JP6452319B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms an image.

  There is known an electrophotographic image forming apparatus that forms toner images of four colors of yellow, magenta, cyan, and black, and forms a color image by superimposing them on a transfer belt. Deviations in the toner image formation positions of the respective colors occur due to misalignment of the assembly positions of the image forming components in the image forming apparatus and speed errors of the photosensitive drum and transfer belt. As a result, there is a problem that color shift occurs in the color image. In order to reduce the color misregistration, there has been proposed a misregistration correction technique in which a pattern of each color is formed, the amount of misregistration of each color forming position is measured by a sensor, and the image exposure condition is corrected (Patent Document 1). .

Japanese Patent Laid-Open No. 1-183676

  A registration sensor for detecting a registration pattern for detecting a positional deviation amount needs to detect a positional deviation amount of several μm to several tens of μm. In the irregular reflection type sensor, if the density of the registration pattern is constant, and the density within the registration pattern is uniform, it is possible to detect the positional deviation amount with an accuracy of several μm. However, in the irregular reflection type sensor, the sensor output level greatly changes due to a change in toner density, and a detection error of several tens of μm may occur due to a change in the density of the registration pattern.

  An object of the present invention is to suppress a decrease in accuracy of positional deviation correction due to a change in density in the surface of a registration patch for positional deviation correction.

An image forming apparatus according to the present invention includes a first image forming unit that forms a first color toner image and a second image that is disposed downstream of the first image forming unit and forms a second color toner image. And an image formed by transferring the first color toner image formed by the first image forming unit and the second color toner image formed by the second image forming unit. A carrier, a sensor that irradiates light toward the image carrier and receives reflected light from the image carrier, an amplifying unit that amplifies an output value of the sensor based on an amplification condition, and the amplified Determining means for determining the positions of the toner images of the first color and the second color from the output values, and the toner of the second color based on the positions of the toner images of the first color and the second color; The position of the image matches the position of the toner image of the first color And control means for controlling the formation position of the second color toner image, the result was an output value of the sensor of test patches formed by said first image forming means and amplified by said amplifying means and the second Adjusting means for adjusting the amplification condition so that the result of amplification by the amplification means of the output value of the sensor of the test patch formed by the image forming means is saturated.

  According to the present invention, it is possible to suppress a decrease in the accuracy of misalignment correction due to a density change in the surface of a registration patch for misalignment correction. Therefore, color misregistration in the output image can be reduced and a high-quality output image can be formed.

1 is a diagram illustrating a configuration of an image forming apparatus. FIG. 5 is a diagram for explaining the arrangement of sensors in a registration / density detection unit 150. It is a figure for demonstrating the structure regarding density correction. It is a figure which shows the structure of a density sensor. It is a flowchart which shows the process sequence of density correction. It is a figure explaining the structure concerning a position shift correction. It is a figure for demonstrating the method of calculating positional offset amount. It is a figure which shows the structure of a registration sensor. It is the figure which showed the relationship between the analog output of a registration sensor, and a binarization process. It is a flowchart which shows the process sequence of position shift correction.

  Hereinafter, an embodiment in which the present invention is applied to an electrophotographic image forming apparatus (printer) will be described.

  FIG. 1 is a diagram illustrating a configuration of an image forming apparatus according to the present exemplary embodiment.

  A document 100 is placed on the document table glass 101. The document 100 placed on the document table glass 101 is irradiated with illumination 103. The light reflected by the original 100 passes through mirrors 104, 105, and 106 and is imaged on the CCD 109 by the optical system 108. The CCD 109 is composed of a three-line CCD line sensor of R (red), B (blue), and G (green). Further, the motor 102 drives the first mirror unit 137 holding the illumination 203 and the mirror 104 at a speed V, and drives the second mirror unit 107 including the mirrors 105 and 106 at a speed 1 / 2V. By driving the first mirror hot water 137 and the second mirror unit 107 in this manner, the entire image of the original 100 is read.

  The image processing unit 110 processes the image information output from the CCD 109 and outputs it as a print signal to the printer control unit 111. Scanners 112, 113, 114, and 115 having semiconductor lasers emit laser light based on output signals output from the printer control unit 111. Laser light emitted from the scanners 112, 113, 114, and 115 scans the photosensitive drums 124, 125, 126, and 127 by the polygon mirrors 116, 117, 118, and 119, and forms a latent image corresponding to the output image. Developing units 123, 122, 121, and 120 develop the latent images formed on the photosensitive drums 127, 126, 125, and 124, and yellow (Y), magenta (M), cyan (C), and black (Bk). The toner image is generated. The yellow (Y), magenta (M), cyan (C), and black (Bk) toner images are transferred to the intermediate transfer belt 128 by the transfer rollers 151, 152, 153, and 154 to form a color toner image. The The intermediate transfer belt is rotated by a driving roller. The intermediate transfer belt 128 is an endless belt wound around a plurality of rollers, and is an image carrier that carries a toner image. A registration / density detection unit 150 for detecting the position and density of the toner image is provided downstream of the black photosensitive drum 124.

  The sheet fed from any of the sheet cassettes 132, 133, 134 and the manual feed tray 131 passes through the registration roller 130 and is conveyed to a nip portion formed by the intermediate transfer belt 128 and the second transfer transfer roller 155. In this nip portion, the color toner image on the intermediate transfer belt 128 is transferred onto a sheet. The photo interrupter 129 is an ITOP sensor for generating a page synchronization signal. The aforementioned paper feed timing and the start of exposure scanning of the photosensitive drum of the laser are performed in accordance with a page synchronization signal (ITOP signal) generated by this ITOP sensor.

  The sheet on which the color toner image is transferred is conveyed to the fixing device 136, and the color toner image is fixed on the sheet by the fixing device 136. The fixing device 136 includes a fixing roller 141 incorporating a halogen heater and a pressure roller 140, and fixes a color toner image on a sheet by heat and pressure. The fixed sheet is discharged onto the discharge tray 135.

  FIG. 2 is a diagram for explaining the arrangement of sensors in the registration / density detection unit 150.

  The registration / concentration detection unit 150 includes registration detection sensors 201, 202, 203 and density sensors 204, 205, 206, 207. Three registration patches and gradation patches of yellow, magenta, cyan, and black are formed at different positions in the width direction on the moving intermediate transfer belt 128. The registration sensors 201, 202, and 203 and the density sensors 204, 205, 206, and 207 are arranged corresponding to the positions of the registration patches and the gradation patches in order to read these patches.

  In the misregistration correction, three registration patches are formed on the intermediate transfer belt 128. Then, the position of the registration patch is detected using the registration sensors 201, 202, and 203. In density correction, density patches of yellow, magenta, cyan, and black are formed on the intermediate transfer belt 128, and density is detected using density sensors 204, 205, and 206.

  FIG. 3 is a diagram for explaining a configuration relating to density correction.

  The density correction is executed under the control of the control unit 206 provided in the printer control unit 111. First, the printer control unit 206 causes the patch generation unit 209 to output gradation patch image data for forming a gradation patch. The scanner is driven based on the gradation patch image data, and a toner image corresponding to the gradation patch is formed on the photosensitive drum. A toner image corresponding to the gradation patch is transferred to the intermediate transfer belt. Then, each of the density sensors 204, 205, 206, and 207 detects the density of each of the gradation patches of black, cyan, magenta, and yellow, and outputs density data.

  The control unit 206 outputs the density data to the LUT generation unit 211 and causes the LUT generation unit 211 to generate a gradation correction table (LUT) corresponding to each of black, cyan, magenta, and yellow. The LUT generation unit 211 generates a gradation correction table for each color based on the density data for each color and the target value for each color. The gradation correction table is a correction condition for correcting the image data so that the density of the toner image formed based on the image data becomes a target density corresponding to the image data. The LUT generation unit 211 sets the generated gradation correction unit 210. During normal image formation, the gradation correction unit 210 performs gradation correction on image data indicating an output image using a gradation correction table.

  FIG. 4 is a diagram showing the configuration of the density sensor. Here, the configuration of the density sensor will be described using the density sensor 207 for detecting the density of the yellow tone patch. The other density sensors 204, 205, and 206 have the same configuration.

  The density sensor 207 is an irregular reflection type density sensor, and includes an LED 404 that emits infrared light, an optical lens 406 that collects reflected light, and a photodiode 406. The light emitted from the infrared light emitting LED 404 is emitted on the intermediate transfer belt 128 at an angle of about 45 degrees.

  When there is no toner image on the intermediate transfer belt 128, the intermediate transfer belt 128 absorbs light, so that the photodiode 205 receives almost no reflected light from the intermediate transfer belt 128. On the other hand, when the toner image 308 is formed on the intermediate transfer belt, irregularly reflected light 409 having a light amount corresponding to the density of the toner image 303 is generated. The photodiode 205 receives the irregularly reflected light 409 and generates a current corresponding to the amount of received light. Then, it is converted into a voltage corresponding to the toner density by a current-voltage conversion unit (not shown) and output. The characteristic of the density sensor 207 is a linear characteristic when the horizontal axis concentration−the vertical axis voltage. Further, the output of the density sensor 207 does not exceed the saturation level Limit even at the highest density formed by the image forming apparatus.

  FIG. 5 is a flowchart showing a density correction processing procedure.

When density correction is started, the control unit 206 causes the patch generation unit 209 to output gradation patch image data for forming a gradation patch. The scanner emits laser light based on the gradation patch image data to expose the photosensitive drum. The toner image formed on the photosensitive drum is transferred to the intermediate transfer belt. In this way, magenta, cyan, yellow, and black gradation patches are formed on the intermediate transfer belt 128 (step 501). The density sensors 204, 205, 206, and 207 detect the density of the gradation patch formed on the intermediate transfer belt. Then, the control unit 206 acquires density data output from the density sensor (step 403). Based on the density data, the controller 206 determines whether the number of gradation patches whose density is detected by the density sensor is a predetermined number (step 404). If the number of gradation patches is different from the predetermined number, there is a possibility that some abnormality has occurred, and the process ends without updating the correction table (step 407). If the number of gradation patches matches the predetermined number,
The LUT generation unit 211 generates a gradation correction table for each color (step 405) and sets it in the gradation correction unit 210 (step 406).

  FIG. 6 is a diagram illustrating a configuration relating to misalignment correction.

  FIG. 6A shows the shape of a registration patch formed on the intermediate transfer belt 128. Three registration patches (601a, 601b, 601c) are formed at different positions in the width direction of the intermediate transfer belt.

  The misregistration correction is executed under the control of the control unit 602 provided in the printer control unit 111. First, the image control unit 604 outputs registration patch image data for forming registration patches to the scanner. The scanner is driven based on the registration patch image data, and a registration patch that is a toner image is formed on the photosensitive drum. The registration patches 601a, 601b, and 601c are transferred to the intermediate transfer belt. Then, each of the registration sensors 201, 202, and 203 detects a registration patch.

  The control unit 602 detects the position of each patch included in the registration patch based on the sensor outputs of the registration sensors 201, 202, and 203. The control unit 602 amplifies the sensor output, binarizes it, and determines the patch position based on the binarized edge. The registration correction parameter calculation unit 603 calculates the amount of misregistration of other colors with respect to the reference color (magenta in the present embodiment) based on the patch position, and determines registration correction parameters for controlling the image formation position. And set in the image control unit 604. At the time of normal image formation, the image control unit 604 performs correction for adjusting the image forming position on the image data output from the gradation correction unit 210 based on the registration correction parameter. Then, the corrected image data is output to the scanner. Although the image data is corrected in order to adjust the tail chart formation position, the timing at which the laser is emitted may be used based on the image data. in this case. The scanner is controlled based on the correction parameter calculated by the registration correction parameter calculation unit.

  A method of calculating the positional deviation amount will be described with reference to FIG.

  Each of FIGS. 7A, 7B, and 7C shows a part of one registration patch formed on the intermediate transfer belt 128. FIG.

  FIG. 7B shows a case where no positional deviation has occurred. The patches 710, 712, 713, and 714 are magenta patches that are reference colors in this embodiment. On the other hand, patches 711 and 714 are patches of the measurement color cyan that is a target for measuring the positional deviation with respect to the reference color. The arrows in FIG. 7 indicate the conveyance direction (rotation direction) of the intermediate transfer belt. The position of the intermediate transfer belt in the width direction indicated by the arrow indicates the measurement position of the registration sensor.

  The distance A1 indicates the distance between the detected position of the patch 710 and the detected position of the patch 711. The distance A2 indicates the distance between the detected position of the patch 711 and the detected position of the patch 712. The distance B1 indicates the distance between the detected position of the patch 713 and the detected position of the patch 714. The distance A2 indicates the distance between the detected position of the patch 714 and the detected position of the patch 715. In FIG. 7B, the distance A1 and the distance A2 are equal, and the distance B1 and the distance B2 are also equal. Therefore, the formation position of the reference color magenta and the formation position of the measurement color cyan coincide with each other with respect to the belt conveyance direction and the direction orthogonal to the belt conveyance direction.

  FIG. 7A shows a case where the measurement color cyan formation position is shifted by ΔVert in a direction orthogonal to the belt conveyance direction with respect to the reference color magenta formation position.

ΔVert is equal to ΔA and ΔB, and the following expression (1) is established.
ΔVert = (ΔA + ΔB) / 2 ………………………… (1)
ΔVert: ΔVert can be calculated using the following equation (2) according to the amount of deviation of the formation position of the measured color cyan from the formation position of the reference color magenta in the direction orthogonal to the belt transport treasure.
ΔVert = {B2-B1) / 2- (A2-A1) / 2} / 2
……………………………… (2)
FIG. 7C shows a case where the formation position of the measurement color cyan is shifted by ΔHori in the belt conveyance direction with respect to the reference color magenta formation position.
ΔHori is equal to ΔA and ΔB, and the following equation (2) is satisfied.
ΔHori = (ΔA ′ + ΔB ′) / 2 (3)
Therefore, ΔHori can be obtained using the following equation (4).
ΔHori = {(B2−B1) / 2 + (A2−A1) / 2} / 2
……………………………… (4)

  FIG. 8 is a diagram illustrating the configuration of the registration sensor. Here, the configuration of the registration sensor will be described using the registration sensor 201. The other registration sensors 202 and 203 have the same configuration.

  The registration sensor 201 is a diffuse reflection type sensor, and includes an infrared light emitting LED 803, an optical lens 805 that collects reflected light, and a photodiode 804. The light emitted from the infrared light emitting LED 803 is emitted onto the transfer belt 701 at an angle of about 45 degrees.

  When there is no toner image on the intermediate transfer belt 128, the intermediate transfer belt 128 absorbs light, so that the photodiode 804 receives almost no reflected light from the intermediate transfer belt 128. On the other hand, when the toner image 808 is formed on the intermediate transfer belt, irregularly reflected light 809 having a light amount corresponding to the density of the toner image 808 is generated. The photodiode 805 receives the irregularly reflected light 809 and generates a current corresponding to the amount of received light. Then, it is converted into a voltage corresponding to the toner density by a current-voltage conversion unit (not shown) and output.

  FIG. 9 is a diagram showing the relationship between the analog output of the registration sensor and the binarization process.

  In FIG. 9A, an analog output 901 is an analog output when the registration patch density is a target density. An analog output 902 is an analog output when the density of the registration patch is lower than the target density.

  A voltage 903 is a reference voltage that serves as a threshold condition for binarization. When the analog output voltage is lower than the reference voltage, “L” is output. When the analog output voltage is higher than the reference voltage, “H” is output. The analog output 904 is a voltage waveform after binarization. Here, ideally, a waveform is shown, and the sensor analog output is a triangular wave. Therefore, even if the peak level changes due to the toner concentration, the center of gravity position 950 does not change. An ideal waveform is obtained when the density in the registration patch is uniform.

  FIG. 9B shows an actual toner concentration waveform. An analog output 906 is an analog waveform of the registration sensor when an actual toner image is detected. Actually, the toner image is not uniform, and density changes occur in a plane where the leading edge is dark and the trailing edge is thin. Furthermore, the ideal triangular wave 905 is not obtained due to error factors such as delay of the electric element and deviation of the sensor optical axis. A voltage waveform 904 shows the result (dotted line) obtained by binarizing the actual toner density waveform 906. The barycentric position is shifted by ΔERR1 (809) with respect to the barycentric position of the ideal waveform binarization result. As an error, for example, it is known from experiments that a deviation of 30 to 50 μm occurs.

  Next, a result obtained by amplifying the analog output by applying a gain is shown in a waveform 911. An output result of the amplifier (AMP) is shown in a waveform 910. Since the portion of the dotted line of the waveform 911 exceeds the output voltage range (saturation level, maximum output value) of the amplifier, it is limited to the maximum output voltage value, and the output result of the amplifier is as shown in the waveform 911. A voltage waveform 912 shows the result of binarization. In the misalignment correction, the edge portion of the output waveform of the registration sensor is important and information on the peak level of the output waveform is not necessary. Thus, by amplifying the analog output and then binarizing, it is possible to detect the edge immediately after the measurement position of the registration sensor enters the front end of the registration patch and immediately before the rear end of the registration patch. When binarization is performed after amplification, the error with respect to the ideal center of gravity is ΔERR2, and ΔERR1 >> ΔERR2. In the binarization result, it is possible to considerably reduce the influence of the density change in the plane where the front end is dark and the rear end is thin.

  FIG. 9C shows processing performed on the analog output of the photodiode. First. The offset correction unit 920 performs zero level correction on the analog output from the photodiode. Next, the amplification processing unit 921 performs amplification processing based on the gain value (amplification condition), and the binarization processing unit performs binarization processing based on the reference voltage.

  The binarized voltage value is output to the control unit 602 as digital data. The control unit 602 detects the rising edge and falling edge of the binary data for each patch, and calculates the position of the center of gravity of the patch from the rising edge position and the falling edge position. The calculated barycentric position becomes the patch formation position data. That is, it is used in equations (2) and (4).

  FIG. 10 is a flowchart illustrating a processing procedure for positional deviation correction.

  When the registration correction is started, first, the control unit 602 causes the registration sensor to read the background of the transfer belt, and sets an offset value for correcting the zero level of the sensor from the reading result of the background (step 1001).

  Next, the control unit 602 forms yellow, magenta, cyan, and black test patches, causes the registration sensor to read them, and obtains the output value of the registration sensor processed by the set offset value (step 1002). At this time, a default gain value is set for the amplifier that amplifies the analog output of the registration sensor. As the test patch, the color patch of the registration patch described in FIG. 6 is used. Note that one set of registration patches may be used.

  The control unit 602 determines whether the output value of the test patch registration sensor is saturated (step 1003). Specifically, it is determined whether or not the maximum output value of each color is the maximum output value of the registration sensor. When it is determined that the output value of at least one color is not saturated (NO in step 1003), the control unit 602 increases the gain value of the amplifier (step 1007). Specifically, the minimum value is determined from the maximum output values that are not saturated, and the gain value is calculated in accordance with the determined minimum value. A predetermined value may be added to the default gain value.

  When it is determined that the output value is saturated, the control unit 602 determines a reference voltage serving as a threshold value used for binarization from the maximum value of the output value of the test patch for each color (step 1004). Specifically, an average value of the maximum output values of the test patches for each color is calculated, and a predetermined ratio of the calculated average value is determined as a threshold value. In this embodiment, 10% is used as the predetermined ratio. This 10% is a value determined based on an experiment at the time of design, and is a value determined according to the product.

  Next, the control unit 602 forms the registration patch shown in FIG. 6 and causes the registration sensor to read the registration patch. The analog output value of the registration sensor is amplified by a gain value set in the amplifier, and binarized at a set threshold value. From the binarization result, yellow, magenta, cyan, and black patch formation positions are calculated. Then, the registration correction parameter calculation unit 603 calculates the positional deviation amounts (ΔVert and ΔHori) of the measurement colors yellow, cyan, and black with respect to the reference color magenta using the above equations (2) and (4), and the image Set in the control unit 604.

  According to this embodiment, due to the developability and transferability peculiar to the image forming apparatus, the accuracy of the misregistration correction is reduced due to the change in density in the surface of the registration patch for misregistration correction. Can be suppressed. Therefore, color misregistration in the output image can be reduced and a high-quality output image can be formed.

201, 202, 203 Registration sensor 602 Control unit 603 Registration correction parameter calculation unit

Claims (5)

First image forming means for forming a first color toner image;
A second image forming unit disposed downstream of the first image forming unit to form a second color toner image;
An image carrier onto which the first color toner image formed by the first image forming unit and the second color toner image formed by the second image forming unit are transferred;
A sensor that irradiates light toward the image carrier and receives reflected light from the image carrier;
Amplifying means for amplifying the output value of the sensor based on amplification conditions;
Determining means for determining the positions of the toner images of the first color and the second color from the amplified output values;
Based on the positions of the first and second color toner images, the second color toner image is positioned so that the position of the second color toner image matches the position of the first color toner image. Control means for controlling the formation position of the toner image ;
The result of amplifying the sensor output value of the test patch formed by the first image forming means by the amplification means and the output value of the sensor of the test patch formed by the second image forming means are amplified. An image forming apparatus comprising: an adjusting unit configured to adjust the amplification condition so that a result amplified by the unit is saturated. The determining means includes
Based on the threshold condition, the amplified output value is binarized,
The image forming apparatus according to claim 1, wherein a center of gravity position of the toner image is obtained as a position of the toner image based on the binarization result. The adjusting means includes a maximum value obtained by amplifying the output value of the sensor of the test patch formed by the first image forming means by the amplifying means and a test patch formed by the second image forming means. 3. The image forming apparatus according to claim 1, wherein the amplification condition is adjusted such that a maximum value obtained by amplifying the output value of the sensor by the amplification unit becomes a maximum output value. 4. Furthermore, an offset correction unit that corrects the output value of the sensor based on an offset value;
Based on the output value from the sensor at the time of receiving the reflected light from the underlying of the image bearing member, according to any one of claims 1 to 3, characterized in that it has a setting means for setting the offset value Image forming apparatus. And density detecting means for detecting the density of the toner images of the first color and the second color;
A gradation correction table of the first color is generated based on the detected density of the first color toner image, and the second color of the second color is corrected based on the detected density of the second color toner image. The image forming apparatus according to claim 1, further comprising: a generation unit configured to generate a gradation correction table.

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