Image forming device

Abstract

To provide an image forming device that can suppress an image from being defectively formed, when approximating fθ characteristics using magnification correction function.SOLUTION: In order to obtain correction characteristics which are used in changing clock frequencies for controlling a timing of outputting a laser beam for scanning a photosensitive drum, different magnification correction functions (quadratic functions) are set respectively for divided areas (S48). Since Fθ characteristics can be approximated regarded as a secondary curve when viewed locally, the magnification correction functions can be set, with a magnified area boundary position (S45) and magnification correction values of secondary curves for the divided areas (S47) being determined as boundary conditions. Thereby, since correction characteristics in which both the area boundary positions and the magnification correction values are continuous, can be realized, by using the magnification correction functions for the respective areas, the correction characteristics can be approximated in a boundary between the areas.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus using electrophotographic technology, such as a printer, a copying machine, a facsimile machine, or a multifunction machine. [Background technology]

[0002] In electrophotographic image forming devices such as digital copiers, an image is formed on a recording material through an exposure process in which an electrostatic latent image is formed on a photosensitive drum, followed by a development process and a transfer process. The exposure process for forming an electrostatic latent image on the rotating photosensitive drum is performed by an optical scanning device that scans a laser beam based on input image data. In the optical scanning device, a laser beam emitted from a light-emitting element is deflected by a rotating polygon mirror to scan the surface of the photosensitive drum in the direction of the rotation axis of the photosensitive drum (main scanning direction). Furthermore, as the photosensitive drum rotates, the laser beam scans the surface of the photosensitive drum in a direction intersecting the main scanning direction (sub-scanning direction). This forms a two-dimensional electrostatic latent image on the surface of the photosensitive drum.

[0003] Conventionally, optical scanning devices have been known in which laser light deflected by a polygon mirror is irradiated onto a photosensitive drum via an fθ lens with high processing accuracy to irradiate the photosensitive drum with laser light while maintaining a uniform scanning speed in the main scanning direction. In contrast, there are optical scanning devices, such as those described in Patent Documents 1 and 2, that irradiate the photosensitive drum with laser light deflected by a polygon mirror without using an fθ lens or using a simple fθ lens with low processing accuracy. In such optical scanning devices, the frequency of a pixel clock (hereinafter referred to as the clock frequency), which controls the output timing of the laser light, is changed according to a magnification correction function that approximates the correction characteristics (hereinafter referred to as the fθ characteristics) of an fθ lens with high processing accuracy, in order to approximate the exposure length (dot width) per pixel in the main scanning direction when an fθ lens with high processing accuracy is used. One cycle of the clock frequency basically corresponds to one pixel, and increasing the clock frequency reduces the pixel dot width, while decreasing the clock frequency increases the pixel dot width.

[0004] Patent Document 1 discloses a linear approximation method in which the clock frequency is corrected for each of a plurality of areas obtained by dividing the scanning range of a photosensitive drum according to a linear magnification correction function. In this way, the fθ characteristic, which is generally expressed as an nth-order function (n≧3), is reproduced using a magnification correction function of second order or less, which is easy to calculate. Furthermore, Patent Document 1 shifts the boundary position between adjacent areas (called the area boundary) for each scanning line, thereby suppressing image defects caused by errors that may occur between scanning lines due to the above-mentioned approximate correction.

[0005] In Patent Document 2, the clock frequency in each area is corrected by first-order approximation, and the area boundaries are matched between two colors, thereby suppressing misalignment of the area boundaries in the main scanning direction and color misalignment between the two colors. [Prior art documents] [Patent documents]

[0006] [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]

[0007] However, in the past, when the fθ characteristics were approximated by a magnification correction function, the magnification correction value sometimes changed significantly at the area boundary, which could result in image defects.

[0008] In view of the above problems, the present invention aims to provide an image forming apparatus that is configured to irradiate a photosensitive drum with laser light without using an fθ lens with high processing accuracy, and that can suppress the occurrence of image defects when the fθ characteristics are approximated by a magnification correction function. [Means for solving the problem]

[0009] An image forming apparatus according to one embodiment of the present invention is an image forming apparatus that forms an image on a recording material, and is equipped with: a photosensitive member; a light source that outputs laser light based on image data; a scanning unit that irradiates the photosensitive member with the laser light output from the light source and scans the laser light in the direction of the rotation axis of the photosensitive member; a modification unit that can change the frequency of a pixel clock to control the output timing of the laser light from the light source; a memory unit that stores a correction function representing information regarding the correction of the width in accordance with the scanning position in the direction of the rotation axis on the photosensitive member so that the frequency of the pixel clock is changed by the modification unit so that the width in the direction of the rotation axis of a latent image corresponding to each pixel of the image data becomes constant; and a setting unit that sets the correction function for each of a plurality of divided areas obtained by dividing a scanning range on the photosensitive member in which the laser light is scanned, and is characterized in that the setting unit sets the correction function for at least two of the plurality of divided areas based on information regarding the deviation of the scanning position in each divided area and information regarding the correction of the width at the boundary between each divided area. [Effects of the Invention]

[0010] According to the present invention, in a configuration in which laser light is irradiated onto a photosensitive drum without using an fθ lens with high processing accuracy, when the fθ characteristics are approximated by a magnification correction function of second order or less, the occurrence of image defects can be suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram illustrating an image forming apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the configuration of an optical scanning device and a control system of the optical scanning device. [Figure 3] FIG. 1 is a block diagram for explaining an overview of laser drive signal generation by a CPU. [Figure 4] FIG. 10 is a diagram showing pixel sizes according to count values. [Figure 5] Graphs showing the relationship between gray scale and pulse width, (a) when the pixel size is 32 divisions, and (b) when the pixel size is 24 divisions. [Figure 6] FIG. 4 is a block diagram for explaining the processing of a pixel size calculation unit. [Figure 7] 10 is a graph showing an example of pixel sizes according to main scanning positions output by a pixel size calculation unit. [Figure 8] 8 is a graph showing changes in pixel size in a partial range of FIG. 7; [Figure 9] 10 is a flowchart showing a process for determining a magnification correction value. [Figure 10] 4 is a flowchart showing initial setting and image forming processing. [Figure 11] 10 is a flowchart showing a model information acquisition process. [Figure 12] 10 is a flowchart showing a machine information acquisition process. [Figure 13] 10 is a flowchart showing a magnification correction function determination process. [Figure 14] FIG. 1A is a diagram showing the arrangement of density sensors, and FIG. 1B is a diagram showing patch images. [Figure 15] 10 is a flowchart showing a process of updating a magnification correction function in response to positional deviation detected by a density sensor. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Image forming device> The present embodiment will be described below. First, an overview of the image forming apparatus of the present embodiment will be described with reference to FIG. 1. As shown in FIG. 1, the image forming apparatus 100 of the present embodiment is a digital full-color printer (color image forming apparatus) that forms images using toners of multiple colors. The image forming apparatus 100 has four image forming units 101Y, 101M, 101C, and 101Bk that form images by color. The image forming units 101Y, 101M, 101C, and 101Bk form images using yellow, magenta, cyan, and black toner, respectively. In the following, the suffixes "Y, M, C, Bk" will be omitted except when describing specific colors.

[0013] The image forming unit 101 has a photosensitive drum 102 as a photosensitive member. Around the photosensitive drum 102, a charging device 103, an optical scanning device 104, a developing device 105, and a cleaning device 106 are arranged. An endless intermediate transfer belt 107 is arranged below the image forming unit 101. The intermediate transfer belt 107 as an intermediate transfer member is stretched by a drive roller 108 and tension rollers 109 and 110, and is driven to rotate by the drive roller 108 so as to move in the direction of arrow B (clockwise) in the figure. A primary transfer device 111 is arranged at a position opposite the photosensitive drum 102 across the intermediate transfer belt 107. The image forming apparatus 100 also includes a secondary transfer device 112 for transferring a toner image on the intermediate transfer belt 107 to a recording material S, and a fixing device 113 for fixing the toner image on the recording material S.

[0014] Here, the image formation process of the image forming apparatus 100 will be described. Since the image formation process is the same in the image forming units 101Y to 101Bk, the image formation process in the image forming unit 101Y will be described below as a representative example. First, the image formation process from the charging step to the developing step will be described. The photosensitive drum 102Y, which is rotated and driven, is charged by the charging device 103Y of the image forming unit 101Y. The charged photosensitive drum 102Y is exposed to laser light irradiated from the optical scanning device 104Y. As a result, an electrostatic latent image is formed on the rotating photosensitive drum 102Y. Thereafter, the electrostatic latent image is developed into a yellow toner image by the developing device 105Y, which serves as a developing unit.

[0015] The toner image formed on the photosensitive drum 102Y is primarily transferred to the intermediate transfer belt 107 by applying a primary transfer voltage to the primary transfer device 111Y. The toner image primarily transferred onto the intermediate transfer belt 107 is secondarily transferred by the secondary transfer device 112 onto the recording material S transported from a manual feed cassette 114 or a cassette 115 to a secondary transfer portion T2. The recording material S that has passed through the secondary transfer portion T2 is then transported to a fixing device 113, where heat and pressure are applied. As a result, the toner image formed on the recording material S is fixed to the recording material S. The recording material S with the fixed toner image is discharged to a discharge tray 116.

[0016] <Optical scanning device> 2 shows the configuration of the optical scanning device 104 and a control system for the optical scanning device 104. The optical scanning device 104 includes a laser light source 201 that emits laser light, a collimator lens 202 that shapes the laser light into parallel light, a cylindrical lens 203 that focuses the laser light that has passed through the collimator lens 202 in the sub-scanning direction, and a polygon mirror 204.

[0017] The laser light source 201 is driven by a laser drive unit 304. The polygon mirror 204 serving as a scanning unit has a rotating polygonal mirror made up of multiple reflective mirrors, and is driven to rotate at a constant speed by a polygon mirror drive unit 305. The optical scanning device 104 and the photosensitive drum 102 are positioned so that the laser light is scanned in the direction of the rotation axis of the photosensitive drum 102. Each time one surface of the reflective mirror of the polygon mirror 204 scans the photosensitive drum 102 (photoconductor) once, a scan line of the laser light irradiated from the laser light source 201 is formed on the photosensitive drum 102. Note that, although the polygon mirror 204 having five reflective mirrors has been exemplified in this embodiment, the number of reflective mirror surfaces is not limited to this.

[0018] The optical scanning device 104 also includes a BD (Beam Detector) sensor 207. The BD sensor 207 detects the laser light deflected by the polygon mirror 204, and outputs a horizontal synchronization signal (BD signal) in response to the detection of the laser light.

[0019] <Optical scanning device control system> 2, the optical scanning device 104 is controlled by a CPU (Central Processing Unit) 303. An image controller (not shown) that generates pixel signals based on input image data is connected to the CPU 303, and pixel signals are input from the image controller. In addition to the image controller, the CPU 303 is also connected to the BD sensor 207, memory 302, laser driver 304, and polygon mirror driver 305. The memory 302 serves as a storage unit and stores various types of information such as a gradation characteristic table and a magnification correction function, which will be described later. The memory 302 may be a storage device such as a ROM (Read Only Memory), a RAM (Random Access Memory), or a hard disk.

[0020] Based on the BD signal output from the BD sensor 207, the CPU 303 detects the writing start position of the scan line on the photosensitive drum 102 and counts the time interval of the BD signal. As a result, the CPU 303 detects the rotation speed of the polygon mirror 204 and transmits an acceleration / deceleration signal to the polygon mirror driver 305 to instruct acceleration or deceleration so that the polygon mirror 204 rotates at a predetermined speed based on the detected rotation speed. The polygon mirror driver 305 drives the polygon mirror 204 in accordance with the input acceleration / deceleration signal.

[0021] The CPU 303 converts the pixel signal input from the image controller into a PWM signal, and can output a laser drive signal that instructs laser light irradiation to the laser drive unit 304. The pixel signal includes bit pattern data (e.g., gradation data of 4 bits or more) that indicates the density of each pixel, and the PWM signal is PWM bit pattern data obtained based on the gradation characteristics table shown in Table 1 below.

[0022] An overview of generation of a laser drive signal by CPU 303 will be described with reference to Fig. 3. In this embodiment, a PWM conversion unit 701, a parallel / serial conversion unit 702, a main scanning counter 703, a gradation characteristics selector 706, a profile calculation unit 707, and a pixel size calculation unit 708 shown in Fig. 3 are realized by CPU 303 as a change unit that can change the frequency of a pixel clock (hereinafter referred to as clock frequency) that controls the output timing of laser light.

[0023] The main scanning counter 703 counts the position in the main scanning direction and outputs the count value to the profile calculation unit 707. The main scanning counter 703 resets the count value every time it receives a BD signal output from the BD sensor 207; in other words, it resets the position in the main scanning direction for each scan line and starts counting the position in the main scanning direction anew. The profile calculation unit 707 determines a magnification correction value in accordance with the position where a pixel is formed by the laser light (referred to as main scanning position x), and outputs the value to the pixel size calculation unit 708. The process of determining the magnification correction value by the profile calculation unit 707 will be described later (see FIG. 9).

[0024] The pixel size calculation unit 708 calculates an integer pixel size S(x) corresponding to the number of divisions of one pixel by feedback control, which will be described later, in accordance with the magnification correction value output from the profile calculation unit 707, and outputs the size to the tone characteristic selector 706. The operation of the pixel size calculation unit 708 will be described later (see FIG. 6).

[0025] The gradation characteristic selector 706 selects and outputs one of the gradation characteristics (1 to N) according to the pixel size S(x). In this embodiment, the pixel size S(x) ranges from 24 to 32, and a gradation characteristic (1 to N (N=9)) is associated with each pixel size S(x). For example, the gradation characteristic selector 706 associates gradation characteristic "1" when the pixel size S(x) is 24, associates gradation characteristic "2" when the pixel size S(x) is 25, and thereafter associates a gradation characteristic that increases by "1" with each increase in pixel size S(x) by "1."

[0026] FIG. 4 shows an example of pixel size S(x) selected according to the count value input from the main scanning counter 703. In FIG. 4, the horizontal axis shows count values (edges, center) that represent positions in the main scanning direction. For example, at both edge positions in the main scanning direction, the pixel size is set to "24" and one pixel is "divided into 24." At the center position in the main scanning direction, the pixel size is set to "32" and one pixel is "divided into 32." The pixel size S(x) represents the number of bits of data per pixel. Note that for ease of viewing, FIG. 4 only shows pixel sizes S(x) at the edge and center positions.

[0027] The gradation characteristics (1 to N) are profile data for associating the pixel values of pixel signals input from an image controller (not shown) with the output density, and can be realized as a table or function. In this embodiment, a PWM signal is output as a laser drive signal to the laser driver 304, so the gradation characteristics are expressed as a bit pattern gradation characteristics table. As an example, Table 1(a) shows a gradation characteristics table when the pixel size S(x) is "32" and the gradation characteristics is "9," and Table 1(b) shows a gradation characteristics table when the pixel size S(x) is "24" and the gradation characteristics is "1." In the gradation characteristics tables shown in Tables 1(a) and 1(b), the left vertical column indicates gradations (0 to 15), and each row corresponding to each gradation indicates the "on / off" pattern of the PWM signal using "1 / 0." [Table 1]

[0028] 5(a) and 5(b) are graphs showing the relationship (gradation characteristics) between the gradation and the pulse width of the PWM signal in the gradation characteristic tables shown in Tables 1(a) and 1(b), with the horizontal axis showing the gradation (16 gradations from 0 to 15) and the vertical axis showing the pulse width of the PWM signal. Here, the pulse width is the ON width of the PWM signal, and is represented by "1" in Table 1. FIG. 5(a) shows the case where the pixel size S(x) is "32", and FIG. 5(b) shows the case where the pixel size S(x) is "24". As can be seen by comparing FIGS. 5(a) and 5(b), in this embodiment, the gradation and the pulse width of the PWM signal are set in the gradation characteristic table so that the gradation characteristics are approximated even when the number of divisions is different.

[0029] Returning to the explanation of FIG. 3, the gradation characteristic selector 706 selects a gradation characteristic table of gradation characteristics (1 to N) corresponding to the pixel size (24 to 32) input from the pixel size calculation unit 708, and outputs the selected gradation characteristic table to the PWM conversion unit 701. The PWM conversion unit 701 converts the pixel signal (including 4-bit density data) into a PWM signal with a bit pattern set in the gradation characteristic table according to the gradation characteristic table (see Table 1) selected by the gradation characteristic selector 706 for each division number (24 to 32) of one pixel. The bit pattern is, for example, data represented by "0" and "1". The PWM conversion unit 701 outputs the converted PWM signal to the parallel / serial conversion unit 702. The parallel / serial conversion unit 702 divides the PWM signal (PWM bit pattern data) output from the PWM conversion unit 701 into a fixed number of bits (for example, 24 bits), converts it into a serial signal, and outputs it to the laser drive unit 304 as a laser drive signal.

[0030] For example, if the pixel size of consecutive pixels is "32, 24, 24" and the corresponding gradation characteristics are "10, 15, 15," the gradation characteristics selector 706 selects the gradation tables of gradation characteristics "9," "1," and "1." The PWM conversion unit 701 outputs a PWM signal with the corresponding gradation characteristics according to the selected gradation characteristics table, and the parallel / serial conversion unit 702 converts the PWM signal into a serial signal bit by bit, and outputs it to the laser driver 304 as a laser drive signal with "1" representing a high level and "0" representing a low level.

[0031] <Pixel size calculation section> The operation of the pixel size calculation unit 708 will be described with reference to Figure 6. A target pixel size Sr(x) for each pixel is input to the pixel size calculation unit 708. The target pixel size Sr(x) is determined by adding a magnification correction value to the basic division number Dbase. For example, if the division number Dbase is "24" and the magnification correction value is "+1.4", the target pixel size Sr(x) will be "25.4 (24 + 1.4)".

[0032] When the pixel size calculation unit 708 receives the target pixel size Sr(x), a subtractor 801 subtracts the quantization error Se(x-1) of the previous pixel in the main scanning direction from the target pixel size Sr(x), resulting in a value Sa(x), which is output to a quantization unit 802. The quantization error Se(x-1) is output from a delay unit 806, which will be described later. Here, "x" indicates the main scanning position of the current pixel. The quantization unit 802 calculates an integer "n" that satisfies the following "Equation 1," and outputs the calculated integer "n" as the pixel size S(x).

number

[0033] The threshold value table 803 outputs "1 / Dbase" (for example, 1 / 24) used in the above "Equation 1" to the quantization unit 802 and the inverse quantization unit 804. The pixel size S(x) is input from the quantization unit 802 to the inverse quantization unit 804, which performs inverse quantization by multiplying the input pixel size S(x) by "1 / Dbase" output from the threshold value table 803. The result of the inverse quantization (S(x) × 1 / Dbase) is output from the inverse quantization unit 804 to the subtractor 805.

[0034] The subtractor 805 outputs the difference ((S(x)×1 / Dbase)−Sr(x)) between (S(x)×1 / Dbase) input from the inverse quantization unit 804 and the target pixel size Sr(x) as the quantization error Se(x−1) to the delay unit 806. The delay unit 806 delays the quantization error Se(x−1) by one pixel and feeds it back to the target pixel size Sr(x) of the next pixel via the subtractor 801. By repeating the above feedback process, the pixel size calculation unit 708 outputs the integer “n” corresponding to the number of divisions of a pixel as the pixel size S(x).

[0035] Fig. 7 shows an example of pixel size S(x) corresponding to main scanning position x output by pixel size calculation unit 708. Fig. 8 shows changes in pixel size S(x) output corresponding to each pixel from "-2480" to "2380" in the main scanning direction shown in Fig. 7. In Figs. 7 and 8, the horizontal axis represents main scanning position x, and the vertical axis represents pixel size S(x) output from pixel size calculation unit 708 corresponding to main scanning position x.

[0036] As can be seen from FIG. 7, the pixel size S(x) output by the pixel size calculator 708 can take on two values due to the feedback process described above, such as "24 and 25" at both ends in the main scanning direction and "31 and 32" at the center. For example, as shown in FIG. 8, for pixels ranging from "-2480" to "2380" in the main scanning direction, the pixel size S(x) fluctuates between "24" and "25." This is because the decimal portion (error) that occurs in the target pixel size Sr(x) due to magnification correction is reflected in the target pixel size Sr(x+1) of the next pixel through feedback processing. Furthermore, as the main scanning position (x) increases, the frequency with which "25" is output as the pixel size S(x) increases. In other words, the frequency with which "24" is output decreases, and the pixel size S(x) gradually transitions from "24" to "25." That is, the subtractor 805 compares the target pixel size Sr(x) with the quantized data to calculate the quantization error, and when calculating the pixel size S(x) of the next pixel, the subtractor 801 incorporates the quantization error from the previous time, so that the target pixel size is reached using multiple pixels.

[0037] <Profile calculation section> Next, the operation of the profile calculation unit 707 will be described using Fig. 9 while also referring to Fig. 3. Fig. 9 is a flowchart showing the process of determining the magnification correction value. The profile calculation unit 707 starts the process of determining the magnification correction value every time it receives a signal from the BD sensor 207.

[0038] As shown in FIG. 9, the profile calculation unit 707 determines whether the count value output from the main scanning counter 703 is equal to or greater than a predetermined value "Th" (S1). If the count value is smaller than the predetermined value "Th" (No in S1), the profile calculation unit 707 waits without proceeding with the process. Here, the scanning range of the laser light on the photosensitive drum 102 is set to, for example, a range of "-2480 (left end) to +2480 (right end)" with the center of the scanning range as the reference. The predetermined value "Th" indicates the position where the laser light starts writing a scanning line scanned from left to right across the photosensitive drum 102. In this embodiment, the predetermined value "Th=200" is set to the position where 200 pixels are counted after the signal of the BD sensor 207 is detected.

[0039] If the count value is equal to or greater than the predetermined value "Th" (Yes in S1), the profile calculation unit 707 performs line initialization (S2). In this embodiment, in order to perform control for each of a plurality of areas (each divided region) obtained by dividing the scanning range in the main scanning direction into predetermined sizes, the area number indicating the divided area is initialized to "n=0", and the area variable indicating the position in the main scanning direction within the area is also initialized to "0". The profile calculation unit 707 determines whether or not it is the beginning of the area depending on whether or not the area variable is "0" (S3).

[0040] If the area variable is "0", that is, if it is the beginning of the area in the main scanning direction (Yes in S3), the profile calculation unit 707 acquires a magnification correction function corresponding to the area from the memory 302 (S4). If the area variable is not "0" (No in S3), the profile calculation unit 707 does not acquire a new magnification correction function, and jumps to the processing of step S5. The magnification correction function is shown in "Equation 2". In this embodiment, the magnification correction function is a quadratic function, and by determining the coefficients (a, b, c) of the magnification correction function for each of the multiple areas as described below, a different magnification correction function for each area is stored in the memory 302 as profile data.

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[0041] The profile calculation unit 707 determines a magnification correction value (M) according to the magnification correction function, corresponding to the main scanning position x where a pixel is formed by the laser light (S5). Thereafter, the profile calculation unit 707 adds "1" to the area variable (S6) and determines whether the count value output from the main scanning counter 703 is smaller than the scanning range (S7). If the count value is equal to or greater than the scanning range (No in S7), the laser light is not irradiated outside the scanning range of the photosensitive drum 102, and the profile calculation unit 707 ends the process of determining the magnification correction value. If the count value is smaller than the scanning range (Yes in S7), the profile calculation unit 707 determines whether the area variable is equal to the area length "areaL" (S8).

[0042] If the area variable is not the same as the area length "areaL" (No in S8), the profile calculation unit 707 returns to the process of step S3. In this case, since the count value has not reached the end position of the area, the area is not changed to the next area and the processes of steps S3 to S7 are repeated. On the other hand, if the area variable is the same as the area length "areaL" (Yes in S8), the profile calculation unit 707 adds 1 to the area number "n" and performs area initialization by clearing the area variable to "0" (S9), and then returns to the process of step S3. In this case, since the count value has reached the end position of the area, the area is changed to the next area and the processes of steps S3 to S7 are performed for the next area.

[0043] In this manner, in this embodiment, a magnification correction value (M) corresponding to the main scanning position x in each area is determined according to a magnification correction function that differs for each area, and is output to the pixel size calculation unit 708. The magnification correction value (M) determined by the magnification correction function is information related to width correction for changing the clock frequency so that the width in the rotation axis direction of the electrostatic latent image corresponding to each pixel of the image data becomes constant.

[0044] <Initial settings and image formation process> Next, the "initial setting and image forming process" will be described using Figs. 10 to 13 with reference to Figs. 2 and 3. Fig. 10 is a flowchart showing the initial setting and image forming process. The initial setting and image forming process are executed by CPU 303 (see Fig. 2). CPU 303 as a setting unit executes the initial setting process shown in steps S11 to S13 when the image forming apparatus 100 is powered on, and executes the image forming process shown in steps S14 and S15 when an image forming job is received.

[0045] 10, when the image forming apparatus 100 is powered on, the CPU 303 acquires model information specific to the image forming apparatus 100 from the memory 302 (S11). The CPU 303 also acquires machine information, which differs for each image forming apparatus 100, from the memory 302 (S12). The CPU 303 then determines a magnification correction function for each of a plurality of areas based on the acquired model information and machine information (S13), and stores the determined magnification correction function for each area (see Equation 17, described below) in the memory 302. Note that, since the magnification correction function stored in the memory 302 can be used from the second time the power is turned on, the above initial setting process need only be executed the first time the power is turned on.

[0046] Thereafter, when the CPU 303 receives an image formation job, it starts image formation processing to form an image on the recording material S (S14). During the image formation processing, the CPU 303 irradiates the photosensitive drum 102 with laser light using the optical scanning device 104, and at that time, it is possible to change the clock frequency using a magnification correction value according to a magnification correction function for each area stored in the memory 302.

[0047] The CPU 303 determines whether to end the image formation job (S15), and if the image formation job is not to be ended (No in S15), the CPU 303 returns to the processing of step S14 to continue the image formation process. If the image formation job is to be ended (Yes in S15), the CPU 303 ends the image formation process.

[0048] <Model information acquisition process> FIG. 11 shows the "machine information acquisition process" (S11 in FIG. 10) for acquiring model information. As shown in FIG. 11, the CPU 303 acquires from the memory 302, for each of the YMCK optical scanning devices 104, a pixel position function x(t) indicating the position on the photosensitive drum 102 relative to the exposure scanning time "t" from the BD sensor 207 (S21). The position on the photosensitive drum 102 here indicates which position in the main scanning direction is scanned according to the exposure scanning time, based on the position detected by the BD sensor 207, and is measured by actually irradiating a laser beam on a jig when assembling the optical scanning device 104. The pixel position function x(t) is expressed by "Equation 3." In this embodiment, the pixel position function x(t) is a position function expressed by an N-th degree polynomial of the time "t." Therefore, for example, in the case of a 13th degree polynomial, the coefficient "kn (n = 0 to 13)" is acquired as the model information.

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[0049] <Machine information acquisition process> Fig. 12 shows the "machine information acquisition process" (S12 in Fig. 10) for acquiring machine information. As shown in Fig. 12, the CPU 303 acquires, from the memory 302, machine information measured by irradiating a laser beam onto the image forming apparatus 100 equipped with the optical scanning devices 104 for each of the YMCK colors before shipping (S31, S32). The CPU 303 acquires a reference time Tp for measuring the scanning timing for three points "p=0 to 2 (left edge, center, right edge)" in the order of scanning the photosensitive drum 102 (S31), and acquires the time difference ΔXp from the reference time at each time as machine information (S32).

[0050] <Magnification correction function determination process> Fig. 13 shows the "magnification correction function determination process" (S13 in Fig. 10) for determining the magnification correction function. As shown in Fig. 13, the CPU 303 generates the correction function xj(t) by interpolating the "machine information (delay time ΔXp)" acquired in the above-mentioned "machine information acquisition process" (S41). In this embodiment, the delay time ΔXp is interpolated using a quadratic function, and therefore the correction function xj(t) as an approximation function approximating the fθ characteristic (predetermined scanning characteristic) expressed by a polynomial is expressed by "Equation 4."

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[0051] Since the simultaneous equations with three unknowns expressed by "Equation 5" are derived according to the acquired "aircraft information (delay time ΔXp)", the coefficients of the correction function xj(t) can be obtained by "Equation 6".

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[0052] Since the correction function xj(t) represents the time lag in the main scanning direction according to the pixel position function x(t), the CPU 303 calculates the corrected pixel position function xc(t) by adding and synthesizing the pixel position function x(t) and the correction function xj(t) (S42). The corrected pixel position function xc(t), which is a synthesized function, is expressed by "Equation 7".

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[0053] Next, the CPU 303 determines area boundary positions Pn for dividing the scanning range of the photosensitive drum 102 into a plurality of areas (S43). For example, if the scanning range (mainL) of the photosensitive drum 102 is "divided into 32" at equal intervals, the area length (areaL) of each area is expressed by "number 8" because "mainL" is a multiple of 32.

number

[0054] In this embodiment, the area boundary position Pn (start position of the area) of each area (area number n=0 to 31) and the area boundary position P32 indicating the end position of the area with area number 31 are expressed by "number 9".

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[0055] The CPU 303 calculates the area boundary time Tn corresponding to the area boundary position Pn (S44). As shown in "Equation 10", the area boundary time Tn corresponding to the area boundary position Pn is calculated by the inverse function xc(t) of the corrected pixel position function xc(t). -1 Calculated from (x). Inverse function xc -1 (x) can be calculated by using Newton's method or the like to find the time t at which "Equation 11" becomes "g(t) = 0".

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[0056] The CPU 303 determines the area boundary position Ptn after scaling as the "position boundary condition" for determining the magnification correction function for each area (S45). The area boundary position Ptn after scaling is information regarding the deviation of the scanning position in each divided area, and indicates the area boundary time Tn converted into distance, and is expressed by "Equation 12." "fave" is the average value over the scanning range of the clock frequency, which means the number of pixels exposed per time, and is determined by "Equation 13."

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[0057] The CPU 303 generates a pixel clock function f(t) for calculating a clock frequency according to the exposure scanning time (S46). The pixel clock function f(t) is generated by differentiating the corrected pixel position function xc(t), as shown in Equation 14.

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[0058] The CPU 303 calculates the area boundary magnification Mn at the area boundary (S47). Since the time to expose one pixel is the reciprocal of the clock frequency f(t), the magnification function M(t) according to the exposure scanning time is expressed by "Equation 15." Therefore, the area boundary magnification Mn can be calculated from the area boundary time Tn using "Equation 16." The area boundary magnification Mn is information regarding width correction for changing the clock frequency at the boundary of each divided area.

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[0059] By obtaining the area boundary position Ptn after scaling (S45) and the area boundary magnification Mn (S47), the CPU 303 can set the magnification correction function Mn(x) shown in "Equation 17" (S48). Then, the CPU 303 stores different magnification correction functions Mn(x) for each of the multiple areas in the memory 302. In this embodiment, the magnification correction function Mn(x) corresponding to the main scanning position x within each area is expressed as a quadratic function with the start position of each area (area boundary position Pn) set to "x=0".

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[0060] The CPU 303 calculates the coefficients "an, bn, cn" of the magnification correction function Mn(x) shown in "Equation 17" for each of a plurality of areas, and stores the different magnification correction functions Mn(x) defined by the calculated coefficients "an, bn, cn" in the memory 302. The following is an explanation.

[0061] The area boundary position Ptn(x) after magnification for each area relative to the area boundary position before magnification for each area is obtained by integrating the magnification correction function Mn(x) shown in "Equation 17" as shown in "Equation 18".

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[0062] In the magnification correction function Mn(x) shown in Equation 17, the "boundary condition of magnification" shown in Equation 19 is substituted for each area (n=0 to 31). Mn(0) indicates the start position of each area.

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[0063] Furthermore, for the area boundary position Ptn(x) after scaling shown in Equation 18, the "boundary condition of the position after scaling" shown in Equation 20 is substituted for each area (n=0 to 31). Ptn(0) indicates the start position of each area after scaling.

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[0064] The coefficients "an, bn, cn" are calculated for each area according to the simultaneous equations shown in "Equation 21" which summarizes the above.

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[0065] The CPU 303 stores the magnification correction function Mn (see Equation 17) defined by the calculated coefficients "an, bn, cn" in the memory 302. The magnification correction function Mn stored in the memory 302 is a quadratic function that differs for each area, and is used during image formation processing (see S5 in FIG. 10). The magnification correction function Mn is set for at least two of the multiple areas.

[0066] As described above, in this embodiment, a magnification correction value corresponding to the main scanning position is determined using a different magnification correction function (quadratic function) for each divided area. Then, based on the determined magnification correction value, the clock frequency that controls the output timing of the laser light when scanning the photosensitive drum 102 is changed. The fθ characteristic expressed by an n-th order function (n≧3) can be locally approximated as a quadratic curve. In this case, in this embodiment, the magnification correction function is set using the area boundary position after magnification change (S45) and the magnification correction value of each quadratic curve for each area (S47) as boundary conditions. This allows the magnification correction function for each area to achieve correction characteristics in which the area boundary position and magnification correction value are continuous. Since the correction characteristics can be continuously approximated at the boundaries between areas, image defects such as image quality degradation are less likely to occur even when the clock frequency is changed. Furthermore, calculations for approximating the correction characteristics by the CPU 303 are easy.

[0067] [Other embodiments] As described above, CPU 303 performs the "initial setting process" (see S11 to S13 in FIG. 10) and stores the magnification correction function determined based on the model information and device information in memory 302. The device information in this case is a fixed value (delay time ΔXp) specific to image forming apparatus 100 measured before shipping. The magnification correction function stored in memory 302 is then referenced during the "image formation process" (see S14 in FIG. 10) when changing the clock frequency that controls the output timing of the laser light.

[0068] However, the machine information (delay time ΔXp) may change due to various factors such as aging of the image forming apparatus 100. If this is the case, there is a risk that image defects such as deterioration in image quality may occur on the recording material S even if the clock frequency is changed using the magnification correction function stored in memory 302. To prevent this, it is preferable to newly acquire the machine information (delay time ΔXp) at an appropriate timing according to the use of the image forming apparatus 100, and update the magnification correction function based on the model information and the newly acquired machine information.

[0069] To newly acquire the machine information (deviation time ΔXp), for example, a detection toner image (called a patch image) of a predetermined shape for detecting deviation is formed on the intermediate transfer belt 107, and the formed patch image is read by a density sensor. This method will be explained below with reference to FIGS. 14(a) to 15. FIG. 14(a) is a diagram showing the arrangement of the density sensor, and FIG. 14(b) is a diagram showing the patch image. In FIG. 14(a), the dotted line indicates the area in the main scanning direction of the intermediate transfer belt 107 where a toner image can be formed, the arrow indicates the direction of movement of the intermediate transfer belt 107, and ">" indicates the patch image for deviation detection.

[0070] As shown in Fig. 1, the density sensor 120 as a detection unit is disposed downstream of the black image forming unit 101Bk in the conveying direction of the recording material S. Here, as shown in Fig. 14(a), three density sensors (120L, 120C, 120R) are arranged side by side at predetermined positions on the left end, center, and right end of the intermediate transfer belt 107 in the main scanning direction so as to face the intermediate transfer belt 107. As shown in Fig. 14(b), the patch image 130 is a wedge-shaped toner image having one side at an angle of 45 degrees to the conveying direction and another side at an angle of 45 degrees to the opposite direction to the conveying direction with respect to the main scanning direction.

[0071] 15 is a flowchart showing the process of updating the magnification correction function in response to the positional misalignment detected by the density sensor. The CPU 303 (see FIG. 2) executes the process of updating the magnification correction function, for example, every time the number of sheets of recording material S on which the image forming apparatus 100 has formed images reaches a predetermined number.

[0072] CPU 303 forms a patch image on intermediate transfer belt 107 (S51). CPU 303 acquires detection results of patch image 130 passing directly below density sensors (120L, 120C, 120R) at a constant speed from the density sensors (120L, 120C, 120R) (S52). That is, when intermediate transfer belt 107 moves in the direction of the arrow relative to the density sensors (120L, 120C, 120R), patch image 130 passes directly below the density sensors (120L, 120C, 120R). CPU 303 (see FIG. 2) can acquire a time difference ΔXp from a reference time Tp based on the time interval between when two sides of patch image 130 pass the density sensors (120L, 120C, 120R).

[0073] Then, the CPU 303 executes the above-mentioned "magnification correction function determination process" (see FIG. 13) (S53). In this way, if the additional correction function for the overall positional misalignment in the main scanning direction is set as a quadratic function from the exposure scanning time corresponding to the arrangement positions of the three density sensors (120L, 120C, 120R) and the positional misalignment information measured by the three density sensors (120L, 120C, 120R), three simultaneous equations can be created for the three unknown coefficients, and the three coefficients (an, bn, cn) can be obtained.

[0074] In the above embodiment, the same magnification correction function is determined for each surface of the reflecting mirror of the polygon mirror 204, but a different magnification correction function may be determined for each surface of the reflecting mirror of the polygon mirror 204.

[0075] The optical scanning device 104 may be a multi-beam type in which multiple scanning lines are simultaneously formed by multiple laser beams emitted from the laser light source 201 each time one surface of the reflecting mirror of the polygon mirror 204 scans the photosensitive drum 102. In this case, a magnification correction function may be determined for each of the multiple laser beams. [Explanation of symbols]

[0076] 100...image forming apparatus, 102...photosensitive member (photosensitive drum), 105...developing unit (developing device), 107...intermediate transfer member (intermediate transfer belt), 120 (120L, 120C, 120R)...detecting unit (density sensor), 201...light source, 204...scanning unit (rotating polygon mirror, polygon mirror), 302...storing unit (memory), 303...setting unit (CPU), 701...changing unit (PWM conversion unit), 702...changing unit (parallel / serial conversion unit), 703...changing unit (main scanning counter), 706...changing unit (tone characteristic selector), 707...changing unit (profile calculation unit), 708...changing unit (pixel size calculation unit), S...recording material

Claims

1. In an image forming apparatus for forming an image on a recording material, A photoreceptor; a light source that outputs laser light based on image data; a scanning unit that irradiates the photosensitive member with the laser light output from the light source and scans the photosensitive member with the laser light in a rotation axis direction; a change unit capable of changing the frequency of a pixel clock for controlling the output timing of the laser light from the light source; a storage unit that stores a correction function that represents information regarding correction of the width in accordance with a scanning position on the photosensitive member in the direction of the rotation axis, so that the frequency of the pixel clock is changed by the changing unit so that the width in the direction of the rotation axis of a latent image corresponding to each pixel of the image data becomes constant; a setting unit that sets the correction function for each of a plurality of divided areas obtained by dividing a scanning range of the photosensitive member in which the laser light is scanned, the setting unit sets different correction functions for at least two of the plurality of divided regions based on information on a deviation of a scanning position in each divided region and information on a correction of the width at a boundary of each divided region. An image forming apparatus characterized by:

2. the correction function is a quadratic function that differs for each of the divided regions; 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. the setting unit acquires the positional deviation at the boundary of each divided region based on an inverse function of a composite function obtained by combining a position function with time as a variable obtained from the scanning characteristics of the laser light stored in advance with an approximation function that approximates predetermined scanning characteristics expressed by a polynomial.

3. The image forming apparatus according to claim 2, wherein the image forming apparatus is a recording medium.

4. the setting unit obtains information regarding correction of the width at the boundary of each divided region based on a clock function that represents the frequency of the pixel clock and that uses time as a variable by differentiating the position function.

4. The image forming apparatus according to claim 3, wherein the image forming apparatus is a recording medium.

5. the setting unit sets the correction function for each of the divided areas using a scanning characteristic of a laser beam stored in advance when the image forming apparatus is turned on.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

6. a developing unit that develops a latent image corresponding to each pixel formed on the photosensitive member in response to scanning by the scanning unit into a toner image; an intermediate transfer body onto which the toner image on the photosensitive body developed by the developing unit is transferred; a detection unit for detecting the toner image transferred to the intermediate transfer body; the position function with time as a variable is obtained by forming a detection toner image of a predetermined shape on the intermediate transfer body and detecting the detection toner image by the detection unit; 4. The image forming apparatus according to claim 3, wherein the image forming apparatus is a recording medium.

7. the scanning unit has a rotating polygon mirror that deflects the laser light output from the light source and rotates at a predetermined speed to scan the photosensitive member; 7. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

Images