JP2017076082A - Image forming apparatus and control method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a correction error caused by a change in peripheral length and peripheral speed of an image carrier, in eliminating reflection unevenness during adjustment of image density.SOLUTION: A CPU 501 performs sampling in synchronization with a belt reference signal, acquires reflection unevenness data including a reflection unevenness component for one cycle of an intermediate transfer belt 31, and sequentially detects five-stage pattern images PT to acquire as pattern detection data. The CPU 501 then converts the pattern detection data after reflection unevenness correction in which the reflection unevenness component is removed from the pattern detection data into a density value on the basis of a density value conversion table, and performs density adjustment by changing various image process parameters to have a proper density on the basis of the calculated density value. The CPU 501 forms a pattern image PT1 with the lowest density at a position closest to a reference position P at the rear side in a belt rotation direction on the basis of an elapsed time T from the time point at which the reference position P reaches the position of a toner density sensor 60.SELECTED DRAWING: Figure 10

Description

  The present invention relates to a technique for adjusting the density of an image formed on an endless image carrier.

  2. Description of the Related Art Conventionally, a technique for adjusting image density in an electrophotographic image forming apparatus such as a printer or a copying machine is known. In this type of apparatus, for example, using an optical sensor having a light emitting element and a light receiving element, the density of the image is adjusted based on the result of reading a measurement image formed on an image carrier such as an intermediate transfer belt. However, when the surface of the image carrier has uneven reflectance (reflection unevenness), the amount of light reflected from the image carrier is not uniform over the entire circumference of the image carrier. Therefore, conventionally, correction processing for reducing the influence of the reflection unevenness on the surface of the image carrier from the data obtained by measuring the reflection unevenness data of the entire circumference in advance and storing it in the memory and detecting the image pattern for density detection is performed.

  For example, the image forming apparatus described in Patent Document 1 includes a storage unit that stores in advance detection results (profile data) corresponding to one rotation of the image carrier, and the measurement results of the measurement image are stored as profile data. Use to correct. Then, an image forming condition for adjusting the image density is generated based on the corrected measurement result.

JP 2006-220845 A

  However, even when the influence of reflection unevenness is reduced by the correction processing according to Patent Document 1, the image density may not be adjusted properly. For example, there is a case where the circumference of the image carrier changes in expansion and contraction or the rotation speed changes due to thermal expansion and contraction of the image carrier and a driving roller that rotationally drives the image carrier. In these cases, when the measurement of uneven reflection data of the entire circumference of the image carrier and when the pattern image is detected, if the circumference and rotational speed do not match, even if sampling is performed at a predetermined time interval, Data at the exact same position cannot be detected. If the detection position is deviated, a correction error in removing the influence of the reflection unevenness on the surface of the image carrier that is received when the measurement image is detected increases, and the accuracy of density adjustment may be reduced. Especially those with large reflection unevenness depending on the material of the image carrier, or those with uneven reflection on the surface of the image carrier due to wear or scratches due to durable use, have a large influence and returned when density adjustment is performed. There is also a risk of increasing the density deviation.

  Here, the above-described problem will be described in detail with reference to FIGS. An intermediate transfer belt is taken as an example of the image carrier.

  FIG. 17 is a waveform diagram for comparing the uneven reflection due to the presence or absence of thermal expansion of the belt. The belt reference signal is a signal for specifying a reference position with respect to the belt rotation direction generated every rotation of the belt. The belt is provided with a reference sealing material, and a signal detected by an optical sensor is a belt reference signal. A belt reference signal indicated by a solid line is a signal when the belt is not thermally expanded. A belt reference signal indicated by a dotted line is a signal when the belt is thermally expanded. Here, a case where the belt circumferential length is shifted by about 1.5 mm due to thermal expansion is taken as an example. If the belt conveyance speed is 300 mm / sec, the time required for one rotation of the belt due to thermal expansion will change by about 5 msec (= 1.5 mm ÷ 300 mm / sec × 1000).

  The belt reflection unevenness signal is a waveform that indicates sensor output fluctuation when the belt surface (background) is detected by an optical sensor for toner density detection. In the belt conveyance direction, near the belt reference position where the belt reference signal is detected, the belt reflection nonuniformity signals substantially coincide with each other when there is no belt thermal expansion and when there is thermal expansion. However, the further away from the belt reference position in the belt conveyance direction, the greater the discrepancy between the belt reflection unevenness signals when there is no belt thermal expansion and when there is thermal expansion. In general, for toner density adjustment, data obtained by sampling a belt reflection unevenness signal when there is no thermal expansion at the time of power-on or belt replacement in advance at a period of 4 msec in synchronization with a belt reference signal is used as 875 profile data. Remember. However, if there is an influence of the thermal expansion of the belt when the density adjustment is performed, the belt position sampled at the same 4 msec cycle is very different from the profile acquisition time. The deviation of the belt detection position increases as the distance from the belt reference position increases.

  FIG. 18A is a timing chart showing how a pattern image is read in the prior art. Conventionally, when the density adjustment is started, a density pattern in which the density gradation is gradually changed from a low density is formed in the order of yellow, magenta, cyan, and black. The formation start timing is arbitrary, and the density pattern formation order is fixed in a predetermined order. In the example of FIG. 18A, the yellow density pattern is read after a time corresponding to approximately the belt half circumference has elapsed from the detection of the belt reference signal, that is, at least half a circle from the belt reference position. It is formed from a distant position.

  FIG. 18B is a diagram showing a comparison of pattern detection data before and after correcting the belt reflection unevenness of the yellow density pattern. In FIG. 18B, the solid line of the pattern detection signal indicates the waveform when there is no belt thermal expansion, and the dotted line indicates the waveform when the belt is thermally expanded. The density levels of the density pattern are set to density levels 1, 2, 3, 4, 5 in order from the thinner. In the low density regions of density levels 1 and 2, the amount of toner on the belt is small and the amount of reflected light from the belt surface is large, so that the waveform is greatly affected by uneven reflection of the belt. On the other hand, the toner level on the belt increases and the amount of reflected light from the belt surface decreases as the density levels 3, 4, and 5 increase, and the influence of uneven belt reflection is reduced.

  Here, in the prior art, since the density pattern is formed in a predetermined order, as illustrated in FIG. 18A, the low density density pattern is located far from the belt reference position in the rear of the belt conveyance direction. May be formed. In this case, it is assumed that correction processing for removing the belt reflection unevenness component is performed using the profile data of the belt reflection unevenness signal acquired in advance when there is no thermal expansion. Then, as shown in FIG. 18B, the correction is actually performed using profile data whose detection position is different from that in the presence of thermal expansion. Therefore, the correction error is particularly large at low density levels 1 and 2. Therefore, correct density adjustment cannot be performed.

  If the profile data of the unevenness of belt reflection is acquired again immediately before the density adjustment control, the correction error can be reduced even for a low density pattern. However, if this is done, a measurement time of more than one round of the belt is required, leading to a reduction in productivity.

  An object of the present invention is to reduce a correction error caused by a change in the peripheral length or peripheral speed of an image carrier when removing uneven reflection during image density adjustment.

  In order to achieve the above object, the present invention provides an endless image carrier that is rotationally driven, an image forming unit that forms an image on the image carrier, and irradiates light toward the image carrier, A sensor that outputs a value corresponding to the amount of reflected light received from the irradiation position irradiated with light, and a first data that acquires first data from the output of the sensor based on the reflected light from the ground of the image carrier. An acquisition unit; a second acquisition unit that acquires second data from an output of the sensor based on reflected light from an image for measurement formed on the image carrier by the image forming unit; and A correction unit that corrects the second data acquired by the second acquisition unit based on the first data acquired by the first acquisition unit with reference to the reference position in the circumferential direction, and the correction unit Based on the corrected second data The adjustment unit for adjusting the density of the image formed by the image forming unit, and when the plurality of measurement images are formed, the measurement image having the lowest density among the plurality of measurement images is And a control unit that controls the image forming unit so as to be formed at a position closest to the reference position in the rotation direction of the image carrier.

  According to the present invention, it is possible to reduce a correction error caused by a change in the peripheral length or peripheral speed of the image carrier when removing the reflection unevenness when adjusting the image density.

1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment. FIG. 3 is a schematic diagram illustrating a schematic configuration of a toner density sensor. It is a schematic diagram which shows schematic structure of a reference mark detection sensor. It is a block diagram which shows the control function of a control unit. FIG. 2 is a schematic diagram (FIG. (A)) of an intermediate transfer belt, and a schematic diagram (FIG. (B)) illustrating a pattern image. FIG. 2 is a schematic diagram (FIG. (A)) of an intermediate transfer belt and a schematic diagram (FIG. (B)) showing a pattern image. FIG. 2 is a schematic diagram (FIG. (B)) showing a pattern image of the intermediate transfer belt (FIG. (A)). It is a flowchart which shows a belt reflection nonuniformity profile measurement process. It is a flowchart which shows a density adjustment process. It is a flowchart which shows a pattern formation process. It is a timing chart which shows the mode of reading a pattern image. It is a figure which shows the waveform before and after correction | amendment of reflection nonuniformity of pattern detection data. It is a schematic diagram which illustrates the pattern image in the modification 1. FIG. 10 is a schematic diagram (FIG. (A)) of an intermediate transfer belt according to a second modification, and a schematic diagram (FIG. (B)) illustrating a pattern image. FIG. 6 is a schematic diagram (FIG. (A)) of an intermediate transfer belt according to a second embodiment, and a schematic diagram (FIG. (B)) illustrating a pattern image. It is a flowchart which shows a pattern formation process. It is a wave form diagram which compares the reflection nonuniformity by the presence or absence of the thermal expansion of a belt. It is a timing chart (figure (a)) which shows the mode of reading of a pattern image in a prior art, and a figure (figure (b)) which contrasts pattern detection data before and after correction of unevenness of belt reflection of a yellow density pattern.

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

(First embodiment)
FIG. 1 is a schematic sectional view of an image forming apparatus according to a first embodiment of the present invention. The image forming apparatus 1000 is a color electrophotographic apparatus in which image forming units 10 (10Y, 10M, 10C, and 10K) serving as a plurality of image forming units are arranged in parallel and adopt an intermediate transfer method. The image forming apparatus 1000 includes a document reading unit 200 that reads a document image and a printer unit 100 that prints an image on a sheet. In addition to the four image forming units 10 arranged side by side, the printer unit 100 includes a paper feeding unit 20, an intermediate transfer unit 30, a fixing unit 40, and a control unit 500 (see FIG. 4, not shown in FIG. 1). Have

  The image forming units 10 (10K, 10C, 10M, and 10Y) are stations for forming images of four colors of black (K), cyan (C), magenta (M), and yellow (Y), respectively. Since the constituent elements of each image forming unit 10 are common, hereinafter, the same reference numerals are used when the constituent elements are not distinguished for each image forming part 10, and K, C, M, Y are used after the reference numerals when distinguishing. Corresponding to, a, b, c, d are attached. In each drawing, colors may be abbreviated as Y, M, C, and K.

  In each image forming unit 10, a photosensitive drum 11 (11 a, 11 b, 11 c, 11 d) that is a cylindrical electrophotographic photosensitive member is rotatably supported and is driven to rotate in an arrow direction (counterclockwise direction in FIG. 1). Is done. A primary charger 12 (12a, 12b, 12c, 12d) and an optical system 13 (13a, 13b, 13c, 13d) are arranged in the rotational direction so as to face the outer peripheral surface of each photosensitive drum 11. Further, a folding mirror 16 (16a, 16b, 16c, 16d), a developing device 14 (14a, 14b, 14c, 14d), and a cleaning device 15 (15a, 15b, 15c, 15d) are arranged.

  The intermediate transfer unit 30 includes an intermediate transfer belt 31 as an example of an endless image carrier that is rotationally driven. The intermediate transfer belt 31 is stretched around a driving roller 32 and driven rollers 33 and 34. The driving roller 32 is operated by a driving motor (not shown) to rotate the intermediate transfer belt 31 in the clockwise direction in FIG. Primary transfer chargers 35 (35a, 35b, 35c, 35d) are arranged at positions facing the photoconductor drums 11 (11a, 11b, 11c, 11d) with the intermediate transfer belt 31 interposed therebetween.

  A plurality of image forming units 10 are arranged in series along the rotation direction of the intermediate transfer belt 31 so that images of a plurality of colors can be formed. In each image forming unit 10, the primary charger 12 applies a uniform charge amount to the surface of the photosensitive drum 11. An electrostatic latent image is obtained by exposing a laser beam emitted from the optical system 13 to the photosensitive drum 11 through the folding mirror 16 based on a signal modulated in accordance with a recording image signal from the document reading unit 200. Is formed. Further, the electrostatic latent image is visualized by the developing devices 14 each containing developer of four colors of yellow, cyan, magenta, and black (hereinafter referred to as “toner”). The visualized visible image is transferred to the intermediate transfer belt 31 in the image transfer region T (Ta, Tb, Tc, Td). The toner remaining on the photosensitive drum 11 without being transferred to the intermediate transfer belt 31 in the image transfer region T is scraped off by the cleaning device 15. By the above process, image formation with toner of each color is sequentially performed.

  A cleaning device 50 for cleaning the image forming surface of the intermediate transfer belt 31 is disposed downstream of the secondary transfer region Te of the intermediate transfer belt 31. The cleaning device 50 includes a cleaning blade 51 for removing toner on the intermediate transfer belt 31 and a collected toner box 52 for storing collected toner. The paper feed unit 20 includes cassettes 21a and 21b and a manual feed tray 27 for storing the transfer material S, and pickup rollers 22a, 22b and 26 for feeding the transfer material S one by one from the cassettes 21a, 21b or the manual feed tray 27. Have The paper feed unit 20 also includes a paper feed roller pair 23 for further transporting the transfer material S sent out from the pickup rollers 22a, 22b, and 26, and a paper feed guide 24. The paper supply unit 20 further includes registration rollers 25 a and 25 b for sending the transfer material S to the secondary transfer region Te in accordance with the image formation timing of each image forming unit 10.

  The fixing unit 40 includes a fixing roller 41a having a heat source such as a halogen heater therein, and a pressure roller 41b that is pressed against the fixing roller 41a (the pressure roller 41b may also have a heat source). The fixing unit 40 also includes a conveyance guide 43 for guiding the transfer material S to a nip portion formed by a roller pair of a fixing roller 41a and a pressure roller 41b. The fixing unit 40 further includes an inner discharge roller 44 and an outer discharge roller 45 for guiding the transfer material S discharged from the roller pair to the outside of the apparatus.

  Further, a toner density sensor 60 is disposed on the downstream side (forward in the rotational direction) of the photosensitive drum 11a in the conveyance direction (belt conveyance direction or belt rotation direction) of the intermediate transfer belt 31. The toner density sensor 60 is a reflective optical sensor (described later in FIG. 2). The toner density sensor 60 is a pattern image (measurement for detecting the toner density) formed on the background (base) of the intermediate transfer belt 31 or the surface (annular outer surface) of the intermediate transfer belt 31 at the reading position Sa serving as the irradiation position. Image). A reference mark detection sensor 61 is disposed on the downstream side of the secondary transfer region Te in the belt conveyance direction. The reference mark detection sensor 61 is a reflective optical sensor that detects a reference mark 83 that is a white sealing material attached to the annular inner surface of the intermediate transfer belt 31 (described later in FIG. 3).

  Next, an image forming operation will be described. When the image forming operation start signal is issued, first, the transfer material S is sent out one by one from the cassette 21a by the pickup roller 22a. Then, the transfer material S is guided between the paper feed guides 24 by the paper feed roller pair 23 and conveyed to the registration rollers 25a and 25b. At that time, the registration rollers 25a and 25b are stopped, and the leading edge of the transfer material S comes into contact with a nip portion formed by the registration rollers 25a and 25b. Thereafter, the registration rollers 25a and 25b start rotating in accordance with the timing at which the image forming unit 10 starts image formation. The rotation timing is set so that the transfer material S and the toner image primary-transferred onto the intermediate transfer belt 31 by the image forming unit 10 coincide in the secondary transfer region Te.

  On the other hand, in the image forming unit 10, when an image forming operation start signal is issued, primary transfer is performed from the photosensitive drum 11 d that is the most upstream in the rotation direction of the intermediate transfer belt 31. First, the toner image formed on the photosensitive drum 11d by the above-described process is primarily transferred to the intermediate transfer belt 31 in the image transfer region Td by the primary transfer charger 35d to which a high voltage is applied.

  The primarily transferred toner image is conveyed to the next primary transfer region Tc. In this case, image formation is performed with a delay by the time during which the toner image is conveyed between the image forming units 10, a resist is put on the already transferred toner image, and the next toner image is transferred. Thereafter, the same process is repeated. As a result, the four color toner images are superimposed on the intermediate transfer belt 31 and primarily transferred. Thereafter, when the transfer material S enters the secondary transfer region Te and contacts the intermediate transfer belt 31, a high voltage is applied to the secondary transfer roller 36 in accordance with the passing timing of the transfer material S. Accordingly, the four-color toner images formed on the intermediate transfer belt 31 are secondarily transferred onto the surface of the transfer material S by the process described above. Thereafter, the transfer material S is accurately guided to the nip portion of the fixing unit 40 by the conveyance guide 43. The toner image is fixed on the surface of the transfer material S by the heat of the fixing roller 41a and the pressure roller 41b and the pressure of the nip portion thereof. The transfer material S on which the image is thus formed is then conveyed by the inner discharge roller 44 and the outer discharge roller 45 and discharged outside the apparatus.

  FIG. 2 is a schematic diagram illustrating a schematic configuration of the toner density sensor 60. The toner density sensor 60 is a reflective optical sensor that includes an LED 71, a regular reflection PD 72, and an irregular reflection PD 73 as optical elements. The LED 71 is a light emitting diode that emits infrared light. The regular reflection PD 72 is a photodiode that receives regular reflection light of light emitted from the LED 71 to the irradiation position of the intermediate transfer belt 31. The irradiation position corresponds to the reading position Sa (FIG. 1). The irregular reflection PD 73 is a photodiode that receives irregular reflection light of light emitted from the LED 71 to the irradiation position of the intermediate transfer belt 31. These optical elements are mounted on an electric board (not shown) including a drive circuit that supplies current to the LED 71, a light receiving circuit that generates a voltage output corresponding to the amount of light received by the regular reflection PD 72 and the irregular reflection PD 73.

  FIG. 3 is a schematic diagram showing a schematic configuration of the reference mark detection sensor 61. The reference mark detection sensor 61 is arranged to face the annular inner surface of the intermediate transfer unit 30. The reference mark detection sensor 61 is a reflective optical sensor that includes an LED 81 and an irregular reflection PD 82 as optical elements. The LED 81 is a light emitting diode that emits infrared light. The irregular reflection PD 82 is a photodiode that receives irregular reflection light of the light emitted from the LED 81 to the reference mark 83. These optical elements generate a drive circuit that supplies current to the LED 81, a light receiving circuit that generates a voltage output corresponding to the amount of light received by the irregular reflection PD 82, and a voltage signal that is binarized depending on whether the received light voltage is equal to or higher than a predetermined voltage It is mounted on an electric board (not shown) composed of a comparator circuit.

  FIG. 4 is a block diagram showing control functions of the control unit 500. A toner density sensor 60 and a reference mark detection sensor 61 are electrically connected to the control unit 500. The control unit 500 includes a CPU 501, a sensor control unit 504, and a pattern image generation unit 502. The CPU 501 is a central arithmetic processing unit, and has a function of executing various command controls and arithmetic processing according to a program stored in its built-in memory. The sensor control unit 504 has functions of turning on / off the power of the toner density sensor 60 and controlling the light emission amount of the LED 71 in accordance with a command from the CPU 501. The pattern image generation unit 502 has a function of generating pattern image data in accordance with an instruction from the CPU 501. A storage unit 503 is a storage memory and has a function of storing and reading data in accordance with instructions from the CPU 501.

  An analog voltage signal, which is a detection result output from the toner density sensor 60, is input to the CPU 501, and is sampled at a cycle of, for example, 4 ms while being converted into a digital value by an AD conversion function built in the CPU 501. The detection result of the toner density sensor 60 sampled by the CPU 501 is stored in the storage unit 503 as pattern detection data (second data). The CPU 501 serves as a second acquisition unit.

  Here, regarding the reflection to the analog voltage signal output from the toner density sensor 60, the method of using the output value of the regular reflection PD 72 and the output value of the irregular reflection PD 73 differs between black and color. The output value of the toner density sensor 60 is calculated by “output value of toner density sensor = output value of regular reflection PD 72−output value of irregular reflection PD 73 × coefficient α−offset voltage value”. The offset voltage value is an output value of the toner density sensor 60 when the LED 71 is turned off. The coefficient α is greater than 0 for color, but 0 for black. Accordingly, only the output value of the regular reflection PD 72 is used for black. The coefficient α may be different depending on the color. This calculation formula is an example, but in this embodiment, it is assumed that at least the output value of the regular reflection PD 72 is reflected in the output of the toner density sensor 60 for all colors.

  On the other hand, a digital voltage signal that is a detection result output from the reference mark detection sensor 61 is input to an interrupt terminal (not shown) of the CPU 501. The sampling control in the CPU 501 is performed in synchronization with a detection signal (belt reference signal) input from the reference mark detection sensor 61. The reference mark detection sensor 61 outputs a Low pulse when the reference mark 83 is detected, and this becomes a belt reference signal. Since the belt reference signal is input every time the intermediate transfer belt 31 makes one rotation, the CPU 501 determines whether or not the intermediate transfer belt 31 has made one rotation using the belt reference signal and determines the time required for one rotation. It also has a function to measure. With the above control function, belt reflection unevenness profile measurement, toner density adjustment control, and the like are performed.

  By the way, the intermediate transfer belt 31 is driven at a speed (process speed) of 300 mm / sec by a drive motor. Although the drive motor rotates at the designed number of rotations, the peripheral speed of the intermediate transfer belt 31 may vary due to thermal expansion and contraction of the drive roller 32. In addition, although the structure which controls the rotational speed of a drive motor using an encoder may be employ | adopted, dispersion | variation may arise in the peripheral speed of the intermediate transfer belt 31 even in that case.

  Next, an aspect of forming a pattern image on the intermediate transfer belt 31 will be described. FIG. 5A is a schematic diagram of the intermediate transfer belt 31. In FIG. 5A, the photosensitive drums 11 of the image forming units 10Y, 10M, 10C, and 10K are indicated by Y, M, C, and K, respectively.

  First, a reference position P in the circumferential direction of the intermediate transfer belt 31 is defined. The reference position P is a position that becomes the reading position Sa when the reference mark detection sensor 61 detects the reference mark 83 in the circumferential direction (rotation direction) of the intermediate transfer belt 31. FIG. 5A shows a point in time when the reference mark 83 is detected, and the position where the toner density sensor 60 faces is the reference position P. FIG. Acquisition of background data (belt reflection unevenness profile), formation of pattern images, and acquisition of pattern detection data are performed with reference position P as a reference. Each region R obtained by dividing the nominal circumference in the circumferential direction of the intermediate transfer belt 31 by 5 is divided into regions R 1, R 2, R 3, Let R4 and R5.

  First, assuming that the intermediate transfer belt 31 rotates at a constant peripheral speed of 300 mm / sec, the nominal time for which the intermediate transfer belt 31 makes one revolution is 3500 ms. When the nominal time for one round is divided into five equal parts, 700 ms is obtained. Each of t1 to t4 shown in FIG. 5A is given at a position corresponding to a rotation length of 700 ms. t1 = 700 ms, t2 = 1400 ms, t3 = 2100 ms, t4 = 2800 ms. The elapsed time T is the time from when the reference position P reaches the position (reading position Sa) of the toner density sensor 60 from when the time is counted (timer count). The time when the reference position P reaches the reading position Sa is the time when the belt reference signal is generated. The timer that counts the elapsed time T is reset every time the reference position P reaches the reading position Sa. Assuming that the intermediate transfer belt 31 has rotated as nominal, the reference position P starts at the reading position Sa and moves to the positions indicated by t1, t2, t3, and t4 every 700 ms. Return to Sa (0).

  In the configuration shown in FIG. 1, it is assumed that the reference position P is located between the image transfer regions Tc and Td when the elapsed time T reaches t4 (2800 ms) in the configuration shown in FIG. Such conditions vary depending on the apparatus, and in particular, differ depending on the arrangement of the image forming unit 10, the reference mark 83, the reference mark detection sensor 61, and the toner density sensor 60, the peripheral length of the intermediate transfer belt 31, and the like. Accordingly, the times t1 to t4 are merely examples and are not limited to these.

  FIG. 5B is a schematic view illustrating a pattern image formed on the intermediate transfer belt 31. In this figure, the intermediate transfer belt 31 is shown by developing one round of the surface starting from the reference position P. An arrow in FIG. 5B indicates the belt rotation direction (conveyance direction).

  Conventionally, gradation patterns with different gradation density settings for each color have been formed. In other words, the gradation pattern of each color in a predetermined color order in the sequence of a plurality of gradation patterns of yellow, a plurality of gradation patterns of magenta, a plurality of gradation patterns of cyan, a plurality of gradation patterns of black, etc. Was forming. In contrast, in the present embodiment, the pattern image is not formed for each color, but is formed for each gradation.

  In the present embodiment, the plurality of pattern images PT (PT1 to PT5) have gradation levels different from each other in stages, and the pattern images PT1, PT2, PT3, and PT4 from the lower one of the five gradation levels. , PT5. The pattern image PT5 has the highest density. The arrangement order of the pattern images PT is thus determined in advance. One pattern image PT is a group of images having the same gradation density level for each color. For example, the pattern image PT1 is formed by continuously forming the lowest density images of the respective colors.

  In the present embodiment, the five-stage pattern image PT is formed such that the pattern image PT1 having the lowest density is closest to the reference position P on the rear side in the rotation direction of the intermediate transfer belt 31. Accordingly, the head position of the pattern image PT1 is located in the region R1. Pattern images PT2, PT3, PT4, and PT5 are formed in order from the lowest density after the pattern image PT1 at the rear in the rotation direction. Therefore, the pattern image PT having a higher density is formed at a position farther from the reference position P in the rear in the rotation direction. As will be described with reference to FIGS. 11 and 12, the pattern image PT is formed in such an order when the reflection unevenness data is measured and the pattern image PT is detected as the distance from the reference position P is further rearward in the rotation direction. This is because the deviation of the detection position becomes larger. That is, the pattern image PT is formed in the order as described above in order to reduce the correction error caused by the change in the peripheral length and peripheral speed of the intermediate transfer belt 31 when removing the reflection unevenness when adjusting the image density.

  Although there is no limitation on the length and the number of gradations of one pattern image PT, the length of one pattern image PT is assumed to be shorter than “the nominal circumference / number of gradations of the intermediate transfer belt 31”. Next, a control example of the formation of the pattern image PT will be described with reference to FIGS. When forming the pattern image PT, the CPU 501 serves as a control unit in the present invention that controls the image forming unit 10.

  FIG. 6A is a schematic diagram of the intermediate transfer belt 31 in a state where the elapsed time T is less than the time t1. The reference position P is upstream from the position corresponding to the passage of time t1, and the region R2 is in a state of straddling the image transfer region Td of the image forming unit 10Y. In order to ensure that the head position of the pattern image PT1 is positioned within the region R1, the CPU 501 forms the pattern image PT1 after the elapsed time T reaches time t4. That is, when T = t4, the reference position P is located at a position corresponding to the elapse of time t4, and the leading position of the region R1 exceeds (is over) the image transfer region Td. Therefore, if the formation of the pattern image PT1 is started after the time t4, the pattern image PT1 of the five stages of the pattern image PT is positioned closest to the reference position P at the rear (upstream) in the rotation direction of the intermediate transfer belt 31. Can be formed.

  FIG. 6B is a schematic diagram showing a pattern image PT formed on the intermediate transfer belt 31 in the case of the example of FIG. Since T <t1 in the example of FIG. 6A, the CPU 501 forms the pattern image PT3 when T = t1. Then, the CPU 501 forms a pattern image PT4 following the pattern image PT3, and further forms a pattern image PT5. Thus, first, until the reference position P reaches the position (reading position Sa) of the toner density sensor 60 (the belt reference signal is output from the reference mark detection sensor 61) and before the elapsed time T is reset. Then, the formation of the pattern images PT3, PT4, PT5 is completed.

  Then, the CPU 501 forms the pattern image PT1 when T = t4, and forms the pattern image PT2 following the pattern image PT1. Since the pattern images PT3 to PT5 have already been formed, the formation of all the pattern images PT is completed after the pattern image PT2.

  FIG. 7A is a schematic diagram of the intermediate transfer belt 31 in a state where the elapsed time T is not less than the time t1 and less than the time t2. The reference position P is a position from the time corresponding to the passage of time t1 to the time corresponding to the passage of time t2, and the leading position of the region R3 is in a state exceeding the image transfer region Td. Also in this example, the CPU 501 forms the pattern image PT1 after the elapsed time T reaches time t4.

  FIG. 7B is a schematic diagram showing a pattern image PT formed on the intermediate transfer belt 31 in the case of the example of FIG. In the example of FIG. 7A, since t1 ≦ T <t2, the CPU 501 forms the pattern image PT4 when T = t2. Then, the CPU 501 forms a pattern image PT5 following the pattern image PT4. Thereby, first, the formation of the pattern images PT4 and PT5 is completed before the reference position P reaches the position (reading position Sa) of the toner density sensor 60 and the elapsed time T is reset. Then, the CPU 501 forms the pattern image PT1 when T = t4, and forms the pattern images PT2 and PT3 following the pattern image PT1. Since the pattern images PT4 and PT5 have already been formed, the formation of all the pattern images PT is completed after the pattern image PT3. Details of such formation control will also be described with reference to FIGS.

  FIG. 8 is a flowchart showing a belt reflection unevenness profile measurement process. A procedure for measuring the belt reflection unevenness profile prior to toner density adjustment control will be described with reference to FIG. The belt reflection unevenness profile measurement process acquires background data including a reflection unevenness component from the output of the toner density sensor 60 based on the reflected light from the background (background) of the intermediate transfer belt 31 using the reference position P as a reference. This processing is stored in 503. This process is performed when the intermediate transfer belt 31 is replaced or immediately after the image forming apparatus 1000 is turned on. In the process of FIG. 8, the CPU 501 serves as a first acquisition unit in the present invention.

  First, in step S <b> 101, the CPU 501 starts driving the intermediate transfer belt 31 and the photosensitive drum 11. In step S102, the CPU 501 turns on the toner density sensor 60 and turns on the LED 71. In step S103, the CPU 501 starts measurement of uneven belt reflection after 500 ms has elapsed since the LED 71 was turned on. In the sampling control for measuring the belt reflection unevenness, the CPU 501 samples the output of the toner density sensor 60 in a cycle of 4 ms in synchronization with the belt reference signal from the reference mark detection sensor 61. In step S104, the CPU 501 waits for the intermediate transfer belt 31 to rotate one or more times from the start of sampling based on the belt reference signal from the reference mark detection sensor 61. When the intermediate transfer belt 31 rotates one or more times, the CPU 501 causes the storage unit 503 to store reflection unevenness data (first data) for one belt rotation. In step S106, the CPU 501 turns off the toner density sensor. In step S107, the CPU 501 stops driving the intermediate transfer belt 31 and the photosensitive drum 11. The belt reflection unevenness profile measurement is thus completed.

  FIG. 9 is a flowchart showing the density adjustment processing. This processing includes toner density adjustment and processing for controlling the order of formation of the pattern image PT. This process is started when a user gives an instruction from an operation unit (not shown), when a predetermined number of sheets are printed, or when the photosensitive drum 11 and the developing device 14 are replaced. When the rotation of the intermediate transfer belt 31 is started, the CPU 501 always resets the elapsed time T and measures the time based on the belt reference signal from the reference mark detection sensor 61. Accordingly, the elapsed time T is reset and timed before the start of the process of FIG. 9, and is continued during the process.

  First, in step S201, the CPU 501 turns on the toner density sensor 60 and turns on the LED 71. Next, in step S <b> 202, the CPU 501 starts control for sampling the detection signal from the toner density sensor 60. Next, in step S203, the CPU 501 executes a pattern formation process (FIG. 10). That is, the CPU 501 controls the image forming unit 10 to form the five pattern images PT1 to PT5 on the intermediate transfer belt 31 in an order determined based on the elapsed time T.

  FIG. 10 is a flowchart showing the pattern forming process. This process is executed in step S203 of FIG. First, in step S301, the CPU 501 determines whether or not the elapsed time T is less than time t1 (700 ms) (T <t1). As a result of the determination, if T <t1, the CPU 501 forms the pattern image PT in the manner illustrated in FIGS. 6A and 6B in steps S302 to S305.

  First, in step S302, the CPU 501 waits until the elapsed time T reaches time t1. When the elapsed time T reaches time t1, the CPU 501 forms a pattern image PT3 in step S303, and further forms pattern images PT4 and PT5 following the pattern image PT3. Next, in step S304, the CPU 501 waits until the elapsed time T reaches time t4 (2800 ms). When the elapsed time T reaches time t4, the CPU 501 forms a pattern image PT1 in step S305, and further forms a pattern image PT2 following the pattern image PT1. Thereby, five pattern images PT are formed in the order shown in FIG. Thereafter, the process of FIG. 10 ends.

  As a result of the determination in step S301, if t1 ≦ T, the CPU 501 determines in step S306 whether the elapsed time T is greater than or equal to time t1 and less than time t2 (1400 ms) (t1 ≦ T <t2). Is determined. As a result of the determination, if t1 ≦ T <t2, the pattern image PT is formed in the manner illustrated in FIGS. 7A and 7B in steps S307 to S310.

  First, in step S307, the CPU 501 waits until the elapsed time T reaches time t2. When the elapsed time T reaches time t2, the CPU 501 forms a pattern image PT4 in step S308, and further forms a pattern image PT5 following the pattern image PT4. Next, the CPU 501 waits until the elapsed time T reaches time t4 in step S309. When the elapsed time T reaches time t4, the CPU 501 forms a pattern image PT1 in step S310, and further sequentially forms pattern images PT2 and PT3 after the pattern image PT1. Thereby, five pattern images PT are formed in the order shown in FIG. Thereafter, the process of FIG. 10 ends.

  As a result of the determination in step S306, if t2 ≦ T, the CPU 501 determines in step S311 whether or not the elapsed time T is not less than time t2 and less than time t3 (2100 ms) (t2 ≦ T <t3). Is determined. As a result of the determination, if t2 ≦ T <t3, in step S312, the CPU 501 waits until the elapsed time T reaches time t3. When the elapsed time T reaches time t3, the CPU 501 forms a pattern image PT5 in step S313. Next, the CPU 501 waits until the elapsed time T reaches time t4 in step S314. When the elapsed time T reaches time t4, the CPU 501 forms a pattern image PT1 in step S315, and further forms pattern images PT2, PT3, and PT4 in order after the pattern image PT1. Thereafter, the process of FIG. 10 ends.

  If it is determined in step S311 that t3 ≦ T, the CPU 501 determines in step S316 whether or not the elapsed time T is equal to or greater than time t3 and less than time t4 (t3 ≦ T <t4). . If it is determined that t3 ≦ T <t4, in step S317, the CPU 501 waits until the elapsed time T reaches time t4. When the elapsed time T reaches time t4, the CPU 501 forms a pattern image PT1 in step S318, and further forms pattern images PT2, PT3, PT4, and PT5 in order after the pattern image PT1. Thereafter, the process of FIG. 10 ends.

  If t3 ≦ T <t4 is not satisfied as a result of the determination in step S316, since t4 ≦ T, the CPU 501 waits until the elapsed time T is reset to 0 in step S319. When T = 0, the CPU 501 forms a pattern image PT2 in step S320, and further forms pattern images PT3, PT4, and PT5 following the pattern image PT2. Next, in step S321, the CPU 501 waits until the elapsed time T reaches time t4. When the elapsed time T reaches time t4, the CPU 501 forms a pattern image PT1 in step S322. Thereafter, the process of FIG. 10 ends.

  In this way, the processing of FIG. 10 arranges the five-step pattern images PT in the determined order in the circumferential direction of the intermediate transfer belt 31, and among them, the pattern image PT1 is behind the rotation direction of the intermediate transfer belt 31. In FIG. By the sampling control started in step S202, each pattern image PT is sequentially detected and captured by the toner density sensor 60.

  Returning to FIG. 9, in step S204, the CPU 501 waits for the sampling of all the pattern images PT to end. When the sampling of all pattern images PT is completed, the CPU 501 stops the sampling control, stores the result in the storage unit 503 as pattern detection data, and turns off the toner density sensor 60. Next, in step S205, the CPU 501 performs an uneven reflection correction process on the pattern detection data acquired by sampling. This uneven reflection correction process uses the uneven belt reflection data acquired in advance and stored in the storage unit 503 to remove the uneven reflection component on the belt surface included in the pattern detection data with reference to the reference position P. Arithmetic processing. The CPU 501 serves as correction means in the present invention.

Data obtained by removing the reflection unevenness component from the pattern detection data is referred to as “pattern detection data after correcting the reflection unevenness”. The corrected pattern detection data is calculated by the following formula 1. In Equation 1, the calculation is performed using belt reflection unevenness data corresponding to each sampling data.
[Equation 1]
Pattern detection data after reflection unevenness correction = pattern detection data− (belt reflection unevenness data−belt reflection unevenness data average) × coefficient k

In the present embodiment, the coefficient k is changed according to the voltage level (pattern detection data average) obtained by converting the pattern detection data into a voltage. The pattern detection data average is an average value of 8 data obtained by removing the maximum and minimum data from the pattern detection data sampled, for example, 10 times for each color pattern of each gradation. Then, the value of the coefficient k to be applied is changed according to the average voltage level (v) of the pattern detection data. As a specific example, cases are classified as follows. The coefficients k are k1 = 0.9, k2 = 0.8, and k3 = 0.7.
In the case of 3.0v ≦ pattern detection data average → coefficient k = k1
2.5v ≦ pattern detection data average <3.0 v → coefficient k = k2
2.0v ≦ pattern detection data average <2.5v → coefficient k = k3
When 0v ≦ pattern detection data average <2.0 v → coefficient k = 0

  The lower the density, the higher the pattern detection data average value (v). Therefore, the lower the pattern detection data, the larger the value of the coefficient k to be applied, and the greater the degree of reflection unevenness correction. On the other hand, since the coefficient k = 0 on the high density side (pattern detection data average <2.0 v), there is substantially no correction of reflection unevenness. Note that the appropriate value of the coefficient k varies depending on the device configuration and the like, and is not limited to this example. Further, reflection unevenness correction may also be performed on the high density side. For example, in Equation 1, the value of the coefficient k may be set to a value larger than 0, and the coefficient k may be set to a smaller value on the higher density side.

  Next, in step S206, the CPU 501 calculates density data. That is, the CPU 501 converts the pattern detection data after the reflection unevenness correction calculated in step S205 into a density value based on, for example, a density value conversion table stored in advance in the internal memory of the CPU 501. Next, in step S207, the CPU 501 updates various image process parameters (primary charging high voltage setting, laser exposure amount, density value conversion table, etc.) so as to obtain an appropriate image density based on the density value calculated in step S206. To adjust the density. Thereafter, the process of FIG. 9 ends. In steps S206 and S207, the CPU 501 plays a role as an adjusting unit in the present invention. As a result, the pattern image PT to be formed first is selected based on the elapsed time T by the processing of FIG.

  FIG. 11 shows timing when the pattern image PT formed in the manner illustrated in FIGS. 7A and 7B is read in steps S307 to S310 in FIG. 10 when t1 ≦ T <t2. It is a chart. FIG. 12 is a diagram showing waveforms before and after correction of reflection unevenness in pattern detection data obtained by reading each pattern image PT shown in FIG.

  In FIG. 11, the belt reference signal indicated by the solid line is a waveform when the belt reflection unevenness profile measurement process (FIG. 8) is performed immediately after the power is turned on (that is, when there is no thermal expansion). On the other hand, the belt reference signal indicated by the dotted line is a waveform when the intermediate transfer belt 31 is thermally expanded by the continuous printing operation of several thousand sheets in the image forming apparatus 1000. Since the intermediate transfer belt 31 is driven at a nominal speed of 300 mm / sec, when the peripheral length of the intermediate transfer belt 31 extends by about 1.5 mm, the period of the belt reference signal changes by about 5 msec. When the sampling period is 4 msec, the generation timing of the belt reference signal is shifted by one sampling period or more when there is no thermal expansion and when there is thermal expansion.

  The belt reflection unevenness signal is a waveform of belt reflection unevenness data based on the belt reference signal, and is a signal indicating output fluctuation when the toner density sensor 60 measures the reflected light on the surface of the intermediate transfer belt 31. As for the waveform of the belt reflection unevenness signal, the solid line indicates the signal waveform when there is no thermal expansion, and the dotted line indicates the signal waveform when there is thermal expansion. In the belt reflection unevenness profile measurement, sampling is performed in synchronization with the belt reference signal. Comparing the profile waveform without thermal expansion with the profile waveform after thermal expansion, there is little difference between the two immediately after the belt reference signal is generated, and it is close to the next belt reference signal generation after a lapse of time from the belt reference signal generation. It can be seen that the difference between the two increases.

  In the example of FIG. 11, the pattern images PT4 and PT5 having relatively high density gradation levels are formed at positions that are read immediately after the belt reference signal is generated rather than immediately after the belt reference signal is generated. . That is, it is formed at a position where the data deviation from the reflection unevenness profile tends to increase. However, the pattern images PT4 and PT5 have a large amount of toner on the belt and are not greatly affected by the amount of light reflected from the belt surface. Therefore, the pattern detection data obtained by reading the pattern images PT4 and PT5 has little influence on density data calculation such as thermal expansion.

  On the other hand, the pattern images PT1, PT2, and PT3 (particularly, the pattern image PT1) having relatively low density gradation levels are susceptible to the amount of reflected light from the belt surface because the amount of toner on the belt is small. However, in the present embodiment, these low-density patterns are formed at positions that can be read immediately after the generation of the belt reference signal. That is, it is formed at a position where data deviation from the reflection unevenness profile is difficult to increase. As a result, the lower the pattern image PT, the smaller the correction error caused by the change in the circumferential length and the circumferential speed when correcting the reflection unevenness.

  Considering the case where the belt circumferential length is extended by about 1.5 mm, the change in length is about 0.3 mm (= 1.5 ÷ 5) from the generation of the belt reference signal until 700 msec. If this is grasped by time, it is about 1 msec (= 0.3 mm ÷ 300 mm / sec × 1000). Since this is a deviation of half or less of the sampling period of 4 msec, it is included in the quantization error. Therefore, the influence of the reflected light from the belt surface included in the pattern detection signal can be sufficiently corrected by removing the reflection unevenness using the reflection unevenness profile acquired in advance.

  According to the present embodiment, the pattern image PT1 having the lowest density among the plurality of pattern images PT is formed at a position closest to the reference position P in the rear of the belt rotation direction. As a result, correction errors caused by changes in the peripheral length and peripheral speed of the intermediate transfer belt 31 can be reduced when removing the reflection unevenness during image density adjustment. Moreover, the pattern image PT having a higher density is formed at a position farther from the reference position P at the rear of the belt rotation direction. As a result, a pattern image PT having a lower density that easily reflects the state of the background of the intermediate transfer belt 31 can be made less susceptible to the influence of changes in the peripheral length and peripheral speed of the intermediate transfer belt 31 when removing the uneven reflection.

  As can be seen from the description of FIGS. 6, 7, and 10, the pattern image PT to be formed first in time is selected based on the elapsed time T from the time when the reference position P reaches the reading position Sa. The In any case, the lowest density pattern image PT1 is formed (time t4) after the reference position P reaches the image forming position (image transfer region Td). When the selected pattern image PT is not the pattern image PT1, the CPU 501 controls to form all pattern images PT other than the pattern image PT1 (other than the lowest density measurement image) before the pattern image PT1 is formed. .

  Specifically, the formation is started from the selected pattern image PT between the reference position P reaching the reading position Sa and the image forming position (Td). Thereafter, up to the pattern image PT5 having a density adjacent to the front in the rotation direction with respect to the pattern image PT1 having the lowest density is sequentially formed. Then, after the reference position P reaches the image forming position (Td), from the lowest density pattern image PT1 to the density pattern image PT whose arrangement order is adjacent to the front in the rotation direction with respect to the selected pattern image PT. Are formed in order. Since the arrangement order of the pattern images PT is determined in advance, the pattern image PT can be formed at an appropriate position by controlling the formation timing based on the elapsed time T. Therefore, the control is not complicated. Here, the “image forming position” is the primary transfer position of the image forming unit 10 that is the most upstream among the plurality of image transfer areas, and is the yellow image transfer area Td in the present embodiment.

  In addition, since it is not necessary to re-acquire profile data of unevenness of belt reflection immediately before the density adjustment control, an excessive decrease in productivity is not caused.

  In the present embodiment, an example in which an image of a plurality of colors is formed is shown, but the present invention can also be applied to an image forming apparatus that forms a monochrome image. In the case of monochrome, the primary transfer position of a single image forming unit is the “image forming position”.

(Modification of the first embodiment)
Next, a modification of the present embodiment will be described with reference to FIGS. FIG. 13 is a schematic view illustrating the pattern image PT in the first modification. According to the forming method described with reference to FIGS. 6 and 7 and the like, the pattern image PT1 is formed when the elapsed time T reaches the time t4. For the other pattern images PT, the pattern image from the first formed pattern image is formed. Up to PT5 was continuously formed without any interval.

  On the other hand, in Modification 1 shown in FIG. 13, one corresponding pattern image PT is formed in each region R. Therefore, the CPU 501 forms the pattern image PT1 when the elapsed time T reaches time t4. When the elapsed time T is reset, the CPU 501 forms the pattern image PT2. Similarly, when the elapsed time T reaches the times t1, t2, and t3, the CPU 501 forms the pattern images PT3, PT4, and PT5. When the fifth pattern image PT is formed from the pattern image PT formed first, the formation of all the pattern images PT is completed.

  FIGS. 14A and 14B are schematic diagrams illustrating the intermediate transfer belt 31 according to the second modification and a schematic diagram illustrating the pattern image PT. In the second modification shown in FIGS. 14A and 14B, a contrivance is made so that the top of the lowest density pattern image PT1 is as close as possible to the reference position P. FIG. However, it is necessary to avoid the pattern image PT1 straddling the reference position P. Therefore, particularly with respect to the formation timing of the pattern image PT1, improvements are added to the above embodiment.

  First, as shown in FIG. 14A, let D be the nominal distance from the reading position Sa to the image transfer region Td, which is the first image forming position in the front (downstream) in the belt rotation direction. The margin value d is a value that takes into consideration all the variation factors such as the change in the peripheral length and peripheral speed of the intermediate transfer belt 31 and the deviation amount of the laser writing position. The margin value d is set so that the position downstream from the reading position Sa by D + d is surely positioned downstream from the image transfer region Td.

  The CPU 501 determines the formation timing of the pattern image PT1 based on the nominal distance D and the elapsed time T from when the reference position P reaches the reading position Sa. Specifically, the CPU 501 forms the pattern image PT1 when the elapsed time T reaches the nominal transport time of “D + d” instead of using the time t4 as a trigger. The formation of the pattern image PT other than the pattern image PT1 is the same as that described in the above embodiment.

  As a result, as shown in FIG. 14B, the pattern image PT1 is formed so that the head is positioned immediately (almost immediately after) the reference position P in the rear in the belt rotation direction. Since the distance between the pattern image PT1 and the reference position P is shortened, the pattern image PT1 is formed in an area where data deviation from the reflection unevenness profile is smaller. Therefore, the accuracy of removing the uneven reflection component is increased, and the density can be adjusted more appropriately and with high accuracy.

(Second Embodiment)
In the first embodiment, each of the plurality of pattern images PT having different densities is composed of images of a plurality of colors (see FIG. 5B). On the other hand, in the second embodiment of the present invention, the plurality of pattern images PT are divided for each color, and each color pattern image PT is composed of a plurality of images having the same color and different densities. Accordingly, the second embodiment will be described with reference to FIGS. 15 and 16 instead of FIGS. 5 to 7 and FIGS. 10 to 13 with respect to the first embodiment.

  FIG. 15A is a schematic diagram of the intermediate transfer belt 31 according to the second embodiment. In the first embodiment, since there are five gradation density stages, the total length of the intermediate transfer belt 31 is considered to be divided into five equal parts. On the other hand, in the second embodiment, the number of pattern images PT is the number of colors, which is four. Therefore, each region R obtained by equally dividing the nominal circumferential length of the intermediate transfer belt 31 into four equal regions R1, R2, R2, R2,. Let R3 and R4.

  In the configuration shown in FIG. 15A, it is assumed that the reference position P is located upstream of the image transfer region Td when the elapsed time T reaches time t3. Such conditions vary depending on the apparatus. In the present embodiment, as an example, t1 = 875 ms, t2 = 1750 ms, and t3 = 2625 ms.

  FIG. 15B is a schematic view illustrating a pattern image formed on the intermediate transfer belt 31 and corresponds to FIG. A plurality of pattern images PT are arranged in the order of black, cyan, magenta, and yellow in the order of the pattern images PT-K, PT-C, PT-M, and PT-Y at the rear of the belt rotation direction. . Regarding the black pattern image PT-K, pattern images PT-K1, PT-K2, PT-K3, PT-K4, and PT-K5 are arranged in order from the lowest density among the five gradation densities. The pattern image PT-K5 has the highest density. For other colors (colors), the order of gradation density is not limited, but for example, it is the same as black.

  In the present embodiment, among the four color pattern images PT, the black pattern image PT-K is formed at a position closest to the reference position P on the rear side in the rotation direction of the intermediate transfer belt 31. Therefore, the head position of the pattern image PT-K1 is located in the region R1. The order of the colors of the pattern image PT following the pattern image PT-K at the rear in the rotation direction is not limited. For example, the pattern images PT-C, PT-M, and PT-Y are sequentially formed.

  Unlike the first embodiment, in the second embodiment, only the output of the irregular reflection PD 73 out of the output of the toner density sensor 60 is used to acquire the color pattern detection data, and the output of the regular reflection PD 72 is used. Is not used. That is, regarding the reflection to the analog voltage signal output from the toner density sensor 60, for black, the output value of the toner density sensor 60 is “output value of the toner density sensor 60 = output value of the regular reflection PD 72−offset voltage value”. ] Is calculated. On the other hand, regarding the color, the output value of the toner density sensor 60 is calculated by “output value of the toner density sensor 60 = output value of the irregular reflection PD 73−offset voltage value”.

  As described above, since the output of the regular reflection PD 72 is not reflected in the color pattern detection data and is not affected by the uneven reflection, it is not necessary to perform a process for removing the uneven reflection component. Therefore, the process of removing the uneven reflection component is performed only for the black pattern detection data. Therefore, the arrangement order and formation position of the pattern images PT-C, PT-M, and PT-Y with respect to the reference position P are not so important. On the other hand, for black pattern detection data in which the output of the regular reflection PD 72 is reflected, the lower the density of the pattern image PT-K, the lower the density of the pattern image PT-K, the reference position P in the belt rotation direction. It is made to form in the position near.

  Processing for forming the pattern image PT as shown in FIG. 15B will be described with reference to FIG. FIG. 16 is a flowchart showing the pattern forming process in the second embodiment. This process is executed in step S203 of FIG.

  First, in step S401, the CPU 501 determines whether or not the elapsed time T is less than time t1 (T <t1). As a result of the determination, if T <t1, the CPU 501 waits until the elapsed time T reaches time t1 in step S402. When the elapsed time T reaches time t1, the CPU 501 forms pattern images PT-C, PT-M, and PT-Y in step S403. Next, the CPU 501 waits until the elapsed time T is reset to 0 in step S404. When T = 0, the CPU 501 forms a pattern image PT-K in step S405. Thereafter, the processing of FIG. 16 ends.

  As a result of the determination in step S401, if t1 ≦ T, the CPU 501 determines in step S406 whether t1 ≦ T <t2. As a result of the determination, if t1 ≦ T <t2, the CPU 501 waits until the elapsed time T reaches time t2 in step S407. When the elapsed time T reaches time t2, the CPU 501 forms pattern images PT-M and PT-Y in step S408. Next, the CPU 501 waits until the elapsed time T becomes 0 in step S409. When T = 0, the CPU 501 forms pattern images PT-K and PT-C in step S410. Thereafter, the processing of FIG. 16 ends.

  As a result of the determination in step S406, if t2 ≦ T, the CPU 501 determines in step S411 whether t2 ≦ T <t3. As a result of the determination, if t2 ≦ T <t3, in step S412, the CPU 501 waits until the elapsed time T reaches time t3. When the elapsed time T reaches time t3, the CPU 501 forms a pattern image PT-Y in step S413. Next, the CPU 501 waits until the elapsed time T becomes 0 in step S414. When T = 0, the CPU 501 forms pattern images PT-K, PT-C, and PT-M in step S415. Thereafter, the processing of FIG. 16 ends.

  If t2 ≦ T <t3 is not satisfied as a result of the determination in step S411, since t3 ≦ T, the CPU 501 waits until the elapsed time T becomes 0 in step S416. When T = 0, the CPU 501 forms pattern images PT-K, PT-CPT-M, and PT-Y in step S417. Thereafter, the processing of FIG. 16 ends.

  In this way, as shown in FIG. 15B, the processing of FIG. 16 arranges the four color pattern images PT in a predetermined order, and among them, the pattern image PT-K is the intermediate transfer belt. It is formed at a position closest to the reference position P at the rear of the rotation direction 31. In the pattern image PT-K, the pattern images PT-K1, PT-K2, PT-K3, PT-K4, and PT-K5 are formed in this order.

  In step S205 in FIG. 9, in this embodiment, the uneven reflection correction process is performed only on the black pattern detection data, and is not performed on the color pattern detection data.

  According to the present embodiment, among the multiple color pattern images PT, the black pattern image PT-K is formed at a position closest to the reference position P at the rear of the belt rotation direction. As a result, the same effect as that of the first embodiment can be obtained with respect to reducing the correction error caused by the change in the peripheral length and peripheral speed of the intermediate transfer belt 31 when removing the reflection unevenness when adjusting the black image density. Can play. Moreover, in the black pattern image PT-K, the higher the density of the pattern images PT-K1, PT-K2, PT-K3, PT-K4, and PT-K5, the farther from the reference position P in the belt rotation direction backward. Formed in position. As a result, an image with a lower density that easily reflects the state of the background of the intermediate transfer belt 31 can be less affected by changes in the peripheral length and peripheral speed of the intermediate transfer belt 31 when removing uneven reflection.

  Also in the present embodiment, the color of the pattern image PT to be formed first is selected based on the elapsed time T as a result of the processing of FIG. As can be seen from the description of FIGS. 15 and 16, the color of the pattern image PT to be formed first in time is selected based on the elapsed time T from the time when the reference position P reaches the reading position Sa. In any case, the black pattern image PT-K is formed after T = 0. When the color of the selected pattern image PT is not black, the CPU 501 controls to form the pattern image PT of a color other than black before the formation of the black pattern image PT-K.

  In the first embodiment, since there are five gradation levels, the formation timing of the pattern image PT is determined using a time t assuming a required time obtained by dividing the entire length of the intermediate transfer belt 31 into five equal parts. did. However, it is not essential to match the number of gradation steps with the number of divisions of the entire length of the intermediate transfer belt 31. As long as the length of each pattern image PT can be made shorter than one region R, it may be equal to or greater than 6 or may be unequal instead of equal.

  The same applies to the second embodiment. That is, since the number of pattern images PT is four, the formation timing of the pattern image PT is determined using a time t that assumes a required time obtained by dividing the entire length of the intermediate transfer belt 31 into four equal parts. However, the number of divisions of the entire length of the intermediate transfer belt 31 may be 5 or more, or may be unequal.

  Although the present invention has been described in detail based on preferred embodiments thereof, the present invention is not limited to these specific embodiments, and various forms within the scope of the present invention are also included in the present invention. included. A part of the above-described embodiments may be appropriately combined.

DESCRIPTION OF SYMBOLS 10 Image formation part 31 Intermediate transfer belt 60 Toner density sensor 501 CPU
PT pattern image P Reference position

Claims (12)

An endless image carrier that is driven to rotate;
An image forming means for forming an image on the image carrier;
A sensor that emits light toward the image carrier and outputs a value corresponding to the amount of reflected light from the irradiation position irradiated with the light;
First acquisition means for acquiring first data from the output of the sensor based on reflected light from the ground of the image carrier;
Second acquisition means for acquiring second data from an output of the sensor based on reflected light from a measurement image formed on the image carrier by the image forming means;
Correction means for correcting the second data acquired by the second acquisition means based on the first data acquired by the first acquisition means with reference to a reference position in the circumferential direction of the image carrier; ,
Adjusting means for adjusting the density of the image formed by the image forming means based on the second data corrected by the correcting means;
When a plurality of measurement images are formed, the lowest density measurement image is formed at a position closest to the reference position in the rotation direction of the image carrier among the plurality of measurement images. An image forming apparatus comprising: a control unit that controls the image forming unit.   2. The image forming apparatus according to claim 1, wherein a measurement image having a higher density among the plurality of measurement images is formed at a position farther from the reference position at a rear side in the rotation direction of the image carrier. The plurality of measurement images have different densities, and the arrangement order of the plurality of density measurement images formed in the circumferential direction of the image carrier is predetermined,
The control means selects a measurement image to be formed first in time among the plurality of measurement images based on an elapsed time from when the reference position reaches the irradiation position, and the lowest density The measurement image is formed after the reference position reaches the image formation position, and when the selected measurement image is not the lowest density measurement image, measurement other than the lowest density measurement image is performed. 3. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled to form an image for use before the formation of the lowest density measurement image. 4.   The control means changes the selected measurement image from the selected measurement image to the lowest density measurement image after the reference position reaches the irradiation position and before the reference position reaches the image forming position. On the other hand, after the reference position reaches the image formation position, the selected measurement is performed from the lowest density measurement image after sequentially forming up to the density measurement image adjacent to the front in the rotation direction. The image forming apparatus according to claim 3, wherein the image forming apparatus performs control so as to sequentially form up to a measurement image having a density adjacent to the front in the rotation direction with respect to the image for use.   The control means is based on a nominal distance from the irradiation position to an image forming position in the forward direction of rotation of the image carrier, and an elapsed time from when the reference position reaches the irradiation position. The image forming apparatus according to claim 1, wherein a timing for forming an image having a density is determined. A plurality of the image forming means are arranged in series along the rotation direction of the image carrier so that a plurality of color images can be formed.
5. The image forming apparatus according to claim 3, wherein each of the plurality of measurement images having different densities is composed of the images of the plurality of colors. An endless image carrier that is driven to rotate;
A plurality of image forming means arranged in series along the rotation direction of the image carrier, and forming a plurality of color images including black on the image carrier;
A sensor that emits light toward the image carrier and outputs a value corresponding to the amount of reflected light from the irradiation position irradiated with the light;
First acquisition means for acquiring first data from the output of the sensor based on reflected light from the ground of the image carrier;
Second acquisition means for acquiring second data of each color from the output of the sensor based on reflected light from a plurality of color measurement images including a black measurement image formed on the image carrier by the image forming means. When,
Correction for correcting the second black data acquired by the second acquisition unit based on the first data acquired by the first acquisition unit with reference to the reference position in the circumferential direction of the image carrier Means,
Adjusting means for adjusting the density of the black image formed by the image forming means based on the second black data corrected by the correcting means;
When the plurality of color measurement images are formed, the black measurement image is formed at a position closest to the reference position behind the image carrier in the rotation direction of the image carrier. An image forming apparatus comprising: control means for controlling the image forming means.   The black measurement image is composed of a plurality of images having different densities from each other, and the higher the density of the plurality of images having different densities, the farther from the reference position in the rotation direction of the image carrier. The image forming apparatus according to claim 7, wherein the image forming apparatus is formed as follows. The arrangement order of the measurement images of the plurality of colors formed in the circumferential direction of the image carrier is predetermined,
The control means selects a color of a measurement image to be formed first in time among the measurement images of the plurality of colors based on an elapsed time from the time when the reference position reaches the irradiation position, A black measurement image is formed after the reference position reaches the irradiation position, and when the selected color measurement image is not a black measurement image, a measurement image of a color other than black is used as the black measurement image. The image forming apparatus according to claim 7, wherein the image forming apparatus is controlled to form the image before measurement. Having a detecting means for detecting a reference mark provided on the image carrier,
10. The reference position according to claim 1, wherein the reference position is a position that becomes the irradiation position when the detection unit detects the reference mark in the circumferential direction of the image carrier. The image forming apparatus described. An endless image carrier that is driven to rotate, an image forming unit that forms an image on the image carrier, and light that is emitted toward the image carrier, and reflected light from an irradiation position where the light is emitted A sensor that outputs a value corresponding to the amount of received light, and a method for controlling the image forming apparatus,
A first acquisition step of acquiring first data from an output of the sensor based on reflected light from a ground of the image carrier;
A second acquisition step of acquiring second data from the output of the sensor based on the reflected light from the measurement image formed on the image carrier by the image forming unit;
A correction step of correcting the second data acquired by the second acquisition step based on the first data acquired by the first acquisition step with reference to a reference position in the circumferential direction of the image carrier; ,
An adjustment step of adjusting the density of the image formed by the image forming unit based on the second data corrected by the correction step;
When a plurality of measurement images are formed, the lowest density measurement image is formed at a position closest to the reference position in the rotation direction of the image carrier among the plurality of measurement images. And a control step of controlling the image forming unit. An endless image carrier that is rotationally driven, and a plurality of image forming units that are arranged in series along the rotation direction of the image carrier and that form images of a plurality of colors including black on the image carrier. A method for controlling an image forming apparatus, comprising: a sensor that emits light toward the image carrier and outputs a value corresponding to a received light amount of reflected light from an irradiation position irradiated with the light,
A first acquisition step of acquiring first data from an output of the sensor based on reflected light from a ground of the image carrier;
A second acquisition step of acquiring second data of each color from the output of the sensor based on reflected light from a plurality of color measurement images including a black measurement image formed on the image carrier by the image forming unit. When,
Correction for correcting the second black data acquired by the second acquisition step based on the first data acquired by the first acquisition step with reference to the reference position in the circumferential direction of the image carrier Steps,
An adjustment step of adjusting a density of a black image formed by the image forming unit based on the second black data corrected by the correction step;
When the plurality of color measurement images are formed, the black measurement image is formed at a position closest to the reference position behind the image carrier in the rotation direction of the image carrier. And a control step for controlling the image forming unit.