JP2019159122A - Image forming apparatus and color shift amount calculation method - Google Patents

Abstract

To provide an image forming apparatus capable of highly accurately detecting the position of a measurement image.SOLUTION: The image forming apparatus includes a color shift detection sensor 40 which detects a measurement image of a plurality of colors for detecting a color shift correction amount formed on an intermediate transfer belt 8 and outputs a detection signal, a comparator 72 which generates a binary signal obtained by binarizing the detection signal according to a set threshold, and a controller 700 which calculates a color shift amount from the binary signal. The image forming apparatus forms the measurement image of each color by a plurality of image forming parts. The controller 700 determines whether or not the threshold set to the comparator 72 is switched according to the change of the temperature of each image forming part.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, or a printing machine that forms an image of a plurality of colors, and more particularly to an image forming apparatus having a color misregistration correction function for each color.

  An electrophotographic image forming apparatus forms yellow (Y), magenta (M), cyan (C), and black (K) images through electrophotographic processes such as charging, exposure, development, and transfer. To do. This type of image forming apparatus includes a tandem system that obtains a color image by superimposing images of respective colors. The tandem image forming apparatus has an advantage that high-speed printing is possible because printing is performed at one time, but color misregistration occurs unless the paper feed and image formation timing are accurately controlled. On the other hand, the image forming apparatus calculates a shift amount (color shift amount) of the image forming position between the respective colors, and performs color shift correction according to the color shift amount. The color misregistration amount is calculated according to the detection result of the measurement image by the optical sensor after an image for color misregistration (measurement image) is formed for each color on the intermediate transfer belt or the like.

  The optical sensor detects the measurement image based on the difference in light reflectance between the intermediate transfer belt and the measurement image. The measurement image is detected by passing through the detection range of the optical sensor by the rotation of the intermediate transfer belt. The value of the detection signal varies depending on the amount of reflected light from the intermediate transfer belt and the measurement image. The position of the measurement image is binarized by comparing the detection signal, which is an analog signal representing the detection result of the optical sensor, with a predetermined threshold, and is measured according to the binarized result. The middle of the rising edge and falling edge of the binarized signal, which is the binarized result, is the position of the measurement image.

It is desirable that the measurement image is formed with a uniform density (toner load amount). The detection signal based on the measurement image formed with a uniform density is a triangular waveform. In this case, the color misregistration amount can be accurately detected from the position of the measurement image. However, when the density of the measurement image is not uniform, the detection signal has a distorted triangular waveform. In this case, the position of the measurement image cannot be accurately detected from the binarized signal, and the accurate color misregistration amount cannot be measured.
On the other hand, Patent Document 1 is a case where the detection result is distorted by changing the threshold value for binarizing the detection result of the optical sensor at a uniform ratio according to the amplitude of the detection result. Also discloses a technique for accurately detecting the amount of color misregistration.

JP 2013-25184 A

  Measurement images of different colors are generally formed at very narrow intervals of about several millimeters. The reason for this is to avoid the influence due to uneven conveyance of the intermediate transfer belt, shorten the detection time, and the like. The threshold value for binarizing the detection result needs to be switched while the measurement image is not detected. However, when the interval between the measurement images becomes narrow, the threshold value cannot be switched in time, and the detection signal may not be accurately binarized. The interval of the measurement image varies, for example, when the optical path of the laser beam during exposure is deflected due to a temperature change in the image forming apparatus. This is generally called a thermal shift. The image forming apparatus needs to perform color misregistration correction accurately even when such a thermal shift occurs.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of accurately detecting a position of a measurement image and performing highly accurate color misregistration correction.

  The image forming apparatus of the present invention detects a plurality of image forming units that form images of different colors, an image carrier on which the images of each color are transferred from the plurality of image forming units, and a temperature of the image forming unit. Detecting means for irradiating light toward the image carrier, receiving reflected light of the light, and outputting a detection signal that is an analog signal corresponding to the received light quantity; and the detection signal Binarizing means for generating a binarized signal binarized according to a threshold set for each color, threshold adjusting means for setting the threshold corresponding to the color of the image in the binarizing means, A timing generation unit that determines whether or not the threshold adjustment unit switches the threshold; and a plurality of image forming units that form and transfer a measurement image of each color for detecting a color shift amount of each color. In each image carrier, Control means for forming a measurement image and acquiring a color misregistration amount for each color in accordance with a binarized signal based on the threshold value of the detection signal for each color output from the detection means for detecting the measurement image for each color And storage means for storing the temperature detected by the temperature detection means, wherein the timing generation means is the current temperature detected by the temperature detection means and the previous color shift stored in the storage means. It is characterized in that it is determined whether or not the threshold adjustment unit is to switch the threshold according to a temperature difference from the temperature when the amount is acquired.

  According to the present invention, it is possible to accurately detect the position of the measurement image and perform highly accurate color misregistration correction.

1 is a configuration diagram of an image forming apparatus. Explanatory drawing of a color shift detection sensor. (A), (b) is explanatory drawing of the influence on the detection signal of the density | concentration of the image for a measurement. Explanatory drawing in the case of making a threshold variable. FIG. 3 is a structural example diagram of an exposure unit. (A), (b) is explanatory drawing of threshold value switching judgment corresponding to a thermal shift. (A), (b) is explanatory drawing of a controller. Explanatory drawing of the amount of color shift. (A), (b) is explanatory drawing of the pattern image for threshold value setting. The flowchart showing a threshold value setting process. 6 is a flowchart showing color misregistration amount detection processing.

  Hereinafter, embodiments will be described in detail with reference to the drawings.

(overall structure)
FIG. 1 is a configuration diagram of an image forming apparatus according to the present embodiment. The image forming apparatus 1 includes a plurality of image forming units IMG-Y, IMG-M, and a plurality of image forming units for forming toner images corresponding to colors of yellow (Y), magenta (M), cyan (C), and black (K). IMG-C and IMG-K are provided. Each of the image forming units IMG-Y, IMG-M, IMG-C, and IMG-K has the same configuration, and only the color of the toner image to be formed is different.

  The image forming unit IMG-Y includes a photosensitive drum 2a that is a drum-shaped photosensitive member. The photosensitive drum 2a is an image carrier on which a yellow toner image is formed, and rotates counterclockwise in the drawing. Around the photosensitive drum 2a, a charger 3a, an exposure device 5a, a developing device 7a, and a cleaner 4a are provided. The charger 3a charges the surface of the photosensitive drum 2a. The exposure device 5a is a laser scanning unit using a semiconductor laser as a light source. The exposure device 5a exposes the surface of the charged photosensitive drum 2a with laser light modulated based on yellow image data, thereby forming an electrostatic latent image on the photosensitive drum 2a. The laser beam scans the photosensitive drum 2a with the direction perpendicular to the rotation direction of the photosensitive drum 2a (the axial direction of the photosensitive drum 2a) as the main scanning direction. The developing device 7a develops the electrostatic latent image with a yellow developer (toner) to form a yellow toner image on the photosensitive drum 2a. The cleaner 4a cleans toner remaining on the photosensitive drum 2a after transfer of a toner image to an intermediate transfer belt 8 described later.

  The image forming unit IMG-M includes a photosensitive drum 2b, a charger 3b, an exposure device 5b, a developing device 7b, and a cleaner 4b. The image forming unit IMG-M forms a magenta toner image on the photosensitive drum 2b. The image forming unit IMG-C includes a photosensitive drum 2c, a charger 3c, an exposure device 5c, a developing device 7c, and a cleaner 4c. The image forming unit IMG-C forms a cyan toner image on the photosensitive drum 2c. The image forming unit IMG-K includes a photosensitive drum 2d, a charger 3d, an exposure unit 5d, a developing unit 7d, and a cleaner 4d. The image forming unit IMG-K forms a black toner image on the photosensitive drum 2d.

  An intermediate transfer belt 8 that is an intermediate transfer member is provided below each of the image forming units IMG-Y, IMG-M, IMG-C, and IMG-K. The intermediate transfer belt 8 is an image carrier that carries a full-color toner image by sequentially transferring the toner images of the respective colors formed on the photosensitive drums 2a to 2d. The intermediate transfer belt 8 is rotationally driven in the clockwise direction in the figure by rollers 10, 11, and 21. Transfer blades (primary transfer portions) 6a to 6d are provided at positions facing the respective photosensitive drums 2a to 2d with the intermediate transfer belt 8 interposed therebetween.

  The yellow toner image formed on the photosensitive drum 2a is transferred to the intermediate transfer belt 8 by a transfer bias applied to the transfer blade 6a. The magenta toner image formed on the photosensitive drum 2b is transferred to the intermediate transfer belt 8 by a transfer bias applied to the transfer blade 6b. The cyan toner image formed on the photosensitive drum 2c is transferred to the intermediate transfer belt 8 by a transfer bias applied to the transfer blade 6c. The black toner image formed on the photosensitive drum 2d is transferred to the intermediate transfer belt 8 by a transfer bias applied to the transfer blade 6d. Transfer of the toner image from each of the photosensitive drums 2 a to 2 d to the intermediate transfer belt 8 is performed at a timing according to the rotation of the intermediate transfer belt 8. As a result, toner images of respective colors are formed on the intermediate transfer belt 8 so as to overlap each other.

  The intermediate transfer belt 8 forms a secondary transfer portion T between the intermediate transfer belt 8 and the secondary transfer roller 22. The intermediate transfer belt 8 conveys the toner images transferred from the photosensitive drums 2a to 2d to the secondary transfer portion T by rotating. A recording medium S such as a sheet is conveyed to the secondary transfer portion T in accordance with the timing at which the toner image is conveyed. The recording medium S is transported between the intermediate transfer belt 8 and the secondary transfer roller 22 so that the toner images of the respective colors are collectively transferred from the intermediate transfer belt 8.

  The recording medium S to which the toner image is transferred is conveyed to the fixing device 23. The fixing device 23 fixes the transferred toner image on the recording medium S. The fixing device 23 fixes the toner image on the recording medium S by heating and pressurizing the recording medium S, for example. The recording medium S on which the toner image is fixed is discharged from the fixing device 23 to the discharge tray 25 by the discharge roller 24.

  The image forming unit IMG-K that forms a black toner image is located downstream of the other chromatic image forming units IMG-Y, IMG-M, and IMG-C in the rotational direction of the intermediate transfer belt 8. Provided. With such an arrangement, when a monochrome image is formed, the time from the image formation instruction to the ejection of the recording medium S on which the image is formed is shortened.

  The recording medium S is fed from the paper feed cassette 17 or the manual feed tray 13 to the paper transport unit. The paper transport unit includes transport rollers such as pickup rollers 14, 15, 18, 19, a vertical pass roller 20, and a registration roller 16 in the transport path. The recording medium S accommodated in the paper feed cassette 17 is fed one by one by the pick-up rollers 18 and 19 and conveyed to the registration roller 16 through the vertical pass roller 20. The recording medium S placed on the manual feed tray 13 is fed one by one by the pickup rollers 14 and 15 and conveyed to the registration roller 16. The fed recording medium S is corrected for skew in the conveyance direction by the electrostatic conveyance unit 30 and is conveyed to the secondary transfer unit T by adjusting the timing by the registration roller 16. Each conveyance roller of the paper conveyance unit is driven by an independent stepping motor in order to realize a high-speed and stable conveyance operation.

  At the time of duplex printing, the recording medium S that has passed through the fixing device 23 is conveyed from the paper discharge roller 24 to the duplex reversing path 27 and then reversed and conveyed to the duplex path 28. The recording medium S that has passed the double-sided pass 28 passes through the vertical pass roller 20 and is conveyed again to the secondary transfer unit T. The recording medium S transported to the secondary transfer portion T has the toner images of the respective colors collectively transferred from the intermediate transfer belt 8 to the back surface. Thereafter, the toner image is fixed on the recording medium S by the fixing device 23 and is discharged to the paper discharge tray 25 via the paper discharge roller 24.

  Among the four photosensitive drums 2 a to 2 d, the image forming apparatus 1 is between a photosensitive drum 2 d that is located most downstream with respect to the rotation direction of the intermediate transfer belt 8 and a roller 10 that supports the intermediate transfer belt 8. A color misregistration detection sensor 40 is provided. The color misregistration detection sensor 40 detects a measurement image for detecting a color misregistration amount of each color transferred from the photosensitive drums 2a to 2d to the intermediate transfer belt 8. The driving timing of the color misregistration detection sensor 40 is controlled by a synchronization unit (not shown). In order to measure the color misregistration amount, the measurement image is formed as a toner image for each color by the image forming units IMG-Y to IMG-K and transferred to the intermediate transfer belt 8.

(Color shift detection sensor)
FIG. 2 is an explanatory diagram of the color misregistration detection sensor 40. The color misregistration detection sensor 40 of the present embodiment is an optical sensor, and includes a light emitting unit 41 having a light emitting element such as a light emitting diode (LED), a light receiving unit 42 having a phototransistor and the like, and a lens 43. Prepare. The light emitting unit 41 irradiates light toward the intermediate transfer belt 8. The light receiving unit 42 receives the reflected light reflected by the intermediate transfer belt 8 and collected by the lens 43. The light receiving unit 42 outputs, as a detection result, a detection signal that is an analog signal whose value changes according to the amount of received light. The intermediate transfer belt 8 has a higher reflectance than the measurement image. The color misregistration detection sensor 40 of the present embodiment uses a detection method in a regular reflection optical system. Therefore, the detection signal output from the color misregistration detection sensor 40 detects the toner image (measurement image) as a value when detecting the surface of the intermediate transfer belt 8 onto which the toner image (measurement image) is transferred. It becomes higher than the value of time. The detection signal is binarized as described later. The position of the measurement image is detected based on the binarized detection signal.

FIG. 3 is an explanatory diagram of the influence of the density of the measurement image on the detection signal. FIG. 3A is an explanatory diagram of a detection result of a measurement image having a uniform density. The detection signal of the measurement image having a uniform density is output as a well-formed triangular waveform. FIG. 3B is an explanatory diagram of the detection result of the measurement image having a non-uniform density. A detection signal of a measurement image having a non-uniform density is output as a distorted triangular waveform.
The detection signal is binarized according to the threshold value to become a binarized signal. Here, a binarized signal when the threshold values are 75%, 50%, and 25% with respect to the amplitude of the detection signal is exemplified. An intermediate point between the rising edge and the falling edge of the binarized signal is detected as the position of the measurement image. In the case of FIG. 3A, the measurement image positions reg_1, reg_2, and reg_3 detected at any threshold are the same. In the case of FIG. 3B, since the detection signal is distorted due to the influence of the non-uniform density, the positions reg-1, reg-2, reg-3 of the detected measurement image vary according to the threshold value. To do. That is, a detection error occurs according to the threshold value.

  With respect to such a detection error, the detection error can be suppressed by making the threshold variable according to the amplitude of the detection signal. FIG. 4 is an explanatory diagram for making the threshold variable. When a plurality of measurement images are detected, the amplitude of the detection signal of each measurement image may be different. This occurs due to variations in the density (toner loading amount) of each measurement image and differences in color reflection characteristics. In FIG. 4, the case of the detection signal a having the amplitude Va and the detection signal b having the amplitude Vb will be described.

  FIG. 4 shows an example in which the threshold value Vth is set to a uniform ratio of 50% (Va / 2, Vb / 2) with respect to the amplitudes Va and Vb of the detection signals a and b. Thus, when the ratio of the threshold value Vth to the amplitudes Va and Vb is constant, the detection error of the position of the measurement image is suppressed even if the detection signals a and b are distorted triangular waveforms. The threshold value Vth needs to be switched after the binarization for the detection signal a is completed and before the binarization for the detection signal b is performed. However, if the threshold value Vth cannot be switched in time, binarization is not performed correctly and a detection error occurs. For example, when the interval between the measurement images becomes narrow, the threshold value cannot be switched in time. The interval between the measurement images is narrowed by deflecting the laser light used when the exposure devices 5a to 5d are exposed due to, for example, a temperature change in the image forming apparatus 1.

(Exposure device)
FIG. 5 is a view showing a configuration example of the exposure devices 5a to 5d. As described above, the electrophotographic image forming apparatus 1 includes the exposure units 5a to 5d that are optical scanning units that expose the photosensitive drums 2a to 2d with laser light in order to form electrostatic latent images on the photosensitive drums 2a to 2d. 5d. The exposure units 5a to 5d have the same configuration. Here, the configuration of the exposure unit 5a will be described, and description of the other exposure units 5b to 5d will be omitted.

  The exposure unit 5 a includes a laser light source 51, a rotary polygon mirror 52, an imaging lens 53, a reflection plate 54, and a laser beam detection sensor 55. The laser light source 51 emits laser light. Therefore, the laser light source 51 includes a semiconductor laser, a collimator lens, an aperture, and the like. The semiconductor laser outputs divergent light. The collimator lens and the aperture convert the divergent light into a parallel light beam to form laser light having a predetermined beam shape. The laser light is emitted toward the rotating polygon mirror 52.

  The rotary polygon mirror 52 is a columnar body having a reflecting mirror provided on a side surface and a polygonal bottom surface (an octagon in FIG. 5). The rotary polygon mirror 52 is configured to rotate about the axis of the column. The laser light emitted from the laser light source 51 is reflected by the reflecting mirror of the rotating polygon mirror 52 that is rotating. The laser beam reflected by the rotary polygon mirror 52 is guided to the photosensitive drum 2 a and the reflection plate 54 according to the rotation of the rotary polygon mirror 52 through the imaging lens 53. The laser light guided to the reflection plate 54 is reflected by the reflection plate 54 and detected by the laser beam detection sensor 55.

  When the laser beam detection sensor 55 detects the laser beam, the laser light source 51 emits a laser beam modulated based on the yellow image data. The modulated laser beam is reflected by the rotary polygon mirror 52 and guided to the photosensitive drum 2a, and scans the photosensitive drum 2a in one direction (main scanning direction) according to the rotation of the rotary polygon mirror 52. As a result, an electrostatic latent image corresponding to yellow image data is formed line by line on the photosensitive drum 2a. The photosensitive drum 2a rotates around the drum axis. Due to the rotation of the photosensitive drum 2a, the exposure spot of the laser beam is shifted in the sub-scanning direction orthogonal to the main scanning direction. As described above, the rotation of the rotary polygon mirror 52 and the rotation of the photosensitive drum 2a form electrostatic latent images in the main scanning direction and the sub-scanning direction using the laser beam.

(Thermal shift)
In the exposure devices 5a to 5d configured as described above, the rotating polygon mirror 52 continuously rotates when performing image formation continuously. For this purpose, the temperature of a drive motor (not shown) that rotates the rotary polygon mirror 52 and its related parts rises. This becomes a factor in which the housing for accommodating the exposure devices 5a to 5d and the imaging lens 53 are expanded and deformed. Such deformation causes a shift in the line where the laser beam scans the photosensitive drum 2a. FIG. 5 shows that the line L is shifted to the line L ′ due to the temperature rise. This is a thermal shift. When the thermal shift occurs, for example, the interval in the sub-scanning direction of the image changes. There are individual differences in the degree of temperature rise in each of the exposure devices 5a to 5d. For this reason, there is a difference in the influence of the thermal shift in each of the exposure devices 5a to 5d, and the amount of shift in the formation position of each color image transferred to the intermediate transfer belt 8 is different. As a result, the interval between the measurement images of the respective colors changes. The exposure units 5a to 5d of the present embodiment include a temperature sensor 50 in the vicinity of the rotary polygon mirror 52 in order to detect such a temperature rise, and cope with a thermal shift. The temperature sensor 50 detects the temperature in the vicinity of the rotary polygon mirror 52.

  FIG. 6 is an explanatory diagram of threshold value switching determination corresponding to the thermal shift. FIG. 6 illustrates a measurement image that is closer than usual due to the influence of the thermal shift and a detection signal thereof. As described above, the influence of the thermal shift has individual differences in each of the exposure devices 5a to 5d. For this purpose, the measurement image is approached or separated.

  FIG. 6A shows a state in which the distance D between the two measurement images 601 and 602, which is originally about 1.7 [mm], approaches about 1.0 [mm] due to thermal shift. For example, if the temperature varies by 1 [° C.] and the gap between the images for measurement of 0.042 [mm] occurs, the temperature fluctuates by about 17 [° C.] so that it is originally about 1.7 [mm]. The interval between the measurement images is about 1.0 [mm]. For example, when the image forming apparatus 1 performs continuous printing, color misregistration correction is performed in a state where the drive motor of the rotary polygon mirror 52 is heated, and after the color misregistration correction value is stored, the image is cooled down by about 17 [° C.]. Thus, when a measurement image is formed using the color misregistration correction value, such an error occurs.

  When the image forming process speed (the rotational speed of the intermediate transfer belt 8) is 500 [mm / s], the interval between the measurement images 601 and 602 when approaching is 2.0 milliseconds in terms of time. When the threshold value is switched in this state, the subsequent measurement image 602 may be detected during the switching, and the threshold value may not be switched in time. In the example of FIG. 6A, the threshold value intersects with the falling value of the detection signal 604 of the subsequent measurement image 602 in the middle of switching from VH_1 to VH_2 after the output of the detection signal 603 of the measurement image 601. Yes.

FIG. 6B illustrates a result of comparison between a case where the threshold switching is in time and a case where the threshold is not in time. If the switching from the threshold value VH_1 to VH_2 is not in time, an error Dif_1 occurs at the center of gravity position of the detected measurement image 602.
When binarization of the detection signal 604 is performed with the threshold value kept at VH_1 without switching the threshold value, an error Dif_2 occurs at the center of gravity position of the detected measurement image 602. The error of the center of gravity position is suppressed from the error Dif_1 to the error Dif_2 as compared with the case where switching is performed. Thus, depending on the temperature difference, the error can be reduced by not switching the threshold value. Therefore, it is possible to suppress the error Dif_1 to the error Dif_2 by predicting the occurrence of a thermal shift according to the temperature detected by the temperature sensor 50 and controlling whether to switch the threshold according to the prediction result. .

(controller)
FIG. 7 is an explanatory diagram of a controller that controls the operation of the image forming apparatus 1 configured as described above. FIG. 7A illustrates the overall configuration of the controller 700. The controller 700 includes a CPU (Central Processing Unit) 70, a ROM (Read Only Memory) 72, and a RAM (Random Access Memory) 73. The controller 700 is also provided with a comparator 72, a threshold adjustment unit 77, a bottom hold unit 76, an image processing control unit 74, and an exposure control unit 75. The color misregistration detection sensor 40 and the temperature sensor 50 are connected to the controller 700.
The controller 700 according to the present embodiment performs color misregistration correction in addition to control of image forming processing by the image forming apparatus 1. The CPU 70 controls the operation of the image forming apparatus 1 by executing a computer program stored in the ROM 73 using the RAM 78 as a work area. In addition, the CPU 70 executes a computer program to execute a threshold adjustment control unit 711, a position detection unit 712, a color misregistration amount calculation unit 713, a light emission control unit 714, an A / D converter 715, a measurement image forming unit 716, and a threshold value switching. It functions as a timing generation unit 717. Such a controller 700 is built in the image forming apparatus 1.

  The color misregistration detection sensor 40 transmits a detection signal that is a detection result of the measurement image to the controller 700. The detection signal output from the color misregistration detection sensor 40 is input to the CPU 70, the comparator 72, and the bottom hold unit 76. The detection signal input to the comparator 72 is binarized according to a preset threshold value. The threshold is set by a threshold signal input from the threshold adjustment unit 77. The threshold signal is generated, for example, by smoothing a PWM signal, which is a pulse signal output from the CPU 70, by a threshold adjustment unit 77 formed of a resistor and a capacitor. The comparator 72 transmits the binarized signal generated from the detection signal according to the threshold value to the CPU 70.

The threshold adjustment control unit 711 inputs a PWM signal for generating a threshold set in the comparator 72 to the threshold adjustment unit 77. FIG. 7B illustrates the threshold adjustment unit 77. The threshold adjustment unit 77 is configured by an RC circuit having a resistor 771 and a capacitor 772. The PWM signal generation unit 7111 is provided in the threshold adjustment control unit 711, generates a PWM signal, and inputs the PWM signal to the threshold adjustment unit 77. The threshold adjustment unit 77 generates a threshold by smoothing the PWM signal using an RC circuit. The threshold value changes according to the duty ratio of the PWM signal. For example, the threshold value is increased by lengthening the ON state of the PWM signal. The threshold adjustment unit 77 sets the threshold generated according to the PWM signal in the comparator 72.
The position detection unit 712 detects the position of the measurement image according to the binarized signal generated by the comparator 72. The position detection unit 712 detects the position of the center of gravity, which is an intermediate position between the rising edge and the falling edge of the binarized signal, as the position of the measurement image. The color misregistration amount calculation unit 713 calculates the color misregistration amount of each color image according to the position of the measurement image for each color detected by the position detection unit 712.

  The light emission control unit 714 performs light emission control of the light emitting unit 41 of the color misregistration detection sensor 40. The A / D converter 715 converts the detection signal of the color misregistration detection sensor 40 that is an analog signal into a digital signal. The measurement image forming unit 716 holds measurement image data for forming a measurement image, and transmits the measurement image data to the exposure control unit 75 when forming the measurement image. The exposure control unit 75 controls blinking of the laser light emitted from the exposure devices 5a to 5d based on the measurement image data, and forms a measurement image. The threshold switching timing generation unit 717 generates a threshold switching timing when switching the threshold according to the temperature detected by the temperature sensor 50. The temperature detected by the temperature sensor 50 is stored in the RAM 78. The threshold switching timing generation unit 717 determines whether or not to switch the threshold according to the temperature difference between the previous color shift correction temperature stored in the RAM 78 and the current temperature acquired from the temperature sensor 50. .

  The RAM 78 stores a threshold value set in the comparator 72, the color misregistration amount calculated by the color misregistration amount calculation unit 713, the temperature detected by the temperature sensor 50, and the like. The CPU 70 sets the image processing control unit 74 and the like with reference to these data when the image forming apparatus 1 is activated. The image processing control unit 74 controls the operation of the exposure control unit 75 by performing image processing according to the set contents at the time of image formation. The bottom hold unit 76 samples the value of the detection signal in a threshold setting process described later. The CPU 70 controls the position of the image formed by the image forming units IMG-Y, IMG-M, IMG-C, and IMG-K based on the detected color misregistration. This is because when the image forming apparatus 1 forms an image (output image) on the recording medium S based on the image data, the image processing control unit 74 performs image processing on the image data based on the color misregistration correction amount stored in the RAM 78. It is realized by executing. That is, image data that has been subjected to image processing by the image processing control unit 74 based on the color misregistration correction amount is transferred to the exposure control unit 75, and an output image in which color misregistration is suppressed is formed on the recording medium S.

(Color misregistration calculation)
A color shift amount is calculated by the position detection unit 712 and the color shift amount calculation unit 713. FIG. 8 is an explanatory diagram of the amount of color misregistration. FIG. 8 illustrates a measurement image for measuring the color misregistration amount and a binarized signal generated from the detection signal. The broken line in the binarized signal indicates the position of the measurement image that is an intermediate position between the rising edge and the falling edge. The measurement image includes yellow pattern images 801 and 811, magenta pattern images 802 and 812, cyan pattern images 803 and 813, and black pattern images 804 and 814. The measurement image is formed with an inclination of 45 degrees with respect to the main scanning direction. The pattern images 801, 802, 803, and 804 and the pattern images 811, 812, 813, and 814 are formed with an inclination of 45 degrees in the reverse direction with respect to the main scanning direction.

  The measurement image is conveyed to the detection range of the color misregistration detection sensor 40 in the order of the pattern image 801, the pattern image 802, the pattern image 803, the pattern image 804, the pattern image 811, the pattern image 812, the pattern image 813, and the pattern image 814. The For this purpose, the color misregistration detection sensor 40 outputs detection signals in the order of a pattern image 801, a pattern image 802, a pattern image 803, a pattern image 804, a pattern image 811, a pattern image 812, a pattern image 813, and a pattern image 814.

  The position detection unit 712 detects the positions of the pattern images 801 to 804 and 811 to 814 of the respective colors from the detection timing of the rising edge and the detection timing of the falling edge of the binarized signal. Therefore, the position of the measurement image is represented by the detection timing. The amount of color misregistration is represented by the detection timing (time interval) of the measurement target color with respect to the reference color detection timing. In FIG. 8, the amount of color misregistration is expressed based on the detection timing of the yellow pattern images 801 and 811. The color misregistration amount of the magenta pattern image 802 is represented by a time interval ym_h1. The color misregistration amount of the magenta pattern image 812 is represented by a time interval ym_h2. The color misregistration amount of the cyan pattern image 803 is represented by a time interval yc_h1. The color misregistration amount of the cyan pattern image 813 is represented by a time interval yc_h2. The color misregistration amount of the black pattern image 804 is represented by a time interval yk_h1. The color misregistration amount of the black pattern image 814 is represented by a time interval yk_h2.

The amount of color misregistration of the magenta pattern images 802 and 812 based on the yellow pattern images 801 and 811 will be described. When the magenta pattern images 802 and 812 are color-shifted in the positive direction of the sub-scanning direction, the time intervals ym_1 and ym_2 are increased by the same amount in proportion to the color-shift amount. When the magenta pattern images 802 and 812 are color-shifted in the negative direction of the sub-scanning direction, the time intervals ym_1 and ym_2 are reduced by the same amount in proportion to the color-shift amount. When the magenta pattern images 802 and 812 are color-shifted in the positive direction of the main scanning direction, the time interval ym_1 increases in proportion to the color shift amount, and the time interval ym_2 decreases by the same amount in proportion to the color shift amount. . When the magenta pattern images 802 and 812 are color-shifted in the negative direction of the main scanning direction, the time interval ym_1 decreases in proportion to the color shift amount, and the time interval ym_2 increases by the same amount in proportion to the color shift amount. . The color misregistration amounts of the magenta pattern images 802 and 812 are expressed by the following equations.
(Color misregistration amount in the sub-scanning direction) = X− (ym — 1 + ym — 2) / 2 * (conveying speed) (Expression 1)
(Color misregistration amount in the main scanning direction) = (ym_1−ym_2) / 2 * (conveying speed) (Expression 2)

  X is the position interval between the yellow pattern images 801 and 811 that are reference colors and the magenta pattern images 802 and 812 that are the target of color misregistration when no color misregistration occurs. Since the time intervals ym_1 and ym_2 are time information, the time intervals ym_1 and ym_2 are converted into distance information by multiplying by the rotation speed of the intermediate transfer belt 8 (the conveyance speed of the measurement image). The color misregistration amounts of other colors can be derived from similar equations. The color misregistration amount calculation unit 713 can calculate the color misregistration amount of each color in the sub-scanning direction and the main scanning direction according to Equations 1 and 2.

(Threshold setting process)
In this embodiment, the threshold value used when the comparator 72 generates the binarized signal is generated using an image formed on the intermediate transfer belt 8. FIG. 9 is an explanatory diagram of a threshold setting pattern image used for threshold setting.

  FIG. 9A illustrates a threshold setting pattern image. The threshold setting pattern image includes a yellow threshold setting pattern image 901, a magenta threshold setting pattern image 902, a cyan threshold setting pattern image 903, and a black threshold setting pattern image 904. The threshold setting pattern images 901, 902, 903, and 904 for each color are formed with an inclination of 45 degrees with respect to the main scanning direction. The threshold setting pattern images 901, 902, 903, and 904 have wider intervals in the sub-scanning direction than the pattern images 801, 802, 803, and 804 in FIG. This is because it is necessary to sample and hold the detection signals of the threshold setting pattern images 901, 902, 903, and 904 as described later.

  The threshold setting pattern image is conveyed to the detection range of the color misregistration detection sensor 40 in the order of the threshold setting pattern image 901, the threshold setting pattern image 902, the threshold setting pattern image 903, and the threshold setting pattern image 904. Therefore, the color misregistration detection sensor 40 outputs detection signals in the order of a threshold setting pattern image 901, a threshold setting pattern image 902, a threshold setting pattern image 903, and a threshold setting pattern image 904. FIG. 9B illustrates detection signals of the threshold setting pattern images 901, 902, 903, and 904. The threshold value is generated according to detection signals of the threshold setting pattern images 901, 902, 903, and 904. For example, the threshold value is 50% of the amplitude of the detection signal of the threshold setting pattern images 901, 902, 903, and 904.

  FIG. 10 is a flowchart showing a threshold setting process performed using such threshold setting pattern images 901, 902, 903, and 904. The CPU 70 performs this process when the main power is turned on or after forming a predetermined number of images after the previous threshold setting process.

  The CPU 70 starts the rotation of the intermediate transfer belt 8 and turns on the light emitting unit 41 of the color misregistration detection sensor 40 (S1001, S1002). The CPU 70 samples a detection signal, which is a detection result of one rotation of the intermediate transfer belt 8 by the color misregistration detection sensor 40, at 100 millisecond intervals (S1003). The CPU 70 calculates an average value Base_ave of the sampled detection signals and stores it in the RAM 78 (S1004). The average value Base_ave is a value obtained from the detection result of the surface on which the image of the intermediate transfer belt 8 is formed. At this time, no image is formed on the intermediate transfer belt 8.

  The CPU 70 forms the threshold setting pattern images 901, 902, 903, and 904 on the intermediate transfer belt 8 by the image forming units IMG-Y, IMG-M, IMG-C, and IMG-K (S1005). At this time, the measurement image forming unit 716 transmits measurement image data for forming the threshold setting pattern images 901, 902, 903, and 904 to the exposure control unit 75. Next, the CPU 70 samples a detection signal that is a detection result of the threshold value setting pattern images 901, 902, 903, and 904 by the color misregistration detection sensor 40 (S1006). Sampling is performed for each color three times at intervals of 5 milliseconds within the period Vhold shown in FIG. 9B for the detection signal that is bottom-held by the bottom-hold unit 76. The CPU 70 stores the average value of the sampled detection signals in the RAM 78 as the bottom voltage of each color detection signal. The RAM 78 stores a yellow bottom voltage Vh_y, a magenta bottom voltage Vh_m, a cyan bottom voltage Vh_c, and a black bottom voltage Vh_k. The detection signal held in the bottom hold unit 76 is hold-reset at the time Trst.

  The CPU 70 compares the difference value between the sampled bottom voltages Vh_y, Vh_m, Vh_c, Vh_k and the average value Base_ave with a predetermined value (S1007). The predetermined value is stored in the ROM 73 in advance. This process is performed in order to prevent an accurate bottom voltage from being detected due to the presence / absence of the formation of a threshold setting pattern image or a scratch at the position where the threshold setting pattern image is formed on the intermediate transfer belt 8. When the difference value is less than the predetermined value (S1007: N), the CPU 70 determines that the bottom voltage cannot be accurately detected. In this case, the CPU 70 determines whether the retry has been completed (S1010). When the retry has not been completed (S1010: N), the CPU 70 returns to the process of S1005 and acquires the bottom voltage again. When the retry has been completed (S1010: Y), the CPU 70 performs an error notification indicating that the bottom voltage cannot be detected by the display or the speaker without performing the retry (S1011).

When the difference value is larger than the predetermined value (S1007: Y), the CPU 70 calculates a threshold value (S1009). Based on the bottom voltages Vh_y, Vh_m, Vh_c, Vh_k and the average value Base_ave of each color, the CPU 70 calculates a threshold value for each color, for example, using the following equation.
(Yellow threshold) = (Base_ave−Vh_y) * 0.5 + Vh_y (Formula 3)

  Expression 3 is an expression that calculates an intermediate value (50%) between the bottom voltage Vh_y of the detection signal of the yellow threshold setting pattern image 901 and the average value Base_ave of the detection signal of the intermediate transfer belt 8 as a yellow threshold. is there. Equation 3 shows an example of calculating a threshold value for yellow, but threshold values for other colors can be similarly derived using the bottom voltages of the respective colors.

  The CPU 70 stores the calculated threshold values for each color in the RAM 78. After calculating the threshold value or notifying the error, the CPU 70 turns off the light emitting portion 41 of the color misregistration detection sensor 40 and stops the rotation of the intermediate transfer belt 8 (S1012, S1013). As described above, the threshold value for each color used for binarization of the detection signal at the time of color misregistration correction is calculated, and the threshold value setting process ends.

(Color shift detection processing)
FIG. 11 is a flowchart showing the color misregistration amount detection process. The CPU 70 performs color misregistration amount detection processing following the threshold setting processing. Here, a case where the pattern image shown in FIG. 8 enters the detection range of the color misregistration detection sensor 40 in the order of yellow, magenta, cyan, and black will be described. Therefore, pattern images are detected in the order of yellow, magenta, cyan, and black.

  When the CPU 70 starts the color misregistration detection process, the threshold adjustment control unit 711 sets a threshold corresponding to the color of the pattern image detected first among the thresholds acquired in the threshold setting process in the comparator 72 (S1101). ). In this embodiment, since a yellow pattern image is detected first, the CPU 70 sets a threshold value for yellow in the comparator 72. After setting the threshold value, the CPU 70 starts to rotate the intermediate transfer belt 8 and turns on the light emitting unit 41 of the color misregistration detection sensor 40 (S1102, S1103).

The CPU 70 forms the pattern image illustrated in FIG. 8 for detecting the color misregistration amount on the intermediate transfer belt 8 by the image forming units IMG-Y, IMG-M, IMG-C, and IMG-K (S1104). ). At this time, the measurement image forming unit 716 transmits measurement image data for forming a color misregistration detection pattern image (FIG. 8) to the exposure control unit 75 in order to form a color misregistration amount. Next, the CPU 70 initializes the pattern order counter N to “1” (S1105). The pattern order counter N specifies the color of the pattern image to be detected. In the present embodiment, the pattern order counter N repeatedly designates yellow, magenta, cyan, and black sequentially from 1. This order is determined according to the order of the colors of the pattern image detected by the color misregistration detection sensor 40.
The comparator 72 binarizes the detection signal of the yellow pattern image acquired from the color misregistration detection sensor 40 with the threshold set in S1101, and generates a binarized signal. The comparator 72 transmits the generated binarization signal to the CPU 70.

The CPU 70 starts detecting the edges (rising edge and falling edge) of the binarized signal acquired from the comparator 72 by the position detector 712 (S1106). The CPU 70 acquires time information between edges used for calculating the color misregistration amount by detecting the edge, and counts the number of detected edges. After detecting the leading edge (rising edge) of the binarized signal of the color pattern image corresponding to the pattern order counter N, the CPU 70 detects the trailing edge (falling edge) of the binarized signal of the pattern image. (S1107: Y, S1108: Y).
When detecting the trailing edge, the CPU 70 determines whether or not the number of detected edges has reached a predetermined number (S1109). The predetermined number is a number corresponding to the number of detected edges when the pattern image is correctly detected. In the present embodiment, ten sets of measurement images exemplified in FIG. 8 are formed on the intermediate transfer belt 8. The color misregistration amount acquired by the CPU 70 is an average value of the color misregistration amounts acquired from the detection results of the pattern images of the respective colors. “160”, which is the number of edges when 10 sets of measurement images are correctly detected, is the predetermined number.

  When the detected number of edges does not reach the predetermined number (S1109: N), the CPU 70 calculates the current temperature detected by the temperature sensor 50 and the temperature when the previous color misregistration correction stored in the RAM 78 is performed. It is determined whether the temperature difference is equal to or greater than a predetermined temperature difference (S1110). The CPU 70 acquires the current detected temperature from the temperature sensor 50 and performs this process. The CPU 70 stores the acquired current detected temperature in the RAM 78. If the temperature difference from the previous color misregistration correction is not greater than or equal to the predetermined temperature difference (S1110: N), the CPU 70 determines that no thermal shift has occurred. In this case, the CPU 70 uses the threshold adjustment control unit 711 to change the threshold set in the comparator 72 to a threshold for the next color (S1111). Specifically, the comparator 72 has a yellow threshold value that is detected first at the start of processing, but since the pattern image to be detected next is magenta, the threshold value for magenta is set. Be changed. Thereafter, until the number of detected edges of the binarized signal reaches a predetermined number, the threshold value is repeatedly set in the comparator 72 in order for each color of yellow, magenta, cyan, and black.

  If the temperature difference from the previous color misregistration correction is equal to or greater than the predetermined temperature difference (S1110: Y), the CPU 70 determines that there is a possibility that a thermal shift has occurred. In this case, the CPU 70 does not switch the threshold value. That is, when the temperature difference is equal to or greater than the predetermined temperature difference, the CPU 70 proceeds to step S1112. After determining whether or not a thermal shift has occurred, the CPU 70 detects the leading edge (rising edge) of the binarized signal of the pattern image of the next color (magenta) by the position detection unit 712 (S1112: Y). The CPU 70 increments the pattern order counter N by 1 (S1113), and repeats the processing of S1108 to S1113 until the number of detected edges of the binarized signal reaches a predetermined number. As a result, the CPU 70 detects the magenta pattern image after detecting the yellow pattern image. After detecting the magenta pattern image, the CPU 70 detects the pattern image in the order of cyan and black. When there is no thermal shift, every time the color of the pattern image to be detected is changed, the CPU 70 causes the threshold adjustment control unit 711 to change the threshold set in the comparator 72 for that color. On the other hand, when there is a possibility that a thermal shift has occurred, the CPU 70 sets the threshold value to be set in the comparator 72 to the threshold value for yellow regardless of the color of the pattern image to be detected.

  The CPU 70 ends the detection of the pattern image when the number of detected edges reaches the predetermined number (S1109: Y). After the detection of the pattern image, the CPU 70 calculates the color misregistration amounts in the sub-scanning direction and the main scanning direction according to the formulas 1 and 2 by the color misregistration amount calculation unit 713 (S1114). The color misregistration amount calculation unit 713 calculates the color misregistration amounts in the sub-scanning direction and the main scanning direction by substituting the time information between the edges acquired in the processing of S1107 to S1113 into Equations 1 and 2 as time intervals. The color misregistration amount calculation unit 713 derives an average value of color misregistration amounts obtained from 10 sets of measurement images. The CPU 70 stores the calculated color misregistration amounts in the main scanning direction and the sub scanning direction in the RAM 78 and uses them for color misregistration correction when performing image forming processing.

  After calculating the color misregistration amount, the CPU 70 turns off the light emitting portion 41 of the color misregistration detection sensor 40 and stops the rotation of the intermediate transfer belt 8 (S1116, S1117). Thus, the color misregistration detection processing in the sub-scanning direction and main scanning direction is completed.

  The image forming apparatus 1 according to the present embodiment as described above accurately switches the threshold value used for binarization of the detection signal according to the amount of change in temperature, so that the color misregistration amount in the main scanning direction and the sub scanning direction can be accurately determined. Can be obtained. Therefore, highly accurate color misregistration correction can be performed. In the above example, the temperature sensor 50 is disposed at a position for detecting the temperature in the vicinity of the rotary polygon mirror 52, but may be any position that can detect the temperature of a part (for example, an image forming unit) that affects the image forming position. , May be placed anywhere. Alternatively, a plurality of temperature sensors 50 may be arranged at a plurality of parts that affect the image forming position, and the threshold value switching may be determined according to the temperature change of each part.

Claims (10)

A plurality of image forming means for forming images of different colors;
An image carrier to which the image of each color is transferred from the plurality of image forming units;
Temperature detecting means for detecting the temperature of the image forming means;
Detecting means for irradiating light toward the image carrier and receiving reflected light of the light and outputting a detection signal which is an analog signal corresponding to the received light amount;
Binarization means for generating a binarized signal obtained by binarizing the detection signal according to a threshold value set for each color;
Threshold adjustment means for setting the threshold corresponding to the color of the image in the binarization means;
Timing generation means for determining whether to cause the threshold adjustment means to switch the threshold;
The measurement image of each color for detecting the color misregistration amount of each color is formed on the plurality of image forming means and transferred, thereby forming the measurement image of each color on the image carrier, and the measurement image of each color. Control means for acquiring a color misregistration amount of each color in accordance with a binarized signal based on the threshold value of each color detection signal output from the detection means that has detected an image;
Storage means for storing the temperature detected by the temperature detection means,
The timing generation unit is provided with the threshold adjustment unit according to a temperature difference between a current temperature detected by the temperature detection unit and a temperature when the previous color shift amount stored in the storage unit is acquired. It is determined whether to switch the threshold value,
Image forming apparatus. The timing generation unit causes the threshold adjustment unit to switch the threshold if the temperature difference is not equal to or greater than a predetermined temperature difference, and does not cause the threshold adjustment unit to switch the threshold if the temperature difference is equal to or greater than the predetermined temperature difference. Characterized by
The image forming apparatus according to claim 1. The timing generation means stores the current temperature detected by the temperature detection means in the storage means.
The image forming apparatus according to claim 1. The temperature detecting means is arranged at a position where the temperature of a part that affects the image forming positions of the plurality of image forming means can be detected.
The image forming apparatus according to claim 1. The plurality of image forming units each include a photoconductor on which an image is formed, an exposure unit that forms an electrostatic latent image by irradiating the photoconductor with laser light, and an electrostatic latent image formed on the photoconductor. Developing means for developing the image with a developer of a corresponding color,
The temperature detection means is arranged at a position where the temperature of the exposure means can be detected,
The image forming apparatus according to claim 1. The exposure means includes a laser light source, a rotating polygon mirror, and an imaging lens,
The temperature detection means is arranged at a position where the temperature in the vicinity of the rotary polygon mirror can be detected,
The image forming apparatus according to claim 5. The threshold adjustment means sets a threshold corresponding to the amplitude of the detection signal in the binarization means,
The image forming apparatus according to claim 1. The control unit causes the image forming unit to form a pattern image for setting a threshold value of each color and transfer the pattern image to form the threshold value setting pattern image on the image carrier. The threshold value for each color is determined according to the detected detection signal output from the detection means and the detection signal output from the detection means that detects the surface of the image carrier to which the image is transferred. It is characterized by
The image forming apparatus according to claim 7. The control means stores the color misregistration amount in a predetermined storage means.
The image forming apparatus according to claim 1. A plurality of image forming means for forming images of different colors, an image carrier to which the image of each color is transferred from the plurality of image forming means, a temperature detecting means for detecting the temperature of the image forming means, and the image Detection means for irradiating light toward the carrier and receiving reflected light of the light and outputting a detection signal that is an analog signal corresponding to the received light quantity; and a threshold value for setting the detection signal for each color A binarization unit that generates a binarized signal in accordance with the threshold, a threshold adjustment unit that sets the threshold according to the color of the image in the binarization unit, and the threshold adjustment unit that sets the threshold. A timing generation unit that determines whether to switch, a storage unit that stores the temperature detected by the temperature detection unit, and a control unit, the method being executed by the image forming apparatus, wherein the control unit By controlling
In response to an instruction from the control unit, the image forming unit forms a measurement image of each color for detecting a color shift amount, and forms the measurement image of each color on the image carrier by transferring the image. Every
The timing generation unit determines the threshold adjustment unit according to a temperature difference between the current temperature detected by the temperature detection unit and the temperature when the previous color shift amount stored in the storage unit is acquired. Determine whether to switch the threshold,
When the threshold adjustment unit determines that the timing generation unit switches the threshold, the threshold set in the binarization unit is switched according to the color,
The binarization means binarizes the image for measurement of each color with a set threshold value to generate the binarized signal;
The control means acquires a color shift amount of each color according to the binarized signal,
Color misregistration calculation method.

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