JP2017064992A - Optical scanning device, and image formation device equipped with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanning device capable of improving suppression effect of uneven light quantity, without excessively enlarging a circuit scale which is used for correction on the light quantity of light beam when exposure scanning the same photoreceptor with a plurality of light beams at the same time.SOLUTION: A light source emits two light beams each time from each semiconductor laser. A scanning optical system performs exposure scanning on the same photoreceptor drum with both light beams. Here, according to an interval along main scanning direction of spots (SP1, SP2) in which both light beams focus on a surface of the same photoreceptor drum, a correction part adjusts a duty ratio of MCL signal. The correction part further selects a correction value for an emitted light quantity from a first light emitting point which a laser oscillator of the semiconductor laser includes by synchronizing with a rising of the MCL signal, and selects a correction value for the emitted light quantity from the second light emitting point by synchronizing with a falling of the MCL signal.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image forming technique, and more particularly, to an exposure technique for a photoreceptor in an electrophotographic system.

  There are electrophotographic image forming apparatuses that use an optical scanning device for exposure scanning of a photosensitive member. The optical scanning device includes a semiconductor laser or a light emitting diode (LED) as a light source, and light rays emitted from the light source are periodically deflected by a scanning optical system including a combination of a polygon mirror and an fθ lens. The image is formed on the surface. In each cycle of this deflection operation, the light spot moves in one direction (hereinafter referred to as “main scanning direction”) on the surface of the photosensitive member, so that the surface is exposed linearly. Since the optical scanning device modulates the exposure amount at this time according to each line of the image represented by the image data, the distribution of the charge amount changes according to the modulation pattern in each linear exposure region. Thus, each line of the electrostatic latent image is formed on the surface of the photoreceptor. When toner adheres to the electrostatic latent image, the toner density changes in accordance with the charge amount distribution. That is, the change in the exposure amount by the optical scanning device is visualized as the density of the toner.

  In recent years, there is an increasing demand for higher speed and higher definition for image forming apparatuses. In order to satisfy both requirements, the following two points have been tried to improve the optical scanning device. One point is to shorten the time required for forming one line by increasing the speed of the deflection operation in the scanning optical system, for example, by increasing the rotational speed of the polygon mirror. The other point is that a plurality of light beams (also referred to as “multi-beams”) are emitted from the light source to form a plurality of lines formed on one photoconductor by one deflection operation of the scanning optical system. (For example, see Patent Documents 1-3).

  In any improvement, it is necessary to take measures against unevenness in the amount of light caused by the scanning optical system. “Light amount“ unevenness ”” means that the scanning optical system is maintained even if the emitted light amount is kept constant as the reflectance and transmittance of the scanning optical system differ strictly depending on the deflection angle. Refers to a phenomenon in which the amount of light applied to the photosensitive member varies depending on the position in the main scanning direction. If this variation is directly reflected in the change in the exposure amount, “unevenness” in the charge amount appears in one line of the electrostatic latent image, and “unevenness” in the toner density appears in one line corresponding to the toner image. . Therefore, in order to improve the image quality on the sheet, it is desirable to remove as much as possible the unevenness in the amount of light by suppressing fluctuations in the amount of light irradiated from the scanning optical system to the photosensitive member due to the change in the deflection angle.

  An existing technique for suppressing unevenness in the amount of light cancels the above-described variation in the amount of light applied to the photosensitive member by correcting the amount of light emitted from the light source (see, for example, Patent Documents 4-8). The correction value at this time is set by the following procedure, for example. (1) The variation in the amount of light applied to the photoconductor is sampled (sampled) while the amount of light emitted from the light source is constant. For example, a plurality of positions are discretely selected from the straight line drawn by the spot of the irradiated light on the surface of the photoconductor, and the ratio of the irradiated light amount to the reference value is calculated from the model of the scanning optical system as a sample value (sample) at each position. Or measured experimentally. (2) A correction value necessary for cancellation, for example, the inverse ratio of the sample is calculated for each sample. (3) Based on these correction values and the position where the sample is obtained, a correspondence relationship between the position of the spot of irradiation light and the correction value to be applied is set.

JP-A-2005-241686 JP 2003-266757 A Japanese Patent Application Laid-Open No. 11-064765 JP 2013-240996 A JP 2011-158761 A JP 2011-158760 A JP 2009-262344 A JP 2009-053466 A

  One improvement aimed at further speeding up and high definition of the image forming apparatus is to irradiate a single photoconductor with a plurality of light beams from a light source to the optical scanning device, and to scan the line on the surface of the optical system. A plurality of deflections are formed per deflection operation. In this case, in order to further increase the definition of the image, it is preferable to increase the line density by narrowing as much as possible the interval (pitch) of a plurality of spots where a plurality of light beams connect to the surface of the photoreceptor. However, the structure of each light emitting element embedded in the light source has a minimum required size. In order to avoid thermal and electrical crosstalk between the light emitting elements, a certain distance must be provided between the light emitting elements. As a result, there is a lower limit of several tens of μm to several hundreds of μm between the spots of the irradiated light beam from the light source to the photosensitive member.

  In order to increase the line density regardless of this lower limit, such as 1200 dpi, 2400 dpi (line spacing = 21 μm, 11 μm), or the inverse ratio of this lower limit, for example, the spot arrangement direction is perpendicular to the main scanning direction. What is necessary is just to incline from a direction (henceforth "sub-scanning direction"). By adjusting this inclination angle, it is possible to narrow the interval of the lines drawn by the spots below the lower limit, while maintaining the interval between the spots themselves at the above lower limit or more.

  In this case, since the timing to reach the same position in the main scanning direction is different for each spot, the correction value change timing is changed for each outgoing light beam in order to correct the outgoing light amount of the light source for the purpose of suppressing unevenness in the light amount. Ingenuity is necessary. For example, the technique disclosed in Patent Document 5 sets a correspondence table between correction values and application timings for each ray. The techniques disclosed in Patent Documents 4 and 6 make the correction value common to the light beams, while changing the correction value delays the timing for the subsequent spot with reference to the timing for the first spot.

  However, any of the existing devices is not preferable in that it requires the optical scanning device to greatly expand the circuit scale. For example, in the technique disclosed in Patent Document 5, the amount of data in the correspondence table increases in proportion to the number of light rays emitted from the light source. Therefore, the optical scanning device must expand the scale of the memory in accordance with the increase. In the techniques disclosed in Patent Documents 4 and 6, the optical scanning apparatus must keep counting the clock until the last spot has passed after the first spot has passed through the exposure target area on the photosensitive member. It is necessary to increase the upper limit of counting by expanding the scale of the counter.

  The scale of this counter may be further increased when the variation in spot spacing (especially in the main scanning direction) is taken into account. Actually, the timing for changing the correction value is designed to coincide with the rise or fall of the clock. Therefore, if the true timing deviates from the rising edge of the clock or the like due to the variation in the spot interval, an error occurs in the correction. In order to eliminate this error, it is necessary to increase or decrease the clock frequency from the design value to displace the rising edge. Since there is a limit to the range in which this frequency can be increased or decreased, the design value of the frequency must be set sufficiently high in advance so that the above error can always be removed. This setting further raises the counter count limit, so the scale of this counter can become excessive.

  The object of the present invention is to solve the above-mentioned problems, particularly when the same photoconductor is exposed and scanned simultaneously with a plurality of light rays without excessively increasing the circuit scale used for correcting the light quantity of those light rays, An object of the present invention is to provide an optical scanning device capable of improving the effect of suppressing unevenness in the amount of light.

  An optical scanning device according to one aspect of the present invention is an optical scanning device that forms an image on a photosensitive member by exposure scanning, and emits a first light beam and a second light beam, and a light source capable of adjusting a light amount for each light beam. Then, the first light beam and the second light beam are imaged on the surface of the photoconductor while periodically deflecting the light beam, so that the distance between the spots of both light beams is maintained in both the main scanning direction and the sub scanning direction. A scanning optical system that moves the spot in the main scanning direction, a modulation unit that modulates the amount of light of each light beam according to image data, and a correction value that is selected according to the position of the spot of each light beam in the main scanning direction. And a correction unit for correcting the light quantity of the light beam. The correction unit generates a clock signal in synchronization with the periodic deflection of the first light beam so as to rise or fall at the timing when the spot of the first light beam reaches a predetermined position, and pulse width modulation The pulse width modulation unit for matching the duty ratio of the clock signal to a target value determined by the interval in the main scanning direction of the spot of the first light beam and the second light beam, and the rising edge of the clock signal modulated by the pulse width modulation unit or A timing generation unit that counts the falling edges and generates a timing signal indicating the timing at which the number of rising times reaches a predetermined number and the timing at which the number of falling times reaches the predetermined number, and the first light beam according to the timing signal Select the correction value for each light quantity of the first and second rays, one of which is synchronized with the rising edge of the clock signal modulated by the pulse width modulator So, the other containing a switching unit which pulse-width modulation unit synchronizes with the falling edge of the clock signal modulated.

  The correction unit sets the coordinates in the main scanning direction in the region on the photoconductor on which the spot of each light beam moves, divides the possible range of the coordinates into a plurality of correction sections, and the coordinates of the spot are included in each correction section. At the arrival timing, the correction value for the light amount of the light beam is changed to the correction value for the correction interval, and the oscillation unit makes the rising or falling edge of the clock signal coincide with the timing when the spot of the first light beam reaches each correction interval. May be.

The optical scanning device may further include a detection unit that detects a timing at which the deflection angle of the first light beam by the scanning optical system reaches a predetermined value. In this case, the oscillation unit may make the rising or falling edge of the clock signal coincide with the timing detected by the detection unit.
The scanning optical system can change the period of deflecting each light beam, and the pulse width modulation unit may change the target value of the duty ratio of the clock signal in accordance with the change of the period. Further, when the pulse width modulation unit changes the target value of the duty ratio of the clock signal, the range that the target value after the change can take may be limited to be narrower than 0 and less than 1.

  The light source further emits a third light beam, and the scanning optical system forms an image between the spots of the first light beam and the second light beam while periodically deflecting the third light beam in the same manner as the first light beam and the second light beam. As a result, the spots of the three light beams are moved in the main scanning direction while maintaining the interval between the spots in both the main scanning direction and the sub-scanning direction, and the oscillating unit causes the third light beam to be periodically deflected. Synchronously, another clock signal is generated so that it rises or falls at the timing when the spot of the third light beam reaches a predetermined position, and the timing generator counts the rise or fall of the other clock signal, and rises Another timing signal indicating the timing at which the number of times or the number of falling times reaches a predetermined number, and the switching unit selects a correction value for the light amount of the third light according to the other timing signal. The may be synchronized with the rising or falling edge of another clock signal.

  The light source further emits from the fourth light beam to the second n light beam (the integer n is 2 or more), and the scanning optical system cycles from the fourth light beam to the second n light beam in the same manner as from the first light beam to the third light beam. By forming an image on the surface of the photoconductor while deflecting it, the spots of the (2m-1) light rays (integer m = 1, 2, 3,..., N) are arranged in order along the main scanning direction. Subsequently, the spots of the second m light rays are arranged in order, and the arrangement is moved in the main scanning direction while maintaining the arrangement interval in both the main scanning direction and the sub-scanning direction. The m-th clock signal including this clock signal and another clock signal is synchronized with the periodic deflection of the (2m-1) -th light beam, and rises or falls at the timing when the spot of the light beam reaches a predetermined position. The pulse width modulation unit generates pulse width modulation Therefore, the duty ratio of the m-th clock signal is made to coincide with a target value determined by the interval in the main scanning direction of the spot of the (2m-1) -th light beam and the second m-th light beam, and the timing generation unit rises or rises the m-th clock signal. The mth timing signal including the above timing signal and another timing signal is counted as the timing at which the number of rising edges of the mth clock signal reaches a predetermined number and the timing at which the number of falling edges reaches a predetermined number. The switching unit selects a correction value for each light amount of the (2m-1) light beam and the second m light beam according to the m-th timing signal, and one of them is the m-th signal modulated by the pulse width modulation unit. The clock signal may be synchronized with the rising edge, and the other may be synchronized with the falling edge of the m-th clock signal modulated by the pulse width modulation unit.

  An image forming apparatus according to one aspect of the present invention is an image forming apparatus that forms a toner image on a sheet, and a photosensitive member whose charge amount changes according to an exposure amount, and an electrostatic latent image formed on the photosensitive member by exposure scanning. And a developing unit that develops the electrostatic latent image with toner, and a transfer unit that transfers the toner image developed by the developing unit from the photoreceptor to the sheet.

  As described above, the optical scanning device according to the present invention emits two light beams from the light source, and sets the duty ratio of the clock signal in accordance with the interval in the main scanning direction of the spots where the two light beams pass through the scanning optical system to the surface of the photosensitive member. The correction value for one light quantity of both rays is selected in synchronization with the rising edge of the clock signal, and the correction value for the other light is selected in synchronization with the falling edge of the clock signal. Thereby, the optical scanning device can appropriately correct the timing of changing the correction value without changing the frequency of the clock signal even if the interval of the spot in the main scanning direction varies. As a result, this optical scanning device has the effect of suppressing unevenness in light quantity without excessively increasing the circuit scale used for correcting the light quantity of the light beams when simultaneously scanning the same photosensitive member with a plurality of light beams. Can be improved.

1 is a front view showing an internal structure of an image forming apparatus according to an embodiment of the present invention. It is a top view of the optical scanning part shown in FIG. (A) is a schematic diagram showing a package of each semiconductor laser shown in FIG. 2 and two light emitting points of a laser oscillator incorporated therein, and (b) is two outgoing beams from the semiconductor laser. FIG. 3 is a schematic diagram showing two spots that are connected to the photosensitive drum through the scanning optical system shown in FIG. 2. FIG. 2 is a block diagram of an electronic control system included in the image forming apparatus shown in FIG. 1. FIG. 3 is a block diagram of a semiconductor laser drive circuit included in the optical scanning unit shown in FIG. 2. FIG. 6 is a block diagram illustrating details of a circuit configuration of an SH unit illustrated in FIG. 5. (A) is a timing chart showing the correspondence between the time when two light beam spots arrive at the same main scanning position on the surface of the photosensitive drum and the pulse waveforms of the CLK signal and the MCL signal. b) is a correspondence table between a combination of an integer value representing a delay time between the spots, a target value of the duty ratio of the MCL signal, and the sheet conveyance speed. 6 is a timing chart of signals related to the timing generation unit shown in FIG. 5. (A) is a graph of a correction curve representing the correspondence between the correction value set by the correction unit shown in FIG. 5 and the main scanning position on the photosensitive drum, and (b) is a graph of the boundary of the correction section. 6 is a correspondence table between main scanning positions and correction values. (A) is a schematic diagram which shows the incident angle of the laser beam of the light source with respect to a polygon mirror. (B) is a schematic diagram showing the incident angle of the reflected light of the polygon mirror with respect to the fθ lens. (C) is a graph which shows the fluctuation | variation which appears in the irradiation light quantity to a photoconductor drum on the conditions which a light source maintains the emitted light quantity constant. (A) is a graph which shows the setting conditions of the boundary of the correction area with respect to the correction curve shown to (a) of FIG. 9, (b) is the one part enlarged view. It is a flowchart of control with respect to the optical scanning part shown in FIG.2, FIG.5, FIG.6. 13 is a flowchart of a subroutine for generating an MCL signal in step S102 shown in FIG. It is a flowchart of the production | generation process of the instruction | indication signal and timing signal by the timing generation part shown in FIG. 5, FIG. (A) is a schematic diagram which shows the light emission point on the laser oscillator built in the semiconductor laser by the modification of embodiment of this invention, (b) is in either boundary of the correction area shown in FIG. It is a timing chart which shows the correspondence between the time when the spot of each light beam from the semiconductor laser shown in (a) arrives and the pulse waveform of the CLK signal and the MCL signal, and (c) shows the control target for each CLK signal. It is a table | surface which shows the correspondence between the pair of light beam, and the target value of duty ratio.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Internal structure of image forming apparatus]
FIG. 1 is a front view schematically showing the internal structure of an image forming apparatus 100 according to an embodiment of the present invention. The image forming apparatus 100 is a color laser printer. In FIG. 1, elements inside the printer 100 are depicted as if the front surface of the housing is seen through. Referring to FIG. 1, the printer 100 includes a feeding unit 10, an image forming unit 20, a fixing unit 30, and a paper discharge unit 40.

-Feeding section-
The feeding unit 10 feeds sheets SHT from the sheet feeding cassette 11 to the image forming unit 20 one by one using the transport roller groups 12, 13, and 14. The material of the sheet SHT that can be stored in the paper feed cassette 11 is paper or resin, and the size is A3, A4, A5, B4, or the like. The timing roller 14 closest to the image forming unit 20 in the transport roller group is generally stopped and rotates according to a drive signal from a main control unit 60 (see FIG. 4) described later. The timing roller 14 sends the sheet SH2 to the image forming unit 20 at the timing indicated by the drive signal.

-Image forming part-
The image forming unit 20 forms a toner image on the sheet SH2 sent from the feeding unit 10.
Specifically, the four image forming units 21Y, 21M, 21C, and 21K first charge the surfaces of the photosensitive drums 25Y, 25M, 25C, and 25K uniformly, and then the optical scanning unit 26 Are exposed and scanned in the axial direction (X-axis direction (normal direction of the paper surface) shown in FIG. 1). At this time, the light scanning unit 26 determines the amount of light applied to the photosensitive drums 25Y,. Modulate based on On the other hand, the charge disappears from the exposed area on the surface of the photosensitive drum 25Y. In this way, an area where the distribution of the charge amount is changed in a pattern corresponding to the change in the gradation values of Y, M, C, and K, that is, one line of the electrostatic latent image is generated on those surfaces.

Next, the image forming units 21Y,... Charge the four color toners of Y, M, C, and K one by one and attach them to one line of the electrostatic latent image. Thus, one line of different color toner images is developed on the surfaces of the different photosensitive drums 25Y,.
The image forming units 21Y,... Repeat such exposure and development for each line represented by the image data while rotating the photosensitive drums 25Y,. Thus, the four-color images represented by the image data are reproduced as toner images of one color on each surface of the four photosensitive drums 25Y,.

  These four color toner images are formed on the surface of the intermediate transfer belt 23 in order from the surface of the photosensitive drum 25Y,... By the electric field between the primary transfer rollers 22Y, 22M, 22C, 22K and the photosensitive drum 25Y,. Transferred to the same position. As a result, one color toner image is formed at that position. When the intermediate transfer belt 23 comes into contact with the sheet SH2 fed from the feeding unit 10 at the nip between the driving roller 23R and the secondary transfer roller 24, the color toner image is further between the two toner images 23R and 24. Is transferred to the surface of the sheet SH2. Thereafter, the driving roller 23 </ b> R and the secondary transfer roller 24 of the intermediate transfer belt 23 send the sheet SH <b> 2 to the fixing unit 30.

  Referring to FIG. 1, one line of the toner image appearing on the surface of the photosensitive drum 25Y,... Is transferred to the surface of the sheet SH2 with respect to the conveyance direction of the sheet SH2 by the secondary transfer roller 24. It is vertical. On the other hand, the one line and the next one generated on the surface of the photosensitive drum 25Y are aligned in the conveyance direction of the sheet SH2 after being transferred to the surface of the sheet SH2. Therefore, in the following, the direction of one line of the toner image, such as the direction in which the optical scanning unit 26 scans the surface of the photosensitive drums 25Y,... With the irradiation light (the X-axis direction (normal direction of the paper surface) shown in FIG. Are collectively referred to as the “main scanning direction”, and in the direction in which the lines of the toner image are aligned, such as the rotation direction of the photosensitive drums 25Y,... And the conveyance direction of the sheet SH2 (Y-axis direction shown in FIG. 1). All the corresponding directions are collectively referred to as “sub-scanning direction”.

-Fixing part-
The fixing unit 30 heat-fixes the toner image on the sheet SH2 sent out from the image forming unit 20. Specifically, when the sheet SH2 is passed through the nip between the fixing roller 31 and the pressure roller 32, the fixing roller 31 applies heat of a built-in heater to the surface of the sheet SH2, and the pressure roller No. 32 applies pressure to the heated portion of the sheet SH <b> 2 and presses it against the fixing roller 31. The toner image is fixed on the surface of the sheet SH <b> 2 by the heat from the fixing roller 31 and the pressure from the pressure roller 32.

-Output section-
The paper discharge unit 40 discharges the sheet SH3 on which the toner image is fixed out of the housing of the printer 100. Specifically, the sheet SH3 first moves along the guide plate 41 from the upper part of the fixing unit 30. At this time, the paper discharge unit 40 rotates a paper discharge roller 43 disposed inside a horizontal slit 42 opened in the housing of the printer 100, and feeds the sheet SH 3 out of the slit 42 on its peripheral surface. As a result, the sheet SH3 is stored in the paper discharge tray 44 included in the upper surface of the printer 100.

These elements 10, 20, 30, and 40 of the printer 100 control the drive motors (not shown in FIG. 1) of the conveyance roller groups 12, 13, 14, 23 R, 24, 31, and 43 to convey the sheet. The speed is maintained at a target value instructed from a main control unit 60 (see FIG. 4) described later.
[Optical scanning unit]
FIG. 1 includes a longitudinal sectional view of the optical scanning unit 26. FIG. 2 is a top view of the optical scanning unit 26. In FIG. 2, for convenience of explanation, the upper plate member that covers the optical scanning unit 26 is removed. 2 also shows the position of the longitudinal section of the optical scanning unit 26 shown in FIG. 1 by a straight line II. Referring to FIGS. 1 and 2, the optical scanning unit 26 includes a light source 260, a scanning optical system, and a control unit 300. The scanning optical system includes a polygon mirror 271, a motor 272, an fθ lens 273, and four sets of folding mirrors (28Y, 29Y), (28M, 29M), (28C, 29C), and 28K.

-Light source-
The light source 260 includes four semiconductor lasers 26 </ b> Y, 26 </ b> M, 26 </ b> C, 26 </ b> K, four mirrors 261-264, and a cylindrical lens 265.
All of the semiconductor lasers 26Y have a common structure. FIG. 3A is a schematic diagram showing a package of one of the semiconductor lasers 26Y. Referring to FIG. 3A, the semiconductor laser 26Y includes a laser oscillator 361. The laser oscillator 361 is, for example, a laser diode (LD), and particularly a semiconductor chip including a PN junction. The laser oscillator 361 can emit, for example, two laser beams having a wavelength of 790 nm or 660 nm with an output of several mW to several tens of mW. The laser oscillator 361 amplifies the light generated by recombination of holes and electrons in the active layer with the application of the forward voltage to the PN junction by reciprocating in the active layer by a reflecting mirror. The laser oscillator 361 is roughly classified into two types: an edge-emitting type whose reciprocating direction is parallel to the surface of the chip and a vertical cavity surface-emitting type (VCSEL) which is vertical. FIG. 3A includes schematic enlarged views EEL and VCS showing two light emitting points (PE1, PE2) and (PS1, PS2) of the laser oscillator 361 incorporated in the package according to these types. It is. Between these light emitting points (PE1, PE2) and (PS1, PS2), the supplied current amounts are independent of each other even if the applied voltage is common, so that the amount of emitted light can be adjusted for each light emitting point.

  When the laser oscillator 361 is of the edge emitting type, as shown by one EEL in the enlarged view of FIG. 3A, the oscillator 361 is arranged so that the end face of the chip is viewed from the emission port 362 of the package, The end face includes two light emitting points PE1 and PE2. Although not shown in FIG. 3A, a light amount sensor is disposed on the opposite side of the laser oscillator 361 from the emission port 362. This light amount sensor is, for example, a photodiode (PD), detects the amount of light emitted from the end surface opposite to the end surface of the laser oscillator 361 viewed from the emission port 362, and outputs the amount of output current proportional to the amount of light in FIG. It feeds back to the control unit 300 shown.

  When the laser oscillator 361 is a VCSEL, as shown by the other VCS in the enlarged view of FIG. 3A, the oscillator 361 is arranged such that the surface of the chip is viewed from the emission port 362 of the package. Includes two light emitting points PS1 and PS2. Unlike the edge-emitting type VCSEL, laser light is emitted only from the surface viewed from the emission port 362. Therefore, the light quantity sensor is not disposed inside the VCSEL, but, for example, on the back side of the fourth mirror 264 shown in FIG. 2 (FIG. 2 is not shown). In this case, the fourth mirror 264 is a half mirror. The light amount sensor is, for example, a PD, and detects the amount of light emitted from the laser oscillator 361 that has passed through the fourth mirror 264 and feeds back the amount of output current proportional to the amount to the control unit 300.

  Referring again to FIG. 2, the first semiconductor laser 26Y, the second semiconductor laser 26M, and the third semiconductor laser 26K have the same emission direction and are arranged at equal intervals in a direction perpendicular to the direction. . On the other hand, the fourth semiconductor laser 26C is arranged so that the emission direction thereof is orthogonal to the emission directions of the other semiconductor lasers 26Y, 26M, and 26K. Although not shown in FIG. 1 and FIG. 2, the four semiconductor lasers 26Y,... Are the height of the exit (in FIG. 1, the vertical position on the paper surface, and in FIG. 2, the normal line position on the paper surface). .) Are different, the outgoing rays LY, LM, LC, LK from them have different path heights.

  The first mirror 261, the second mirror 262, and the third mirror 263 are provided one by one from the exit port before each exit port of the first semiconductor laser 26Y, the second semiconductor laser 26M, and the third semiconductor laser 26K. Are arranged such that only the outgoing light beams LY, LM, or LK are incident. Each of the mirrors 261,..., 263 reflects the light rays LY,..., LK emitted from the semiconductor lasers 26Y,. The fourth mirror 264 is installed so as to reflect the reflected light from the other three mirrors 261,..., 263 and the outgoing light LC from the fourth semiconductor laser 26C in the same direction.

  The cylindrical lens 265 transmits the reflected light beam LL from the fourth mirror 264 and irradiates the polygon mirror 271 with it. The cylindrical lens 265 forms an image on the side surface of the polygon mirror 271 particularly in the direction of the rotation axis of the polygon mirror 271 (in FIG. 1, the vertical direction of the paper surface and in FIG. 2 the normal direction of the paper surface). At the same time, in the direction orthogonal to both the direction and the irradiation direction (in FIG. 1, the horizontal direction of the paper surface, in FIG. 2, the direction parallel to the paper surface and perpendicular to the outgoing light beam LL). Convert to As will be described later, the rotation axis direction of the polygon mirror 271 is the sub-scanning direction, and the direction orthogonal to both the direction and the traveling direction of the reflected light beam LL from the fourth mirror 264 is the main scanning direction.

-Scanning optical system-
The polygon mirror 271 is a regular polygonal column (regular heptagonal column in the example of FIG. 2) member, and any side surface is mirror-finished. Thereby, each side surface reflects and deflects incident light. Hereinafter, these side surfaces are referred to as “deflection surfaces”.
The polygon mirror 271 is supported so as to be rotatable around its central axis. The motor 272 applies a driving force to the polygon mirror 271 to rotate around its central axis. In particular, while the light beam LL is emitted from the light source 260 to the polygon mirror 271, the motor 272 maintains the angular velocity of the polygon mirror 271 at a predetermined value.

Referring to FIG. 2, the polygon mirror 271 reflects and deflects the outgoing light beam LL from the light source 260, and at the same time, the angle formed by the traveling direction of the outgoing light beam LL and the reflected light beam RL by its rotation, that is, the outgoing light beam. The deflection angle of LL is changed. Specifically, while the polygon mirror 271 rotates by the rotation angle θ, the deflection angle changes by twice the angle 2θ. Further, as the polygon mirror 271 rotates, the deflection surface that actually reflects the light beam LL periodically changes, so that the deflection angle is continuously and periodically in the range from the minimum value φ _L to the maximum value φ _R. To change. In particular the deflection angle in the case where the polygon mirror 271 rotates at a constant angular velocity, from the maximum value phi _R to the minimum value phi _L changes at a constant rate, changes instantaneously from the minimum value phi _L to a maximum value phi _R .

  The fθ lens 273 is arranged so as to transmit the reflected light beam RL from the polygon mirror 271 and irradiate the folding mirrors 28Y, 28M, 28C, and 28K. The two first folding mirrors 28Y and 29Y, the two second folding mirrors 28M and 29M, the two third folding mirrors 28C and 29C, and the one fourth folding mirror 28K are all in the shape of an elongated plate. As shown in FIGS. 1 and 2, the longitudinal direction is arranged in parallel with the axial direction of the photosensitive drums 25Y,. Referring to FIG. 1, one of the first folding mirror 28Y, one of the second folding mirror 28M, one of the third folding mirror 28C, and the fourth folding mirror 28K are at a height (in FIG. 1, the position in the vertical direction of the paper surface). FIG. 2 shows the position in the normal direction of the paper surface). As a result, only the outgoing light beams LY from the different semiconductor lasers 26Y of the reflected light beams RL from the polygon mirror 271 fall on the different folding mirrors 28Y. The other 29Y of the first folding mirror, the other 29M of the second folding mirror, the other 29C of the third folding mirror, and the fourth folding mirror 28K are disposed one by one immediately below the photosensitive drum 25Y,. The outgoing light beam LY from the first semiconductor laser 26Y is reflected by the polygon mirror 271 and transmitted through the fθ lens 273, and then is reflected by the first folding mirrors 28Y and 29Y and is incident on the photosensitive drum 25Y of the first image forming unit 21Y. Irradiated. Similarly, the outgoing light beam LM from the second semiconductor laser 26M is reflected by the second folding mirrors 28M and 29M and applied to the photosensitive drum 25M of the second image forming unit 21M, and the outgoing light beam LC from the third semiconductor laser 26C. Is reflected by the third folding mirrors 28C and 29C and applied to the photosensitive drum 25C of the third image forming unit 21C, and the outgoing light beam LK from the fourth semiconductor laser 26K is reflected by the fourth folding mirror 28K and is reflected by the fourth production mirror. Irradiated to the photosensitive drum 25K of the image unit 21K.

The fθ lens 273 is composed of, for example, two aspheric lenses, and the reflected light beam RL from the polygon mirror 271 is applied to the surface of each photosensitive drum 25Y,... in both the axial direction and the rotational direction (X shown in FIG. 1). Both the axial direction and the Y-axis direction) are imaged. Thereby, the position of the spot of the reflected light beam RL is exposed on the surface. When the deflection angle is changed by rotation of the polygon mirror 271, the position at which the folding mirrors 28 Y,..., 28 K reflect the transmitted light from the fθ lens 273 moves on the mirrors 28 Y,. . Therefore, the spot where the reflected light from the folding mirrors 28Y,... Connects to the surface of the photosensitive drum 25Y,. In particular, during a period in which the deflection angle changes from the maximum value φ _R to the minimum value φ _L at a constant speed, the spot moves in the main scanning direction on the surface of one of the photosensitive drums 25Y,. As a result, the exposed portions on the surface are connected in a straight line to form one line of the electrostatic latent image.

  Hereinafter, the period from the beginning of the area where one line of the electrostatic latent image is to be formed to the beginning of the area where the next line is to be formed is referred to as “main scanning” on the surface of the photosensitive drum 25Y,. Period. Further, from the above definitions of “main scanning direction” and “sub-scanning direction”, the longitudinal direction of the folding mirrors 28Y,... And the rotation direction of the polygon mirror 271 correspond to the main scanning direction, and the folding mirrors 28Y,. The height direction and the axial direction of the polygon mirror 271 correspond to the sub-scanning direction.

Further, the fθ lens 273 has a characteristic that “the incident angle of transmitted light is proportional to the image height (distance from the optical axis of the image forming point)”, so that the polygon mirror 271 can change the deflection angle of the outgoing light beam LL from the light source 260. The amount to be changed is proportional to the distance that the spot of the irradiated light beam moves on the surface of the photosensitive drum 25Y,. Referring to FIG. 2, when the deflection angle changes from the maximum value φ _R to the angle 2θ, the mirrors 28Y,... Reflect the transmitted light from the fθ lens 273 accordingly. , Move on. The moving distances ρY, ρM, ρC, and ρK at this time are proportional to the change amount of the angle at which the reflected light beam RL from the polygon mirror 271 enters the fθ lens 273, that is, the change amount 2θ of the deflection angle, due to the characteristics of the fθ lens 273. . These movement distances ρY,... Are proportional to the movement distance of the spot of the irradiated light on the surface of each photosensitive drum 25Y,..., And the deflection angle change amount 2θ is proportional to the rotation angle change amount θ of the polygon mirror 271. Therefore, linearity is established between the position of each spot in the main scanning direction and the rotation angle of the polygon mirror 271. In particular, when the polygon mirror 271 rotates at a constant angular speed, each spot moves at a constant speed in the main scanning direction.

In FIG. 3B, two outgoing beams LL1, LL2 from the first semiconductor laser 26Y are connected to the photosensitive drum 25Y of the first image forming unit 21Y through the scanning optical systems 271, 273, 28Y, 29Y. It is a schematic diagram which shows two spots SP1, SP2. For the purpose of simplifying the drawing, FIG. 3B shows the two first folding mirrors 28Y and 29Y shown in FIG. 2 as a single mirror. Referring to FIG. 3B, the two outgoing light beams LL1 and LL2 represent the outgoing light beams from the two light emitting points (PE1, PE2) and (PS1, PS2) shown in FIG. . These outgoing rays LL1 and LL2 are reflected by the polygon mirror 271 to the deflection angles φ ₁ and φ ₂ and converted into two reflected rays RL1 and RL2. These reflected rays RL1 and RL2 are transmitted through the fθ lens 273, reflected by the first folding mirrors 28Y and 29Y, and connect the two spots SP1 and SP2 to the surface of the photosensitive drum 25Y. As shown in FIG. 3B, these spots SP1 and SP2 have a spacing PB in both the main scanning direction and the sub-scanning direction (both the X-axis direction and the Y-axis direction in FIG. 3B). PL is placed. The diameter of each of the spots SP1 and SP2 is about several μm to several tens of μm. The interval PB in the main scanning direction is about several tens of μm to several hundreds of μm. The interval PL in the sub-scanning direction is equal to the interval between lines and is about several μm to several tens of μm. The spots SP1 and SP2 move in the axial direction on the surface of the photosensitive drum 25Y while maintaining the intervals PB and PL as the polygon mirror 271 rotates. Thus, while the outgoing beams LL1 and LL2 from the semiconductor laser 26Y are reflected by one deflecting surface of the polygon mirror 271, the spots SP1 and SP2 cause two lines LN1 and LN2 on the surface of the photosensitive drum 25Y. Draw at the same time. Similarly, two outgoing beams from the other semiconductor lasers 26M,..., 26K also have spots similar to the two spots shown in FIG. 3B on the other photosensitive drums 25M,. Tying, these spots draw two lines simultaneously.

-Control unit-
The control unit 300 includes a group of electronic circuits mounted on one or a plurality of printed circuit boards built in the optical scanning unit 26, and using these, the scanning optical system such as the motor 272 of the polygon mirror 271 and the light source 260. Drive control is performed with the semiconductor laser 26Y including. In the former case, the control unit 300 particularly controls the motor 272 so that the rotational speed of the polygon mirror 271 matches the target value instructed from the main control unit 60 (see FIG. 4) described later. In the latter case, the control unit 300 modulates the amount of light emitted from the semiconductor lasers 26Y,.

  In this modulation, the control unit 300 first receives image data from a main control unit 60 (see FIG. 4), which will be described later, and based on the tone values of each color Y, M, C, K represented by the image data, The blinking pattern of the semiconductor lasers 26Y,. For example, the higher the gradation value of a pixel, the longer the emission time of the semiconductor laser for that pixel is adjusted. Next, the control unit 300 delays the timing at which this blinking pattern is modulated based on the gradation value of one line represented by the image data by a predetermined time between the semiconductor lasers 26Y,. During this time, one point on the surface of the intermediate transfer belt 23 moves from between one pair (22Y, 25Y) of the primary transfer roller and the photosensitive drum to between the next pair (22M, 25M). Determined by the time required for

Referring back to FIG. 2, the controller 300 includes a first mirror 301, a second mirror 302, and a scan start (SOS) sensor 303. The first mirror 301 is installed on the path of the light beam reflected by the polygon mirror 271 to the maximum deflection angle φ _R and reflects the light beam toward the second mirror 302. The second mirror 302 reflects the reflected light from the first mirror 301 and irradiates the SOS sensor 303 installed inside the control unit 300. The SOS sensor 303 includes a photodetector, detects the amount of light emitted from the second mirror 302, and notifies the control unit 300 with a signal (hereinafter referred to as “SOS signal”). The SOS sensor 303 detects the reflected light beam RL from the polygon mirror 271 and validates the SOS signal each time the polygon mirror 271 rotating at a constant angular velocity reflects the light beam LL emitted from the light source 260 to the maximum deflection angle φ _R ( Active). That is, if the SOS signal is a positive logic signal, the pulse is raised, and if the SOS signal is a negative logic signal, the pulse is lowered. Based on the timing at which the SOS signal becomes valid, the controller 300 synchronizes the blinking of the semiconductor lasers 26Y,... With the rotation of the polygon mirror 271.

In addition, the control unit 300 samples the amount of emitted light fed back from the semiconductor lasers 26Y, and adjusts the amount of light based on the obtained sample. At this time, the control unit 300 further corrects the amount of light to be emitted from the semiconductor lasers 26Y,... For each deflection angle. Details of adjustment and correction of the emitted light amount will be described later.
[Electronic control system of image forming apparatus]
FIG. 4 is a block diagram illustrating the configuration of the electronic control system of the printer 100. Referring to FIG. 4, in this electronic control system, in addition to the feeding unit 10, the image forming unit 20, and the fixing unit 30, the operation unit 50 and the main control unit 60 are communicably connected to each other through a bus 90. .

  The operation unit 50 receives a job request and data to be processed such as an image through a user operation or communication with an external electronic device, and transmits them to the main control unit 60. Referring to FIG. 4, the operation unit 50 includes an operation panel 51, a memory interface (I / F) 52, and a network (LAN) I / F 53. Operation panel 51 includes a push button, a touch panel, and a display. The operation panel 51 displays a GUI screen such as an operation screen and an input screen for various parameters on a display. The operation panel 51 also identifies the position of the push button or touch panel operated by the user, and transmits information related to the identification to the main control unit 60 as operation information. The memory I / F 52 includes a USB port or a memory card slot, through which data to be processed is taken directly from an external storage device such as a USB memory or a hard disk drive (HDD). The LAN / I / F 53 is connected to an external network NTW by wire or wireless, and receives data to be processed from other electronic devices connected to the network NTW.

  The main control unit 60 is an electronic circuit mounted on a single board, and the board is installed inside the printer 100. Referring to FIG. 4, the main control unit 60 includes a CPU 61, a RAM 62, and a ROM 63. The CPU 61 is constituted by one MPU, and implements various functions as a control body for the other elements 10, 20,... Connected to the bus 90 by executing various firmware. The RAM 62 is a volatile semiconductor memory device such as a DRAM or an SRAM, and provides a work area when the CPU 61 executes firmware to the CPU 61 and stores data to be processed received by the operation unit 50. The ROM 63 is configured by a combination of a non-writable nonvolatile storage device and a rewritable nonvolatile storage device. The former stores firmware, and the latter includes a semiconductor memory device such as an EEPROM and a flash memory, or an HDD, and provides the CPU 61 with a storage area for environment variables and the like.

  When the CPU 61 executes various types of firmware, the main control unit 60 first controls other elements in the printer 100 based on operation information from the operation unit 50. Specifically, the main control unit 60 displays an operation screen on the operation unit 50 to accept an operation by the user. In response to this operation, the main control unit 60 determines an operation mode such as an operation mode, a standby mode, a sleep mode, etc., notifies the operation mode to other elements with a drive signal, and performs processing according to the operation mode. Let each element execute.

  For example, when the operation unit 50 receives a print job from the user, the main control unit 60 first causes the operation unit 50 to transfer image data to be printed to the RAM 62. Next, the main control unit 60 designates the type of sheet to be fed and the timing of the feeding in the feeding unit 10 according to the printing conditions indicated by the job, and the toner to be formed in the image forming unit 20. Image data representing an image is provided, and the fixing unit 30 is designated with the surface temperature of the fixing roller 31 to be maintained. In particular, the main control unit 60 selects target values for the sheet conveyance speed and the rotation speed of the polygon mirror 271 according to the image quality, power consumption, or sheet type or sheet thickness indicated by the printing conditions, and sets these target values. Is instructed to each element 10 of the printer 100. For example, when the paper type to be printed is thick paper, the sheet conveyance speed is set to be lower than the value for plain paper. Thereby, the power consumption of the drive motor 272 of the polygon mirror 271 is reduced, while the power consumption of the drive motors of the transport rollers 12,... Is stabilized regardless of the weighing of the sheet to be transported.

[Configuration for light source drive control]
The control unit 300 includes four drive circuits that individually control the four semiconductor lasers 26Y,. Each drive circuit is an electronic circuit such as an MPU / CPU, ASIC, or FPGA, and is incorporated in a single chip or a plurality of chips.
FIG. 5 is a block diagram of the drive circuit 300Y of the first semiconductor laser 26Y. Referring to FIG. 5, the drive circuit 300Y includes a sample hold (SH) unit 510, a correction unit 520, and a modulation unit 530, and controls supply currents IC1 and IC2 to the first semiconductor laser 26Y using these. To do. The amount of light emitted from the first semiconductor laser 26Y and the timing of blinking are determined by the current amounts IC1 and IC2 and the timing of flowing them. Although not shown in FIG. 5, the drive circuits of the other semiconductor lasers 26M,..., 26K have the same configuration.

  The SH unit 510 is provided one-to-one with the two light emitting points LD1 and LD2 included in the laser oscillator of the first semiconductor laser 26Y, and is supplied to the corresponding light emitting points LD1 and LD2 based on the output current amount IFB of the light quantity sensor PD. Reference values IB1 and IB2 of the amount of current to be determined are determined. The light quantity sensor PD is built in the first semiconductor laser 26Y when the laser oscillator is of the edge emitting type, and is installed on the back side of the fourth mirror 264 shown in FIG. 2, for example, when the laser oscillator is a VCSEL. In either case, the output current amount IFB of the light amount sensor PD is proportional to the sum of the light amounts actually emitted from the two light emitting points LD1 and LD2. On the other hand, the reference values IB1 and IB2 of the supply current amount correspond to the reference values of the emitted light amounts from the light emitting points LD1 and LD2, for example, the maximum values that the gradation value represented by the image data can take.

  The correction unit 520 cancels out unevenness in the amount of light caused by the scanning optical system by correcting the amount of light emitted from the two light emitting points LD1 and LD2. More specifically, the correction unit 520 first determines, based on the SOS signal, two spots SP1 and SP2 (FIG. 3 (b) in FIG. ))) In the main scanning direction. Next, the correction unit 520 selects a correction value for the amount of light emitted from each of the light emitting points LD1 and LD2 according to these positions, and outputs the output from the SH unit 510 so that the amount of light emitted is corrected by the correction value. The reference values IB1 and IB2 of the supply current amount are corrected to the actually required supply current amounts IC1 and IC2.

  The modulation unit 530 is provided in a one-to-one relationship with the two light emission points LD1 and LD2, and based on the gradation values of yellow (Y) of two adjacent lines represented by the image data VDS, each light emission point LD1 from the correction unit 520. The currents IC1 and IC2 actually supplied to the LD2 are modulated. Each light emitting point LD1, LD2 blinks in accordance with this modulation, so that the average amount of emitted light is adjusted to a value corresponding to the Y gradation value.

[SH part]
FIG. 6 is a block diagram illustrating details of the circuit configuration of the SH unit 510. Referring to FIG. 6, the SH unit 510 is associated with one of the two light emitting points LD1 and LD2 included in the first semiconductor laser 26Y (hereinafter referred to as “first light emitting point”) and supplies the light to the LD1. A reference value IB1 for the power amount is determined. The SH unit 510 includes a resistor 511, a reference voltage source 512, a differential amplifier 513, a switch 514, a capacitor 515, and a voltage / current (VI) converter 516. Although not shown in FIG. 6, the SH unit associated with the other light emitting point LD1, LD2 (hereinafter referred to as “second light emitting point”) has the same configuration.

  The resistor 511 is connected between the output terminal of the light quantity sensor PD and the ground conductor. Since one resistor 511 is provided for each of the two SH units 510, the output current IFB of the light quantity sensor PD flows through each resistor 511 in a ratio of the inverse ratio of the resistance value. Therefore, the voltage drop amount VFB at the resistor 511 is proportional to the output current IFB of the light amount sensor PD, and is therefore proportional to the sum of the light amounts emitted from the two light emitting points LD1 and LD2.

The reference voltage source 512 is a constant voltage source, and its output voltage VRF is equal to the voltage drop amount VFB of the resistor 511 under the condition that the amount of light emitted from any of the light emitting points LD1 and LD2 is equal to a predetermined reference value.
One of the two input terminals of the differential amplifier 513 is connected to the output terminal of the light quantity sensor PD, and the other is connected to the reference voltage source 512. As a result, the differential amplifier 513 outputs a constant current ISH in an amount proportional to the difference between the voltage drop amount VFB of the resistor 511 and the output voltage VRF of the reference voltage source 512. In particular, the sign of the difference between both voltages VFB and VRF determines the direction of the output current ISH. Since the difference between the two voltages VFB and VRF is proportional to the difference between the sum of the amounts of light actually emitted from the two light emitting points LD1 and LD2 and the sum of the reference values of the amounts of emitted light, the amount of the output current ISH Is proportional to the difference, and the direction represents the sign of the difference.

The switch 514 connects or disconnects between the output terminal of the differential amplifier 513 and one end of the capacitor 515 in accordance with a first instruction signal SHS1 from a timing generation unit 524 described later. First indication signal SHS1 is instantaneously change the deflection angle of the outgoing light ray LL from the first light emitting point LD1 from the end of each main scan period over the early next main scan period from a minimum phi _L to a maximum value phi _R The time period during which the deflection angle changes from the maximum value φ _R to the minimum value φ _L at a constant speed (hereinafter referred to as “effective scanning period”). ) Is disabled. Accordingly, the switch 514 is closed during the blanking period to maintain the connection between the output terminal of the differential amplifier 513 and one end of the capacitor 515, and is opened during the effective scanning period to disconnect the connection.

  Since the capacitor 515 holds a predetermined amount of charge in advance, the voltage VSH between both ends thereof is maintained at a predetermined value while the switch 514 is open. Since the switch 514 is closed during the retrace period, the capacitor 515 is charged or discharged by the output current ISH of the differential amplifier 513, and the voltage VSH across the terminal fluctuates accordingly. The amount of variation is determined by the amount of output current ISH, and the polarity is determined by the direction of output current ISH. On the other hand, since the switch 514 is opened during the effective scanning period, the capacitor 515 substantially keeps the voltage VSH between both ends constant at the value after the fluctuation.

  The VI converter 516 makes the output current amount IB1 proportional to the voltage VSH across the capacitor 515. A reference value for the amount of light emitted from the first light emitting point LD1 is determined by the output current amount IB1. Since the voltage VSH across the capacitor 515 fluctuates in accordance with the change in the output current ISH of the differential amplifier 513 in the blanking period, the VI converter 516 changes the output current amount IB1 according to the fluctuation. In particular, since the output current ISH of the differential amplifier 513 is proportional to the difference between the sum of the amounts of light emitted from the two light emitting points LD1 and LD2 and the sum of their reference values, the VI converter 516 cancels the difference. The output current amount IB1 is adjusted as follows. On the other hand, the voltage VSH across the capacitor 515 is kept constant during the effective scanning period, so the VI converter 516 maintains the output current amount IB1 at a value corresponding to the voltage VSH. By performing this maintenance in both of the two SH units 510, the sum of the amounts of light emitted from the two light emitting points LD1 and LD2 before being modulated by the image data VDS is maintained at the sum of their reference values. .

[Correction section]
Referring back to FIG. 5, the correction unit 520 includes a storage unit 521, an oscillation unit 522, a pulse width modulation (PWM) unit 523, a timing generation unit 524, and a switching unit 525.
-Storage unit-
The storage unit 521 includes a rewritable nonvolatile storage element such as an EEPROM or a flash memory, and manages data stored in these. This data includes spot interval information PTC, correction section information CRP, and correction value information CRV. The “spot interval information” PTC is an interval in the main scanning direction between two spots SP1 and SP2 that the light beams LL1 and LL2 emitted from the two light emitting points LD1 and LD2 included in the first semiconductor laser 26Y connect to the surface of the photosensitive drum 25Y. PB (see FIG. 3B) is defined. The “correction section information” CRP defines the position of the boundary of the correction section. “Correction value information” CRV defines a correction value for the amount of light emitted from each light emitting point LD1, LD2 for each boundary of the correction section.

The “correction section” refers to each section when a range that can be taken by coordinates in the main scanning direction (hereinafter referred to as “main scanning position”) associated with the surface of the photosensitive drum 25Y is divided into a plurality of sections. Say. The spot interval defined by the spot interval information PTC is represented by the difference in the main scanning position, and the position of the boundary of the correction section defined by the correction section information CRP is represented by the value of the main scanning position. Particularly in the region on the surface of the photosensitive drum 25Y to which the entire correction section is associated, the deflection angles φ ₁ of the outgoing rays LL1 and LL2 from the light emitting points LD1 and LD2 in the effective scanning period of each main scanning period, phi ₂ is includes the entire range the spot SP1, SP2 is moving at a constant speed in the main scanning direction according to the change at a constant rate from the maximum value phi _R to the minimum value phi _L. Each spot SP1, the main scanning position is the deflection angle phi ₁ of the outgoing light beam LL1, LL2 of SP2, uniquely determined by phi _2, the deflection angle phi _1, phi ₂ is the elapsed time of the main scan period, i.e., from the polygon mirror 271 It is uniquely determined by the time that has elapsed since the reflected light rays RL1 and RL2 were detected by the SOS sensor 303. Therefore, the main scanning position can be identified with the local part itself on the surface of the corresponding photosensitive drum 25Y and the elapsed time of the main scanning period until the spot reaches the local part. Those skilled in the art are unlikely to confuse them. Therefore, in the following description, the term “correction section” is used to refer to the meaning of the area on the surface of the corresponding photosensitive drum 25Y. It is also used for the meaning of the time zone when.

-Oscillator-
The oscillating unit 521 generates a clock (CLK) signal having a constant frequency of about several MHz to several tens of MHz using a crystal oscillator or the like, and synchronizes the CLK signal with the SOS signal using a phase synchronization circuit (PLL) or the like. Specifically, for example, the oscillating unit 521 shapes the CLK signal into a rectangular pulse wave with a duty ratio of 50% and validates the rising edge of the SOS signal (that is, if the positive logic signal, the rising edge of the pulse, the negative logic signal) If so, it is made to coincide with the falling timing. This timing is equal to the detection timing of the outgoing light beam LL1, LL2 from the two light emitting points LD1, LD2 which is detected earlier by the SOS sensor 303. This is detected by the SOS sensor 303 between the two light beams LL1 and LL2 because the spots SP1 and SP2 connecting the two light beams LL1 and LL2 on the surface of the photosensitive drum 25Y have a space PB in the main scanning direction. This is due to the difference in timing. Hereinafter, the one detected first by the SOS sensor 303 is an outgoing light beam LL1 from the first light emitting point LD1, and the other one is an outgoing light beam LL2 from the second light emitting point LD2. In this case, the rising number of the CLK signal after this detection time is treated as the elapsed time of the effective scanning period for the first light emitting point LD1. That is, this elapsed time is expressed by the number of pulses (number of clocks) in units of one cycle (clock cycle) of the CLK signal, that is, by clock units.

-PWM part-
The PWM unit 523 performs PWM on the CLK signal, and matches the duty ratio of the modulated clock (MCL) signal with the target value. The PWM unit 523 determines the target value based on the spot interval PB defined by the spot interval information PTC. Thereby, when the spot SP1 (hereinafter referred to as “first spot”) of the light beam emitted from the first light emitting point LD1 reaches a certain main scanning position on the surface of the photosensitive drum 25Y when the MCL signal rises, A spot SP2 (hereinafter referred to as “second spot”) of the light beam emitted from the second light emitting point LD2 reaches the position at the time of one falling edge of the MCL signal.

  FIG. 7A is a timing chart showing the correspondence between the time when the spots SP1 and SP2 arrive at the same main scanning position on the surface of the photosensitive drum 25Y and the pulse waveforms of the CLK signal and the MCL signal. . Referring to FIG. 7A, at this position, the first spot SP1 reaches one rising time TS of the CLK signal. On the other hand, the second spot SP2 arrives at this position later than the first spot SP1 by a time DLY. The delay time DLY represents the time required for the second spot SP2 to move a distance equal to the spot interval PB.

  The interval between the light emitting points LD1, LD2 in the laser oscillator of the first semiconductor laser 26Y, the rotational speed of the polygon mirror 271 and the scanning so that the delay time DLY is equal to a half integer multiple (odd / 2 times) of the clock period PCL. The clock cycle PCL is set based on the magnification of the optical system. Therefore, ideally, the arrival time of the second spot SP2 coincides with one falling time TS of the CLK signal. However, actually, since the interval between the light emitting points actually includes an error, the actual arrival time deviates from the falling time TS. Therefore, the target value of the duty ratio of the MCL signal is set so that the falling time of the MCL signal coincides with the actual arrival time. That is, this target value is set to the decimal part of the value representing the delay time DLY in clock units. The PWM unit 523 makes the actual duty ratio of the MCL signal coincide with this target value. As a result, the second spot SP2 reaches the main scanning position on the surface of the photosensitive drum 25Y where the first spot SP1 reaches when the CLK signal rises, when the MCL signal falls.

  The setting of the target value of the duty ratio of the MCL signal is performed, for example, based on the interval PB in the main scanning direction between the two spots SP1 and SP2 measured at the time of manufacturing the optical scanning unit 26. Specifically, when the arrival time TSH of the second spot SP2 is earlier than the falling time TS of the CLK signal, the difference ΔS = TSH−TS between the two times with respect to the clock cycle PCL (in FIG. 7A) 20%), the target value is set to be smaller than the duty ratio of the CLK signal (50% -20% = 30% in FIG. 7A). Conversely, when the arrival time TSL of the second spot SP2 is later than the falling time TS of the CLK signal, the difference between the two times ΔL = the ratio of TS−TSL to the clock period PCL (20% in FIG. 7A). ), The target value is set larger than the duty ratio of 50% (50% + 20% = 70% in FIG. 7A).

  As described above, the target value of the duty ratio of the MCL signal corresponds to the decimal part of the value representing the delay time DLY of the second spot SP2 with respect to the first spot SP1 in units of clocks. Therefore, the delay time DLY is reproduced by multiplying the sum of the integer part of the value (hereinafter referred to as “the number of delay clocks”) and the target value by the clock period PCL. Further, the spot interval PB is obtained by multiplying the delay time DLY by the spot moving speed. Therefore, the spot interval information PTC defines the spot interval PB by a combination of the target value of the duty ratio of the MCL signal and the number of delayed clocks. In this case, the PWM unit 523 may apply the target value read from the spot interval information PTC to the PWM as it is.

  However, since the delay time DLY is a time required for the second spot SP2 to move a distance equal to the spot interval PB, it depends on the moving speed of the second spot SP2. This moving speed is proportional to the rotational speed of the polygon mirror 271, and this rotational speed is set higher as the sheet conveyance speed is higher. This conveyance speed generally varies depending on the image quality indicated by the printing conditions, power consumption, or the paper type or thickness of the sheet. Therefore, the spot interval information PTC generally defines the delay time DLY for each sheet conveyance speed. In particular, the delay time DLY is set to a shorter value as the sheet conveyance speed is higher.

  FIG. 7B is a correspondence table between the delay time DLY of the second spot SP2 with respect to the first spot SP1 and the sheet conveyance speed. For example, the spot interval information PTC defines the delay time DLY in the form of this table. Referring to FIG. 7B, in this table, the combination of the number of delay clocks and the target value of the duty ratio of the MCL signal differs for each sheet conveyance speed. Specifically, a combination of the delay clock number “3” and the target value 70% is assigned to the transport speed index “high speed”, and the delay clock number “4” is targeted to the index “low speed”. A combination with the value 30% is assigned. These indicate that the delay time DLY is equal to the values “3.7” and “4.3” in units of clocks. As described above, the higher the sheet conveyance speed, the shorter the delay time DLY is.

  The storage unit 521 selects in advance a value to be referred to the PWM unit 523 from among these delay times DLY defined by the spot interval information PTC. For the purpose of instructing this selection, the control unit 300 acquires the currently set sheet conveyance speed from the main control unit 60 before the PWM unit 523 accesses the storage unit 521, and a delay corresponding to this speed. The storage unit 521 is made to search for the value of the time DLY.

-Timing generator-
The timing generation unit 524 is, for example, a single logic element, generates instruction signals SHS1 and SHS2 based on the SOS signal and the MCL signal, and generates timing signals TMS1 and TMS2 based on the correction interval information CRP and the MCL signal. To do. In the example of FIG. 5, the instruction signals SHS1 and SHS2 and the timing signals TMS1 and TMS2 are all the same in number as the light emitting points LD1 and LD2 of the laser oscillator included in the first semiconductor laser 26Y. One type of instruction signals SHS1 and SHS2 are sent to the SH unit 510 individually corresponding to the light emitting points LD1 and LD2, and one type of timing signals TMS1 and TMS2 are sent to the switching unit 525.

  Although not shown in FIG. 5, the timing generation unit 524 includes a counter. This counter counts the rise or fall of the MCL signal. The timing generation unit 524 monitors the value of this counter, and uses this value to indicate the instruction signals SHS1, SHS2 or the timing signals TMS1, TMS2 in accordance with the timing at which the number of rises or falls of the MCL signal reaches a predetermined number. Enable or disable.

FIG. 8 is a timing chart of signals related to the timing generation unit 524. In the example of FIG. 8, the SOS signal, the instruction signal SHS1, and the instruction signal SHS2 are negative logic signals, and the timing signals TMS1 and TMS2 are positive logic signals.
<SOS signal>
One cycle SCT of the SOS signal (a period from one falling time T0 to the next falling time T3) represents one main scanning period. In this period SCT, both deflection angles φ ₁ and φ ₂ of the outgoing beams LL1 and LL2 from the two light emitting points LD1 and LD2 make one round trip in the range from the maximum value φ _R to the minimum value φ _L (FIG. 3). (See (b)).

  The timing generation unit 524 synchronizes the SOS signal with the count of the number of pulses of the MCL signal (equal to the number of clocks) by a built-in counter. Specifically, the timing generation unit 524 resets the counter value CNT to “0” in response to the fall of the SOS signal. In the example of FIG. 8, the counter then increments the value CNT by 1 each time the MCL signal rises. That is, this value CNT represents the number of rises of the MCL signal in the current main scanning period SCT.

<Instruction signal>
In each main scanning period SCT, the effective period FBR (the period from one falling time to the next rising time) of the first instruction signal SHS1 is the blanking period (the deflection angle is the minimum value φ _L ) for the first light emitting point LD1. Represents an instantaneous change period from the first to the maximum value φ _R ), and an invalid period ESR (a period from one rising time T1 to the next falling time T2) is an effective scanning period (deflection angle) for the first light emitting point LD1. Represents a period during which changes from the maximum value φ _R to the minimum value φ _L at a constant speed). Further, the effective period FBR of the second instruction signal SHS2 represents a blanking period for the second light emitting point LD2, and the invalid period ESR (the period from one rising time T1 to the next falling time T2) is the second light emitting point. It represents the effective scanning period for LD2. The SH unit 510 corresponding to each of the light emitting points LD1 and LD2 turns on the switch 514 in response to the fall of the instruction signals SHS1 and SHS2, and turns off in response to the rise. Therefore, the ON period of the switch 514 coincides with the blanking periods FBR and FBR for the corresponding light emitting point.

  The timing generator 524 generates the first instruction signal SHS1 as follows. First, the timing generation unit 524 matches the rise of the MCL signal when the counter value CNT reaches the first threshold value (“2” in the example of FIG. 8) (in the example of FIG. 8, the third scan in the main scanning period SCT). At the time of rising (T1), the first instruction signal SHS1 is raised. The first threshold “2” is equal to a value representing the time length from the start point T0 of the main scanning period SCT to the start point T1 of the effective scanning period ESR for the first light emission point LD1 in units of clocks when the SOS signal falls. . Next, the timing generation unit 524 matches the rising edge of the MCL signal when the counter value CNT reaches the second threshold value “N” (>> “1”) (in the example of FIG. 8, (N + 1) in the main scanning period SCT). The first instruction signal SHS1 is lowered at the second rise time T2. The second threshold “N” represents the time length from the start point T0 of the main scanning period SCT to the end point T2 of the effective scanning period ESR for the first light emission point LD1 in units of clocks.

In this way, the timing generation unit 524 sets the time length from the start point T1 and the end point T2 of the effective scanning period ESR for the first light emission point LD1 represented by the first instruction signal SHS1, and the time length from the start point T0 of each main scanning period to any main scanning. The common values (clock numbers “2” and “N” in the example of FIG. 8) are also maintained in the period SCT.
The timing generation unit 524 generates the second instruction signal SHS2 as follows. First, the timing generation unit 524 reads one of the number of delay clocks defined by the spot interval information PTC from the storage unit 521. The number of delay clocks is selected in advance by the control unit 300 to a value (see FIG. 7B) corresponding to the sheet conveyance speed set by the main control unit 60 at the present time. Next, the timing generator 524 sets the sum (“5”) of the number of delayed clocks (“3” in the example of FIG. 8) and the first threshold (“2”) as the third threshold, and this delayed clock. The sum (“N + 3”) of the number (“3”) and the second threshold (“N”) is set as the fourth threshold. Thereafter, when the counter value CNT reaches the third threshold value (“5”), the timing generator 524 synchronizes with the next falling edge of the MCL signal (in the example of FIG. 8, the sixth rising edge in the main scanning period SCT). The second instruction signal SHS2 is raised at the time T1. Accordingly, the start point T1 of the effective scanning period ESR for the second light emitting point LD2 is set after the delay time DLY has elapsed from the start point T1 of the effective scanning period ESR for the first light emitting point LD1. This delay time DLY is equal to the sum of the delay clock number “3” and the duty ratio of the MCL signal (= pulse width α of the MCL signal / clock cycle PCL) in units of clocks. Next, when the counter value CNT reaches the fourth threshold value (“N + 3”), the timing generation unit 524 synchronizes with the next falling edge of the MCL signal (in the example of FIG. 8, (N + 4) in the main scanning period SCT). The second instruction signal SHS2 is lowered at the second falling time T2. Thus, the end point T2 of the effective scanning period ESR for the second light emitting point LD2 is set after the elapse of the delay time DLY from the end point T2 of the effective scanning period ESR for the first light emitting point LD1.

Thus, in any main scanning period SCT, the timing generation unit 524 sets the start point T1 and the end point T2 of the effective scanning period for the second light emitting point LD2, and the starting point T1 of the effective scanning period ESR for the first light emitting point LD1, respectively. Delayed by a common delay time DLY from the end point T2.
<Timing signal>
The rise of the first timing signal TMS1 represents the timing at which the spot SP1 of the outgoing light beam LL1 from the first light emitting point LD1 reaches one of the boundaries of the correction section. Each rising edge of the second timing signal TMS2 represents the timing at which the spot SP2 of the outgoing light beam LL2 from the second light emitting point LD2 reaches one of the boundaries of the correction section. The switching unit 525 corresponding to each light emitting point LD1, LD2 changes the correction value for the amount of light emitted from the light emitting point LD1, LD2 in response to the rise of the timing signals TMS1, TMS2.

  The timing generation unit 524 generates timing signals TMS1 and TMS2 as follows. First, the timing generation unit 524 reads the correction section information CRP from the storage unit 521 in accordance with the falling edge of the SOS signal. In the example of FIG. 8, the correction section information CRP defines main scanning positions CP1 = 4, CP2 = 6, CP3 = 8,. Next, the timing generator 524 raises the first timing signal TMS1 at a time point TC1 when the counter value CNT reaches the main scanning position CP1 = 4 at the first boundary. When the counter value CNT reaches the value “7” which is larger than the main scanning position CP1 = 4 by the delay clock number “3”, the timing generation unit 524 generates the second timing signal at the next falling time TC1 of the MCL signal. Launch TMS2. Both timing signals TMS1 and TMS2 fall when the clock cycle PCL elapses from the rise. Subsequently, the timing generation unit 524 raises the first timing signal TMS1 at a time point TC2 when the counter value CNT reaches the main scanning position CP2 = 6 of the next boundary. When the counter value CNT reaches the value “9” which is larger by the delay clock number “3” than the main scanning position CP2 = 6, the timing generator 524 outputs the second timing signal at the next falling time TC2 of the MCL signal. Launch TMS2. Thereafter, similarly, the timing generator 524 reaches the time point when the counter value CNT reaches the main scanning position CPk (integer k = 3,..., N, n + 1,... (Integer n ≧ 4)) defined by the correction section information CRP. The first timing signal TMS1 is raised at TCk, and the second timing signal TMS2 is raised at the next falling time TCk of the MCL signal when the delay clock number reaches a value larger than the main scanning position CPk by “3”. .

  Each rising edge of the timing signals TMS1 and TMS2 particularly indicates the following. The time TCk at which the spot SP2 of the outgoing light beam from the second light emitting point LD2 reaches the kth boundary of the correction interval is more common than the time TCk at which the spot SP1 of the outgoing light beam from the first light emitting point LD1 reaches the same boundary. Delay time DLY = (delay clock number “3” + duty ratio of MCL signal) × clock period PCL = 3 × PCL + pulse width α of MCL signal. Furthermore, the rising interval TCk−TC (k + 1) of the first timing signal TMS1 represents the kth correction section CSk in the correction for the amount of light emitted from the first light emitting point LD1. On the other hand, the rising interval TCk−TC (k + 1) of the second timing signal TMS2 represents the kth correction section CSk in the correction for the amount of light emitted from the second light emitting point LD2. Therefore, the second light emitting point LD2 passes through each correction section CSk with a delay time DLY = 3 × PCL + α from the first light emitting point LD1.

-Switching section-
Similar to the SH unit 510, the switching unit 525 is provided in one-to-one correspondence with the two light emitting points LD1 and LD2 included in the laser oscillator of the first semiconductor laser 26Y. As shown in FIG. 6, the switching unit 525 includes a digital-analog converter (DAC) 600. In the switching unit 525 corresponding to the first light emission point LD1, the DAC 600 first sequentially reads out the correction values indicated by the correction value information CRV from the storage unit 521 in response to the rise of the first timing signal TMS1. Next, the DAC 600 amplifies the output current IB1 of the VI converter 516 of the SH unit 510 at a rate based on this correction value. As a result, the correction value that is a digital value is converted into the actual supply current amount IC1 to the first light emitting point LD1 that is an analog value. The switching unit corresponding to the second light emitting point LD2 operates in the same manner according to the rising edge of the second timing signal TMS2.

  The switching unit 525 corresponding to the different light emitting points LD1 and LD2 synchronizes the timing for reading the correction value from the storage unit 521 with the rise of the different timing signals TMS1 and TMS2. Thereby, the change of the correction value by the DAC 600 is synchronized with the rise of the MCL signal with respect to the amount of light emitted from the first light emission point LD1, and at the fall of the MCL signal with respect to the amount of light emitted from the second light emission point LD2. Synchronize.

  The correction value defined by the correction value information CRV is expressed by the ratio of the corrected emitted light quantity with respect to the reference value of the emitted light quantity from the first light emitting point LD1 for each boundary of the correction section CSk. On the other hand, the SH unit 510 corresponding to the first light emitting point LD1 sets the output current amount IB1 of the VI converter 516 when the amount of emitted light matches the reference value during the ON period of the switch 514 (that is, the blanking period FBR). The value is adjusted and maintained during the off period of switch 514 (ie, the effective scanning period ESR). Therefore, the switching unit 525 corresponding to the first light emission point LD1 matches the amplification factor of the DAC 600 with the correction value at each rising edge of the first timing signal TMS1. Thereby, when the spot SP1 of the emitted light from the first light emitting point LD1 reaches each boundary of the correction section CSk, the amount of emitted light is a correction value (= amplified current amount IC1 / before amplification). Of the current amount IB1). This change cancels fluctuations in the amount of irradiated light that would appear at each boundary of the correction section CSk if the amount of emitted light remains the reference value. As a result, the reference value of the actual irradiation light quantity is made uniform at each boundary, that is, unevenness in the light quantity is removed. The same applies to the amount of light emitted from the second light emitting point LD2.

  Further, the switching unit 525 linearly interpolates the correction values defined by the correction value information CRV with respect to the boundaries at both ends of each correction section CSk, and linearly changes the amplification factor of the DAC 600 according to the obtained interpolation values. Specifically, for example, the switching unit 525 first obtains the time length of each correction section CSk from the correction section information CRP, and calculates the ratio of this time length to a fixed minute time. Next, the switching unit 525 equally divides the difference in correction values between the boundaries at both ends of the correction section. The switching unit 525 sets the number of divisions at this time to the calculated ratio. Subsequently, the switching unit 525 changes the amplification factor of the DAC 600 from the correction value at the start of the correction section by the value divided from the difference per minute time. As a result, the amplification factor of the DAC 600 changes to a correction value for the next boundary between each rising edge of the timing signals TMS1 and TMS2, strictly speaking, in a fine step shape, approximately linearly. .

  FIG. 9A is a graph showing the correspondence between the correction value set by the switching unit 525 and the main scanning position on the surface of the photosensitive drum 25Y. Referring to FIG. 9A, the horizontal axis of the graph represents the main scanning position as a counter value CNT (= 0, 1, 2,...) Built in the timing generation unit 524. This value CNT is equivalent to a value representing the elapsed time of each main scanning period in units of clocks. On the other hand, the vertical axis of the graph represents the correction value for the amount of light emitted from the first light emitting point LD1 as a ratio to the minimum value “1.00”. Further, black points CP1, CP2, CP3,..., CP (n-1), CPn, CP (n + 1),... Plotted in the graph are correction intervals CS1, CS2, CS3,. The boundary position of CSn, CS (n + 1),... And the correction value there are shown. FIG. 9B is a correspondence table between the main scanning position of the boundary of the correction section and the correction value. The combinations of values shown in this table correspond to the coordinates of the black points CP1,... Plotted in the graph of FIG. Of each coordinate, the main scanning position is defined by the correction section information CRP, and the correction value is defined by the correction value information CRV.

  A broken line CRC connecting the black points CP1,... Shown in FIG. 9A is hereinafter referred to as a “correction curve”. This correction curve CRC represents the correspondence between the correction value for the amount of light emitted from the first light emitting point LD1 and the main scanning position, which is really necessary to cancel the fluctuation of the amount of light applied to the photosensitive drum 25Y. When the amount of emitted light is corrected along the correction curve CRC, the variation in the amount of light applied to the photosensitive drum 25Y that would appear if the amount of emitted light is constant is canceled out in the entire exposure target range. The correction curve CRC is estimated by calculation from the refractive indexes of the optical elements included in the scanning optical system on the optical path from the light source 260 to the photosensitive drum 25Y, such as the polygon mirror 271 and the fθ lens 273, or these optical paths are actually measured by experiments. Measured from the amount of light that passed through. From the correction values continuously indicated by the correction curve CRC, the discrete correction values indicated by the black points CP1,... Are sampled, and the main scanning position from which these samples are obtained is recorded in the correction section information CRP. The correction value shown is recorded in the correction value information CRV. Details of the correction curve CRC will be described later.

  The black spots CP1,... Are further connected by a solid broken line CRB. This broken line CRB represents the correspondence between the linear interpolation value calculated by the switching unit 525 from the correction values indicated by the black points CP1,. Hereinafter, this broken line CRB is referred to as an “interpolation line”. The interpolation line CRB is a straight line between the black points CP1,. This is because the switching unit 525 linearly changes the amplification factor IC1 / IB1 in the DAC 600 from each rising edge of the first timing signal TMS1 to the next rising edge. On the other hand, at each black point CP1,..., The angle of the interpolation line CRB changes discontinuously. This is because the switching unit 525 changes one of the correction values to be interpolated to the next correction value indicated by the correction value information CRV in response to each rising edge of the first timing signal TMS1. As shown in FIG. 9A, the interpolation line CRB has a high degree of approximation to the correction curve CRC. As a result, the change in the amount of light emitted from the first light emission point due to the amplification of the output current IC1 by the DAC 600 can substantially cancel the variation in the amount of light irradiated to the photosensitive drum 25Y in the entire exposure target range. .

  As indicated by the intervals between the black spots CP1,..., The interval between the boundaries of the correction section CSk is not uniform and varies depending on the main scanning position. That is, each boundary has a different distance to the adjacent boundary according to the main scanning position. In the example of FIG. 9A, the boundary interval ΔPD located in the second region STR of “16” or less is smaller than the boundary interval ΔPS located in the first region GNR where the main scanning position exceeds “16”. narrow. This is because the slope of the correction curve CRC is generally steeper in the second region STR than in the first region GNR, that is, the average value, maximum value, intermediate value, or maximum value of the change rate of the correction value with respect to the main scanning position. This is due to large statistical values such as frequent values. Details of the setting conditions of the correction section CSk will be described later.

[Modulation section]
5 and 6, the modulation unit 530 corresponding to the first light emission point LD1 opens and closes the built-in switching unit, and supplies the supply current IC1 between the switching unit 525 and the first light emission point LD1. Or shut off. Particularly in the effective scanning period ESC, the modulation unit 530 synchronizes the opening / closing operation of the switching unit with the CLK signal, and performs the pattern based on the Y gradation value represented by the image data VDS. As a result of the intermittent change in the supply current IC1, the blinking pattern of the first light emitting point LD1 is modulated into a pattern based on the Y gradation value. On the other hand, in the blanking period FBR, the modulation unit 530 maintains the closed state in the switching unit, so that the supply current IC1 constantly flows between the switching unit 525 and the first light emitting point LD1. As a result, the amount of light emitted from the first light emitting point LD1 is maintained at a value corresponding to the current amount IC1.

[Relationship between fluctuations in amount of light applied to photoconductor drum and correction curve]
FIG. 10A is a schematic diagram showing an angle at which the outgoing light beam LL from the light source 260 enters the polygon mirror 271. Referring to FIG. 10A, as the polygon mirror 271 rotates, the incident angles θ ₁ and θ ₂ of the outgoing light beam LL to the one deflection surface 701 change, so that the deflection surface 701 becomes a light beam RL. The angles θ ₁ and θ ₂ for reflecting the light also change.

FIG. 10B is a schematic diagram showing an angle at which the reflected light beam RL of the polygon mirror 271 enters the fθ lens 273. Referring to FIG. 10B, incident angles ξ ₁ and ξ _{2 of} the reflected light beam RL to the fθ lens 273 change with the rotation of the polygon mirror 271, so that the light beam RL passes through the fθ lens 273. Also the refraction angles η ₁ and η ₂ change.
In general, when light is obliquely incident on the boundary surface between the media, a part of the light is reflected at the boundary surface, while the other part is transmitted through the boundary surface. Furthermore, the ratio of the amount of light between the two portions varies depending on the incident angle. Therefore, the reflectance for the light beams LL with different incident angles θ ₁ and θ ₂ is different on the deflection surface 701 of the polygon mirror 271, and the transmittance for the light beams RL with different incident angles ξ ₁ and ξ ₂ is different on the lens surface of the fθ lens 273. Is different. Similarly, the reflection rates for light rays having different incident angles are different on the reflecting surfaces of the folding mirrors (28Y, 29Y),. Further, in the fθ lens 273, if the refraction angles η ₁ and η ₂ are different, the distance through which the lens material, for example, a transparent resin is transmitted, is different, so that the attenuation rate due to the light absorption by the material is different.

  As a result, the reflectance and transmittance of the optical elements included in the scanning optical system such as the polygon mirror 271 and the fθ lens 273 vary depending on the rotation angle of the polygon mirror 271, that is, the deflection angle with respect to the outgoing light beam LL from the light source 260. In this case, even if the light source 260 keeps the emitted light amount constant at the light emitting point of each laser oscillator during the main scanning period, the irradiated light amount to the photosensitive drums 25Y,... Changes in the deflection angle of the emitted light beam LL. It fluctuates with it.

  FIG. 10C is a graph showing fluctuations that appear in the amount of light applied to the photosensitive drum 25Y under the condition that the light source 260 keeps the amount of light emitted from the first light emitting point LD1 constant. The horizontal axis of this graph represents the main scanning position, and the vertical axis represents the fluctuation range of the irradiation light amount as a ratio to the maximum value. In the example of FIG. 10C, the following can be understood from the curve EXC (hereinafter referred to as “variation curve”) represented by this graph. First, the irradiation light quantity reaches a peak (= 100%) in the vicinity of the main scanning position “6”. Next, on both sides of this peak, in the range of “± 6” in units of clocks, as the distance from the peak increases, the amount of irradiated light suddenly attenuates by about 10% to 20%, and is minimal around the main scanning position “16”. Indicates the value. Further, at the main scanning position “20” or more, the irradiation light amount gradually changes at about 75 to 80% of the peak value. If this variation in the amount of irradiation light is directly reflected in the change in the exposure amount of the photosensitive drum 25Y, unevenness in the amount of light appears in each line of the toner image.

A curve obtained by graphing a change in the amount of emitted light from the first light emitting point LD1 necessary to cancel out this variation in the amount of irradiated light is a correction curve CRC shown in FIG. Specifically, the fluctuation curve EXC shown in FIG. 10C represents the fluctuation of the irradiation light quantity as a ratio with respect to the maximum value, so that the correction curve CRC can be obtained by plotting the inverse ratio for each main scanning position.
As described above, the unevenness in the amount of light is caused by the change in reflectance and transmittance accompanying a change in the optical path in the scanning optical system. Therefore, the shape of the correction curve CRC is also determined by how the optical path changes in the scanning optical system. Here, between the light beams emitted from the two light emitting points LD1 and LD2 included in the laser oscillator of the first semiconductor laser 26Y, the positions of the spots SP1 and SP2 connected to the surface of the photosensitive drum 25Y are the main scanning direction and the sub scanning. Since it differs in both directions (see FIG. 3B), the optical path in the scanning optical system is also strictly different. However, the difference that appears in the amount of light applied to the photosensitive drum 25Y due to the difference between these optical paths can be sufficiently ignored as compared with the magnitude of the undulation of the fluctuation curve EXC. Therefore, the correction curve CRC is considered to be common to the light rays emitted from the two light emitting points LD1 and LD2, and the correction unit 520 uses a common correction value in correcting the light quantity. Similarly, a common correction value is used for correcting the amount of emitted light between light emitting points included in the same laser oscillator.

[Conditions for setting the boundary of correction section]
A common correction value for the amount of light emitted from the two light emitting points LD1 and LD2 is set as follows. First, the fluctuation curve EXC shown in FIG. 10C is calculated from the model of the scanning optical system or measured by experiment. Next, the correction curve CRC shown in FIG. 9A is calculated from the inverse ratio of the fluctuation curve EXC, and the boundary CP1,... Of the correction section is set based on the correction curve CRC, and at each boundary CP1,. The correction value indicated by the correction curve CRC is sampled.

  The following conditions should be considered in setting the boundary CP1,. First, the smaller the interval between the boundaries CP1,..., The smaller the error of the interpolation line CRB with respect to the correction curve CRC, that is, the sampling error, between these boundaries. Therefore, it is desirable that the boundary interval be as narrow as possible. However, if the boundary interval is narrowed, the number of boundaries increases. On the other hand, since there is an upper limit on the capacity of the storage unit 521, there is an upper limit on the total amount of data of the correction section information CRP and the correction value information CRV that can be stored therein. This upper limit limits the total number of samples, ie the total number of boundaries. Next, when the boundary interval is constant, the sampling error is smaller as the undulation of the correction curve CRC between the boundaries is more gradual. Accordingly, in order to suppress the sampling error in the entire correction curve CRC without excessively increasing the total number of boundaries, the boundary interval may be narrowed preferentially for a region where the undulations of the correction curve CRC are severe.

  (A) of FIG. 11 is a graph which shows the setting conditions of the boundary of the correction area with respect to the correction curve CRC. Referring to FIG. 11A, the undulation of the correction curve CRC is larger in the second region STR than in the first region GNR. Therefore, the boundary interval is set narrower in the second region STR than in the first region GNR. Specifically, the first condition is applied to the first region GNR and the second condition is applied to the second region STR as the setting condition of the boundary of the correction section. The first condition is that “the difference between the correction values is within an allowable range between two adjacent boundaries”. The second condition is that “in the portion of the correction curve that changes monotonously, the difference between the correction values between two adjacent boundaries is made uniform”.

-Setting of correction section according to the first condition-
First, the front end CPI of the first region GNR and the point where the main scanning position differs from the rear end CPL by a constant value, for example, “8”, are set as the boundaries of the correction section. Next, the difference in the correction value between two adjacent boundaries is compared with an allowable upper limit, for example 2%. In FIG. 11A, the difference in correction value exceeds the allowable upper limit of 2% between the leading boundary CPI and the two adjacent boundaries CPk and CP (k + 1). On the other hand, it is below the allowable upper limit between other boundaries. Therefore, a new boundary is added between the three boundaries CPI, CPk, CP (k + 1), and the difference in the correction values between these boundaries is suppressed to an allowable upper limit or less.

  FIG. 11B is an enlarged view of a portion including the boundaries CPk and CP (k + 1) in the correction curve CRC shown in FIG. Referring to FIG. 11B, the difference between the correction values exceeds the allowable upper limit of 2% between the boundaries CPk and CP (k + 1). In this case, a new boundary CP + is added between the boundaries CPk and CP (k + 1). The new boundary CP + is set such that the difference between the correction value of the boundary CPk and CP (k + 1) from the correction value is 2% or less. Similarly, a new boundary is added between the boundaries CPI and CPk.

If a new boundary cannot be added due to the limitation on the total number of correction sections, the allowable upper limit of 2% for the difference in correction values between the boundaries can be increased under the condition that the sampling error remains within the allowable range. That's fine.
-Setting the correction interval according to the second-
First, boundaries CPT and CPB are set at the peak and valley bottom of the correction curve CRC in the second region STR, and the difference between the correction values, for example, about 25% is the number of boundaries that can be set between them. For example, it is equally divided into four. Next, a point where the correction value differs from the valley bottom CPB or the peak CPT by a division unit, for example, 25% / 4 = 5%, is set as a boundary. Thus, in the portion of the correction curve CRC that monotonously increases from the valley bottom CPB to the peak CPT, the difference between the correction values between two adjacent boundaries is equal to a constant value of 5%. When the second region STR extends to the outside of the peak CPT and the valley bottom CPB, the boundary is set in the same manner on the outside.

Note that the number of boundaries that can be set for the portion of the correction curve that changes monotonically is determined so that the total number of boundaries does not increase and the sampling error estimated from the correction value division unit falls within an allowable range. The
Regardless of which of the first condition and the second condition is applied, the boundary interval, that is, the width of the correction section is set to an integral multiple of the clock period. As a result, the timing generation unit 524 can easily detect the timing at which the spots SP1 and SP2 of the emitted light rays from the light emitting points LD1 and LD2 reach the respective boundaries from the timing of the rise and fall of the MCL signal. Therefore, the circuit configuration of the timing generation unit 524 is simplified.

[Control Flowchart for Optical Scanning Unit]
FIG. 12 is a flowchart of control for the optical scanning unit 26. This process is started in response to the start of the print job.
In step S101, the main control unit 60 causes the light source 260 to emit the semiconductor lasers 26Y,..., And activates the motor 272 in the scanning optical system control unit 300 to rotate the polygon mirror 271. As a result, the SOS sensor 303 enables the SOS signal in the period of the main scanning period. Thereafter, the process proceeds to step S102.

In step S102, the correction unit 520 generates an MCL signal from the SOS signal. Details of this processing will be described later. Thereafter, the process proceeds to step S103.
The loop from step S103 to step S110 is between four drive circuits 300Y,..., And in each drive circuit 300Y,..., Two light emitting points LD1, LD2 included in the laser oscillator of the semiconductor laser 26Y,. Processing is performed in parallel between two sets of the SH unit 510, the switching unit 525, and the modulation unit 530 that correspond one-to-one.

In step S103, the controller 300 monitors whether the instruction signals SHS1 and SHS2 generated by the drive circuits 300Y,... Are valid. For the SH unit 510 in which the instruction signal is valid, the process proceeds to step S104, and for the SH unit 510 in which the instruction signal is invalid, the process proceeds to step S105.
In step S104, the blanking period FBR or FBR for the light emitting point LD1 or LD2 corresponding to the SH unit 510 in which the instruction signal is valid. Therefore, in the SH unit 510, the switch 514 maintains the connection between the differential amplifier 515 and the capacitor 515. As a result, the capacitor 515 is charged and discharged by the output current ISH of the differential amplifier 515. Further, the VI converter 516 converts the voltage VSH between both ends after charging / discharging of the capacitor 515 into the supply current IB1. At this time, the output current ISH of the differential amplifier 515 is proportional to the difference VFB−VRF between the voltage drop amount VFB of the resistor 511 and the output voltage VRF of the reference voltage source 512, and this difference VFB−VRF is the two light emitting points LD1. , Proportional to the difference between the sum of the amounts of light emitted from the LD 2 and the sum of the reference values of the amounts of light emitted. Therefore, the supply currents IB1 and IB2 are adjusted so that the amounts of light emitted from the light emitting points LD1 and LD2 both coincide with the reference value. Thereafter, the process is repeated from step S103.

  Step S105 is an effective scanning period ESR or ESR for the light emitting point LD1 or LD2 corresponding to the SH unit 510 in which the instruction signal SHS1 or SHS2 is invalid. Therefore, in the SH unit 510, the switch 514 disconnects the connection between the differential amplifier 515 and the capacitor 515. As a result, the voltage VSH across the capacitor 515 is maintained at a constant value, so that the VI converter 516 maintains the supply current amount IB1 or IB2 at the reference value. Thereafter, the process proceeds to step S106.

In step S106, the control unit 300 monitors whether or not the timing signals TMS1 and TMS2 generated by the drive circuits 300Y,... Are valid. For the switching unit 525 for which the timing signal is valid, the process proceeds to step S107, and for the switching unit 525 for which the timing signal is invalid, the process proceeds to step S108.
In step S107, the spot SP1 or SP2 of the light beam emitted from the light emitting point LD1 or LD2 corresponding to the switching unit 525 for which the timing signal is valid reaches a new correction section. At this time, the switching unit 525 reads the correction value indicated by the correction value information CRV from the storage unit 521, and changes one of the correction values to be interpolated to the read correction value. Thereafter, the process proceeds to step S108.

In step S108, the switching unit 525 linearly interpolates the correction values for the boundaries at both ends of the new correction section, and linearly changes the amplification factor of the DAC 600 according to the obtained interpolation value. Thereafter, the process proceeds to step S109.
In step S109, the modulation unit 530 modulates the currents IC1 and IC2 that are actually supplied from the correction unit 520 to the light emitting points LD1 and LD2 based on the tone values of the respective colors represented by the image data VDS. Thereafter, the process proceeds to step S110.

  In step S110, the control unit 300 confirms whether or not unprocessed image data remains for the tone values of the colors Y, M, C, and K. For the drive circuits 300Y,..., Or 300K where unprocessed image data remains, the process is repeated from step S103, and for the drive circuits 300Y,. .

In step S111, the control unit 300 confirms that no unprocessed image data remains in all the drive circuits 300Y,... And notifies the main control unit 60 accordingly. In response to this notification, the main controller 60 causes the light source 260 to stop emitting the semiconductor lasers 26Y,... And causes the controller 300 to stop the rotation of the motor 272. Thus, the process ends.
[PWM processing for CLK signal]
FIG. 13 is a flowchart of a subroutine for generating an MCL signal in step S102 shown in FIG.

In step S <b> 121, the control unit 300 acquires the currently set sheet conveyance speed from the main control unit 60. Thereafter, the process proceeds to step S122.
In step S122, the control unit 300 stores the value of the delay time DLY corresponding to the conveyance speed acquired in step S121 from the spot interval information PTC in the storage unit 521, that is, the combination of the number of delay clocks and the target value of the duty ratio of the MCL signal. To search. Thereafter, the process proceeds to step S123.

In step S123, the oscillating unit 521 generates the CLK signal, and makes the rising edge of the CLK signal coincide with the timing of activation of the SOS signal. Thereafter, the process proceeds to step S124.
In step S124, the PWM unit 523 first acquires the target value of the duty ratio of the MCL signal searched in step S122 from the storage unit 521. Next, the PWM unit 523 performs PWM on the CLK signal to match the duty ratio of the MCL signal with the acquired target value. Thereafter, the processing returns to the flow shown in FIG. 12, and proceeds to step S103.

[Flowchart of signal processing by timing generator]
FIG. 14 is a flowchart of signal processing by the timing generation unit 524. This process is started in response to the start of the print job.
In step S201, the timing generation unit 524 first reads the number of delay clocks retrieved from the spot interval information PTC in step S122 from the storage unit 521. Next, the timing generation unit 524 sets the value obtained by adding the number of delayed clocks to the first threshold value with respect to the counter value CNT as the third threshold value, and sets the value added to the second threshold value as the fourth threshold value. Thereafter, the process proceeds to step S202.

In step S202, the timing generator 524 resets the counter value CNT to “0” in response to the fall of the SOS signal. Thereafter, the counter increments the value CNT by 1 every time the MCL signal rises. Thereafter, the process proceeds to step S203.
In step S203, the timing generation unit 524 reads out the main scanning position CPk (integer k = 1, 2, 3,...) At the boundary of the correction section defined by the correction section information CRP from the storage unit 521, and performs each main scanning. A main scanning position CPk that is larger than the position CPk by the number of delay clocks is specified. Thereafter, the process proceeds to step S204 and step S214.

Steps S204 to S210 are processes for generating the first instruction signal SHS1 and the first timing signal TMS1, and steps S214 to S220 are processes for generating the second instruction signal SHS2 and the second timing signal TMS2. is there. These processes are executed in parallel.
In step S204, the timing generation unit 524 checks whether or not the counter value CNT has reached the first threshold value T1-T0 (“2” in the example of FIG. 8) (CNT ≧ T1-T0). If it has reached, the process proceeds to step S205, and if not, the process repeats step S204.

  In step S205, since the counter value CNT has reached the first threshold value T1-T0, the timing generation unit 524 immediately disables the first instruction signal SHS1 (“rise” in the example of FIG. 8). As a result, the first light emitting point LD1 shifts from the blanking period FBR to the effective scanning period ESC. The timing generation unit 524 also sets the integer value variable n to the initial value “1”. Thereafter, the process proceeds to step S206.

In step S206, the timing generation unit 524 determines whether or not the counter value CNT has reached the main scanning position CPn of the nth boundary among the boundaries of the correction section defined by the correction section information CRP (CNT ≧ CPn). Check. If it has reached, the process proceeds to step S207, and if not, the process repeats step S206.
In step S207, since the counter value CNT has reached the main scanning position CPn at the nth boundary, the timing generator 524 immediately enables the first timing signal TMS1 (“rise” in the example of FIG. 8). . Thereafter, the process proceeds to step S208.

In step S208, the timing generation unit 524 checks whether or not the counter value CNT has reached the second threshold T2-T0 (“N” in the example of FIG. 8) (CNT ≧ T2-T0). If so, the process proceeds to step S209. If not, the process proceeds to step S210.
In step S209, since the counter value CNT has reached the second threshold value T2-T0, the timing generator 524 immediately enables the first instruction signal SHS1 ("fall" in the example of FIG. 8). As a result, the first light emitting point LD1 shifts from the effective scanning period ESC to the blanking period FBR. Thereafter, the process ends.

In step S210, since the counter value CNT has not reached the second threshold value T2-T0, the timing generation unit 524 increases the integer value variable n by “1”. Thereafter, the process is repeated from step S206.
In step S214, the timing generation unit 524 checks whether or not the counter value CNT has reached the third threshold value T1-T0 (“5” in the example of FIG. 8) (CNT ≧ T1-T0). If it has reached, the process proceeds to step S215; otherwise, the process repeats step S214.

  In step S215, since the counter value CNT has reached the third threshold value T1-T0, the timing generator 524 invalidates the second instruction signal SHS2 in accordance with the next falling edge of the MCL signal (in the example of FIG. 8). "Launch"). As a result, the second light emitting point LD1 shifts from the blanking period FBR to the effective scanning period ESC. The timing generation unit 524 also sets the integer value variable n to the initial value “1”. Thereafter, the process proceeds to step S216.

  In step S216, whether the counter value CNT has reached the main scanning position CPn that is larger than the main scanning position CPn of the nth boundary defined by the correction section information CRP by the number of delay clocks (“3” in the example of FIG. 8). The timing generation unit 524 checks whether or not (CNT ≧ CPn). If so, the process proceeds to step S217. If not, the process repeats step S216.

In step S217, since the counter value CNT has reached the main scanning position CPn, the timing generator 524 enables the second timing signal TMS2 in accordance with the next falling edge of the MCL signal (in the example of FIG. 8, “ Launch"). Thereafter, the process proceeds to step S218.
In step S218, the timing generation unit 524 checks whether or not the counter value CNT has reached the fourth threshold value T2-T0 (“N + 3” in the example of FIG. 8) (CNT ≧ T2-T0). If so, the process proceeds to step S219. If not, the process proceeds to step S220.

In step S219, since the counter value CNT has reached the fourth threshold value T2-T0, the timing generator 524 enables the second instruction signal SHS2 in accordance with the next falling edge of the MCL signal (in the example of FIG. 8). "Fall down"). As a result, the second light emitting point LD2 shifts from the effective scanning period ESC to the blanking period FBR. Thereafter, the process ends.
In step S220, since the counter value CNT has not reached the fourth threshold value T2-T0, the timing generation unit 524 increases the integer value variable n by “1”. Thereafter, the process is repeated from step S216.

[Advantages of the embodiment]
In the optical scanning unit 26 according to the embodiment of the present invention, as described above, the light source 260 emits two light beams from each of the semiconductor lasers 26Y,..., And the scanning optical systems 271, 273,. ,..., Or 25K is exposed and scanned. At this time, the correction unit 520 adjusts the duty ratio of the MCL signal in accordance with the interval PB in the main scanning direction between the spots SP1 and SP2 where the two light beams are connected to the surface of the same photosensitive drum 25Y. The correction unit 520 further selects a correction value for the amount of light emitted from the first light emitting point LD1 included in the laser oscillator of the semiconductor laser 26Y in synchronization with the rise of the MCL signal, and corrects the amount of light emitted from the second light emitting point LD2. The value is selected in synchronization with the falling edge of the MCL signal. Thereby, even if the interval PB in the main scanning direction of the spots SP1 and SP2 varies between products, the optical scanning unit 26 can appropriately correct the timing of changing the correction value without changing the frequency of the CLK signal. In this way, the optical scanning unit 26 improves the effect of suppressing the unevenness of the light amount without increasing the circuit scale of the correction unit 520 such as the capacity of the storage unit 521. As a result, the print image quality of the printer 100 can be improved.

[Modification]
(A) The image forming apparatus 100 shown in FIG. 1 is a color laser printer. In addition, the image forming apparatus according to the embodiment of the present invention may be any one of a monochrome laser printer, an ink jet printer, a facsimile machine, a copier, and a multifunction machine.
(B) The values of the wavelength and output of the semiconductor lasers 26Y shown in FIG. 3 are merely examples, and other values may be used. Further, instead of the semiconductor lasers 26Y,..., LEDs may be used for the light source 260.

  (C) The structure of the scanning optical system is not limited to the combination of the polygon mirror 271 and the fθ lens 273 shown in FIG. 2, and may be a combination of other optical elements. For example, the number of deflection surfaces of the polygon mirror may be an integer value other than the number “7” of the number 271 shown in FIG. The polygon mirror is arranged on the left side of the four photosensitive drums 25Y as viewed from the front of the printer 100 as shown in FIG. 1 271 and may be on the right side or the center. . The arrangement of other optical elements such as an fθ lens and a folding mirror may be changed in accordance with the position of the polygon mirror.

(D) In FIG. 5, the switching unit 525 corrects the amount of current supplied to the laser oscillators LD1 and LD2 before the modulation unit 530. In addition, the switching unit may correct the supply current amount after the modulation unit 530, that is, after the modulation by the image data.
(E) As shown in FIG. 7A, the PWM unit 523 performs MCL according to the error between the times TSH and TSL when the second spot SP2 reaches the specific main scanning position and the TS at the falling edge of the CLK signal. The target value of the duty ratio of the signal is adjusted before and after the duty ratio of the CLK signal is 50%. Thereby, the falling time of the MCL signal is displaced so as to coincide with the arrival times TSH and TSL. At this time, the PWM unit 523 may limit the range that can be taken by the target value to be narrower than the range that the duty ratio can be originally taken, that is, from 0% to 100%. Specifically, for example, the PWM unit 523 limits the range that the target value can take to 10% or more and 90% or less. As a result, the time length (that is, the pulse width) during which the logic level of the MCL signal is maintained true (H) or false (L) is secured at least up to 10% of the clock period PCL. By ensuring the lower limit of the pulse width of the MCL signal in this way, the timing generation unit 524 can reliably detect both the rise and fall of the MCL signal.

  (F) In FIG. 8, the rising timings of the different timing signals TMS1 and TMS2 represent the timing at which the spots SP1 and SP2 of the light rays emitted from the different light emitting points LD1 and LD2 reach the boundary of the correction section. In addition, the same timing signal may represent these timings. For example, a multilevel signal may be used as the timing signal, and the timing at which the spot of the outgoing light beam from a different light emission point reaches the boundary of the correction section may be represented by a transition of this timing signal to a different logic level. In this case, the switching unit corresponding to each light-emitting point may identify the timing of changing the correction value by identifying this logical level.

  (G) In the correction section information CRP, as shown in FIG. 9, the main scanning position of each boundary of the correction section is irradiated with the emitted light from the semiconductor laser 26Y,... At the start point T0 of the effective scanning period ESR (exactly Will be irradiated if the first mirror 301 is removed). The main scanning position is defined by the value of the coordinate in the main scanning direction with the origin “0”. In addition, the correction section information CRP may define the main scanning position of the leading boundary and the width of each correction section. In this case, the timing generation unit 524 may calculate the main scanning position of the second and subsequent boundaries from the main scanning position of the leading boundary and the width of each correction section.

  (H) The correction value information CRV indicates a correction value for the amount of light emitted from the first light emitting point LD1 included in the laser oscillator after amplification with respect to the current amount IB1 before amplification by the DAC 600, as shown in FIG. Current ratio IC1, that is, the ratio of the corrected amount of emitted light to the reference value of the amount of emitted light. The correction value may also be defined by the value of another parameter that can specify the value of the amount of emitted light necessary for canceling the reflectance and transmittance fluctuations accompanying the change in the optical path in the scanning optical system. Examples of such parameters include the corrected emission light amount value itself, the difference between the value and the reference value, and the amount of current required to emit the light amount of that value from the first light emitting point LD1.

(I) The switching unit 525 performs linear interpolation on the correction value defined by the correction value information CRV, and linearly changes the amplification factor of the DAC 600 along the interpolation line CRB shown in FIG. In addition, the switching unit 525 may change the amplification factor of the DAC 600 stepwise, and in particular, the amplification factor may be kept constant at the correction value with respect to the boundary of the starting edge in each correction section.
(J) The switching unit 525 performs digital processing of linear interpolation of correction values, that is, subdivides each correction section into minute sections, and changes the amplification factor of the DAC 600 at a constant rate per minute section. In addition, the switching unit may perform this linear interpolation by analog processing. Specifically, the switching unit includes, for example, an analog integration circuit in the subsequent stage of the DAC 600. The DAC 600 keeps the amplification factor constant in each correction section, while the analog integration circuit integrates the amount of current after amplification with time. Then, an amount of current equal to the sum or difference between the amount proportional to the integral value and the initial value is output. If the time constant of the integration circuit is set sufficiently longer than the time length of each correction section, the output current amount can be changed linearly.

(K) As shown in FIG. 9A, the boundary between the correction sections is a second area where the undulation is relatively more severe than the value ΔPS in the first region GNR where the undulation of the correction curve CRC is relatively gentle. The value ΔPD in the region STR is set small. In addition, the boundary between the correction sections may be set to be constant regardless of whether the correction curve CRC is uneven.
As shown in FIG. 9A, the boundary of the correction section is set over the entire range of coordinates in the main scanning direction, as well as part of the range, for example, the second region STR. The correction curve CRC may be set only in a region where the undulations are severe. On the other hand, for example, in a region where the undulation of the correction curve CRC is gradual as in the first region GNR, the correction value for the amount of light emitted from the light source may be made uniform.

  If there is no substantial difference in the shape of the correction curve CRC when the light beam emitted from the light source is reflected by any deflection surface of the polygon mirror 271, the boundary of the correction section and the correction value are common to all the deflection surfaces. Is set. Thereby, any data amount of the correction section information CRP and the correction value information CRV can be reduced. On the other hand, depending on the deflection surface on which the outgoing light beam is reflected due to variations in shape (surface tilt) between the deflection surfaces of the polygon mirror and surface irregularities such as fine irregularities on the surface. When a substantial difference appears in the shape of the correction curve CRC, one or both of the correction section information CRP and the correction value information CRV may be set for each deflection surface. Thereby, the suppression effect of light quantity nonuniformity can further be improved.

  In the above embodiment, the shape of the correction curve is considered to be substantially common between the two light emitting points LD1 and LD2 included in the laser oscillators of the same semiconductor laser 26Y,. Correction values are used. In addition, one or both of the correction section information CRP and the correction value information CRV may be set for each light emission point according to the difference in the shape of the correction curve between these light emission points. Even in this case, the correction unit 520 synchronizes the selection of the correction value for the emitted light amount with respect to the rise of the MCL signal for the first light emission point LD1 and with the fall of the MCL signal for the second light emission point LD2. As a result, the correction unit 520 uses the common CLK signal and the common counter for correction of the amount of light emitted from both light emitting points, and corrects the correction value according to the variation in the interval PB in the main scanning direction between the spots SP1 and SP2. The timing of the change can be appropriately corrected. Therefore, the circuit configuration of the timing generation unit 524 can be simplified.

(L) Two light beams are emitted from each semiconductor laser 26Y, as shown in FIG. In addition, the number of emitted light beams may be more than two per one semiconductor laser. Even in this case, the correction unit can improve the effect of suppressing the unevenness of the light amount without operating the circuit scale by operating each element as follows.
[When there are three beams]
A single semiconductor laser emits three light beams, a first light beam, a second light beam, and a third light beam, and a spot that connects these three light beams to the surface of the same photosensitive drum extends along the main scanning direction. Assume that the light beam, the third light beam, and the second light beam are arranged in this order.

  The oscillating unit generates a first CLK signal and a second CLK signal, synchronizes the rising edge of the first CLK signal with the timing at which the SOS sensor 303 detects the first light beam, and causes the SOS sensor 303 to emit the third light beam at the rising edge of the second CLK signal. Synchronize with the detection timing. In addition, the oscillating unit delays the phase of the first CLK signal in accordance with the time that the third light beam spot is delayed due to the interval in the main scanning direction with respect to the first light beam spot. May be converted into a second CLK signal.

  The PWM unit performs PWM on the first CLK signal to generate an MCL signal, and matches the duty ratio with the target value. This target value corresponds to the fractional part of the value representing the time in which the second light spot is delayed due to the interval in the main scanning direction with respect to the first light spot in units of clocks. Thereby, the spot of the second light beam reaches the main scanning position where the spot of the first light beam reaches when the MCL signal rises, and when the spot of the MCL signal falls.

  The timing generation unit counts the rising edges of the MCL signal and generates a first timing signal and a second timing signal based on the number of rising edges. The rising edge of the first timing signal indicates the timing at which the spot of the first light beam reaches the main scanning position CPk at the boundary of the correction section defined by the correction section information. The rise of the second timing signal indicates the timing at which the spot of the second light beam reaches the main scanning position CPk which is larger than each main scanning position CPk by the number of delay clocks. The number of delay clocks corresponds to an integer part of a value representing the delay time of the second light spot with respect to the first light spot in clock units.

The timing generation unit further counts rising edges of the second CLK signal and generates a third timing signal based on the number of rising edges. The rise of the third timing signal indicates the timing at which the spot of the third light beam reaches the main scanning position CPk at the boundary of the correction section defined by the correction section information.
The switching unit corresponding to the different light beam changes the correction value according to the rise of the different timing signal.

[When there are 2n rays (integer n ≧ 2)]
FIG. 15A is a schematic diagram showing a matrix of light emission points included in a laser oscillator 371 built in a semiconductor laser. This laser oscillator 371 differs from that shown in FIG. 3A in that it forms a matrix MTR having 8 × 4 emission points. Referring to FIG. 15A, in this matrix MTR, the light emitting points are arranged in a straight line with eight equal intervals in each column, and four in each row are arranged in a straight line at equal intervals. Further, the row direction is slightly inclined from the direction (X-axis direction shown in FIG. 15A) perpendicular to the column direction (Y-axis direction shown in FIG. 15A). On the surface of the photosensitive drum, the spots of the emitted light from the eight light emitting points in each row are arranged at intervals in the sub-scanning direction, while the main scanning position is common. Therefore, the correction unit may count the rise of the common CLK signal and measure the timing of changing the correction value as is well known in the correction for the amount of light emitted from the eight light emitting points in the same row. On the other hand, the spots of the emitted light rays from the four light emitting points in each row are arranged at intervals in the main scanning direction. Therefore, the correction unit measures the timing of changing the correction value by using a plurality of CLK signals as follows in the correction for the amount of light emitted from the four light emitting points in the same row.

  FIG. 15B is a schematic diagram showing 2n spots SP1, SP2,..., SP2n that 2n (integer n ≧ 2) emitted light beams from one semiconductor laser are connected to the surface of the same photosensitive drum. It is. Referring to FIG. 15B, these spots SP1,... Are arranged at intervals in the main scanning direction. Here, the identification number is set as follows in the order of the spot having the largest value of the main scanning position with respect to the light beam connecting each spot SPk (integer k = 1, 2, 3,..., 2n). First, odd numbers are assigned in order from “1” to “2n−1”, and then even numbers are assigned in order from “2” to “2n”. With such numbering, when 2n light emitting points from which 2n light beams are emitted are arranged in the same row of the matrix as shown in FIG. 15A, the spot SP of the (2m-1) light beam ( 2m-1) and the spot SP2m of the second m-ray beam are substantially uniform. In particular, for any pair of spots, the ratio of error to the interval can be suppressed to the same extent.

The correction unit treats the pair of the (2m-1) light beam and the second m light beam in the same manner as the pair of the first light beam and the second light beam in the above-described embodiment. Specifically, the correction unit operates each element as follows.
The oscillating unit generates the mCLK signal (hereinafter abbreviated as “CLKm”; integers m = 1, 2, 3,..., N), and the SOS sensor 303 detects the (2m−1) th light beam at the rising edge of CLKm. Synchronize with the timing. In particular, the period of CLKm is set such that the delay time of the spot SP2m of the second m light beam with respect to the spot SP (2m-1) of the (2m-1) light beam is equal to a half integer multiple of the clock period. In addition, the oscillation unit delays the spot of the (2p−1) light beam (integer p = 2, 3,..., N) with respect to the spot of the first light beam due to the interval in the main scanning direction of both. CLK1 may be converted to CLKp by delaying the phase of CLK1 according to time.

  FIG. 15C shows the time when the first light beam spot SP1, the second light beam spot SP2,... Reach the same main scanning position on the surface of the photosensitive drum, and the CLK1, CLK2,. It is a timing chart which shows the correspondence between the pulse waveform of signal (MCL1), 2nd MCL signal (MCL2), .... Referring to (c) of FIG. 15, the first light spot SP1 arrives at this position at one rising edge TS1 of CLK1, and the second light spot SP2 arrives after the delay time DLY1. The delay time DLY1 represents the time required for the second light spot SP2 to travel a distance equal to the distance from the first light spot SP1. Similarly, a spot SP (2m-1) of the (2m-1) light beam (hereinafter referred to as "(2m-1) spot") arrives at this position at one rise time TSm of CLKm, and further. After the elapse of the delay time DLYm, a spot SP2m (hereinafter referred to as “second m spot”) of the second m light beam arrives. This delay time DLYm represents the time required for the second m spot SP2m to move a distance equal to the distance from the (2m-1) th spot SP (2m-1).

  The PWM unit performs PWM on CLKm to generate the mMCL signal (MCLm), and matches the duty ratio with the mth target value. The m-th target value corresponds to a decimal part of a value representing the delay time of the second m spot SP2m with respect to the (2m-1) spot SP (2m-1) in clock units. For example, in FIG. 15C, the arrival time TS2 of the second spot SP2 is delayed from the time TR1 at the fall of CLK1, so the first target value is only 20% of the difference TR1-TS2 between the two times with respect to the clock cycle PCL1. A value 70% larger than the duty ratio of CLK1 is set to 70%. On the other hand, since the arrival time TS4 of the fourth spot SP4 is earlier than the time TR2 at the fall of CLK2, the second target value is a duty ratio 50% of CLK2 by a ratio 15% to the clock period PCL2 of the difference TR2-TS4 between the two times. The smaller value is set to 35%. In this way, the PWM unit matches the duty ratio of MCLm with the mth target value, so that the (2m-1) th spot SP (2m-1) reaches the second scanning position at the main scanning position at which MCLm rises. The spot SP2m arrives when the mMCL signal falls.

  FIG. 15D is a table showing combinations of the number of delay clocks and the target value of the duty ratio of the MCL signal for each CLK signal. These delay clock numbers correspond to the integer part of the value representing the delay time of the 2m spot with respect to the (2m-1) spot in clock units. For example, this table is defined by the spot interval information and is referred to by the PWM unit. Referring to FIG. 15D, a combination of a delayed clock number “2” and a target value 70% is assigned to CLK1, and a delayed clock number “3” and a target value 35% are assigned to CLK2. A combination with is assigned. These represent that the delay times DLY1 and DLY2 are equal to the values “2.7” and “3.35” in units of clocks.

  The timing generation unit counts rising edges of MCLm, and generates a (2m-1) th instruction signal, a second m instruction signal, a (2m-1) th timing signal, and a second m timing signal based on the number of rising times. Specifically, the timing generation unit first raises the (2m−1) instruction signal in accordance with the rise of MCLm when the value of the built-in counter reaches the first threshold value. Next, the timing generation unit causes the (2m-1) th instruction signal to fall in accordance with the rise of the MCL signal when the counter value reaches the second threshold value. Here, since the oscillation unit matches the phase difference between CLKm in advance with the interval of the (2m-1) -th spot in the main scanning direction, the first threshold value and the second threshold value may be common between MCLm.

  The timing generation unit further sets the sum of the number of delayed clocks defined by the spot interval information and the first threshold as the third threshold, and sets the sum of the number of delayed clocks and the second threshold as the fourth threshold. Here, since the number of delay clocks generally differs between CLKm, the third threshold value and the fourth threshold value generally differ between MCLm. Thereafter, when the value of the counter that counts the rise of MCLm reaches the third threshold, the timing generation unit raises the second m instruction signal in accordance with the next fall of MCLm. Further, when the counter value reaches the fourth threshold value, the timing generation unit causes the second m instruction signal to fall in accordance with the next fall of MCLm.

  A timing generation unit that matches the rise of MCLm when the value of the counter that counts the rise of MCLm reaches the main scanning position CPk (integer k = 1, 2, 3,...) Defined by the correction section information. Raises the (2m-1) th timing signal. When the counter value reaches a value larger than the main scanning position CPk by the number of delay clocks, the timing generator raises the second m timing signal at the next fall of MCLm.

Thus, the rising edge of the (2m-1) timing signal indicates the timing at which the (2m-1) spot reaches the main scanning position CPk at the boundary of the correction section, and the rising edge of the second m timing signal indicates each main scanning position. The timing at which the spot of the second light beam reaches the main scanning position larger than CPk by the number of delay clocks with respect to CLKm is shown.
The switching unit corresponding to each of the (2m−1) th light beam and the second mth light beam changes the correction value in accordance with each rising edge of the (2m−1) th timing signal and the second mth timing signal. Accordingly, the selection of the correction value for the light amount is synchronized with the rise of MCLm for the (2m-1) light beam and is synchronized with the fall of MCLm for the second light beam.

[When there are (2n + 1) rays (integer n ≧ 2)]
On the surface of the photosensitive drum, (2n + 1) spots SP1, SP2,..., SP (2n + 1) connected by (2n + 1) outgoing beams from one semiconductor laser are arranged at intervals in the main scanning direction. For the outgoing light beam connecting 2n spots excluding the spot SP (n + 1) located at the center of the spot row, the correction unit may correct the same as in the case of 2n light beams. On the other hand, with respect to the outgoing light beam connecting the central spot SP (n + 1), the correction unit may perform correction in the same manner as the third light beam when there are three light beams.

  Note that the two combinations using the same MCL signal for correction of the light quantity among a plurality of light beams may be other than the combination shown in FIG. However, in this case, a pair of spots having a shorter interval than the interval in the main scanning direction of the spots shown in FIG. Since this pair has a relatively high error rate with respect to the interval, the CLK signal frequency may have to be set relatively high for this pair. Therefore, since the combination shown in FIG. 15B can suppress the frequency of any CLK signal relatively lower than the other combinations, simplification of the circuit configuration of the correction unit and electromagnetic noise caused by the CLK signal. This is advantageous in terms of reduction of.

  The present invention relates to an optical scanning device, and as described above, the duty ratio of a clock signal is adjusted in accordance with the interval between two spots where two outgoing beams from a semiconductor laser are connected to the surface of the same photosensitive member, thereby correcting the light quantity. The timing of changing the value is synchronized with the rising edge of the clock signal for one of the outgoing beams and is synchronized with the falling edge of the clock signal for the other. Thus, the present invention is clearly industrially applicable.

DESCRIPTION OF SYMBOLS 100 Laser printer 25Y, 25M, 25C, 25K Photosensitive drum 26 Optical scanning part 260 Light source 26Y, 26M, 26C, 26K Semiconductor laser 271 Polygon mirror 273 f (theta) lens 28Y-K, 29Y-C Folding mirror 300 Control part 301 1st mirror 302 Second mirror 303 SOS sensor 361 Laser oscillator 362 Outlet EEL Enlarged view of end-emitting laser oscillator PE1, PE2 Emission point of end-emitting laser oscillator VCS Enlarged view of VCSEL laser oscillator PS1, PS2 VCSEL laser oscillation Light emitting points SP1 and SP2 Spots of emitted light from the light emitting points of the laser oscillator PB and PL Spacing in the main scanning direction and sub-scanning direction LN1 and LN2 Lines drawn on the surface of the photosensitive drum CLK CLK Click signal MCL delay time between the clock signal DLY spot after the PWM

Claims (8)

An optical scanning device that forms an image on a photoconductor by exposure scanning;
A light source that emits a first light beam and a second light beam, the amount of light being adjustable for each light beam;
The first light beam and the second light beam are imaged on the surface of the photoconductor while being periodically deflected, so that the distance between the spots of both light beams is maintained in both the main scanning direction and the sub-scanning direction. A scanning optical system for moving the spot in the main scanning direction;
A modulator that modulates the amount of each light according to the image data;
A correction unit that selects a correction value according to the position of each light spot in the main scanning direction, and corrects the light amount of the light beam with the correction value;
With
The correction unit is
An oscillation unit that generates a clock signal in synchronization with the periodic deflection of the first light beam so that the clock signal rises or falls at a timing when the spot of the first light beam reaches a predetermined position;
A pulse width modulation unit that matches the duty ratio of the clock signal by pulse width modulation to a target value determined by an interval in the main scanning direction between spots of the first light beam and the second light beam;
The timing of generating the timing signal indicating the timing at which the number of rises reaches the predetermined number and the timing at which the number of falls reaches the predetermined number by counting the rising or falling of the clock signal modulated by the pulse width modulation unit A generator,
In accordance with the timing signal, selection of correction values for the respective light amounts of the first light beam and the second light beam is synchronized with the rising edge of the clock signal modulated by the pulse width modulation unit, and the other is the pulse width modulation. A switching unit that synchronizes with the falling edge of the clock signal modulated by the unit;
Including an optical scanning device. The correction unit sets a coordinate in the main scanning direction in an area on the photoconductor on which the spot of each light beam moves, divides a range that the coordinate can take into a plurality of correction sections, and the coordinate of the spot corresponds to each correction. Change the correction value for the light quantity of the light beam to the correction value for the correction section at the timing of reaching the section,
2. The optical scanning device according to claim 1, wherein the oscillation unit matches the rising edge or the falling edge of the clock signal with a timing at which the spot of the first light beam reaches each correction section. A detector that detects a timing at which a deflection angle of the first light beam by the scanning optical system reaches a predetermined value;
3. The optical scanning device according to claim 1, wherein the oscillating unit makes the rising edge or the falling edge of the clock signal coincide with the timing detected by the detecting unit. The scanning optical system can change the period for deflecting each light beam,
4. The optical scanning device according to claim 1, wherein the pulse width modulation unit changes a target value of a duty ratio of the clock signal in accordance with the change of the period. 5.   5. The pulse width modulation unit according to claim 4, wherein when the target value of the duty ratio of the clock signal is changed, a range that the target value after the change can take is limited to be narrower than 0 and less than 1. Optical scanning device. The light source further emits a third light beam;
The scanning optical system forms an image between the spots of the first light beam and the second light beam while periodically deflecting the third light beam in the same manner as the first light beam and the second light beam. The spot is moved in the main scanning direction while keeping the interval between the spots of the light beams in both the main scanning direction and the sub-scanning direction,
The oscillator generates another clock signal in synchronization with the periodic deflection of the third light beam so as to rise or fall at a timing when the spot of the third light beam reaches a predetermined position,
The timing generator counts the rising or falling edge of the other clock signal, and generates another timing signal indicating the timing at which the number of rising edges or the number of falling edges reaches a predetermined number;
6. The switching unit according to claim 1, wherein the switching unit synchronizes selection of a correction value for the light amount of the third light beam with a rising edge or a falling edge of the another clock signal in accordance with the another timing signal. The optical scanning device according to any one of the above. The light source further emits from the fourth light to the second n light (the integer n is 2 or more),
The scanning optical system forms an image on the surface of the photoconductor while periodically deflecting the fourth light beam to the second n light beam similarly to the first light beam to the third light beam. The spots of the (2m−1) light rays (integer m = 1, 2, 3,..., N.) Are arranged in order, and then the spots of the second m light rays are arranged in order, The arrangement is moved in the main scanning direction while maintaining the interval in both the main scanning direction and the sub-scanning direction,
The oscillating unit synchronizes the m-th clock signal including the clock signal and the other clock signal with the periodic deflection of the (2m-1) light beam, so that the spot of the light beam reaches a predetermined position. Generate to rise or fall at the timing,
The pulse width modulation unit matches the duty ratio of the m-th clock signal by pulse width modulation to a target value determined by an interval in the main scanning direction of the spot of the (2m-1) light beam and the second m light beam,
The timing generator counts the rise or fall of the m-th clock signal, and the m-th timing signal including the timing signal and the other timing signal reaches the predetermined number of rises of the m-th clock signal. To indicate the timing to perform and the timing when the number of falling times reaches a predetermined number,
The switching unit selects a correction value for the light amount of the (2m-1) light beam and the second m light beam according to the mth timing signal, and one of the mth clock signal modulated by the pulse width modulation unit. 7. The optical scanning device according to claim 6, wherein the optical scanning device is synchronized with the rising edge, and the other is synchronized with the falling edge of the m-th clock signal modulated by the pulse width modulation section. An image forming apparatus for forming a toner image on a sheet;
A photoconductor whose charge amount changes according to the exposure amount;
The optical scanning device according to any one of claims 1 to 7, wherein an electrostatic latent image is formed on the photoconductor by exposure scanning.
A developing unit for developing the electrostatic latent image with toner;
A transfer unit that transfers the toner image developed by the developing unit from the photoreceptor to a sheet;
An image forming apparatus including:

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