JP2011085870A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus to perform electric correction of positional deviation, wherein positional deviation is accurately detected while a result of detection is made stable when detecting the positional deviation. <P>SOLUTION: A positional deviation detecting pattern is formed based on image data to which coordinate correction is performed in areas other than a level difference suppressing area 51 without performing the coordinate correction of pixel unit of the image data in a subscanning direction in the level difference suppressing area 51 in a main scanning direction. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that forms an image on a transfer material based on image information.

  A positional shift (color shift) is known as a problem of the tandem color image forming apparatus. The causes include non-uniformity and attachment position deviation of the deflection scanning device lens, and assembly position deviation of the deflection scanning device to the color image forming apparatus main body. In that case, an inclination or a curve is generated in the scanning line, and the degree is different for each color, thereby causing an image position shift (color shift).

  As a method of dealing with this problem, there is a method of measuring the inclination and curvature of the scanning line using an optical sensor, correcting the bitmap image data so as to cancel them, and forming the corrected image. (Patent Document 1).

  On the other hand, in a color image forming apparatus, a misregistration detection pattern for each color is formed on an image carrier such as a photosensitive member or an intermediate transfer member, or a transfer material carrier such as a conveyor belt, and this is formed on a downstream portion. Detection is performed by the optical sensors on both sides, and correction of the positional deviation is performed according to the detection result.

JP 2004-170755 A

  However, when a misregistration detection pattern is formed by the misregistration correction method using the method of Patent Document 1, the pattern detection accuracy may deteriorate. Hereinafter, the reason why the detection accuracy deteriorates will be described.

  When obtaining a positional deviation (color deviation) using a positional deviation detection pattern, a pattern composed of horizontal lines and diagonal lines is formed, and the position of the line is determined by the optical sensor passing over the line. The color shift is calculated based on the detected line position. FIG. 11A shows an example of a positional deviation detection pattern. Assuming that the detection timings of the K1, K2, and Y lines by the optical sensor are t1, t2, and t3, and the conveyance speed of the intermediate transfer member is v, the color shift in the sub-scanning direction between black and yellow is {t3− (t1 + t2 ) / 2} × v. The pattern of this example is composed only of black and yellow, but a color shift in the sub-scanning direction between other colors can be obtained with a similar pattern.

  Moreover, the detection timing of each line is calculated | required from the signal of an optical sensor. FIG. 11B is a diagram showing a horizontal line formed as a part of a pattern for detecting misalignment and a scanning line of an optical sensor, and FIG. 11C is a horizontal line of FIG. It is a figure showing the signal from the optical sensor at the time of detecting along a scanning line. As shown in FIG. 11C, the timing at which the signal and the threshold intersect is set as the detection timing of the edge of the line, the detection timing at both ends of the line is averaged, and the detection timing of the center position in the sub-scanning direction of the line is obtained. This center position detection timing is the above-described line detection timing.

  Here, when detecting a misregistration detection pattern that has been subjected to electrical misregistration correction using the method of Patent Document 1 by the above method, there is a concern that the detection accuracy may deteriorate. . This will be described with reference to FIG.

  FIG. 12A is a diagram showing an image formation image when electrical displacement correction is performed on a horizontal line, and FIG. 12B is detected by an optical sensor with respect to the line of FIG. It is the figure which showed the detection signal in the case of doing. When electrical displacement correction is performed as shown in FIG. 12A, in addition to pixel-by-pixel correction, a small pixel of less than one pixel is used at the edge portion of the line.

  Here, in general, it is difficult for an electrophotographic image forming apparatus to stably form small pixels, and therefore, an edge portion of a line composed of small pixels tends to become unstable. In the case of forming a small pixel, if the amount of light at the time of exposure is reduced or the exposure time is shortened, the latent image becomes shallow, and the amount of applied toner is not stabilized. Therefore, as shown in FIG. 12B, the edge is shifted in the actual signal as compared with the ideal detection signal. As a result, since the center position of the line is also calculated with a deviation, the detection accuracy of the pattern for detecting the deviation is deteriorated.

  In addition, regardless of the electrical correction described above, even when correction is performed on a pixel-by-pixel basis, for example, a problem may occur in the misregistration detection result due to the correction. As described above, when electrical displacement correction is performed to correct the image data itself in relation to the sub-scanning direction, a problem occurs with respect to the detection result of the displacement detection pattern. An object of the present invention is to make it possible to accurately detect a misregistration detection pattern in an image forming apparatus that performs electrical misregistration correction.

  The image forming apparatus according to the present invention performs image position correction in the sub-scanning direction by performing image correction in the sub-scanning direction at each position in the main scanning direction on the image data in order to correct image tilt or image curvature. An image position correcting unit to perform, an image forming unit for forming an image on a transfer material based on the image data subjected to the image correction, a control unit for forming a misregistration detection pattern on the image forming unit, and the position A displacement detection means for detecting a displacement detection pattern, and the control means include a correction means for correcting displacement based on a detection result by the detection means, and the displacement detection pattern is at least in the main scanning direction. The image is not subjected to the image correction at a predetermined pixel position corresponding to the central portion.

  According to the present invention, in an image forming apparatus that performs electrical misregistration correction, a misregistration detection pattern can be detected with high accuracy.

It is a figure which shows one Embodiment of a color image forming apparatus cross section. It is a figure which shows one Embodiment of the signal processing in a functional block diagram and this functional block diagram. FIG. 6 is a diagram illustrating a relationship between a positional deviation amount of an image in the sub-scanning direction and a positional deviation correction amount Δy with respect to each position in the main scanning direction when the scanning line is inclined or curved. It is a figure for demonstrating one Embodiment of the image correction at the time of normal image formation. It is a figure which shows an example of the detection pattern for position shift. It is a flowchart of the switching process of an image correction method. It is a figure for demonstrating that the level | step difference of a line appears when image correction is performed without smoothing. It is a figure for demonstrating the positional relationship of a detection pattern and a level | step difference suppression area | region. It is a figure for demonstrating one Embodiment of the image correction at the time of detection pattern formation. It is a figure for demonstrating the difference in the position shift detection precision by each image correction method. It is a figure for demonstrating about the color shift detection between two colors of a subscanning direction. It is the figure which showed the detection signal at the time of detecting the horizontal line after electric position shift correction | amendment with the optical sensor.

  FIG. 1 is a diagram showing an embodiment of a cross section of a color image forming apparatus provided with image forming means for four colors, that is, yellow Y, magenta M, cyan C, and black K. The image forming apparatus includes an image forming unit shown in FIG. 1 and an image processing unit (not shown).

  The operation of the image forming unit in the electrophotographic color image forming apparatus will be described with reference to FIG. The color image forming apparatus in FIG. 1 forms an electrostatic latent image with exposure light that is turned on based on the exposure time converted by the image processing unit, and develops the electrostatic latent image to form a single color toner image. A single color toner image is superimposed to form a multicolor toner image. Further, the multicolor toner image is transferred to the transfer material 11 and the multicolor toner image on the transfer material 11 is fixed. The image forming unit includes a paper feeding unit 21, a photosensitive drum (22Y, 22M, 22C, 22K), an injection charger (23Y, 23M, 23C, 23K), a scanner unit (24Y, 24M, 24C, 24K), a toner cartridge ( 25Y, 25M, 25C, 25K), a developing device (26Y, 26M, 26C, 26K), an intermediate transfer belt 27, a transfer roller 28, and a fixing unit 30.

  The photosensitive drums 22Y, 22M, 22C, and 22K rotate by receiving a drive transmission from a motor (not shown). The motor rotates the photosensitive drums 22Y, 22M, 22C, and 22K in a counterclockwise direction according to an image forming operation. Rotate. Around the photosensitive drums 22Y, 22M, 22C, and 22K, there are injection charging devices 23Y, 23M, 23C, and 23K for charging the photosensitive members, and developing devices 26Y, 26M, 26C, and 26K that perform development. It has been.

  The intermediate transfer belt 27 is rotated in the clockwise direction by the rotation of an intermediate transfer belt drive roller 32 (hereinafter referred to as drive roller), and the drive roller 32 is rotated by receiving drive transmission from a motor (not shown). .

  In image formation, first, the rotating photosensitive drum is charged by the injection chargers 23Y, 23M, 23C, and 23K, and then the surface of the photosensitive drum is selectively exposed from the scanner units 24Y, 24M, 24C, and 24K. An electrostatic latent image is formed. Then, the latent image is developed with toner by the developing devices 26Y, 26M, 26C, and 26K to form a visible image. The single color toner image is superimposed and transferred onto the intermediate transfer belt 27 that rotates in the clockwise direction as the photosensitive drum rotates. Thereafter, the transfer roller 28 comes into contact with the intermediate transfer belt 27 to nipping and transferring the transfer material 11, and the multicolor toner image on the intermediate transfer belt 27 is transferred to the transfer material 11. Further, the transfer material 11 holding the multicolor toner image is applied with heat and pressure at the fixing unit 30, and the toner is fixed on the transfer material. After the toner image is fixed, the transfer material 11 is discharged to a discharge tray (not shown) by a discharge roller (not shown). The toner remaining on the intermediate transfer belt 27 is cleaned by the cleaning unit 29, and the cleaned toner is stored in a cleaner container.

  The image detection unit 31 includes two image detection sensors 31L and 31R provided on both sides of the intermediate transfer belt 27, and detects a positional deviation amount of each color by a method described later.

  FIG. 2 is a functional block diagram and a diagram showing an embodiment of signal processing in the functional block diagram.

  In FIG. 2, the host 301, the controller 302, and the engine 303 have independent main control units (CPUs 312, 313, and 314) for controlling each block in each device. Each CPU controls the operation in each device and the communication between the devices. In the image forming apparatus of the present embodiment, the controller unit and the engine unit are configured separately, and are configured to be closed between the devices so that the devices are individually controlled.

  An RGB image signal is transmitted from the host 301 and input to the controller 302. The input RGB signal is subjected to masking and UCR processing by the color conversion processing unit 304. In addition, color correction and under color removal are performed and converted into image signals of yellow Y, magenta M, cyan C, and black K. Further, correction is performed by the γ correction unit 305 so that the output density curve becomes linear. Thereafter, the image signal of each color is corrected by the image correction unit 315 to correct the image position in the sub-scanning direction, and is input to the engine 303.

  A laser driving unit 306 turns on and off the laser in accordance with an image data signal 307 from the controller 302 to the engine 303. Reference numeral 308 denotes a horizontal synchronization signal from the engine 303 to the controller 302, and reference numeral 309 denotes a vertical synchronization signal from the engine 303 to the controller 302. Image data is output from the controller 301 to the engine 303 in accordance with these synchronization signals. Reference numeral 310 denotes various command transmission signals from the controller 302 to the engine 303, and 311 denotes various status signals transmitted from the engine 303 to the controller 302.

  Next, a method for correcting image data in the image correction unit 315 will be described.

  FIG. 3 is a diagram illustrating the relationship between the positional deviation amount of the image in the sub-scanning direction and the positional deviation correction amount Δy with respect to each position in the main scanning direction when the scanning line is inclined or curved. Image tilt and image curvature are corrected in accordance with this positional deviation correction amount Δy. The positional deviation correction amount Δy is a correction amount for canceling the positional deviation in the sub-scanning direction. As shown in FIG. 3, the positional deviation amount in the sub-scanning direction and the positional deviation correction amount Δy have an inverse relationship. . For example, in the device manufacturing process, the amount of misalignment is measured for each device, and the misalignment correction amount Δy is obtained in advance based on the measured misalignment amount. The method of obtaining the positional deviation correction amount Δy is not limited to this, and a positional deviation detection pattern formed on the image carrier such as the intermediate transfer belt 27 as in the positional deviation detection method described later is provided in the image detection unit 31. May be detected, and Δy may be calculated from the detection result.

  FIG. 4 is a diagram for explaining that image correction is performed on the image data in the sub-scanning direction at each position in the main scanning direction and image position correction in the sub-scanning direction is performed during normal image formation. Note that normal image formation refers to image formation according to user-desired image data input from the outside, not to form a calibration (positional deviation detection pattern) pattern, which will be described later. Hereinafter, the image correction method will be described with reference to FIG. 4, taking as an example a portion in which a position deviation of the image inclination that rises to the right occurs. The image correction method shown in FIG. 4 is executed by the image correction unit 315 described with reference to FIG.

  FIG. 4A is an image of a scanning line in a portion where a positional deviation with an upward slope is generated. In this example, an inclination of 1 dot is generated per 4 dots in the main scanning direction of the scanner unit. FIG. 4B is an example of a horizontal straight-line bitmap image before gradation value conversion, and represents a 2-dot line. FIG. 4C is a corrected bitmap image obtained by correcting the bitmap image in FIG. 4B in order to cancel the positional deviation due to the inclination of the scanning line in FIG. In order to realize the correction image of FIG. 4C, image data of pixels before and after in the sub-scanning direction is adjusted. FIG. 4D is a table showing the relationship between the positional deviation correction amount Δy and the gradation value conversion parameter for each pixel in the main scanning direction of FIG. Note that Δy in FIG. 4D corresponds to the positional deviation correction amount Δy described in FIG. k is an integer part (truncated after the decimal point) of the positional deviation correction amount Δy, and represents the correction amount in the sub-scanning direction in units of one pixel. In the image position correction in units of one pixel, the pixels are offset (coordinate correction) in the sub-scanning direction in units of one pixel according to the correction amount.

β and α are image data adjustment distribution rates for correcting the image position in the sub-scanning direction of less than one pixel. Represents the share,
β = Δy−k
α = 1−β
Is calculated by α is a distribution ratio at a position offset by k pixels from the original pixel in the sub-scanning direction, and β is a distribution ratio at a position offset by k + 1 pixels. The gradation values distributed according to the distribution ratios α and β are added to the gradation values at the respective distribution positions.

  FIG. 4E is a bitmap image obtained by performing pixel gradation value conversion before and after in the sub-scanning direction according to the image correction parameter of FIG. FIG. 4F shows an exposure image of an image carrier of a bitmap image that has undergone tone value conversion, and the inclination of the main scanning line is canceled out to form a horizontal straight line.

  The above is the description of the image correction method. Note that the correction method described here is a method at the time of normal image formation, and a method different from this correction method is used as described later at the time of detecting a positional deviation.

  Next, the displacement detection operation of this embodiment will be described.

  A misregistration detection pattern (hereinafter referred to as a detection pattern or pattern) as shown in FIG. 5 is formed on the intermediate transfer belt 27, and image detection sensors 31 L and 31 R provided on both sides of the intermediate transfer belt 27. Read and detect the amount of misregistration of each color. Reference numerals 41 to 44 in FIG. 5 are patterns for detecting the amount of positional deviation in the main scanning direction and the sub-scanning direction. In this example, the inclination is 45 degrees, and K1 to K4 are reference colors (black), Y, M, and C. Indicates a detected color (Y: yellow, M: magenta, C: cyan). The patterns 41 and 43 are detected by the image detection sensor 31L, and the patterns 42 and 44 are detected by the image detection sensor 31R. Further, tYL1, tML1, tCL1, tK1L1, tK2L1, tK3L1, tK4L1 indicate the detection timing of the pattern 41, and tYR1, tMR1, tCR1, tK1R1, tK2R1, tK3R1, and tK4R1 indicate the detection timing of the pattern 42. In addition, tYL2, tML2, tCL2, tK1L2, tK2L2, tK3L2, and tK4L2 indicate the detection timing of the pattern 43, and tYR2, tMR2, tCR2, tK1R2, tK2R2, tK3R2, and tK4R2 indicate the detection timing of the pattern 44. An arrow indicates the moving direction of the intermediate transfer belt 27. Each pattern of the pattern 41 is arranged at equal intervals, and the patterns 42 to 44 are similarly arranged at equal intervals. The interval between the detection timings of the patterns 41 and 43 and the interval between the detection timings of the patterns 42 and 44 in an ideal state with no positional deviation is Tdt. That is, the difference tK1L2−tK1L1 between the detection timings tK1L1 and tK1L2 of the patterns 41K1 and 43K1 is Tdt in an ideal state with no positional deviation. The same applies to other detection timing intervals.

  With respect to the sub-scanning direction, the amount of misregistration of each color on both sides of the intermediate transfer belt detected by the detection pattern is calculated.

First, based on the detection timing after correction of the reference color, the detection timing shift after correction in the detection color is obtained. Deviations in detection timing of marks 41Y-44Y ΔYL1, ΔYR1, ΔYL2, ΔYR2, deviations in detection timing of marks 41M-44M ΔML1, ΔMR1, ΔML2, ΔMR2, deviations in detection timing of marks 41C-44C ΔCL1, ΔCR1, ΔCL2, ΔCR2 Is obtained by the following equation.
ΔYL1 = tYL1- (tK1L1 + tK2L1) / 2
ΔYR1 = tYR1− (tK1R1 + tK2R1) / 2
ΔYL2 = tYL2− (tK1L2 + tK2L2) / 2
ΔYR2 = tYR2− (tK1R2 + tK2R2) / 2
ΔML1 = tML1- (tK2L1 + tK3L1) / 2
ΔMR1 = tMR1- (tK2R1 + tK3R1) / 2
ΔML2 = tML2− (tK2L2 + tK3L2) / 2
ΔMR2 = tMR2- (tK2R2 + tK3R2) / 2
ΔCL1 = tCL1− (tK3L1 + tK4L1) / 2
ΔCR1 = tCR1− (tK3R1 + tK4R1) / 2
ΔCL2 = tCL2- (tK3L2 + tK4L2) / 2
ΔCR2 = tCR2− (tK3R2 + tK4R2) / 2

From the above detection timing shift, the amount of positional shift of each color on both sides of the intermediate transfer belt detected by the detection pattern is
ΔpYL = v × (ΔYL1 + ΔYL2) / 2
ΔpYR = v × (ΔYR1 + ΔYR2) / 2
ΔpML = v × (ΔML1 + ΔML2) / 2
ΔpMR = v × (ΔMR1 + ΔMR2) / 2
ΔpCL = v × (ΔCL1 + ΔCL2) / 2
ΔpCR = v × (ΔCR1 + ΔCR2) / 2
The positional deviation amounts ΔpY, ΔpM, ΔpC in the sub-scanning direction of yellow, magenta, and cyan are
ΔpY = (ΔpYL + ΔpYR) / 2
ΔpM = (ΔpML + ΔpMR) / 2
ΔpC = (ΔpCL + ΔpCR) / 2
It becomes. Here, v is the moving speed of the intermediate transfer belt 27.

Regarding the main scanning direction, the amount of positional deviation of each color on both sides of the intermediate transfer belt detected by the detection pattern is
ΔsYL = v × (ΔYL1−ΔYL2)
ΔsYR = v × (ΔYR1−ΔYR2)
ΔsML = v × (ΔML1-ΔML2)
ΔsMR = v × (ΔMR1-ΔMR2)
ΔsCL = v × (ΔCL1-ΔCL2)
ΔsCR = v × (ΔCR1−ΔCR2)
It becomes.

From these, the average positional deviation amounts ΔsY, ΔsM, ΔsC in the main scanning direction of yellow, magenta, and cyan are
ΔsY = (ΔsYL + ΔsYR) / 2
ΔsM = (ΔsML + ΔsMR) / 2
ΔsC = (ΔsCL + ΔsCR) / 2
It becomes.

  By the above-described method, the misalignment is corrected based on the detected misalignment amounts in the main scanning direction and the sub scanning direction. The misregistration correction here is different from the bitmap image coordinate correction and the image misregistration correction described above with reference to FIGS. 3 and 4. For example, in the case of the misalignment correction for yellow, the image forming timing in the sub-scanning direction is adjusted based on the average misalignment amount ΔpY in the sub-scanning direction, and the scanning line is based on the average misalignment amount ΔsY in the main scanning direction. Adjust the export timing. Further, by finely adjusting the frequency of the image data signal 207 based on ΔsYR−ΔsYL (when the scanning width is long, the frequency is increased), and by changing the length of the scanning line, the magnification in the main scanning direction is changed. Correct. If there is an error in the magnification in the main scanning direction, the writing position is calculated not only by ΔsY but also by taking into account the amount of change in the image frequency that has changed with the magnification correction in the main scanning direction. The same correction is performed for magenta and cyan. The mechanism of various misregistration corrections (also referred to as color misregistration correction) based on the detection results of these misregistration detection patterns is a well-known matter and will not be described in detail here.

[Image correction method switching flowchart]
Next, an image correction method for the misregistration detection pattern in the image correction unit 315 will be described. When the misregistration detection pattern is formed, image data that is different from the normal image formation is corrected. FIG. 6 is a flowchart showing the switching determination of the image correction method, and shows processing by the image correction unit 315. Here, the switching determination is performed for all image formations. Hereinafter, the switching determination will be described with reference to FIG.

  First, before image formation, it is determined whether the image to be printed is a misregistration detection pattern (S1). If the image to be printed is not a misregistration detection pattern, an image correction method at the time of normal image formation is used (S2). If the image to be printed is a misregistration detection pattern, an image correction method for forming the misregistration detection pattern is used (S3). In this way, the image correction method is switched.

[Image correction method when misalignment is detected]
Next, a description will be given of portions where the image correction method at the time of forming the misregistration detection pattern is different from that at the time of normal image formation. The following two are given as different parts.

  The first is that image position correction of less than one pixel in the sub-scanning direction is not performed when a detection pattern is detected.

  In an image correction method at the time of normal image formation, image smoothing (smoothing) is performed using small pixels of less than one dot at the time of gradation value conversion. For this reason, small pixels (halftone dots) unstable in the electrophotographic method are formed near the edge of the line, and the edge portion of the line may not have the intended density. Therefore, when forming a detection pattern, image correction of the detection pattern is performed without applying smoothing, so that a stable pattern can be formed and edges can be detected with high accuracy.

  The second is to perform processing so as not to generate a step in the sub-scanning direction.

  Here, when image correction is simply performed without smoothing, one or more steps of 1 dot occur on the horizontal line as shown in FIG. If there is a step near the scanning line of the image detection sensor, the image detection sensor 31 may read this step, and the positional deviation may not be detected accurately.

  Therefore, in the present embodiment, as shown in FIG. 8, a step suppression region 51 is provided in the vicinity of the scanning line of the image detection sensor 31, and measures are taken so that no line step is generated in the step suppression region 51. In an ideal situation where no positional deviation has occurred, the vicinity of the scanning line of the image detection sensor 31 coincides with the central portion of the detection pattern in the main scanning direction. Hereinafter, an image correction method when forming a detection pattern in consideration of these two points will be described with reference to FIG.

[Detection pattern to be formed]
FIG. 9A shows an image of a scanning line having an upward slope. Specifically, for example, the detection pattern of tK1L1 shown in FIG. 8 can be used. In addition, here, as in FIG. 4A, an inclination of 1 dot occurs per 4 dots in the main scanning direction.

  FIG. 9B is an example of a horizontal straight line bitmap image before correction, and represents a 2-dot wide line.

  FIG. 9C shows a corrected bitmap image obtained by correcting the bitmap image shown in FIG. 9B in order to cancel the positional deviation caused by the inclination of the scanning line shown in FIG. In order to realize the corrected image of FIG. 9C, the image data of the pixels before and after in the sub-scanning direction are adjusted. The adjustment of the image data here refers to the coordinate correction of the bitmap image and the adjustment of the image gradation value described above.

  FIG. 9D is a table showing the relationship between the positional deviation correction amount Δy and the gradation value conversion parameter. In the table of FIG. 9D, pixels marked in the step suppression row represent pixels in the step suppression region 51. As shown in FIG. 9D, a plurality of predetermined pixel positions in the main scanning direction of the detection pattern correspond to the step suppression region 51. In addition, k represents an integer part of the misregistration correction amount Δy (truncated after the decimal point), and k ′ represents a correction amount in the sub-scanning direction in units of one pixel. The pixels are offset in the sub-scanning direction in units of pixels according to the correction amount k ′. Unlike image correction at the time of normal image formation, the value of k is not directly used as a correction amount so that the value of the correction amount does not change within the step suppression region. That is, in the step suppression region, the image data coordinate correction in the sub-scanning direction is not performed at a predetermined pixel position (first predetermined position) in the main scanning direction, and other than the predetermined pixel position (outside of the step suppression region). In the second predetermined position), coordinate correction is performed.

  Outside the step suppression region, the value of k and the value of k ′ are the same. The reason for allowing the change of the value of k ′ outside the step suppression region is that the values of k ′ are all the same. In other words, if the coordinate correction is not executed for all of the detection patterns, it overlaps with the adjacent pattern. Because there is. This will be described in detail with reference to FIGS.

  FIG. 9E is a bitmap image obtained by adjusting the positions of the front and rear pixels in the sub-scanning direction according to the image correction parameter of FIG. A hatched portion in FIG. 9E represents the step suppression region 51. Since the image correction method is different from that at the time of normal image formation, even if the inclination amount is the same, the correction result is different from that in FIG.

  FIG. 9F shows how the pattern is formed when no correction is performed on the image data that is the basis of the pattern for detecting the amount of misalignment. In this case, when the degree of image inclination and image curvature is large, the patterns overlap each other. When the deviation in the main scanning direction is large, the overlapped portion is detected by the image detection sensors 31L and 31R, and as a result, erroneous misalignment correction may be performed.

  On the other hand, FIG. 9G shows that the image data corresponding to the step suppression region 51 is not corrected with respect to the image data that is the basis of the pattern for detecting the displacement amount, and the step suppression region 51 is not corrected. A case where image data corresponding to an outer pattern is corrected is shown. As described above, as shown in FIG. 9G, it is possible to effectively prevent the positional deviation amount detection patterns of different colors from overlapping.

  That is, a pattern outside the step suppression region 51 can be detected when a large positional deviation has occurred. In such a case, if a pattern for detecting the positional deviation amount is formed as shown in FIG. 9G, the image detection sensor 31 may detect the step described with reference to FIG. However, as compared with the case where the pattern is formed as shown in FIG. 9 (f), it is possible to avoid detecting a portion where at least the positional deviation amount detection patterns of different colors overlap, which is better than the case of FIG. 9 (f). A detection result can be obtained.

  Based on the image data of the misregistration detection pattern subjected to the image correction shown in FIG. 9, a pattern is formed in the shape of the intermediate transfer belt 27 by the electrophotographic configuration described in FIG.

[Difference in misalignment detection accuracy]
Next, an image correction method at the time of normal image formation and an image correction method at the time of formation of a displacement detection pattern are compared, and a difference in displacement detection accuracy when a detection pattern is formed by each correction method will be described. .

  FIG. 10 shows the state of a line when image correction is performed on a horizontal line and printing, and a detection signal when the line is detected by the image detection sensor 31. 10A performs image correction during normal image formation, and FIG. 10B performs image correction during formation of a misregistration detection pattern. Also, as shown in FIG. 10, the timing at which the signal crosses a predetermined threshold value is the detection timing of the line edge, and the detection timing of the line is the average value of the detection timings at both ends of the line (detection timing of the center position). .

  In the case of FIG. 10A, since smoothing is performed on the edge portion of the line, an unstable small pixel smaller than one pixel is used. Therefore, the edge portion becomes unstable, and the edge is detected with a deviation from an ideal signal. In FIG. 10B, since smoothing is not performed, the edge portion is more stable than that in FIG. 10A, and the error during edge detection is small. Therefore, the line detection accuracy is improved in the case of FIG. 10B compared to the case of FIG.

  As described above, when detecting patterns are formed, smoothing is not applied, and control is performed so that the amount of correction does not change within the step suppression area. It becomes. In addition, it is possible to prevent a situation in which misregistration detection patterns between different colors are overlapped and a misregistration detection is performed completely, and more stable misregistration detection can be performed.

[Modification 1]
In the above description, only the step suppression region 51 is described as being not subjected to image correction in the sub-scanning direction, but is not limited thereto. For example, when the degree of positional deviation due to mechanical factors is not so severe that the detection patterns overlap each other, image correction in the sub-scanning direction may not be performed for all the positional deviation detection patterns.

[Modification 2]
Further, in the description related to FIGS. 8 and 9 above, the image correction unit 315 has been described to generate image data of a positional deviation detection pattern. However, it is not limited to this. For example, the image itself that has been subjected to the image processing described with reference to FIG. 9 is stored in advance in a non-illustrated non-volatile memory, and when the misregistration detection pattern is formed, the image data is read from the non-volatile memory. A pattern for detecting reading and misalignment may be formed. In this case, for example, at the factory, the inclination or curvature as described in FIG. 3 is measured with a measuring instrument, and the image data of the inverse characteristics of the measured inclination or curvature (the corrected image data described in FIG. 9). ) May be stored in a non-volatile memory provided in the video controller 302 (not shown). The processing after reading out the image data of the misregistration detection pattern subjected to image correction in advance from the nonvolatile memory and using it to form the misregistration detection pattern is the same as in the above embodiment. Detailed explanation in is omitted.

[Modification 3]
In the above description, as an example of performing image correction in the sub-scanning direction at each position in the main scanning direction on the image data and performing image position correction in the sub-scanning direction, the image data is related to the sub-scanning direction of less than one pixel. A case has been described in which image correction and image correction in the sub-scanning direction in pixel units are performed. However, for example, even in an image forming apparatus that executes only image correction in the sub-scanning direction in units of pixels, a certain effect can be obtained in terms of improving detection accuracy of a misregistration detection pattern.

[Modification 4]
Further, in the above description, the video controller 302 (image correction unit 315) performs image correction of the displacement detection pattern (coordinate correction in units of one pixel in the sub-scanning direction and adjustment of the density of each pixel in the sub-scanning direction. In the above description, the image position correction in the sub-scanning direction of less than one pixel is performed. However, the image correction is not limited to the video controller 302. For example, part or all of the image correction processing may be performed by the engine 303. Further, when image correction processing is executed on either the video controller 302 or the engine 303 side, the CPU 313 or CPU 314 may perform the processing, or part or all of the processing may be performed by an ASIC (integrated circuit). Needless to say, you can make it happen.

301 Host computer 302 Video controller 303 Engine 304 Color conversion processing unit 305 Gamma correction unit 306 Laser drive unit 307 Image data signal 308 Vertical synchronization signal 309 Horizontal synchronization signal 310 Command signal 311 Status signal 312 to 314 CPU
315 Image correction unit

Claims (4)

Image position correction means for performing image position correction in the sub-scanning direction by performing image correction in the sub-scanning direction at each position in the main scanning direction on the image data in order to correct image tilt or image curvature;
Image forming means for forming an image on a transfer material based on the image data subjected to the image correction;
Control means for forming a misregistration detection pattern in the image forming means;
A displacement detection means for detecting the displacement detection pattern, and
The control means includes a correction means for correcting a displacement based on a detection result by the detection means,
The image forming apparatus, wherein the misregistration detection pattern is an image that has not been subjected to the image correction at a predetermined pixel position corresponding to at least a central portion in the main scanning direction. The predetermined pixel position in the main scanning direction includes a plurality of predetermined pixel positions,
The misregistration detection pattern is not subjected to the image correction at the first predetermined position corresponding to the central portion in the main scanning direction, and at the second predetermined position corresponding to the outside of the central portion. The image forming apparatus according to claim 1, wherein the image data is an image subjected to image correction.   The image forming apparatus according to claim 1, wherein the image correction is coordinate correction in units of one pixel in the sub-scanning direction at a predetermined pixel position in the main scanning direction in the image data. The image position correcting means further adjusts the pixel density in the sub-scanning direction at each pixel position in the main scanning direction to correct image tilt or image curvature, and corrects the image position in the sub-scanning direction by less than one pixel. ,
Further, the image position correction means does not perform image position correction in the sub-scanning direction of less than one pixel when forming the misregistration detection pattern, and in the sub-scanning direction of less than one pixel during normal image formation. The image forming apparatus according to claim 1, wherein the image position correction is performed.

Images