100% and sub scanning magnification is 100% B: Main scanning magnification of first surface (10100)
C: Main scanning magnification at a main scanning magnification of 100% (10000)
D: BD signal period of the first surface (10100)
E: BD signal period when sub-scan magnification is 100% (10000)

  In this embodiment, the values of the main scanning magnification (symbol B) on the first surface and the BD signal period (symbol D) on the first surface are given by the system control unit 101 as described above. Further, the values of the write clock cycle (symbol A), the main scan magnification (symbol C), and the BD signal cycle (symbol E) at the main scanning magnification of 100% and the sub scanning magnification of 100% are stored in the writing control unit 102. Assume that the input is input from the write control unit 102 to the frequency calculation unit 109. Note that the values of the symbols A, C, and E are stored in the memory of the system control unit 101 and may be output to the frequency calculation unit 109 by the system control unit 101 via the write control unit 102 as necessary. Alternatively, a configuration stored in advance in the frequency calculation unit 109 may be used. In addition, the meanings of symbols A, C, and E, and their values are the same as in formula (1) in formulas (2) and (4) described later.

Then, the frequency calculation unit 109 outputs the calculated write clock cycle M _n of the first surface to the write clock generation unit 110. The write clock generation unit 110 generates a write clock signal corresponding to the write clock cycle of the first surface output from the frequency calculation unit 109 and outputs the write clock signal to the image signal generation unit 111.

  In S3, image formation is performed on the first surface of the transfer material. Based on the transfer material detection signal from the transfer material detection unit 100, the system control unit 101 sends a sub-scanning image signal gate signal to the writing control unit 102 at the timing when an image is formed at a predetermined position on the transfer material. At the same time, the image signal stored in the image memory is output. The sub-scanning image signal gate signal is a signal indicating whether or not an image signal can be output from the writing control unit 102 to the image signal generation unit 111. A high level indicates that image signal output is permitted, and a low level indicates an image signal. This means that output is prohibited. Then, since the sub-scanning image signal gate signal is at a high level, the writing control unit 102 outputs the received image signal to the image signal generation unit 111. The image signal generation unit 111 outputs to the LD driver 105 a laser emission pattern signal obtained by synchronizing the write clock signal generated by the write clock generation unit 110 and the image signal from the write control unit 102. In the LD driver 105, the laser diode 106 is turned on or off based on the laser emission pattern signal output from the image signal generation unit 111. Note that the writing control unit 102 outputs a laser control signal to the LD driver 105 so that the laser diode 106 is turned on when the sub-scanning image signal gate signal becomes high level. The laser light emitted from the laser diode 106 is deflected by the mirror of the rotating polygon mirror 104, passes through various mirror lenses, and irradiates the photosensitive drum 11, thereby forming an electrostatic latent image on the photosensitive drum 11. Is done. Thereafter, image formation on the first surface of the first transfer material is performed through the image forming process described above. When the image formation on the first surface of the first transfer material is completed, the system control unit 101 sets the sub-scanning image signal gate signal to a low level.

  In S4, the system control unit 101 determines whether the print job is still continued. If the print job continues, the process proceeds to S5. If the print job ends, the system control unit 101 discharges the transfer material, executes a print job end sequence, and shifts to the print standby mode.

  In S5, the system control unit 101 determines whether or not the surface of the transfer material to be printed next is the same as the surface printed immediately before, in order to determine whether or not the image forming conditions need to be changed. That is, when image formation on the first surface (second surface) is performed similarly to the image formation on the first surface (second surface), the process proceeds to S3 and image formation is performed under the same image formation conditions. Accordingly, when printing only one side of the transfer material, only the image forming sequence of S3 to S5 is repeated as many times as necessary. On the other hand, when image formation on the second surface (first surface) is performed subsequent to image formation on the first surface (second surface), the process proceeds to S6. In the present embodiment, in order to form an image on the second surface of the first transfer material, the second sheet is transferred while the first transfer material passes through the conveyance paths 22, 23, and 24 by the flapper 20. In order to perform image formation on the first surface of the third transfer material, the image forming sequence of S3 to S5 is repeated twice. In this case, image formation on the first surface of the second and third transfer materials is performed based on the main scanning magnification and the sub-scanning magnification used for image formation on the first surface of the first transfer material. .

  In S6, image forming conditions for image formation on the second surface (or first surface) of the transfer material are set. In S6 to S12 in FIG. 3, not only the image formation on the second surface of the transfer material but also the image formation on the first surface may be performed, but in the following, the first transfer material according to the order of image formation The case of image formation on the second side will be described. Before the image formation on the second surface of the first transfer material, the system control unit 101 sets the main scanning magnification (second scanning surface) in order to set the main scanning magnification and the sub scanning magnification on the second surface to 100%. 10000) and the value of the BD signal cycle (10000) are output to the write control unit 102.

  In S <b> 7, the system control unit 101 changes the write clock scan cycle synchronization gate signal from low level to high level and outputs it to the write control unit 102, thereby synchronizing the write clock and the scan cycle by the rotary polygon mirror 104. Start control. Here, synchronous control of the scanning cycle by the writing clock and the rotary polygon mirror 104 will be described with reference to FIG. FIG. 4A shows a transfer material printing order, a sub-scanning image signal gate signal, a scanning cycle counter value, a write clock scanning cycle synchronization gate signal when performing double-side printing on a plurality of transfer materials in this embodiment. 3 is a timing chart schematically showing the relationship between write clock cycles.

  As described above, the sub-scanning image signal gate signal becomes high level at the timing when image information is output, and as shown in FIG. 4A, it is high level while image formation of each transfer material is being performed. Thus, the transfer material conveyance interval time is at a low level.

  The count value of the scan cycle counter schematically shows the count value of the scan cycle counter provided in the above-described writing control unit 102 for measuring the BD signal cycle. The target count value of the scanning cycle counter at the time of image formation on the first surface of the transfer material is 10100 in order to set the sub-scan magnification to 101%, and the target count value of the scanning cycle counter at the time of image formation on the second surface is In order to make the sub-scanning magnification 100%, it is 10,000. When the surface on which image formation is performed is changed, change control is performed so that the scanning cycle of the rotary polygon mirror 104 becomes the target scanning cycle of the corresponding surface. However, as described above, since the scanning cycle change of the rotary polygon mirror 104 requires time, it does not end within the transfer material conveyance interval. In FIG. 4A, the scanning cycle change control is performed on the transfer material image. It shows how it ends after formation has started.

  The writing clock scanning period synchronization gate signal changes from a low level to a high level when the image formation of the transfer material changes from the first side to the second side or from the second side to the first side. While the write clock scan period synchronization gate signal is at a high level, the write clock period is calculated based on the count value of the scan period counter that measures the BD signal period, and the obtained write clock period is calculated. Image formation is performed using a writing clock. When the BD signal period measured by the scanning period counter is stabilized within a predetermined error range of the BD signal period setting value input from the system control unit 101, the write clock scanning period synchronization gate signal is changed from the high level. Become low level.

  The write clock cycle schematically shows the cycle of the write clock signal calculated by the frequency calculation unit 109 in the same manner as the count value of the scanning cycle counter. The writing clock cycle (counter value) when image formation on the first surface of the transfer material is performed becomes long (becomes large) in order to set the main scanning magnification to 101%. Conversely, the write clock cycle (counter value) when the image formation on the second surface of the transfer material is being performed is shortened (decreased) in order to set the main scanning magnification to 100%. The write clock cycle is held at a constant clock cycle value when the scanning cycle of the rotary polygon mirror 104 is stable within a predetermined error range. When the write clock scanning cycle synchronization gate signal is at a high level, scanning cycle change control of the rotary polygon mirror 104 is performed. Since the write clock cycle is calculated based on the count value of the scan cycle counter described above, the write clock cycle shows a change waveform of the count value similar to that of the scan cycle counter as shown in FIG. .

The writing control unit 102 outputs an acceleration signal or a deceleration signal to the rotation control unit 103 so that the scanning cycle counter value becomes equal to the value of the BD signal cycle (10000) of the second surface, and the rotation control unit 103 Based on the acceleration signal or the deceleration signal, the rotational speed of the rotary polygon mirror 104 is controlled. The writing control unit 102 outputs the count value of the scanning cycle counter to the frequency calculation unit 109 every time a BD signal is input from the BD sensor 107. The frequency calculation unit 109 uses the current input counter value T _n and the previous input counter value T _n−1 each time the count value of the scanning cycle counter is input, and the write clock is calculated from the following equation (2). The period _Mn is calculated.

_{M n = A × F / C} × (2T n -T n-1) / E (2)
M _n : Write clock cycle A: Write clock cycle when the main scanning magnification is 100% and the sub scanning magnification is 100% F: Main scanning magnification of the second surface (10000)
C: Main scanning magnification at a main scanning magnification of 100% (10000)
T _n : Count value of the latest scanning cycle counter T _n-1 : Count value of the scanning cycle counter input last time E: BD signal cycle (10000) when the sub-scanning magnification is 100%

  Note that the value of the main scanning magnification (symbol F) of the second surface is input from the system control unit 101 to the writing control unit 102 in S6 as described above.

Here, as shown in Expression (2), a method of calculating a predicted writing clock period M _n when performing next image formation from a plurality of past scanning period counter values is shown in FIG. ). FIG. 4B shows the rotation of the rotating polygon mirror 104 in FIG. 4A in order to change the scanning period of the rotating polygon mirror 104 from the BD signal period of the second surface to the BD signal period of the first surface. It is the figure which expanded the change of the count value of the scanning period counter when decelerating speed. The vertical axis of FIG. 4B shows the count value of the scanning cycle counter, the latest scanning cycle counter value currently obtained is T _n , the previous scanning cycle counter value is T _n−1 , and the previous scanning cycle counter value. _Let T _n−2 be the value, and T _{n + 1} be the next (predicted) scan cycle counter value. Also, the horizontal axis of FIG. 4B shows the elapsed time, and the time required for the count value of the scanning cycle counter to change from T _{n− 2} to T _n−1 is Δt _n−1 , and similarly T _n−. time Delta] t _n required from ₁ to _{T n,} the time required from _{T n} to _{T n + 1} and Delta] t _{n + 1. T n} and _{T n-1} of the count value of the difference (the fluctuation) [Delta] T _n scanning cycle count value, when the difference between the count value of the _{T n + 1} and _{T n} (the variation amount) and [Delta] T _{n + 1,} the next scanning cycle The counter value T _{n + 1} is
T _{n + 1} = T _n + ΔT _{n + 1}
= T _n + ΔTn × Δt _{n + 1} / Δt _n
And the scanning period of the rotary polygon mirror 104 is substantially the same (Δt _n ≈Δt _{n + 1} ),
T _{n + 1} ≈T _n + ΔT _n
From ΔT _n = T _n −T _n−1 ,
_{T n + 1 ≒ T n +} (T n -T n-1)
≒ 2T _n -T _n-1 (3)
It becomes. Therefore, according to the above calculation formula (3), the count value T _n (second period) of the latest scanning period counter and the count value T _n−1 (first period) of the previous scanning period counter are The count value T _{n + 1} (third period) of the scanning period counter can be predicted.

Then, the frequency calculation unit 109 outputs the calculated write clock cycle M _n of the second surface to the write clock generation unit 110. The write clock generation unit 110 generates a write clock having the write clock cycle M _n output from the frequency calculation unit 109 as a clock cycle, and outputs the write clock to the image signal generation unit 111.

  In S <b> 8, the system control unit 101 sets the sub-scanning image signal gate signal to a high level and outputs the image signal to the writing control unit 102. The writing control unit 102 outputs the image signal to the image signal generation unit 111. The image signal generation unit 111 outputs a laser emission pattern signal obtained by synchronizing the image signal output from the write control unit 102 with the write clock output from the write clock generation unit 110 to the LD driver 105. The LD driver 105 turns on and off the laser diode 106 by turning on and off the drive current of the laser diode 106 according to the laser emission pattern signal from the image signal generation unit 111, and forms an electrostatic latent image on the photosensitive drum 11. Do.

In S9, the writing control unit 102 confirms that the speed of the rotary polygon mirror 104 is stable and the count value of the scanning cycle counter is stable within a predetermined error range compared to the value of the BD signal cycle (10000). Judge whether. If the writing control unit 102 determines that the speed of the rotary polygon mirror 104 is stable, the writing control unit 102 notifies the system control unit 101 and proceeds to S10. If it is determined that the state is not stable, the writing control unit 102 performs the next process described above, and then returns to the process of S8. That is, the writing control unit 102 reduces the difference in counter value so that the scanning cycle counter value becomes the value of the BD signal cycle (10000) of the second surface every time a BD signal is input from the BD sensor 107. An acceleration signal or a deceleration signal is output to the rotation control unit 103. Further, each time a BD signal is input, the writing control unit 102 outputs the count value of the scanning cycle counter at that time to the frequency calculation unit 109, resets the scanning cycle counter, and sets the subsequent BD signal cycle. Start measurement. Each time the count value of the scanning period counter is input, the frequency calculation unit 109 calculates the write clock period M _n from the above-described equation (2), and outputs the calculation result to the write clock generation unit 110. The write clock generation unit 110 generates a write clock having the write clock cycle M _n output from the frequency calculation unit 109 as a clock cycle, and outputs the write clock to the image signal generation unit 111.

In S <b> 10, the system control unit 101 changes the write clock scan cycle synchronization gate signal from the high level to the low level and outputs it to the write control unit 102, so that the write clock and the scan cycle of the rotary polygon mirror 104 are changed. End synchronous control. Then, the frequency calculation unit 109 uses the write clock cycle M _n of the second surface as the write clock cycle M _n (fixed value) of the second surface calculated by the following equation (4). Output to. The write clock generation unit 110 generates a write clock having the write clock cycle M _n output from the frequency calculation unit 109 as a clock cycle, and outputs the write clock to the image signal generation unit 111.

M _n = A × F / C × G / E (4)
M _n : Write clock cycle A: Write clock cycle when the main scanning magnification is 100% and the sub scanning magnification is 100% F: Main scanning magnification of the second surface (10000)
C: Main scanning magnification at a main scanning magnification of 100% (10000)
G: BD signal period of the second surface (10000)
E: BD signal period when sub-scan magnification is 100% (10000)

  Note that the main scanning magnification (symbol F) of the second surface and the BD signal cycle (symbol G) of the second surface are input from the system control unit 101 to the writing control unit 102 in S6 as described above. .

  In S11, the image formation on the second surface of the first transfer material is performed through the image forming process described above. When the image formation on the second surface of the first transfer material is completed, the first transfer material is discharged to the outside of the image forming apparatus by the discharge roller pair 18. Then, the system control unit 101 sets the sub-scanning image signal gate signal to a low level.

  In S12, the system control unit 101 determines whether or not the print job is finished. If the print job is finished, the system control unit 101 discharges all of the transfer material, shifts to the print standby mode through the print job stop sequence, When image formation is continued, the process proceeds to S5.

In the process of S5, the system control unit 101 determines whether or not the next image formation is performed on the same surface as the previous image formation, and S3 to S4 (image formation on the same surface) and S6 to S11 (different surfaces). Image formation) is selected in which sequence. In this embodiment, double-sided continuous printing is performed, and the above-described steps S6 to S11 are repeated in order to print the first surface of the fourth transfer material. However, the description of S6 to S11 described above is for printing the second surface of the transfer material. In the case of printing the first surface, the basic control is the same, but the write clock cycle is the same. It is necessary to change the values of the symbols F and G used in the equations (2) and (4) for calculating M _n . That is, the values of symbols F and G in the equations (2) and (4) when printing the first surface are respectively the main scanning magnification (10100) of the first surface and the BD signal period (10100) of the first surface. Use the value. Therefore, in S6, the system control unit 101 sets the write clock scanning period synchronization gate signal to a high level, and also sets the main scanning magnification (10100) of the first surface of the fourth transfer material and the BD of the first surface. The value of the signal period (10100) is output to the write control unit 102. When the image formation on the first surface of the fourth transfer material is completed, the second surface of the second transfer material, the first surface of the fifth transfer material, and the second surface of the third transfer material. Image formation of the surface,..., And the first surface and the second surface is performed alternately. Since the last transfer material is formed on the same surface (second surface) as the transfer material imaged immediately before, the last transfer material is image-formed by the sequence of S3 to S4. Is called. At the end of the print job, the system control unit 101 discharges all the transfer material, and shifts to the print standby mode through the print job end sequence.

  As described above, according to this embodiment, the correction accuracy of the image magnification is improved by correcting the image magnification in the main scanning direction based on the prediction of the scanning period when the speed of the rotary polygon mirror fluctuates. Images can be provided. That is, when double-sided printing of a transfer material is performed, the image scanning magnification may be changed between the first surface and the second surface due to the reduction of the transfer material by the fixing device. In this case, since the scanning cycle change of the rotary polygon mirror is not completed within the transfer material conveyance interval, the cycle change of the rotary polygon mirror is continued even during image formation. Therefore, in order to perform image formation even during periodic fluctuations, the scanning period of the rotating polygon mirror can be measured, and the scanning period of the rotating polygon mirror at the next scanning can be predicted from the tendency of a plurality of periodic changes. The write clock cycle is changed based on the predicted cycle. As a result, it is possible to form an image with a main scanning magnification close to the original main scanning magnification, and thus it is possible to provide an image with little magnification variation even when the rotary polygon mirror has a periodic variation.

102 Writing control unit 103 Rotation control unit 104 Rotating polygon mirror 107 BD (beam detect) sensor 109 Frequency calculation unit (corresponding to clock cycle calculation means)
110 Write clock generator

Claims (5)

An optical scanning device that forms an electrostatic latent image on a rotationally driven photoconductor and deflects the light beam so that a light beam emitted from a light source scans the photoconductor in a predetermined direction.
Generating means for generating a clock signal;
Driving means for emitting the light beam from the light source based on the clock signal and image data;
In order to change the magnification of the electrostatic latent image formed by the light beam in the rotation direction of the photoconductor, the rotation speed of the rotary polygon mirror that deflects the light beam is controlled to a rotation speed corresponding to the magnification. Control means;
Output means for outputting a detection signal in response to detection of the light beam deflected by the rotary polygon mirror;
The length of the electrostatic latent image in the predetermined direction is set to a predetermined length based on the amount of change in the period of the detection signal that changes due to the rotation speed of the rotary polygon mirror being changed by the control means. An optical scanning device comprising: modulation means for modulating in a period such that Based on the amount of variation between T _n−1 that is the first period of the detection signal and T _n that is the second period that follows the first period, the third period that follows the second period. 2. The optical scanning device according to claim 1, wherein T _{n + 1} that is a period of is calculated by a calculation formula of T _{n + 1} = 2 × T _n −T _n−1 . The period of the clock signal is
Both the sub-scanning magnification which is the magnification of the electrostatic latent image in the rotation direction of the photoconductor and the main scanning magnification which is the magnification of the electrostatic latent image in the direction orthogonal to the rotation direction of the photoconductor are both 100%. A period of the clock signal generated by the generating means, and
The difference between the period of the detection signal and the third period when the main scanning magnification is 100%;
The optical scanning device according to claim 2, wherein the optical scanning device is calculated based on a difference between the length of the electrostatic latent image and the predetermined length when the main scanning magnification is 100%.   The optical scanning device according to claim 1, wherein the electrostatic latent image formed on the photosensitive member by the optical scanning device is developed as a toner image by a developing unit and developed. An image forming apparatus for forming an image of the toner image on a transfer material. A transfer means for transferring the transfer material in order to reverse the transfer material on which one side is imaged and to form an image on one side different from the one side;
After the transfer material having an image formed on one side thereof is reversed by the conveying means, the rotation speed of the rotary polygon mirror is changed to form the electrostatic latent image having a magnification different from that of the one surface, and the transfer material The image forming apparatus according to claim 4, wherein an image is formed on one side.

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