JP6447058B2 - 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 a surface of a built-in photoconductor by uniformly charging the surface and performing exposure scanning. The apparatus further develops the latent image with toner and transfers the toner image that appears to a sheet (paper or film resin), thereby forming an image on the sheet.
The image forming apparatus includes an optical scanning device as a configuration for exposing and scanning the photosensitive member. The optical scanning device irradiates the photosensitive member by deflecting light from a light source such as a semiconductor laser or LED with a scanning optical system. The scanning optical system periodically deflects light from the light source by reflecting it with, for example, a rotating polygon mirror. The scanning optical system further forms an image of the reflected light of the polygon mirror on the surface of the photosensitive member, and moves the image forming point in one direction according to the change in the deflection angle of the polygon mirror. Thereby, the surface of the photoreceptor is exposed linearly. The optical scanning device further modulates the exposure amount according to the image data, thereby changing the distribution of the charge amount in the exposure region of the photoreceptor. 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.

  Strictly speaking, the light reflectance and light transmittance of the scanning optical system vary depending on the deflection angle, and therefore, even if the light amount is kept constant by the light source, the irradiation light amount of the scanning optical system varies as the deflection angle changes. This variation is one of the causes of exposure “unevenness” in the scanning range of the scanning optical system. That is, if the variation in the amount of irradiation light of the scanning optical system is directly reflected in the variation in the exposure amount, “unevenness” of 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 the above-described variation in the amount of irradiation light accompanying the change in the deflection angle of the scanning optical system.

  With respect to a conventional optical scanning device, a technique for correcting the light amount of a light source for each deflection angle of a scanning optical system is known for the purpose of suppressing exposure unevenness. (For example, refer to Patent Document 1-5.) The correction value at this time is set so that the above-described variation in the amount of irradiation light accompanying the change in the deflection angle of the scanning optical system is offset by the change in the light amount of the light source. , Exposure unevenness is suppressed.

JP 2008-033318 A JP 2009-053466 A JP 2009-262344 A JP 2011-158760 A JP 2011-158761 A

  The correction value for the light amount of the light source for the purpose of suppressing exposure unevenness is set in the following procedure as disclosed in, for example, Patent Literature 1-5 for a conventional optical scanning device. First, when the deflection angle is changed in the scanning optical system in a state where the light amount is kept constant in the light source, the fluctuation of the exposure amount that appears in the scanning range is sampled. The sample value (sample) at this time is at each end point (hereinafter referred to as “main correction position”) of a plurality of sections (hereinafter referred to as “correction sections”) equally divided from the deflection range of the scanning optical system. On the other hand, it is calculated from the model or measured experimentally. Next, the correction value in the time zone in which the deflection angle moves in each correction section is aligned with a value that cancels the sample at the main correction position in the correction section.

Aligning the correction intervals to the same width has the following advantages. Since the scanning optical system generally changes the deflection angle at a constant speed, if the correction section has the same width, the deflection angle reaches each 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. The width of the correction section is preferably set so that the sampling error is within an allowable range. Sampling errors occur between the fluctuation range of the exposure amount that appears while the deflection angle moves through one correction section under the condition that the light amount of the light source is constant, and the sample at the main correction position in the correction section. It is a difference. Therefore, the narrower the correction interval, the smaller the sampling error. When the correction sections have the same width, the width is set so that the sampling error falls within the allowable range in the correction section where the exposure rate change rate with respect to the deflection angle is the highest. In this case, the total number of correction sections increases as the maximum value of the change rate increases. However, since there is an upper limit on the memory capacity that can be used to store correction values, there is also an upper limit on the total number of correction values. Therefore, there is also an upper limit to the change rate of the exposure amount that can keep the sampling error within the allowable range. As a result, in the conventional optical scanning device, it is difficult to further reduce the exposure unevenness without increasing the memory capacity.

  An object of the present invention is to solve the above-described problems, and in particular, to improve the effect of suppressing uneven exposure by correcting the light amount of the light source without increasing the memory capacity necessary for storing the correction value for the light amount 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 photoconductor by exposing and scanning the photoconductor, and periodically deflects light from the light source that can change the amount of light. When the scanning optical system scans each of the scanning optical system for exposing and scanning the photosensitive member, the modulation unit for modulating the light amount of the light source according to the image data, and the plurality of correction sections divided from the scanning range of the scanning optical system And a correction unit that corrects the light amount of the light source with a correction value for the correction section . When the photosensitive member is exposed and scanned by the scanning optical system while maintaining the light amount of the light source constant without correction, the scanning range of the scanning optical system includes a first region and a second region. In the second area, the representative value of the change rate of the exposure amount with respect to the position in the scanning direction is larger than that in the first area. For the first area, the end points of the correction section are set at regular intervals in the scanning direction, and if the difference between the correction values exceeds the allowable upper limit between two adjacent end points, a new end point is added between the two end points. Thus, the difference between the correction values is within an allowable range in two adjacent correction sections. Thereby, each width of the plurality of correction sections is set to a different value depending on the position within the scanning range of the scanning optical system. For the second region, the difference between the correction values for the two adjacent correction sections in the monotonically changing portion of the correction curve that indicates the correction value that is truly necessary for the light amount of the light source is made uniform. Thereby, each width of the plurality of correction sections is set to a different value depending on the position within the scanning range of the scanning optical system .

In this optical scanning device, among the plurality of correction sections, the correction section divided from the second area may be set narrower than the correction section divided from the first area.

The correction value for each correction section may be set so that the exposure amount becomes the same value at the main correction position that is the start point or end point of each correction section when the image data shows a constant gradation value.
The optical scanning device may further include a storage unit that stores information on the width and the correction value in association with each other for each correction section. In this case, each time the scanning position of the scanning optical system reaches each correction section, the correction unit reads the correction value for the correction section from the storage unit until the scanning position of the scanning optical system reaches the next correction section. During this time, the light amount of the light source may be corrected with the correction value.

The width of each correction section is the exposure amount when the image data shows a certain gradation value when the scanning optical system is exposed and scanned with the correction section after correcting the light amount of the light source with the correction value for the correction section. The fluctuation range may be set to be equal to or less than a predetermined upper limit value.
Of the scanning range of the scanning optical system, a correction value for two adjacent correction sections in a portion where the exposure amount changes monotonously when the light amount of the light source is kept constant and the photosensitive member is exposed and scanned by the scanning optical system. The width of each correction section may be set so that the difference between the two becomes constant.

  The scanning optical system is a polygon mirror that periodically changes the deflection angle of the light by reflecting the light from the light source while rotating at a constant speed, and an image that forms the reflected light of the polygon mirror on the surface of the photoreceptor. The correction unit includes a timing generation unit that generates a signal indicating the timing at which the imaging point of the reflected light from the polygon mirror reaches each correction section, and corrects each correction section at the timing indicated by the signal. A value may be acquired.

The width of each correction section may be an integer multiple of the minimum value.
The correction unit subdivides at least one of the plurality of correction sections into a plurality of subsections, and interpolates a correction value for each subsection with a correction value for the correction section before the subdivision and a correction value for a correction section adjacent thereto. When the scanning optical system scans each small section, the light quantity of the light source may be corrected with a correction value for the small section. In this case, the width of each small section may be set to be 1 / integer of the width of the correction section before subdivision.

  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, an optical scanning device that forms an electrostatic latent image on the photosensitive member by exposing and scanning the photosensitive member, and development for developing the electrostatic latent image with toner And a transfer unit that transfers the toner image developed by the developing unit from the photoreceptor to the sheet.

  As described above, when the scanning optical system scans each correction section, the optical scanning device according to the present invention corrects the light amount of the light source with the correction value for the correction section. In particular, the width of each correction section is set to a different value depending on the position within the scanning range of the scanning optical system. As a result, this optical scanning device can improve the effect of suppressing unevenness of 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 an embodiment 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. (A) is a graph showing the relationship between the correction value set by the correction unit shown in FIG. 4 and the main scanning position on the photosensitive drum, and (b) is a black point CP1, plotted on this graph, Is a list of coordinates of CP2, CP3,..., CP (n-1), CPn, CP (n + 1),. (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 graph which shows the setting conditions of the correction area which makes the black point of FIG. 6 a starting point, (b) is the one part enlarged view. (A) is a graph which shows the relationship between the correction area set according to the conditions shown to (a) of FIG. 8, and a correction value, (b) shows the sampling error in the correction shown to (a). It is a graph. (A) is a graph which shows the relationship between the correction area of the same width | variety, and a correction value, (b) is a graph which shows the sampling error in the correction | amendment which (a) shows. 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. It is a block diagram of the electronic circuit which the optical scanning part by the modification of embodiment of this invention contains. (A) is a graph which shows the interpolation method of the correction value by the electronic circuit shown in FIG. 13, (b) is the one part enlarged view. It is a flowchart of control with respect to the optical scanning part shown in FIG. 2, FIG.

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

-Feeding section-
The feeding unit 10 feeds sheets SHT from the sheet feeding cassette 11 to the image forming unit 20 one by one using the 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 roller group is generally stopped and rotates in accordance with 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 on which the toner image is fixed out of the housing of the printer 100. Specifically, the sheet SH3 first moves from the upper part of the fixing unit 30 along the guide plate 41 toward the horizontal slit 42 opened in the housing of the printer 100. At this time, the paper discharge unit 40 rotates the paper discharge roller 43 disposed inside the slit 42 and feeds the sheet SH3 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, scanning optical systems 271, 272, 273, (28Y, 29Y), (28M, 29M), (28C, 29C), 28K, and a control unit. 300 is included. The light source 260 and the scanning optical system 271,... Expose and scan the photosensitive drums 25Y of the image forming units 21Y,. The controller 300 modulates the light amount of the light source 260 according to the image data.

-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 second semiconductor lasers 26Y, 26M, and 26K. Although not shown in FIGS. 1 and 2, between the four semiconductor lasers 26Y,..., The height of the laser beam exit (in FIG. 1, the vertical position of the paper surface, and in FIG. Since the laser beam 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 reflective surface of the polygon mirror 271. In particular, the cylindrical lens 265 forms an image of the irradiation light on the reflection 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 direction is the left-right direction of the paper surface, in FIG. 2, the direction is parallel to the paper surface and perpendicular to the laser beam LL). Convert to As will be described later, the rotation axis direction of the polygon mirror 271 is the sub-scanning direction, and the direction orthogonal to both the direction and the irradiation direction of the laser light LL is the main scanning direction.

-Scanning optical system-
The scanning optical system includes a polygon mirror 271, a motor 272, an fθ lens 273, and four sets of folding mirrors (28Y, 29Y), (28M, 29M), (28C, 29C), and 28K.
The polygon mirror 271 is a regular polygonal column (regular heptagonal column in the example of FIG. 2) member, and any side surface is a reflecting surface. 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 light LL from the light source 260, and at the same time, the angle formed by the traveling direction of the laser light LL and the reflected light RL by rotation, that is, the deflection angle of the laser light LL. To change. Specifically, while the polygon mirror 271 rotates by the rotation angle θ, the deflection angle changes by twice the angle 2θ. Further, the reflection surface of the polygon mirror 271 is periodically changed from one side surface to the adjacent side surface with the rotation, so that the deflection angle is continuously and periodically in the range from the minimum value φ _L to the maximum value φ _R. Changes. Especially when the polygon mirror 271 rotates at a constant angular velocity, the deflection angle is the maximum value phi _R to the minimum value phi _L changes at a constant rate, changes instantaneously from the minimum value phi _L to a maximum value phi _R .

  The fθ lens 273 is arranged so as to transmit the reflected light 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, the height between 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 (in FIG. 1, the vertical direction of the page) In FIG. 2, the position in the normal direction of the paper surface). Thereby, only the outgoing lights LY of the different semiconductor lasers 26Y including 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). Thereby, the imaging point on the surface is 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,. Accordingly, the point at which the transmitted light forms an image on the surface of each photosensitive drum 25Y, that is, the exposure point moves in the axial direction of the drum. Particularly result the deflection angle of the polygon mirror 271 is four photosensitive drums 25Y to the maximum value phi continuously varying period to _R from the minimum value phi _L, the imaging point to either surface of ... is exposure scanning in one direction The exposure points on the surface are connected in a straight line to form one line of the electrostatic latent image. Therefore, hereinafter, this period is referred to as a “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 rotation axis direction of the polygon mirror 271 correspond to the sub-scanning direction.

The fθ lens 273 further has a characteristic that “the incident angle of transmitted light and its image height (distance from the optical axis of the image forming point) are proportional”, and the amount of change in the deflection angle of the polygon mirror 271 and the change thereof. The moving distance of the imaging point of transmitted light is made proportional. Referring to FIG. 2, when the deflection angle of the polygon mirror 271 changes from the minimum value φ _L to the angle 2θ, the reflection point of the transmitted light of the fθ lens 273 moves on the folding mirrors 28Y,. To do. The moving distances ρY, ρM, ρC, and ρK at this time depend on the characteristics of the fθ lens 273, and 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 of the polygon mirror 271. Is proportional to These moving distances ρY,... Are proportional to the moving distance of the image forming point on the surface of each photosensitive drum 25Y,..., And the deflection angle variation 2θ of the polygon mirror 271 is proportional to the rotational angle variation θ. The linearity between the position of the imaging point in the main scanning direction and the rotation angle of the polygon mirror 271 is established. In particular, when the polygon mirror 271 rotates uniformly, the imaging point moves on the surface of each photosensitive drum 25Y,... At a constant speed 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,. In particular, the control unit 300 receives image data from a main control unit, which will be described later, and, based on the gradation values of each color Y, M, C, K represented by the image data, the semiconductor lasers 26Y,. Modulate the blinking 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 position 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”). While the polygon mirror 271 rotates at a constant speed, every time the laser light LL of the light source 260 is reflected to the maximum deflection angle φ _R , the SOS sensor 303 detects the reflected light and validates, that is, asserts the SOS signal. (If the signal is a positive logic signal, the pulse is raised, and if the signal is a negative logic signal, the pulse is lowered.) Based on this assertion timing, the control unit 300 synchronizes the blinking of the semiconductor lasers 26Y,... With the rotation of the polygon mirror 271. Let

  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 laser 26Y,... For each deflection angle of the polygon mirror 271. 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 print job request and image data to be printed 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 image data to be printed is directly taken 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 wirelessly, and receives image data to be printed from another electronic device 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 a work area for the CPU 61 to execute the firmware to the CPU 61 and stores image data to be printed 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 other electronic circuits 300M, 300C, and 300K have the same configuration.

The electronic circuit 300Y includes a sample and hold (SH) unit 310, a storage unit 320, 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 emission amount fed back from the light amount sensor PD in the first semiconductor laser 26Y. The storage unit 320 individually stores correction values for the light amount of the laser oscillator LD for a plurality of sections divided from the deflection ranges φ _{L to} φ _R of the polygon mirror 271. Hereinafter, these sections are referred to as “correction sections”. Each time the deflection angle of the polygon mirror 271 reaches each correction section, the correction unit 330 reads the correction value for the correction section from the storage unit 320, and while the deflection angle moves through the correction section, the correction value SH is used as the correction value. The amount of current IBS determined by unit 310 is corrected. The correction unit 330 further corrects the light emission amount by supplying the corrected current ICR to the laser oscillator LD. The modulation unit 340 modulates the blinking pattern of the laser oscillator LD by modulating the output current ICR to the laser oscillator LD based on the Y gradation value represented by the image data.

-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, The direction represents the sign of the difference.

  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 asserted in a time zone in which the photosensitive drum 25Y of the first image forming unit 21Y can be exposed every main scanning period, so that the switch 314 is closed and the differential amplifier 313 is closed every time zone. The connection between the output terminal and one end of the capacitor 315 is maintained.

  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. When the switch 314 is closed, the capacitor 315 is charged / discharged by the output current ISH of the differential amplifier 313, so that the voltage VSH between both ends thereof varies. The amount of the variation is determined by the amount of the output current ISH, and the polarity of the variation is determined by the direction of the output current ISH.

  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. When the voltage VSH across the capacitor 315 varies according to the change in the output current ISH of the differential amplifier 313, the VI converter 316 changes the amount of the output current IBS according to the variation. 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 out the difference. Adjust.

-Storage unit-
Referring to FIG. 4, the storage unit 320 includes a correction interval register 321 and a correction value register 322. These registers 321 and 322 are memory areas mounted on the electronic circuit 300Y. The correction section register 321 stores correction section information CRP. The correction section information CRP is information regarding the width of each correction section, and in particular, the time required for the polygon mirror 271 rotating at a constant speed to move the deflection angle from end to end of the correction section, specifically, the width. It is defined by the number of pulses (clock number) of the clock (CLK) signal. The correction value register 322 stores correction value information CRV. The correction value information CRV is a value that should be maintained as a correction value for the light emission amount of the laser oscillator LD while the deflection angle of the polygon mirror 271 moves in each correction section. It is specified as a ratio.

-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, and generates the instruction signal SHS and the timing signal TMS according to the SOS signal 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 SOS signal is a negative logic signal, and a period SCT from one falling edge to the next falling edge represents one main scanning period. The length of the period SCT is equal to, for example, 74 times the period of the CLK signal (clock period). The timing generation unit 331 reads the correction interval information CRP from the correction interval register 321 every time the SOS signal is asserted, and recounts the number of pulses (clock number) of the CLK signal from “0”. The timing generation unit 331 further generates an instruction signal SHS and a timing signal TMS according to the width of the correction section indicated by the correction section information CRP. The instruction signal SHS is a positive logic signal, and indicates the timing when the switch 314 of the SH unit 310 is turned on and off by the rising edge and the falling edge thereof. The timing signal TMS is a positive logic signal, and indicates the timing at which the deflection angle of the polygon mirror 271 reaches the start point of each correction section (hereinafter referred to as “main correction position”) by its rising edge.

In general, the deflection angle that allows the fθ lens 273 to form an image of the transmitted light on the surface of the photosensitive drum 25Y of the first image forming unit 21Y is larger than the deflection range φ _{L to} φ _{R of the} polygon mirror 271 shown in FIG. The range of is narrow. Accordingly, a time zone in which the deflection angle of the polygon mirror 271 belongs to this range, that is, a time zone during which the transmitted light of the fθ lens 273 can expose the surface of the photosensitive drum 25Y (hereinafter referred to as “exposure possible period”) is one. It is shorter than the main scanning period. For example, in FIG. 5, the exposure possible period SHR starts from a time T1 when four times the clock period has elapsed from the start point T0 of the main scanning period SCT, and ends at the end of the main scanning period SCT (the time when the SOS signal falls next). ) The end point is a time point T2 earlier than T3 by a time three times the clock cycle.

  The timing generation unit 331 starts counting the number of clocks from the time T0 when the SOS signal falls, in the example of FIG. 5, starts counting the number of rises of the CLK signal, and instructs at the time T1 when the count reaches “4”. The signal SHS is raised, and the instruction signal SHS is lowered at time T2 when “74-3 = 71” is reached. Thus, the timing generation unit 331 defines the exposure possible period for the photosensitive drum 25Y of the first image forming unit 21Y by the effective period SHR of the instruction signal SHS.

  More generally, the range in which one line of the electrostatic latent image is allowed to be formed on the surface of the photosensitive drum 25Y of the first image forming unit 21Y is limited to be narrower than the range in which the fθ lens 273 can form the transmitted light. Is done. For example, in FIG. 5, the time zone ESC (hereinafter referred to as “effective scanning period”) in which the image forming point of the transmitted light of the fθ lens 273 moves within this range is twice the clock period from the start point T1 of the exposure possible period SHR. The time point CP1 at which the time elapses is set as the start point, and the time point CPL that is earlier by the clock cycle than the end point T2 of the exposure possible period SHR is set as the end point.

  In this case, the timing generation unit 331 first raises the timing signal TMS at a time CP1 when the count of the number of clocks is increased by “2” from the time T1 when the instruction signal SHS is raised. 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. Next, the timing generation unit 331 reads the width of the first correction section from the correction section information CRP. The timing generator 331 raises the timing signal TMS again at the time point CP2 when the count of the number of clocks has increased by the width from the time point CP1 when the timing signal TMS is first raised, and the second correction interval from the correction interval information CRP. Read the width. Thereafter, similarly, the timing generator 331 generates a timing signal at each time point CP3,..., CP (n-1), CPn, CP (n + 1),. The rise of TMS and the reading of the width of the next correction section are repeated. In this way, the timing generation unit 331 defines the timing at which the deflection angle of the polygon mirror 271 reaches each correction section by the rise of the timing signal TMS.

<DAC>
The DAC 332 reads the correction value from the correction value information CRV every time the timing signal TMS rises, and amplifies the output current IBS of the VI converter 316 at the rate of the correction value until the timing signal TMS rises next time. Thereby, while the deflection angle of the polygon mirror 271 moves in each correction section, the current amplification factor of the DAC 332 is maintained at the correction value in the correction section. Here, the current IBS before amplification is adjusted by the SH unit 310 so that the light emission amount of the laser oscillator LD matches the reference value. Therefore, the amplified current ICR is an amount necessary to change the light emission amount of the laser oscillator LD from the reference value so as to cancel the fluctuation of the exposure amount that appears if the light emission amount is maintained at the reference value. Can be considered: ICR = CRV × IBS.

  FIG. 6A is a graph showing the relationship between the correction value set by the correction unit 330 and the main scanning position on the photosensitive drum 25Y of the first image forming unit 21Y. The “main scanning position” represented by the horizontal axis of the graph is that the imaging point of the transmitted light of the fθ lens 273, that is, the exposure point, moves with the change of the scanning range of the scanning optical system, that is, the deflection angle of the polygon mirror 271. The position in the main scanning direction in the range on the photosensitive drum 25Y. On the other hand, the vertical axis of this graph represents the 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. FIG. 6B shows a list of coordinates of black spots CP1, CP2, CP3,..., CP (n−1), CPn, CP (n + 1),... Plotted on this graph, that is, deflection of the polygon mirror 271. It is a correspondence table between the main scanning position when a corner reaches each correction section and the correction value for the correction section. Since the correspondence between the deflection angle of the polygon mirror 271 and the main scanning position is linear due to the characteristics of the fθ lens 273, the main scanning direction starting from these points CP1,. These sections are also called “correction sections”. In this case, those points CP1,... (Main scanning position) correspond to the main correction position of each correction section.

  As described above, the exposure point moves at a constant speed in the main scanning direction by rotating the polygon mirror 271 at a constant speed. Accordingly, in FIG. 6, the main scanning position is represented by the number of clocks counted from the start time point CP1 of the effective scanning period ESC (clock number = 64) shown in FIG. 5 to the time point when the exposure point reaches the main scanning position. . In this case, the main correction position CP1,... Of each correction section is equal to the cumulative value from the width of the first correction section defined by the correction section information CRP to the width of the correction section immediately before the main correction position. The item “interval” shown in FIG. 6B is equal to the width of the correction section defined by the correction section information CRP.

  A broken line CRV in the graph shown in FIG. 6A represents the transition of the correction value set by the correction unit 330. Referring to the broken line CRV, the correction value is changed every time the exposure point reaches one of the main correction positions CP1,..., And the correction value is changed until the exposure point reaches the next main correction position CP2,. Maintained. This is because the DAC 332 changes the current amplification factor ICR / IBS to the next correction value indicated by the correction value information CRV according to the rise of the timing signal TMS, and continues to amplify the current IBS at that ratio until the next rise. The item “correction value” shown in FIG. 6B indicates a correction value specified by the correction value information CRV.

  Further referring to FIG. 6A, the broken line CRV is an approximation of a curve CRC (hereinafter referred to as “correction curve”) that smoothly connects the main correction positions CP1,. This correction curve CRC is a plot of correction values that are truly necessary for the light emission amount of the laser oscillator LD over the scanning range of the scanning optical system. That is, when the light emission amount of the light source 260 is changed for each main scanning position of the exposure point along the correction curve CRC, that is, for each deflection angle of the polygon mirror 271, the exposure amount that should appear if the light emission amount is constant. Can be canceled over the entire scanning range of the scanning optical system. Details of the correction curve CRC will be described later.

  The correction value at the main correction position CP1,..., That is, the correction value defined by the correction value information CRV is a sample sampled from the correction curve CRC. Referring to FIG. 6A, in the correction section divided from the first region GNR where the main scanning position exceeds “18”, the correction section divided from the second region STR where the main scanning position is “18” or less. The width is narrower than the section. In other words, the interval ΔPD between the main correction positions CP1, belonging to the second region STR is finer than the interval ΔPS between the main correction positions belonging to the first region GNR, CP (n−1), CPn,. This density 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, intermediate value of the change rate of the correction value with respect to the main scanning position, or This is because the representative values in statistics such as the mode are large. As described above, the embodiment of the present invention is characterized in that the width of the correction section is not uniform and varies depending on the main scanning position, that is, the deflection angle of the scanning optical system. Details of the correction section setting conditions will be described later.

-Modulator-
Referring to FIG. 4, the modulation unit 340 includes a switching unit 341. The switching unit 341 conducts or interrupts the current ICR supplied from the DAC 332 of the correction unit 330 to the laser oscillator LD by opening and closing. The switching unit 341 further 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. As a result of the intermittent change in current ICR, the blinking pattern of the laser oscillator LD is modulated into a pattern based on the Y gradation value for each exposure point.

[Significance of correction for light quantity of light source]
7A is a schematic diagram illustrating the incident angle of the laser light LL of the light source 260 with respect to the polygon mirror 271, and FIG. 7B is a schematic diagram illustrating 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. 7A, the incident angles θ ₁ and θ ₂ of the laser beam LL of the light source 260 with respect to the reflecting surface 701 change with the rotation of the polygon mirror 271, so the reflection angles θ ₁ and θ ₂ are also changed. Change. Further, as shown in FIG. 7B, the incident angles θ ₁ and θ ₂ of the reflected light RL of the polygon mirror 271 with respect to the fθ lens 273 change due to the 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 between the two parts varies with the angle of incidence. Accordingly, the reflectivity for light with different incident angles θ ₁ and θ ₂ is different on the reflection 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 scanning optical system such as the polygon mirror 271 and the fθ lens 273 vary depending on the rotation angle of the polygon mirror 271, that is, its deflection angle. In this case, even if the light source 260 is kept constant during the main scanning period, the amount of light irradiated from the scanning optical system to the photosensitive drums 25Y,... Fluctuates as the deflection angle changes.
(C) of FIG. 7 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,... As in the graph of FIG. On the other hand, 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%) in the vicinity of the main scanning position “6”, and on both sides thereof, in the range of the clock number “± 6”, the exposure amount is abruptly attenuated by about 10 to 20% as the distance from the peak increases. On the other hand, at the main scanning position “16” 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 of the scanning optical system. It is necessary to keep it uniform. For this purpose, the light amount of the light source 260 may be corrected so as to cancel out the cause of the variation in exposure amount, that is, the change in reflectance / transmittance accompanying the change in the deflection angle of the scanning optical system. Specifically, the correction value for the amount of light may be an inverse ratio to the exposure amount variation 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 correction curve CRC is estimated by calculation from the refractive indexes of optical elements in the scanning optical system such as the polygon mirror 271 and the fθ lens 273, or is measured from the actual light quantity of the scanning optical system by experiment.

[Setting conditions for correction section]
When sampling the correction value for the light amount of the light source 260 from the correction curve CRC of FIG. 6A, the error in the correction curve CRC of the broken line CRV connecting them, that is, the sampling error is smaller as the main correction positions CP1,. . 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 correction sections also has an upper limit. Therefore, there is a limit to the suppression of sampling error due to the increase in the total number of correction sections.

  On the other hand, the sampling error is smaller as the width of the correction section is narrower. If the width of the correction section is constant, the sampling error is smaller as the change of the correction value in the section is more gradual. Therefore, in order to suppress the sampling error in the entire correction curve CRC without increasing the total number of correction sections, the width of the correction section is preferentially reduced with respect to the area where the slope of the correction curve CRC is steep. In this correction interval, the sampling error may be suppressed within an allowable range, for example, within a few percent.

  (A) of FIG. 8 is a graph which shows the setting conditions of the correction area with respect to the correction curve CRC. Referring to FIG. 8A, 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 a correction section having the same width is set in the first region GNR and the second region STR, the sampling error exceeds the allowable range in any one of them. Accordingly, the main correction positions CPI, CPk, CP (k + 1),... In the first region GNR are more closely spaced than the main correction positions CP2, CPm, CP (m + 1),. Set to.

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

-First setting condition-
For example, the first setting condition is adopted in the first region GNR shown in FIG. Specifically, 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 the main correction position. Next, the difference in correction value between two adjacent main correction positions is compared with an allowable upper limit, for example, “2%”. In FIG. 8A, the difference between the correction values exceeds the allowable upper limit “2%” between the main correction position CPI at the head and the two main correction positions CPk and CP (k + 1) adjacent thereto. Is within the allowable upper limit between the main correction positions. Therefore, a new main correction position is added between the three main correction positions CPI, CPk, CP (k + 1), and the difference in correction values between the main correction positions is suppressed to an allowable upper limit or less.

  FIG. 8B is an enlarged view of a portion including the main correction positions CPk and CP (k + 1) in the correction curve CRC shown in FIG. Referring to FIG. 8B, the difference between the correction values exceeds the allowable upper limit “2%” between the main correction positions CPk and CP (k + 1). In this case, a new main correction position CP + is added between the main correction positions CPk and CP (k + 1). The new main correction position CP + is set such that the difference between the correction value from the correction values of the main correction positions CPk and CP (k + 1) is not more than the allowable upper limit “2%”. Similarly, a new main correction position is added between the main correction positions CPI and CPk.

If a new main correction position cannot be added due to the limitation on the total number of correction sections, the allowable upper limit “2%” of the difference in correction values between the main correction positions is within the allowable range of the sampling error. Just raise it.
-Second setting condition-
For example, the second setting condition is adopted in the second region STR shown in FIG. Specifically, first, main correction positions CPT and CPB are set at the highest point and the lowest point of the correction curve CRC in the second region STR, and the difference between the correction values, for example, about “25%” The number of main correction positions that can be set between them is divided into, for example, “4”. Next, a point where the correction value differs from the lowest point CPB or the highest point CPT of the correction curve CRC by a division unit, for example, “25% / 4 = 5%”, is set as the main correction position. Thus, in the portion of the correction curve CRC that monotonously increases from the lowest point CPB to the highest point CPT, the difference between the correction values for two adjacent correction sections is equal to the constant value “5%”. When the second region STR extends to the outside between the end points CPT and CPB, the main correction position is similarly set for the outside.

Note that the number of main correction positions that can be set in the portion of the correction curve that changes monotonically does not increase the total number of correction sections, and the sampling error estimated from the correction value division unit falls within an allowable range. To be determined.
Regardless of which setting condition is adopted, the width of the correction section is set to an integral multiple of the minimum value (the number of clocks = “2” in FIG. 8B). In this case, the timing generation unit 331 can only synchronize the rising edge of the timing signal TMS with the timing at which the exposure point reaches each correction section only by counting the CLK signal by its minimum value (for example, “2”). Therefore, the circuit configuration of the timing generation unit 331 is simplified.

[Effect of correction]
FIG. 9A is a graph showing the relationship between the correction interval and the correction value set according to the condition shown in FIG. Referring to FIG. 9A, since the slope of the correction curve CRC is generally steeper in the second region STR than in the first region GNR, the correction section is in the second region STR than in the first region GNR. Many are preferentially arranged. Thereby, the width ΔPD (for example, “2”) of the correction section in the second area STR is narrower than the width ΔPS (for example, “8”) of the correction section in the first area GNR.

  On the other hand, (a) of FIG. 10 is a graph which shows the relationship between the correction area of the same width | variety, and a correction value. Referring to FIG. 10A, the width ΔPE of the correction section is aligned to a constant value (for example, “4”) throughout the correction curve CRC. This constant value is a correction interval in which the change rate of the exposure amount with respect to the main scanning position is the highest in the main scanning position range 0 to 64 of the exposure point, for example, from the lowest point BT to the highest point PK in FIG. Is determined so that the sampling error is within an allowable range.

  In order to compare the broken lines CRV and EQV indicating the transition of the correction value between FIG. 9A and FIG. 10A, in the first region GNR in which the inclination of the correction curve CRC is generally gentle, Since the width ΔPE shown in FIG. 10 (a) is narrower than the width ΔPS of the correction section shown in FIG. 9 (a), the broken line EQV shown in FIG. 10 (a) is smaller than the broken line CRV shown in FIG. 9 (a). Has a high degree of approximation to the correction curve CRC. On the other hand, in the second region STR where the slope of the correction curve CRC is steep, the width ΔPD of the correction section shown in FIG. 9A is narrower than the width ΔPE shown in FIG. The broken line CRV indicated by a) has a higher degree of approximation to the correction curve CRC than the broken line EQV indicated by (a) in FIG.

  FIGS. 9 and 10B are graphs showing sampling errors in the correction shown in FIGS. 9 and 10A, that is, errors of the broken lines CRV and EQV with respect to the correction curve CRC, respectively. When comparing FIG. 9B and FIG. 10B, the sampling error in the second region STR is more dominant than the sampling error in the first region GNR in any graph. However, the sampling error fluctuation width FRW shown in FIG. 9B is considerably smaller than the fluctuation width EQW shown in FIG. 10B. Since the sampling error represents the “unevenness” of the toner density that remains without being removed even after correction, the reduction in the fluctuation range means an improvement in the effect of suppressing unevenness in exposure. That is, as shown in FIG. 9A, the width of the correction section that is set narrower as the change rate of the exposure amount with respect to the main scanning position is higher than the uniform width shown in FIG. It is advantageous for improvement.

[Control Flowchart for Optical Scanning Unit]
FIG. 11 is a flowchart of control for the optical scanning unit 26. This process is started by starting a print job.
In step S101, the main controller 60 activates the semiconductor lasers 26Y,... Of the light source 260 and the motor 272 of the scanning optical system. 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 asserts the SOS signal in the period of the main scanning period. Thereafter, the process proceeds to step S102.

In step S102, 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 S103, and if it is not valid, step S102 is repeated.
In step S103, since the instruction signal SHS is valid, the switch 314 of the SH unit 310 maintains the connection between the differential amplifier 313 and the capacitor 315. At this time, the capacitor 315 is charged and discharged by the output current ISH of the differential amplifier 313. As a result, the voltage VSH between both ends is based on 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 light emission amount of the laser oscillator LD approaches the reference value. Adjusted to do. Thereafter, the process proceeds to step S104.

In step S104, the VI converter 316 converts the voltage VSH across the capacitor 315 into the output current IBS. Thereafter, the process proceeds to step S105.
In step S105, the control unit 300 monitors whether or not the timing signal TMS has risen for each of the four electronic circuits 300Y,. When the timing signal TMS rises, the process proceeds to step S106, and when it does not rise, the process proceeds to step S107.

In step S106, the DAC 332 of the correction unit 330 changes the correction value, that is, the current amplification factor to the next correction value indicated by the correction value information CRV in response to the rise of the timing signal TMS. Thereafter, the process proceeds to step S107.
In step S107, the DAC 332 amplifies the output current IBS of the VI converter 316 at a correction value ratio indicated by the correction value information CRV. Thereafter, the process proceeds to step S107.

  In step S108, 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 amplified current ICR supplied 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 S109.

In step S109, the control unit 300 confirms whether unprocessed image data remains. If unprocessed image data remains, the process is repeated from step S102, and if not, the process proceeds to step S110.
In step S110, the control unit 300 notifies the main control unit 60 that no unprocessed image data remains. 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. 12 is a flowchart of signal processing by the timing generation unit 331. This process is started each time the SOS sensor 303 asserts the SOS signal.
In step S201, the timing generation unit 331 starts counting the number of clocks in response to the assertion of the SOS signal. When the count reaches a value from the start point T0 of the main scanning period STC shown in FIG. 5 to the start point T1 of the exposure possible period SHR, for example, “4”, the timing generator 331 asserts the instruction signal SHS. Thereafter, the process proceeds to step S202.

  In step S202, the timing generator 331 generates the timing signal TMS when the clock count reaches a value from the starting point T0 of the main scanning period STC to the starting point CP1 of the effective scanning period ESC, which is “6” in the example of FIG. Is asserted. On the other hand, the timing generation unit 331 reads the width of the head correction section from the correction section information CRP, “2” in the example of FIG. 5, and sets the number of clocks until the next correction section. Thereafter, the process proceeds to step S203.

In step S203, the timing generation unit 321 checks whether the count of the number of clocks from the start point of the correction interval has increased by the width of the correction interval, that is, by the number of clocks up to the next correction interval. If so, the process proceeds to step S204. If not, the process proceeds to step S205.
In step S204, since the count of the number of clocks has increased by more than the width of the correction section from the start point of the correction section, the timing generation unit 321 asserts the timing signal TMS. On the other hand, the timing generation unit 331 reads the width of the next correction section from the correction section information CRP and sets the number of clocks until the next correction section. Thereafter, the process proceeds to step S206.

  In step S205, since the count of the number of clocks has not yet increased from the start point of the correction interval to the width of the correction interval, the timing generation unit 321 invalidates the timing signal TMS, that is, negates it. (If the signal is a positive logic signal, the pulse is lowered, and if the signal is a negative logic signal, the pulse is raised.) If already invalid, the state of the timing signal TMS is maintained. Thereafter, the process proceeds to step S206.

In step S206, the timing generation unit 321 checks whether or not the count of the number of clocks has reached a value up to the end point CPL of the effective scanning period ESC. If the value has been reached, the process proceeds to step S207, and if not, the process is repeated from step S203.
In step S207, when the count of the number of clocks further reaches a value up to the end point T2 of the exposure possible period SHR, which is “71” in the example of FIG. 5, the timing generator 331 negates the instruction signal SHS. Thereafter, the process ends.

[Advantages of the embodiment]
As described above, the optical scanning unit 26 according to the embodiment of the present invention reaches the main correction position CP1,..., In which the deflection angle of the polygon mirror 271 is plotted along the correction section, that is, along the correction curve CRC in FIG. Each time the correction value is read from the correction value register 322, the light amount of the light source 260 is corrected with the correction value until the deflection angle reaches the next correction section. The correction value is set so as to cancel out fluctuations in the reflectance of the polygon mirror 271 and the transmittance of the fθ lens 273 accompanying the change in the deflection angle of the polygon mirror 271. In particular, in the deflection range of the polygon mirror 271, the correction interval width ΔPD in the second region STR having a steep inclination is narrower than the correction interval width ΔPS in the first region GNR in which the inclination of the correction curve CRC is gentle. Is set. Accordingly, the optical scanning unit 26 can improve the effect of suppressing uneven exposure without increasing the capacity of the correction value register 322, and can improve the image quality of the printer 100.

[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.

(C) 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.
(D) The waveform of each signal shown in FIG. 5 is merely an example, and its logic may be positive or negative. The edge of the pulse to be counted and the edge of the pulse to be regarded as the end point of each period STC, SHR, ESC are rising. And either falling or falling.

(E) The correction section information CRP defines the width of each correction section by the time during which the deflection angle of the polygon mirror 271 moves through the correction section. In addition, the correction section information CRP may be information that can specify the width of the correction section, such as the main scanning position of the start point of each correction section.
(F) The correction value information CRV defines the correction value for the light emission amount of the laser oscillator LD by the ratio of the light emission amount after correction to the reference value of the light emission amount. 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 scanning optical system with a change in the deflection angle of the polygon mirror 271.

  (G) In FIG. 6, the start point of each correction section is set as the main correction position. In addition, the end point of each correction section may be set as the main correction position. In FIG. 6, the correction value is set over the entire scanning range (more precisely, the effective scanning period ESC) of the scanning optical system. In addition, the light amount 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 has a steep slope, such as the second region STR. On the other hand, for example, in a region where the slope of the correction curve CRC is gentle, such as in the first region GNR, the light amount of the light source may be made constant to a value corrected with the representative value of the correction value in that region.

  (H) The cause of the variation in the exposure amount shown in FIG. 7C is the amount of transmitted light of the scanning optical system, such as the reflectivity accompanying the change in the deflection angle of the polygon mirror 271 or the variation in the transmittance of the fθ lens 273. It is assumed that this is a fluctuation. In this case, if the structure of the scanning optical system is common, the correction curve CRC can be regarded as common, and therefore the correction value information CRV may be common. In addition, a manufacturing error of an optical element such as the polygon mirror 271 and the fθ lens 273 may be assumed as a cause of the exposure amount fluctuation. In this case, the correction value information CRV may be set for each product by measuring the actual light amount of the light source 260 for each product in the manufacturing process of the optical scanning unit 26.

  (I) In FIG. 8, as the setting condition of the correction section, the first setting condition is adopted for the first area GNR of the correction curve CRC, and the second setting condition is adopted for the second area STR. In addition, the setting condition may be limited to only one of the first and second in the entire correction curve. Further, the setting conditions for the correction section are not limited to the above two types, and any correction conditions can be adopted as long as the sampling error can be within an allowable range without increasing the capacity of the correction value register 322.

(J) In the above embodiment, the control unit 300 uses only the sample indicated by the correction value information CRV as the correction value. In addition, the control unit 300 may interpolate these samples and use the obtained interpolation value as a correction value.
FIG. 13 is a block diagram of an electronic circuit 400 included in the optical scanning unit according to this modification. This electronic circuit 400 differs from the electronic circuit 300Y shown in FIG. 4 only in that a sample interpolation function is implemented. Referring to FIG. 13, the correction unit 330 of the electronic circuit 400 further includes an interpolation unit 333. The interpolation unit 333 reads the next correction value from the correction value information CRV in response to the assertion of the timing signal TMS, notifies the DAC 332 of the correction value, and performs interpolation on the correction value and the correction value before the change. . Further, the correction value is updated to the interpolation value at the main correction position every time the deflection angle of the polygon mirror 271 reaches the new main correction position added by interpolation until the timing signal TMS is next asserted.

  FIG. 14A is a graph showing a correction value interpolation method by the interpolation unit 333 of the electronic circuit 400, and FIG. 14B is an enlarged view of a part thereof. Referring to FIG. 14A, when the interpolation unit 333 performs interpolation on correction values at two adjacent main correction positions on the broken line CRV shown in FIG. The main correction positions CP2 and CP3 are connected by a single line segment INT. Next, the interpolating unit 333 subdivides the line segment INT into two small sections and sets a new main correction position CPN at the boundary. In this case, as shown in FIG. 14B, the main scanning position of the new main correction position CPN is an intermediate value “3” between the main scanning positions “2” and “4” of the main correction positions CP2 and CP3 before and after that. The correction value at the new main correction position CPN is equal to the intermediate value (CV2 + CV3) / 2 between the correction values CV2 and CV3 at the previous and subsequent main correction positions CP2 and CP3. Similarly, the other correction sections are subdivided into subsections, and the width of each subsection is set to be an integral fraction of the width of the correction section before subdivision. Since the calculation required for this interpolation is simple, the interpolation unit 333 can repeat the calculation every time the timing signal TMS is asserted.

FIG. 15 is a flowchart of control for the optical scanning unit according to this modification. Comparing FIG. 15 with FIG. 11, this flowchart differs from that of FIG. 11 in that it further includes steps S151, S152, and S153.
In step S151, the interpolation unit 333 performs interpolation on the next correction value indicated by the correction value information CRV and the correction value before the change, so that one correction section has a width that is 1 / integer of the width. Subdivide into small sections. Thereafter, the process proceeds to step S107.

In step S152, since the next assertion of the timing signal TMS has not yet been performed, the interpolation unit 333 checks whether or not the count of the number of clocks has reached the new main correction position CPN. If it has reached, the process proceeds to step S153, and if not, the process proceeds to step S107.
In step S153, since the count of the number of clocks has reached the new main correction position CPN, the interpolation unit 333 updates the correction value to the correction value at the main correction position CPN. Thereafter, the process proceeds to step S107.

  Referring again to FIG. 14A, the interpolated broken line NCV including the new main correction position CPN has a higher degree of approximation to the correction curve CRC than the interpolated broken line CRV. On the other hand, since the correction value at the new main correction position is added by calculation when necessary, the capacity of the correction value register 322 may not be added. In this way, the optical scanning unit 26 can further improve the effect of suppressing exposure unevenness by correcting the light amount of the light source 260 without increasing the capacity of the correction value register 322.

  The present invention relates to an optical scanning device. As described above, the interval for changing the correction value for the light amount of the light source is changed between different portions of the scanning range of the scanning optical system. 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 CRV correction value information CP1, CP2, ... Main correction position of correction section GNR First area of correction curve STR correction curve second area 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 the photosensitive member by exposing and scanning the photosensitive member;
A light source with variable light intensity,
A scanning optical system for periodically scanning the photosensitive member by deflecting light of the light source periodically;
A modulator for modulating the light quantity of the light source according to image data;
When the scanning optical system scans each of a plurality of correction sections divided from the scanning range of the scanning optical system, a correction unit that corrects the light amount of the light source with a correction value for the correction section;
With
When the scanning optical system exposes and scans the photosensitive member without correcting the light amount of the light source, the scanning range of the scanning optical system is the first region and the exposure amount with respect to the position in the scanning direction. A second region having a representative value of the rate of change larger than the first region;
For the first region, the end points of the correction section are set at regular intervals in the scanning direction, and if the difference between the correction values exceeds the allowable upper limit between two adjacent end points, a new end point is set between the two end points. Is added, and the difference between the correction values falls within the allowable range in the two adjacent correction sections.
Regarding the second region, the difference between correction values for two adjacent correction sections in a monotonically changing portion of the correction curve that indicates the correction value that is truly necessary for the light amount of the light source is made uniform. By
The width of each of the plurality of correction sections is set to a different value depending on the position within the scanning range of the scanning optical system. 2. The light according to claim 1 , wherein among the plurality of correction sections, a correction section divided from the second area is set to be narrower than a correction section divided from the first area. Scanning device.   The correction value for each correction section is set so that the exposure amount becomes the same value at the main correction position that is the start point or end point of each correction section when the image data shows a constant gradation value. The optical scanning device according to claim 1 or 2. A storage unit that stores information on the width and the correction value in association with each other for each correction section,
The correction unit reads a correction value for the correction section from the storage unit every time the scanning position of the scanning optical system reaches each correction section, and the scanning position of the scanning optical system reaches the next correction section. 4. The optical scanning device according to claim 1, wherein the light amount of the light source is corrected with the correction value during the period up to.   The width of each correction section is the case where the image data shows a constant gradation value when the scanning optical system is exposed and scanned with the correction section after correcting the light amount of the light source with the correction value for the correction section. 5. The optical scanning device according to claim 1, wherein the exposure fluctuation range is set to be equal to or less than a predetermined upper limit value. 6.   Of the scanning range of the scanning optical system, in the portion where the exposure amount changes monotonously when the scanning optical system exposes and scans the photoconductor while maintaining the light amount of the light source constant, two adjacent corrections 6. The optical scanning device according to claim 1, wherein the width of each correction section is set so that a difference in correction values with respect to the section becomes constant. The scanning optical system includes:
A polygon mirror that periodically changes the deflection angle of the light by reflecting the light of the light source while rotating at a constant speed;
An imaging optical system that forms an image of the reflected light of the polygon mirror on the surface of the photoreceptor;
Including
The correction unit is
A timing generation unit that generates a signal indicating the timing at which the imaging point of the reflected light of the polygon mirror reaches each correction section;
The optical scanning device according to claim 1, wherein a correction value for each correction section is acquired at a timing indicated by the signal.   8. The optical scanning device according to claim 1, wherein the width of each correction section is an integral multiple of the minimum value. The correction unit is
At least one of the plurality of correction sections is subdivided into a plurality of subsections, and a correction value for each subsection is determined by interpolation between a correction value for a correction section before the subdivision and a correction value for a correction section adjacent thereto. Including an interpolator,
9. The optical scanning according to claim 1, wherein when the scanning optical system scans each small section, the light amount of the light source is corrected with a correction value for the small section. apparatus.   10. The optical scanning device according to claim 9, wherein the width of each small section is set to be 1 / integer of the width of the correction section before subdivision. 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 any one of claims 1 to 10, wherein an electrostatic latent image is formed on the photoconductor by exposing and scanning the photoconductor.
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