JP2019171822A - Image formation device - Google Patents

Abstract

To suitably decide timing for changeover of a threshold.SOLUTION: An image formation device 1 has: output means 40 which outputs an output value corresponding to a detection result of reflectance from a pattern which is formed on an image carrier 8 by image formation means; and control means 70 which converts a first output value, which is corresponding to a first color shift detection pattern of a first color formed by the image formation means, on the basis of a first threshold, converts a second output value, which is corresponding to a second color shift detection pattern of a second color, on the basis of a second threshold, and controls color shift of an image, which is formed by the image formation means, on the basis of the converted first output value and the converted second output value. The control means decides timing for converting a threshold from the first threshold to the second threshold on the basis of the output value corresponding to a pattern for setting a threshold of the first color formed by the image formation means, the output value corresponding to a pattern for setting a threshold of the second color, and the output value corresponding to the detection result of reflectance from the image carrier.SELECTED DRAWING: Figure 8

Description

  The present invention relates to color misregistration correction control of an image forming apparatus.

  The electrophotographic image forming apparatus forms yellow (Y), magenta (M), cyan (C) and black (K) images by an electrophotographic process including charging, exposure, development and transfer, and superimposes these images. To form a full color image. The image forming apparatus measures a color misregistration detection pattern for detecting a color misregistration to prevent a misregistration (color misregistration) of each color image, and detects the image forming positions of a plurality of colors as the detected color. Control based on the deviation. The detection of the color misregistration amount of the color misregistration detection pattern is performed as follows, for example. A color shift detection pattern is formed on the image carrier, and reflected light from the color shift detection pattern is measured by a color shift detector. Then, by detecting the color misregistration detection pattern, the detection timing of the color misregistration detection pattern is obtained based on the detection signal output from the color misregistration detector, and the color misregistration amount is calculated based on the detection timing for each color.

  FIG. 9 is an explanatory diagram of the influence of the light intensity distribution of the light irradiated on the image carrier from the color misregistration detector on the detection signal. FIGS. 9A and 9D are diagrams showing a light intensity distribution by a color misregistration detection pattern and a color misregistration detector formed on the image carrier. The light intensity distribution is formed by connecting points of the same light intensity with lines like contour lines in a map, and the light intensity increases toward the center. FIG. 9B and FIG. 9E are diagrams illustrating detection signals output from the color misregistration detector. 9C and 9F show detection signals as threshold values (b) -1, (b) -2, (b) -3, (e) -1, (e) -2, and (e). It is a figure which shows the binarization signal binarized by -3. The color misregistration detector uses a detection system using a regular reflection optical system. For this reason, since the reflected light from the image carrier having a high glossiness has a large amount of light, the detection signal of the color misregistration detector that receives the reflected light from the image carrier has a high voltage level. On the other hand, when the color misregistration detection pattern enters the light irradiation area of the color misregistration detector, the reflected light from the image carrier is blocked, and the voltage level of the detection signal from the color misregistration detector decreases. In order to obtain the position information of the color misregistration detection pattern from the waveform of the detection signal output from the color misregistration detector, the waveform is binarized using a threshold value, and an intermediate between the rising edge and the falling edge of the binarized signal Generally, a point is acquired as the position of the color misregistration detection pattern. Here, the light intensity distribution shown in FIG. 9A is axisymmetric with respect to a direction (dotted line in FIG. 9A) perpendicular to the passing direction of the color misregistration detection pattern. When the color misregistration detection pattern passes on this light intensity distribution, the detection signal of the color misregistration detector has a symmetrical waveform as shown in FIG. On the other hand, the light intensity distribution shown in FIG. 9D is not line symmetric with respect to the direction perpendicular to the passing direction of the color misregistration detection pattern (dotted line in FIG. 9D). When the color misregistration detection pattern passes on this light intensity distribution, the detection signal of the color misregistration detector has a left-right asymmetric waveform as shown in FIG. At a point where the color misregistration detection pattern starts to enter a region of light intensity distribution that is not line symmetric, the decrease in the amount of light received by the color misregistration detector is small, and the decrease in the detection signal of the color misregistration detector is gentle. Thereafter, when the color misregistration detection pattern advances in the region of the light intensity distribution, the decrease in the amount of light received by the color misregistration detector increases rapidly, and the detection signal of the color misregistration detector decreases sharply.

  The voltage levels of the threshold values (b) -1, (b) -2, and (b) -3 shown in FIG. 9B are set to 75% level, 50% level, and 25% level with respect to the amplitude of the detection signal, respectively. . When the detection signal having a symmetrical waveform shown in FIG. 9B is binarized by threshold values (b) -1, (b) -2, and (b) -3, binarization is performed as shown in FIG. 9C. Signals (c) -1, (c) -2, and (c) -3 are generated. Positions (c) -reg-1, (c) -reg-2, and (c) -reg of color misregistration detection patterns obtained from the binarized signals (c) -1, (c) -2, and (c) -3 -3 remains at the same position even if the threshold value changes.

  Similarly, the voltage levels of the threshold values (e) -1, (e) -2, and (e) -3 shown in FIG. 9 (e) are set to 75%, 50%, and 25% with respect to the amplitude of the detection signal, respectively. Level. When the detection signal having a left-right asymmetric waveform shown in FIG. 9E is binarized by threshold values (e) -1, (e) -2, and (e) -3, a binary signal is obtained as shown in FIG. 9F. Signals (f) -1, (f) -2 and (f) -3 are generated. Positions (f) -reg-1, (f) -reg-2, and (f) -reg of the color misregistration detection patterns obtained from the binarized signals (f) -1, (f) -2, and (f) -3 -3 changes when the threshold value changes. That is, in the case of a detection signal with a left-right asymmetric waveform, an error occurs in the detection position of the color misregistration detection pattern due to a change in the threshold value.

  Here, when a plurality of color misregistration detection patterns are detected, the amplitude of the detection signal output from the color misregistration detector may be different. FIG. 10 is a diagram illustrating detection signals having different amplitudes. The amplitude of the detection signal varies depending on the variation in the amount of applied toner in the color misregistration detection pattern and the reflection characteristics of the color misregistration detection pattern for each color. FIG. 10A shows a case where the amplitude Va of the detection signal PTa of the color misregistration detection pattern and the amplitude Vb of the detection signal PTb of the color misregistration detection pattern have a relationship of Va = Vb × 1.3 as an example. In this case, when the detection signals PTa and PTb are binarized by the threshold value Vth that is 50% of the amplitude Va of the detection signal PTa, the threshold value Vth becomes 38% of the amplitude Vb of the detection signal PTb. As described above, the threshold Vth is different in the ratio of the detection signal PTa to the amplitude Va and the ratio of the detection signal PTb to the amplitude Vb. If the ratio of the threshold to the amplitude of the detection signal is different, a detection error of the position of the color misregistration detection pattern occurs.

  For such a problem, as shown in FIG. 10B, Patent Document 1 discloses that when the amplitude of the detection signal changes from the amplitude Va to the amplitude Vb, the threshold Vth is a constant ratio of the amplitude (for example, The position is controlled to be 50% of the amplitude). This reduces the occurrence of a detection error in the position of the color misregistration detection pattern even if the detection signal has a left-right asymmetric waveform.

JP 2013-25184 A

However, as illustrated in FIG. 10C, when the threshold value Vth_a is switched to the next threshold value Vth_b immediately after the binarization of the detection signal PTa of the color misregistration detection pattern is completed, the next detection signal PTb is detected. The next threshold value Vth_b crosses the detection signal PTa before. Therefore, an unnecessary pulse is generated in the binarized signal. If the position of the color misregistration detection pattern is detected based on the unnecessary pulse, the color misregistration amount cannot be calculated correctly. In addition, the amplitude of the detection signal of the color misregistration detector may differ due to variations in the amount of applied toner in the color misregistration detection pattern. FIG. 11 is a diagram showing detection signals and binarized signals having different amplitudes Vc and Vd. As described above, by setting the threshold value Vth_c and the threshold value Vth_d at a constant ratio of the amplitude to the amplitude Vc and the amplitude Vd, the width of the waveform of the binarized signal does not change. However, the ratio of the next threshold value Vth_next to the amplitude Vc of the detection signal PTc is different from the ratio of the next threshold value Vth_next to the amplitude Vd of the detection signal PTd. Therefore, the time T1 for the detection signal PTc to reach the next threshold value Vth_next is different from the time T2 for the detection signal PTd to reach the next threshold value Vth_next.
Accordingly, an object of the present invention is to appropriately determine the timing for switching the threshold value.

An image forming apparatus according to an embodiment of the present invention includes:
An image carrier that rotates in a predetermined direction;
A plurality of image forming means for forming images of different colors on the image carrier;
A transfer portion where the image formed on the image carrier is transferred to a sheet;
Output means for detecting reflected light from the pattern formed on the image carrier by the plurality of image forming means and outputting an output value corresponding to the detection result;
Conversion means for converting the output value of the output means based on a threshold;
The plurality of image forming means form different color misregistration detection patterns, the output means outputs an output value corresponding to the detection result of reflected light from the color misregistration detection pattern, and the conversion means outputs the color misregistration pattern. The first output value corresponding to the first color misregistration detection pattern of the first color in the detection pattern is converted based on the first threshold value, and is different from the first color in the color misregistration detection pattern by the converting means. A second output value corresponding to the second color misregistration detection pattern of the second color is converted based on a second threshold value, and the plurality of the output values are converted based on the converted first output value and the converted second output value. Control means for controlling color misregistration of an image formed by the image forming means,
The first color misregistration detection pattern is formed downstream of the second color misregistration detection pattern in the predetermined direction,
The second color misregistration detection pattern is formed adjacent to the first color misregistration detection pattern in the predetermined direction;
The control unit causes the plurality of image forming units to form threshold value setting patterns of different colors, causes the output unit to output an output value corresponding to a detection result of reflected light from the threshold value setting pattern, and the conversion An output value corresponding to the first color threshold setting pattern in the threshold setting pattern, an output value corresponding to the second color threshold setting pattern in the threshold setting pattern, and the image The timing for changing the threshold value of the conversion means from the first threshold value to the second threshold value is determined based on an output value corresponding to a detection result of reflected light from the carrier.

  According to the present invention, it is possible to appropriately determine the timing for switching the threshold.

1 is a cross-sectional view of an image forming apparatus. The figure which shows a color shift detector. 1 is a diagram illustrating a control system of an image forming apparatus. The figure which shows a color shift detection pattern and a detection signal. The figure which shows the threshold value and threshold value switching time setting sequence which are performed by CPU. The figure which shows the pattern for threshold value setting, and a detection signal. 6 is a flowchart showing a color misregistration amount acquisition sequence executed by a CPU. The figure which shows the analog detection signal of a color shift detector, and the detection signal of a comparator. Explanatory drawing of the influence which the light intensity distribution of the light output from a color shift detector has on a detection signal. The figure which shows the detection signal which has a different amplitude. The figure which shows the detection signal and binarization signal which have different amplitude.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings.

(Image forming device)
With reference to FIG. 1, an image forming apparatus 1 as an example of an embodiment of the present invention will be described. The image forming apparatus 1 is a color image forming apparatus including a plurality of color image forming units such as a copying machine, a printer, or a facsimile. In this embodiment, the image forming apparatus 1 will be described by taking a tandem color electrophotographic printer as an example. FIG. 1 is a cross-sectional view of the image forming apparatus 1. The image forming apparatus 1 includes a plurality of image forming units 31Y, 31M, 31C, and 31K. The plurality of image forming units 31Y, 31M, 31C, and 31K form toner images of respective colors on the intermediate transfer belt 8 as an image carrier. The image forming unit 31Y forms a yellow image using yellow toner. The image forming unit 31M forms a magenta image using magenta toner. The image forming unit 31C forms a cyan image using cyan toner. The image forming unit 31K forms a black image using black toner. The four image forming units 31Y, 31M, 31C, and 31K have the same structure except for the color of the developer (toner). Therefore, in the following description, the subscripts Y, M , C and K are omitted.

  The image forming unit 31 includes a rotatable photosensitive member (hereinafter referred to as a photosensitive drum) 2. The photosensitive drum 2 is rotated in a predetermined direction indicated by an arrow R1 in FIG. 1 during image formation. Around the photosensitive drum 2, a charger 3, an optical scanning device (laser scanner) 5, a developing device 7, a primary transfer member 6 and a drum cleaning device 4 are arranged. An intermediate transfer belt 8 is disposed below the photosensitive drum 2. The rotatable intermediate transfer belt 8 is stretched around a support roller 10, a drive roller 11, and a secondary transfer counter roller 21. The intermediate transfer belt 8 includes a belt cleaner 12 having an elastic blade. The intermediate transfer belt 8 is rotated in a predetermined direction indicated by an arrow R2 in FIG. 1 during image formation. A yellow image forming unit 31Y, a magenta image forming unit 31M, a cyan image forming unit 31C, and a black image forming unit 31K are sequentially arranged along the rotation direction of the intermediate transfer belt 8 indicated by the arrow R2.

  The primary transfer member 6 is disposed to face the photosensitive drum 2 via the intermediate transfer belt 8, and forms a primary transfer portion PTR between the intermediate transfer belt 8 and the photosensitive drum 2. The secondary transfer roller 22 is disposed to face the secondary transfer counter roller 21 with the intermediate transfer belt 8 interposed therebetween. The secondary transfer roller 22 forms a secondary transfer portion STR between the intermediate transfer belt 8 and the secondary transfer roller 22. A fixing device 26 is provided downstream of the secondary transfer portion STR in the conveyance direction CD of a recording medium (hereinafter referred to as a sheet) S. A discharge roller 24 and a discharge tray 25 are provided downstream of the fixing device 26 in the discharge direction DD of the sheet S discharged from the fixing device 26.

  The feeding cassette 17 in which the sheet S is stored is disposed in the lower part of the image forming apparatus 1. The sheets S stored in the feeding cassette 17 are fed in order from the uppermost sheet by the pickup roller 18. The sheets S fed by the pickup roller 18 are separated one by one by the separation roller 19. The sheet S is conveyed to the electrostatic conveyance means 30 by the vertical pass roller 20. The pickup roller 18, separation roller 19, vertical pass roller 20 and registration roller 16 are each driven by an independent stepping motor in order to realize a high-speed and stable conveying operation. The pickup roller 18, the separation roller 19, the vertical pass roller 20, and the registration roller 16 constitute a sheet conveyance unit that conveys the sheet S from the feeding cassette 17 to the conveyance path 32.

  The manual feed tray 13 is disposed on the side portion of the image forming apparatus 1. The sheets S placed on the manual feed tray 13 are sequentially fed from the uppermost sheet by the pickup roller 14. The sheets S fed by the pickup roller 14 are separated one by one by the separation roller 15 and conveyed to the electrostatic conveying means 30. The pickup roller 14 and the separation roller 15 are driven by independent stepping motors. The pickup roller 14 and the separation roller 15 constitute a sheet conveyance unit that conveys the sheet S from the manual feed tray 13 to the conveyance path 32.

  The electrostatic conveyance unit 30 corrects the position of the sheet S in the sheet width direction orthogonal to the conveyance direction CD. The sheet S is conveyed to the stopped registration roller pair 16. The leading end of the sheet S hits the nip of the registration roller pair 16, and the sheet S forms a loop and is temporarily stopped. As the sheet S forms a loop, the skew of the sheet S is corrected.

(Image formation process)
Since the image forming processes in the four image forming units 31Y, 31M, 31C, and 31K are the same, the image forming process in the yellow image forming unit 31Y will be mainly described. The photosensitive drum 2Y is rotated in a predetermined direction indicated by an arrow R1. The charger 3Y uniformly charges the surface of the photosensitive drum 2Y. The optical scanning device 5Y emits laser light (hereinafter referred to as a light beam) LY modulated in accordance with yellow image information from a light source (semiconductor laser) to the surface of the uniformly charged photosensitive drum 2Y, and onto the photosensitive drum 2Y. An electrostatic latent image is formed. The developing device 7Y develops the electrostatic latent image with yellow toner (developer) to form a yellow toner image. The primary transfer member 6Y primarily transfers the yellow toner image on the photosensitive drum 2Y onto the intermediate transfer belt 8 at the primary transfer portion PTRY. The toner remaining on the photosensitive drum 2Y after the primary transfer is removed by the drum cleaning device 4Y.

  After a predetermined time has elapsed since the scanning of the light beam LY on the photosensitive drum 2Y, the optical scanning device 5M starts scanning the light beam LM modulated according to the magenta image information on the photosensitive drum 2M, and the photosensitive drum 2M. An electrostatic latent image is formed thereon. The electrostatic latent image is developed with magenta toner by the developing unit 7M to become a magenta toner image. The magenta toner image is transferred onto the yellow toner image on the intermediate transfer belt 8 by the primary transfer member 6M at the primary transfer portion PTRM.

  After a predetermined time has elapsed since the scanning of the light beam LM on the photosensitive drum 2M, the optical scanning device 5C starts scanning the light beam LC modulated according to the cyan image information on the photosensitive drum 2C, and the photosensitive drum 2C. An electrostatic latent image is formed thereon. The electrostatic latent image is developed with cyan toner by the developing device 7C to become a cyan toner image. The cyan toner image is transferred onto the magenta toner image on the intermediate transfer belt 8 by the primary transfer member 6C at the primary transfer portion PTRC.

  After a predetermined time has elapsed since the scanning of the light beam LC on the photosensitive drum 2C, the optical scanning device 5K starts scanning the light beam LK modulated according to the black image information on the photosensitive drum 2K, and the photosensitive drum 2K. An electrostatic latent image is formed thereon. The electrostatic latent image is developed with black toner by the developing device 7K to become a black toner image. The black toner image is transferred onto the cyan toner image on the intermediate transfer belt 8 by the primary transfer member 6K in the primary transfer portion PTRK.

  In this way, four color toner images are superimposed on the intermediate transfer belt 8. The sheet S conveyed from the feeding cassette 17 or the manual feed tray 13 is conveyed by the registration roller pair 16 to the secondary transfer unit STR in synchronization with the toner image on the intermediate transfer belt 8. The four color toner images superimposed on the intermediate transfer belt 8 are secondarily transferred onto the sheet S at once by the secondary transfer roller 22 in the secondary transfer portion STR. The toner remaining on the intermediate transfer belt 8 without being transferred to the sheet S in the secondary transfer portion STR is removed by the belt cleaner 12. The sheet S on which the four-color toner images are collectively transferred by the secondary transfer unit STR is conveyed to the fixing device 26. The fixing device 26 heats and pressurizes the sheet S to thermally fix the toner image on the sheet S. A full-color image is formed on the sheet S. The sheet S on which the image is formed is discharged to the discharge tray 25 by the discharge roller 24.

  When the duplex printing mode for forming images on both sides of the sheet S is set, the sheet S that has passed through the fixing device 26 is transported by the flapper 33 and is temporarily conveyed from the discharge roller 24 to the duplex reversing path 27. Is done. Thereafter, the sheet S is conveyed in the reverse direction and conveyed to the duplex path 28. Thereby, the front and back of the sheet S are reversed. The sheet S that has passed the double-sided path 28 passes through the vertical path roller 20 again and is conveyed to the secondary transfer unit STR. The toner images are collectively transferred from the intermediate transfer belt 8 to the back surface of the sheet S at the secondary transfer portion STR. The toner image is fixed on the back surface of the sheet S by the fixing device 26. In this way, the sheet S on which images are formed on both sides is discharged to the discharge tray 25 by the discharge roller 24.

(Color shift detector)
In the image forming apparatus 1, an optical sensor (hereinafter referred to as a color misregistration detector) 40 is provided above the intermediate transfer belt 8 between the support roller 10 and the photosensitive drum 2K. The image forming units 31Y, 31M, 31C, and 31K form color misregistration detection patterns on the photosensitive drums 2Y, 2M, 2C, and 2K as toner patterns of the respective colors. The color misregistration detection patterns on the photosensitive drums 2Y, 2M, 2C, and 2K are transferred onto the intermediate transfer belt 8. The color misregistration detection pattern is a detection image used for detecting color misregistration. The color misregistration detector (output means) 40 detects a color misregistration detection pattern transferred from the photosensitive drums 2Y, 2M, 2C, and 2K onto the intermediate transfer belt 8. The amount of color misregistration is calculated based on the detection result of the color misregistration detector 40.

  FIG. 2 is a diagram illustrating the color misregistration detector 40. The color misregistration detector 40 includes a light emitting unit 51 and a light receiving unit 52 as shown in FIG. The light receiving unit 52 receives the regular reflection light of the object that has received the light emitted from the light emitting unit 51. The light emitted from the light emitting unit 51 is reflected by the intermediate transfer belt 8 or the color misregistration detection pattern on the intermediate transfer belt 8 at the position facing the light emitting unit 51. Reflected light from the intermediate transfer belt 8 or the color misregistration detection pattern is collected by the lens 53 and enters the light receiving unit 52. The light receiving unit 52 outputs an analog detection signal having a voltage (output value) corresponding to the amount of reflected light received. The reflectance of the intermediate transfer belt 8 is higher than the reflectance of the color misregistration detection pattern. Accordingly, the voltage output from the color misregistration detector 40 that has received reflected light from the color misregistration detection pattern is lower than the voltage output from the color misregistration detector 40 that has received reflected light from the intermediate transfer belt 8. The waveform of the voltage (analog detection signal) output from the color misregistration detector 40 drops at a position corresponding to the color misregistration detection pattern. The position of the color misregistration detection pattern is calculated by binarizing the analog voltage waveform output from the color misregistration detector 40 based on a predetermined threshold.

(Control system)
Next, the control system 101 of the image forming apparatus 1 will be described with reference to FIG. FIG. 3 is a diagram illustrating the control system 101 of the image forming apparatus 1. The control system 101 includes a CPU 70, a color misregistration detector 40, a comparator 72, a ROM 73, an image processing control unit 74, a laser scanning unit control unit 75, a bottom hold circuit 76, a threshold adjustment unit 77, and a RAM 78. The color misregistration detector 40 detects a color misregistration detection pattern on the intermediate transfer belt 8 and outputs an analog detection signal to the comparator 72. The comparator 72 converts the analog detection signal into a binarized signal based on the threshold signal generated by the threshold adjustment unit 77 and outputs the binarized signal to the CPU 70. If the analog detection signal is equal to or greater than the threshold value, the binarized signal is at the low level. If reflected light from the intermediate transfer belt 8 is detected by the color misregistration detector 40, the comparator 72 outputs a low level binary signal. On the other hand, if the analog detection signal is less than the threshold value, the binarized signal is at a high level. If reflected light from the pattern is detected by the color misregistration detector 40, the comparator 72 outputs a high level binarized signal.

  The CPU 70 includes a threshold adjustment control unit 711, a pattern reading unit 712, a color misregistration amount calculation unit 713, a light emission control unit 714, an A / D converter 715, a color misregistration pattern formation unit 716, and a threshold switching timing generation unit 717. The threshold adjustment control unit 711 controls the threshold adjustment unit 77 that generates a threshold signal to the comparator 72. The threshold adjustment unit 77 is controlled by the threshold adjustment control unit 711 so as to set a threshold to the comparator 72 for each color of the plurality of color misregistration detection patterns. The pattern reading unit 712 detects the timing at which the binarized signal changes from Low level to High level (rising edge) and the timing at which the binarized signal changes from High level to Low level (falling edge). The color misregistration amount calculation unit 713 calculates the color misregistration amount based on the rising edge and the falling edge calculated by the pattern reading unit 712. The ROM 73 stores the threshold value set in the comparator 72 and the color misregistration amount calculated by the color misregistration amount calculation unit 713. The ROM 73 is a memory that can be written and read. The ROM 73 is, for example, an EEPROM. The CPU 70 reads the color misregistration amount from the ROM 73 at startup and sets the color misregistration amount in the image processing control unit 74. The image processing control unit 74 performs image processing on the image data to control color misregistration. For example, the image processing control unit 74 performs image processing on the image data so that the writing position of the image to be formed on the sheet is corrected. For example, the image processing control unit 74 performs image processing on the image data so that the magnification of the image to be formed on the sheet is corrected. The image forming apparatus 1 of the present embodiment has a configuration in which the color misregistration is controlled by the image processing control unit 74. However, for example, color correction is performed by correcting the inclination of the lens and mirror included in the optical scanning device 5 based on the color misregistration amount. The deviation may be controlled. Since the color misregistration correction is a well-known technique, a detailed description thereof is omitted here. The light emission control unit 714 controls light emission of the light emitting unit 51 of the color misregistration detector 40. The A / D converter 715 converts an analog signal into a digital signal. The color misregistration pattern forming unit 716 has pattern image data when forming a color misregistration detection pattern and a threshold setting pattern, and sends the pattern image data to the laser scanning unit control unit 75 when forming the color misregistration detection pattern. . The bottom hold circuit 76 is used when sampling the value of the detection signal in a threshold value and threshold value switching time setting sequence described later. The threshold switching timing generation unit 717 is used in threshold switching control described later. The RAM 78 is used as a temporary data storage area.

(Color misregistration calculation method)
Here, a color misregistration calculation method performed by the color misregistration amount calculation unit 713 will be described with reference to FIG. FIG. 4 is a diagram illustrating the color misregistration detection patterns 801, 802, 803, 804, 811, 812, 813, and 814 and the detection signal 800. FIG. 4A is a diagram illustrating the color misregistration detection patterns 801, 802, 803, 804, 811, 812, 813, and 814 in the present embodiment. In FIG. 4A, the main scanning direction is a direction in which the light beam of the optical scanning device 5 scans the photosensitive drum 2. Alternatively, the main scanning direction is a direction (axial direction) parallel to the rotation axis of the photosensitive drum 2. In FIG. 4A, the sub-scanning direction is a direction orthogonal to the main scanning direction. Note that the sub-scanning direction on the intermediate transfer belt 8 is a direction in which an image is conveyed. FIG. 4B is a diagram illustrating a detection signal (binary signal) 800 obtained by binarizing the analog detection signal output from the color shift detector 40 that has read the color shift detection patterns 801 to 804 and 811 to 814. is there. A broken line in the detection signal 800 indicates a center position between the rising edge and the falling edge. The color misregistration detection patterns 801 to 804 and 811 to 814 are formed from a yellow pattern (801, 811), a magenta pattern (802, 812), a cyan pattern (803, 813), and a black pattern (804, 814). Is done. Each of the color misregistration detection patterns 801 to 804 and 811 to 814 is formed with an inclination of 45 ° with respect to the main scanning direction. The inclination direction of the color misregistration detection patterns 801 to 804 is opposite to the inclination direction of the color misregistration detection patterns 811 to 814. The time information ym_1, yc_1, and yk_1 indicate the time difference between the detection signal of the yellow color shift detection pattern 801, which is the reference color in this embodiment, and the detection signals of the other color shift detection patterns 802, 803, and 804. Similarly, the time information ym_2, yc_2, and yk_2 indicate the time difference between the detection signal of the yellow color shift detection pattern 811 that is the reference color in this embodiment and the detection signals of the other color shift detection patterns 812, 813, and 814. . The color misregistration amount calculation unit 713 calculates the color misregistration amount based on the time information ym_1, yc_1, yk_1, ym_2, yc_2, and yk_2.

For example, calculation of a magenta color misregistration amount will be described. When the magenta color misregistration detection patterns 802 and 812 are deviated to the (+) side in the sub-scanning direction, the time information ym_1 and the time information ym_2 are proportional to the color misregistration amount and increase by the same amount. When the magenta color shift detection patterns 802 and 812 are shifted to the (−) side in the sub-scanning direction, the time information ym_1 and the time information ym_2 are reduced by the same amount in proportion to the color shift amount. On the other hand, when the magenta color misregistration detection patterns 802 and 812 are deviated to the (+) side in the main scanning direction, the time information ym_1 increases in proportion to the color misregistration amount, and conversely, the time information ym_2 is proportional to the color misregistration amount. And get smaller. When the magenta color misregistration detection patterns 802 and 812 are deviated to the (−) side in the main scanning direction, the time information ym_1 is decreased in proportion to the color misregistration amount, and conversely, the time information ym_2 is proportional to the color misregistration amount. growing. Accordingly, the sub-scanning color misregistration amount and the main scanning color misregistration amount are calculated from the following equations (1) and (2).
Sub-scanning color misregistration amount = X− (ym — 1 + ym — 2) / 2 × conveying speed (1)
Main scanning color misregistration amount = (ym_1−ym_2) / 2 × conveying speed (2)

  X in the equation (1) is distance interval information in the sub-scanning direction of the yellow color misregistration detection pattern 801 (or 811) and the magenta color misregistration detection pattern 802 (or 812) when no color misregistration occurs. The time information ym_1 (sec) and the time information ym_2 (sec) are obtained by multiplying the conveyance speed (mm / sec) of the intermediate transfer belt 8 on which the color misregistration detection patterns 801 to 804 and 811 to 814 are formed by multiplying the distance information (mm ). Expressions (1) and (2) are described by taking magenta color misregistration amount as an example, but the time information yc_1, yk_1, yc_2, yk_2 of other colors is similarly used for the color misregistration amounts of other colors. Can be derived.

[Threshold and threshold switching time setting sequence]
Next, using FIG. 5, the threshold for binarizing the analog detection signal of the color misregistration detector 40 and the analog detection signal reach the next threshold for binarizing the next analog detection signal. A sequence for setting a threshold switching time which is a waiting time until will be described. FIG. 5 is a flowchart showing a threshold value and threshold value switching time setting sequence executed by the CPU 70. The CPU 70 executes a threshold value and threshold value switching time setting sequence in accordance with a program stored in the ROM 703. The CPU 70 starts a threshold value and threshold value switching time setting sequence when the main power is turned on or after a predetermined number of images have been formed.

  When the threshold value and threshold value switching time setting sequence is started, the CPU 70 starts the rotation of the intermediate transfer belt 8 (S1001). The CPU 70 turns on the light emitting unit 51 of the color misregistration detector 40 (S1002). The CPU 70 samples the detection signal (image carrier analog detection signal) output from the color misregistration detector 40 that receives the reflected light from the intermediate transfer belt 8 for one rotation of the intermediate transfer belt 8 at 100 msec intervals (S1003). ). After the sampling is completed, the CPU 70 calculates an average value Base_ave of the data acquired from the detection signal for one round (S1004). The average value Base_ave is stored in the ROM 73. The CPU 70 forms a plurality of threshold setting patterns 901, 902, 903, and 904 as toner patterns on the intermediate transfer belt 8 (S1005). The average value Base_ave is a detection signal (image carrier) when the plurality of threshold setting patterns 901, 902, 903, and 904 do not pass through the detection position of the color misregistration detector 40. (Analog detection signal).

  FIG. 6 is a diagram showing threshold setting patterns 901, 902, 903, and 904 and a detection signal 900. FIG. 6A is a diagram showing threshold setting patterns 901, 902, 903, and 904 in the present embodiment. FIG. 6B is a diagram illustrating a detection signal 900 output from the color misregistration detector 40 that has read the threshold setting patterns 901, 902, 903, and 904. In the threshold value and threshold value switching time setting sequence, the threshold value setting patterns 901, 902, 903, and 904 shown in FIG. 6A are detected by the color misregistration detector 40, and the threshold value and the threshold value switching time for each color are set. Similar to the color misregistration detection patterns 801 to 804 shown in FIG. 4A, the yellow, magenta, cyan, and black threshold setting patterns 901, 902, 903, and 904 shown in FIG. 6A are 45 in the main scanning direction. It is formed in a tilted state. The threshold setting patterns 901, 902, 903 and 904 are different from the color misregistration detection patterns 801 to 804 shown in FIG. 4A in that the interval in the sub-scanning direction between the patterns is compared with the color misregistration detection patterns 801 to 804. Long. This is because it is necessary to sample and hold the detection signals 900 of the threshold setting patterns 901, 902, 903, and 904.

  The CPU 70 samples the bottom levels Vh_y, Vh_m, Vh_c, and Vh_k of the analog detection signals of the threshold setting patterns 901, 902, 903, and 904 for yellow, magenta, cyan, and black (S1006). In the sampling, the output held by the bottom hold circuit 76 is sampled for each color three times at intervals of 5 msec within the period indicated by Vhold in FIG. The average value of the three times is stored in the ROM 73 as the bottom level of the detection signal for each color. The output signals of the respective colors that are held at the bottom are held and reset at the timings of TrstY, TrstM, TrstC, and TrstK.

  In S1007, the CPU 70 determines whether or not the difference between the average value Base_ave of the intermediate transfer belt 8 and each of the bottom levels Vh_y, Vh_m, Vh_c, and Vh_k of the detection signal 900 is greater than a predetermined value. When the difference between the average value Base_ave of the intermediate transfer belt 8 and each of the bottom levels Vh_y, Vh_m, Vh_c, and Vh_k of the detection signal 900 is equal to or less than a predetermined value stored in the ROM 73 (NO in S1007), the CPU 70 performs processing. To S1013. When the difference is equal to or smaller than a predetermined value, the bottom levels Vh_y, Vh_m, Vh_c, and Vh_k are because the threshold setting pattern is not formed or the position where the threshold setting pattern of the intermediate transfer belt 8 is formed is scratched. May not have been detected correctly. In S1013, the CPU 70 determines whether or not the formation of the threshold setting patterns 901, 902, 903, and 904 and the sampling of the bottom levels Vh_y, Vh_m, Vh_c, and Vh_k have been retried. If the retry has not been completed (NO in S1013), the CPU 70 returns the process to S1005. The CPU 70 retries formation of threshold setting patterns 901, 902, 903, and 904 (S1005) and sampling of bottom levels Vh_y, Vh_m, Vh_c, and Vh_k (S1006). On the other hand, if the retry has been completed (YES in S1013), the CPU 70 outputs an error signal without retrying (S1014). The CPU 70 advances the process to S1011.

If the difference between the average value Base_ave of the intermediate transfer belt 8 and each of the bottom levels Vh_y, Vh_m, Vh_c, and Vh_k of the detection signal 900 is larger than a predetermined value (YES in S1007), the CPU 70 advances the process to S1009. In step S <b> 1009, the CPU 70 calculates threshold values Vth_y, Vth_m, Vth_c, and Vth_k for each color from the average value Base_ave of the intermediate transfer belt 8 and the bottom levels Vh_y, Vh_m, Vh_c, and Vh_k of the detection signal 900.
Vth_y = (Base_ave−Vh_y) × 0.5 + Vh_y (3)

  Expression (3) is an expression for calculating the threshold value Vth_y for yellow, but the threshold values Vth_m, Vth_c, and Vth_k for other colors can be calculated in the same manner. Formula (3) calculates, for example, a threshold value Vth_y at an intermediate (50%) position between the bottom level Vh_y of the detection signal 900 and the average value Base_ave of the intermediate transfer belt 8. For analog detection signals corresponding to a plurality of color misregistration detection patterns 801 to 804 and 811 to 814, threshold values Vth_y, Vth_m, Vth_c, and Vth_k are set for each color.

In S1010, the CPU 70 calculates threshold switching times Tch_y, Tch_m, Tch_c, and Tch_k for each color. The threshold switching times Tch_y, Tch_m, Tch_c, and Tch_k are calculated from the bottom levels Vh_y, Vh_m, Vh_c, and Vh_k, the average value Base_ave, and the thresholds Vth_y, Vth_m, Vth_c, and Vth_k.
Tch_y = ΔVth_y ÷ α_y (4)

Equation (4) is an equation for calculating the yellow threshold switching time Tch_y, but the threshold switching times Tch_m, Tch_c, and Tch_k for other colors can be calculated in the same manner. Here, ΔVth_y is the difference between the yellow threshold value Vth_y and the magenta threshold value Vth_m, and is obtained by the following equation (5).
ΔVth_y = Vth_m−Vth_y (5)

A yellow threshold setting pattern (second threshold setting pattern) 901 is formed downstream of the magenta threshold setting pattern (first threshold setting pattern) 902 in the rotation direction of the intermediate transfer belt 8.
α_y is the slope of the detection signal when the detection signal of the yellow threshold setting pattern 901 increases from the bottom value (bottom level) to the average value of the intermediate transfer belt 8, and is obtained by the following equation (6).
α_y = (Base_ave−Vh_y) / tr (6)
The rise time tr is the time taken for the detection signal of the color misregistration detector 40 to change from the bottom value to the average value of the intermediate transfer belt 8. In this embodiment, the rise time tr is a design value obtained from the spot diameter of the color misregistration detector 40 and the conveyance speed of the intermediate transfer belt 8.

  The threshold switching time Tch_y is used to determine the timing for switching the threshold value Vth_y of the yellow color shift detection patterns 801 and 811 to the threshold value Vth_m of the magenta color shift detection patterns 802 and 812. The threshold switching time Tch_y is calculated from the thresholds Vth_y and Vth_m calculated from the bottom levels Vh_y and Vh_m and the slope α_y calculated from the bottom level Vh_y. The threshold switching time Tch_m is used to determine the timing for switching the threshold Vth_m of the magenta color misregistration detection patterns 802 and 812 to the threshold Vth_c of the cyan color misregistration detection patterns 803 and 813. The threshold switching time Tch_m is calculated from the thresholds Vth_m and Vth_c calculated from the bottom levels Vh_m and Vh_c and the slope α_m calculated from the bottom level Vh_m. The same applies to the threshold switching times Tch_c and Tch_k. The threshold switching time Tch is used to determine the timing for switching the second threshold of the color misregistration detection pattern formed on the downstream side of the target color misregistration detection pattern to the first threshold of the target color misregistration detection pattern. . The threshold switching time Tch is set to the second bottom level of the second analog detection signal of the color shift detection pattern formed on the downstream side and the first bottom level of the first analog detection signal of the target color shift detection pattern. To be determined. The color misregistration detection pattern formed on the downstream side passes through the detection position of the color misregistration detector 40 before the target color misregistration detection pattern.

  In step S1011, the CPU 70 turns off the light emitting unit 51 (S1011). In S1012, the CPU 70 stops the rotation of the intermediate transfer belt 8 (S1012). The CPU 70 ends the threshold value and threshold value switching time setting sequence.

[Color misregistration acquisition sequence]
The color misregistration amount acquisition sequence will be described with reference to FIG. FIG. 7 is a flowchart showing a color misregistration amount acquisition sequence executed by the CPU 70. The CPU 70 executes a color misregistration amount acquisition sequence according to a program stored in the ROM 703. The CPU 70 starts a color misregistration amount acquisition sequence following the threshold value and threshold value switching time setting sequence when the main power source is turned on or after a predetermined number of images have been formed.

  When the color misregistration amount acquisition sequence is started, the CPU 70 sets the threshold value Vth_y of the leading yellow color misregistration detection pattern 801 among the threshold values Vth_y, Vth_m, Vth_c, and Vth_k acquired in the threshold value and threshold value switching time setting sequence. (S1101). The CPU 70 starts to rotate the intermediate transfer belt 8 (S1102). The CPU 70 turns on the light emitting unit 51 of the color misregistration detector 40 (S1103). The CPU 70 forms a plurality of color misregistration detection patterns 801 to 804 and 811 to 814 on the intermediate transfer belt 8 (S1104). The CPU 70 sets the value N of the pattern order counter built in the CPU 70 to 1 (initial value) (S1105). The CPU 70 detects the rising edge (hereinafter referred to as the leading edge) and the falling edge (hereinafter referred to as the trailing edge) of the detection signal obtained by binarizing the analog detection signal output from the color misregistration detector 40 by the comparator 72. Start (S1106). The CPU 70 counts time detection times E of the leading edge and the trailing edge detected as time information ym_1, yc_1, yk_1, ym_2, yc_2, and yk_2 used for calculating the color misregistration amount.

CPU70 determines whether it has detected the leading edge of the color shift detection patterns PT _N corresponding to the value N of pattern order counter (S1107). It does not detect the leading edge of the color shift detection patterns PT _N (NO in S1107), CPU 70 repeats the processing of S1107. When detecting the leading edge of the color shift detection patterns PT _N (YES in S1107), CPU 70 determines whether it has detected the trailing edge of the color shift detection patterns PT _N (S1108). It does not detect the trailing edge of the color shift detection patterns PT _N (NO in S1108), CPU 70 repeats the processing of S1108. When detecting a trailing edge of the color shift detection patterns PT _N (YES in S1108), CPU 70 is number of detected edges E is equal to or a predetermined number (S1109). The predetermined number is the number of designations of the leading edge and the trailing edge detected when the color misregistration detection patterns 801 to 804 and 811 to 814 are correctly detected. In this embodiment, ten sets of color misregistration detection patterns 801 to 804 and 811 to 814 shown in FIG. 4A are formed. A value obtained by averaging the color misregistration amounts calculated from the respective color misregistration detection patterns 801 to 804 and 811 to 814 is the final detection result of the color misregistration amount. When ten sets of color misregistration detection patterns 801 to 804 and 811 to 814 are correctly detected, the number of edge detections is 160. Therefore, in this embodiment, the predetermined number is set to 160.

If number of detected edges E is not a predetermined number (at S1109 NO), and modify the waits threshold switching time Tch_ _N threshold Vth_ _N to a threshold Vth_ _{N + 1} to detect the following color shift detection patterns _{PT N + 1} (S1110 ). Specifically, when the color misregistration amount acquisition sequence is started, the threshold value Vth_y of the leading yellow color misregistration detection pattern 801 is set. When the leading edge and trailing edge of the yellow color misregistration detection pattern 801 are detected, the CPU 70 waits for the threshold switching time Tch_y. Thereafter, the CPU 70 changes the threshold value from the threshold value Vth_y of the yellow color misregistration detection pattern 801 to the threshold value Vth_m of the magenta color misregistration detection pattern 802. Here, if the immediate threshold without waiting for the elapse of the threshold switching time Tch_ _N immediately after the trailing edge of the color shift detection patterns PT _N is detected switches from the threshold Vth_y to threshold Vth_m, FIG 10 (c) As shown, an unnecessary pulse is generated in the detection signal (binarized signal) 800. The unnecessary pulse causes erroneous detection of the next color misregistration detection pattern PTN _{+ 1} . In contrast, according to this embodiment, switching the threshold when the threshold switching time Tch_ _N has elapsed after the trailing edge of the color shift detection patterns PT _N is detected from the threshold Vth_y to threshold Vth_m. FIG. 8 is a diagram illustrating an analog detection signal 400 output from the color misregistration detector 40 and a detection signal 800 output from the comparator 72. As illustrated in FIG. 8, for example, the threshold corresponds to the second color misregistration detection pattern after the threshold switching time Tch has elapsed after the first output value corresponding to the first color misregistration detection pattern has reached the first threshold. To the second threshold value. If the interval between the color misregistration detection patterns is not extremely narrow, as shown in FIG. 8, the threshold is changed from the first threshold before the timing when the second output value corresponding to the second color misregistration detection pattern falls below the threshold. The change to 2 thresholds can be finished. The first color misregistration detection pattern is formed downstream of the second color misregistration detection pattern in the transport direction of the intermediate transfer belt 8, and the second color misregistration detection pattern is adjacent to the first color misregistration detection pattern in the transport direction. It is formed as follows. As shown in FIG. 8, an unnecessary pulse is prevented from occurring in the detection signal (binarized signal) 800 output from the comparator 72.

The CPU 70 determines whether or not the leading edge of the next color misregistration detection pattern PTN _{+ 1} has been detected (S1111). When the leading edge of the color misregistration detection pattern PTN _{+ 1} is not detected (NO in S1111), the CPU 70 repeats the process of S1111. When the leading edge of the next color misregistration detection pattern PTN _{+ 1} is detected (YES in S1111), the CPU 70 advances the process to S1112. CPU70 is not at S1112, whether the time interval tp from when detecting the trailing edge of the color shift detection patterns PT _N until it detects the next color shift detection patterns _{PT N + 1} of the leading edge is equal to or less than the predetermined time tc Is determined (S1112). When the time interval tp is equal to or shorter than the predetermined time tc (YES in S1112), the leading edge of the next color misregistration detection pattern PTN _{+ 1} is not correctly detected by detecting the unnecessary pulse shown in FIG. Accordingly, the CPU 70 stores the next color misregistration detection pattern PTN _{+ 1} as an NG pattern in the RAM 78 (S1113). The CPU 70 advances the process to S1114. On the other hand, when the time interval tp is larger than the predetermined time tc (NO in S1112), the CPU 70 advances the process to S1114 without executing the step of S1113. In S1114, the CPU 70 increments the value N of the pattern order counter, and returns the process to S1108. CPU70 continues the detection of the color shift detection patterns PT _N.

  In S1109, when the edge detection count E is a predetermined number (YES in S1109), the CPU 70 uses the equations (1) and (2) from the detection result of the color misregistration detection pattern PT excluding the NG pattern. The amount is calculated (S1115). As described above, the average value of the color misregistration amounts calculated from the detection results of the ten sets of color misregistration detection patterns 801 to 804 and 811 to 814 is calculated. The detection result of the color misregistration detection pattern that has become an NG pattern in S1113 is removed, and the amount of color misregistration is calculated. When the calculation of the color misregistration amount is completed, the CPU 70 turns off the light emitting unit 51 (S1116). The CPU 70 stops the rotation of the intermediate transfer belt 8 (S1117). The CPU 70 ends the color misregistration amount acquisition sequence.

In the present embodiment, the color misregistration detection patterns 801 to 804 and 811 to 814 are so-called lower triangular waveforms in which the output voltage decreases. However, the color misregistration detection patterns 801 to 804 and 811 to 814 are not limited to this. The color misregistration detection patterns 801 to 804 and 811 to 814 are so-called upper triangular waveforms in which the output voltage of the color misregistration detector 40 increases when the color misregistration detection patterns 801 to 804 and 811 to 814 are detected, for example. Also good.
According to the present embodiment, it is possible to appropriately determine the timing for switching the threshold.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 8 ... Intermediate transfer belt 31Y, 31M, 31C, 31K ... Image forming part 40 ... Color misregistration detector 51 ... Light emitting part 52 ... Light receiving part 70 ...・ CPU
72: Comparator 77: Threshold adjustment unit

Claims (5)

An image carrier that rotates in a predetermined direction;
A plurality of image forming means for forming images of different colors on the image carrier;
A transfer portion where the image formed on the image carrier is transferred to a sheet;
Output means for detecting reflected light from the pattern formed on the image carrier by the plurality of image forming means and outputting an output value corresponding to the detection result;
Conversion means for converting the output value of the output means based on a threshold;
The plurality of image forming means form different color misregistration detection patterns, the output means outputs an output value corresponding to the detection result of reflected light from the color misregistration detection pattern, and the conversion means outputs the color misregistration pattern. The first output value corresponding to the first color misregistration detection pattern of the first color in the detection pattern is converted based on the first threshold value, and is different from the first color in the color misregistration detection pattern by the converting means. A second output value corresponding to the second color misregistration detection pattern of the second color is converted based on a second threshold value, and the plurality of the output values are converted based on the converted first output value and the converted second output value. Control means for controlling color misregistration of an image formed by the image forming means,
The first color misregistration detection pattern is formed downstream of the second color misregistration detection pattern in the predetermined direction,
The second color misregistration detection pattern is formed adjacent to the first color misregistration detection pattern in the predetermined direction;
The control unit causes the plurality of image forming units to form threshold value setting patterns of different colors, causes the output unit to output an output value corresponding to a detection result of reflected light from the threshold value setting pattern, and the conversion An output value corresponding to the first color threshold setting pattern in the threshold setting pattern, an output value corresponding to the second color threshold setting pattern in the threshold setting pattern, and the image An image forming apparatus comprising: a timing for changing the threshold value of the conversion unit from the first threshold value to the second threshold value based on an output value corresponding to a detection result of reflected light from the carrier.   The control means includes a bottom value of the output value corresponding to the threshold setting pattern of the first color in the threshold setting pattern by the converting means, and a threshold of the second color in the threshold setting pattern. The timing is determined based on a bottom value of the output value corresponding to a setting pattern and the output value corresponding to a detection result of reflected light from the image carrier. Image forming apparatus. The control means determines the first threshold based on the output value corresponding to the threshold setting pattern for the first color and the output value corresponding to the detection result of the reflected light from the image carrier,
The control means determines the second threshold based on the output value corresponding to the threshold setting pattern for the second color and the output value corresponding to the detection result of the reflected light from the image carrier. The image forming apparatus according to claim 1, wherein:   The control means outputs the difference between the determined first threshold value and the determined second threshold value, and the output corresponding to the first color threshold setting pattern in the threshold setting pattern by the converting means. The image forming apparatus according to claim 3, wherein the timing is determined based on a bottom value of the value and the output value corresponding to a detection result of reflected light from the image carrier.   The image forming apparatus according to claim 1, wherein the control unit causes the plurality of image forming units to form the threshold setting pattern before the color misregistration detection pattern is formed.

Images