JPH11212328A - Image forming device and color smear correction method therefor and color smear detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a detection error due to a reference-image color, in the detection of a reference image (pattern) in each color. SOLUTION: By means of a system control part 21, one color whose detection sensitivity characteristic with respect to a detection sensor 19 is satisfactory is set for each of image output parts one of which is composed of a developing unit 52 and a laser writing part 54 and the other of which is composed of a developing unit 53 and a laser writing part 55. In the case color smear is actually detected, the patterns formed on an intermediate transfer belt 60 by the two image output parts (52 and 54, and 53 and 55) according to the colors set by the system control part 21 are detected by the detection sensor 19 and, using correction data computed by a color smear arithmetic-part 20 based on the result of the detection, the system control part 21 corrects the timing of image writing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for multiplexing a plurality of color images as one color image by using a plurality of image output units, and more particularly, to an image forming apparatus between a plurality of image output units. The present invention relates to a technique for correcting a color shift (a color registration shift) due to a position shift.

[0002]

2. Description of the Related Art Conventionally, as an image forming apparatus for multiplexing a plurality of color images into a single color image using a plurality of image output units, for example, a color superposition method using two photosensitive drums (hereinafter, referred to as a color superimposing method). (2 tandem type) color copying machines are known. FIG. 19 is a schematic configuration diagram of a color copying machine employing a two-tandem system. In FIG. 19, the developing units 5 are individually provided for the two photosensitive drums 50 and 51, respectively.
2, 53 are provided. Each developing unit 52, 53
Perform development color switching by rotating each by 180 °. One development unit 52 has development colors of yellow (Y) and magenta (M), and the other development unit 53 has
It has black (K) and cyan (C) development colors.

[0003] Above the two photosensitive drums 50 and 51, laser writing units (ROS) 54 and 55 are provided, respectively.
Are arranged. Each laser writing unit 54, 55
Is for writing an electrostatic latent image on the photosensitive drums 50 and 51 in accordance with write control signals from image write control units (ROS-I / F) 56 and 57, respectively. An image signal read by an image reading unit (IIT) 58 is converted into an image processing unit (IPS) 59.
The data subjected to the image processing is adopted.

On the other hand, below the two photosensitive drums 50 and 51, an intermediate transfer belt 60 is stretched so as to be in contact with the surfaces of the respective photosensitive drums 50 and 51. The intermediate transfer belt 60 is stretched in a loop by a driving roll 61 and two driven rolls 62 and 63, and rotates in one direction according to the driving of the driving roll 61. Further, in addition to the developing units 52 and 53 described above, a charger, a transfer unit, a cleaner, a static eliminator, and the like (not shown) are individually provided around each of the photosensitive drums 50 and 51.

A transfer roll 64 is located near one of the two driven rolls 62 and 63.
Are arranged. The transfer roll 64 is supported so as to be able to move toward and away from the driven roll 63. On the other hand, the paper to be transferred is accommodated in a paper feed tray 65, and the paper P fed from the paper feed tray 65 is transported by a transport roll 66 to a driven roll 63 and a transfer roll 6.
4 (secondary transfer position).

In the color copying machine having the above configuration, the image is read by the image reading unit 58 and
9 is given to image writing control units 56 and 57, respectively, and a writing control signal based on the image data is sent to each of the image writing units 54 and 55.
Is output to

Thus, in the first rotation of the belt, an electrostatic latent image corresponding to the first color, for example, yellow (Y), is written on the surface of the photosensitive drum 50 by the image writing section 54, and the second color, For example, an electrostatic latent image corresponding to cyan (C) is written on the surface of the photosensitive drum 51 by the image writing unit 54. The electrostatic latent images written on the two photoreceptor drums 50 and 51 are developed into yellow and cyan toner images by developing units 52 and 53, respectively, and are sequentially superimposed on the intermediate transfer belt 60. Transcribed.

Subsequently, in the second round of the belt, a third color, for example, an electrostatic latent image corresponding to magenta (M) is written on the surface of the photosensitive drum 50 by the image writing unit 54, and the fourth color, For example, an electrostatic latent image corresponding to black (K) is written on the surface of the photosensitive drum 51 by the image writing unit 54. The electrostatic latent images written on the two photosensitive drums 50 and 51 are developed into magenta and black toner images by developing units 52 and 53, respectively, and then sequentially superimposed on the intermediate transfer belt 60. Transcribed.

At this time, a total of four color toner images are transferred onto the intermediate transfer belt 60, thereby forming one color image in a multiplex manner. In the meantime, the transfer roll 64 is held in a state separated from the driven roll 63,
At the stage where the transfer of the toner image for the color is completed, the toner image is brought into pressure contact with the driven roll 63.

In this state, the color image formed on the intermediate transfer belt 60 is transferred between the driven roll 63 and the transfer roll 64 (roll pressing position) as the belt rotates. On the other hand, the paper P is fed out from the paper feed tray 65, and the fed-out paper P is sent out by the transport roller 66 in accordance with the transfer timing of the color image. As a result, the color image on the intermediate transfer belt 60 is collectively transferred onto the sheet P between the driven roll 63 and the transfer roll 64. Thereafter, the sheet P is sent to a fixing device 67, where the image is subjected to an image fixing process (heating and pressurizing process), and then discharged out of the apparatus.

Here, in the case of the above-described two-tandem color copying machine, errors in dimensions and mounting positions of each mechanical part (image writing unit, photosensitive drum, intermediate transfer belt, etc.) involved in image formation, The positional relationship between the toner images of each color on the intermediate transfer belt 60 is relatively shifted due to the driving accuracy and the like, and the color shift occurs due to the positional shift.

Therefore, generally, the following measures are taken to prevent the occurrence of color misregistration. That is, a latent image pattern is formed on the two photosensitive drums 50 and 51 by the respective image writing units 56 and 57 according to a predetermined pattern signal. At this time, while developing each latent image pattern into a toner pattern of each color (Y, M, C, K) in order by developing machines 52 and 53, each toner pattern is transferred to the intermediate transfer belt 60, and this is transferred. Detected by the detection sensor. Further, the shift amount between the colors is obtained based on the detection result of the detection sensor, and the color shift is corrected by controlling, for example, the writing timing of each of the image writing units 54 and 55 according to the shift amount.

[0013]

By the way, an inexpensive photosensor is adopted as a detection sensor as the above-mentioned color misregistration correction system, and the sensor output is taken out as a binary value using a threshold to detect a pattern edge. That is being done. Specifically, the image position of each color is detected according to the timing when the sensor output becomes equal to or greater than the threshold. In this case, the position of the pattern edge detected based on the sensor output is shifted with respect to the original image position (center of gravity), but if the shift amount is the same for each color, in principle, No detection error occurs.

However, since each color material of YMCK has its own light transmission characteristic and reflection characteristic, when a pattern of each color output by the color material is detected by a photo sensor, as shown in FIG. Some have good S / N and others have poor S / N as shown in FIG. For example, when a YMCK pattern is formed on the fading-type intermediate transfer belt 60 and read by a photo sensor, when the sensor outputs corresponding to the patterns of the respective colors are compared, the Y pattern has the best S / N ratio. The next best one is the C pattern. Then, when the pattern becomes M, the S / N ratio becomes poor, and in the K pattern, the S / N becomes the highest.
/ N worsens. This is an intermediate transfer belt 6 serving as a base.
This is because there is a large difference in the contrast of each color material with respect to the color of 0. For example, the sensor output of the Y pattern having the best S / N and the sensor output of the K pattern having the worst S / N are respectively different. In the case of detecting a shift, the former has a shift amount A between the original image position and the pattern edge detection position, whereas the latter has a smaller shift amount B, and the difference between these A and B is the detection error. Appears as.

Further, when the S / N ratio is not good, since the noise component has a large effect on the threshold value, the variation of B increases, and the variation also causes a fundamental deterioration in the detection accuracy. . As a countermeasure, the number of times the pattern is sampled by the photosensor is increased, and the average of the sampled data is reduced to reduce the influence of noise components. This causes another problem that the customer wait time for image quality adjustment increases.

In order to detect all the toner patterns of each color transferred onto the intermediate transfer belt 60 with high accuracy, a CCD sensor may be used as a detection sensor.
This is because, when a CCD sensor is used, the shape profile of the toner pattern of each color can be easily grasped based on the sensor output, and the data of each color component can be appropriately emphasized and extracted. This is because the image position (center of gravity) can be accurately detected.

However, the CCD sensor itself is expensive, and the circuit configuration for processing the sensor output signal becomes complicated and large-scale, so that the entire system becomes very expensive.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a main object of the present invention is to reduce a detection error due to the color of a reference image when detecting a reference image (pattern) of each color. Correcting the color misregistration caused by the misregistration between
An object of the present invention is to make it possible to accurately detect color misregistration between image output units with a simple configuration without employing a CD sensor or a complicated and large-scale circuit configuration.

[0019]

According to the first aspect of the present invention,
Image formation in which N image output units each having a plurality of colors assigned to at least one are provided, and the N image output units are configured to form an image of N + 1 or more colors on the image carrier as one color image. In the apparatus, setting means for setting one color for each of the N image output units, and sequentially forming the color set by the setting means on the image carrier by the N image output units. Detecting means for detecting the reference image thus obtained, and control means for controlling writing of the N image output units to the image carrier based on the detection result of the detecting means.

In this image forming apparatus, for each of the N image output units, one color, preferably one color having a good detection output condition by the detection means, is set by the setting means. A reference image is formed on an image carrier such as a transfer belt or a photoconductor by N image output units according to the color. The formed reference image is detected by the detection unit, and the writing of the image on the image carrier is controlled by the control unit based on the detection result. Thereby, when the detection unit detects the reference image formed on the image carrier,
The detection error due to the color of the reference image is reduced. Therefore, it is possible to accurately correct the color shift caused by the position shift between the image output units.

According to a fourth aspect of the present invention, there are provided N image output units each having a plurality of colors assigned to at least one of them, and using the N image output units, N + 1 or more colors are formed on an image carrier. In the color misregistration correction method of the image forming apparatus for multiplex forming the image as one color image, of the colors allocated to the N image output units, the image output units are used to form the image on the image carrier. The color misregistration caused by the misregistration between the N image output units is corrected by using one color each having a good detection output condition of the reference image.

In the color misregistration detecting method of the image forming apparatus, the color of the reference image formed on the image carrier by using the plurality of image output units is used to apply the color to the image carrier by using those image output units. Since the detection output condition of the reference image at the time of formation, for example, a single color or one color each having good detection sensitivity is used in combination, the detection error due to the color of the reference image is reduced, and when the reference image is detected. It is less susceptible to noise components. Therefore, it is possible to accurately correct the color shift caused by the position shift between the image output units.

According to a fifth aspect of the present invention, there is provided detecting means for photoelectrically converting the reference image of each color output by the plurality of image output units, and detecting the photoelectrically converted waveform by comparing it with a threshold value. A color shift detection device for outputting color shift detection data corresponding to the detected value, wherein the color shift detection is performed in accordance with a difference between a distance between an edge of a photoelectrically converted waveform of each color and a center of gravity based on a detection result of the detection means. Correction means for correcting the application data.

In this color misregistration detection device, the color misregistration detection data is corrected according to the difference between the edge of the photoelectrically converted waveform of each color and the distance between the center of gravity based on the detection result of the detection means. It is possible to eliminate a detection error with respect to the reference image. Therefore, the reference image can be accurately detected with a simple configuration using a photosensor or the like without using a CCD or the like.

According to a sixth aspect of the present invention, there is provided detecting means for photoelectrically converting the reference image of each color output from the plurality of image output units, and detecting the photoelectrically converted waveform by comparing it with a threshold. A color misregistration detection device that outputs color misregistration detection data corresponding to a detection value, comprising: means for switching a threshold value such that a distance between an edge and a center of gravity with respect to the threshold value of the photoelectrically converted waveform is equal between colors. is there.

In this color misregistration detecting device, the threshold value is switched so that the distance between the edge and the center of gravity of the photoelectrically converted waveform becomes equal between the colors, so that the detection error of each color with respect to the reference image can be eliminated. It becomes possible. Therefore, the reference image can be accurately detected with a simple configuration using a photosensor or the like without using a CCD or the like.

According to a seventh aspect of the present invention, there is provided an amplifying means for photoelectrically converting a reference image of each color output from a plurality of image output units and amplifying a result of the photoelectric conversion, and comparing an output of the amplifying means with a threshold value. A color misregistration detecting device that outputs data for color misregistration detection corresponding to the detected value, so that the distance between the edge and the center of gravity with respect to the threshold value of the output waveform of the amplifying means is equal for each color. Means for switching the gain of the amplifying means.

In this color misregistration detection device, the gain of the amplifying means is switched so that the distance between the edge and the center of gravity with respect to the threshold value of the output waveform of the amplifying means becomes equal between the colors, so that the detection error of each color with respect to the reference image is reduced. It can be eliminated. Therefore, the reference image can be accurately detected with a simple configuration using a photosensor or the like without using a CCD or the like.

When the above-described color misregistration detecting apparatus is used in an image forming apparatus, the reference image can be detected with high accuracy, and the correction using the reference image can be accurately corrected. Control can also be simplified.

Further, when the color misregistration detecting device according to the fifth, sixth or seventh aspect is used as the detecting means according to the first aspect, it is possible to more accurately correct color misregistration caused by misregistration between image output units.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description of embodiments of the present invention, a technique for correcting color misregistration in a two-tandem type image forming apparatus to which the present invention is applied will be described.

FIG. 1 is a schematic configuration diagram of a two-tandem image forming apparatus to which the present invention is applied. First, as a basic mechanical configuration of a two-tandem system, reference numeral 71 denotes a first image forming unit, 72 denotes a second image forming unit, 73 denotes a first laser writing unit (ROS # 1), and 74 denotes a first laser writing unit. Photoconductor drum,
75 is a second laser writing unit (ROS # 2), 76 is a second photosensitive drum, 77 is an intermediate transfer belt, 78 is a driving roll for rotating the belt, 100a is a first developing device, 10a
0b is a second developing device.

Of these, the first and second laser writing units 7
Reference numerals 3 and 74 denote first and second photosensitive drums 74 and 74, respectively.
A laser beam is applied to the surface of the lens 75 to write an electrostatic latent image therein. Although not shown, a polygon mirror for scanning a laser beam corresponding to an image signal of each color, and the polygon mirror are rotated. In addition, a polygon motor for causing the laser beam to be scanned, and an SOS sensor for detecting the timing of starting the scanning of the laser beam by the polygon mirror are incorporated. First and second developing units 100a, 10
Numeral 0b develops the electrostatic latent images written on the first and second photosensitive drums 74 and 75 by the first and second laser writing units 73 and 74 into toner images. Each of the first and second developing units 100a and 100b has two developing colors (for example, the first developing unit 10a).
Yellow and magenta are assigned to 0a, and cyan and black are assigned to the second developing device 100b, and these developing colors are switched by a rotation operation (rotary method). Further, a home sensor 79 is provided in the middle of the rotation path of the intermediate transfer belt 77 so as to be opposed to the home sensor 79. The home sensor 79 is connected to the intermediate transfer belt 77
When the belt rotates, it detects a reference mark formed on the belt and outputs a belt reference signal Belt Home. The belt reference signal Belt Home is output every one rotation of the belt. .

On the other hand, as a configuration of the control system, the main controller 80 outputs various control signals for controlling each section of the two-tandem type image forming apparatus. The first and second image writing control units 81 and 82 perform the processing according to the Y, M, C and K signals generated by the image processing unit 84 based on the R, G and B signals input by the image input unit 83. The first and second laser writing units (ROS # 1 and ROS # 2) 73 and 75 control image writing timing.

The control unit 85 controls the image writing operation of the first and second laser writing units 73 and 74 via the first and second image writing control units 81 and 82.
It has a plurality of counters A to D inside. The control unit 85 includes a belt reference signal Belt Home output from the belt home sensor 79, and a scanning start (SOS) signal output from each of the SOS sensors incorporated in the first and second laser writing units 73 and 75. Scanning signal LS # synchronized with
1, LS # 2 are input. On the other hand, the control unit 85 detects the phase difference between the scanning cycle of each polygon mirror and the rotation cycle of the intermediate transfer belt 77 from these input signals, and increases or decreases the rotation speed of the polygon mirror based on the detection result. Correction signal is output to the first and second polygon motor control units 86 and 87.

Further, a detection sensor 88 is provided downstream of the second image forming section 72 in the belt rotation direction. The detection sensor 88 includes first and second image forming units 71 and 72.
This reads a pattern for detecting color misregistration formed on the intermediate transfer belt 77, and the read data is input to the control unit 85. In response to this, the control unit 85 obtains the amount of color misregistration of the first and second image forming units 71 and 72 based on the read data from the detection sensor 88, and determines the second amount of color misregistration in accordance with the obtained amount of color misregistration. The image writing timing of the image forming unit 72 is controlled.

Next, the color misregistration correction processing executed by the control unit 85 will be described. The color misregistration correction processing by the control unit 85 is executed by a combination of a first processing step based on the flowchart of FIG. 2 and a second processing step based on the flowchart of FIG. The processing procedure will be described with reference to the timing chart of FIG.

First, when the belt reference sensor Belt Home is output from the belt home sensor 79 in a state where the intermediate transfer belt 77 is rotated according to the rotation of the drive roll 78, the first reference is output in synchronization with the output timing. A reference signal TR0 # 1, which is a trigger for image writing in the laser writing unit (ROS # 1) 73, rises. At this time, in the first image writing control section 81, the scanning signal LS # 1 synchronized with the scanning start (SOS) signal of the first laser writing section 73 is changed from the rising timing of the reference signal TR0 # 1 to an arbitrary timing. It rises after time Ta. This time Ta corresponds to the phase difference between the scanning cycle of the polygon mirror in the first laser writing unit 73 and the rotation cycle of the intermediate transfer belt 77, and changes every time the intermediate transfer belt 77 makes one rotation.

In the "first processing step", first, the time Ta (TR0 # 1-LS # 1) is counted by a counter reference clock (Counter REF Clock) (step S11). At this time, simultaneously with the rising timing of the reference signal TR0 # 1, the virtual scanning signal Virtual LS #
Start 1 This virtual scanning signal Virtual LS # 1
Is active in the same cycle as the scanning cycle used as a reference in the first laser writing unit 73, and the counter A starts counting the number of active times.

Next, the rotation speed of the polygon mirror in the first laser writing unit 73 is increased or decreased via the first polygon motor control unit 86, thereby correcting the surface phase of the polygon mirror (step S12). In this surface phase correction, the counter reference clock (Counter REF Clo
ck), based on the time (phase difference) Ta counted by the virtual scanning signal Virtual LS # 1 from the rising timing of the virtual scanning signal Virtual LS # 1.
For example, true at the timing of a half cycle of the scanning signal LS # 1.
The surface phase is corrected by increasing or decreasing the rotation speed of the polygon mirror so that the (True) scanning signal LS # 1 rises.

At this time, in order to increase or decrease the rotation speed of the polygon mirror, the driving conditions of the polygon motor must be temporarily changed via the first polygon motor control unit 86. When undershoot or the like occurs, the number of times of activation of the scanning signal LS # 1 changes randomly, and it may not be possible to grasp the increase or decrease in the number of scanning signals LS # 1 due to the surface phase correction of the polygon mirror. Therefore, considering the time required until the rotation speed of the polygon motor is stabilized after correcting the surface phase of the polygon mirror, the correction period is defined as the time until the count value of the counter A reaches m.

Thus, the count value of the counter A becomes m
At this time, the polygon mirror is adjusted so that the phase relationship between the virtual scanning signal Virtual LS # 1 and the true scanning signal LS # 1 becomes constant (a shift of 1/2 cycle) by the above-described surface phase correction of the polygon mirror. Since the surface phase is corrected, a new reference signal TR0 # 1 'rises at this time (step S13). Thereafter, the counter B and the counter C start counting the number of times of activation of the scanning signal LS # 1 (step S14). When the count value of one of the counters B reaches a predetermined value n, the write start signal PS # 1 rises. (Step S15). Accordingly, the first image writing control unit 11 writes the electrostatic latent image from the first laser writing unit 73 to the second photosensitive drum 74 simultaneously with the rising timing of the writing start signal PS # 1. Will start. As described above, even when the increase or decrease in the number of the scanning signals LS # 1 due to the surface phase correction of the polygon mirror cannot be grasped, the correction period is managed by the count value of the counter A so that the first reference signal TR0 # 1 can be obtained.
After a predetermined time from the rising timing of the write start signal PS # 1, the write operation of the image can be started by raising the write start signal PS # 1.

Thereafter, when the count value of the counter C reaches a predetermined value x, the second laser writing unit (ROS #
2) Reference signal TR0 that triggers image writing at 75
# 2 stands up. At this time, in the second image writing control unit 82, the scanning signal LS # 2 synchronized with the scanning start (SOS) signal of the second laser writing unit 75 generates an arbitrary time from the rising timing of the reference signal TR0 # 2. T
b, that is, after the time Tb corresponding to the phase difference between the scanning cycle of the polygon mirror in the second laser writing unit 75 and the rotation cycle of the intermediate transfer belt 77, rises. Therefore, the time Tb is counted by the counter reference clock (Counter REF Clock) as in step S11, and the virtual scanning signal Virtual LS # 2 rises simultaneously with the output timing of the reference signal TR0 # 2. Is started by the counter A.

Thereafter, as in steps S12 and S13, the second polygon motor controller 87 is controlled based on the time Tb until the count value of the counter A reaches m.
A predetermined motor control signal to correct the surface phase of the polygon mirror in the second laser writing unit 75, thereby obtaining a phase relationship between the virtual scanning signal Virtual LS # 2 and the true scanning signal LS # 2. Is controlled to be constant (a shift of 1/2 cycle). When the count value of the counter A becomes m, a new reference signal TR0 # 2 '
Start up. Next, as in steps S14 and S15, the number of active scan signals LS # 2 is counted by the counter B.
, The writing start signal PS # 1 rises when the count value of the counter B reaches n. Accordingly, the second image writing control unit 82 writes the electrostatic latent image from the second laser writing unit 75 to the second photosensitive drum 76 simultaneously with the rising timing of the writing start signal PS # 1. Will start.

As described above, the first and second laser writing units 7
By correcting the surface phase of the polygon mirror based on the phase difference between the rotation cycle of each polygon mirror in 3 and 75 and the rotation cycle of the intermediate transfer belt 77, the first and second image forming portions 71 and 72 The image writing position for each rotation of the belt coincides. As a result, the first image forming unit 71
Position difference between the first color and the third color toner images transferred to the intermediate transfer belt 77, and the second image forming unit 7
2 can reduce the displacement between the second color and the fourth color toner images transferred to the intermediate transfer belt 77.

Based on this, in the "second processing step", first, when the belt reference signal Belt Home from the belt home sensor 79 is detected (step S21), the polygon is processed in accordance with the "first processing step" described above. While correcting the surface phase of the mirror, the first image forming unit 71 forms a pattern for detecting a color shift on the intermediate transfer belt 77 (steps S22 and S23). Next, the counter C / D
Is set to the initial value (default), the second image forming unit 72 corrects the surface phase of the polygon mirror according to the “first processing step” in the same manner as described above.
Thus, a color misregistration detection pattern is formed on the intermediate transfer belt 77 (steps S24 to S26).

Subsequently, the pattern formed on the intermediate transfer belt 77 is read by the detection sensor 88 (step S2).
7). As a result, the pattern reading data is fetched from the detection sensor 88, and the color shift amounts of the first and second image forming units 71 and 72 are obtained based on the read data. Is calculated (step S28). Specifically, the count value of the counter C / D initialized earlier is set so that the image writing position of the second image forming unit 72 matches the image writing position of the first image forming unit 71. The setting is changed based on the correction data (step S29).

Here, the color shift amounts of the first and second image forming units 71 and 72 determined as described above are the same as the scanning signals LS # 1 and L
If it exceeds one cycle of S # 2 (one image line), the correction is made by increasing or decreasing the count value x of the counter C that defines the rising timing of the reference signal TR0 # 2. For the color shift amount of one cycle (one line of image) or less of the scanning signals LS # 1 and LS # 2, for example, the scanning signal LS # 1
16 values (y = 0)
15) Preparing and correcting by appropriately selecting the count value of the counter D according to the color shift of one line or less.

Thus, each reference signal TR0 #
1, the time from the rising timing of TR0 # 2 to the rising timing of the writing start signals PS # 1 and PS # 2 is always fixed, and the mechanical accuracy of the first and second laser writing units 73 and 74 is improved. The resulting color shift of the first and second image forming units 71 and 72 can be corrected.
As a result, the images of the first to fourth colors can be transferred onto the intermediate transfer belt 77 while being superimposed on each other with high accuracy, so that the color shift in the two-tandem system can be reduced.

In such a color misregistration correction technique,
When the pattern for color shift detection output using the second laser writing units 73 and 75 is detected by the detection sensor 88 composed of, for example, a photo sensor, the S / N is reduced as described in the prior art. Since a detection error occurs between a good product and a bad product, an accurate color shift amount cannot be obtained.

FIG. 5 is a schematic structural view of a two-tandem type color copying machine employed in the first embodiment of the image forming apparatus according to the present invention. In FIG. 5, reference numerals 50 and 51 indicate photosensitive drums, 52 and 53 indicate developing units, 54 and 55 indicate laser writing units (ROS), and 56 and 57 indicate image writing control units (RO).
S-I / F), 59 is an image processing unit (IPS), 58 is an image reading unit (IIT), 60 is an intermediate transfer belt, 61 is a drive roll, 62 and 63 are driven rolls, 64 is a transfer roll, and 65 is a transfer roll. A paper feed tray, 66 is a transport roll, 67 is a fixing device, and the basic configuration thereof is as described in the related art. Further, the photosensitive drums 50 and 51 correspond to the first and second photosensitive drums 74 and 76 of FIG.
Reference numerals 2 and 35 denote first and second developing units 100a and 100 in FIG.
b, the laser writing sections 54 and 55 are the first and second laser writing sections 73 and 75 of FIG. 1, the intermediate transfer belt 60 is the intermediate transfer belt 77 of FIG. 1, the drive roll 61 is the drive roll 78 of FIG. Each corresponds.

In addition to the above configuration, the intermediate transfer belt 1
A detection sensor (detection means) 19 composed of a photo sensor is disposed on one travel route. This detection sensor optically reads a pattern (reference image) for detecting a color shift transferred onto the intermediate transfer belt 11, and the output of the sensor is input to the color shift calculating unit 20.

The color misregistration calculating section 20 calculates a color misregistration amount based on the sensor output input from the detection sensor 19 and calculates correction data corresponding to the color misregistration amount. The correction data calculated by the color misregistration calculation unit 20 is provided to the system control unit 21.

When detecting a color shift, the system control unit 21 gives a pattern output command to the laser writing units 5 and 6 and the image writing control units 7 and 8 and also from the color shift calculating unit 20 by the color shift detection processing. The image writing is controlled based on the corrected data. Further, the system control unit 21 provides a developing color switching signal to each of the developing units 3 and 4 in a predetermined printing order during a normal copying operation. Give a signal.

By the way, in this embodiment, the laser writing section 5
6, the writing timing of the main scanning and the sub-scanning is controlled, so that a correction command for the timing correction is given from the system control unit 21 to the laser writing units 5 and 6. ing. In addition, the scanning cycle of the polygon mirror is also controlled. In addition to such direct writing control,
The indirect image writing control targets include skew correction by adjusting the angle of the scanning mirror in the laser writing units 5 and 6, laser driving frequency (main scanning period) of the laser writing units 5 and 6 using a VCO (Voltage Controlled Oscillator). : Magnification), position control in the sub-scanning direction by adjusting the rotation speed of the photosensitive drums 50 and 51, and the like.

Reference numeral 21 in the figure denotes a controller / printer interface serving as an interface for printing and outputting texts, images, and the like created by a terminal device such as a personal computer. Image signals are handled in the same way as when copying.

Here, in the first embodiment, of the two photosensitive drums 50, 51 which are the basis of the two-tandem system, a developing unit 52 and a laser writing unit provided corresponding to one of the photosensitive drums 50 are provided. One image output unit is constituted by the unit 54, and a developing unit 53 and a laser writing unit 55 similarly provided for the other photosensitive drum 51.
Constitutes another image output unit. Then, two colors of Y (yellow) and M (magenta) are assigned to an image output unit including the developing unit 52 and the laser writing unit 54, and the image output unit including the developing unit 53 and the laser writing unit 55 is assigned to the image output unit. For C (cyan) and K
(Black).

On the other hand, the system control unit 21
In outputting the pattern for detecting color misregistration, the developing device 52 and the laser writing unit 54
The color of the pattern to be output on the upper side and the color of the pattern to be output on the photosensitive drum 51 by the developing unit 53 and the laser writing unit 55 are set one by one.

More specifically, if the intermediate transfer belt 60 is of a red type (including a brown type), for example,
One color “Y” is set as the color of the pattern to be output on 0, and one color “C” is set as the color of the pattern to be output on the other photosensitive drum 51. The reason for selecting the pattern output color here is based on the following contents.

That is, when a pattern output in each color of YMCK is detected by the detection sensor (photo sensor) 19,
When the intermediate transfer belt 60 is red, the contrast is high in the Y and C patterns and low in the M and K patterns. From this, the detection sensitivity of the photo sensor is not the same for the Y and C patterns as shown in FIG.
As shown in FIGS. 6A and 6B, the S / N ratio is improved, whereas the S / N ratio is not improved in the M and K patterns as illustrated in FIGS. 6C and 6D.

From this point of view, the image output unit (5
2, 54), one color “Y” having the highest sensor detection sensitivity (contrast) is set as the output color of the pattern, and for the image output units (53, 55) at the subsequent stage,
One color “C” having the next highest sensor detection sensitivity (contrast) after “Y” is set as the output color of the pattern.

Next, the color misregistration correction processing executed by the system control unit 21 will be described with reference to the flowchart of FIG. First, in step S31, it is repeatedly determined whether or not a color shift detection cycle has been reached. In this color misregistration detection cycle, for example, it is determined whether or not environmental conditions such as temperature and humidity have changed beyond an allowable level, or a preset allowable time has elapsed, based on the time when the previous color misregistration correction process was performed. The process proceeds to step S32 when the color misregistration detection cycle is started.

In step S32, a pattern output command is given to each of the image writing control units 56 and 57. In step S33, each developing unit 52 outputs a pattern using the color material set as described above. , 53 are supplied with a switching signal of a developing color. As a result, from the respective image writing control units 56 and 57, pattern signals corresponding to the color misregistration detection patterns are supplied to the laser writing units 54 and 55, and the developing color in one developing unit 52 is set to “Y”. The developing color in the other developing unit 53 is switched to "C".

Subsequently, in step S34, the latent image pattern written on the photosensitive drum 50 by one of the laser writing units 54 is developed into a "Y" toner pattern by the developing device 52, and the other laser writing is performed. The latent image pattern written on the photosensitive drum 51 by the unit 55 is developed by the developing unit 53 into a “C” toner pattern. Further, the toner patterns on the respective photosensitive drums 50 and 51 are sequentially transferred onto the intermediate transfer belt 60, whereby a color misregistration detection pattern is output.

Next, in steps S35 and S36, the "Y" and "C" patterns transferred on the intermediate transfer belt 60 as described above are sequentially detected by the detection sensor 19, and the detection of these two-color patterns is completed. Then, the process proceeds to step S37. In step S37, the detection sensor 19
The color shift amount between the two image output units (52, 54 and 53, 55) is obtained by the color shift calculating unit 20 based on the sensor output from the CPU, and a correction value corresponding to the color shift amount is calculated.

At this time, as a sensor output given from the detection sensor 19 to the color misregistration calculating section 20, any pattern for color misregistration detection has a contrast (sensor detection sensitivity).
6A and 6B, the S / N becomes good as shown in FIGS. 6A and 6B.

Thus, when the sensor output when the "Y" pattern is detected is extracted in binary using the threshold value, the shift amount between the original image position and the pattern edge detection position is A. On the other hand, when the sensor output at the time of detecting the pattern “C” is extracted in binary using the threshold value in the same manner as described above, the shift amount between the original image position and the pattern edge detection position is A ′. At this time, since both the shift amounts A and A 'are close to each other, these shift amounts A and A'
The color misregistration amount calculated based on A 'hardly includes a detection error.

Therefore, in the final step S38, the correction data calculated by the color misregistration calculating section 20 is
By giving the image writing timing to the image writing control units 56 and 57 as a color shift correction command for correcting the image writing timing, it is possible to output a high-quality copy image without color shift in a normal copy operation. Become.

As described above, in the first embodiment, 2
Of the colors (YMCK) assigned to the two image output units (52, 54 and 53, 55), "Y" is used for one image output unit, and "C" is used for the other image output unit. More specifically, the colors with high detection sensitivity (colors with good detection output conditions) detected by the detection sensor 19 are set in the system control unit 21 as pattern output colors for color shift detection.
In the actual color misregistration correction process, a pattern for color misregistration detection is output by each image output unit according to a preset pattern output color, and the output pattern is detected based on a result detected by the detection sensor 19. Color shift calculator 2
Since the color misregistration amount is calculated with 0 and the image writing timing is controlled by the system control unit 21, the cost is high without employing expensive sensor components such as a CCD sensor or a complicated and large-scale circuit configuration. It is possible to detect a color shift between image output units with detection accuracy and correct this.

In the first embodiment, an example of application to a system configuration using the red intermediate transfer belt 60 has been described. However, when a belt other than the red type is used, the following Table 1 is used. As shown in (1), the pattern output colors for detecting color misregistration are different in correspondence with the belt colors.

[0071]

[Table 1]

That is, when a transparent belt is used as the intermediate transfer belt 60, the output color of the color misregistration detection pattern is set by a combination of "K" and "C" or "K" and "M". When a black belt is used, the output color of the color misregistration detection pattern is set by a combination of “Y” and “C” or “M” and “C”. However, since the contrast of each color pattern with respect to the color of the intermediate transfer belt (the color of the base) slightly differs depending on the light receiving sensitivity characteristics of the sensor and the color of the sensor light source, the sensor detection is performed in consideration of the light receiving sensitivity characteristics and the light source color. It is more preferable to set the pattern output color preferentially from the color with the highest sensitivity.

Further, from the viewpoint of reducing the error of color shift detection, colors having low contrast, for example, “K” and “M” in FIGS. 6C and 6D are output as a pattern output for color shift detection. It can also be realized in principle by setting the color. However, although the colors “K” and “M” have the same detection sensitivity of the detection sensor 19, they are easily affected by noise components due to poor S / N. Therefore, as adopted in the above embodiment, setting a color having a high detection sensitivity with a single color and a uniform detection sensitivity as the pattern output color can minimize the influence of noise components as much as possible.
Very suitable.

When a black belt is used for the intermediate transfer belt 60, if the color misregistration detection pattern is output as K, the K color can be detected by the photo sensor because both colors are the same. Extremely difficult, but K
And the other colors are combined to form a multiple pattern, so that a pattern with good sensor detection sensitivity can be output.

More specifically, when the intermediate transfer belt 60 is of a black color, as shown in FIG. 8A, a pattern for detecting color misregistration is output in a combination of K and Y. FIG.
(B) shows another example of the shape of the color misregistration detection pattern. In this case, as shown in the upper part of FIG. 8A, with respect to the pattern image in which the base part is Y and the pattern part is K, the solid image of Y (base pattern) in the preceding image output unit (52, 54). Is formed thereon, and a K chevron pattern (arrow-shaped pattern) is multiplex-formed thereon using the image output units (53, 55) at the subsequent stage.

On the other hand, as shown in the lower part of FIG. 8A, in the case of a pattern image in which the base portion is K and the pattern portion is Y, when the intermediate transfer belt 60 is a black type, the belt is formed of a K solid pattern. As an alternative, it can be obtained by forming a Y chevron pattern directly on the belt by the image output unit of the preceding stage (52, 54).

On the other hand, when the intermediate transfer belt 60 is of a type other than the black type, for example, a transparent type and is detected by a reflection type sensor (not transmitting), first, in the first rotation of the belt,
A solid pattern of K is formed by using the image output units (53, 55) at the subsequent stage, and Y is then formed on the solid pattern of K by using the image output units (52, 54) at the preceding stage in the second round of the belt. Is obtained by forming multiple chevron patterns.

When the belt is formed by one rotation, a solid Y pattern is formed by using the preceding image output unit, and a K-chevron-shaped hollow pattern is formed from above by using the subsequent image output unit. It is obtained by multiplex formation. However, in this case, the position of the K pattern is actually detected, although the pattern is apparently a Y pattern.

By outputting a pattern for detecting color misregistration by combining a plurality of colors in this manner, a pattern having exactly the same detection sensitivity of the sensor can be obtained. The detected error can be eliminated. Other examples (for example, Y and M) can be considered as a combination of a plurality of colors.
Since the pattern based on the combination of K and Y described above has a very high contrast, it is possible to detect color misregistration with a pattern having the best sensor detection sensitivity (S / N). Further, when the image quality of the output image is given the highest priority, for example, when the color of the intermediate transfer belt 60 or the color assigned to the image output unit is determined, the pattern output color is set under the optimum condition for performing the color misregistration detection. It is assumed that it is not possible, but in such a case, by outputting a pattern by combining a plurality of colors, it is possible to achieve both high image quality of an output image and high accuracy of color misregistration detection.

In the first embodiment, an example of application to a two-tandem type image forming apparatus (color copying machine) using the intermediate transfer belt 60 has been described. However, the present invention is not limited to this. As shown in FIG. 9, a so-called direct transfer method is adopted in which a sheet is supplied to the transfer belt 101 in the direction of the arrow in the figure, and an image is directly transferred from the two photosensitive drums 102 and 103 onto the sheet on the transfer belt 101. The same can be applied to the image forming apparatus described above.

In addition to the two-tandem system, as shown in FIG. 10, for example, as shown in FIG. 10, two image output units (105, 106 and 107, 108), a three-tandem type image forming apparatus (not shown) in which two developing colors are assigned to each of three photosensitive drums (not shown), or as shown in FIG. , Four photosensitive drums 109, 110, 11
The present invention can be similarly applied to a four-tandem-type image forming apparatus in which two developing colors are assigned to each of 1, 112.

Here, for reference, Table 2 below shows an example of the assignment of colors to the image output units and the setting of pattern output colors for detecting color misregistration in the three tandem type and single type image forming apparatuses. Of the three tandem colors assigned, gold and silver are examples of special colors. In addition, fluorescent colors, colors that cannot be reproduced with standard color materials, or low-efficiency color materials for specific applications are special colors. It may be adopted as.

[0083]

[Table 2]

Next, a description will be given of a second embodiment of the present invention in the case where the present invention is applied to the above-described two-tandem-type image forming apparatus (see FIG. 5). In the second embodiment, first, the configuration shown in FIG. 12 is employed in the color misregistration calculating section 20 in order to correct a sensor detection error due to a difference in the characteristics of each color material of YMCK when calculating a sensor output or a color misregistration amount. doing.

In FIG. 12, a CPU (Central Processing Unit) 20a performs a calculation process for detecting a color shift in accordance with a calculation program stored in a ROM 20b, and data necessary for the calculation process is stored in a RAM 20 as needed.
c. The correction data obtained by the arithmetic processing in the CPU 20a is given to the system control unit 21, and the correction data becomes a correction command and is output from the system control unit 21 to each of the image writing control units 7, 8. Has become.

Here, an offset value for each color is stored in the ROM 20b as one of the processing data for detecting a color shift. The offset value is obtained by incorporating data experimentally obtained in advance into an arithmetic program. Specifically, the sensor output when the pattern of each color is detected by the photo sensor is set to 2
Retrieve by value. At this time, the shift amount between the detected position of the pattern edge detected corresponding to each color pattern and the original image position, for example, difference data between A and B in FIGS. 20A and 20B is obtained. Set the difference data as the offset value. The offset value in this case is shown in FIG.
Assuming that a pattern color having a good S / N among the two colors corresponding to (a) and (b) is used as a reference color for color misregistration correction, it is set as an offset value of the other pattern color with respect to the reference color. You.

Further, sensor outputs (analog signal / digital signal) from two detection sensors (photosensors) 190a and 190b are given to the CPU 20a as input data for detecting a color shift. Each of the detection sensors 190a and 190b is disposed on each side of the intermediate transfer belt 60 in order to detect a skew of an image. Also, the CPU 20a includes:
A transmission / reception line (TX, RX) for communication with an external device is connected.

Here, a configuration is adopted in which the correction data calculated by the color misregistration calculation unit 20 is output to each of the image writing control units 56 and 57 via the system control unit 21. Also input / output ports to the color misregistration calculation unit 20.
(I / O) 20d, and the correction data can be output directly from the color misregistration calculating section 20 to the image writing control sections 56 and 57 via the input / output port 20d.

Next, in the second embodiment, the color misregistration correction processing executed by the system control unit 21 will be described.
This will be described with reference to the flowchart of FIG. First, in step S41, it is repeatedly determined whether or not a color misregistration detection cycle has been reached. When the color misregistration detection cycle has been reached, the process proceeds to step S42. In step S42, a pattern output command is given to each of the image writing control units 56 and 57 to output an output of a color misregistration detection pattern on the intermediate transfer belt 60. In subsequent step S43, the intermediate transfer belt 60 is output. The upper color misregistration detection pattern is detected by the detection sensors 190a and 190b.

Next, in step S44, the color misregistration calculating section 20 uses the two image output sections (52, 54 and 53, 53) based on the sensor outputs from the detection sensors 190a and 190b.
55), and calculates correction data according to the color shift amount. At this time, in the color misregistration calculation unit 20, two image output units (52, 54) obtained by sensor outputs from the detection sensors 190a, 190b or by calculation.
And 53, 55) are offset by the offset value stored in the ROM 20b as described above.

For example, when sensor outputs as shown in FIGS. 20A and 20B are obtained, the S / N ratio is good.
With respect to the sensor output of, the position shifted by A from the original image position is detected as a pattern edge as it is, and S / S
Regarding the sensor output of FIG. 20B where N is not good,
With respect to the detection position of the pattern edge shifted by B from the original image position, a position further shifted by (AB) is detected as the pattern edge.

Thus, it is possible to eliminate a sensor detection error for each color pattern, so that correction data suitable for an actual color shift can be obtained. Therefore, in the final step S45, the correction data calculated by the color misregistration calculation unit 20 is given to each of the image writing control units 56 and 57 as a color misregistration correction command.
In a normal copy operation, a high-quality copy image without color shift can be output.

FIG. 14 is a configuration diagram of a color misregistration detecting device provided with a threshold value adjusting function for detecting a pattern edge as an application example of the second embodiment of the present invention. First, the detection sensor 1
Reference numeral 9 denotes a photo sensor unit 19a that actually detects a pattern for color shift detection by photoelectric conversion, a comparator 19b that receives an analog signal from the photo sensor unit 19a as one input, and a color shift calculation unit 20. And a D / A converter 19c for converting the digital signal into an analog signal.

On the other hand, an A / D converter 201 for converting an analog signal from the photosensor unit 19a into a digital signal and a digital signal generated from the A / D converter 201 are input to the color misregistration calculating unit 20. CPU 202 is provided. The CPU 202 includes an A / D converter 2
A digital signal for setting a threshold value is sent to the detection sensor 19 side in accordance with the digital signal from the digital signal 01, and the sent digital signal is converted into an analog signal by the D / A converter 19c, 19b is the other input. The output of the comparator 19b is taken into the CPU 202 as a sensor output for detecting a pattern edge.

FIG. 15 shows a sequence of color misregistration correction processing.
6 is a flowchart illustrating a processing procedure mainly focusing on threshold adjustment. First, in step S51, it is repeatedly determined whether or not a color misregistration detection cycle has been started. At the time of the detection cycle, the detection sensor 19 detects the surface of the intermediate transfer belt 60 which is a background portion of pattern detection (step S51). S52). Next, after substituting 1 for N in step S53, in step S54, a solid patch (a rectangular pattern filled with one color material) of the Nth color (the first color at this time) is output onto the intermediate transfer belt 60. Then, the solid patch is detected by the detection sensor 19 (Step S
55). At this time, an analog signal corresponding to the solid patch output as described above is output from the detection sensor 19 from the photo sensor unit 19a. Since the analog signal in this case shows a rectangular wave profile corresponding to the shape of the solid patch, the maximum level thereof can be easily detected. At this time, the effect of the density difference can be avoided by making the density of the solid patch the same as that of the color misregistration detection pattern.

Subsequently, in step S56, the color shift calculating section 20 determines a threshold value corresponding to the analog signal. Specifically, the maximum level of the analog signal corresponding to the solid patch is set as a patch detection value, and using this patch detection value and the previously obtained sensor detection value of the background portion, for example, according to the following equation (1): Determine the threshold. (Background detection unit) + {(patch detection value) − (background detection value)} × coefficient (1)

In the above equation (1), the “coefficient” is selected in accordance with the patch detection value obtained from the detection result of the previous step S55, and the table data indicating the corresponding relationship is, for example, color data. ROM of the shift calculator 20
Etc. are stored. Therefore, the CP of the color misregistration calculation unit 20
In U202, for example, when the patch detection value is high as shown in FIG. 16A, “0.5” is selected as a coefficient corresponding to the patch detection value, and the patch detection value is selected as shown in FIG. If the value is low, “0.3” is selected as a coefficient corresponding to the patch detection value.

As a result, in FIG. 16A, a half (middle) between the patch detection value and the background detection value is obtained.
A threshold value is determined for the level of... In FIG.
A threshold value is determined between the patch detection value and the background detection value at a level that is one-third of the background detection value. Note that the coefficient for determining the threshold value should be appropriately set according to the MTF characteristic, the detection sensitivity characteristic, and the like of the detection sensor 19, and may be a fixed value or an NVM value by combination.

Thereafter, in step S57, the value of N is incremented (+1), and thereafter, in step S58
It is determined whether or not the value of N has reached a predetermined value. By the way, in the case of the two-tandem type color copier employed in the second embodiment, one color misregistration detection pattern image is obtained by multiplexing two color images among the four colors of YMCK. Thus, "2" is set as the predetermined value in a form corresponding to the number of colors. If the value of N has not reached the predetermined value, the process proceeds to step S54.
And the same process is repeated. When the value of N reaches a predetermined value, the process proceeds to step S59.

In step S59, "1" is substituted for N again, and then in step S60 the Nth color (1
Output a color misregistration detection pattern for
In a succeeding step S61, a threshold value of the Nth color is set. The threshold value set here is the threshold value corresponding to the Nth color among the threshold values determined for each color in step S56. At this time, a digital signal corresponding to the set threshold level is output from the CPU 202, and this is a D / A signal.
The signal is converted into an analog signal by the converter 19c and input to the comparator 19b.

Next, sampling of the color misregistration detection pattern corresponding to the Nth color is started by the detection sensor 19 (step S62), and when the sampling is completed (YES in step S63), the process proceeds to step S64. At this time,
The analog signal from the photo sensor unit 19a is input to the comparator 19b, where the analog signal from the sensor is compared with the analog signal corresponding to the threshold level.
Then, when the analog signal from the sensor becomes larger than the analog signal corresponding to the threshold level, a detection signal for pattern edge detection is output from the comparator 19c to the CPU 202.

After that, in step S64, the value of N is incremented (+1), and then in step S65
It is determined whether or not the value of N has reached a predetermined value (2). If the value of N has not reached the predetermined value, the process returns to the previous step S60 and repeats the same processing. When the value of N reaches the predetermined value, a series of processes relating to the threshold adjustment ends. Here, in order to improve the detection accuracy, when sampling the above pattern in a plurality of blocks,
Sample repeatedly for the number of blocks.

By switching and adjusting the threshold value in accordance with the color of the pattern to be detected as described above, for example, FIG.
If the S / N is good as shown in FIG. 16A, the position where the sensor output exceeds the threshold level set accordingly is detected as a pattern edge, and as shown in FIG. Is not good, the position where the sensor output exceeds the threshold level set accordingly is detected as the pattern edge. At this time, in both of FIGS. 16A and 16B, the shift amounts A and A 'between the original image position and the pattern edge detection position are equal. Therefore,
Even if the S / N ratio is good or bad depending on the color of the color misregistration detection pattern, the detection error associated therewith can be eliminated.

In the above-mentioned application example, the threshold value for edge detection is switched according to the color of the pattern to be detected. However, other than this, the same effect can be obtained by adopting the following configuration. Obtainable.

FIG. 17 is a block diagram of a color misregistration detecting device having a gain adjusting function. In the illustrated configuration, the analog signal photoelectrically converted by the photo sensor unit 191 of the detection sensor 19 is connected to the non-inverting (+) input of the operational amplifier 192. The operational amplifier 192 amplifies the result of the photoelectric conversion performed by the photosensor unit 191. The analog switch (SW) 193 performs a switching operation in accordance with a switching signal from the CPU 194.

The feedback resistor group 195 has a plurality of (eight in the illustrated example) feedback resistors connected in parallel with each other, and the feedback resistance value is switched by the switching operation of the analog switch 193. This feedback resistor group 19
5 is connected between the inverted (-) input and the output of the operational amplifier 192. A resistor 191a is connected between the inverting input of the operational amplifier 192 and a reference potential (earth). On the other hand, the comparator 196 is
9 is one input. Also, the CPU 194
, A digital signal for setting a threshold value is transmitted, and this digital signal is converted into an analog signal by the D / A converter 197 and becomes the other input of the comparator 196.

Incidentally, in the example of FIG.
, A 3-bit digital signal is input, and one of the eight feedback resistors constituting the feedback resistor group 195 is selected by the switching operation of the analog switch 193 according to the input signal. I have.

In the actual color misregistration correction processing, a switching signal is output from the CPU 194 according to the pattern color for color misregistration detection detected by the detection sensor 19, and the analog switch 193 performs a switching operation in accordance with the switching signal. As a result, the feedback resistance of the feedback resistance group 195 is set to a value corresponding to the color of the pattern.

When the detection sensor 19 detects a pattern in this state, an analog signal corresponding to the pattern is output from the photo sensor unit 191. This analog signal becomes one input of the operational amplifier 192 via the input resistor 191a. At this time, the amplification degree of the operational amplifier 192 is determined by the ratio between the resistance value of the input resistor 191a and the feedback resistance value corresponding to the color of the pattern, and an analog signal corresponding to the amplification degree is output from the operational amplifier 192. At this time, the analog signal output from the operational amplifier 192 is input to the comparator 196. On the other hand, a digital signal for threshold setting corresponding to each color is sent from the CPU 194, and this digital signal is input to the comparator 196 via the D / A converter 197. Therefore, the comparator 196 compares the analog signal output from the operational amplifier 192 with the analog signal corresponding to the threshold level. When the analog signal from the operational amplifier 192 becomes larger than the analog signal corresponding to the threshold level, the data for detecting the color shift is output to the comparator 196.
Output from

By adopting the color misregistration detecting device having the gain adjusting function as described above, for example, FIG.
As shown in FIG. 18B, no gain is applied to the sensor output of the pattern having a good S / N, and the output of the detection sensor 19 is extracted as a binary value using the threshold value set by the CPU 194. ), An appropriate amount of gain is applied to a sensor output of a pattern with a poor S / N ratio, and a threshold value corresponding to the gain is set to C.
By setting the value by the PU 194 and taking out the output of the detection sensor 19 in binary, the deviation amount between the original image position and the detection position of the pattern edge becomes equal in both (A =
A '). Thus, even if the S / N ratio is good or bad depending on the color of the color misregistration detection pattern, it is possible to eliminate the detection error associated therewith.

In the second embodiment, the image forming apparatus is not limited to the two-tandem type image forming apparatus using the intermediate transfer as in the above-described first embodiment. 9), an image forming apparatus employing a single system (see FIG. 10), an image forming apparatus employing a 3-tandem system, and an image forming apparatus employing a 4-tandem system (see FIG. 11). is there. In addition, by using the color misregistration detection function of the second embodiment as the color misregistration detection function of the first embodiment, it is possible to correct color misregistration due to misregistration between image output units with higher accuracy.

Further, in the case of the first embodiment described above, at least one of the N colors (for example, 2
) Image output units, and it is assumed that an image forming apparatus that multiplexes images of N + 1 colors (eg, YMCK) or more using the N image output units is used. In the case of the second embodiment, Since it is not necessary that a plurality of colors are assigned to any one image output unit, an image forming apparatus (for example, 4 A tandem system in which four-color images of YMCK are multiplexed is widely applicable.

[0113]

As described above in detail, according to the first aspect of the present invention, each of the N image output units has a good detection output condition for each one color, preferably in the detection means. Since the reference image is formed by setting one color and the reference image is detected by the detecting means, it is possible to reduce a detection error due to the color of the reference image. Therefore, by controlling the writing of the image on the image carrier based on the detection result of the detecting means, it becomes possible to correct the color shift caused by the positional shift between the image output units with high accuracy.

According to the fourth aspect of the present invention, the reference image formed on the image carrier using the N image output units is formed on the image carrier using the image output units. By using one color each having a good detection output condition of the reference image at the time, the detection error due to the color of the reference image can be reduced, and the influence of the noise component when detecting the reference image can be reduced. It is possible to correct the color misregistration caused by the misregistration with high accuracy.

According to the fifth aspect of the present invention, the color misregistration detection data is corrected according to the difference between the distance between the edge of the photoelectrically converted waveform of each color and the center of gravity based on the detection result of the detection means. Since the detection error of each color with respect to the reference image can be eliminated, the reference image can be accurately detected with a simple configuration using a photosensor or the like without using a CCD or the like. Therefore, it is possible to accurately correct a color shift caused by a position shift between the image output units.

According to the sixth aspect of the present invention, the threshold value is switched so that the distance between the edge and the center of gravity with respect to the threshold value of the photoelectrically converted waveform is equal between the colors.
The reference image can be accurately detected with a simple configuration using a photo sensor or the like without using such a method. Therefore, it is possible to accurately correct a color shift caused by a position shift between the image output units.

According to the seventh aspect of the present invention, the gain of the amplifying means is switched so that the distance between the edge and the center of gravity with respect to the threshold value of the output waveform of the amplifying means becomes equal for each color, so that a CCD or the like is not used. The reference image can be accurately detected with a simple configuration using a photosensor or the like. Therefore, it is possible to correct the color shift caused by the position shift between the image output units with high accuracy.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a two-tandem image forming apparatus to which the present invention is applied.

FIG. 2 is a flowchart (part 1) of a color misregistration correction process adopted in a two-tandem system.

FIG. 3 is a flowchart (part 2) of a color misregistration correction process adopted in a two-tandem system.

FIG. 4 is a timing chart corresponding to a color misregistration correction process adopted in a two-tandem system.

FIG. 5 is a schematic configuration diagram of a two-tandem type color copying machine employed in the first embodiment of the image forming apparatus according to the present invention.

FIG. 6 is a diagram illustrating an example of a sensor output corresponding to each color pattern.

FIG. 7 is a flowchart of a color misregistration correction process according to the first embodiment.

FIG. 8 is a diagram showing an example of a pattern shape based on a combination of a plurality of colors.

FIG. 9 shows a target of application of the present invention;
FIG. 2 is a diagram illustrating a tandem type image forming apparatus.

FIG. 10 is a diagram illustrating a configuration of a single type image forming apparatus to which the present invention is applied.

FIG. 11 is a diagram illustrating a configuration of a 4-tandem image forming apparatus to which the present invention is applied.

FIG. 12 is a diagram illustrating a configuration of a color misregistration calculating unit according to a second embodiment of the present invention.

FIG. 13 is a flowchart of a color misregistration calculation process according to the second embodiment of the present invention.

FIG. 14 is a configuration diagram of a color misregistration detection device having a threshold adjustment function.

FIG. 15 is a flowchart of a color misregistration correction process mainly based on threshold adjustment in the second embodiment of the present invention.

FIG. 16 is a diagram illustrating the effect of threshold adjustment.

FIG. 17 is a configuration diagram of a color misregistration detection device having a gain adjustment function.

FIG. 18 is a diagram illustrating the effect of gain adjustment.

FIG. 19 is a schematic configuration diagram of a two-tandem type color copying machine.

FIG. 20 is a diagram for explaining a problem of the related art.

[Explanation of symbols]

51, 51: photosensitive drum, 52, 53: developing device, 5
4, 55: laser writing unit, 56, 57: image writing control unit, 60: intermediate transfer belt, 19: detection sensor, 20
... Color misregistration calculation unit, 21 ... System control unit

Claims (9)

[Claims] 1. An image processing apparatus comprising: N image output units each having at least one assigned a plurality of colors, wherein the N image output units use an N + 1 or more color image as one color image on an image carrier. In an image forming apparatus for performing multiplex formation, setting means for setting one color for each of the N image output units, and the image holding unit using the color set by the setting means by the N image output units Detecting means for detecting reference images sequentially formed on the body, and control means for controlling writing of the N image output units to the image carrier based on a detection result of the detecting means. Characteristic image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the setting unit sets a color having a good detection output condition by the detection unit as an output color of the reference image. 3. The setting means includes: a single color having a high detection sensitivity, a color having a high detection sensitivity when a plurality of colors are combined and multiplexed, and
3. The image forming apparatus according to claim 2, wherein at least one of the colors having the same detection sensitivity is satisfied. 4. An image processing apparatus comprising: N image output units each having a plurality of colors assigned to at least one of the plurality of color images. In the color misregistration correction method for an image forming apparatus that forms multiple images as images, detecting a reference image when colors are assigned to the N image output units and formed on an image carrier using these image output units A color shift correction method for an image forming apparatus, wherein a color shift caused by a positional shift between the N image output units is corrected using one color having good output conditions. 5. A detecting means for photoelectrically converting a reference image of each color output from a plurality of image output units and comparing the photoelectrically converted waveform with a threshold to detect the color, the color corresponding to the detected value is provided. A color shift detection device that outputs shift detection data, wherein the color shift detection data is corrected according to a difference between a distance between an edge and a center of gravity of a photoelectrically converted waveform of each color based on a detection result of the detection unit. A color misregistration detection device comprising: 6. A detecting means for photoelectrically converting a reference image of each color output from a plurality of image output units and comparing the photoelectrically converted waveform with a threshold value to detect a color corresponding to the detected value. A color shift detection device that outputs shift detection data, comprising: means for switching the threshold so that the distance between the edge and the center of gravity with respect to the threshold of the photoelectrically converted waveform is equal for each color. Detection device. 7. An amplifying means for photoelectrically converting a reference image of each color output from a plurality of image output units and amplifying a result of the photoelectric conversion, and a detecting means for comparing the output of the amplifying means with a threshold for detection. A color misregistration detecting device for outputting color misregistration detection data corresponding to the detected value, wherein a gain of the amplifying means is set so that a distance between an edge and a center of gravity with respect to a threshold value of an output waveform of the amplifying means becomes equal between colors. A color shift detecting device, comprising: means for switching the color shift. 8. An image forming apparatus comprising the color misregistration detecting device according to claim 5, 6 or 7. 9. An image forming apparatus comprising: the color misregistration detection device according to claim 5, 6 or 7 as the detection means according to claim 1.