JP2019086669A - Image forming apparatus, method for controlling color shift in image forming apparatus, and program - Google Patents

Abstract

To provide an apparatus that can prevent erroneous detection of the amount of color shift, when setting a threshold voltage for binarization different for every color in color shift detection control, and the relationship between color shift detection patterns and the timing to switch a threshold voltage is broken down.SOLUTION: When the relationship between color shift detection patterns and the timing to switch a threshold voltage is broken down, the image forming apparatus detects the occurrence of an error in color shift detection, and prevents a reduction in accuracy of color shift correction due to the erroneous detection. When an instruction to switch the threshold voltage is issued while a binarization signal is in a high level, the image forming apparatus determines that switching of the threshold voltage is executed during detection of the color shift detection patterns.SELECTED DRAWING: Figure 13

Description

  The present invention relates to control for suppressing color misregistration of each color in an image forming apparatus including an image forming unit of plural colors such as a copying machine, a printer, and a facsimile.

In an electrophotographic image forming apparatus, an image is formed for each of yellow (Y), magenta (M), cyan (C), and black (K) colors through an electrophotographic process of charging, exposure, development, and transfer. There is a tandem type that superimposes them to obtain a color image. In the tandem method, printing is performed at one time, and thus there is an advantage that the printing speed is high, but there is also a disadvantage that color misregistration will occur unless the timing of sheet feeding and image formation is accurately controlled.
In response to this defect, a color misregistration detection pattern for detecting color misregistration is formed on the intermediate transfer belt etc., and this color misregistration detection pattern is detected by an optical sensor etc. A method of calculating the amount) is generally used in an image forming apparatus.

FIGS. 3A to 3F are diagrams showing the relationship between the color misregistration detection pattern and the output of an optical sensor (color misregistration detection sensor) for pattern position detection. (A) and (d) show a state in which toner is stacked on the intermediate transfer belt in a state where the color misregistration detection pattern is viewed from the horizontal direction. (B) and (e) are output signals of the color misregistration detection sensor, and (c) and (f) are output signals of (b) -1 to (b) -3 and (e) -1 to (e)- The signal binarized with the threshold voltage of 3 is shown. When pattern position information is acquired from the output waveform of the color misregistration detection sensor, it is general to binarize the waveform and acquire an intermediate point between the rising edge and the falling edge of the binarized waveform as a pattern position. .
The output of the optical sensor when a pattern having a uniform density (toner application amount) is read as shown in FIG. 3A has a symmetrical waveform as shown in FIG. 3B. At this time, the position of the threshold voltage to be binarized is 75% of the signal amplitude ((b) -1), 50% of the position ((b) -2), 25% of the position ((b) -3) When it changes like ()), a binarized waveform will become like (c) -1, (c) -2, (c) -3, respectively. The pattern positions obtained therefrom are (c) -reg-1, (c) -reg-2, and (c) -reg-3. It can be seen that the detected pattern position does not change even if the threshold voltage changes.

  On the other hand, when the density (toner coverage) of the color misregistration detection pattern is nonuniform as shown in FIG. 3D, the waveform is asymmetric as shown in FIG. 3E. Therefore, when the position of the threshold voltage to be binarized similarly changes 75% to 25% ((e) -1, (e) -2, (e) -3) with respect to the signal amplitude, (f) The pattern detection position changes as indicated by -reg-1, (f) -reg-2, and (f) -reg-3. That is, a detection error occurs depending on the position of the threshold voltage with respect to the waveform.

  For this reason, in Patent Document 1, even if the signal amplitude changes to, for example, Va or Vb as shown in FIG. 4, the threshold voltage Vth is at a uniform ratio (50% of the amplitude in FIG. 4). There has been proposed a technique for suppressing detection errors due to waveform asymmetry by performing control as described above. By changing the threshold voltage in accordance with the signal amplitude in this manner, it is possible to suppress detection errors due to waveform asymmetry.

JP, 2013-25184, A

However, as shown in FIG. 5, when the relationship between the color misregistration detection pattern between each color and the threshold voltage switching timing is broken, the sensor output of the color misregistration detection sensor is binarized with different threshold voltages for rising and falling. An error occurs in the detection position of the color misregistration detection pattern. Specifically, when the sensor output of the left-right symmetrical and color misregistration detection sensor is binarized with the same threshold voltage for both rising and falling, the threshold voltage may be 25% to 75% as described above with reference to FIG. There is no error in the detected position of the pattern even if it fluctuates to. However, when binarization is performed with different threshold voltages for rising and falling, a detection error occurs as shown in FIG.
In FIG. 6, 6- (b) is a sensor output of the color misregistration detection sensor, (c) -6-1 is a signal binarized with the same threshold voltage at the rise and fall, (c) -6-2 Is a signal binarized with different threshold voltages at the rise and fall. Then, (c) -reg-6-1 and (c) -reg-6-2 are detection positions calculated from respective binarized signals. Here, (c) -reg-6-1 is a detection position that is originally desired to be detected, but (c) -reg-6-2 is a detection position shifted by a detection error reg-error. That is, when binarization is performed with different threshold voltages for rising and falling, a detection error occurs in both of the case of waveform asymmetry and the case of symmetry.

  The present invention has been made to solve the above-mentioned problems. An object of the present invention is to provide a mechanism capable of preventing a color shift detection error.

  The present invention comprises a plurality of image forming means for forming toner images of different colors, and a pattern forming means for forming color misregistration detection patterns for detecting color misregistration of the toner images on an image carrier. A light emitting portion for emitting light to the toner image or the surface of the image carrier, and a light reception detection signal having an output level corresponding to the light amount of the reflected light by receiving the reflected light from the toner image or the image carrier A detection unit having a light receiving unit for outputting, a binarization unit for binarizing the light reception detection signal with a certain threshold voltage, and a threshold for adjusting the threshold voltage from the threshold voltage of the first color to the threshold voltage of the second color Determining means for associating the timing to be adjusted from the threshold voltage of the first color to the threshold voltage of the second color and the value of the binarized light reception detection signal at the timing; Equipped with It is an image forming apparatus according to claim.

  Even when the relationship between the color misregistration detection patterns and the switching timing of the threshold voltage is broken, it is detected that a color misregistration detection error has occurred, and the decrease in color misregistration correction accuracy due to false detection is suppressed.

FIG. 1 is a cross-sectional view of an image forming apparatus of the present invention. It is a schematic diagram of the misregistration detection sensor of this invention. It is an explanatory view of a color shift detection pattern and various signals. It is explanatory drawing of the sensor output and threshold voltage of a color shift detection sensor. It is explanatory drawing of the sensor output and threshold voltage of a color shift detection sensor. It is a figure explaining the detection error by the threshold voltage switching timing error. It is a control block diagram of the image forming apparatus of the present invention. It is the schematic of the pattern for color misregistration detection of this invention, and a binarization signal. It is the schematic of the pattern for threshold voltage setting of this invention, and a bottom hold output. FIG. 7 is an explanatory diagram of a threshold voltage setting sequence executed in the first embodiment. FIG. 7 is an explanatory diagram of a color shift amount detection sequence executed in the first embodiment. FIG. 7 is an explanatory diagram of an edge detection sequence performed in the first embodiment. FIG. 7 is an explanatory diagram of a threshold voltage switching timing error determination sequence executed in the first embodiment. FIG. 6 is a schematic view of sensor output of the color misregistration detection sensor and threshold voltage switching timing in the first embodiment. FIG. 14 is an explanatory diagram of a color shift amount detection sequence executed in the second embodiment. FIG. 10 is an explanatory diagram of an edge detection sequence performed in the second embodiment. FIG. 10 is a schematic view of sensor output and threshold voltage of the color misregistration detection sensor in Embodiment 2.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

  FIG. 1 is a schematic cross-sectional view for explaining an image forming apparatus which is an example of an embodiment of the present invention.

The image forming apparatus 1 of the present invention has image carriers 2a to 2d which are means for forming an image of each color of yellow, magenta, cyan and black.
The laser scanning units 5a to 5d as pattern forming means use semiconductor lasers as light sources. The laser scanning units 5a to 5d irradiate the laser to the respective surfaces of the image carriers 2a to 2d to form electrostatic latent images on the drum surfaces.
The electrostatic latent images of the image carriers 2a to 2d are developed by the developing devices 7a to 7d to form toner images. The toner images of the respective colors are sequentially superimposed on the intermediate transfer belt (intermediate transfer member) 8 at the primary transfer portions 6a to 6d.
The intermediate transfer belt 8 is supported and conveyed via the rollers 10, 11, 21. The toner images of the respective colors superimposed on the intermediate transfer belt 8 are conveyed to the secondary transfer portion 22, and the four colors are collectively transferred onto the sheet conveyed to the secondary transfer portion 22.
Reference numerals 3a to 3d denote chargers for charging the surfaces of the image carriers 2a to 2d, reference numerals 4a to 4d denote cleaners for removing toner remaining on the surfaces of the image carriers 2a to 2d, and reference numeral 12 denotes an intermediate transfer belt 8 It is a cleaner that removes the toner remaining in the

The sheet on which the four color toner images have been collectively transferred by the secondary transfer unit 22 is conveyed to the fixing unit 23 and the unfixed toner image is thermally fixed, and then the sheet is conveyed to the sheet discharge tray 25 via the sheet discharge roller 24. Exhausted.
On the other hand, the sheet is fed from the sheet feeding cassette 17 or the manual feed tray 13 to the conveyance path, the lateral registration position is corrected by the electrostatic conveyance means 30, and registration timing is taken by the registration roller 16 to the secondary transfer portion 22. It is transported.
At that time, the sheet conveyance units such as the pickup rollers 18, 19 for feeding the sheet from the sheet feeding cassette 17 to the conveyance path, the vertical pass roller 20, and the registration roller 16 each realize high speed and stable conveyance operation. It is driven by an independent stepping motor.
In addition, the sheet transport units such as the pickup rollers 14 and 15 for feeding the sheet from the manual feed tray 13 are similarly driven by independent stepping motors.

Further, at the time of double-sided printing, a sheet that has passed through the fixing device 23 is guided from the paper discharge roller 24 to the both-side reversing path 27 and is reversely transported in the reverse direction and transported to the both-side pass 28. The sheet having passed the duplex path 28 is conveyed again to the secondary transfer portion 22 through the longitudinal pass roller 20 in the same manner as described above.
The toner images of the respective colors are collectively transferred from the intermediate transfer belt 8 on the back surface of the sheet conveyed to the secondary transfer unit 22, and the sheet after transfer is transferred to the sheet discharge tray 25 via the fixing device 23 and the sheet discharge roller 24. Exhausted.

  In the image forming apparatus 1 of the present invention, the color misregistration detection sensor 40 is disposed between the image carrier 2 d positioned on the right side of the intermediate transfer belt 8 and the support roller 10 of the intermediate transfer belt 8. The color misregistration detection sensor 40 is for detecting a color misregistration detection pattern for detecting a color misregistration amount transferred from the image carriers 2 a to 2 d to the intermediate transfer belt 8. It is controlled. The color misregistration detection pattern is formed as a toner image in the image forming unit such as the image carriers 2 a to 2 d for each color and transferred to the intermediate transfer belt 8.

As shown in FIG. 2, the misregistration detection sensor 40 as a detection unit has a light emitting unit 51 and a light receiving unit 52. The light receiving unit 52 is configured to detect regular reflection light of an object that has received light from the light emitting unit 51. The light emitted from the light emitting unit 51 hits the intermediate transfer belt 8 located at the opposite position of the light emitting unit 51 or the misregistration detection pattern on the intermediate transfer belt, and the reflected light is collected by the lens 53 and is received by the light receiving unit 52. It will be incident. Further, the output potential of the light receiving unit 52 changes in accordance with the amount of light incident on the light receiving unit 52. As shown in FIG. 3, since the intermediate transfer belt 8 has a higher reflectance than the color shift detection pattern, the sensor output when the color shift detection pattern is read has a waveform in which the voltage decreases.
The output waveform is binarized to calculate each pattern position of the color misregistration detection pattern.

Next, the control system of the image forming apparatus of the present invention will be described with reference to FIG.
As described above, the color misregistration detection sensor 40 outputs the sensor output of the color misregistration detection pattern as a detection signal. The sensor output is input to the comparator 72 and binarized with a threshold voltage set by the CPU 70 in advance. The threshold voltage is output from the threshold voltage adjustment unit 711 of the CPU 70. The binarized signal binarized by the comparator 72 is input to the CPU 70.

The CPU 70 has a threshold voltage adjustment unit 711, a pattern reading unit 712, and a color shift amount calculation unit 713. The threshold voltage adjustment unit 711 generates a threshold voltage by a D / A converter.
The pattern reading unit 712 detects the rising edge and the falling edge of the binarized binarized signal and calculates the timing. The color misregistration amount calculation unit 713 calculates the color misregistration amount from the detection timing calculated by the pattern reading unit 712.
Further, the bottom hold circuit 76 is used when sampling the value of the sensor output in a threshold setting sequence described later. The threshold voltage switching timing generation unit 717 is used in threshold voltage switching control, and generates a predetermined time Ta as described later. The RAM 78 is used as a temporary data storage area.

Here, the color misregistration calculation method performed by the color misregistration amount calculation unit 713 will be described.
FIG. 8 shows a schematic view of the color misregistration detection pattern in the present embodiment, and a binarized signal binarized by the comparator from the color misregistration detection sensor 40 which has read the pattern. The broken line in the binarized signal indicates the central position between the rising edge and the falling edge.
The misregistration detection patterns are formed from yellow patterns 801 and 811, magenta patterns 802 and 812, cyan patterns 803 and 813, and black patterns 804 and 814. The respective patterns are formed to be inclined by 45 ° with respect to the main scanning direction, and the inclination directions are opposite between 801 to 804 and 811 to 814. Further, ym_1, yc_1, yk_1, ym_2, yc_2, yk_2 indicate time information between detection signals of a yellow pattern which is a reference color and patterns of other colors in this embodiment. The color misregistration amount is calculated from this time information.

  For example, regarding magenta color shift amount calculation, when magenta shifts color in the (+) direction of sub scanning, ym_1 and ym_2 become proportional to the color shift amount and increase by the same amount. When it shifts to the (−) side, it changes (decreases) by the same amount as ym_1 and ym_2. On the other hand, when the color shift is made in the (+) direction of the main scanning, ym_1 becomes larger in proportion to the color shift amount, and ym_2 becomes smaller by the same amount. When it shifts to the (−) side, ym_1 becomes smaller and ym_2 becomes larger by the same amount.

Therefore, the color misregistration amount is calculated from the following equation.
Sub-scanning color shift = X− (ym_1 + ym_2) / 2 × conveyance speed (1)
Main scanning color shift = (ym_1−ym_2) / 2 × conveyance speed (2)
In the above equation, X is distance interval information of the yellow pattern and the magenta pattern in the sub-scanning direction when no color misregistration has occurred. Further, since ym_1 and ym_2 are time information (sec), they are converted into distance information by multiplying the transport speed (mm / sec) of the intermediate transfer belt 8 on which the color misregistration detection pattern is formed.
Although this equation is described taking magenta color shift as an example, the color shift amounts of other colors can be derived similarly by using time information between yellow patterns.

  The CPU 70 further includes a light emission control unit 714 that controls the light emission of the light emission unit 51 of the color shift detection sensor, an A / D converter 715 that records the output level of the color shift detection sensor 40, and a color shift pattern forming unit 716. The color shift pattern forming unit 716 has data at the time of forming a color shift detection pattern and a threshold setting pattern, as will be described later, and controls a laser scanning unit that controls laser emission when forming a color shift pattern. Send data to section 75. The RAM 78 stores a threshold set in the comparator 72, a color shift amount calculated by the color shift amount calculation unit 713, and the like, and the CPU 70 refers to it at startup or the like to perform setting in the image processing control unit 74 or the like.

[Threshold setting sequence]
Next, a threshold voltage setting sequence for binarizing the output signal of the color misregistration detection sensor 40 will be described with reference to the flowchart of FIG. The processing of this flowchart is realized by the CPU 70 reading out and executing a program stored in the ROM 73 as necessary.

In this sequence, the threshold voltage of each color is set using the threshold voltage setting pattern shown in FIG. FIG. 9B is an output signal of the color misregistration detection sensor 40 when the threshold voltage setting pattern is read.
In the threshold voltage setting pattern shown in FIG. 9A, the yellow 901, magenta 902, cyan 903 and black 904 patterns are inclined 45 ° in the main scanning direction, similarly to the color shift detection pattern of FIG. It is formed. A different point is that in the threshold voltage setting pattern, since it is necessary to sample and hold the pattern detection value described later, the interval in the sub scanning direction between the patterns is relatively long.

When the sequence starts, the CPU 70 starts the rotation of the intermediate transfer belt 8 (S1001), and causes the light emitting unit 51 of the color misregistration detection sensor 40 to emit light (S1002).
Then, the CPU 70 performs sampling for one rotation of the intermediate transfer belt 8 at an interval of 100 msec at the detection level when the color misregistration detection sensor 40 receives the reflected light from the intermediate transfer belt 8 (S1003).
After the end of sampling, the average value (Base_ave) of the acquired data is calculated and stored in the RAM 78 (S1004).

Then, the CPU 70 forms a threshold voltage setting pattern on the intermediate transfer belt 8 (S1005).
Next, the CPU 70 samples the bottom voltage at the time of detecting the pattern with the yellow, magenta, cyan and black patterns (S1006). The sampling is performed for each color three times at intervals of 5 msec within the period indicated by Vhold in FIG. 9B, with the output signal bottom-held by the bottom hold circuit 76. The RAM 78 stores the average value of three times as the bottom voltage of each color. The bottom voltage of each color is set to Vh_y for yellow, Vh_m for magenta, Vh_c for cyan, and Vh_k for black. The held output signal is hold reset at the timing of Trst.

  Next, the CPU 70 compares the sampled bottom voltage and the previously acquired Base_ave. If the difference amount is equal to or less than a predetermined value set beforehand and stored in the RAM 78, it is determined that the bottom voltage can not be accurately detected (S1007). The reason is that the toner pattern is not formed, and the read position of the intermediate transfer belt 8 is scratched.

If there is no problem, the process proceeds to the retry completion determination (S1011).
If the CPU 70 determines in S1011 that the process has already been retried, the CPU 70 does not retry and sends an error notification (S1012). Thereafter, the process proceeds to turn off the light emitting unit (S1010).
If it is determined in S1011 that the retry has not been completed, the process returns to S1005, and a threshold voltage setting pattern is formed.

If there is no problem in S1007 (Yes in S1007), the CPU 70 calculates the threshold voltage of each color from the acquired bottom voltage of each color pattern and the previously acquired Base_ave (S1009).
The calculation formula is as follows.
Yellow threshold voltage = (Base_ave−Vh_y) × 0.5 + Vh_y ··· (3)
This equation is an equation for calculating a threshold voltage at an intermediate (50%) position between the yellow bottom voltage Vh_y and the detection level Base_ave of the intermediate transfer belt 8. Although the threshold voltage calculation formula of yellow is described as an example, other colors can be derived similarly using bottom voltages of the respective colors.
After calculating the threshold voltage in step S1009, the CPU 70 turns off the light emitting unit 51 (S1010) and stops the rotation of the intermediate transfer belt (S1013). Then, the threshold voltage setting sequence ends.

[Color shift amount detection sequence]
The color misregistration amount detection sequence will be described using the flowchart of FIG. The processing of this flowchart is executed by the CPU 70.

  The CPU 70 starts the color shift amount detection sequence when the main power is turned on or after an image having a specified number or more is formed. The aforementioned threshold voltage setting sequence and the start condition are the same, and the threshold voltage setting sequence is performed in advance.

When the sequence is started, the CPU 70 starts rotation of the intermediate transfer belt (S1101), turns on the light emitting unit 51 (S1102), and forms a color shift detection pattern on the intermediate transfer belt 8 (S1103).
Then, the edge detection sequence (S1104) and the threshold voltage switching timing error determination sequence are started (S1105).

Here, the edge detection sequence (S1104) is executed to detect the color misregistration detection pattern of each color. In the edge detection sequence, the CPU 70 sets the threshold voltages of the respective colors to detect the leading and trailing edges of the misregistration detection pattern, and calculates time information of the yellow pattern which is the reference color and the patterns of the other colors. . Details of the edge detection sequence will be described later.
The threshold voltage switching timing error determination sequence (S1105) is executed to determine whether threshold voltage threshold voltage switching has been performed during pattern detection. The details of the threshold voltage switching timing error determination sequence will also be described later.

When these sequences are completed, the CPU 70 determines whether there is a threshold voltage switching timing error (S1106).
If there is an error, the CPU 70 forms a color shift detection pattern on the intermediate transfer belt 8 again. At this time, the pattern interval of each color is expanded to form a color misregistration detection pattern (S1107), and the process shifts to S1104. Here, the reason for widening the pattern interval of each color is that the pattern of the next color has been detected before switching the threshold voltage due to the narrowing of the interval of the color misregistration detection patterns. By widening the pattern interval of each color in this manner, the threshold voltage switching timing error can be eliminated.

If there is no error in S1106 (Yes in S1106), the CPU 70 calculates the amount of color misregistration for each color (S1108).
When the calculation of the color misregistration amount is completed, the light emitting unit 51 is turned off (S1109), the intermediate transfer belt 8 is stopped (S1110), and the color misregistration amount acquisition sequence is completed.

Edge detection sequence
The edge detection sequence will be described using the flowchart of FIG.

  The CPU 70 starts an edge detection sequence in S1104 (FIG. 11) of the color shift amount detection sequence.

When the sequence starts, the CPU 70 first sets a threshold voltage (S1201).
In this embodiment, as shown in FIG. 8, since the pattern 801 of yellow of the reference color is formed, the threshold voltage of yellow is set.
Then, the leading edge of the pattern, that is, the rising edge of the signal in which the sensor output of the color misregistration detection sensor 40 is binarized by the comparator 72 is detected (S1202).
Then, the trailing edge of the pattern, that is, the falling edge of the binarized signal is detected (S1203).

Next, the CPU 70 determines whether all patterns have been detected (S1204).
If it is determined that the process is not completed, the CPU 70 waits for a predetermined time Ta, and then proceeds to S1201 to set the threshold voltage of the next color. Here, the reason for waiting for the predetermined time Ta is that, if the trailing edge of the pattern is detected in S1203 immediately after switching to the threshold voltage of the next color, the trailing edge may be detected twice. It is because there is.

  In the present embodiment, the yellow pattern 801 is followed by the magenta pattern 802, and steps S1201 to S1205 are similarly executed until the cyan pattern 813. Then, when S1201 to S1203 are executed for the black pattern 814, detection of all the patterns ends.

Then, the CPU 70 determines that the pattern detection is completed (S1204).
Then, ym_1, yc_1, yk_1, ym_2, yc_2, yk_2 which are time information between patterns of each color with respect to the reference color are calculated (S1206), and the edge detection sequence is ended.

  Next, threshold switching timing error determination will be described.

  FIG. 14 is a diagram showing the relationship between the sensor output of the color misregistration detection sensor 40 at the time of detecting the color misregistration detection pattern, the binarized signal obtained by binarizing the signal with the threshold voltage in the comparator 72, and the threshold voltage. It is.

FIG. 14 (a) shows the case where switching of the threshold voltage in a normal state is performed, and FIG. 14 (b) shows the case where a switching timing error of the threshold voltage occurs.
When detecting the color misregistration detection pattern, the CPU 70 switches the threshold voltage in the threshold voltage adjustment unit 711 so that the threshold voltage is switched between the patterns. The switching of the threshold voltage is performed after a predetermined time Ta with reference to the falling edge of the binarized signal of the previous pattern.
In both FIGS. 14A and 14B, the cyan threshold voltage Va / 2 (1.7 V) to the black threshold voltage Vb / 2 (1.6 V) when cyan and black patterns are detected. Fig. 6 illustrates switching of the threshold voltage to
The CPU 70 determines whether the binarization signal is at the low level or the high level at timing Ta when the threshold voltage is switched from the threshold voltage of cyan to the threshold voltage of black. Thereby, the switching of the threshold voltage is associated with the value of the binarized signal.

  Focusing on black, the switching of the threshold voltage is completed before detecting the black pattern in FIG. 14A, and the black binarized signal has both the rising edge and the falling edge the black threshold voltage Vb / 2. Is binarized. That is, switching from the threshold voltage Va / 2 of cyan to the threshold voltage Vb / 2 of black is performed when the binary signal is at the low level. In this case, it is determined that there is no threshold voltage switching timing error.

  On the other hand, in FIG. 14B, switching of the threshold voltage is performed while detecting the black pattern. Therefore, while the threshold voltage is set to the cyan threshold voltage Va / 2 at the rising edge of the black binarized signal, it is set to the black threshold voltage Vb / 2 at the falling edge. That is, switching from the threshold voltage Va / 2 of cyan to the threshold voltage Vb / 2 of black is performed when the binarized signal is at the Hi level. In this case, it is determined that a threshold voltage switching timing error has occurred.

[Threshold voltage switching timing error determination sequence]
The sequence of the threshold switching timing error determination described above will be described with reference to the flowchart of FIG.

  The CPU 70 starts a threshold switching timing error determination sequence in S1105 (FIG. 11) of the color shift amount detection sequence.

When the sequence is started, the CPU 70 first determines whether the binarized signal is at the Hi level (S1301).
If it is determined that the binarization signal is at the low level, the process returns to step S1301 to wait until it becomes the high level.
On the other hand, when it is determined that the binarized signal is at the Hi level, the CPU 70 determines whether or not there is an instruction to switch the threshold voltage (S1302).
If it is determined that there is a threshold voltage switching instruction, the CPU 70 determines that the threshold voltage has been switched during pattern detection, and notifies a threshold voltage switching timing error (S1303).
On the other hand, when it is determined that there is no instruction to switch the threshold voltage, the CPU 70 determines whether all the pattern detection has ended (S1304). If it is determined that the pattern detection is not completed, the process returns to S1301 to perform threshold value switching timing error determination of the next pattern.

  In the present embodiment, as shown in FIG. 8, yellow patterns 801 to S1301 to S1302 of the reference color are executed, and S1301 to S1302 are similarly executed from the magenta pattern 802 to the black pattern 814. When the CPU 70 determines that all the patterns have been detected, it ends the threshold switching timing error determination sequence.

  Next, a second embodiment which is a modification of the first embodiment will be described.

In the second embodiment, the determination of the threshold voltage switching timing error is not performed, and the threshold voltage switching is not performed during detection of the color misregistration detection pattern. That is, the threshold voltage is switched only when the binarized signal is at the low level, which is a state in which the color misregistration detection pattern is not detected.
On the other hand, the threshold voltage is not switched when the binarized signal is at the Hi level in a state where the color misregistration detection pattern is detected. As a result, binarization is not performed by different threshold voltages at the rise and fall of the binarized signal, and the detection error of the misregistration detection pattern is reduced.

  The second embodiment will be described below because the color shift amount detection sequence and the edge detection sequence are different from the first embodiment. The threshold voltage setting sequence is the same as that of the first embodiment, and thus the description thereof is omitted. This will be described below with reference to FIGS. 15 and 16.

[Color shift amount detection sequence]
The misregistration amount detection sequence according to the second embodiment will be described with reference to the flowchart of FIG. The processing of this flowchart is executed by the CPU 70.

  The CPU 70 starts the color shift amount detection sequence when the main power is turned on or after an image having a specified number or more is formed. The aforementioned threshold voltage setting sequence and the start condition are the same, and the threshold voltage setting sequence is performed in advance.

When the sequence starts, the CPU 70 starts rotation of the intermediate transfer belt (S1501), turns on the light emitting unit 51 (S1502), and forms a color shift detection pattern on the intermediate transfer belt 8 (S1503).
Then, the edge detection sequence is started (S1504). When the edge detection sequence is completed, the color misregistration amount of each color is calculated (S1505).
When the calculation of the color misregistration amount is completed, the CPU 70 turns off the light emitting unit 51 (S1506), and stops the intermediate transfer belt 8 (S1507). Thus, the color shift amount acquisition sequence ends.

Edge detection sequence
The edge detection sequence of the second embodiment will be described with reference to the flowchart of FIG.

The CPU 70 starts an edge detection sequence at S1504 (FIG. 15) of the color shift amount detection sequence.
When the sequence starts, the CPU 70 first sets a threshold voltage (S1601). In this embodiment, as shown in FIG. 8, since the pattern 801 of yellow of the reference color is formed, the threshold voltage of yellow is set.

Thereafter, the CPU 70 detects the leading edge of the pattern, that is, the rising edge of the signal obtained by the comparator 72 converting the sensor output of the color misregistration detection sensor 40 into binary data (S1602).
Thereafter, the trailing edge of the pattern, that is, the falling edge of the binarized signal is detected (S1603).

Next, the CPU 70 determines whether all pattern detection is completed (S1604).
If it is determined that the process has not ended, it is determined whether a predetermined time Ta has elapsed (S1605).

If it is determined that the predetermined time Ta has elapsed, the CPU 70 shifts to S1601 and sets the threshold voltage of the next color.
On the other hand, when it is determined that the predetermined time Ta has not elapsed, the CPU 70 determines whether the binarized signal is at the Hi level (S1606). If it is determined that the binarization signal is at the low level, the process returns to step S1605, and it is determined again whether the predetermined time Ta has elapsed.
On the other hand, if it is determined that the binarized signal is at the Hi level regardless of whether the predetermined time Ta has elapsed, the process proceeds to S1602. In this case, the rising edge of the pattern tip is detected without switching the threshold voltage. Thus, the timing at which the threshold voltage should be adjusted after the predetermined time Ta has elapsed is associated with the value of the binarized signal at that time.

That is, as shown in FIG. 17, when the CPU 70 detects the edge of the pattern rear end and the binarization signal becomes Hi level before the predetermined time Ta has elapsed, the threshold is detected when the predetermined time Ta has elapsed. Do not switch voltage. This is because the color misregistration detection pattern of the next color is detected before the predetermined time Ta elapses.
As described above, by detecting the leading end and the trailing end of the color misregistration detection pattern using the threshold voltage of the previous color, detection by binarizing the rising and falling of the binarized signal with different threshold voltages The error is avoided.

In the present embodiment, the yellow pattern 801 is followed by the magenta pattern 802, and steps S1601 to S1606 are similarly executed until the cyan pattern 813. Then, when S1601 to S1603 are executed for the black pattern 814, detection of all the patterns ends.
If it is determined that all pattern detection is completed (S1604), the CPU 70 calculates ym_1, yc_1, yk_1, ym_2, yc_2, yk_2 which are time information between patterns of respective colors with respect to the reference color (S1607). Then, the edge detection sequence ends.

  As described above, in the second embodiment, when the relationship between the color misregistration detection patterns and the switching timing of the threshold voltage is broken, the threshold voltage is not switched and color misregistration detection is performed using the threshold voltage of the previous color. By detecting the leading end and the trailing end of the pattern, it is possible to suppress the decrease in color misregistration correction accuracy due to false detection.

Other Embodiments
The present invention supplies a program that implements one or more functions of the above-described embodiments to a system or apparatus via a network or storage medium, and one or more processors in a computer of the system or apparatus read and execute the program. Can also be realized. It can also be implemented by a circuit (eg, an ASIC) that implements one or more functions.
Further, the present invention is not limited to the above embodiments, and various modifications (including organic combinations of the respective embodiments) are possible based on the spirit of the present invention, and they are excluded from the scope of the present invention It is not something to do. That is, the configuration in which each of the above-described embodiments and their modifications are combined is also included in the present invention.

40: Misregistration detection sensor 70: CPU
711: threshold voltage adjustment unit 713: color shift amount calculation unit 717: threshold voltage switching timing generation units 801 to 804, 811 to 814, 901 to 904 color shift correction patterns

Claims (9)

A plurality of image forming means for forming toner images of different colors respectively;
Pattern forming means for forming a color misregistration detection pattern for detecting color misregistration of the toner image on an image carrier;
A light emitting portion for emitting light to the toner image or the surface of the image carrier, and a light reception detection signal having an output level corresponding to the light amount of the reflected light by receiving the reflected light from the toner image or the image carrier Detection means comprising a light receiving unit for outputting;
Binarizing means for binarizing the light reception detection signal with a threshold voltage;
Threshold voltage adjusting means for adjusting the threshold voltage from the threshold voltage of the first color to the threshold voltage of the second color;
Have
It is characterized by comprising a determination means for correlating the timing to be adjusted from the threshold voltage of the first color to the threshold voltage of the second color, and the value of the light reception detection signal binarized at the timing. , Image forming apparatus.   The threshold voltage switching timing generation unit adjusts the threshold voltage of the first color to the threshold voltage of the second color when a predetermined time Ta elapses after the rise of the binarized light reception detection signal is detected. The image forming apparatus according to claim 1, wherein the timing to be set is set.   The threshold voltage adjusting means adjusts the threshold voltage of the first color to the threshold voltage of the second color at the timing to adjust from the threshold voltage of the first color to the threshold voltage of the second color. The image forming apparatus according to 1 or 2.   When the value of the light reception detection signal binarized at the timing to be adjusted from the threshold voltage of the first color to the threshold voltage of the second color is a low level, the determination means does not have a threshold voltage switching timing error The image forming apparatus according to any one of claims 1 to 3, wherein the determination is made.   When the value of the light reception detection signal binarized at the timing to be adjusted from the threshold voltage of the first color to the threshold voltage of the second color is Hi level, the determination means generates a threshold voltage switching timing error The image forming apparatus according to any one of claims 1 to 4, wherein it is determined that   When the value of the light reception detection signal binarized at the timing to be adjusted from the threshold voltage of the first color to the threshold voltage of the second color is the low level, the threshold voltage adjustment means sets the threshold voltage to the first threshold voltage. 3. The image forming apparatus according to claim 1, wherein the threshold voltage of the color is adjusted to the threshold voltage of the second color.   When the value of the light reception detection signal binarized at the timing to be adjusted from the threshold voltage of the first color to the threshold voltage of the second color is the Hi level, the threshold voltage adjustment means performs the first threshold voltage adjustment. 7. The image forming apparatus according to claim 1, wherein no adjustment is made from the threshold voltage of color to the threshold voltage of second color. A plurality of image forming steps for forming toner images of different colors respectively;
Forming a color misregistration detection pattern for detecting color misregistration of the toner image on an image carrier;
A light emitting process of irradiating light to the surface of the toner image or the image carrier, and a light reception detection signal having an output level corresponding to the light amount of the reflected light by receiving the reflected light from the toner image or the image carrier A detection step including a light receiving step of outputting;
A binarization step of binarizing the light reception detection signal with a threshold voltage;
Adjusting the threshold voltage from the threshold voltage of the first color to the threshold voltage of the second color;
Have
A determination step of correlating the timing to be adjusted from the threshold voltage of the first color to the threshold voltage of the second color and the value of the light reception detection signal binarized at the timing is characterized. , Color shift control method of the image forming apparatus.   A program for making a computer execute the color shift control method according to claim 8.

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