Image forming device

Abstract

To provide an image forming device capable of correctly correcting exposure scanning of a photosensitive drum in which a scanning speed changes by image processing even when scanning position shifting occurs due to a temperature or the like.SOLUTION: This image forming device for scanning a photosensitive drum through exposure scanning in which a scanning speed changes to form an image comprises: scanning position correction means for correcting a scanning position by variable-power processing image data input on the basis of magnification characteristics corresponding to the scanning speed for exposure-scanning the photosensitive drum; scanning position shifting detection means for detecting further scanning position shifting after the scanning position correction of the scanning position correction means; and magnification characteristic correction means for correcting the magnification characteristics on the basis of the scanning position shifting detected by the position shifting detection means.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, printer, facsimile machine, or multifunction machine having a plurality of functions of these machines, which uses an electrophotographic or electrostatic recording system. [Background technology]

[0002] In an electrophotographic image forming apparatus, exposure is controlled by a laser in accordance with an image signal to form an electrostatic latent image on a photosensitive drum, and the image is formed through the steps of development, transfer, and fixing.

[0003] The laser exposure on the photosensitive drum is performed by deflecting and scanning the light in the longitudinal direction of the photosensitive drum (hereinafter referred to as the main scanning direction) using the rotation of a polygon mirror. Furthermore, a two-dimensional electrostatic latent image is formed by scanning in a direction perpendicular to the main scanning direction (hereinafter referred to as the sub-scanning direction) using the rotation of the photosensitive drum. Furthermore, when deflected by the rotation of the polygon mirror, the laser is irradiated onto the photosensitive drum via an fθ lens, ensuring a uniform main scanning speed on the photosensitive drum surface.

[0004] On the other hand, as described in Patent Document 1, a method has been proposed in which all or part of the magnification correction is electrically corrected as an optical configuration that does not use an fθ lens or that uses a simple fθ lens in pursuit of low costs.

[0005] Patent Document 1 proposes a method of correcting magnification by dividing the main scanning direction into predetermined areas and modulating the clock frequency according to the magnification for each area. Methods for approximating the correction characteristics within an area include zero-order approximation, which corrects the area at a constant magnification, and a method of correcting the relationship between the position within the area and the pixel clock by linear approximation using a linear function.

[0006] Furthermore, Patent Document 1 proposes a method of softening image quality degradation due to approximation errors by shifting the area boundary position for each scan.

[0007] Patent Document 2 shows that the correction characteristics are an nth-order function of the main scanning position, and then proposes a method of dividing the main scanning direction into areas and correcting the areas using linear approximation, while aligning the positions of the area boundaries between colors to eliminate color shift at the area boundaries and prevent large color shifts across the entire main scanning direction. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-338280 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-213841 Summary of the Invention [Problem to be solved by the invention]

[0009] As mentioned above, even if the fθ lens is omitted and the exposure scanning speed on the photosensitive drum is corrected by image processing, further deviations in the scanning position can occur due to temperature rises inside the machine, etc. In particular, in color printing devices, even slight deviations in position can be recognized as color deviations, which can be a visually noticeable phenomenon.

[0010] The present invention has been made in consideration of the above circumstances, and aims to provide an image forming apparatus that forms an image by exposing and scanning a photosensitive drum, and that can correctly correct any deviation in scanning position due to temperature or the like when correcting exposure scanning, which is performed using a simple fθ lens or without an fθ lens and involves changing the scanning speed of the photosensitive drum, using image processing. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention provides an image forming apparatus that forms an image by scanning a photosensitive drum with an exposure scan that changes the scanning speed, the image forming apparatus comprising: a scanning position correction means that changes the magnification of input image data according to a magnification characteristic that corresponds to the scanning speed at which the photosensitive drum is exposed and scanned, and corrects the scanning position; a scanning position deviation detection means that detects further deviation of the scanning position after the scanning position correction by the scanning position correction means; and a magnification characteristic correction means that corrects the magnification characteristic according to the deviation of the scanning position detected by the position deviation detection means. [Effects of the Invention]

[0012] According to the present invention, in an image forming apparatus that forms an image by exposing and scanning a photosensitive drum, when exposure scanning that changes the scanning speed of the photosensitive drum performed using a simple fθ lens or without an fθ lens is corrected by image processing, it is possible to provide an image forming apparatus that can correctly correct any deviation in the scanning position due to temperature, etc. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic front cross-sectional view of a digital full-color printer that forms images using toners of multiple colors according to a first embodiment of the present invention. [Figure 2] 1 is a schematic diagram illustrating a configuration of a photosensitive drum and an optical scanning device according to a first embodiment of the present invention. [Figure 3] 1 is a block diagram showing an image signal generation flow for converting image data into a PWM signal according to a first embodiment of the present invention. FIG. [Figure 4] 3 is an explanatory diagram showing a change in the number of pixel divisions depending on the scanning position of the optical scanning device 104 according to the first embodiment of the present invention. FIG. [Figure 5] FIG. 3 is an explanatory diagram showing gradation characteristics according to a bit pattern according to the first embodiment of the present invention. [Figure 6] 4 is a graph showing a change in PWM duty, in which the gradation characteristics according to the first embodiment of the present invention are expressed in terms of input gradation and pulse width. FIG. [Figure 7]FIG. 2 is a block diagram for calculating an actual pixel size from a target pixel size according to the first embodiment of the present invention. [Figure 8] FIG. 2 is an explanatory diagram showing pixel sizes for each pixel position according to the first embodiment of the present invention. [Figure 9] 9 is an explanatory diagram showing pixel sizes at respective pixel positions in an enlarged manner of the pixel positions in FIG. 8 according to the first embodiment of the present invention. [Figure 10] 5A and 5B are explanatory diagrams illustrating a method for measuring positional misalignment of a toner image formed on a transfer belt by a registration detection sensor according to the first embodiment of the present invention. [Figure 11] 5 is a flowchart showing a registration detection operation according to the first embodiment of the present invention. [Figure 12] 4 is a graph showing magnification characteristics according to the first embodiment of the present invention. [Figure 13] FIG. 10 is an explanatory diagram showing a method for measuring a scanning position deviation during product manufacturing according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. Note that the components described in the following embodiments are merely examples, and various conditions such as the configuration, function, dimensions, materials, shape, and relative arrangement of the device to which the present invention is applied can be appropriately modified or changed without departing from the spirit of the present invention, and are not limited to the following embodiments.

[0015] [First embodiment] FIG. 1 is a schematic front cross-sectional view of a digital full-color printer (color image forming apparatus) that forms an image using toners of multiple colors according to a first embodiment of the present invention.

[0016] 1, image forming apparatus 100 is equipped with four image forming units (image forming means) 101Y, 101M, 101C, and 101Bk that form images by color. Here, Y, M, C, and Bk represent yellow, magenta, cyan, and black, respectively. Image forming units 101Y, 101M, 101C, and 101Bk form images using yellow, magenta, cyan, and black toner, respectively.

[0017] Image forming units 101Y, 101M, 101C, and 101Bk are equipped with photosensitive drums 102Y, 102M, 102C, and 102Bk, which are photosensitive bodies. Charging devices 103Y, 103M, 103C, and 103Bk, optical scanning devices 104Y, 104M, 104C, and 104Bk, and developing devices 105Y, 105M, 105C, and 105Bk are respectively provided around photosensitive drums 102Y, 102M, 102C, and 102Bk. Furthermore, drum cleaning devices 106Y, 106M, 106C, and 106Bk are provided around photosensitive drums 102Y, 102M, 102C, and 102Bk.

[0018] An endless belt-like intermediate transfer belt 107 is disposed below the photosensitive drums 102Y, 102M, 102C, and 102Bk. The intermediate transfer belt (intermediate transfer body) 107 is stretched around a drive roller 108 and driven rollers 109 and 110, and rotates in the direction of arrow B in the figure during image formation. Primary transfer devices 111Y, 111M, 111C, and 111Bk are provided at positions facing the photosensitive drums 102Y, 102M, 102C, and 102Bk across the intermediate transfer belt 107.

[0019] In addition, the image forming apparatus 100 of this embodiment is equipped with a secondary transfer device 112 for transferring the toner image on the intermediate transfer belt 107 to the recording medium S, and a fixing device 113 for fixing the toner image on the recording medium S.

[0020] Here, we will explain the image formation process from the charging process to the developing process of the image forming apparatus 100 having such a configuration. Since the image formation process is the same in each of the image forming units 101Y, 101M, 101C, and 101Bk, we will explain the image formation process of the image forming unit 101Y as an example, and will omit explanations of the image formation processes in the image forming units 101M, 101C, and 101Bk.

[0021] First, the photosensitive drum 102Y, which is driven to rotate in the image forming unit 101Y, is charged by the charging device 103Y. The charged photosensitive drum 102Y is exposed to and scanned by a laser beam emitted from the optical scanning device 104Y. As a result, an electrostatic latent image is formed on the rotating photosensitive drum 102Y. The electrostatic latent image is then developed into a yellow toner image by the developing device 105Y.

[0022] Next, the primary transfer devices 111Y, 111M, 111C, and 111Bk apply a transfer bias to the intermediate transfer belt 107, whereby the yellow, magenta, cyan, and black toner images formed on the photosensitive drums 102Y, 102M, 102C, and 102Bk of the image forming units 101Y, 101M, 101C, and 101Bk are transferred to the intermediate transfer belt 107. As a result, the toner images of each color are superimposed on each other on the intermediate transfer belt 107.

[0023] When the four-color toner image is transferred onto the intermediate transfer belt 107, the four-color toner image transferred onto the intermediate transfer belt 107 is transferred again (secondary transfer) by the secondary transfer device 112 onto the recording medium S transported to the secondary transfer section from the manual feed cassette 114 or the paper feed cassette 115.

[0024] The toner image on the recording medium S is then heated and fixed by a fixing device 113 to form a fixed full-color image on the recording medium S, and the recording medium S with the fixed full-color image is discharged from a paper discharge section 116 .

[0025] In addition, a registration detection sensor 120 that detects the image on the intermediate transfer belt 107 in order to detect misalignment of the exposure scanning position in the optical scanning devices 104Y, 104M, 104C, and 104Bk described later is arranged downstream of the image forming unit 101Bk.

[0026] Next, we will explain the configurations of the photosensitive drums 102Y, 102M, 102C, and 102Bk, optical scanning devices 104Y, 104M, 104C, and 104Bk, and the control units of the optical scanning devices 104Y, 104M, 104C, and 104Bk in Fig. 1. Note that the photosensitive drums 102Y, 102M, 102C, and 102Bk and the optical scanning devices 104Y, 104M, 104C, and 104Bk in each image forming unit 101Y, 101M, 101C, and 101Bk have the same configuration, so in the following explanation, the suffixes Y, M, C, and Bk indicating colors will be omitted.

[0027] FIG. 2 is a schematic diagram showing the configuration of the photosensitive drum and the optical scanning device periphery.

[0028] 2, the optical scanning device 104 includes a multi-beam laser light source 201 that generates a plurality of laser beams (light beams), a collimator lens 202 that shapes the laser beam into parallel beams, a cylindrical lens 203 that focuses the laser beam that has passed through the collimator lens 202 in the sub-scanning direction, and a polygon mirror (rotating polygonal mirror) 204. Note that in this embodiment, the laser light source 201 is described as an example of a multi-beam light source in which a plurality of beams are arranged, but the same operation is also possible when a single light source is used.

[0029] The polygon mirror 204 is made up of a rotating motor and a reflecting mirror attached to the motor shaft. In this embodiment, the mirror has five faces, but any number of faces may be used. The polygon mirror 204 is driven by a polygon mirror driver 305. Furthermore, the polygon mirror 204 is provided with a beam detector 207 (hereinafter referred to as BD sensor 207), which is a signal generating means that detects the laser light deflected by the polygon mirror 204 and outputs a horizontal synchronization signal (hereinafter referred to as BD signal) in response to the detection of the laser light.

[0030] The laser light emitted from the optical scanning device 104 scans and exposes the surface of the photosensitive drum 102. The optical scanning device 104 and the photosensitive drum 102 are positioned so that the scanning direction of the laser light is parallel to the rotation axis of the photosensitive drum 102. Each time the mirror surface of the polygon mirror 204 scans the photosensitive drum once, it simultaneously forms as many scanning lines as there are beams of the multi-beam laser.

[0031] Next, we will explain the control unit (CPU 303) of the optical scanning device 104. Image data is input to the CPU 303 from an image controller (not shown) that generates image data. The CPU 303 is also connected to the BD sensor 207, memory 302, laser driver 304, and polygon mirror driver 305.

[0032] CPU 303 detects the write start position of the scan line based on the BD signal output from BD sensor 207, and detects the rotation speed of polygon mirror 204 by counting the time interval of the BD signal, and supplies an acceleration / deceleration signal to polygon mirror driver 305 to instruct acceleration or deceleration so that polygon mirror 204 reaches a predetermined speed. Polygon mirror driver 305 supplies a drive current to the motor of polygon mirror 204 in response to the input acceleration / deceleration signal, thereby driving the motor.

[0033] The CPU 303 also converts the image data into a PWM signal and supplies it to the laser driver. The flow of this conversion into a PWM signal will be explained with reference to the block diagram of FIG.

[0034] FIG. 3 is a block diagram showing an image signal generation flow for converting image data into a PWM signal.

[0035] In FIG. 3, a main scanning counter 703 that is reset for each BD signal counts for each pixel and outputs a count value.

[0036] Profile calculation unit 707, which functions as scanning position correction means and magnification characteristic correction means, receives the count value of main scanning counter 703 and outputs an ideal magnification correction amount for each main scanning position to pixel size calculation unit 708. In this embodiment, exposure scanning of photosensitive drum 102 is performed without using an fθ lens, and changes in the exposure scanning speed during this process are corrected by profile calculation unit 707. A method for generating a magnification correction amount by profile calculation unit 707 will be described later.

[0037] The pixel size calculation unit 708 outputs a pixel size by feedback calculation so as to conform to the ideal amount of magnification correction output by the profile calculation unit 707. In this embodiment, the pixel size ranges from 24 to 32, and each pixel size is associated with a gradation characteristic 1 to N (N=9). For example, a pixel size of 24 corresponds to gradation characteristic 1, a pixel size of 25 corresponds to gradation characteristic 2, and so on, with each increase in pixel size by +1 increasing the gradation characteristic number by +1.

[0038] FIG. 4 is an explanatory diagram showing the change in the number of pixel divisions depending on the scanning position of the optical scanning device 104.

[0039] In Figure 4, the pixels on the horizontal axis are divided into 24 at both ends in the main scanning direction of the horizontal axis, and 32 in the center. The number of divisions for the pixels in between is changed and set to match the characteristics of the optical system. Note that the number of pixel divisions has been simplified in Figure 4 to make it easier to see.

[0040] The gradation characteristic is a profile that associates input pixel values ​​with output densities, and can be realized using a table or function. In this embodiment, since the output is PWM, the gradation characteristic is expressed as a bit pattern table. Examples of gradation characteristic 9 and gradation characteristic 1 are shown in Figure 5.

[0041] FIG. 5 is an explanatory diagram showing the gradation characteristics based on the bit pattern.

[0042] In Figure 5, Figure 5(A) shows the bit pattern of gradation characteristic 9, and Figure 5(B) shows the bit pattern of gradation characteristic 1. The left column indicates the input gradation, and each row corresponding to each input gradation indicates the PWM ON / OFF pattern using 1 / 0.

[0043] Figures 6(A) and (B) are graphs showing the change in PWM duty, with the gradation characteristics of Figures 5(A) and (B) expressed in terms of input gradation and pulse width, respectively. In this embodiment, the gradation characteristics are set to be approximated even with different division numbers.

[0044] The gradation characteristic selector 706 selects and outputs gradation characteristics 1 to N according to the input pixel size 24 to 32. The PWM conversion 701 outputs a PWM bit pattern according to the table selected for each pixel by the gradation characteristic selector 706 according to the gradation of each pixel.

[0045] The parallel / serial converter 702 extracts the PWM bit pattern output by the PWM converter 701 in units of a fixed number of bits (24 bits in this embodiment), converts it into a serial signal, and outputs it to the laser driver 304 .

[0046] For example, when consecutive pixels have pixel sizes of 32, 24, and 24 and gradations of 10, 1, and 5, the gradation characteristic selector 706 selects gradation characteristic 9 (FIG. 5(A)), gradation characteristic 1 (FIG. 5(B)), and gradation characteristic 1 in that order. The PWM converter 701 outputs PWM data of the corresponding gradation, and the parallel / serial converter 702 converts this into a serial string and outputs a PWM signal with 1 being High and 0 being Low.

[0047] In this embodiment, the profile data of the profile calculation unit 707 and the gradation characteristics 705 are stored on a hard disk (not shown), and the CPU 303 copies them to the memory 302 at startup, and controls the memory 302 to be accessed during image processing for high-speed processing.

[0048] The operation of the pixel size calculator 708 will be described below with reference to FIG.

[0049] FIG. 7 is a block diagram for calculating the actual pixel size from the target pixel size.

[0050] 7, when a target pixel size Sr(x) is input for each pixel, a value Sa(x) obtained by subtracting the quantization error Se(x-1) (output of delay 806) carried over from the previous pixel by a subtractor 801 is input to a quantizer 802. The quantizer 802 outputs n that satisfies the condition of the following equation (Math. 1) as the pixel size S(x).

[0051]

number

[0052] Inverse quantization 804 performs inverse quantization by multiplying the input value by 1 / Dbase output by threshold table 803. Subtractor 805 outputs a quantization error component Se(x) based on the difference between inverse quantization 804 and the target pixel size Sr(x), which is delayed by one pixel by delay 806 and fed back to the next target pixel size Sr(x+1) via subtractor 801. While repeating the above feedback process, pixel size calculation unit 708 outputs an integer corresponding to the number of divisions of the pixel as the pixel size.

[0053] FIG. 8 shows the output result of the pixel size calculation unit 708 in the entire main scanning direction.

[0054] Figure 8 is an explanatory diagram showing pixel size for each pixel position, with the horizontal axis of Figure 8 dividing the pixel size into 24 at the ends in the main scanning direction and 32 in the center, showing how the pixel size changes between the two types through feedback control. Figure 9 also shows the horizontal axis of Figure 8 enlarged to show how the pixel size changes for each pixel.

[0055] By controlling the pixel size as described above, the subtractor 805 compares the target value with the quantized data to calculate the quantization error. By incorporating the error from the previous time when calculating the next pixel in the subtractor 801, the target pixel size is reached in multiple pixels.

[0056] In this embodiment, the registration detection sensor 120 shown in FIG. 1 serves as a scanning position misalignment detection means for detecting misalignment of the scanning position, and measures the misalignment of the toner image formed on the intermediate transfer belt 107, and reflects the measurement result in the correction function.

[0057] As a method for correcting the print position in the main scanning direction, for example, there is a method in which the toner image on the transfer belt is read by a density sensor to detect the positional deviation in the main scanning direction.

[0058] 10A and 10B are explanatory diagrams illustrating a method for measuring the positional misalignment of a toner image formed on intermediate transfer belt 107 by registration detection sensor 120, and Fig. 10A shows a part of the rotation direction of intermediate transfer belt 107. In Fig. 10A, the dotted line indicates the print frame in the main scanning direction, 120L, 120C, and 120R indicate density sensors, and the shape ">" indicates a patch for detecting positional misalignment, and the center positions of each are images drawn at timings t1, t3, and t5 from the reference time of scanning.

[0059] The intermediate transfer belt 107 rotates in the direction of the arrow relative to the density sensors 120L, 120C, and 120R, which are fixed in position, and the positional deviation D is measured as the difference between the reference time interval T0 and the time interval T at which the patch passes directly below the density sensor and the two sides of the patch pass the density sensor.

[0060] Figure 10(B) is an enlarged view of the shape ">", which is open 45 degrees above and below the horizontal in the figure, so the amount of positional misalignment in the driving direction of the intermediate transfer belt 107 can be read as the amount of positional misalignment in the main scanning direction (misalignment distance), and positional misalignment information e1, e3, e5 measured corresponding to exposure scanning times t1, t3, t5 corresponding to the sensor positions can be obtained from the detection information of the three density sensors 120L, 120C, 120R.

[0061] FIG. 11 is a flowchart showing the operation of register detection.

[0062] In FIG. 11, when registration detection starts, the patch shown in FIG. 10 is generated (S1101), this patch is read by density sensors 120L, 120C, and 120R (S1102), and converted into the amount of misalignment (S1103), and the detection operation ends. The profile calculation unit 707 calculates the post-correction magnification characteristic Mh(x) as follows:

[0063] Figure 12 is a graph showing magnification characteristics, with the horizontal axes of Figures 12(A), (C), and (D) representing time t from the scanning reference, the horizontal axis of Figure 12(B) representing magnification M, the vertical axes of Figures 12(A) and (B) representing the main scanning position x in the exposure scanning direction, the vertical axis of Figure 12(C) representing the scanning speed v, and the vertical axis of Figure 12(D) representing the positional deviation e.

[0064] FIG. 12A shows the main scanning position versus time, which is expressed by the following polynomial (Equation 2).

[0065]

number

[0066]

number

[0067]

number

[0068]

number

[0069] Furthermore, the positional deviation amounts e2, e4, e0, and e6 at the intermediate values ​​t2 and t4 and both ends t0 and t6 can be obtained by substituting t2, t4, t0, and t6 into e(t).

[0070] By increasing the number of sections on the x-axis and interpolating using a cubic spline curve from x0 to x6 and e0 to e6 to derive the deviation function E(x), a correction close to the function e(t) interpolated on the time axis can be achieved.

[0071] Moreover, by differentiating the function of the amount of deviation, the magnification correction amount ΔM(x) can be obtained by equation (6). In the case of a cubic spline curve, the magnification correction amount ΔM(x) is expressed as a quadratic function for each section.

[0072]

number

[0073]

number

[0074] In this embodiment, a cubic spline curve is used, but other interpolation methods such as a spline curve of another degree or linear interpolation may also be used.

[0075] In addition, the exposure scanning time corresponding to the positions of the three density sensors 120L, 120C, and 120R may be converted using equation (1), and a quadratic function may be generated from the exposure position and the positional deviation information measured by the three density sensors 120L, 120C, and 120R and used for magnification correction.

[0076] Second Embodiment Next, a second embodiment of the present invention will be described.

[0077] In the second embodiment, the scanning position deviation is measured during product manufacturing to correct the magnification characteristics.

[0078] FIG. 13 is an explanatory diagram showing a method for measuring the scanning position deviation during product manufacturing.

[0079] In FIG. 13, optical sensors 401, 402, and 403 are placed on the surface of the photosensitive drum at the time of shipment from the factory, and the scanning time deviations dt0 to dt2 from the reference position are measured, and the positional deviation amounts e0, e1, and e2 of the scanning positions are calculated using the following equation (Equation 8).

[0080]

number

[0081] According to each of the above embodiments, when exposing and scanning a photosensitive drum using a simple fθ lens or without using an fθ lens, even if the high-order function defined by time is used, the deviation in the scanning position due to thermal changes or changes over time is efficiently synthesized into a high-order function with time as a parameter, and the error is quickly corrected, allowing the exposure and scanning to be performed at the accurate position, thereby obtaining good image quality.

[0082] In the above embodiment, the same correction function is used for correction on each surface of the polygon mirror that reflects the laser, but to improve the correction accuracy, correction may be performed using an optimum correction function for each polygon surface.

[0083] Furthermore, in a multi-beam system using a plurality of exposure elements, correction may be performed using an optimum correction function for each beam.

[0084] In addition, the results obtained from the correction information calculation may be stored in a non-volatile memory, and from the second startup onwards, the values ​​stored in the non-volatile memory may be read out to reduce the average time required to calculate the correction information.

[0085] Furthermore, in the above embodiment, the main scanning is processed without being divided, but the scanning range may be divided into a plurality of areas, and position correction may be performed by performing variable magnification processing using the magnification characteristics of each area. [Explanation of symbols]

[0086] 100...Image forming apparatus 101Y, 101M, 101C, 101Bk...Image forming section 102Y, 102M, 102C, 102Bk...Photosensitive drum 104Y, 104M, 104C, 104Bk...Optical scanning device 107...Intermediate transfer belt 120...Cash register detection sensor 120L, 120C, 120R...concentration sensor 207...BD sensor 303...Control unit 401, 402, 403...optical sensors 701...PWM conversion 702...Parallel / serial conversion 705... Gradation characteristics 706... Gradation characteristics selector 707...Profile calculation unit 708...Pixel size calculation unit

Claims

1. In an image forming apparatus, a photosensitive drum is scanned by exposure scanning at a variable scanning speed to form an image, a scanning position correcting means for correcting the scanning position by changing the magnification of the input image data according to a magnification characteristic corresponding to the scanning speed at which the photosensitive drum is exposed and scanned; a scanning position deviation detection means for detecting a further deviation of the scanning position after the scanning position correction means has corrected the scanning position; a magnification characteristic correcting means for correcting the magnification characteristic based on the deviation of the scanning position detected by the position deviation detecting means; An image forming apparatus comprising:

2. 2. The image forming apparatus according to claim 1, wherein the magnification characteristic correcting means corrects the magnification characteristic in accordance with the amount of deviation of the scanning position every time the scanning position deviation detecting means detects a deviation of the scanning position.

3. 2. The image forming apparatus according to claim 1, wherein the magnification characteristic correction means generates a scanning position misalignment characteristic by interpolating the detection information of the scanning position misalignment detection means in the exposure scanning direction, and corrects the magnification characteristic using the generated scanning position misalignment characteristic.

4. 2. The image forming apparatus according to claim 1, wherein said scanning position deviation detecting means detects deviation distances at a plurality of predetermined scanning positions during exposure scanning.

5. 2. The image forming apparatus according to claim 1, wherein the scanning position deviation detection means detects position deviations from a reference position of exposure scanning for a plurality of predetermined scanning times.

6. 2. The image forming apparatus according to claim 1, wherein the scanning position correcting means divides the scanning range into a plurality of areas, and corrects the scanning position by changing the magnification characteristics of each of the areas.

Images