JP2012025051A - Image forming apparatus, and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a scanning line interval in a sub scan direction becomes uneven in a forming image and an image quality decreases due to surface tilting which is an inclination difference at each reflecting surface of a polygon mirror in an electrophotographic process.SOLUTION: A chart recording control section 15 forms on a recording medium a plurality of test charts in which a combination of reflecting surfaces started to be used for every color in the polygon mirror is changed. A chart evaluating section 16 evaluates an unevenness degree of a main scanning line density in the sub scan direction for all test charts. A reflecting surface combination selecting section 17 selects a combination of reflecting surfaces started to be used for every color to the test chart whose evaluation value is within an allowable range. In each color unit, a use-start-reflecting-surface designating section 14 designates the reflecting surfaces started to be used which accord with the selected combination, controls a polygon motor and performs image formation. Image quality degradation caused by surface tilting can be reduced while an image forming speed is maintained without requiring a special constitution such as a position deviation measuring unit.

Description

  The present invention relates to an image forming apparatus that performs image formation by an electrophotographic process using a polygon mirror, and a control method thereof.

  Conventionally, a so-called electrophotographic system such as a laser beam printer or a copying machine that forms an image by converting laser light into scanning light using a polygon mirror (rotating polygon mirror) and scanning on an image carrier. An image forming apparatus is known. In general, an electrophotographic image forming apparatus forms an image through a plurality of processes such as charging, exposure, development, transfer, fixing, and cleaning.

  Here, a general electrophotographic process will be described first using an image forming apparatus that forms a single color image as an example. After the photosensitive drum is uniformly charged by the charging means provided in the apparatus, the photosensitive drum is exposed in accordance with the image signal by the exposure means including a polygon mirror. An electrostatic latent image is formed. Thereafter, the electrostatic latent image on the photosensitive drum is developed by the developing unit to become a toner image, and the toner image on the photosensitive drum is transferred to the recording medium by the transfer unit. The toner image transferred to the recording medium is fixed by a fixing unit to obtain a single color image. The transfer residual toner remaining on the photosensitive drum is collected by a cleaning unit.

  An image forming apparatus that forms a color image on a recording medium by superimposing toner images of a plurality of colors takes the following process. First, an electrostatic latent image is formed for each color on the photosensitive drum, and this is formed into one of a plurality of colors, for example, C (cyan), M (magenta), Y (yellow), or K (black). A toner image is obtained by sequential development by a developing means. The toner image is transferred onto the intermediate transfer belt by the primary transfer means. By repeating this process for each color, a plurality of color toner images are superimposed on the intermediate transfer belt. Further, a plurality of color toner images superimposed on the intermediate transfer belt are collectively transferred onto a recording medium by a secondary transfer unit, and fixed by a fixing unit to obtain a color image.

  Further, in recent years, so-called tandem multiple transfer systems have been widely adopted in which process units such as a photoconductor and a developing device are independently provided for each color and a full color image can be formed in one pass. In the tandem multiple transfer system, a unit is provided for each of a plurality of colors, for example, C, M, Y, and K, and charging, exposure, development, and transfer are performed. The toner image developed for each color is superimposed and transferred onto the intermediate transfer belt by the primary transfer unit for each color, further transferred to the recording medium by the secondary transfer unit, and fixed by the fixing unit, thereby obtaining a color image. It is done. The transfer residual toner remaining on the photosensitive drum of each color is removed and collected from the intermediate transfer belt by the cleaning unit.

  Here, a technique for scanning the photosensitive member by an exposure unit including a polygon mirror in the general electrophotographic process as described above will be described. When forming an electrostatic latent image on the photoreceptor, the polygon mirror is rotated at a constant speed by the polygon motor control means. The laser control unit controls the laser according to the image signal from the image formation control unit, and the laser beam is scanned on the photosensitive member by reflecting the emitted laser beam to the polygon mirror. At this time, the image writing timing is determined by the signal from the main scanning direction synchronization timing detection sensor. The polygon mirror has a plurality of reflecting surfaces. For example, in the case of four surfaces, four scanning lines are formed by one rotation of the polygon mirror.

  When an electrostatic latent image is formed by reflecting the laser beam on a polygon mirror and scanning on the photosensitive member, a difference in inclination occurs between the reflecting surfaces of the polygon mirror, so that the interval between the main scanning lines in the sub-scanning direction is reduced. This causes the problem of non-uniformity. Such a difference in inclination between the reflecting surfaces of the polygon mirror is generally referred to as “surface collapse”. Due to the occurrence of this surface tilt, even in the image formed on the recording medium, the scanning line interval in the sub-scanning direction becomes non-uniform, density unevenness and color unevenness occur, and the image quality deteriorates.

  In order to reduce the image quality deterioration due to such a tilt, the following techniques have been proposed. First, there is a technique in which a reflective surface with less surface tilt is selected from the reflective surfaces of a polygon mirror, and a laser beam is scanned using only the selected specific surface (see, for example, Patent Document 1). Also, there is a technique for detecting the positional deviation of a plurality of polygon mirrors and correcting the phase of the reflecting surface to be used so that the periodic deviation of the plurality of polygon mirrors is minimized (see, for example, Patent Document 2). ).

JP 2005-28705 A JP 2007-232770 A

  However, the conventional technique for reducing the image quality deterioration due to the tilting of the polygon mirror has the following problems.

  First, in a technique that selectively uses a reflective surface with less surface tilt, a surface that is not used for scanning occurs. Therefore, when the rotation speed of the polygon mirror is constant, it is necessary to reduce the rotation speed of the photoconductor in order to maintain the resolution in the sub-scanning direction, and the printing speed is reduced. In this case, it is conceivable to increase the printing speed by increasing the rotation speed of the polygon mirror, but further increasing the rotation speed of the polygon mirror is realized in view of the cost increase of the member, power consumption, and noise. Is not easy.

  In addition, in the technology that controls the reflective surface used based on the positional shift period between multiple polygon mirrors, phase control is performed based on positional shift information detected by the BD slit provided in the BD sensor that detects laser light. I do. However, since the final image quality of an actual image is not taken into consideration, a combination of usage surfaces that is optimal for the image quality is not always selected. In addition, a sensor for measuring the positional deviation of the scanning line is essential.

  An object of the present invention is to reduce image quality degradation due to a difference in inclination at each reflection surface of a polygon mirror with a simple configuration and without reducing the image forming speed.

  In order to achieve the above object, an image forming apparatus of the present invention comprises the following arrangement. That is, an image forming apparatus that forms a color image by superimposing a plurality of color images formed by an electrophotographic process using a polygon mirror having a plurality of reflecting surfaces on a recording medium, and superimposing all of the plurality of colors. A plurality of test charts formed by changing the combination of the reflective reflective start surfaces when forming the chart of the polygon mirror for each of the plurality of colors for chart image data for repeatedly forming recording lines and blank lines in the sub-scanning direction The degree of positional deviation of the recording line recorded in the test chart with respect to the chart image data for each of a plurality of test charts formed on the recording medium and chart recording control means for forming the recording line on the recording medium A chart evaluation means for evaluating the position, and the position by the chart evaluation means Combination selecting means for selecting a combination of the polygon mirrors for starting use corresponding to the test chart evaluated to be within a predetermined allowable range, and the polygon mirror for each of the plurality of colors And an image formation control means for performing image formation by controlling the reflection start surface to be the combination selected by the combination selection means.

  According to the present invention, it is possible to reduce image quality degradation due to a difference in inclination at each reflection surface of a polygon mirror without reducing the image forming speed with a simple configuration.

FIG. 3 is a diagram illustrating a configuration of an image forming apparatus according to the first embodiment. The figure which shows the block structure for exposure control in the image forming apparatus of this embodiment. A diagram for explaining the occurrence of image quality degradation due to troublesomeness, The figure which shows the correspondence (for 1 color) of each scanning line which continues in a subscanning direction, and the reflective surface used for the scanning, The figure which shows the correspondence (for 4 colors) of each scanning line which continues in a subscanning direction, and the reflective surface used for the scanning, Schematic diagram of a recording medium on which a plurality of test charts in this embodiment are recorded, The figure which shows the structural example of each test chart in this embodiment. The figure which shows an example of the main scanning line density in the subscanning direction of a test chart, The flowchart which shows the exposure control processing in this embodiment, The figure which shows the block structure for exposure control in the image forming apparatus of 2nd Embodiment. The flowchart which shows the exposure control processing in 2nd Embodiment, The figure which shows a mode that a user selects one from several test charts in 2nd Embodiment. FIG. 10 is a diagram illustrating a configuration of an image forming apparatus having four color developing devices and one photoconductor as another embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following embodiments do not limit the present invention related to the scope of claims, and all combinations of features described in the present embodiments are essential to the solution means of the present invention. Not necessarily.

<First Embodiment>
Apparatus Configuration FIG. 1 is a diagram showing an electrophotographic process configuration of an image forming apparatus in the present embodiment. This configuration is a so-called tandem multiple transfer system, and is provided with units a, b, c, and d corresponding to the number of colors forming an image of each color, that is, one unit per color.

  In the figure, reference numerals 1a, 1b, 1c and 1d denote photoconductors, 2a, 2b, 2c and 2d denote chargers, 3a, 3b, 3c and 3d denote exposure units, and 4a, 4b, 4c and 4d denote developing units. 51 is an intermediate transfer belt, 53a, 53b, 53c and 53d are primary transfer units, 55 is an intermediate transfer belt cleaner, 56 and 57 are secondary transfer units, 6a, 6b, 6c and 6d are cleaners, and 7 is a fixing device. , P is a recording medium.

  After the photoreceptors 1a, 1b, 1c, 1d are uniformly charged by the chargers 2a, 2b, 2c, 2d, the exposure units 3a, 3b, 3c, 3d are exposed according to the image signal, Electrostatic latent images are formed on the photoreceptors 1a, 1b, 1c, 1d. Thereafter, the electrostatic latent images are developed by the developing devices 4a, 4b, 4c, and 4d to obtain toner images. The toner images on the four photoconductors 1a, 1b, 1c, and 1d are multiplex-transferred onto the intermediate transfer belt 51 by the primary transfer portions 53a, 53b, 53c, and 53d, and further, the recording medium P is transferred by the secondary transfer portions 56 and 57. Is transferred to. Transfer residual toner remaining on the photoreceptors 1a, 1b, 1c, and 1d is collected by the cleaners 6a, 6b, 6c, and 6d. Further, the transfer residual toner remaining on the intermediate transfer belt 51 is collected by the intermediate transfer belt cleaner 55. The toner image transferred to the recording medium P is fixed by the fixing device 7 to obtain a color image.

Generally, four colors of cyan (C), magenta (M), yellow (Y), and black (K) are used as color toners used in the image forming apparatus. For example, toner of C in the developing device 4a, M in the developing device 4b, Y in the developing device 4c, and K in the developing device 4d can be mounted. Note that the order of colors may be arbitrary. FIG. 2 is a diagram showing a block configuration for exposure control in the image forming apparatus of the present embodiment. Here, in order to simplify the description, the block configuration of the unit a for one color will be described in detail, but the other units b, c, and d have the same configuration.

  As shown in FIG. 2, the unit a includes an exposure unit 3a for the photoreceptor 1a. In the exposure unit 3a, a laser beam LB emitted from the laser L is a plurality of reflecting surfaces (four surfaces x, y, z, and w in this embodiment) of the polygon mirror PMa that can rotate at a constant speed in the direction of the arrow in the figure. ) To scan the photoreceptor 1a.

  The main scanning direction synchronization timing in the scanning is detected by the main scanning direction synchronization timing detection sensor BDSa, and the main scanning direction synchronization control unit 10a determines the writing timing in the main scanning direction based on the detection signal. Then, the image formation control unit 11a controls the laser control unit 12a according to the writing start timing, thereby causing the laser L to emit a laser beam LB based on the image information. The laser beam LB is reflected on the reflecting surface (x, y, z, w surface) of the polygon mirror PMa rotating at a constant speed under the control of the polygon motor control unit 13a, and scans on the photoreceptor 1a. At this time, since there are four reflecting surfaces of the polygon mirror PMa, four scanning lines can be formed by one rotation of the polygon mirror PMa.

  Based on the same chart image data for each color unit of the present embodiment, the chart recording control unit 15 in the controller generates a test chart formation signal for detecting the optimum combination of reflecting surfaces of the respective polygon mirrors. Output. Details of this test chart will be described later. The chart evaluation unit 16 measures and evaluates a plurality of test charts formed on the recording medium under the control of the chart recording control unit 15. Based on the evaluation result by the chart evaluation unit 16, the reflection surface combination selection unit 17 selects the optimum combination of the reflection start surfaces of the polygon mirrors in the units a to d of the respective colors. The details of the method of selecting the combination of the start-use reflective surfaces will be described later. Then, according to the selected combination, for example, information for designating the use start reflecting surface in the polygon mirror PMa is given to the use start reflecting surface designating unit 14a of the unit a.

  When the use start reflection surface designating unit 14a of the unit a receives the above information from the reflection surface combination selection unit 17 of the controller, the use start reflection surface designating unit 14a designates the polygon motor control unit 13a with the use start reflection surface of the polygon mirror PMa. Then, the polygon motor control unit 13a controls the rotation of the polygon mirror PMa so that a specific scanning line is scanned on the designated use start reflecting surface. Details of the image formation by the selected use start reflecting surface will be described later.

  The configuration of the unit a shown in FIG. 2 may be provided for each of the units a, b, c, and d as shown, but may be a common configuration for each unit. In FIG. 2, “a” is added to the number of each component to indicate that the unit a is configured. However, by omitting this “a”, each component provided in each unit, Or a common structure shall be shown. For example, the polygon motor control unit 13 is used as a general term for the polygon motor control units 13a, 13b, 13c, and 13d in each unit.

Degradation of image quality due to surface tilt of polygon mirror Next, with reference to FIG. 3, a difference in inclination between the reflecting surfaces of the polygon mirror, that is, image quality degradation caused by surface tilt will be described. FIG. 3 (a) shows how the reflection direction of the laser beam changes when the polygon mirror in the exposure unit 3 is tilted, that is, when the reflection surface of the polygon mirror is inclined with respect to the ideal surface. FIG. When each of the four reflecting surfaces x, y, z, and w included in the polygon mirror has a slight inclination with respect to the ideal surface, the reflected laser beam LB as shown in FIG. The direction of is different for each surface. FIG. 3B is a diagram showing a shift of the scanning line in the sub-scanning direction that occurs when the surface tilt as shown in FIG. 301 shows an ideal example in which the scanning lines are arranged at equal intervals when the four surfaces x, y, z, and w coincide with the ideal surface. On the other hand, 302 shows an example of image quality deterioration in which the scanning line intervals are non-uniform when the x, y, z, and w planes do not coincide with the ideal plane and surface tilt occurs.

Polygon Mirror Reflecting Surface FIG. 4 is a diagram showing the correspondence (for one color) between each scanning line connected in the sub-scanning direction and the reflecting surface used for the scanning. In the figure, each scanning line is numbered 1, 2, 3, 4,..., And each scanning line in the case where there are four types of x, y, z, w surfaces of the polygon mirror as the start reflective surface. The reflective surface used for the scanning of is shown. Here, the use starting reflection surface is defined as a reflection surface used when scanning a scanning line whose scanning line No. is 1. As shown in FIG. 4, the x, y, z, and w planes are repeatedly used sequentially in accordance with the rotation of the polygon mirror PM in any of the four types of use-start reflective surfaces.

  FIG. 5 is a diagram illustrating a correspondence relationship (for four colors) between each scanning line continuous in the sub-scanning direction and a reflecting surface used for the scanning. Since the use start reflection surface can be set independently for each of the polygon mirrors corresponding to each color, the use start reflection surfaces of the respective polygon mirrors can be arbitrarily combined for each color. Here, the case where the use start reflecting surfaces for forming the C, M, Y, and K images are the w, x, z, and y surfaces, respectively, is shown as an example. In the present embodiment, each of the four colors has four polygon mirror reflecting surfaces, and therefore, there are 64 combinations (= 4 × 4 × 4 × 4/4) of starting use reflecting surfaces that can be taken for each color.

  Here, the reason for dividing by 4 when calculating the combination of the use-start reflecting surfaces that can be obtained for each color will be described. The combination of the start-of-use reflecting surfaces of the respective colors in the scanning line No. 1 can occur in the same manner when the scanning line No. is 2, 3, or 4. However, in any combination, since the same repetition is performed in the sub-scanning direction, it can be considered that they are the same as the combination of the reflecting surfaces from the viewpoint of the circular permutation. Therefore, all combinations of the start-use reflective surfaces that can be obtained for each color are obtained by dividing 4 times of 4 as described above by 4.

Test Chart A test chart recorded for selecting an optimal polygon mirror use start reflecting surface combination before actual image formation in the present embodiment will be described in detail below.

  FIG. 6 is a schematic diagram showing a state of a recording medium on which a plurality (64) of test charts are recorded. Each square part in the figure represents each test chart. Since the polygon mirror for each color of C, M, Y, and K has four reflective surfaces of x, y, z, and w, respectively, there are 64 combinations of the reflective surfaces for starting use as described above. In the example shown in FIG. 6, the Y use starting reflection surface is fixed to the x plane, and the M use starting reflection surface is the x, y, z, and w surfaces in the order of upper left, upper right, lower left, and lower right in the figure. In this case, the four combinations of C and K are shown. Here, the Y-use starting reflective surface is fixed to the x plane, as described above, when the Y-use starting reflective surface is the x plane and the y, z, and w planes are sub-scanned. This is because the repeated patterns of directions are the same. Therefore, in this embodiment, as shown in FIG. 6, a total of 64 test charts are recorded on the recording medium, each of which has a different combination of use-reflecting reflective surfaces when forming a polygon mirror chart for each color. Note that the combination of the four colors of use start reflecting surfaces with respect to the recording medium and the arrangement on the recording medium are not particularly limited and are arbitrary.

  FIG. 7 is a diagram showing a configuration example of each of the 64 test charts shown in FIG. As shown in the figure, the test chart in the present embodiment has a pattern in which one recording line + 4 blank lines are repeated in the sub-scanning direction based on the chart image data. In FIG. 7, the notation (n, n + 1, n + 2,...) On the left side of the pattern is a number assigned to each scanning line, and n is an integer of 1 or more. The test chart of this embodiment is common to the four colors C, M, Y, and K. Therefore, the recording lines that are actually recorded in the test chart on the recording medium are recorded by overlapping all the colors of CMYK. Is done. At this time, the resolution in the sub-scanning direction of the test chart is equal to the scanning line interval in the sub-scanning direction, the recording duty of the recording line portion is 100%, and the recording duty of the blank line portion is 0%. The image size of the test chart is assumed to be 15 mm × 15 mm, for example. In one test chart, the pattern is repeated for every five scanning lines, whereas the polygon mirror of each color has four reflecting surfaces, so the recording line in the test chart has four reflecting surfaces one by one. It is used and recorded in sequence while shifting.

  In the present embodiment, an example in which one recording line + 4 blank lines, that is, a repetition of a pattern in which one recording line has the same number of blank lines as the reflecting surface of the polygon mirror is used for all colors as a test chart is shown. . However, the test chart used in the present invention is not limited to this example, and other types of charts may be used as long as a difference in image quality due to the combination of the reflective mirrors for starting use of the polygon mirrors of the respective colors can be detected.

Evaluation of Test Chart In this embodiment, the image quality is measured and evaluated for each of the plurality of test charts formed as described above as follows. Specifically, first, a recording medium on which a plurality of test charts are recorded is read by reading means (not shown) such as a scanner provided inside or outside the image forming apparatus. For each test chart, the average of the density values of each pixel (hereinafter, the main scanning line density) is obtained for each line in the main scanning direction, so that the position of the recording line is shifted, that is, the position from the ideal position based on the chart image data. Evaluate the degree of deviation. Specifically, the main scanning line density variation σ in the sub-scanning direction is calculated for each test chart, and this is used as an evaluation value indicating the degree of positional deviation in the sub-scanning direction. Here, the main scanning line density variation σ indicates a variation in the difference between the actually detected main scanning line density value (actually measured density value) and the ideal density value that is an ideal density value for the main scanning line. It may be calculated as a value (variance or standard deviation). The ideal density value can be calculated in advance as an ideal value of the main scanning line density from chart image data, for example. At this time, tilt correction or magnification conversion accompanying reading may be performed as necessary.

  Here, FIG. 8 shows an example in which the main scanning line density in the sub-scanning direction is detected for the test chart shown in FIG. 7 formed by a certain combination of polygon mirror use start reflection surfaces. In the figure, the horizontal axis indicates the position of the test chart in the sub-scanning direction, and the vertical axis indicates the calculation result of the average density (main scanning line density) for each main scanning. FIG. 8 is an enlarged view of the vicinity of a certain position in the sub-scanning direction with respect to the main scanning line density obtained for three combinations A, B, and C among 64 combinations of the reflecting surfaces of the polygon mirror. It is a thing. The recording lines in each test chart are formed so that all the colors overlap, but the combinations of the reflecting surfaces of the polygon mirror for each color used at that time are different from each other. Therefore, as shown in FIG. 8, the main scanning line density in each test chart does not necessarily match at each position in the sub-scanning direction, and the degree of overlap of each color in the recording line portion and the interval (blank line portion). Features such as non-uniformity are different.

  In the present embodiment, as described above, an image with little positional deviation in the sub-scanning direction and with less unevenness and color unevenness will be formed from the combination of the reflective surfaces where the polygon mirrors for each color are used. A combination is selected. Since recording is performed with the selected combination at the time of actual image formation, it is possible to reduce unevenness and color unevenness in the sub-scanning direction due to the tilting of the polygon mirror.

  In this embodiment, since it is necessary to detect high-frequency unevenness in the sub-scanning direction, it is possible to ensure a sufficient reading resolution with respect to the scanning resolution in the sub-scanning direction in the image forming apparatus when measuring the test chart. desirable. Therefore, in this embodiment, it is assumed that the resolution in the sub-scanning direction of the image forming apparatus, that is, the resolution in the sub-scanning direction of the test chart is 600 dpi, and the reading resolution of the scanner that reads this is 1200 dpi. When it is desired to detect macroscopic image stripe unevenness, it is possible to detect a relatively low frequency stripe unevenness component in the sub-scanning direction by reducing the reading resolution of the scanner to, for example, about 150 dpi. Alternatively, a low-pass filter corresponding to visual characteristics may be applied to an image after reading to evaluate an image corresponding to human vision. In addition, other well-known method for evaluating a stripe with respect to an image can be applied.

  In addition, as density information for each pixel used for calculating an evaluation value for a test chart, parameters that can express density and color, such as brightness, luminance, optical density, and L * a * b * values, are generally used. Is possible. Therefore, in this embodiment, for example, even if the test chart is monochrome, it is possible to appropriately evaluate the unevenness.

  Here, in order to compare the positional deviation in the sub-scanning direction for each test chart, the method using the variation σ of the main scanning line density is shown, but the comparison may be performed by other methods. For example, a comparison method using various parameters such as the difference between the maximum value and the minimum value of the main scanning line density and the integrated value of the power spectrum when Fourier transformed is applicable.

● Exposure control processing (surface tilt correction)
In the following, the exposure control process with surface tilt correction in the present embodiment will be described with reference to the flowchart of FIG.

  First, in S1001, the chart recording control unit 15 selects one of the above 64 methods as the combination of the use-start reflecting surfaces between the colors of the polygon mirrors. In S1002, the chart recording control unit 15 performs control so that a test chart is formed on the recording medium by the selected combination. That is, the use start reflection surface designation unit 14 in each color unit is notified of the use start reflection surface at the time of chart formation of each polygon mirror according to the combination selected in S1001. At this time, the chart image data held in advance is sent to the image formation control unit 11 of each unit. Then, each unit operates in accordance with the selected combination of the start-of-use reflective surfaces, whereby a full-color test chart as shown in FIGS. 6 and 7 is recorded on the recording medium.

  Next, in S1003, the chart recording control unit 15 determines whether or not a test chart has been recorded in S1002 for all 64 combinations of the start-use reflective surfaces. If an unprocessed combination remains, the process returns to S1001 to select the next use start reflecting surface combination, and the test chart is recorded in S1002. Note that the order of selecting combinations in S1001 is not particularly defined. On the other hand, if all combinations are selected in S1003, the process proceeds to S1004.

  In S1004, the chart evaluation unit 16 measures and evaluates the image quality of each of the plurality of test charts recorded on the recording medium. That is, as described above, the recording medium on which a plurality of test charts are recorded is read by a scanner or the like, and the main scanning line density is calculated for each test chart, thereby obtaining the measurement result as shown in FIG. Then, as described above, the variation σ of the main scanning line density in the sub-scanning direction is calculated as the evaluation value.

  Next, in S1005, the reflection surface combination selection unit 17 selects a test chart with less occurrence of unevenness and color unevenness based on the evaluation value for each test chart obtained in S1004. Specifically, one test chart is selected in which the variation σ of the main scanning line density in the sub-scanning direction is not more than a predetermined value as an assumed allowable range. Ideally, the test chart selected here has the smallest variation σ in all the test charts. However, even if the variation σ is not necessarily the minimum, the test chart may be selected as long as it is within an allowable range in which it is determined that the occurrence of unevenness and color unevenness is sufficiently small. Then, the reflection surface combination selection unit 17 selects a combination of the use start reflection surfaces of the polygon mirrors of the respective colors when the selected test chart is recorded, and information on the use start reflection surfaces of the respective colors corresponding to the combinations is obtained for each unit. Is notified to the reflecting surface designating unit 14.

  In step S1006, actual image recording by the image formation control unit 11 of each unit is executed by a combination of use-reflecting reflective surfaces of polygon mirrors of respective colors selected in step S1005. That is, the use start reflecting surface designation unit 14 of each unit notified of the use start reflecting surface of the polygon mirror records the first line in the sub-scanning direction with the designated use start reflecting surface with respect to the polygon motor control unit 13. The rotation of the polygon motor is controlled. In this way, the polygon mirror start-use reflection surface in each unit is set so that the formed image has a high image quality, so in the image recorded here, the image quality degradation due to the surface tilt of the polygon mirror of each color Is maximally suppressed. In S1006, the example in which the polygon motor control unit 13 controls the rotation of the polygon mirror according to the designated use start reflecting surface is shown. However, the laser control unit 12 may control the emission of the laser beam. .

  As described above, according to the present embodiment, by performing test chart printing using a combination of a plurality of reflecting surfaces of a polygon mirror, it is possible to detect a combination that can reduce image quality deterioration due to surface tilt. When an image is formed by the detected combination, the reflection surfaces of the polygon mirrors are all used, so there is no adverse effect that the printing speed is reduced. In addition, since it is not necessary to provide a special sensor for directly measuring each scanning line in the formed image, there is no adverse effect such as high cost.

  Furthermore, according to the present embodiment, the polygon mirror, which has conventionally been regarded as a defective product that causes surface tilt due to insufficient surface accuracy, in the manufacturing stage of the polygon mirror is not uniformly used, and the method of the present invention is used. It may become usable by applying. Therefore, an effect of improving the yield rate when manufacturing the polygon mirror can be obtained.

<Second Embodiment>
Hereinafter, a second embodiment according to the present invention will be described.

  In the first embodiment described above, an example is shown in which the image quality of a plurality of test charts recorded by changing the combination of the reflection mirrors for starting use of the polygon mirror is measured and evaluated using reading means such as a scanner. The second embodiment shows an example in which the image quality determination of the test chart is left to the user of the image forming apparatus, and the user instructs the polygon mirror use start reflecting surface from the user interface (UI).

  The configuration of the image forming apparatus in the second embodiment is a tandem multiple transfer system similar to that in the first embodiment described above, and a description thereof is omitted. Only portions that are particularly different from the first embodiment will be described below.

  FIG. 10 is a diagram showing a block configuration for exposure control in image formation according to the second embodiment. As shown in FIG. 10, the unit “a” of the second embodiment is merely a replacement of the chart evaluation unit 16 in the controller with the chart evaluation UI 26 in the configuration shown in FIG. 2 in the first embodiment described above. The other configurations are the same.

  The chart evaluation UI 26 inputs the evaluation results by the user for each of the test charts recorded on the recording medium by the chart recording control unit 15 with respect to each image quality, that is, the degree of occurrence of uneven stripes and color unevenness.

  Hereinafter, the exposure control process accompanied by surface tilt correction in the second embodiment will be described with reference to the flowchart of FIG. In FIG. 11, steps similar to those in FIG. 9 described in the first embodiment are given the same step numbers, and description thereof is omitted.

  First, in S1001 to S1003, as in the first embodiment, a test chart for all combinations of polygon mirror usage start reflecting surfaces for each color is printed out on a recording medium. In S2004, the user evaluates the image quality of the plurality of test charts, selects one test chart with little unevenness and color unevenness, and specifies identification information indicating the test chart via the chart evaluation UI 26. Then, the reflection surface combination selection unit 17 receives the user instruction information from the chart evaluation UI 26, and acquires the combination of the use start reflection surfaces of the polygon mirrors of the respective colors when the test chart designated by the information is recorded. Then, each unit is notified of information on the use start reflecting surface of each color according to the combination. As a result, the polygon mirror use start reflection surface in each unit becomes a combination that allows high image quality to be expected, and actual image recording is performed based on the combination of the use start reflection surfaces in S1006.

  The test chart designation process in the second embodiment will be specifically described below. In the first embodiment described above, a plurality of test charts are output in the format as shown in FIG. 6, and the user evaluates them to select the optimum one. At this time, as identification information for designating the selected test chart, it is possible to use information on the reflection start surface of the polygon mirror of each color corresponding to the test chart. For example, when the user selects a test chart surrounded by a thick frame shown in FIG. 12, the user may directly input information on the start reflective surface of the polygon mirror of each color when the test chart is formed. In the case of FIG. 12, it is only necessary to specify from the chart evaluation UI 26 that yellow is the x plane, magenta is the x plane, and cyan and black are the z planes. Of course, the symbols x, y, z, and w indicating the reflective surface may be expressed as numerical values such as 1, 2, 3, and 4, for example.

  Note that the association between the test chart and the polygon mirror start-use reflection surface is not limited to the example shown in FIG. For example, by attaching an ID to each test chart and holding a table that holds the correspondence between the ID and the start-use reflective surface in the apparatus, the user only has to specify the ID of the test chart. The chart evaluation UI 26 is not necessarily provided in the image forming apparatus of the present embodiment. If the input can be referred to from the apparatus, the chart evaluation UI 26 is provided on an external device such as a PC connected to the apparatus. It may be done.

  Note that the user can easily determine the image quality of the test chart by visual observation, but it is also possible to perform image quality determination using a measuring instrument or the like. That is, it suffices if information on the test chart that the user has determined to be optimal can be input from the UI.

  As described above, according to the second embodiment, even when the image forming apparatus does not include an image reading unit such as a scanner, the combination of polygon mirror use start surfaces is determined according to a user instruction. The same effects as those of the first embodiment described above can be obtained.

<Other embodiments>
In the first and second embodiments described above, as a configuration of the image forming apparatus, a process unit such as a photoconductor and a developing device is independently provided for each color so that a full color image can be formed in one pass. An example of the structure of the transfer system is shown. However, the present invention is not limited to such a configuration, and can be applied to image forming apparatuses having other configurations as long as a color image is formed using a plurality of polygon mirrors. For example, even in a configuration in which a polygon mirror is shared between colors, for example, a configuration in which Y and K, and C and M each have a common exposure unit, unevenness in the sub-scanning direction and color unevenness due to surface tilt are reduced. Possible combinations of polygon mirror reflecting surfaces can be obtained.

  In addition, a configuration in which one polygon mirror is shared by each color, for example, a configuration in which an electrostatic latent image is formed on one photoconductor using a developing device for four colors as shown in FIG. The present invention is applied. In the figure, 1 is a photoreceptor, 2 is a charger, 3 is an exposure unit, 4a, 4b, 4c and 4d are development units, 51 is an intermediate transfer belt, 53 is a primary transfer unit, 55 is an intermediate transfer belt cleaner, 56 , 57 are secondary transfer sections, 6 is a cleaner, 7 is a fixing device, and P is a recording medium. After the photoreceptor 1 rotating at a constant speed is uniformly charged by the charger 2, the exposure unit 3 performs exposure according to the image signal, and an electrostatic latent image is formed on the photoreceptor 1. This is developed by any of the developing devices 4a, 4b, 4c, and 4d for four colors to obtain a toner image, and the toner image is transferred onto the intermediate transfer belt 51 by the primary transfer unit 53. By repeating this process for each color, a plurality of color toner images are superimposed on the intermediate transfer belt 51. Further, the secondary transfer unit 57 collectively transfers these onto the recording medium P and fixes them by the fixing device 7, thereby obtaining a color image. The developing devices 4a, 4b, 4c, and 4d perform development using toners of colors C, M, Y, and K, respectively, but the order of these colors is not limited.

  In the configuration shown in FIG. 13, since there is only one image forming unit common to each color, a polygon mirror common to all four colors is used. However, even in such a configuration, a test chart can be formed by changing the use start reflection surface for each color of the plurality of reflection surfaces of the polygon mirror. Therefore, even in a configuration in which image formation is performed using a single polygon mirror, it is possible to determine the combination of reflective surfaces for each color so that unevenness in the sub-scanning direction and color unevenness are reduced, resulting in surface tilt. Image quality degradation can be reduced.

  The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, etc.) of the system or apparatus reads the program. It is a process to be executed.

Claims (7)

An image forming apparatus for forming a color image by superimposing a plurality of color images formed by an electrophotographic process using a polygon mirror having a plurality of reflecting surfaces on a recording medium,
For the chart image data for repeatedly forming the recording lines that overlap all of the plurality of colors and the blank lines in the sub-scanning direction, the combination of the start reflective surfaces when forming the chart of the polygon mirror for each of the plurality of colors is changed. A chart recording control means for forming a plurality of test charts formed on the recording medium;
For each of a plurality of test charts formed on the recording medium, chart evaluation means for evaluating the degree of positional deviation of the recording line recorded on the test chart with respect to the chart image data;
A combination selection means for selecting a combination of the polygon mirrors to start use corresponding to the test chart evaluated by the chart evaluation means as the degree of positional deviation being within a predetermined allowable range;
Image formation control means for performing image formation by controlling the polygon mirror start-use reflective surface for each of the plurality of colors to be the combination selected by the combination selection means;
An image forming apparatus comprising: Furthermore, for each of the plurality of test charts, there is a measuring means for measuring a density value for each main scanning line in the sub-scanning direction,
The chart evaluation unit is configured to calculate, for each of the plurality of test charts, a difference between a density value for each main scanning line measured by the measuring unit and an ideal density value of the main scanning line calculated from the chart image data. A value indicating variation is calculated as an evaluation value indicating the degree of positional deviation,
2. The image forming apparatus according to claim 1, wherein the combination selection unit selects a combination of the use-reflecting reflective surfaces of the polygon mirror corresponding to a test chart whose evaluation value is within the allowable range.   3. The combination selection unit selects a combination of the polygon mirrors to start use corresponding to the test chart having the smallest evaluation value among the plurality of test charts. 5. Image forming apparatus. The chart evaluation means inputs user instruction information indicating a test chart designated by a user among the plurality of test charts,
2. The image forming apparatus according to claim 1, wherein the combination selection unit selects a combination of the use-reflecting reflective surfaces of the polygon mirror according to a test chart specified by the user instruction information.   5. The chart image data is data in which a pattern in which the same number of blank lines as the reflection surface of the polygon mirror continues following one recording line is repeated. The image forming apparatus described in the item. A chart recording control means, a chart evaluation means, a combination selection means, and an image formation control means, and superimposing a plurality of color images formed by an electrophotographic process using a polygon mirror having a plurality of reflecting surfaces on a recording medium A method of controlling an image forming apparatus for forming a color image
The chart recording control unit is formed by changing the combination of the polygon mirror for starting use of the polygon mirror for each of the plurality of colors for the chart image data in which the recording lines and blank lines that overlap all of the plurality of colors are repeated in the sub-scanning direction. A chart recording control step of forming a plurality of test charts on a recording medium;
A chart evaluation step in which the chart evaluation means evaluates a degree of positional deviation of the recording line recorded in the test chart with respect to the chart image data for each of the plurality of test charts formed on the recording medium; ,
A combination in which the combination selection unit selects a combination of the reflection mirrors to start using the polygon mirror corresponding to the test chart in which the degree of the positional deviation is evaluated by the chart evaluation unit to be within a predetermined allowable range. A selection step;
An image formation control step in which the image formation control means performs image formation by controlling the use start reflection surface of the polygon mirror for each of the plurality of colors to be a combination selected by the combination selection means;
A control method characterized by comprising:   A program for causing a computer device to function as each unit of the image forming apparatus according to any one of claims 1 to 5 when executed by the computer device.

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