JP6439544B2 - Optical scanning device and image forming apparatus having the same - Google Patents

Description

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

An electrophotographic image forming apparatus forms an electrostatic latent image on the surface of the built-in photoconductor by uniformly charging the surface of the photosensitive member, and developing the latent image with toner. The appearing toner image is transferred to a sheet. Thus, an image is formed on the sheet.
An optical scanning device is used for exposure scanning of the photoreceptor. The optical scanning device periodically changes the deflection angle of the light by reflecting the light of the light source at the deflection surface while rotating the polygon mirror, and forms an image of the light on the surface of the photoconductor by the optical system. Since the image formation point moves in one direction on the surface of the photosensitive member as the deflection angle changes, the surface is exposed linearly. The optical scanning device modulates the exposure amount according to the image data, thereby changing the distribution of the charge amount in the exposure region. Thus, one line of the electrostatic latent image is formed on the photosensitive member, and a change in toner density corresponding to a change in exposure amount appears in one line of the toner image developed from this one line.

  In an optical system such as a polygon mirror, the reflectance and transmittance differ strictly depending on the deflection angle. Therefore, even if the light amount is kept constant by the light source, the amount of light applied to the photosensitive member varies with the change of the deflection angle. This variation is one of the causes of exposure “unevenness” in the scanning range. That is, if the variation in the amount of light applied to the photosensitive member is directly reflected in the variation in the exposure amount, “unevenness” in the charge amount appears in one line of the electrostatic latent image, and the toner image developed from the latent image “Unevenness” of toner density appears on the corresponding one line. Therefore, in order to improve the image quality on the sheet, it is desirable to eliminate exposure unevenness as much as possible by suppressing fluctuations in the amount of irradiation light accompanying changes in the deflection angle.

  A technique for correcting the amount of light of a light source for each deflection angle is known for suppressing exposure unevenness. (For example, refer to Patent Documents 1-7.) With this technology, fluctuations in the amount of irradiation light accompanying changes in the deflection angle are offset by changes in the amount of light from the light source accompanying correction. Thereby, uneven exposure is suppressed.

JP 2009-262344 A JP 2009-053466 A JP 2011-158760 A JP 2011-158761 A JP 2013-240996 A JP 2011-148142 A JP 2008-126644 A Japanese Patent Laid-Open No. 06-079919 JP 2002-273936 A

  The correction value for the light amount of the light source for the purpose of suppressing exposure unevenness is set by the following procedure, as disclosed in Patent Documents 1 and 2, for example. First, a change in exposure amount due to a change in deflection angle is sampled (sampling) in a state where the light source is kept constant. At this time, the sample value (sample) is a boundary of a plurality of sections (hereinafter referred to as “correction sections”) equally divided from the range that can be taken by the coordinates in the main scanning direction associated with the scanning range on the photosensitive member. Each is calculated from the model or measured experimentally. Next, the correction value in the time zone in which the coordinate in the main scanning direction representing the scanning position moves in each correction section is aligned with a value that cancels the sample at the boundary of the correction section.

Aligning the correction intervals to the same width has the following advantages. Since the optical scanning device generally changes the deflection angle at a constant speed, if the correction section has the same width, the deflection angle reaches the next correction section at a uniform timing. Therefore, the light source control circuit only needs to change the correction value for the light amount of the light source at a uniform timing, so that the circuit configuration is simplified.
On the other hand, the correction section having the same width has the following problems. Since the correction value for each correction section is based on the sample at the boundary, it is strictly different from the correction value that can truly cancel the fluctuation of the irradiation light amount in the entire correction section. The difference between these correction values decreases if the correction interval is narrowed. However, since the total number of correction sections increases, the total number of correction values increases. On the other hand, since there is an upper limit on the memory capacity that can be used for storing correction values, there is an upper limit on the total number of correction values. Accordingly, there is a limit to increasing the accuracy of correction by shortening the correction section, and the upper limit of the memory capacity must be increased to further reduce the exposure unevenness. This is not preferable in terms of manufacturing cost.

  Furthermore, strictly speaking, the variation in the amount of irradiation light accompanying the change in the deflection angle has a different pattern for each deflection surface of the polygon mirror. This is mainly due to variations in the shape of the deflection surface. For example, the technique disclosed in Patent Documents 5-7 is known for the inclination (hereinafter referred to as “surface tilt”) of the polygonal mirror with respect to the rotation axis direction of the polygon mirror. In this technique, when the variation in the amount of irradiation light due to surface tilt is canceled out by correcting the light amount of the light source, the correction value is changed for each deflection surface in accordance with the difference of the variation pattern for each deflection surface. However, more precisely, the irradiation is not only caused by the surface tilt but also by the degree of unevenness of the deflection surface (hereinafter referred to as “tilt error”) in the circumferential direction of the polygon mirror, that is, in the direction of each side of the regular polygon that forms the circumference. The variation pattern of the amount of light is different. In particular, the maximum point and the minimum point in the main scanning direction are displaced for each deflection surface. In this case, even if the boundary of the correction section coincides with the maximum point or the like for a certain deflection surface, it deviates from the maximum point or the like for another deflection surface. Since the correction value is set so as to cancel the sample at the boundary of the correction section, the shift of the boundary from the maximum point or the like means the shift of the correction value from the value that truly cancels the fluctuation of the irradiation light amount. Techniques for reducing this deviation for all deflecting surfaces are not yet known.

  An object of the present invention is to solve the above-mentioned problems, and in particular, to improve the effect of suppressing uneven exposure by correcting the light quantity of the light source without increasing the memory capacity necessary for storing the correction value for the light quantity of the light source. It is an object of the present invention to provide an optical scanning device capable of satisfying the requirements.

  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 a light source having a variable amount of light and a light beam emitted from a light source on each deflection surface while rotating a polygon mirror. A deflecting unit that periodically deflects the light by reflection, an optical system that forms an image of the reflected light of the polygon mirror on the surface of the photosensitive member, a modulation unit that modulates the light amount of the light source according to image data, and a polygon mirror And a correction unit that corrects the light amount of the light source before or after the modulation by the modulation unit when the photosensitive member is scanned with the reflected light. Coordinates in the main scanning direction are associated with the areas of the photoconductor scanned with the reflected light from the respective deflection surfaces of the polygon mirror, and the range that the coordinates can take is divided into a plurality of correction sections. A correction value for the light quantity of the light source is assigned to each boundary, and while the photosensitive member region corresponding to each correction section is scanned with the reflected light from each deflection surface of the polygon mirror, the correction unit Based on a correction value assigned to at least one of the boundaries, a correction value for the light amount of the light source is determined, and the plurality of correction sections depend on which deflection surface of the polygon mirror corresponds to the scanning range of the reflected light. These are set for each deflection surface of the polygon mirror so that the combination of coordinates in the main scanning direction representing the boundary is different.

In this optical scanning device, the light amount of the light source is kept constant, and the exposure interval that appears when the photosensitive member is exposed and scanned with the reflected light from each deflection surface of the polygon mirror is corrected to the maximum or minimum point in the main scanning direction. The boundary may be set.
The boundary of the correction section may be set so that the distance to the adjacent boundary differs according to the coordinates in the main scanning direction. In particular, when the photosensitive member is exposed and scanned with reflected light from one deflecting surface of a polygon mirror while maintaining the light quantity of the light source constant without being corrected, the scanning range of the reflected light includes the first area, the main area, and the main area. The second region includes a second region in which a representative value of the change rate of the exposure amount with respect to the position in the scanning direction is larger than that of the first region. Even if the width of the corresponding correction section is set narrower than that of the first region Good. Further, the first region included in the scanning range of reflected light from one deflection surface of the polygon mirror and the first region included in the scanning range of reflected light from another deflection surface share the same coordinates in the main scanning direction. The boundary of the correction section corresponding to the common part may be set to the same coordinate in the main scanning direction.

The correction unit includes a timing generation unit that generates a signal indicating the timing at which the coordinates of the imaging point of the optical system in the main scanning direction reach the boundary of each correction section, and the timing at which the signal indicates the correction value assigned to the boundary You may get it at
The optical scanning device includes a storage section that stores a correspondence table between correction section boundaries and correction values for each deflection surface of the polygon mirror, a measurement section that measures the attribute of the reflected light of the polygon mirror, and a measurement by the measurement section. Based on the result, an identification unit that identifies which deflection surface of the polygon mirror reflects the reflected light may be further provided. In this case, the timing generation unit sets the timing to be indicated by the signal based on the boundary of the correction section indicated by the correspondence table regarding the deflection surface identified by the identification unit, and the correction unit corrects the correction value at the timing indicated by the signal. May be updated to the correction value assigned to the next boundary indicated by the correspondence table.

  The optical scanning device includes a measurement unit that measures the attribute of the reflected light of the polygon mirror, and a calculation unit that calculates a correspondence table between the boundary of the correction section and the correction value for each deflection surface of the polygon mirror based on the measurement result of the measurement unit. And an identification unit for identifying which deflection surface of the polygon mirror the reflected light is based on the measurement result by the measurement unit. In this case, the timing generation unit sets the timing to be indicated by the signal based on the boundary of the correction section indicated by the correspondence table regarding the deflection surface identified by the identification unit, and the correction unit corrects the correction value at the timing indicated by the signal. May be updated to the correction value assigned to the next boundary indicated by the correspondence table.

  The optical scanning device includes an actual measurement unit that actually measures the amount of light irradiated from the optical system to the photosensitive member, an arithmetic unit that determines a boundary of each correction section based on an actual measurement result by the actual measurement unit, and calculates a correction value for the boundary; May be further provided. In this case, the timing generation unit sets the timing to be indicated by the signal based on the boundary determined by the calculation unit, and the correction unit calculates the correction value at the timing indicated by the signal and the next boundary calculated by the calculation unit. You may update to the correction value with respect to.

When scanning the area of the photosensitive member corresponding to each correction section with the reflected light from one deflection surface of the polygon mirror, the correction unit interpolates the correction values assigned to the boundaries at both ends of the correction section. You may correct | amend the light quantity of a light source.
An image forming apparatus according to an aspect of the present invention is an image forming apparatus including an image forming unit that forms a toner image on a sheet and a fixing unit that thermally fixes the toner image. A photosensitive member whose charge amount changes in accordance with the above, a light scanning device that forms an electrostatic latent image on the photosensitive member by exposure scanning, a developing unit that develops the electrostatic latent image with toner, and a developing unit. And a transfer unit that transfers the developed toner image from the photoreceptor to the sheet.

  As described above, when the optical scanning device according to the present invention scans the area of the photoreceptor corresponding to each correction section with the reflected light of the polygon mirror, the light amount of the light source is determined based on the correction value assigned to the boundary of the correction section. to correct. At this time, each correction plane of the polygon mirror is such that the combination of coordinates in the main scanning direction representing the boundary differs depending on which deflection surface of the polygon mirror corresponds to the scanning range of the reflected light. Is set to As a result, the optical scanning device can improve the effect of suppressing the uneven exposure by correcting the light amount of the light source without increasing the memory capacity necessary for storing the correction value.

1 is a front view showing an internal structure of an image forming apparatus according to Embodiment 1 of the present invention. It is a top view of the optical scanning part shown in FIG. FIG. 2 is a block diagram of an electronic control system included in the image forming apparatus shown in FIG. 1. FIG. 4 is a block diagram of an electronic circuit included in the optical scanning unit shown in FIG. 3. 6 is a timing chart of signals related to the timing generation unit shown in FIG. 4. It is a timing chart which shows the waveform of a timing signal according to the deflection surface of a polygon mirror. FIGS. 4A and 4B show correction values set by the correction unit shown in FIG. 4 and main scanning on the photosensitive drum when the photosensitive drum is scanned with reflected light from the deflection surfaces 1 and 2 of the polygon mirror. It is a graph which shows the relationship between a position. It is a table | surface which shows the correspondence between the main scanning position of the boundary of a correction area, and a correction value for every deflection surface of a polygon mirror. (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 exposure amount of a photoconductive drum under the conditions where a light source maintains light quantity constant. (A) is a schematic diagram for demonstrating the relationship between the surface tilt of a polygon mirror and the outgoing direction of reflected light, and (b) shows one neighborhood of the apex of a regular heptagon that forms the circumference of the polygon mirror. FIG. 9C is an enlarged partial top view, and FIG. 9C is a graph showing a difference appearing in the exposure amount fluctuation pattern on the photosensitive drum shown in FIG. 9C according to the difference in the shape of the deflection surface of the polygon mirror. It is. (A) is a graph which shows the setting conditions of the boundary of the correction area with respect to a correction curve, (b) is the one part enlarged view. (A) is a graph showing the vicinity of the peak of the correction curve adopted when scanning the photosensitive drum with the reflected light from the deflecting surface 1 of the polygon mirror and the boundary of the correction section set for the correction curve. Yes, (b) shows the case where the boundary of the correction section is set at the same main scanning position as shown in (a) with respect to the vicinity of the peak of the correction curve for the reflected light from the deflecting surface 2 of the polygon mirror. (C) is a graph showing a case where the boundary of the correction section is displaced from that shown in (b) with respect to the vicinity of the peak of the correction curve shown in (b). It is a flowchart of control with respect to the optical scanning part shown in FIG. 2, FIG. It is a flowchart of the signal processing by the timing generation part shown in FIG. (A), (b) shows the relationship between the correction value set by the correction unit and the main scanning position on the drum when the photosensitive drum is scanned with the reflected light from the deflection surfaces 1 and 2 of the polygon mirror. It is a graph which shows the modification of. It is a block diagram of the electronic circuit which the optical scanning part by Embodiment 2 of this invention contains. (A) is a schematic diagram which shows an example of the actual measurement part by Embodiment 3 of this invention, (b) is a perspective view which shows another example of the actual measurement part typically, (c) is ( It is a partial enlarged view of the cross section along the straight line cc shown by b). It is a block diagram of the electronic circuit which the optical scanning part by Embodiment 3 of this invention contains. It is a flowchart of control with respect to the optical scanning part shown in FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Embodiment 1
[Internal structure of image forming apparatus]
FIG. 1 is a front view schematically showing an internal structure of an image forming apparatus 100 according to Embodiment 1 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 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, each of the four image forming units 21Y, 21M, 21C, and 21K first charges the surfaces of the photosensitive drums 25Y, 25M, 25C, and 25K uniformly, and then applies a laser to the optical scanning unit 26. The surface of each drum is exposed and scanned with light in the axial direction (the X-axis direction shown in FIG. 1 (the normal direction of the paper surface)). At this time, the optical scanning unit 26 modulates the laser light amount based on the gradation values of yellow (Y), magenta (M), cyan (C), and black (K) represented by one line of the image data. On the other hand, the charge disappears from the exposed area on the surface of each photosensitive drum 25Y,. Thus, on the surface, 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.

Next, each of the image forming units 21Y,... Charges the toner of each color of Y, M, C, and K and adheres it to one line of the electrostatic latent image. As a result, one line of four color toner images is developed on the surfaces of the four 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. The color toner image is further transferred onto the surface of the sheet SH2 that has been fed from the feeding unit 10 to the nip between the two transfer belts 23 and 24 by an electric field between the intermediate transfer belt 23 and the secondary transfer roller 24. Thereafter, the secondary transfer roller 24 sends the sheet SH <b> 2 to the fixing unit 30.

  Referring to FIG. 1, one line of the toner image generated on the surface of the photosensitive drum 25Y,... By the scanning of the laser light is transferred to the surface of the sheet SH2, and then the sheet SH2 by the secondary transfer roller 24 is transferred. It is perpendicular to the transport direction. 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, a direction corresponding to the direction of one line of the toner image, such as a direction in which the laser beam scans the surface of the photosensitive drum 25Y,... (X-axis direction (normal direction of the paper surface) shown in FIG. Both are collectively referred to as “main scanning direction”, and any direction corresponding to the direction in which the lines of the toner image are arranged, such as the rotation direction of the photosensitive drums 25Y,... And the conveyance direction of the sheet SH2 (Y-axis direction shown in FIG. Is also 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.

[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. 1 and 2, the optical scanning unit 26 includes a light source 260, a polygon mirror 271, a motor 272, an optical system 273, (28Y, 29Y), (28M, 29M), (28C, 29C), 28K, and A control unit 300 is included.

-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.
Each semiconductor laser 26Y includes a laser oscillator such as a laser diode and a light amount sensor such as a photodiode. (Not shown in FIG. 2) The laser oscillator can emit, for example, one laser beam having a wavelength of 780 nm or 655 nm with an output of several mW to several tens of mW, and the light emission amount can be changed. The light quantity sensor monitors the light emission amount of the laser oscillator and feeds back an output current in an amount proportional to the light emission amount to the control unit 300.

  Referring 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 laser beams LY, LM, LC, and LK have different optical 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. It arrange | positions so that only laser beam LY, LM, or LK may hit. Each of the mirrors 261,..., 263 reflects the emitted light LY,..., LK of the semiconductor lasers 26Y,. The fourth mirror 264 is installed so as to reflect the reflected light of the other three mirrors 261,..., 263 and the emitted light LC of the fourth semiconductor laser 26C in the same direction.

  The cylindrical lens 265 transmits the reflected light LL of the fourth mirror 264 and irradiates the polygon mirror 271 with it. In particular, the cylindrical lens 265 forms an image of the irradiation light on the side surface of the polygon mirror 271 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). In addition, 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 laser beam LL), the parallel light. 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 irradiation direction of the laser light LL is the main scanning direction.

-Polygon mirror-
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 source 260 irradiates the polygon mirror 271 with the laser beam LL, 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 laser beam LL of the light source 260, and at the same time, an angle formed by the rotation of the laser beam LL and the reflected beam RL by the rotation of the polygon mirror 271, that is, the laser beam LL. Change the deflection angle. 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 laser beam LL periodically changes, so that the deflection angle continuously and periodically in the range from the minimum value φ _L to the maximum value φ _R. Changes. 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 .

-Optical system-
The optical system includes an fθ lens 273 and four sets of folding mirrors (28Y, 29Y), (28M, 29M), (28C, 29C), and 28K.
The fθ lens 273 is arranged so as to transmit the reflected light RL of the polygon mirror 271 and irradiate the folding mirrors 28Y, 28M, 28C, and 28K. The first folding mirrors 28Y and 29Y, the second folding mirrors 28M and 29M, the third folding mirrors 28C and 29C, and the fourth folding mirror 28K are all long and thin plate-shaped, as shown in FIG. 1 and FIG. The longitudinal direction is arranged in parallel to 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 have heights (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). Thereby, only the outgoing lights LY of the different semiconductor lasers 26Y of the reflected light RL of the polygon mirror 271 strike the different folding mirrors 28Y. The other one of the first folding mirror 29Y, the other of the second folding mirror 29M, the other of the third folding mirror 29C, and the fourth folding mirror 28K are disposed one by one immediately below the respective photosensitive drums 25Y,. The emitted light LY of the first semiconductor laser 26Y is reflected by the polygon mirror 271 and transmitted through the fθ lens 273, and then reflected by the first folding mirrors 28Y and 29Y to irradiate the photosensitive drum 25Y of the first image forming unit 21Y. Is done. Similarly, the emitted light LM of the second semiconductor laser 26M is reflected by the second folding mirrors 28M and 29M and irradiated to the photosensitive drum 25M of the second image forming unit 21M, and the emitted light LC of the third semiconductor laser 26C is the first emitted light LC. The reflected light is reflected on the third folding mirrors 28C and 29C and applied to the photosensitive drum 25C of the third imaging unit 21C, and the emitted light LK of the fourth semiconductor laser 26K is reflected on the fourth folding mirror 28K and is reflected on the fourth imaging unit 21K. The photosensitive drum 25K is irradiated.

The fθ lens 273 is composed of, for example, two aspheric lenses, and the reflected light RL of the polygon mirror 271 is applied to the surface of each photosensitive drum 25Y,... in both the axial direction and the rotational direction (the X axis shown in FIG. 1). Image in both direction and Y-axis direction). As a result, the imaging point on the surface and the vicinity thereof are exposed. When the deflection angle is changed by the rotation of the polygon mirror 271, the transmitted light of the fθ lens 273 moves in the longitudinal direction on the folding mirrors 28 Y,. It moves in the axial direction on the surface of the body drum 25Y. In particular, during a period in which the deflection angle changes at a constant speed from the maximum value φ _R to the minimum value φ _L , the imaging point scans one of the surfaces of the four 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 when the imaging point on the surface of the photosensitive drum 25Y,... Scans one line of the electrostatic latent image from the head to the head of the next line is referred to as “main scanning 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.

The fθ lens 273 further changes the deflection angle of the light LL of the light source 260 by the characteristic that “the incident angle of transmitted light is proportional to the image height (distance from the optical axis of the imaging point)”. The amount is proportional to the distance that the imaging point of the transmitted light moves with the change. Referring to FIG. 2, when the deflection angle changes from the maximum value φ _R to the angle 2θ, the irradiation point of the transmitted light of the fθ lens 273 moves on the folding mirrors 28Y,. 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 RL of 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. Since these moving distances ρY,... Are proportional to the moving distance of the imaging point 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. A linearity is established between the position of the image point 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 velocity, the image formation point moves at a constant velocity in the main scanning direction.

-Control unit-
The control unit 300 is an electronic circuit dedicated to light emission control for the four semiconductor lasers 26Y,. The control unit 300 receives image data from a main control unit, which will be described later, and blinks the semiconductor lasers 26Y corresponding to the respective colors based on the gradation values of the respective colors Y, M, C, and K represented by the image data. Modulate the pattern. For example, the higher the gradation value of a pixel, the longer the emission time of the semiconductor laser for that pixel is adjusted.

The control unit 300 also delays the timing of the blinking pattern based on the gradation value of one line represented by the image data by a predetermined time between the four semiconductor lasers 26Y,. This fixed time is determined by the time required for one point on the surface of the intermediate transfer belt 23 to move from between the pair of primary transfer rollers 22Y and the photosensitive drum 25Y to the next pair 22M, 25M. .
The controller 300 further includes a first mirror 301, a second mirror 302, and a scanning start (SOS) sensor 303. Referring to FIG. 2, the first mirror 301 is installed on the path of the laser light reflected by the polygon mirror 271 to the maximum deflection angle φ _R and reflects the light toward the second mirror 302. The second mirror 302 further reflects the reflected light of 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 irradiation light of the second mirror 302, and notifies the control unit 300 with a signal (hereinafter referred to as “SOS signal”). Each time the polygon mirror 271 rotating at a constant angular velocity reflects the laser beam LL of the light source 260 to the maximum deflection angle φ _R , the SOS sensor 303 detects the reflected light and validates the SOS signal. 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 controller 300 samples the amount of laser light LY fed back from the semiconductor lasers 26Y, and adjusts the amount of light emitted from the semiconductor lasers 26Y based on the samples. At this time, the controller 300 further corrects the light emission amount of the semiconductor lasers 26Y,... For each deflection angle. Details of adjustment and correction of the light emission amount will be described later.
[Electronic control system of image forming apparatus]
FIG. 3 is a block diagram illustrating a configuration of the electronic control system of the printer 100. Referring to FIG. 3, in this electronic control system, in addition to the feeding unit 10, the image forming unit 20, and the fixing unit 30, an operation unit 50 and a 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. 3, 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. 3, the main control unit 60 includes a CPU 61, a RAM 62, and a ROM 63. The CPU 61 controls the other elements 10, 20,... Connected to the bus 90 according to the firmware. The RAM 62 provides the CPU 61 with a work area when the CPU 61 executes the firmware, and stores processing target data received by the operation unit 50. The ROM 63 includes a non-writable semiconductor memory device and a rewritable semiconductor memory device such as an EEPROM or an HDD. The former stores firmware, and the latter 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.

[Configuration of electronic circuit of optical scanning unit]
FIG. 4 is a block diagram of an electronic circuit included in the control unit 300 of the optical scanning unit 26. Referring to FIG. 4, the controller 300 includes four electronic circuits 300Y, 300M, 300C, and 300K. Each of the electronic circuits 300Y,... Individually controls the current ICR supplied to one of the semiconductor lasers 26Y,. The amount of light emitted from the semiconductor laser 26Y,... And the timing of blinking are determined by the amount of the current ICR and the timing of flowing it. FIG. 4 illustrates the configuration of the electronic circuit 300Y for the first semiconductor laser 26Y. The configurations of the other electronic circuits 300M, 300C, and 300K are the same.

The electronic circuit 300Y includes a sample hold (SH) unit 310, a storage unit 320, an identification unit 325, a correction unit 330, and a modulation unit 340.
The SH unit 310 determines the amount of current IBS corresponding to the light amount to be output to the laser oscillator LD in the first semiconductor laser 26Y based on the light amount fed back from the light amount sensor PD in the first semiconductor laser 26Y.

  The storage unit 320 stores a correspondence table between the correction value for the light amount of the laser oscillator LD and the boundary of the correction section for each deflection surface of the polygon mirror 271. The “correction section” is divided from the possible range of coordinates in the main scanning direction associated with the range on the surface of the photosensitive drum 25Y,... Scanned with the reflected light from each deflection surface of the polygon mirror 271. Each section. “Boundary of the correction section” means an end point of the correction section. Hereinafter, since it is considered that there is little possibility of confusion for those skilled in the art, the “correction section” is the area on the surface of the photosensitive drum 25Y,... Corresponding to the area of the coordinate in the main scanning direction represented by the correction section. The term “correction section boundary” is also used to mean the position on the surface of the photosensitive drum 25Y corresponding to the coordinates of the end points of the correction section.

The identification unit 325 identifies which deflection surface of the polygon mirror 271 detects the reflected light from the SOS sensor 303 based on the attribute of the SOS signal, for example, the period. The identification unit 325 further causes the storage unit 320 to search a correspondence table regarding the identified deflection surface.
The correction unit 330 reads the correction value for the next boundary from the storage unit 320 at the timing when the reflected light of the polygon mirror 271 is irradiated to the boundary of each correction section (corresponding position on the surface of the photosensitive drum 25Y). While the reflected light is applied to the correction section (corresponding area on the surface of the photosensitive drum 25Y), the correction unit 330 corrects the amount of current IBS determined by the SH unit 310 based on the read correction value. Then, the corrected current ICR is supplied to the laser oscillator LD. Thereby, the light emission amount is corrected.

The modulation unit 340 modulates the output current ICR to the laser oscillator LD based on the Y gradation value represented by the image data. When the laser oscillator LD blinks in accordance with this modulation, the average light emission amount is adjusted to a value corresponding to the Y gradation value.
-SH part-
Referring to FIG. 4, the SH unit 310 includes a resistor 311, a reference voltage source 312, a differential amplifier 313, a switch 314, a capacitor 315, and a voltage / current (VI) converter 316.

The resistor 311 is connected between the output terminal of the light quantity sensor PD and the ground conductor. Since the voltage drop amount VFB in the resistor 311 is proportional to the output current IFB of the light quantity sensor PD, it is proportional to the light emission amount of the laser oscillator LD.
The reference voltage source 312 is a constant voltage source, and its output voltage VRF is equal to the voltage drop amount VFB of the resistor 311 when the light emission amount of the laser oscillator LD is equal to a predetermined reference value.

  One of the two input terminals of the differential amplifier 313 is connected to the output terminal of the light quantity sensor PD, and the other is connected to the reference voltage source 312. As a result, the differential amplifier 313 outputs a constant current ISH in an amount proportional to the difference between the voltage drop amount VFB of the resistor 311 and the output voltage VRF of the reference voltage source 312. 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 light emission amount of the laser oscillator LD and its reference value, the amount of the output current ISH is proportional to the difference between the light emission amount and the reference value, and the direction of the difference is Represents a sign.

The switch 314 connects or disconnects the output terminal of the differential amplifier 313 and one end of the capacitor 315 in accordance with the instruction signal SHS from the correction unit 330. As will be described later, the instruction signal SHS is a time period in which the deflection angle of the laser beam LL instantaneously changes from the minimum value φ _L to the maximum value φ _R from the end of each main scanning period to the beginning of the next main scanning period (hereinafter, It is validated in the “return line period”) and invalidated in the time zone in 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 scan period”). . Accordingly, the switch 314 is closed during the blanking period to maintain the connection between the output terminal of the differential amplifier 313 and one end of the capacitor 315, and is opened during the effective scanning period to disconnect the connection.

  Since the capacitor 315 holds a predetermined amount of charge in advance, the voltage VSH between both ends thereof is maintained at a predetermined value while the switch 314 is open. Since the switch 314 is closed during the retrace period, the capacitor 315 is charged / discharged by the output current ISH of the differential amplifier 313, and the voltage VSH between both ends thereof varies. 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 314 is opened during the effective scanning period, the capacitor 315 maintains the voltage VSH between both ends substantially constant after the fluctuation.

  The VI converter 316 makes the amount of the output current IBS proportional to the voltage VSH across the capacitor 315. The reference value of the light emission amount of the laser oscillator LD is determined by the amount of the output current IBS. Since the voltage VSH across the capacitor 315 fluctuates in accordance with the change in the output current ISH of the differential amplifier 313 during the blanking period, the VI converter 316 changes the amount of the output current IBS according to the fluctuation. In particular, since the output current ISH of the differential amplifier 313 corresponds to the difference between the light emission amount of the laser oscillator LD and its reference value, the VI converter 316 adjusts the amount of the output current IBS so as to cancel the difference. . On the other hand, since the voltage VSH across the capacitor 315 is kept constant during the effective scanning period, the VI converter 316 maintains the amount of the output current IBS at a value corresponding to the voltage VSH. Thereby, the light emission amount of the laser oscillator LD before being modulated by the image data is maintained at the reference value.

-Storage unit-
The storage unit 320 includes a single or a plurality of memory elements mounted on the electronic circuit 300Y, and manages data stored in these. The data includes a correspondence table between the correction value for the light quantity of the laser oscillator LD and the boundary of the correction section, and these are preferably stored in a rewritable nonvolatile memory area. Referring to FIG. 4, the storage unit 320 further includes a correction interval register 321 and a correction value register 322. Each of these registers 321 and 322 is an area in the above memory device, and is preferably volatile. When the storage unit 320 receives the identification information of the deflection surface of the polygon mirror 271 from the identification unit 325, the storage unit 320 searches the correspondence table regarding the deflection surface, and based on the table, stores the correction interval information CRP in the correction interval register 321. The correction value information CRV is set in the correction value register 322. “Correction section information” CRP is information relating to the boundary of the correction section. In particular, the coordinates in the main scanning direction where the boundary is set are displayed next to the time point when the reflected light RL of the polygon mirror 271 is detected by the SOS sensor 303. It is defined by the length of time until the boundary is irradiated, specifically, the number of pulses of the clock (CLK) signal (number of clocks). The “correction value information” CRV defines a correction value for the laser oscillator LD for each correction section boundary by a ratio of the corrected light amount to the reference value of the light amount.

-Identification part-
Each time the identification unit 325 receives a pulse of the SOS signal, the identification unit 325 first compares the attribute with predetermined data to identify which deflection surface of the polygon mirror 271 causes the reflected light, and then The identification information of the deflection surface is passed to the storage unit 320. This data to be collated indicates the attribute of the SOS signal for each deflection surface of the polygon mirror 271 and is stored in advance in a memory element mounted on the electronic circuit 300Y such as a memory element of the storage unit 320, for example.

  Strictly speaking, the attributes of the SOS signal, such as the pulse period and level, differ for each deflection surface of the polygon mirror 271. This is mainly due to variations in the shape of the deflection surface. Therefore, it is possible to identify from which deflection surface of the polygon mirror 271 the light detected by the SOS sensor 303 is reflected from the difference in the attributes of the SOS signal. Specifically, since the tilt error (degree of unevenness in the circumferential direction) of the deflection surface of the polygon mirror 271 differs for each deflection surface, the position at which light is reflected at an angle incident on the SOS sensor 303 differs for each deflection surface. As a result, the pulse period of the SOS signal differs for each deflection surface. Further, since the tilt of the polygon mirror 271 with respect to the rotation axis direction, that is, the surface tilt differs for each deflection surface, the amount of light incident on the SOS sensor 303 differs for each deflection surface. As a result, the pulse level of the SOS signal differs for each deflection surface. Therefore, if the cycle or level of the pulse of the SOS signal is previously converted into data for each deflection surface of the polygon mirror 271, the data is detected by the SOS sensor 303 by collating the data with the actual cycle or level of the SOS signal. It can be specified which deflection surface of the polygon mirror 271 reflects the light.

-Correction part-
Referring to FIG. 4, the correction unit 330 includes a timing generation unit 331 and a digital / analog converter (DAC) 332. The timing generation unit 331 is, for example, a single logic element, generates an instruction signal SHS based on the SOS signal and the CLK signal, and generates a timing signal TMS based on the correction interval information CRP and the CLK signal. The DAC 332 amplifies the output current IBS of the VI converter 316 at a variable rate.

<Timing generator>
FIG. 5 is a timing chart of signals related to the timing generation unit 331. Referring to FIG. 5, the CLK signal is generated by an oscillation circuit (not shown in FIGS. 2 and 4) mounted on the control unit 300. All of the four electronic circuits 300Y-300K shown in FIG. 4 synchronize their operations with this CLK signal. The SOS signal is a negative logic signal, and the period SCT from one falling time T0 to the next falling time T3 is one main scanning period, that is, the deflection angle of the laser light LL of the light source 260 is from the maximum value φ _R. the range to the minimum value phi _L represents the period of reciprocating once. Instruction signal SHS is a negative logic signal, the fall period FBR one blanking period, i.e. represents a period in which the deflection angle changes instantaneously from the minimum value phi _L to a maximum value phi _R, its rising period ESR This represents an effective scanning period, that is, a period during which the deflection angle changes from the maximum value φ _R to the minimum value φ _L at a constant speed. In particular, the fall of the instruction signal SHS indicates the on timing of the switch 314 of the SH unit 310, and the rise indicates the off timing. The timing signal TMS is a positive logic signal, and its rising edge represents the timing at which the reflected light RL of the polygon mirror 271 is applied to each boundary of the correction section of each photosensitive drum 25Y,.

First, the timing generation unit 331 synchronizes the count of the number of pulses (the number of clocks) of the CLK signal with the SOS signal. Specifically, the timing generator 331 resets the number of clocks to “0” in response to the fall of the SOS signal. Thereafter, the timing generation unit 331 raises the SOS signal and increases the number of clocks by one each time the CLK signal falls.
Next, the timing generation unit 331 raises the instruction signal SHS in accordance with the next rise of the CLK signal when the number of clocks reaches a predetermined number, and the next of the CLK signal when the number of clocks further increases by a certain number. The instruction signal SHS is lowered in accordance with the rise. Thus, the start point and the end point of the effective scanning period ESR are the same number of clocks in any main scanning period SCT, that is, in the period in which the photosensitive drum 25Y is scanned with the reflected light from any deflection surface of the polygon mirror 271. It is aligned. In the example of FIG. 5, the instruction signal SHS rises at time T1 when the CLK signal rises immediately after the number of clocks reaches “2”, and the instruction signal at time T2 when the CLK signal rises immediately after the number of clocks reaches “64”. SHS falls. Thereby, in any main scanning period SCT, the start point T1 of the effective scanning period ESR corresponds to the clock number “2”, and the end point T2 corresponds to the clock number “64”.

  In parallel with these processes, the timing generation unit 331 reads the correction section information CRP from the correction section register 321 every time the SOS signal falls, and generates a timing signal TMS according to the coordinates of the boundary of the correction section indicated by the correction section information CRP. . Specifically, in the example of FIG. 5, the coordinate of the first boundary indicated by the correction section information CRP is “4”. Therefore, from the time when the count of the clock number reaches “4”, that is, from the rising time T1 of the instruction signal SHS. The timing generation unit 331 raises the timing signal TMS at the time point CP1 increased by “2”. Since the timing signal TMS has a pulse width equal to the clock period, the timing signal TMS falls when the clock period elapses from the rising edge. Since the coordinate of the next boundary indicated by the correction section information CRP is “6”, the timing generation unit 331 increases the count of the number of clocks by 6−4 = 2 from the time point CP1 when the timing signal TMS is first raised. At the time point CP2, the timing signal TMS is raised again. Thereby, the range from the clock number “4” to “6” is defined as the first correction section CS1. Thereafter, similarly, the timing at each time point CP3,..., CP (n-1), CPn, CP (n + 1),... (Integer n.gtoreq.4) at which the clock number counts coincide with the boundary coordinates indicated by the correction section information CRP. The generation unit 331 raises the timing signal TMS. Thus, the timing generation unit 331 defines the timing at which the reflected light of the polygon mirror 271 reaches each boundary CP1,... Of the correction section and the time zone in which each correction section CS1,. .

  FIG. 6 is a timing chart showing the waveform of the timing signal TMS for each deflection surface of the polygon mirror 271. Referring to FIG. 6, correction is made according to which of the seven deflecting surfaces (hereinafter referred to as “deflecting surfaces 1, 2,..., 7”) of the polygon mirror 271 reflects the light LL of the light source 260. The combination of coordinates representing the boundaries of the sections CS1, CS2,. This is because the correspondence table between the correction value for the light quantity of the laser oscillator LD managed by the storage unit 320 and the boundary of the correction interval differs for each deflection surface, and is set in the correction interval register 321 according to the identification information from the identification unit 325. This is because the corrected correction section information CRP is switched for each deflection surface. In particular, since the tilt error is slightly different between the deflection surfaces 1-7, the timing of reflecting light toward the SOS sensor 303 is strictly different. As a result, the starting point of the first correction section CS1 differs for each deflection surface. The reason why the other boundaries of the correction section are different for each deflection surface will be described later.

<DAC>
The DAC 332 reads the correction value indicated by the correction value information CRV from the correction value register 322 every time the timing signal TMS rises, and amplifies the output current IBS of the VI converter 316 based on the correction value. As a result, the correction value, which is a digital value, is converted into the amount of the amplified output current ICR, which is an analog value. Here, the correction value represents the ratio of the corrected light emission amount to the reference value of the light emission amount of the laser oscillator LD for each boundary of the correction section. On the other hand, the amount of output current IBS before amplification is adjusted by SH unit 310 to a value when the light emission amount of laser oscillator LD matches the reference value. Therefore, the DAC 332 adjusts the amplification factor of the output current IBS to the correction value particularly at each rising point of the timing signal TMS. Thereby, at the time when the reflected light of the polygon mirror 271 is irradiated to each boundary of the correction section, the light emission amount of the laser oscillator LD is corrected with respect to the reference value (= the amount of current ICR after amplification / current IBS before amplification). The amount). This change cancels fluctuations in the exposure amount that would appear at each boundary of the correction interval if the light emission amount remains the reference value. As a result, the exposure amount at each boundary when the modulation unit 340 does not modulate the amplified output current ICR is made uniform.

  The DAC 332 further linearly interpolates the correction values assigned to the boundaries at both ends of each correction section, and linearly changes the amplification factor according to the interpolation value. Specifically, the DAC 332 equally divides the difference in correction values between the boundaries at both ends of each correction section by a ratio of a fixed minute time to the time length of each correction section, and the amplification factor is the boundary at the start of the correction section. From the correction value for, a certain amount of division is changed per minute time. As a result, the amplification factor changes to a correction value for the next boundary between each rising edge of the timing signal TMS, strictly in a fine step shape, and approximately linearly.

  7A and 7B respectively show correction values set by the DAC 332 and main scanning on the drum 25Y when the photosensitive drum 25Y is scanned with reflected light from the deflection surfaces 1 and 2 of the polygon mirror 271. FIG. It is a graph which shows the relationship between a position. 8 shows the coordinates of the black dots CP1, CP2, CP3,..., CP (n-1), CPn, CP (n + 1),... Plotted in the graph of FIG. 4 is a table showing the correspondence between values for each deflection surface of a polygon mirror 271.

Referring to FIG. 7, the horizontal axis of the graph indicates the “main scanning position” on the photosensitive drum 25Y, that is, the coordinates in the main scanning direction, and the number of clocks (0 to 64) that the timing generation unit 331 counts during the effective scanning period ESC. ). On the other hand, the vertical axis of the graph represents a correction value for the light emission amount of the laser oscillator LD of the first semiconductor laser 26Y as a ratio to the minimum value of the light emission amount.
The black dots CP1,... Are connected by a broken curve CRC. This curve CRC shows the correspondence between the correction value for the light emission amount of the laser oscillator LD that is really necessary for canceling the exposure amount variation and the main scanning position. Hereinafter, this curve CRC is referred to as a “correction curve”. When the light emission amount of the laser oscillator LD is corrected along the correction curve CRC, the exposure amount fluctuation that would appear if the light emission amount was constant can be canceled out in the entire scanning range. The correction curve CRC is estimated by calculation from the refractive index of an optical element existing 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 the optical path is actually measured by experiment. Measured from the amount of light that has passed. The correction values indicated by the black points CP1,..., That is, the correction values defined by the correction value information CRV shown in the table of FIG. 8, represent samples sampled from the correction curve CRC. Details of the correction curve CRC will be described later.

  The black spots CP1,... Are further connected by a solid broken line CRB. The broken line CRB indicates the correspondence between the linear interpolation value by the DAC 332 and the main scanning position. In particular, at each black point CP1,..., The angle of the broken line CRB generally changes discontinuously. This is because the DAC 332 linearly changes the amplification factor ICR / IBS in accordance with the interpolation value from one rising edge to the next rising edge of the timing signal TMS, while at one rising edge, one of the correction values to be interpolated is corrected value information. By changing to the next value indicated by CRV. As shown in FIG. 7, since the broken line CRB has a high degree of approximation to the correction curve CRC, the change in the light emission amount of the laser oscillator LD accompanying the amplification of the output current ICR by the DAC 332 substantially changes the exposure amount and the entire scanning range. Can be offset.

  Referring to FIG. 7, the interval between the main scanning positions indicated by the black points CP1,..., That is, the interval between the boundaries of the correction section, is not uniform, but 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. Further referring to FIG. 7A, the scanning range is divided into a first region GNR in which the main scanning position exceeds “16” and a second region STR of “16” or less, depending on the boundary of the correction section. Divided. The same applies to (b) of FIG. The interval ΔPD between the boundaries CP1,... Located in the second region STR is narrower than the interval ΔPS between the boundaries..., CP (n−1), CPn,. That is, the width of the correction section is narrower in the second region STR than in the first region GNR. The difference in width is that 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, and intermediate value of the change rate of the correction value with respect to the main scanning position. Or because the representative value in statistics such as mode is large. Details of the correction section setting conditions will be described later.

  7A and 7B, the light irradiated onto the photosensitive drum 25Y is reflected from the deflection surface 1 of the polygon mirror 271 and the deflection surface 2 as apparent. In this case, the black spots CP1,. This is because the correction curve CRC is deformed depending on which of the deflecting surfaces 1-7 of the polygon mirror 271 is the irradiation light to the photosensitive drum 25Y. The black points CP1,... Are set so that the degree of approximation of the broken line CRB with respect to the correction curve CRC is sufficiently high, so that the black points CP1,. As a result, as shown in FIG. 8, since the correspondence table between the correction value and the boundary of the correction section is different for each deflection surface, the correction section information CRP set in the correction section register 321 according to the identification information from the identification unit 325. Switches for each deflection surface. Therefore, since the timing generation unit 331 changes the rising point of the timing signal TMS for each deflection surface, the combination of the main scanning positions representing the boundary of the correction section differs for each deflection surface. Similarly, the correction value information CRV set in the correction value register 322 is switched for each deflection surface, and accordingly the DAC 332 changes the amplification factor in a different pattern for each deflection surface, so that correction at each boundary of the correction section is performed. The value differs for each deflection surface. In this way, the broken line CRB has a high degree of approximation to the correction curve CRC regardless of whether the irradiation light to the photosensitive drum 25Y is reflected light from any deflection surface of the polygon mirror 271. Therefore, the change in the light emission amount of the laser oscillator LD accompanying the amplification of the output current ICR by the DAC 332 can substantially cancel the fluctuation of the exposure amount in the entire scanning range in any main scanning period. Details of the reason why the correction curve CRC is deformed for each deflection surface of the polygon mirror 271 will be described later.

-Modulator-
Referring to FIG. 4, the modulation unit 340 includes a switching unit 341. The switching unit 341 conducts or shuts off the output current ICR from the DAC 332 of the correction unit 330 to the laser oscillator LD by opening and closing. In particular, in each effective scanning period ESC, the switching unit 341 performs the opening / closing operation in a pattern based on the Y gradation value represented by the image data VDS in synchronization with the CLK signal. Due to the intermittent change of the output current ICR accompanying this, the blinking pattern of the laser oscillator LD is modulated into a pattern based on the Y gradation value for each exposure point. On the other hand, in each blanking period FBR, the switching unit 341 maintains a closed state. Thereby, since the output current ICR is constantly maintained, the light emission amount of the laser oscillator LD is maintained at a value corresponding to the amount of the current ICR.

[Exposure variation in the main scanning direction]
FIG. 9A is a schematic diagram showing the incident angle of the laser light LL of the light source 260 with respect to the polygon mirror 271, and FIG. 9B is a schematic diagram showing the incident angle of the reflected light RL of the polygon mirror 271 with respect to the fθ lens 273. FIG. As shown in FIG. 9A, as the polygon mirror 271 rotates, the incident angles θ ₁ and θ ₂ of the laser beam LL of the light source 260 with respect to the deflection surface 701 change, so the reflection angles θ ₁ and θ ₂ are also Change. Furthermore, as shown in FIG. 9B, the change causes the incident angles θ ₁ and θ ₂ of the reflected light RL of the polygon mirror 271 to the fθ lens 273 to change, so that the refraction angles φ ₁ and φ ₂ also change.

In general, when light is incident obliquely on the boundary surface between the media, a part of the light is reflected at the boundary surface, while the other portion is refracted and 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 reflectivity for light with different incident angles θ ₁ and θ ₂ is different on the deflection surface of the polygon mirror 271, and the transmittance for light with different incident angles θ ₁ and θ ₂ is different on the lens surface of the fθ lens 273. Similarly, the reflectivity for light of different incident angles is different on the reflecting surfaces of the folding mirrors (28Y, 29Y),. Further, in the fθ lens 273, if the refraction angles φ ₁ and φ ₂ are different, the distance through which the lens material, for example, a transparent resin is transmitted, is different.

  As a result, the reflectance and transmittance of the optical elements 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 of the laser light LL of the light source 260. In this case, even if the light source 260 is kept constant during the main scanning period, the amount of light applied to the photosensitive drums 25Y,... Varies with the change in the deflection angle of the laser light LL.

  (C) of FIG. 9 is a graph which shows the fluctuation | variation which appears in the exposure amount of the photoconductive drum 25Y ...... on condition that the light source 260 maintains light quantity constant. The horizontal axis of this graph represents the main scanning position on the surface of the photosensitive drum 25Y,..., And the vertical axis represents the fluctuation range of the exposure amount on the surface as a ratio to the maximum value. The curve EXC of this graph shows the following. First, the exposure amount has a peak (= 100%) near the main scanning position “6”. On both sides, in the range of the number of clocks “± 6”, as the distance from the peak increases, the exposure amount abruptly attenuates by about 10 to 20%, and shows a minimum value around the main scanning position “16”. Further, at the main scanning position “20” or more, the exposure amount gradually changes at about 75 to 80% of the peak value.

  In order to improve the image quality of the toner image, the “unevenness” of the charge amount in one line of the electrostatic latent image due to this fluctuation of the exposure amount is suppressed, and the exposure amount for a certain gradation value is set over the scanning range on the photoreceptor. It is necessary to keep it uniform. For this purpose, the light quantity of the light source 260 may be corrected so as to cancel out the cause of the fluctuation of the exposure amount, that is, the change in the reflectance / transmittance of the optical element accompanying the change in the deflection angle of the laser beam. Specifically, the correction value for the amount of light may be an inverse ratio to the exposure amount fluctuation ratio indicated by the curve EXC in FIG. A curve obtained by plotting the inverse ratio for each main scanning position is a correction curve CRC shown in FIG. The variation ratio of the exposure amount indicated by the curve EXC is calculated from the refractive indexes of the optical elements such as the polygon mirror 271 and the fθ lens 273, or is measured from the actual irradiation light amount on the photosensitive drum 25Y,. A correction curve CRC is obtained by calculating the inverse ratio of.

[Relationship between polygon mirror deflecting surface shape and exposure]
Each deflection surface of the polygon mirror 271 actually deviates from an ideal shape. This deviation fluctuates the distribution of the irradiation light amount on the photosensitive drums 25Y,. Specifically, the tilting of the deflection surface, that is, the inclination of the polygon mirror 271 with respect to the rotation axis direction changes the irradiation light amount itself. The tilt error of the deflection surface, that is, the unevenness in the circumferential direction of the polygon mirror 271 gives an error in the correspondence between the rotation angle of the polygon mirror 271 and the main scanning position on the photosensitive drum 25Y,. As a result, the main scanning positions of the maximum point (peak) and the minimum point (valley bottom) of the irradiation light amount are changed.

−Relationship between surface tilt and exposure amount−
FIG. 10A is a schematic diagram for explaining the relationship between the surface tilt of the polygon mirror 271 and the direction in which the reflected light is emitted. Referring to FIG. 10A, a solid line 701N indicates a deflection surface parallel to the rotation axis RS of the polygon mirror 271, and a broken line 701B and a one-dot chain line 701F are inclined in opposite directions with respect to the rotation axis RS. The deflection surface is shown. The tilt angles + ψ and −ψ of these tilted deflection surfaces 701B and 701F represent the magnitude of the surface tilt.

  The laser beam LL from the light source 260 enters the polygon mirror 271 from a direction perpendicular to the rotation axis RS. At this time, the reflected light NRL from the parallel deflection surface 701N is emitted in a direction perpendicular to the rotation axis RS. On the other hand, the reflected lights BRL and FRL from the tilted deflecting surfaces 701B and 701F are emitted in a direction tilted from a direction perpendicular to the rotation axis RS by angles + 2ψ and −2ψ which are twice the surface tilt. Since the reflectivity of the deflecting surface is different depending on the emission direction in this way, the amount of light irradiated from the inclined deflecting surfaces 701B and 701F to the photosensitive drum 25Y,. . In this way, the tilting of the deflecting surface changes the irradiation light amount on the photosensitive drums 25Y,.

-Relationship between deflection surface tilt error and exposure amount-
FIG. 10B is a partial top view enlarging the vicinity of one of the apexes of a regular heptagon that forms the circumference of the polygon mirror 271. Referring to FIG. 10B, a solid line 701N represents an ideal deflection surface having a tilt error of “0”, that is, a completely smooth deflection surface, and a one-dot chain line 701P bulges outside the polygon mirror 271. A broken surface 701H represents a deflection surface that is recessed inward. When the polygon mirror 271 is positioned at a rotation angle θ (0 <θ <2π / 7) with respect to the reference direction RD shown in FIG. 10B, an ideal deflection surface 701N and a deflection surface 701P inclined with respect to this are provided. The incident angle of the laser beam LL is different from 701H. As a result, a difference occurs in the deflection angle of the laser beam LL between these deflection surfaces 701N, 701P, and 701H. As a result, the reflected light NRL from the ideal deflection surface 701N advances to the deflection angle φ _R and enters the SOS sensor 303, whereas the reflected light PRL and HRL from the inclined deflection surfaces 701P and 701H are both Advancing in a direction different from the deflection angle φ _R deviates from the SOS sensor 303.

Thus, the tilt error, that is, the unevenness in the circumferential direction of the polygon mirror 271 gives an error to the deflection angle φ _R that should correspond to the specific rotation angle θ of the polygon mirror 271. Further, since the tilt error exists at various locations on the deflection surface and varies from location to location, the error in the correspondence between the rotation angle of the polygon mirror 271 and the deflection angle varies for each rotation angle.
-Difference in exposure fluctuation pattern between different deflection surfaces-
Any of the seven deflection surfaces of the polygon mirror 271 actually deviates from the ideal shape, and is different for each deflection surface. Therefore, the distribution of the irradiation light amount on the photosensitive drum 25Y,... Varies depending on which deflection surface reflects the laser light LL of the light source 260. Specifically, since the amount of surface tilt differs for each deflection surface, the amount of light applied to the same place in the main scanning direction on the photosensitive drum 25Y,. Also, since the tilt error differs for each deflection surface, the rotation angle of the polygon mirror 271 when reflecting the laser beam LL to the deflection angle that forms an image at the same location in the main scanning direction on the photosensitive drum 25Y,. Different for each. As a result of synthesizing these differences, the variation pattern of the exposure amount on the photosensitive drums 25Y,... Shown in the graph of FIG.

  FIG. 10C is a graph showing the difference appearing in the exposure amount fluctuation pattern EXC on the photosensitive drum 25Y shown in FIG. 9C according to the difference in the shape of the deflection surface. Curves EXS and EXG shown in (c) of FIG. 10 are similar to the curve EXC shown in (c) of FIG. 9, and the exposure amount variation pattern that appears under the condition that the light source 260 keeps the light quantity constant, that is, the photoconductor. This represents the relationship between the main scanning position on the drum 25Y,... And the variation ratio of the exposure amount to the maximum value.

  The broken curve EXG shown in FIG. 10C has a smaller fluctuation range at each main scanning position than the solid curve EXC. This difference is mainly due to a decrease in the amount of light applied to the photosensitive drums 25Y,... Accompanying the deflection of the deflection surfaces 701B and 701F shown in FIG. Although not shown in FIG. 10C, when the amount of light applied to the photosensitive drums 25Y,... Such expansion / contraction of the fluctuation range of the exposure amount does not change the main scanning position between the maximum point (peak) PKP and the minimum point (valley bottom) MNP of the fluctuation.

  The alternate long and short dash line curve EXS shown in FIG. 10C is moved in the main scanning position where the variation ratio is the same as that of the solid curve EXC. This difference is mainly caused by the fluctuation of the deflection angle accompanying the tilt error of the deflection surfaces 701P and 701H shown in FIG. In this case, both the peak PKP and valley bottom MNP of the fluctuation of the exposure amount are displaced in the main scanning direction. In general, the direction and amount of this displacement are different at the peak and the valley bottom.

[Conditions for setting the boundary of correction section]
The exposure fluctuation pattern shown by the curve EXC in FIG. 9C is calculated from the refractive index of the optical elements 271, 273,..., Or measured by experiment, and the correction curve shown in FIG. CRC is obtained. Further, the correction value for the light amount of the light source 260 is sampled from the correction curve CRC for each boundary of the correction section indicated by the black points CP1,. In this case, the more the boundaries CP1,..., The smaller the error of the broken line CRB connecting them to the correction curve CRC, that is, the sampling error. Therefore, it is desirable that the number of samples is as large as possible. However, since the capacity of the correction value register 322 has an upper limit, the total number of boundaries also has an upper limit. Therefore, there is a limit to the suppression of sampling error due to the increase in the number of samples.

  On the other hand, if the boundary interval is constant, the sampling error is smaller as the slope of the correction curve CRC is gentler in the correction section between them. Therefore, in order to keep the sampling error small in the entire correction curve CRC without increasing the total number of boundaries, all the correction sections are preferentially reduced by narrowing the boundary interval with respect to the region where the inclination of the correction curve CRC is steep. In this case, the sampling error may be suppressed within an allowable range, for example, within a few percent.

  (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 slope of the correction curve CRC is generally larger in the second region STR than in the first region GNR. In this case, if the boundary of the correction section is set at the same interval in the first region GNR and the second region STR, the sampling error exceeds the allowable range in either of them. Therefore, the intervals between the boundaries in the second region STR, CPB, CPm, CP (m + 1),... Are set more densely than the intervals between the boundaries CPI, CPk, CP (k + 1),.

  For example, there are the following two conditions for setting the boundary of the correction section. The first setting condition is that “the difference between correction values is within an allowable range between two adjacent boundaries”. The second setting condition is that “in the portion of the correction curve that changes monotonically, the difference between the correction values between two adjacent boundaries is made uniform”. Under this condition, the difference between the correction values is constant between any two adjacent boundaries.

-First setting condition-
For example, the first setting condition is adopted in the first region GNR shown in FIG. Specifically, 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 “8” are set as boundaries. Next, the difference in 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), while it is allowable between the other boundaries. Below the upper limit. Therefore, a new boundary is added between the three boundaries CPI, CPk, CP (k + 1), and the difference in correction values between the 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% of the difference in correction values between the boundaries may be increased within a range where the sampling error is within the allowable range.
-Second setting condition-
For example, the second setting condition is adopted in the second region STR shown in FIG. Specifically, first, boundaries CPT and CPB are set at the peak and valley of the correction curve CRC in the second region STR, and a difference between the correction values, for example, about 25% can be set between them. It is equally divided into the number of large boundaries, for example, four. Next, the point where the correction value differs from the valley CPB or peak CPT of the correction curve CRC 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 for the outside.

Note that the number of boundaries that can be set in the monotonically changing correction curve portion 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 the allowable range. Is done.
Regardless of which setting condition is adopted, the boundary interval is set to an integral multiple of the minimum value (the number of clocks = “2” in FIG. 11B). In this case, the timing generator 331 only counts the CLK signal by its minimum value (for example, “2”), and synchronizes the rising edge of the timing signal TMS with the timing when the reflected light of the polygon mirror 271 reaches each correction section. Can do. Therefore, the circuit configuration of the timing generation unit 331 is simplified.

−Movement of boundary due to fluctuation of correction curve for each deflection surface−
As shown in FIG. 10 (d), the exposure fluctuation patterns EXC, EXG, EXS on the photosensitive drums 25Y,... Differ depending on the shape of the deflection surface of the polygon mirror 271. Therefore, the shape of the correction curve CRC representing the inverse ratio of the pattern also differs for each deflection surface.
FIG. 12A shows the vicinity of the peak PK1 of the correction curve CR1 employed when scanning the photosensitive drum 25Y with the reflected light from the deflecting surface 1 of the polygon mirror 271, and is set for that. It is a graph which shows boundary CP4, CP5, CP6 of a correction area. Referring to FIG. 12A, boundaries CP4, CP5, and CP6 are set on the peak PK1 of the correction curve CR1 and on both sides thereof. In the correction sections CS4 and CS5 between them, the correction value changes along the broken line CB1 indicating linear interpolation. Since this broken line CB1 has a high degree of approximation with respect to the correction curve CR1, the sampling error ES1 can be kept small.

In FIG. 12 (a), the alternate long and short dash line curve CR2 shows the vicinity of the peak PK2 of the correction curve CR2 employed when scanning the photosensitive drum 25Y,... With the reflected light from the deflecting surface 2 of the polygon mirror 271. Show. As shown in FIG. 12A, this peak PK2 is particularly different in the main scanning position from the peak PK1 of the correction curve CR1 for the reflected light from the deflecting surface 1.
FIG. 12B shows the boundary CP4, CP5, CP6, CP7 of the correction section with respect to the vicinity of the peak PK2 of the correction curve CR2 for the reflected light from the deflecting surface 2 as shown in FIG. It is a graph which shows the case where it sets to the same main scanning position. Referring to FIG. 12B, the peak PK2 of the correction curve CR2 at this time is different in the main scanning position from the peak PK1 shown in FIG. 12A, so any boundary CP4,. It deviates from the peak PK2. As a result, the broken line CB2 indicating the transition of the correction value in the correction sections CS4,..., CS6 between them has a low degree of approximation to the correction curve CR2, particularly in the vicinity of the peak PK2, and the sampling error ES2 is shown in FIG. Is greater than the value ES1 indicated by

  (C) in FIG. 12 shows the boundaries CQ4, CQ5, and CQ6 of the correction section with respect to the vicinity of the peak PK2 of the correction curve CR2 shown in (b), from CP4, CP5, and CP6 shown in (b) in FIG. It is a graph which shows the case where it is displaced. As indicated by black points CQ4,..., CQ6 in FIG. 12C, one of the boundaries CQ5 after the displacement is different from that before the displacement indicated by the white circle CP5 in FIG. The peak PK2 is set. As a result, the degree of approximation with respect to the correction curve CR2 of the broken line CB3 indicating the transition of the correction values in the correction sections CS4, CS5 between the boundaries CQ4,..., CQ6 after the displacement is improved particularly near the peak PK2, and the sampling error ES3 Decreases from the value ES2 shown in FIG.

[Control Flowchart for Optical Scanning Unit]
FIG. 13 is a flowchart of control for the optical scanning unit 26. This process is started by starting a print job.
In step S <b> 101, the main controller 60 activates the semiconductor lasers 26 </ b> Y of the light source 260 and the motor 272. As a result, the semiconductor lasers 26Y,... Start to emit light and the polygon mirror 271 starts to rotate, so that the SOS sensor 303 validates the SOS signal in the period of the main scanning period. Thereafter, the process proceeds to step S102.

In step S102, the identification unit 325 measures the period of the SOS signal, and identifies from which deflection surface of the polygon mirror 271 the reflected light from the SOS sensor 303 is detected based on the measured value. The identification unit 325 further causes the storage unit 320 to search a correspondence table regarding the identified deflection surface. Thereafter, the process proceeds to step S103.
In step S103, the control unit 300 of the optical scanning unit 26 monitors whether or not the instruction signal SHS is valid for each of the four electronic circuits 300Y,. If the instruction signal SHS is valid, the process proceeds to step S104, and if not, the process proceeds to step S105.

  In step S104, since the instruction signal SHS is valid, the blanking period FBR. At this time, since the switch 314 of the SH unit 310 maintains the connection between the differential amplifier 313 and the capacitor 315, the capacitor 315 is charged / discharged by the output current ISH of the differential amplifier 313. Further, the VI converter 316 converts the voltage VSH between both ends after charging / discharging of the capacitor 315 into an output current IBS to the semiconductor laser 26Y,. The output current ISH of the differential amplifier 313 is proportional to the difference VFB−VRF between the voltage drop amount VFB of the resistor 311 and the output voltage VRF of the reference voltage source 312, and the voltage drop amount VFB is a light amount sensor PD of the semiconductor laser 26Y,. Is proportional to the output current IFB. In this way, the output current IBS to the laser oscillator LD is adjusted so that the light emission amount of the laser oscillator LD of the semiconductor laser 26Y,... Matches the reference value corresponding to the output voltage VRF of the reference voltage source 312. Thereafter, the process is repeated from step S102.

  In step S105, since the instruction signal SHS is not valid, the effective scanning period ESR. At this time, since the switch 314 of the SH unit 310 disconnects the connection between the differential amplifier 313 and the capacitor 315, the voltage VSH across the capacitor 315 is maintained at a constant value. Therefore, since the VI converter 316 maintains the output current IBS to the laser oscillator LD at a constant amount, the light emission amount of the laser oscillator LD before modulation by the image data is maintained at the reference value. Thereafter, the process proceeds to step S106.

In step S106, the control unit 300 monitors whether or not the timing signal TMS is valid for each electronic circuit 300Y,. If the timing signal TMS becomes valid, the process proceeds to step S107, and if not valid, the process proceeds to step S108.
In step S107, since the timing signal TMS is valid, the reflected light of the polygon mirror 271 reaches a new correction section. At this time, the DAC 332 of the correction unit 330 reads the correction value indicated by the correction value information CRV from the correction value register 322, and linearly interpolates the correction values for the boundaries at both ends of the new correction section. Thereafter, the process proceeds to step S108.

In step S108, the DAC 332 amplifies the output current IBS of the VI converter 316 to the output current ICR to the laser oscillator LD, and changes the amplification factor linearly according to the interpolation value calculated in step S107. Thereby, the light emission amount of the laser oscillator LD is corrected. Thereafter, the process proceeds to step S109.
In step S109, the switching unit 341 of the modulation unit 340 opens and closes with a pattern based on the gradation value of each color represented by the image data VDS. As a result, the output current ICR from the DAC 332 to the laser oscillator LD changes intermittently, so that the blinking pattern of the laser oscillator LD is modulated into a pattern based on the gradation value of each color. Thereafter, the process proceeds to step S110.

In step S110, the control unit 300 confirms whether unprocessed image data remains. If unprocessed image data remains, the process is repeated from step S103, and if not, the process proceeds to step S111.
In step S111, the control unit 300 notifies the main control unit 60 that unprocessed image data does not remain. Accordingly, the main controller 60 stops the semiconductor lasers 26Y,... Of the light source 260 and the motor 272 of the scanning optical system. As a result, the emission of the semiconductor lasers 26Y,... Stops, and the rotation of the polygon mirror 271 stops. Thus, the process ends.

[Flowchart of signal processing by timing generator]
FIG. 14 is a flowchart of signal processing by the timing generation unit 331. This process is started each time the SOS sensor 303 validates the SOS signal.
In step S201, since the SOS signal is valid, the timing generator 331 resets the number of clocks to “0” and starts counting, and reads the correction section information CRP from the correction section register 321. The timing generation unit 331 further invalidates the SOS signal. Thereafter, the process proceeds to step S202.

In step S202, the timing generation unit 331 monitors the number of clocks and confirms whether or not the number of clocks has reached the main scanning position at the start point T1 of the effective scanning period ESC, that is, “2” in the example of FIG. If the number of clocks has reached the main scanning position of the start point T1, the process proceeds to step S203, and if not, step S202 is repeated.
In step S203, since the number of clocks has reached the main scanning position “2” of the start point T1 of the effective scanning period ESC, the timing generation unit 331 invalidates the instruction signal SHS. As a result, the main scanning period SCT shifts from the blanking period FBR to the effective scanning period ESC. Thereafter, the process proceeds to step S204.

In step S204, the timing generation unit 331 checks the number of clocks with the main scanning position of the next boundary of the correction section indicated by the correction section information CRP. If the number of clocks has reached the main scanning position, the process proceeds to step S251. If not, the process proceeds to step S252.
In step S251, since the number of clocks has reached the main scanning position on the next boundary, the timing generation unit 321 enables the timing signal TMS. Thereafter, the process proceeds to step S206.

In step S252, since the number of clocks has not reached the main scanning position of the next boundary, the timing generation unit 321 invalidates the timing signal TMS, and maintains the state if it has already been invalidated. Thereafter, the process proceeds to step S206.
In step S206, the timing generation unit 321 confirms whether the number of clocks has reached the main scanning position at the end point T2 of the effective scanning period ESC, or “64” in the example of FIG. If the number of clocks has reached the main scanning position of the end point T2, the process proceeds to step S207, and if not, the process is repeated from step S203.

In step S207, since the number of clocks has reached the main scanning position “64” at the end point T2 of the effective scanning period ESC, the timing generation unit 331 enables the instruction signal SHS. As a result, the main scanning period SCT shifts from the effective scanning period ESC to the blanking period FBR. Thereafter, the process ends.
[Advantages of Embodiment 1]
As described above, when the optical scanning unit 26 according to the first embodiment of the present invention scans each correction section on the surface of the photosensitive drum 25Y,... With the reflected light of the polygon mirror 271, the light source 260 has a correction value for the correction section. Correct the amount of light. At this time, as shown in FIG. 7, the boundary CP1,... Of the correction section is set so that the combination of the main scanning positions is different for each deflection surface of the polygon mirror 271. As a result, even if the correction curve CRC is deformed depending on which of the deflecting surfaces 1-7 of the polygon mirror 271 is irradiated to the photosensitive drums 25Y,. The broken line CRB indicating the linear interpolation of the correction values can be set so that the degree of approximation to the correction curve CRC is sufficiently high without increasing the total number of boundaries of the correction sections. As a result, the correction of the light emission amount of the laser oscillator LD by the DAC 332 can substantially cancel the fluctuation of the exposure amount in the entire scanning range in any main scanning period.

In this way, the optical scanning unit 26 can improve the effect of suppressing uneven exposure by correcting the light amount of the light source 260 without increasing the memory capacity necessary for storing the correction value, such as the correction value register 322. As a result, the 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. 2 are merely examples, and other values may be used. Further, the number of lasers that can be output from the semiconductor lasers 26Y,... Is not limited to one, but may be two or more multilasers. Furthermore, instead of the semiconductor laser 26Y,..., An LED may be used.
(C) The structure of the optical system such as the polygon mirror 271 and the fθ lens 273 shown in FIG. 2 is merely an example, and other structures may be used. For example, the number of side 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. . In accordance with the position of the polygon mirror, the arrangement of other optical elements such as an fθ lens and a folding mirror may be changed.

(D) In FIG. 4, the correction unit 330 corrects the output current amount to the laser oscillator LD before the modulation unit 340. In addition, the correction unit may correct the output current amount after the modulation by the image data after the modulation unit 340.
(E) The waveform of each signal shown in FIG. 5 is merely an example, and its logic may be positive or negative. The pulse to be counted and the pulse to be regarded as each end point of the main scanning period STC and the effective scanning period ESR The edge may be either rising or falling.

  (F) The correction section information CRP indicates the main scanning position of each boundary of the correction section as reflected by the polygon mirror 271 at the boundary from the start of the effective scanning period ESR for each deflection surface of the polygon mirror 271 as shown in FIG. It is defined by the number of clocks until the time when the light RL is irradiated. In addition, the correction section information CRP may define the main scanning position of the leading boundary and the width of each correction section by the number of clocks or the like. In this case, the timing generation unit 331 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.

  (G) The correction value information CRV is a correction value for the light emission amount of the laser oscillator LD, as shown in FIG. 8, for each deflection surface of the polygon mirror 271, the output current ICR after amplification with respect to the amount of current IBS before amplification by the DAC 332. It is defined by the ratio of the amount of light, that is, the ratio of the light emission after correction to the reference value of the light emission of the laser oscillator LD. In addition, the correction value includes the corrected light emission value itself, the difference between the value and the reference value, the amount of current necessary for causing the laser oscillator LD to emit light with that value, and the like. It is sufficient that the amount is defined by a value that can be matched with a value necessary for canceling a change in reflectance / transmittance of the optical system accompanying a change in the deflection angle of the polygon mirror 271.

  (H) Since the tilt error differs for each deflection surface, as shown in FIG. 10B, the rotation angle of the polygon mirror 271 when the laser beam LL is reflected toward the SOS sensor 303 differs for each deflection surface. As a result, the period of the SOS signal differs for each deflection surface. The identification unit 325 identifies which deflection surface of the polygon mirror 271 has detected the reflected light from the difference in the cycle. In addition, the identification unit 325 may identify the deflection surface based on the level of the SOS signal.

  For example, since the magnitude of the surface tilt differs for each deflection surface, the deviation amount of the reflected light RL in the axial direction of the polygon mirror 271 differs for each deflection surface, as shown in FIG. Therefore, since the reflectivity differs for each deflection surface, if the level of the SOS signal is changed by the SOS sensor 303 according to the detected amount of reflected light, the identification unit 325 can identify the deflection surface based on the level difference. it can.

In addition, the identification unit 325 identifies only a specific deflection surface of the polygon mirror 271 from the attribute of the SOS signal, and counts the pulses of the SOS signal after the point in time when the pulse of the SOS signal caused by the deflection surface is detected. Other deflection surfaces may be identified.
(I) The DAC 332 performs linear interpolation on the correction value, and changes the amplification factor for the output current IBS of the VI converter 316 substantially linearly along the broken line CRB shown in FIG. In addition, the DAC 332 may change the amplification factor stepwise by rough interpolation, and in particular, the amplification factor may be kept constant at the correction value for the boundary of the start edge in each correction section.

  (J) The DAC 332 performs linear interpolation of correction values by digital processing. That is, the DAC 332 subdivides each correction section into minute sections, and changes the amplification factor at a constant rate per minute section. In addition, the DAC may perform linear interpolation of correction values by analog processing using an analog integration circuit incorporated in the output unit. Specifically, the main body of the DAC first keeps the amplification factor constant at the correction value for each correction interval. On the other hand, the analog integration circuit time-integrates the amount of current amplified by the main body, and outputs an amount of current equal to the sum or difference between the amount proportional to the integrated value and the initial value. If the time constant of the integration circuit is set sufficiently longer than the time length of each correction section, the output current amount changes linearly.

  (K) As shown in FIG. 7, the scanning range on the photosensitive drum 25Y,... Is divided into a first region GNR in which the undulation of the correction curve CRC is relatively gentle and a second region STR in which the undulation is more severe. In addition, the interval ΔPD in the second region STR is set narrower than the interval ΔPS of the boundary of the correction section in the first region GNR. In addition, the boundary of the correction section may be set constant regardless of whether the correction curve CRC is undulating or not. In this case, as shown in FIG. 7, for any deflection surface, if the boundary main scanning position is set for each deflection surface so that the boundary of the correction section is located between the peak CPP and the valley bottom CP2 of the correction curve CRC. Good. As a result, as shown in FIG. 12, the sampling error can be kept small in the vicinity of the peak CPP and the valley bottom CP2 of the correction curve CRC.

  (L) In the first embodiment, as shown in FIG. 7, correction values are set over the entire scanning range on the photosensitive drums 25Y,. In addition, the light quantity of the light source may be corrected only in a part of the scanning range, for example, in a region where the correction curve CRC is undulating, such as the second region STR. On the other hand, for example, in a region where the undulation of the correction curve CRC is gradual, such as the first region GNR, the light amount of the light source may be uniformly set to a value corrected with the representative value of the correction value in that region. In that region, the boundary of the correction section may be set in common for any deflection surface of the polygon mirror 271.

  FIGS. 15A and 15B show correction values set by the DAC 332 and main scanning on the drum 25Y when the photosensitive drum 25Y is scanned with the reflected light from the deflecting surfaces 1 and 2 of the polygon mirror 271, respectively. It is a graph which shows the modification of the relationship between a position. The scan range shown in FIG. 15 is divided into a first region GNR having a comparatively wide boundary interval ΔPS and a second region STR narrower than that, as shown in FIG. However, unlike FIG. 7, FIG. 15 differs from FIG. 7 in the first region GNR regardless of whether the light irradiated to the photosensitive drum 25Y is reflected light from the deflection surfaces 1 and 2 of the polygon mirror 271. The main scanning position of the boundary is common.

  This is because no sharp peaks PK1, PK2 or deep valley bottoms as shown in FIG. 12 appear in the first region GNR. In this case, as shown in (a) and (b) of FIG. 12, the main scanning positions of the correction curves CR1 and CR2 peaks PK1 and PK2 are different between the deflection surfaces 1 and 2, so that the boundary CP5 of the correction section is the peak. Even if it deviates from PK2, the sampling error ES2 is small enough to be ignored. Therefore, in the first region GNR, the degree of approximation of the broken line CRB with respect to the correction curve CRC is maintained sufficiently high even if the boundary of the correction section is common regardless of any of the deflection surfaces 1 and 2.

  For any deflection surface, in the first region GNR, the main scanning position at the boundary of the correction section is set in common, thereby reducing the size of the correspondence table between the correction value and the correction section as shown in FIG. Is generally possible. Therefore, the memory capacity of the storage unit 320 can be further reduced. In addition, the degree of approximation of the broken line CRB with respect to the correction curve CRC is further increased by increasing the boundary of the correction section located in the second region STR using the memory capacity reduced for the correction section located in the first region GNR. Can be improved.

<< Embodiment 2 >>
The optical scanning unit according to the second embodiment of the present invention is mounted on a color laser printer, similar to the one according to the first embodiment 26. This optical scanning unit is different from that according to the first embodiment 26 only in that the control unit 300 includes a calculation unit, and other elements are the same as those of the first embodiment. Accordingly, only the differences will be described below, and the description of the first embodiment is used for similar elements.

  In the first embodiment, as shown in FIG. 8, a correspondence table between correction sections and correction values is set for all the deflection surfaces of the polygon mirror 271 and stored in the storage unit 320. On the other hand, in the second embodiment, the correspondence table is set only for a specific deflection surface of the polygon mirror 271 and stored in the storage unit 320, and the correspondence table relating to the other deflection surfaces is the operation unit corresponding to the specific deflection surface. Calculate from the table.

  FIG. 16 is a block diagram of an electronic circuit 400 included in the control unit of the optical scanning unit according to the second embodiment. Referring to FIG. 16, this electronic circuit 400 is different from each electronic circuit 300Y,... Shown in FIG. Further, in the electronic circuit 400, unlike the electronic circuits 300Y,... Shown in FIG. 4, the storage unit 320 only associates the boundary of the correction section and the correction value with respect to a specific deflection surface of the polygon mirror 271, for example, the deflection surface 1. The table is saved.

  The arithmetic unit 350 operates in response to the activation signal INT from the main control unit 60 when the printer 100 is turned on or when the image is stabilized. In the initialization period or the image stabilization period of the printer 100, the SOS sensor 303 activates the laser LL of the semiconductor laser 26Y,... Is detected to generate an SOS signal. Each time the identification unit 325 identifies the deflection surface of the polygon mirror 271 from the attribute of the SOS signal, the calculation unit 350 acquires the identification result, and the storage unit 320 saves the result based on the attribute of the SOS signal indicated by the result. The correspondence table for the other deflection surfaces 2-7 is calculated from the correspondence table for the deflection surface 1. The calculation unit 350 further stores the calculated correspondence table in the storage unit 320 at least while the printer 100 is powered on or until the next image stabilization time. Once the correspondence tables for all the deflection surfaces 1-7 are thus prepared, the storage unit 320 thereafter relates to the deflection surfaces in accordance with the deflection surface identification information from the identification unit 325, as in the first embodiment. The correspondence table is searched, and based on the table, correction interval information CRP is set in the correction interval register 321 and correction value information CRV is set in the correction value register 322.

  The computing unit 350 calculates the correspondence table related to the deflection surface 2-7 based on the attribute of the SOS signal indicated by the identification result from the identifying unit 325 as follows. As shown by the three curves EXC, EXG, and EXS in FIG. 10C, the change in the variation pattern of the irradiation light amount accompanying the change of the deflection surface is mainly composed of the following two types of synthesis. (1) Expansion and contraction of the fluctuation range caused by the tilting of the deflection surface shown in FIG. (2) Displacement in the main scanning direction due to the tilt error of the deflection surface shown in FIG. The amount of expansion / contraction of the fluctuation range can be estimated from the difference in the level of the SOS signal, and the amount of displacement in the main scanning direction can be estimated from the deviation of the timing at which the SOS signal becomes valid (falling in FIG. 5). Accordingly, the calculation unit 350 increases or decreases the correction value indicated by the correspondence table related to the deflection surface 1 by the ratio of the expansion / contraction amount estimated from the level of the SOS signal, and only the displacement amount estimated from the deviation of the timing at which the SOS signal becomes valid. The main scanning position (number of clocks) at the boundary of the correction section indicated by the correspondence table is increased or decreased. In this way, the calculation unit 350 calculates a new correspondence table and stores it in the storage unit 320 as a correspondence table regarding different deflection surfaces for each cycle (or level) of the SOS signal.

[Advantages of Embodiment 2]
As in the first embodiment, the optical scanning unit according to the second embodiment of the present invention sets the correction interval for each deflection surface of the polygon mirror 271 so that the combination of the main scanning positions representing the boundary is different. Thus, even if the correction curve CRC is deformed depending on which of the deflecting surfaces 1-7 of the polygon mirror 271 is applied to the photosensitive drums 25Y,. The broken line CRB indicating the linear interpolation can be set so that the degree of approximation to the correction curve CRC is sufficiently high without increasing the total number of boundaries of the correction sections. As a result, the correction of the light emission amount of the laser oscillator LD by the DAC 332 can substantially cancel the fluctuation of the exposure amount in the entire scanning range in any main scanning period. In this way, the optical scanning unit can improve the effect of suppressing uneven exposure by correcting the light amount of the light source 260 without increasing the memory capacity necessary for storing the correction value. As a result, the image quality of the printer 100 can be improved.

  As described above, the optical scanning unit according to the second embodiment is different from the one according to the first embodiment, and a correspondence table between correction boundary boundaries and correction values stored in advance in the storage unit 320 is used as a specific deflection of the polygon mirror 271. The calculation unit 350 calculates the correspondence table related to other deflection surfaces. Thereby, the memory capacity to be secured in the storage unit 320 as a storage area for the correspondence table can be reduced.

[Modification]
(M) The computing unit 350 calculates a correspondence table for another deflection surface from a correspondence table for a specific deflection surface based on the SOS signal. In addition, among the parameters representing the operation state of the printer 100 managed by the main control unit 60, the calculation unit 350 irradiates the photosensitive drums 25Y,..., Such as the bias potential of the photosensitive drums 25Y,. The correspondence table may be calculated based on what can estimate the fluctuation of the light amount. Further, in the manufacturing process of the printer 100, variations in the shape of the deflection surface 1-7 of the polygon mirror 271 such as surface tilt and tilt error, or fluctuations in the main scanning direction of the amount of light applied to the photosensitive drums 25Y,. Measurement may be performed, and the obtained measurement values may be stored in the storage unit 320, and the calculation unit 350 may calculate a correspondence table regarding other deflection surfaces based on the measurement values.

<< Embodiment 3 >>
The optical scanning unit according to the third embodiment of the present invention is mounted on a color laser printer, similar to the one according to the first embodiment 26. This optical scanning unit is different from that according to the first embodiment 26 only in that an actual measurement unit is mounted and the identification unit and the storage unit are replaced with a calculation unit, and other elements are different from those in the first embodiment. It is the same. Accordingly, only the differences will be described below, and the description of the first embodiment is used for similar elements.

In the first embodiment, as illustrated in FIG. 8, a correspondence table between the boundaries of the correction sections and the correction values is stored in the storage unit 320 in advance. On the other hand, in the third embodiment, the actual measurement unit actually measures the amount of light applied to the photosensitive drums 25Y,..., And the calculation unit uses the deflection surface of the polygon mirror 271 and the boundary of the correction section based on the fluctuation of the actual measurement value. Calculate the correction value.
FIG. 17A is a schematic diagram illustrating an example 500 of an actual measurement unit. Referring to FIG. 17A, the actual measurement unit 500 includes a half mirror 501 and a sensor array 502. The half mirror 501 is an optical element that replaces the folding mirrors 29Y, 29M, 29C, and 28K arranged one by one immediately below the respective photosensitive drums 25Y,... Shown in FIGS. The longitudinal direction is arranged in parallel to the axial direction of each photosensitive drum 25Y, that is, the main scanning direction. However, in FIG. 17A, for convenience of illustration, the half mirror 501 is shown above the photosensitive drum 25Y. Reflected light RL of the polygon mirror 271 is incident on the half mirror 501 through the fθ lens 273. The half mirror 501 reflects a part of the reflected light RL in the same manner as the folding mirrors 29Y,... Irradiates the photosensitive drum 25Y, and transmits the rest. The sensor array 502 is an array of a plurality of photodetectors arranged in parallel with the half mirror 501 on the opposite side of the half mirror 501 from the side on which the reflected light RL of the polygon mirror 271 is incident, and the reflected light RL. Among them, the main scanning position and the light amount of the light transmitted through the half mirror 501 are detected.

  The actual measurement unit 500 causes the light source 260 to emit a constant amount of light continuously for a predetermined time, for example, at the start or interval of job processing by the printer 100, and measures the amount of light transmitted through the half mirror 501 for each main scanning position. Even if this predetermined time is short, the time required for scanning the photosensitive drums 25Y,... With the reflected light from all the deflecting surfaces of the polygon mirror 271 by one main scanning period, that is, the polygon mirror 271 rotating at an equiangular speed. The rotation period, for example, 7 times the main scanning period is set. The actual measurement unit 500 further converts the actual measurement value at each main scanning position of the half mirror 501 into the amount of light applied to the photosensitive drum 25Y according to the transmittance at each main scanning position, and passes the actual measurement result to the calculation unit 360. .

  FIG. 17B is a perspective view schematically showing another example 510 of the actual measurement unit, and FIG. 17C is an enlarged view of a part of the cross section along the line cc shown in FIG. is there. Referring to FIGS. 17B and 17C, the actual measurement unit 510 includes an emission frame 511 and a shutter 512. The emission frame 511 is a rectangular plate-like member arranged one by one below each photosensitive drum 25Y, and the plate surface is parallel to the axial direction of each photosensitive drum 25Y. An exit window WND for opening the reflected light RL of the polygon mirror toward the photosensitive drum 25Y is opened in the exit frame 511. The exit window WND is a slit parallel to the axial direction of the photosensitive drum 25Y, that is, the main scanning direction. The shutter 512 is an opaque plate-like member elongated in the main scanning direction similarly to the shape of the exit window WND, and is attached to the exit frame 511 so that the exit window WND can be opened and closed. By opening / closing the shutter 512, the reflected light RL of the polygon mirror 271 is irradiated on or blocked from the photosensitive drum 25Y. As shown in FIG. 17C, a photodetector array 513 is installed at the center of the shutter 512. While the shutter 512 closes the emission window WND, the reflection of the polygon mirror 271 is reflected on this array 513. Light RL enters. This array 513 detects the main scanning position and the amount of light of the reflected light RL.

  The actual measurement unit 510 causes the light source 260 to emit a constant amount of light continuously for a predetermined time, for example, at the start or interval of job processing by the printer 100, closes the emission window WND with the shutter 512, and determines the amount of light incident on the shutter 512. Measured for each main scanning position. As in the case of the actual measurement unit 500 shown in FIG. 17A, the predetermined time is set to a rotation period of the polygon mirror 271 that rotates at a constant angular velocity, for example, seven times the main scanning period, at the shortest. Further, the attenuation of the reflected light RL due to diffusion or the like between the exit window WND and the photosensitive drums 25Y,... Is normally negligible, so the actual measurement unit 510 directly measures the actual measurement value and the actual measurement result of the amount of light applied to the photosensitive drum 25Y. To the calculation unit 360.

FIG. 18 is a block diagram of an electronic circuit 600 included in the control unit of the optical scanning unit according to the third embodiment. 18, this electronic circuit 600 is different from each electronic circuit 300Y,... Shown in FIG. 4 in that a calculation unit 360 is mounted instead of the storage unit 320 and the identification unit 325.
The computing unit 360 analyzes the variation pattern of the irradiation light amount to the photosensitive drum 25Y, indicated by the actual measurement unit 500 or 510 according to the actual measurement result between the job processes by the printer 100 or at the time of transition of the operation mode. . As a result, the calculation unit 360 associates the difference in the variation pattern with the difference in the deflection surface of the polygon mirror 271, determines the boundary of the correction section for each variation pattern corresponding to the different deflection surface, and calculates the correction value at each boundary. . Thus, the correction section is set so that the combination of the main scanning positions representing the boundary differs for each deflection surface of the polygon mirror 271.

  Referring to FIG. 18, the calculation unit 360 includes a correction interval register 361 and a correction value register 362. These registers 361 and 362 are both memory areas, and are preferably volatile. In parallel with the job processing of the printer 100, the calculation unit 360 identifies the deflection surface of the polygon mirror 271 from the attributes of the SOS signal, for example, the period. The calculation unit 360 further sets the correction section information CRP in the correction section register 361 and the correction value in the correction value register 362 based on the correction value for each correction section calculated from the variation pattern corresponding to the identified deflection surface. Information CRV is set. By referring to these registers 361 and 362, the correction unit 330 can operate in the same manner as in the first embodiment.

FIG. 19 is a flowchart of control over the optical scanning unit according to the third embodiment. This control differs from the control shown in FIG. 13 only in that it includes steps S201 to S203, and the other steps are the same. Therefore, only the details of steps S201 to S203 will be described below, and the description of FIG. 13 is used for details of similar steps.
In step S101, the main control unit 60 activates the semiconductor lasers 26Y,... Of the light source 260 and the motor 272, and the SOS sensor 303 validates the SOS signal in the period of the main scanning period accordingly. Thereafter, the process proceeds to step S102.

  In step S201, the measurement units 500 and 510 determine whether or not to measure the amount of light applied to the photosensitive drums 25Y,. In this determination, it is confirmed whether or not a condition for performing the actual measurement, such as “job has started” and “the elapsed time from the previous actual measurement has exceeded the threshold”, is satisfied. If this actual measurement is performed, the process proceeds to step S202, and if not, the process proceeds to step S102.

  In step S202, the actual measurement units 500 and 510 actually measure fluctuations in the amount of light applied to the photosensitive drums 25Y,. Specifically, the measurement units 500 and 510 cause the light source 260 to emit a constant amount of light continuously for a predetermined time, and the amount of light transmitted through the half mirror 501 during that time, or the amount of light incident on the shutter 512 blocking the exit window WND. Are measured for each main scanning position. Thereafter, the process proceeds to step S203.

  In step S203, the calculation unit 360 analyzes the variation pattern of the amount of light applied to the photosensitive drums 25Y,... According to the actual measurement results obtained by the actual measurement units 500 and 510 in step S202. As a result, the difference in the variation pattern is associated with the difference in the deflection surface of the polygon mirror 271, and the boundary of the correction section and the correction value there are calculated for each variation pattern corresponding to the different deflection surface. Thereafter, the process proceeds to step S102.

  In steps S <b> 102 to S <b> 109, processing similar to that according to the first embodiment is performed except that the calculation unit 360 plays the role of the identification unit 325. That is, the calculation unit 360 identifies the deflection surface of the polygon mirror 271 that has reflected light to the SOS sensor 303 from the cycle of the SOS signal, and the correction interval information CRP and the correction value information CRV related to the deflection surface are stored in the correction interval register 361. It is set in the correction value register 362. If the instruction signal SHS is valid, the blanking period FBR, so that the SH unit 310 is based on the output current IFB from the light quantity sensor PD of the semiconductor laser 26Y,..., And the light emission amount of the laser oscillator LD matches the reference value. Adjust the output current IBS to it. On the other hand, if the instruction signal SHS is not valid, the effective scanning period ESR is set, so the SH unit 310 maintains the output current IBS to the laser oscillator LD at a constant amount. In the effective scanning period ESR, the timing generation unit 331 generates the timing signal TMS according to the correction section information CRP. Each time the timing signal TMS becomes valid, the DAC 332 reads the correction value indicated by the correction value information CRV. Based on the correction value, the DAC 332 amplifies the output current IBS of the VI converter 316 to the output current ICR to the laser oscillator LD while linearly changing the amplification factor. As a result, the light emission amount of the laser oscillator LD is corrected, and in particular, linearly changes with a different slope for each correction section. The modulation unit 340 modulates the output current ICR to the laser oscillator LD based on the gradation value of each color represented by the image data VDS. As the laser oscillator LD of each color blinks in accordance with this modulation, the average light emission amount is adjusted to a value corresponding to the gradation value. Thereafter, the process proceeds to step S110.

In step S110, the control unit 300 confirms whether unprocessed image data remains. If unprocessed image data remains, the process is repeated from step S201, and if not, the process proceeds to step S111.
In step S111, the control unit 300 notifies the main control unit 60 that unprocessed image data does not remain. Accordingly, the main controller 60 stops the semiconductor lasers 26Y,... Of the light source 260 and the motor 272 of the scanning optical system. Thus, the process ends.

[Advantages of Embodiment 3]
As in the first embodiment, the optical scanning unit according to the third embodiment of the present invention sets the correction section for each deflection surface of the polygon mirror 271 so that the combination of the main scanning positions representing the boundary is different. Thus, even if the correction curve CRC is deformed depending on which of the deflecting surfaces 1-7 of the polygon mirror 271 is applied to the photosensitive drums 25Y,. The broken line CRB indicating the linear interpolation can be set so that the degree of approximation to the correction curve CRC is sufficiently high without increasing the total number of boundaries of the correction sections. As a result, the correction of the light emission amount of the laser oscillator LD by the DAC 332 can substantially cancel the fluctuation of the exposure amount in the entire scanning range in any main scanning period. In this way, the optical scanning unit can improve the effect of suppressing uneven exposure by correcting the light amount of the light source 260 without increasing the memory capacity necessary for storing the correction value. As a result, the image quality of the printer 100 can be improved.

  As described above, the optical scanning unit according to the third embodiment is different from the one according to the first embodiment. The measurement units 500 and 510 measure the amount of light applied to the photosensitive drums 25Y,. The correction value is calculated for each correction section. As a result, there is no need for a memory area for storing a correspondence table between correction section boundaries and correction values for a long period of time, such as the storage unit 320 according to the first embodiment. Reduction is possible.

[Modification]
(N) The actual measurement units 500 and 510 shown in FIG. 17 are both well-known technologies (see, for example, Patent Documents 8 and 9). In addition, the actual measurement unit is configured to measure any amount of light that can be converted into the amount of light irradiated to the photosensitive drum, such as the amount of light reflected on the surface of the light irradiated to the photosensitive drum. May be.

  The present invention relates to an optical scanning device. As described above, the boundary of a correction section is displaced for each deflection surface of a polygon mirror. Thus, the present invention is clearly industrially applicable.

DESCRIPTION OF SYMBOLS 100 Color laser printer 26 Optical scanning part 260 Light source 271 Polygon mirror 273 f (theta) lens 28Y-K, 29Y-C Folding mirror CRC correction curve CRB Folding line approximating the correction curve CP1, CP2, ... Correction section boundary GNR First correction curve Area STR Second area of correction curve ΔPS Width of correction section in first area ΔPD Width of correction section in second area

Claims (11)

An optical scanning device that forms an image on a photoconductor by exposure scanning;
A light source with variable light intensity,
A deflecting unit that periodically deflects the light by reflecting the light of the light source at each deflection surface of the polygon mirror while rotating the polygon mirror;
An optical system for imaging the reflected light of the polygon mirror on the surface of the photosensitive member;
A modulator for modulating the light quantity of the light source according to image data;
A correction unit that corrects the light amount of the light source before or after the modulation unit modulates when scanning the photosensitive member with the reflected light of the polygon mirror;
With
Coordinates in the main scanning direction are associated with regions of the photoconductor that scan with reflected light from each deflection surface of the polygon mirror, and the range that the coordinates can take is divided into a plurality of correction sections, and the plurality of corrections. A correction value for the light quantity of the light source is assigned to each boundary of the section,
While the region of the photoconductor corresponding to each correction section is scanned with the reflected light from each deflection surface of the polygon mirror, the correction unit is based on a correction value assigned to at least one of the boundaries of the correction section. Determine a correction value for the light quantity of the light source;
The correction intervals of the polygon mirror are different so that the combination of coordinates in the main scanning direction representing the boundary differs depending on which of the deflection surfaces of the polygon mirror corresponds to the region scanned with the reflected light. An optical scanning device characterized by being set for each deflection surface.   The boundary of the correction section at the maximum or minimum point in the main scanning direction of the exposure amount that appears when the photosensitive member is exposed and scanned with the reflected light from each deflection surface of the polygon mirror while maintaining the light quantity of the light source constant The optical scanning device according to claim 1, wherein: is set.   3. The optical scanning device according to claim 1, wherein the boundary of the correction section is set such that a distance to an adjacent boundary differs according to coordinates in the main scanning direction. When the photosensitive member is exposed and scanned with reflected light from one deflection surface of the polygon mirror while maintaining the light amount of the light source constant without correction, the scanning range of the reflected light includes the first region, A second region having a representative value of a change rate of an exposure amount with respect to a position in the main scanning direction being larger than the first region;
4. The optical scanning device according to claim 3, wherein a width of a corresponding correction section is set narrower in the second area than in the first area. The first region included in the scanning range of reflected light from one deflection surface of the polygon mirror and the first region included in the scanning range of reflected light from another deflection surface have common coordinates in the main scanning direction. Including parts,
The optical scanning device according to claim 4, wherein the boundary of the correction section corresponding to the common portion is set to the same coordinate in the main scanning direction. The correction unit is
A timing generation unit that generates a signal indicating the timing at which the coordinates in the main scanning direction of the imaging point by the optical system reach the boundary of each correction section;
6. The optical scanning device according to claim 1, wherein a correction value assigned to the boundary is acquired at a timing indicated by the signal. A storage unit storing a correspondence table between the boundary of the correction section and the correction value for each deflection surface of the polygon mirror;
A measurement unit for measuring the attribute of the reflected light of the polygon mirror;
Based on the measurement result by the measurement unit, an identification unit for identifying the reflected light from which deflection surface of the polygon mirror, the identification unit;
Further comprising
The timing generation unit sets a timing to be indicated by the signal based on a boundary of a correction section indicated by a correspondence table related to the deflection surface identified by the identification unit;
The correction unit updates the correction value at a timing indicated by the signal to a correction value assigned to a next boundary indicated by a correspondence table related to the deflection surface identified by the identification unit. Optical scanning device. A measurement unit for measuring the attribute of the reflected light of the polygon mirror;
A calculation unit that calculates a correspondence table between a boundary of a correction section and a correction value based on a measurement result by the measurement unit for each deflection surface of the polygon mirror;
Based on the measurement result by the measurement unit, an identification unit for identifying which deflection surface of the polygon mirror the reflected light is reflected from;
Further comprising
The timing generation unit sets a timing to be indicated by the signal based on a boundary of a correction section indicated by a correspondence table related to the deflection surface identified by the identification unit;
The correction unit updates the correction value at a timing indicated by the signal to a correction value assigned to a next boundary indicated by a correspondence table related to the deflection surface identified by the identification unit. Optical scanning device. An actual measurement unit for actually measuring the amount of light irradiated from the optical system to the photosensitive member;
An operation unit that determines a boundary of each correction section based on an actual measurement result by the actual measurement unit, and calculates a correction value for the boundary;
Further comprising
The timing generation unit sets the timing to be indicated by the signal based on the boundary determined by the calculation unit,
The optical scanning apparatus according to claim 6, wherein the correction unit updates a correction value to a correction value for a next boundary calculated by the calculation unit at a timing indicated by the signal.   When scanning the region of the photoconductor corresponding to each correction section with the reflected light from one deflection surface of the polygon mirror, the correction unit interpolates correction values assigned to the boundaries at both ends of the correction section. The optical scanning device according to claim 1, wherein the light amount of the light source is corrected with the calculated value. An image forming unit for forming a toner image on a sheet;
A fixing unit for thermally fixing the toner image;
An image forming apparatus comprising:
The image forming unit
A photoconductor whose charge amount changes according to the exposure amount;
The optical scanning device according to claim 1, 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:

Images