JP2008185622A - Color shift detection method and device, and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high quality image free of color shift. <P>SOLUTION: In a color shift detection method for detecting color shift of each color when forming a color image by superposing a plurality of colors, color shift detection patterns of the corresponding colors superposed one on another are formed on a medium. Then, the formed color shift detection patterns are optically read, and threshold values are set for the detected waveforms of the read colors. The amount of shift between the colors is calculated based on two times in the set threshold values of the detected waveforms of the corresponding colors. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention includes a plurality of image forming units that form images of different colors, and a color misregistration detection method for forming a color image by superimposing a plurality of images of each color formed on a transfer medium. And an image forming apparatus such as a copier, a printer, a facsimile, or a multifunction machine having these functions.

  2. Description of the Related Art Conventionally, in a color image forming apparatus, a tandem type image forming apparatus that forms a color image by arranging a plurality of image bearing members such as a photoreceptor along an endless transfer belt as a transfer medium is known. Yes. In this tandem image forming apparatus, an electrostatic latent image is formed for each color of yellow (Y), magenta (M), cyan (C), and black (K) for each image carrier, and development, etc. An electrophotographic process is performed. Then, the images of different colors on the respective image carriers are superposed to finally form a color image on the transfer paper.

  In such a tandem color image forming apparatus, it is necessary to accurately align the toner transfer positions between the color toner images in order to form a high-quality image. If there is a positional shift between the color toner images, a high-quality color image cannot be obtained due to the color shift between the colors in the superimposed toner image formed on the transfer paper.

  Such color misregistration includes sub-scanning resist misalignment from the ideal image forming line (toner image forming position) in the sub-scanning direction, skew misalignment tilting from the ideal image forming line in the sub-scanning direction, and the like. is there.

  In view of this, an apparatus has been proposed in which a mark pattern for alignment is formed and then the position is detected using a sensor, and the position is corrected to match an ideal image forming line according to the detection result. (Patent Documents 1 and 2).

In Patent Documents 1 and 2, a mark pattern is formed on each end of the intermediate transfer belt, which is an array of mark images of toners arranged in the belt moving direction. Then, each mark image of each mark pattern is detected by a sensor, and a deviation amount between the detected position and the ideal position is calculated to correct the toner transfer position of each color toner.
JP-A-1-142671 Japanese Patent No. 3351435

  By the way, as described above, when the amount of deviation between the position detected by the sensor and the ideal position is calculated to correct the toner transfer position of each color toner, the sensor has a large sensitivity difference with respect to a plurality of colors of the color deviation correction pattern. When the output voltage value fluctuates significantly, the detection error increases. Further, when the color misregistration correction pattern is not a symmetric waveform, the detection accuracy is deteriorated and the detection error is increased.

  The present invention has been made in view of such a situation of the prior art, and the problem to be solved is detection accuracy even when the sensitivity difference of the sensor is large or the symmetry of the color misregistration correction pattern is not good. This is to prevent deterioration.

  In order to solve the above problems, a first means is a color misregistration detection method for detecting a color misregistration of each color when a color image is formed by superimposing a plurality of colors. Optically reading the formed color misregistration detection pattern, setting a threshold value for each read detection waveform of each color, and two times in the set threshold value of the detection waveform of each color The color misregistration amount between each color is calculated based on the above.

  According to a second means, in the first means, the threshold value is set in two or more stages including an upper value and a lower value for each color for each of the plurality of colors of the color misregistration detection pattern, and reading is performed with each threshold value. The midpoint value of each time is obtained from the obtained time, and the symmetry of the detected waveform is determined from the difference between the midpoint values.

  The third means is characterized in that analog / digital conversion is performed on the waveform determined as an asymmetric waveform in the second means by using the upper value of the threshold value.

  According to a fourth means, in any one of the first to third means, the color misregistration detection pattern is formed on each of the plurality of image carriers, and the formed pattern is transferred to the transfer medium. It is characterized by that.

  A fifth means includes a pattern forming means for forming a color misregistration detection pattern of each color to be superimposed on a medium in a color misregistration detection apparatus that detects a color misregistration of each color when a color image is formed by superimposing a plurality of colors. Reading means for optically reading the color misregistration detection pattern formed by the pattern forming means; setting means for setting a threshold value for each color detection waveform read by the reading means; and the setting means. Calculating means for calculating a color shift amount between the respective colors based on two times at the set threshold value.

  A sixth means is the fifth means, wherein the setting means sets two or more threshold values including an upper value and a lower value for each color for each of a plurality of colors of the transferred pattern. It is characterized in that a midpoint value is obtained from the values read by the values and the symmetry of the detected waveform is determined from the difference value.

  The seventh means is characterized in that, in the sixth means, the setting means obtains a midpoint value by using the upper value of the threshold value for the waveform determined to be an asymmetric waveform.

  The eighth means is characterized in that, in any one of the fifth to seventh means, the setting means sets a threshold value independently for a plurality of colors.

  According to a ninth means, the sixth means further comprises display means for displaying an error when the setting means determines that the detected waveform is abnormal from the difference value of the midpoint values. Features.

  The tenth means is characterized in that, in any of the fifth to ninth means, the pattern forming means forms a plurality of rows of the patterns of the plurality of colors.

  An eleventh means is characterized in that, in any of the fifth to tenth means, the medium is an intermediate transfer belt or a paper transport belt.

  A twelfth means is characterized in that the image forming apparatus includes a color misregistration detection apparatus according to any one of the fifth to eleventh means.

  A thirteenth means is characterized in that, in the twelfth means, four image carriers are provided, and images of a plurality of colors are formed by a quadruple tandem method.

  A fourteenth means is characterized in that, in the twelfth or thirteenth means, the color misregistration correction is executed by a command signal from an operation panel or a personal computer connected to the apparatus.

  In the embodiment described later, the pattern forming means is used for the image forming stations 3Y, 3C, 3M, and 3K and the optical writing unit 4, the image carrier is used for the photosensitive members 10Y, 10C, 10M, and 10K, and the medium and the transfer medium are used. The intermediate transfer belt 20, the transfer means are primary transfer rollers 24Y, 24C, 24M and 24K, the reading means are sensors 20f, 20c and 20r, the setting means, the calculation means and the determination means are processing circuits 83, and analog / digital conversion. The means corresponds to the A / D converter 82, and the error display means corresponds to the liquid crystal display section of the operation panel.

  According to the present invention, since the threshold value is set for each color detection waveform, even when the fluctuation or symmetry of the output voltage value is broken, the threshold value corresponding to the fluctuation or the fluctuation is set. Deterioration of detection accuracy can be suppressed.

  Embodiments of the present invention will be described below with reference to the drawings.

  An embodiment in which the present invention is applied to a printer as an image forming apparatus will be described. In the present embodiment, a so-called intermediate transfer type tandem image forming apparatus will be described as an example, but the present invention is not limited to this.

  FIG. 1 is a schematic configuration diagram showing a printer according to an embodiment of the present invention, and FIG. 2 is a configuration diagram showing a schematic configuration of a yellow (Y) imaging station.

  This printer is connected to a personal computer PC, and includes a printer main body 1 that prints print data sent from the personal computer PC, and a paper feed cassette 2 that can be pulled out from the apparatus main body 1. In the central portion of the printer main body 1, image forming stations 3Y, 3C for forming toner images (visible images) of yellow (Y), cyan (C), magenta (M), and black (K) colors. 3M, 3K. Hereinafter, the subscripts Y, C, M, and K of the respective symbols indicate members for yellow, cyan, magenta, and black, respectively.

  As shown in FIGS. 1 and 2, the image forming stations 3Y, 3C, 3M, and 3K include drum-shaped photoconductors 10Y, 10C, 10M, and 10K as latent image carriers that rotate in the direction of the arrows in the drawing. Yes. Each of the photoreceptors 10Y, 10C, 10M, and 10K includes an aluminum cylindrical substrate having a diameter of 40 mm and an OPC (organic photo semiconductor) photosensitive layer that covers the surface of the photoreceptor. Each of the image forming stations 3Y, 3C, 3M, and 3K has charging devices 11Y, 11C, 11M, and 11K that charge the photoconductor around the photoconductors 10Y, 10C, 10M, and 10K, and a latent image formed on the photoconductor. Developing devices 12Y, 12C, 12M, and 12K as developing means for developing an image, and cleaning devices 13Y, 13C, 13M, and 13K for cleaning residual toner on the photoreceptor are provided. Below each image forming station 3Y, 3C, 3M, 3K, an optical writing unit 4 as an optical writing means, which is an optical scanning device that can irradiate the photoconductors 10Y, 10C, 10M, 10K with the writing light L. Is provided. Above each of the image forming stations 3Y, 3C, 3M, and 3K, an intermediate transfer unit 5 including an intermediate transfer belt 20 to which a toner image formed by each of the image forming stations 3Y, 3C, 3M, and 3K is transferred is provided. ing. Further, a secondary transfer roller 25 for transferring the toner image transferred to the intermediate transfer belt 20 to the transfer paper P as a recording material, and the toner image transferred to the transfer paper P are fixed along the transfer paper conveyance path. A fixing unit 6 is provided.

  Furthermore, toner bottles 7Y, 7C, 7M, and 7K that contain toner of each color of yellow (Y), cyan (C), magenta (M), and black (K) are loaded on the upper portion of the printer body 1. . The toner bottles 7 </ b> Y, 7 </ b> C, 7 </ b> M, and 7 </ b> K are configured to be detachable from the printer main body 1 by opening a paper discharge tray 8 formed on the upper portion of the printer main body 1.

  FIG. 2 shows a yellow (Y) imaging station, but the imaging stations of other colors are the same.

  The optical writing unit 4 deflects writing light (laser light) L emitted from a laser diode as a light source by a polygon mirror or the like, and sequentially scans the photosensitive members 10Y, 10C, 10M, and 10K while irradiating them. Detailed description of the optical writing unit 4 will be described later.

  The intermediate transfer belt 20 of the intermediate transfer unit 5 is wound around a driving roller 21, a tension roller 22, and a driven roller 23, and is driven to rotate counterclockwise in the drawing at a predetermined timing. The intermediate transfer unit 5 includes primary transfer rollers 24Y, 24C, 24M, and 24K that transfer the toner images formed on the photoreceptors 10Y, 10C, 10M, and 10K to the intermediate transfer belt 20. The intermediate transfer unit 5 includes a secondary transfer roller 25 that transfers the toner image transferred onto the intermediate transfer belt 20 to the transfer paper P, and a transfer residue on the intermediate transfer belt 20 that has not been transferred onto the transfer paper P. A belt cleaning device 26 for cleaning the toner is provided.

The image forming process in the printer configured as described above is as follows.
First, in the image forming stations 3Y, 3C, 3M, and 3K, the photoreceptors 10Y, 10C, 10M, and 10K are uniformly charged by the charging devices 11Y, 11C, 11M, and 11K. Thereafter, the optical writing unit 4 scans and exposes the laser beam L based on the image information, and forms latent images on the surfaces of the photoconductors 10Y, 10C, 10M, and 10K that rotate in the direction indicated by the arrow A in the drawing. The latent images on the photoreceptors 10Y, 10C, 10M, and 10K are formed by the toners of the respective colors carried on the developing rollers 15Y, 15C, 15M, and 15K that rotate in the direction indicated by the arrow B of the developing devices 12Y, 12C, 12M, and 12K. Developed and visualized as a toner image. The toner images on the photoreceptors 10Y, 10C, 10M, and 10K are sequentially transferred onto the intermediate transfer belt 20 that is rotated counterclockwise by the action of the primary transfer rollers 24Y, 24C, 24M, and 24K. The image forming operation of each color at this time is shifted in timing from the upstream side in the moving direction of the intermediate transfer belt 20 toward the downstream side so that the toner image is transferred to the same position on the intermediate transfer belt 20. Executed. The surfaces of the photoreceptors 10Y, 10C, 10M, and 10K after the completion of the primary transfer are cleaned by the cleaning blades 13a of the cleaning devices 13Y, 13C, 13M, and 13K, and are prepared for the next image formation. A predetermined amount of toner filled in the toner bottles 7Y, 7C, 7M, and 7K is supplied to the developing devices 12Y, 12C, 12M, and 12K of the image forming stations 3Y, 3C, 3M, and 3K through a conveyance path (not shown) as necessary. To be replenished.

  On the other hand, the transfer paper P in the paper feed cassette 2 is transported into the printer main body 1 by a paper feed roller 27 disposed in the vicinity of the paper feed cassette 2 and is secondary by a registration roller pair 28 at a predetermined timing. It is conveyed to the transfer unit. Then, the toner image formed on the intermediate transfer belt 20 is transferred to the transfer paper P in the secondary transfer portion. The transfer paper P onto which the toner image has been transferred is image-fixed in the process of passing through the fixing unit 6, and is discharged to the paper discharge tray 8 by the discharge roller 29. Similar to the photoconductor 10, the transfer residual toner remaining on the intermediate transfer belt 20 is cleaned by a belt cleaning device 26 that contacts the intermediate transfer belt 20.

  The printer illustrated in FIGS. 1 and 2 is an indirect transfer tandem type image forming apparatus, and therefore the transfer medium is an intermediate transfer belt 20. In contrast, in the case of a direct transfer tandem type image forming apparatus, a color image is directly transferred to a sheet, so that a color misregistration detection pattern, which will be described later, is formed on a paper conveyance belt that contacts a photoconductor. . Therefore, the transfer medium becomes a paper conveyance belt.

  Next, an example of a color misregistration correction pattern will be described.

  As shown in FIG. 3, when performing color misregistration correction control, a color misregistration detection pattern is formed on the intermediate transfer belt 20. That is, the rear end (rear) of the direction x orthogonal to the moving direction of the intermediate transfer belt 20 is cyan (Acr), and black (Abr), yellow (Ayr), and magenta (Amr) are equally spaced in the direction y. Are formed on the transfer belt 20 in this order. Next, diagonal lines are similarly formed on the transfer belt 20 in the order of cyan, black, yellow, and magenta at equal intervals. Here, four horizontal lines and four diagonal lines, a total of eight lines, are set as one set, and are further formed in the seven-set direction y (the intermediate portion is omitted in FIG. 3). It is read by the sensor 20r. Similarly, a color misregistration correction pattern having the same number of sets is formed at the center (center) and is read by the sensor 20c. The tip (front) is also the same as the center, and is read by the sensor 20f.

  4 and 5 are diagrams for explaining color misregistration detection. As shown in FIG. 3, the detection patterns Acf, Abf, Ayf, Amf, Acr, Abr, Ayr, Amr, etc. are formed on the intermediate transfer belt 20, and the color misregistration amount (in this case) The color shift amount in the moving direction (sub-scanning direction) of the intermediate transfer belt 20 is detected. 4 and 5 show an example in which the detection patterns Acf, Abf, Ayf, Amf of four colors are detected by the front sensor 20f. When the set distances between the detection patterns Acf, Abf, Ayf, and Amf for each color are Xmm, and all mechanical accuracy and laser writing accuracy are at ideal positions, the detection patterns Acf, Abf, Ayf, Amf are formed. When these detection patterns Acf, Abf, Ayf, Amf are read by the sensor 20f, the midpoints Acfm, Abfm, Ayfm, Amfm of the read pattern are read at Xmm intervals at the time of pattern formation.

  FIG. 5 shows that the detection patterns Acf, Abf, Ayf, Amf are written at intervals of X mm in the same manner as shown in FIG. This is an example in the case where there is a deviation in the detection patterns Acf, Abf, Ayf, Amf. When the detection pattern Acf, Abf, Ayf, Amf shown in FIG. 5 is read by the sensor 20f, the color misregistration amount of each color with respect to the black reference (Abfm) of the middle point Acfm, Abfm, Ayfm, Amfm of each pattern is at the ideal position. In contrast, the cyan detection pattern Acf has a color shift amount of −0.2 * X mm, and the yellow detection pattern Ayf has a shift amount of −0.3 * X mm. In FIG. 5, the arrow Y direction is indicated as +.

  The detected value of the deviation amount is a case where the waveform read by the sensor 20 is symmetrical, and when the waveform is asymmetric as shown in FIGS. 12 to 14 to be described later, the color deviation amount is accurately detected by the midpoint value. I can't do it.

  In a printer (image forming apparatus) that forms a color image, a color misregistration correction pattern as shown in FIG. 3 is periodically formed, and the amount of color misregistration is detected and corrected when written to minimize the occurrence of color misregistration as much as possible. It is controlled to become. However, when the symmetrical detection pattern cannot be obtained due to the characteristics of the sensor 20, the color misregistration amount cannot be accurately calculated, and therefore, the color misregistration corresponding to the offset amount caused by the left / right asymmetry remains. Also, the problem of color and sensor sensitivity remains. Note that the offset amount due to the left-right asymmetry of the detection output of each sensor 20f, 20c, 20r is determined by the characteristics of the sensor. Therefore, the offset amount is detected and corrected when the sensor is replaced in the manufacturing process or after installation.

  The color misregistration amount correction control or the color misregistration correction process itself is a known technique as described in, for example, Patent Documents 1 and 2.

Hereinafter, the waveform read by the sensor 20f on the front end side will be described.
FIG. 6 shows a waveform read by the sensor 20f. The vertical axis is the voltage value, and the vertical axis is the time axis. Since the sensor 20f has different spectral sensitivities for each color, the degree of waveform drop differs even at the same density. In FIG. 6, the sensitivity is good for the color of the sensor 20f black (Abf), but for yellow (Ayf), the read waveform (detection waveform) of the sensor is poor. After reading such an analog waveform, next, in order to A / D convert the analog waveform, a certain threshold value is set, and the read analog signal is converted into a digital signal based on the threshold value. Note that the reference symbol Acf is a cyan and the reference symbol Amf is a magenta color misregistration detection pattern.

  The example of FIG. 6 is a case where the threshold value Vth (denoted as “threshold level” in the figure) cannot be set for each color. In this way, when the threshold value Vth can be set to only one value, if the yellow (Ary) signal varies, the read signal may not be processed with the threshold value Vth. If the threshold value Vth is increased in consideration of such a case, even a scratch on the intermediate transfer belt 20 may be read. If the threshold value Vth is increased in this way, a malfunction is caused, and the threshold value Vth cannot be increased.

  In the present embodiment, in view of such setting of the threshold value Vth, as shown in FIG. 7, individual threshold values Vthb, Vthy, Vthc, Vthm can be set for each color. If an individual threshold value can be set for each color in this way, it becomes possible to set a threshold value Vth that matches the spectral sensitivity of the sensor, thereby increasing the margin for noise included in the read waveform. Can do. In addition, even when the output fluctuation is large, it is possible to cope with the change of the threshold value, and it is possible to reliably perform digital conversion with the threshold value.

  FIG. 8A is a circuit diagram of the sensor 20f including the light emitting element 20a and the light receiving element 20b in the present embodiment, and FIG. 8B is a diagram illustrating the light emitting element 20a and the light receiving element 20b of the sensor 20f in FIG. It is a figure which shows arrangement | positioning of both at the time of pattern reading. The sensor 20f is a light reflection type sensor, and the light receiving element 20b is a regular reflection type optical sensor that detects regular reflection of light. Although not particularly mentioned in this embodiment, the left light receiving element 20g in the drawing detects diffuse reflection of light. The sensor 20f is the same for the rear end sensor 20r and the center sensor 20c.

  In the sensor 20f, light having an output corresponding to the voltage applied to the transistor 20t is emitted from the light emitting element 20a, the emitted light is applied to the pattern, and the reflected light is detected by the light receiving element 20b. At this time, the irradiation diameter to the transfer medium is 202 mm, and the reflection diameter is 0.5 to 0.6 mm. When the light receiving element 20b receives the reflected light, it outputs a voltage corresponding to the received light quantity. Since the received light amount varies depending on the color and width of the reflected pattern, an output corresponding to the received light amount is output from the output terminal 20d. This output waveform is as shown in FIG. 6 and FIG.

FIG. 9 is a diagram showing an example of an output waveform from the output terminal 20d when a positional deviation correction pattern is read by the front sensor 20f, and is a symmetrical waveform. In the present embodiment, a midpoint value (time) of a signal is obtained using values (time) t11 (falling) and t21 (rising) that are read at a preset threshold value Vth of 1 below, The amount of color misregistration between each color is calculated based on the point value. That is, in the example of FIG.
(T11 + t21) / 2 (1)
To obtain the midpoint value of the waveform. As described with reference to FIGS. 4 and 5, the midpoint value is periodically detected and corrected using the midpoint value.

  FIG. 10 is a block diagram illustrating a circuit configuration of a correction processing circuit that sets the threshold value, obtains a midpoint value represented by the equation (1), detects and corrects color misregistration. In FIG. 10, a correction processing circuit 80 amplifies a sensor output inputted from an input terminal 20e connected to the output terminal 20d, and A / D converts the signal amplified by the amplification circuit 81. The processing circuit 83 includes a / D conversion circuit 82. The processing circuit 83 includes each unit necessary for processing including a CPU and a memory, and executes detection processing and correction processing described below.

  In such a configuration, for example, a problem when the read waveform is not symmetrical will be described. FIG. 11 shows a case where the read waveform is a symmetric waveform. As can be seen from FIG. 11, the respective shift amounts are detected by the midpoint value (yellow Z11) tcy and tcm (magenta Z12), and the colors are overlapped. As a result, there is almost no deviation at Z13 on the right side of the figure indicated by the arrow. However, when there is an asymmetrical read waveform as shown in FIG. 12, specifically, yellow is symmetric as indicated by reference numeral Z11, and when magenta indicated by reference numeral Z12 is asymmetric, yellow Z11 and magenta Z12. If the midpoints of the respective midpoint values tcy and tcm1 are overlapped, the color misregistration amount Z14 cannot be completely corrected in the diagram indicated by the symbol Z13 on the right side indicated by the arrow.

  As described above, it can be seen that when the waveform is asymmetric, the shift of each color cannot be completely detected and the correction is incomplete. Therefore, in order to eliminate such a waveform defect, it is necessary to determine whether the waveform is asymmetric in advance.

Returning to FIG. 9, when Vth 1 is newly added and the respective midpoints read at Vth 1 and Vth 1 are obtained, when the waveforms are symmetrical, (t1 + t2) / 2 = (t11 + t21) / 2 ... (2)
become.

Similarly, in the case of a waveform that is not symmetrical, as shown in FIG. 13, (t3 + t4) / 2 ≠ (t31 + t41) / 2 (3)
Therefore, the midpoint value read by each threshold value is compared, and if it is equal (=) as in equation (2), it is determined as a symmetrical waveform and the process proceeds to the next process in the normal process. When the midpoint values are not equal (≠) as shown in Expression (3), the waveform is determined to be asymmetric.

  FIG. 14 is an explanatory diagram for explaining correction when it is determined as an asymmetric waveform. In the case of the symmetric waveform 31a, the waveform that is A / D converted by the threshold value Vth is 31b, and the midpoint value is 31c (t = tc1).

On the other hand, when the same processing is performed with the asymmetric waveform 30a, the waveform becomes 30b after A / D conversion, the midpoint value thereof is 30c (t = tc2), and the midpoint value 30c and the midpoint value 31c are An error of Δt2 occurs. However, if it is determined that the waveform is asymmetric, the threshold value is changed to Vth and the same processing is performed. After A / D conversion, the waveform becomes 30d, and the midpoint value is 30e (t = tc3). Therefore, it becomes an error of Δt1,
Δt1 = | tc1-tc3 |
Δt2 = | tc1−tc2 |
From
Δt1 <Δt2
It becomes. This shows that the error is reduced.

  Therefore, each color has a plurality of threshold values, the center value is obtained from the value read by each threshold value, and the symmetry of the detected waveform is determined from the difference between the center values. This determination is performed based on Expression (2) and Expression (3). Since the amount of color misregistration varies depending on each printer (image forming apparatus), the determination of the symmetry is also performed based on the color matching accuracy of each printer.

  In this way, it is possible to detect the distortion of the waveform caused by the sensitivity of each sensor 20f, 20c, 20r, the lens of the optical system, or the like. At this time, for the waveform determined to be an asymmetric waveform, A / D conversion is performed using the value of the upper level among a plurality of threshold values. Thereby, as described above, a midpoint value with less error can be obtained. That is, the center point can be accurately detected by selecting a threshold value for each color.

  At that time, if the threshold value Vth is larger than a certain value Δt between the midpoint values obtained above and below one value, the characteristic value of the sensor is not at a practical level, so that it is regarded as abnormal. To do. If the state is regarded as abnormal in this way, the service man call and the content of the abnormality are displayed on the liquid crystal display unit of the operation panel (not shown) of the printer 1 to notify the user so that the current machine abnormality can be known.

FIG. 15 is a flowchart showing the processing procedure of the CPU of the processing circuit 83 at this time. In the figure, when reading of the correction pattern is started, A / D conversion is performed at the respective threshold values below Vth and above Vth (step S1).
Midpoint value below Vth = Midpoint value above Vth (4)
Is determined, and if not, further,
Midpoint value below Vth ≤ or ≥ Midpoint value above Vth (5)
Whether or not is established is determined. If this determination is negative, a subroutine for the next process is executed in step S4.

  On the other hand, if the determination (4) is possible, a reading process is performed under the threshold value Vth (step S5), and a subroutine of the next process is executed in step S6. On the other hand, if determination (5) is possible, it is considered that the midpoint value indicates an abnormality, and an abnormality processing subroutine is executed in step S7.

  As described above, in this embodiment, the threshold value Vth, midpoint value calculation, and abnormality determination are executed by the CPU in the processing circuit 83.

  The color misregistration detection and correction control is executed in the printer. The color misregistration correction is executed by a command signal from the operation panel of the printer or a personal computer (PC) shown in FIG. 1 connected to the printer.

As described above, according to this embodiment,
1) Since a threshold value can be set for each color of the detection pattern for color misregistration, it is possible to absorb variations due to the spectral sensitivity of the sensor.
2) By having two or more threshold values for each color of the color misregistration detection pattern, the symmetry of the detected waveform can be accurately determined.
3) By using the value (upper value) on the upper side of the threshold value for the waveform determined to be an asymmetric waveform, the detection pattern for color misregistration can be read with high accuracy.
4) Since threshold values of two or more levels can be set independently for each color, it is possible to absorb variations due to the spectral sensitivity of the sensor.
5) When it is determined that there is an abnormality, an error display or the like can be displayed and notified to the user through the operation panel, so that the user can accurately grasp the current state of the machine.
6) Since the color misregistration correction can be performed by setting the threshold value for each color, it is possible to obtain a high-quality image without color misregistration.
7) Since a threshold value can be set for each color and color misregistration correction can be performed for a quadruple tandem image forming apparatus, a high-quality image without color misregistration can be obtained.
8) In the case of an intermediate transfer type tandem type image forming apparatus provided with an intermediate transfer belt, a color misregistration correction pattern is formed on the intermediate transfer belt. Therefore, color misregistration correction is performed by reading the pattern on the intermediate transfer belt. It can be carried out.
9) In the case of a direct transfer type image apparatus provided with a paper transport belt, color misregistration correction can be performed by forming a color misregistration correction pattern on the paper transport belt.
10) Since the color misregistration correction can be executed by a command signal from an operation panel or a personal computer connected to the apparatus, the color misregistration correction can be executed when the user desires.
There are effects such as.

1 is a schematic configuration diagram illustrating a printer according to an embodiment. It is a block diagram which shows schematic structure of the image forming station of yellow (Y). FIG. 4 is a diagram illustrating a color misregistration detection pattern formed on an intermediate transfer belt. It is a figure for demonstrating color misregistration amount detection, and shows the state in which the pattern for color misregistration detection is formed with the ideal space | interval. It is a figure for demonstrating color misregistration amount detection, and shows the state of the color misregistration detection pattern when color misregistration arises. It is a wave form diagram which shows the reading waveform and the fixed threshold value which were read with the sensor. It is a wave form diagram which shows the reading waveform read with the sensor, and the threshold value variable for every color. It is a figure which shows the circuit structure and structure of a sensor which reads the pattern for color misregistration detection. It is a wave form diagram which shows the waveform of the symmetrical waveform read with the sensor. It is a figure which shows the circuit structure of the circuit which performs the process in this embodiment. It is a figure which shows the relationship between the waveform pattern and waveform of a symmetrical read waveform. It is a figure which shows the relationship (error state) of the waveform of a waveform whose read waveform is a non-target form, and a waveform. It is a figure which shows the relationship between the read waveform determined to be an asymmetric waveform, and a midpoint value. It is a figure which shows the correction method of the read waveform determined to be an asymmetric waveform. It is a flowchart which shows the process sequence of CPU in the processing circuit of FIG.

Explanation of symbols

3Y, 3C, 3M, 3K Image forming station 4 Optical writing unit 10Y, 10C, 10M, 10K Photoconductor 11Y, 11C, 11M, 11K Charging device 12Y, 12C, 12M, 12K Developing device 20 Intermediate transfer belt 24Y, 24C, 24M, 24K Primary transfer roller 20f, 20c, 20r Sensor 80 Correction processing circuit 81 AMP
82 A / D converter 83 Processing circuit

Claims (14)

In a color shift detection method for detecting a color shift of each color when forming a color image by superimposing a plurality of colors,
A color misregistration detection pattern for each color to be superimposed is formed on the medium,
Optically reading the formed color misregistration detection pattern,
Set the threshold value for each color detection waveform
A color misregistration detection method, comprising: calculating a color misregistration amount between colors based on two times in the set threshold value of the detection waveform of each color. The color misregistration detection method according to claim 1.
The threshold value is set in two or more stages including an upper value and a lower value for each color for each of the plurality of colors of the color misregistration detection pattern,
From the time read at each threshold value, find the midpoint value of each time,
A color misregistration detection method, comprising: determining symmetry of a detected waveform from a difference between the midpoint values. The color misregistration detection method according to claim 2.
A color misregistration detection method characterized by performing analog / digital conversion using an upper value of a threshold value for a waveform determined to be an asymmetric waveform. The color misregistration detection method according to any one of claims 1 to 3,
The color misregistration detection pattern is formed on each of a plurality of image carriers, one color,
A method of detecting color misregistration, wherein the formed pattern is transferred to the transfer medium. In a color misregistration detection apparatus that detects a color misregistration of each color when a color image is formed by superimposing a plurality of colors,
Pattern forming means for forming a color misregistration detection pattern of each color to be superimposed on the medium;
Reading means for optically reading the color misregistration detection pattern formed by the pattern forming means;
Setting means for setting a threshold value for each color detection waveform read by the reading means;
Calculating means for calculating a color shift amount between each color based on two times in the threshold value set by the setting means;
A color misregistration detection apparatus comprising: The color misregistration detection apparatus according to claim 5.
A determination means for determining the symmetry of the detected waveform;
The setting means sets two or more threshold values including an upper value and a lower value for each color for each of the plurality of colors of the transferred pattern,
The calculation means obtains a midpoint value of two times in the at least two stages of threshold values for each color,
The color misregistration detection apparatus characterized in that the determination means determines the symmetry of the detection waveform for each color from the midpoint value. The color misregistration detection apparatus according to claim 6.
The color misregistration detection apparatus, wherein the calculation means obtains a midpoint value by using an upper value of a threshold value for a waveform determined to be an asymmetric waveform. The color misregistration detection apparatus according to any one of claims 5 to 7,
The color misregistration detection apparatus, wherein the setting means sets a threshold value independently for a plurality of colors. The color misregistration detection apparatus according to claim 6.
An error display means;
The determination means determines whether or not the detected waveform is abnormal from the difference value of the midpoint values, and when it determines that the detection waveform is abnormal, an error display is performed on the error display means. Detection device. The color misregistration detection apparatus according to any one of claims 5 to 9,
The color misregistration detection apparatus, wherein the pattern forming unit forms a plurality of rows of the plurality of color patterns. The color misregistration detection apparatus according to any one of claims 5 to 10,
A color misregistration detection apparatus, wherein the medium is an intermediate transfer belt or a paper conveyance belt.   An image forming apparatus comprising the color misregistration detection device according to claim 5. The image forming apparatus according to claim 12.
An image forming apparatus comprising four image carriers and forming a multi-color image by a quadruple tandem method. The image forming apparatus according to claim 12 or 13,
An image forming apparatus, wherein color misregistration correction is executed by a command signal from an operation panel or a personal computer connected to the apparatus.

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