Image forming apparatus

Abstract

To form an image while preventing image density unevenness due to phase shift.SOLUTION: An image forming apparatus comprises: a photoconductor drum 4d on which a black first image is formed; a photoconductor drum 4a on which a yellow second image is formed; an intermediate transfer belt 5 to which the first and second images are transferred from the photoconductor drum 4d and photoconductor drum 4a; a first image reading sensor 8 that reads the first image formed on the photoconductor drum 4d; and a second image reading sensor 9 that reads the first and second images carried on the intermediate transfer belt 5. A CPU 109 adjusts image density unevenness in a direction in which the photoconductor drum 4d rotates on the basis of a result of reading of the first image read by the first image reading sensor 8, and adjusts image density unevenness in a direction in which the photoconductor drum 4a rotates on the basis of a result of reading of the second image read by the second image reading sensor 9 and the amount of phase shift caused by slipping between the photoconductor drum 4a and the intermediate transfer belt 5.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an image forming apparatus such as a copying machine, a multifunction peripheral, a printer, etc.

Background Art

[0002] An image forming apparatus includes a plurality of rotating bodies such as a photosensitive drum, a charging roller, a developer carrier, a paper transport roller, etc. Among these, the photosensitive drum, the charging roller, and the developer carrier may cause uneven image density due to rotational vibration, uneven sensitivity, uneven resistance, etc. Such uneven image density is a periodic variation in image density that occurs in the rotational direction of the photosensitive drum. Since the photosensitive drum, the charging roller, and the developer carrier have a short rotation period, visible and periodic uneven image density occurs within one page of an image.

[0003] For example, when rotational vibration occurs in the photosensitive drum, the distance between the photosensitive drum and the developer carrier becomes unstable. The developer carrier uses the electric field generated between it and the photosensitive drum when a development bias is applied to attach toner to the photosensitive drum and form a toner image on the photosensitive drum. When the distance between the photosensitive drum and the developer carrier becomes unstable, the strength of the electric field fluctuates, so the amount of toner adhering to the photosensitive drum fluctuates. Due to the rotational vibration of the photosensitive drum, periodic uneven image density occurs in the image (toner image) formed on the photosensitive drum. Even when rotational vibration occurs in the developer carrier, uneven image density occurs for the same reason.

[0004] The photosensitive drum may have uneven sensitivity in the photosensitive layer on its surface due to factors such as environmental variations and changes over time. When uneven sensitivity occurs in the photosensitive drum, even when exposed to a certain exposure amount, a difference occurs in the bright potential, which is the potential after exposure. The difference in bright potential also causes uneven image density.

[0005] The charging roller uniformly charges the photosensitive layer of the photosensitive drum by the discharge current flowing when a charging bias is applied. When unevenness (resistance unevenness) occurs in the resistance value of the charging roller, the amount of discharge current fluctuates, and a difference occurs in the dark potential, which is the potential of the photosensitive layer of the photosensitive drum after charging. Even if the photosensitive layer of the photosensitive drum in a state where a difference occurs in the dark potential is exposed with a certain exposure amount (laser power), a difference occurs in the bright potential, which is the potential after exposure. Due to the difference in the bright potential, image density unevenness occurs.

[0006] In order to suppress the periodic image density unevenness caused by such a rotating body, Patent Document 1 discloses a technique of forming an image having a size of one or more rounds in the rotation direction of the photosensitive drum, detecting the periodic image density unevenness based on the reading result of this image, and suppressing the detected image density unevenness. Thereby, the association between the image density unevenness and the position in the rotation direction of the photosensitive drum is made. The image density unevenness is suppressed by correcting the image forming conditions such as the charging bias, the developing bias, and the exposure amount conditions.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] In Patent Document 1, although the correction amount of the image forming conditions for suppressing the image density unevenness is detected, no consideration is given to which timing (phase) of the image formation the image forming conditions are corrected. That is, no consideration is given to the phase adjustment for matching the period of the image density unevenness and the period of the image forming conditions to be corrected. For example, when correcting the image forming conditions indicating the exposure amount, the timing of correcting the exposure amount may deviate from the period of the image density unevenness. Therefore, even if the image density unevenness corresponding to the position in the rotation direction of the photosensitive drum can be detected, since the period of correcting the image forming conditions does not match the generation period of the image density unevenness, there is a possibility that the image density unevenness remains due to the phase shift during correction.

[0009] In view of the above problems, a main object of the present invention is to provide an image forming apparatus that suppresses uneven image density due to phase shift and forms an image.

Means for Solving the Problems

[0010] The image forming apparatus of the present invention is an image forming apparatus that forms an image on a sheet, and includes a first photoreceptor, a first image forming unit that forms an image of a first color on the first photoreceptor, a second photoreceptor, and a second image forming unit that forms an image of a second color different from the first color on the second photoreceptor, a transfer member onto which the image of the first color and the image of the second color are transferred, a first image reading unit that reads the test image of the first color formed on the first photoreceptor by the first image forming unit, a second image reading unit that reads the test image of the second color on the transfer member formed by the second image forming unit, an adjustment unit that adjusts uneven image density in the rotation direction of the first photoreceptor of the image formed by the first image forming unit based on the reading result of the test image of the first color read by the first image reading unit, and adjusts uneven image density in the rotation direction of the second photoreceptor of the image formed by the second image forming unit based on the reading result of the test image of the second color read by the second image reading unit and the amount of phase shift caused by the slip between the second photoreceptor and the transfer member.

Effects of the Invention

[0011] According to the present invention, it is possible to suppress uneven image density due to phase shift and form an image.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Mode for Carrying Out the Invention

[0013] Embodiments of the present invention will be described with reference to the drawings.

[0014] FIG. 1 is a configuration diagram of the main part of the image forming apparatus according to the present embodiment. The image forming apparatus 100 forms images of four colors: yellow (Y), magenta (M), cyan (C), and black (K). In FIG. 1, the configuration of a paper feeding mechanism for feeding the paper on which the image is formed and a fixing device for fixing the image on the paper are omitted.

[0015] The image forming apparatus 100 includes image forming units 10a to 10d for forming images of different colors in order to form images of four colors. The image forming units 10a to 10d include photosensitive drums 4a to 4d, charging rollers 1a to 1d, exposure devices 2a to 2d, and developing devices 3a to 3d.

[0016] The photosensitive drums 4a to 4d are drum-shaped photoreceptors having a photosensitive layer on their surfaces, and rotate in the direction of arrow R1 around the drum axis. The rotating photosensitive drums 4a to 4d are uniformly charged on their surfaces by the charging rollers 1a to 1d. The exposure devices 2a to 2d form an electrostatic latent image on the charged surface of the photosensitive drums 4a to 4d by exposing the charged surface. The developing devices 3a to 3d form a toner image on the surface of the photosensitive drums 4a to 4d by attaching a developer (for example, toner) to the electrostatic latent image.

[0017] A yellow toner image is formed on the surface of the photosensitive drum 4a. A magenta toner image is formed on the surface of the photosensitive drum 4b. A cyan toner image is formed on the surface of the photosensitive drum 4c. A black toner image is formed on the surface of the photosensitive drum 4d.

[0018] In this embodiment, the drum diameters of the photosensitive drums 4a, 4b, and 4c for forming a color (colored) image are configured to be smaller than the drum diameter of the photosensitive drum 4d for forming a black (achromatic) image. Generally, it is assumed that the frequency of black image formation in the image forming apparatus 100 is high. Therefore, the photosensitive drum 4d is formed with a larger drum diameter than the drum diameters of the other photosensitive drums 4a, 4b, and 4c, thereby extending the product life. Near the photosensitive drum 4d, a first image reading sensor 8 for detecting the image density of the toner image formed on the photosensitive drum 4d is provided. The first image reading sensor 8 is provided between the developing device 3d and the transfer position of the toner image.

[0019] The first image reading sensor 8 may be provided on at least one of the plurality of photosensitive drums 4a to 4d. The photosensitive drum on which the first image reading sensor 8 is provided is not limited to the photosensitive drum 4d used for black image formation. The conditions for providing the first image reading sensor 8 do not include the drum diameter. The first image reading sensor 8 is, for example, an optical sensor.

[0020] The image forming apparatus 100 includes an intermediate transfer belt 5 onto which the toner images of respective colors formed on the photosensitive drums 4a to 4d are transferred. The intermediate transfer belt 5 is an endless belt member and a transfer body that rotates in the direction of arrow R2. The photosensitive drums 4a to 4d are arranged in the order of the photosensitive drum 4a, the photosensitive drum 4b, the photosensitive drum 4c, and the photosensitive drum 4d from the upstream side in the rotation direction of the intermediate transfer belt 5. The transfer of the toner image is performed such that the yellow toner image, the magenta toner image, the cyan toner image, and the black toner image are superimposed in order from the upstream side in the rotation direction of the intermediate transfer belt 5. As a result, the intermediate transfer belt 5 carries a color toner image 6 in which the toner images of respective colors are superimposed.

[0021] The color toner image 6 carried on the intermediate transfer belt 5 is transferred onto the paper fed by the feeding mechanism. The paper onto which the color toner image 6 is transferred is discharged outside the image forming apparatus 100 after the color toner image 6 is fixed by the fixing device. In this way, the image forming apparatus 100 generates a printed matter with an image printed thereon.

[0022] Note that the photosensitive drums 4a to 4d of the present embodiment are provided with home position (HP) sensors 7a to 7d for detecting the rotational phase. The HP sensors 7a to 7d are constituted by, for example, a flag and a photo interrupter. The flag is provided at an end portion of the photosensitive drums 4a to 4d in the drum axis direction, and the photo interrupter is provided at a position where the flag passes due to the rotation of the photosensitive drums 4a to 4d.

[0023] On the downstream side of the photosensitive drum 4d in the rotation direction of the intermediate transfer belt 5, a second image reading sensor 9 for detecting the image density of the color toner image 6 (or test image) formed on the intermediate transfer belt 5 is provided. The second image reading sensor 9 is, for example, an optical sensor.

[0024] The image density of the color toner image 6 varies depending on the usage environment (temperature, humidity) of the image forming apparatus 100 and the usage frequency of the image forming units 10a to 10d for each color. In order to correct this variation, image density correction is performed. At the time of image density correction, a test image for detecting the image density is formed, and the test image is read by the second image reading sensor 9. The image density of the test image is detected based on the reading result of the test image by the second image reading sensor 9. The image forming conditions are adjusted (corrected) based on the detected image density. The image forming conditions are, for example, the charging bias applied to the charging rollers 1a to 1d, the developing bias applied to the developing units 3a to 3d, the conditions for controlling the exposure units 2a to 2d, and the like. Details of the image density correction will be described later.

[0025] FIG. 2 is a configuration explanatory diagram of a controller for controlling the operation of the image forming apparatus 100 having such a configuration. The controller of the present embodiment includes a CPU (Central Processing Unit) 109 and an image forming control unit 101. The CPU 109 has an A / D converter 108. The CPU 109 is connected to an image forming control unit 101, HP sensors 7a to 7d, a first image reading sensor 8, a second image reading sensor 9, a ROM (Read Only Memory) 110, and a RAM (Random Access Memory) 111. The image forming control unit 101 has an exposure control unit 112, a developing unit control unit 113, a photosensitive drum control unit 114, and an intermediate transfer belt control unit 115. The image forming control unit 101 is connected to exposure devices 2a to 2d, developing units 3a to 3d, photosensitive drums 4a to 4d, and an intermediate transfer belt 5.

[0026] The CPU 109 controls the overall operation of the image forming apparatus 100 by executing a computer program stored in the ROM 110. The RAM 111 provides a work area when the CPU 109 executes processing. The CPU 109 controls the operations of the exposure devices 2a to 2d, the developing units 3a to 3d, the photosensitive drums 4a to 4d, and the intermediate transfer belt 5 to perform image formation by transmitting instructions based on image formation conditions to the image forming control unit 101.

[0027] The image forming control unit 101 performs image formation processing by the exposure control unit 112, the developing unit control unit 113, the photosensitive drum control unit 114, and the intermediate transfer belt control unit 115 in response to instructions from the CPU 109. The exposure control unit 112 controls the exposure timing, exposure amount (laser power), etc. of the exposure devices 2a to 2d. The developing unit control unit 113 controls the application of developing bias to the developing units 3a to 3d, the developing timing, etc. The photosensitive drum control unit 114 controls the rotation of the photosensitive drums 4a to 4d. The intermediate transfer belt control unit 115 controls the rotation of the intermediate transfer belt 5.

[0028] The CPU 109 detects the home positions of the photosensitive drums 4a to 4d based on the output signals obtained from the HP sensors 7a to 7d. The CPU 109 can detect the positions (phases) in the respective rotational directions of the photosensitive drums 4a to 4d with reference to the detected home positions. The CPU 109 controls the operations of the first image reading sensor 8 and the second image reading sensor 9 to obtain the respective reading results.

[0029] The first image reading sensor 8 has a light emitting part and a light receiving part. The light emitting part is controlled by the CPU 109 to irradiate light onto the surface of the photosensitive drum 4d. The light receiving part receives the reflected light of the light from the light emitting part by the surface of the photosensitive drum 4d. The light receiving part outputs an electrical signal corresponding to the amount of the received reflected light as a reading result. The CPU 109 converts the reading result output from the first image reading sensor 8 into a digital signal by the A / D converter 108 and obtains it. When a toner image is formed on the photosensitive drum 4d, the reading result of the first image reading sensor 8 is the reading result of the toner image. When no toner image is formed on the photosensitive drum 4d, the reading result of the first image reading sensor 8 is the reading result of the surface of the photosensitive drum 4d. The image density of the toner image formed on the photosensitive drum 4d is detected based on the reading result of the toner image.

[0030] The second image reading sensor 9 has a light emitting part and a light receiving part. The light emitting part is controlled by the CPU 109 to irradiate light onto the surface of the intermediate transfer belt 5 (the surface that carries the toner image). The light receiving part receives the reflected light of the light from the light emitting part by the surface of the intermediate transfer belt 5. The light receiving part outputs an electrical signal corresponding to the amount of the received reflected light as a reading result. The CPU 109 converts the reading result output from the second image reading sensor 9 into a digital signal by the A / D converter 108 and obtains it. When the intermediate transfer belt 5 carries a toner image, the reading result of the second image reading sensor 9 is the reading result of the toner image. When the intermediate transfer belt 5 does not carry a toner image, the reading result of the second image reading sensor 9 is the reading result of the surface of the intermediate transfer belt 5.

[0031] When correcting the image density, test images of each color are not superimposed on the surface of the intermediate transfer belt 5 and are carried in single colors respectively. Therefore, based on the reading result of the test image (toner image), the image density of the toner images of each color formed on the intermediate transfer belt 5 is detected.

[0032] (Image density correction) When correcting the image density, the CPU 109 forms a test image for detecting the image density on the photosensitive drums 4a to 4d by the image formation control unit 101. The test images formed on the photosensitive drums 4a to 4d are transferred to the intermediate transfer belt 5. The test image data for forming the test image is stored in the ROM 110 in advance. The test image is, for example, a solid image.

[0033] The CPU 109 reads the test image formed on the photosensitive drum 4d by the first image reading sensor 8 and detects the image density of the test image. The CPU 109 reads the test images of each color carried by the intermediate transfer belt 5 by the second image reading sensor 9 and detects the image density of the test images of each color. The CPU 109 derives the difference from the image density gradation characteristics that should actually be output based on the detected image densities, and corrects the image formation conditions based on this difference. The CPU 109 corrects the image density by controlling the image forming units 10a to 10d by the image formation control unit 101 based on the corrected image formation conditions. Here, the image formation conditions to be corrected are, for example, the exposure amount (laser power) of the exposure devices 2a to 2d, the development conditions (development bias) of the developing devices 3a to 3d, the charging conditions (charging bias) of the photosensitive drums 4a to 4d, etc.

[0034] (Image density unevenness correction) As described above, in the image forming apparatus 100, periodic fluctuations in image density may occur due to rotating bodies used for image formation such as the photosensitive drums 4a to 4d. Such periodic fluctuations in image density are image density unevenness. The image forming apparatus 100 can perform image density unevenness correction based on the image density of the test image detected during the above image density correction.

[0035] FIG. 3 is an explanatory diagram of image density unevenness correction. FIG. 3(a) explains image density unevenness data representing image density unevenness. FIG. 3(b) explains image density unevenness correction based on the image density unevenness data. In FIGS. 3(a) and 3(b), when no distinction is made by color, the a, b, c, d at the end of the reference signs such as the image forming units 10a to 10d are omitted.

[0036] FIG. 3(a) shows the output signal 17 of the HP sensor 7, the exposure amount 12 of the exposure device 2, the output signal 24d of the first image reading sensor 8, and the output signal 25 of the second image reading sensor 9. The output signal 25 indicates the reading result of the black test image.

[0037] The image forming apparatus 100 forms test images for a plurality of circumferences of the photosensitive drum 4 by outputting light with a constant exposure amount 12 from the exposure device 2 based on the output signal 17 of the HP sensor 7. In FIG. 3(a), test images for two circumferences are formed. With test images for two or more circumferences, processing using the average value of the image density at each position in the rotational direction of the photosensitive drum 4 becomes possible.

[0038] The reading result of the black test image formed on the photosensitive drum 4d is output as the output signal 24d by the first image reading sensor 8. The black test image is read by the second image reading sensor 9 after being transferred from the photosensitive drum 4d to the intermediate transfer belt 5. The reading result of the black test image transferred to the intermediate transfer belt 5 is output as the output signal 25.

[0039] The output signal 24d and the output signal 25 are averaged for each position in the rotational direction of the photosensitive drum 4. By averaging, image density unevenness data of the black test image on the photosensitive drum 4d and image density unevenness data of the black test image on the intermediate transfer belt 5 are obtained. Note that the test images formed on the photosensitive drums 4a, 4b, 4c and transferred to the intermediate transfer belt 5 are also read by the second image reading sensor 9 in the same manner. In this case, since the output signal 25 is generated for each of the yellow, magenta, and cyan colors, the image density unevenness data is also acquired for each of the yellow, magenta, and cyan colors.

[0040] The image formation conditions are corrected based on the correction amount determined from the image density unevenness data. The correction amount is determined so that the image density unevenness data (output signals 24d and 25) becomes a certain target density. Here, the unevenness of the image density is corrected by adjusting the exposure amount from the exposure device 2. The correction amount becomes the correction amount of the exposure amount. In addition to the exposure amount, the correction of the image formation conditions includes the correction of the development bias applied during development by the developing device 3 and the charging bias applied during charging of the surface of the photosensitive drum 4. Further, in a configuration in which the image forming apparatus 100 has a gradation correction table (conversion conditions) for converting the input signal (image signal value) of the image data for each of a plurality of regions in the rotation axis direction of the photosensitive drum 4, the plurality of conversion conditions described above may be generated as the image formation conditions.

[0041] FIG. 3(b) illustrates the exposure amount 12 after correction, the output signals 24d and 25 of the test image formed after correction of the image density unevenness, when correcting the image density unevenness of the image formed by the image forming unit 10d using the image density unevenness data. The image density unevenness data is the image density unevenness data of the black test image on the photosensitive drum 4d. The image forming apparatus 100 outputs, from the exposure device 2, light having an exposure amount 12 corrected by a correction amount that has an inverse relationship with the image density unevenness data, based on the output signal 17 of the HP sensor 7. A test image for a plurality of circumferences (here, two circumferences) of the photosensitive drum 4 is formed by the exposure amount 12 corrected based on the image density unevenness data (correction amount).

[0042] The reading result of the black test image (after correction) formed on the photosensitive drum 4d is output as the output signal 24d by the first image reading sensor 8. The reading result of the black test image (after correction) transferred to the intermediate transfer belt 5 is output as the output signal 25. As shown by the output signal 24d and the output signal 25, by correcting the exposure amount 12 with the image density unevenness data, the image density unevenness of the image formed by the image forming unit 10d is reduced.

[0043] However, when correcting image formation conditions (exposure amount in the above example) based on image density unevenness data prior to the image formation operation, there are the following concerns. That is, in order to correct the image density unevenness over one rotation of the photosensitive drum 4, the image density unevenness data needs to match the rotational phase of the photosensitive drum 4 with reference to the output signal 17 of the HP sensor 7. If the image density unevenness data does not match the rotational phase of the photosensitive drum 4, there is a possibility that the image density unevenness may not be properly corrected even if the image formation conditions are corrected based on the image density unevenness data.

[0044] A general method for matching the image density unevenness data with the rotational phase of the photosensitive drum 4 is to set the acquisition timing of the image density unevenness data to be after a predetermined time has elapsed with reference to the output signal 17 of the HP sensor 7. The predetermined time is calculated from the distance that the test image travels from the exposure position of the photosensitive drum 4 by the exposure device 2 to the reading positions of the first image reading sensor 8 and the second image reading sensor 9, and the speed at which the test image travels.

[0045] In the image forming unit 10d that forms a black image, both the exposure position and the reading position of the first image reading sensor 8 are on the photosensitive drum 4d and are relatively close to each other. Therefore, the image density unevenness data and the rotational phase of the photosensitive drum 4d do not deviate significantly.

[0046] In the image forming units 10a to 10c that form other full-color images, the exposure position and the reading position of the second image reading sensor 9 are on different members. That is, the exposure position is on the photosensitive drums 4a to 4c, and the reading position is on the intermediate transfer belt 5, and they are relatively far apart. Also, image transfer slippage may occur during the transfer from the photosensitive drums 4a to 4c to the intermediate transfer belt 5. For these reasons, in the image forming units 10a to 10c, the deviation between the image density unevenness data and the rotational phase of the photosensitive drums 4a to 4c is larger than that in the image forming unit 10d. Generally, image transfer slippage is mainly caused by the toner loading amount of the image data and the frictional force depending on the respective materials of the photosensitive drums 4a to 4c and the intermediate transfer belt 5.

[0047] In this embodiment, the deviation (phase deviation) between the image density unevenness data and the rotational phase of the photosensitive drum 4 is corrected. FIG. 4 is an explanatory diagram of the phase deviation correction. FIG. 4(a) explains the amount of phase deviation. FIG. 4(b) explains the correction of image density unevenness.

[0048] FIG. 4(a) shows the output signal 17d of the HP sensor 7d, the exposure amount 12d of the exposure device 2d, the output signal 24d which is the reading result of the test image of the photosensitive drum 4d, and the output signal 25d which is the reading result of the black test image of the intermediate transfer belt 5. Further, FIG. 4(a) shows the output signal 17a of the HP sensor 7a, the exposure amount 12a of the exposure device 2a, and the output signal 25a which is the reading result of the yellow test image of the intermediate transfer belt 5. In FIG. 4(a), the phase deviation amount 30d between the black image density unevenness data and the photosensitive drum 4d and the phase deviation amount 30a between the yellow image density unevenness data and the photosensitive drum 4a are shown.

[0049] Let the original time from the start of exposure by the exposure device 2d (start of image formation) to the start of output of the output signal 24d of the first image reading sensor 8 be time t1. Let the original time from the start of exposure by the exposure device 2d to the start of output of the output signal 25d of the second image reading sensor 9 be time t2. Time t1 is determined from the distance and speed when the test image is conveyed from the exposure position of the photosensitive drum 4d (start position of formation of the test image) to the reading position of the first image reading sensor 8. Time t2 is determined from the distance and speed when the test image is conveyed from the exposure position of the photosensitive drum 4d (start position of formation of the test image) to the reading position of the second image reading sensor 9. Note that the start of exposure by the exposure device 2d is synchronized with the timing of home position detection by the HP sensor 7d.

[0050] Let the time from the start of exposure (start of image formation) by the exposure device 2a to the start of output of the output signal 25a of the second image reading sensor 9 be time t3. Time t3 is determined from the distance and speed when the test image is conveyed from the exposure position of the photosensitive drum 4a (start position of formation of the test image) to the reading position of the second image reading sensor 9. Note that the start of exposure by the exposure device 2a is synchronized with the timing of home position detection by the HP sensor 7a.

[0051] The image forming apparatus 100 forms a black test image for a plurality of circumferences (here, two circumferences) of the photosensitive drum 4d by outputting light with a certain exposure amount 12d from the exposure device 2d based on the output signal 17d of the HP sensor 7d. After the elapse of time t1, the black test image formed on the photosensitive drum 4d is started to be read by the first image reading sensor 8 (output signal 24d). After the elapse of time t2, the black test image carried on the intermediate transfer belt 5 is read by the second image reading sensor 9 (output signal 25d).

[0052] At this time, the actual output signal 25d output from the second image reading sensor 9 is phase-shifted by a phase shift amount 30d with respect to the rotation phase of the photosensitive drum 4d from the output signal 25dt of the original timing indicated by the dotted line. The original output signal 25dt is output when time t2 has elapsed since the detection of the home position. Therefore, by comparing the output signal 24d and the output signal 25d, the phase shift amount 30d between the photosensitive drum 4d and the intermediate transfer belt 5 can be detected. That is, the phase shift amount 30d is determined by the difference between the original timing (25dt) at which the output signal is output from the second image reading sensor 9 and the actual timing (25d) at which the output signal is output from the second image reading sensor 9.

[0053] As described above, the image density unevenness data and the rotational phase of the photosensitive drum 4d do not deviate significantly. Therefore, the image density unevenness data of the photosensitive drum 4d is obtained from the output signal 24d of the first image reading sensor 8. Based on the image density unevenness data obtained without shifting the phase from the output signal 24d, a correction amount for correcting the image forming conditions of the black image is determined. By correcting the image forming conditions with the determined correction amount, it is possible to reduce the image density unevenness of the black image. For example, as the image forming condition, the exposure amount by the exposure device 2d is corrected.

[0054] The photosensitive drum 4a without the first image reading sensor 8 will be described. Since the photosensitive drums 4a to 4c have the same configuration, the same processing will be applied to the photosensitive drums 4b and 4c. Therefore, the description of the photosensitive drums 4b and 4c will be omitted.

[0055] The image forming apparatus 100 forms a test image for a plurality of circumferences (here, two circumferences) of the photosensitive drum 4a by outputting light with a constant exposure amount 12a from the exposure device 2a based on the output signal 17a of the HP sensor 7a. After the elapse of time t3, the yellow test image carried on the intermediate transfer belt 5 is read by the second image reading sensor 9 (output signal 25a). As described above, the image density unevenness data of the photosensitive drum 4a is obtained based on the output signal 25a.

[0056] At this time, the output signal 25a is phase-shifted by a phase shift amount 30a with respect to the rotational phase of the photosensitive drum 4a from the output signal 25at of the original timing indicated by the dotted line. The original output signal 25at is output when the time t3 has elapsed since the detection of the home position. Since the photosensitive drums 4a to 4d are made of the same material and the toner loading amounts of the test images are the same for all colors, the phase shift amount 30a can be regarded as equivalent to the phase shift amount 30d. By correcting the image forming conditions based on the image density unevenness data (output signal 25a → output signal 25at) with the phase shifted according to the phase shift amount 30a, it is possible to reduce the image density unevenness of the yellow image. For example, as the image forming condition, the exposure amount by the exposure device 2a is corrected.

[0057] Figure 4(b) shows the exposure amount 12a and the output signal 25a when correcting the yellow image density unevenness using the yellow image density unevenness data. The image density unevenness data (output signal 25at) obtained by phase-correcting the yellow image density unevenness data (output signal 25a) in Figure 4(a) by the phase shift amount 30d becomes the corrected image density unevenness data. Note that the correction of the image density unevenness for each color of magenta, cyan, and black is also performed in the same manner. The description of the correction of the image density unevenness for each color of magenta, cyan, and black is omitted.

[0058] The image forming apparatus 100 outputs light whose exposure amount 12a is corrected from the exposure device 2a based on the output signal 17a of the HP sensor 7a so as to have an inverse relationship with the corrected image density unevenness data. Based on the exposure amount 12a corrected based on the corrected image density unevenness data, a test image for a plurality of circumferences (here, two circumferences) of the photosensitive drum 4a is formed.

[0059] Since the test image is a solid image, the reading result of the yellow test image carried on the intermediate transfer belt 5 is output as the output signal 25a (Figure 4(b)) by the second image reading sensor 9. As shown by the output signal 25a, the corrected image density unevenness data (output signal 25at) can reduce the image density unevenness more than when using the image density unevenness data (output signal 25a). That is, even when there is a phase shift in the image density unevenness data, it is possible to correct the phase shift by the phase shift amount 30d detected at the time of detecting the image density unevenness on the photosensitive drum 4d. Therefore, it is possible to reduce the image density unevenness with the correct phase.

[0060] Due to the change over time of the photosensitive drums 4a to 4d and the intermediate transfer belt 5, the image transfer slip amount changes over time. Therefore, the phase shift correction control of the image density unevenness data is preferably performed at the time of starting the image forming apparatus 100 or during the main body adjustment sequence.

[0061] FIG. 5 is a flowchart showing the process of creating image formation conditions by correcting image density unevenness. In this process, the image density unevenness and the amount of phase shift of a predetermined single color (here, black) are detected using a test image of the single color, and the image density unevenness of other colors is detected using test images of other colors. Since the image density unevenness of the predetermined single color is detected in a state where the phase shift is negligible, the image formation conditions for the predetermined single color are created according to the detected image density unevenness. Since the image density unevenness of other colors is detected in a state where there is a phase shift, the image formation conditions for other colors are created according to the image density unevenness shifted by the amount of phase shift.

[0062] The CPU 109 forms a black test image on the photosensitive drum 4d (S1001). The formation of the black test image is performed based on the output signal 17d of the HP sensor 7d (based on the home position of the photosensitive drum 4d). After forming the test image, the CPU 109 waits until a predetermined time elapses (S1002: Y). The predetermined time is the above-mentioned time t1. When the predetermined time elapses (S1002: Y), the CPU 109 stores an output signal 24d representing the reading result of the test image on the photosensitive drum 4d by the first image reading sensor 8 in the RAM 111 (S1003).

[0063] After storing the output signal 24d in the RAM 111, the CPU 109 waits until a predetermined time elapses (S1004: N). The predetermined time is the difference (t2 - t1) between the above-mentioned time t2 and time t1. When the predetermined time elapses (S1004: Y), the CPU 109 stores an output signal 25d representing the reading result of the test image on the intermediate transfer belt 5 by the second image reading sensor 9 in the RAM 111 (S1005).

[0064] The CPU 109 determines the amount of phase shift 30d by comparing the output signal 24d and the output signal 25d and stores it in the RAM 111 (S1006). For example, the CPU 109 determines the amount of phase shift 30d by comparing the waveform obtained by delaying the output signal 24d by the difference between the predetermined time of S1004 and the predetermined time of S1002 with the output signal 25d. Thus, the amount of phase shift 30d is derived.

[0065] Next, the CPU 109 forms test images of a plurality of colors (yellow, magenta, cyan) on the photosensitive drums 4a to 4c (S1007). The formation of each color test image is performed based on the output signals 17a, 17b, 17c of the HP sensors 7a, 7b, 7c (based on the home positions of the photosensitive drums 4, 4b, 4c). After forming the test images, the CPU 109 waits until a predetermined time elapses (S1008: Y). The predetermined time is the above-mentioned time t3. When the predetermined time elapses (S1008: Y), the CPU 109 stores the output signals 25a, 25b, 25c representing the reading results of the test images of each color on the intermediate transfer belt 5 by the second image reading sensor 9 in the RAM 111 (S1009).

[0066] For each of the output signals 25a, 25b, 25d stored in the RAM 111, the CPU 109 generates correction image density unevenness data with the phase shifted by the phase shift amount 30d and stores it in the RAM 111 (S1010). Based on the black output signal 24d (black image density unevenness data) stored in the RAM 111, the CPU 109 creates black image formation conditions (S1011). Based on the correction image density unevenness data of each color stored in the RAM 111, the CPU 109 creates image formation conditions for each color (S1012).

[0067] (Modification example) In the above example, the case where the test image is a solid image has been described. In contrast, in the modification example, the black test image is an image in which the image density of the solid image varies with a predetermined sine wave period. In such a test image, it becomes easy to detect the phase shift amount 30d based on the output signals 24d, 25d which are the reading results of the first image reading sensor 8 and the second image reading sensor 9 respectively. The variation amount of the sine wave period of the image density is an amount that does not affect the image density unevenness correction.

[0068] FIG. 6 is an explanatory diagram of the detection of the phase shift amount. Each reference numeral in FIG. 6 is the same as in FIG. 4(a).

[0069] The image forming apparatus 100 forms a black test image for a plurality of circumferences (here, two circumferences) of the photosensitive drum 4d by outputting light with an exposure amount 12d that varies sinusoidally at a predetermined frequency based on the output signal 17d of the HP sensor 7d. Note that a similar test image can be formed by keeping the exposure amount constant and varying the developing bias during development sinusoidally with a predetermined frequency component.

[0070] The test image is an image in which the image density varies sinusoidally with the period of a predetermined frequency. After the elapse of time t1, the black test image formed on the photosensitive drum 4d is started to be read by the first image reading sensor 8 (output signal 24d). After the elapse of time t2, the black test image carried on the intermediate transfer belt 5 is read by the second image reading sensor 9 (output signal 25d).

[0071] Both the output signals 24d and 25d are signals that reflect the sinusoidal variation of the image density. That is, due to the variation of the image density, the output signals 24d and 25d are signals that include the frequency components of a sine wave. The frequency components of the sine wave are extracted from each of the output signals 24d and 25d. From each of the extracted frequency components, the phase information of each of the output signals 24d and 25d is obtained. The phase information is, for example, the phase of a sine wave with reference to the output signal 17d of the HP sensor 7d. By comparing the respective phase information, the phase shift amount 30d is detected.

[0072] Based on the phase shift amount 30d detected in this way, correction of image density unevenness is performed in the same manner as the process described with reference to FIG. 4(b). The process of creating image forming conditions by correcting image density unevenness will be described with reference to the flowchart of FIG. 5.

[0073] The CPU 109 forms a black test image on the photosensitive drum 4d (S1001). The formation of the black test image is performed based on the output signal 17d of the HP sensor 7d (based on the home position of the photosensitive drum 4d). As described above, the test image is an image in which the image density of the solid image varies with a sine wave period. After forming the test image, the CPU 109 waits until a predetermined time elapses (S1002: Y). The predetermined time is the above-mentioned time t1. When the predetermined time elapses (S1002: Y), the CPU 109 stores the output signal 24d representing the reading result of the test image on the photosensitive drum 4d by the first image reading sensor 8 in the RAM 111 (S1003).

[0074] After storing the output signal 24d in the RAM 111, the CPU 109 waits until a predetermined time elapses (S1004: N). The predetermined time is the difference (t2 - t1) between the above-mentioned time t2 and time t1. When the predetermined time elapses (S1004: Y), the CPU 109 stores the output signal 25d representing the reading result of the test image on the intermediate transfer belt 5 by the second image reading sensor 9 in the RAM 111 (S1005).

[0075] The CPU 109 extracts the frequency component of the sine wave indicating the variation of the image density by comparing the output signal 24d and the output signal 25d, determines the phase shift amount 30d, and stores it in the RAM 111 (S1006). Thus, the phase shift amount 30d is derived.

[0076] Next, the CPU 109 forms test images of a plurality of colors (yellow, magenta, cyan) on the photosensitive drums 4a to 4c (S1007). The formation of each color test image is performed based on the output signals 17a, 17b, 17c of the HP sensors 7a, 7b, 7c (based on the home positions of the photosensitive drums 4, 4b, 4c). The color test images are solid images. After forming the test images, the CPU 109 waits until a predetermined time elapses (S1008: Y). The predetermined time is the above time t3. When the predetermined time elapses (S1008: Y), the CPU 109 stores the output signals 25a, 25b, 25c representing the reading results of the test images of each color on the intermediate transfer belt 5 by the second image reading sensor 9 in the RAM 111 (S1009). The subsequent processing (S1010 to S1012) is as described above.

[0077] As described above, the image forming apparatus 100 according to the present embodiment can form an image with suppressed image density unevenness. In particular, in the present embodiment, by synchronizing the image density unevenness data with the rotation phase of the photosensitive drum 4, the phase shift of the image density unevenness can be corrected, and the image density unevenness can be suppressed with higher accuracy than in the prior art.

Claims

1. An image forming apparatus for forming an image on a sheet, comprising: a first image forming means having a first photoreceptor and forming an image of a first color on the first photoreceptor; a second image forming means having a second photoreceptor and forming an image of a second color different from the first color on the second photoreceptor; a transfer member onto which the image of the first color and the image of the second color are transferred; a first image reading means for reading a test image of the first color formed on the first photoreceptor by the first image forming means; a second image reading means for reading a test image of the second color on the transfer member formed by the second image forming means; adjustment means for adjusting unevenness in image density in the direction in which the first photoreceptor rotates of the image formed by the first image forming means based on a reading result of the test image of the first color read by the first image reading means, and adjusting unevenness in image density in the direction in which the second photoreceptor rotates of the image formed by the second image forming means based on a reading result of the test image of the second color read by the second image reading means and a phase shift amount caused by slippage between the second photoreceptor and the transfer member; An image forming apparatus.

2. The second image reading means further reads a test image of the first color on the transfer member formed by the first image forming means, and the adjustment means determines the phase shift amount based on a reading result of the test image of the first color by the first image reading means and a reading result of the test image of the first color by the second image reading means. The image forming apparatus according to claim 1.

3. The adjustment means performs the adjustment by shifting the reading result of the test image of the second color by the second image reading means by the phase shift amount. The image forming apparatus according to claim 1.

4. The adjustment means determines a first correction amount of a first image forming condition of the image of the first color for adjusting the unevenness in image density in the direction in which the first photoreceptor rotates based on a reading result of the test image of the first color by the first image reading means. The image forming apparatus according to claim 1.

5. further comprising first exposure means for exposing the first photoreceptor and second exposure means for exposing the second photoreceptor, and the adjustment means exposes the first photoreceptor by the first exposure means under the first image forming condition corrected based on the first correction amount. ​ The adjustment means determines a second correction amount for the second image formation condition of the test image of the second color based on the phase shift amount and the reading result of the test image of the second color by the second image reading means, and exposes the second photoreceptor to the second exposure means under the second image formation condition corrected based on the second correction amount. The image forming apparatus according to claim 4.

6. The adjustment means corrects the exposure amount by the first exposure means based on the first correction amount and corrects the exposure amount by the second exposure means based on the second correction amount. The image forming apparatus according to claim 5.

7. The adjustment means determines the phase shift amount based on the difference between the original time from the start of image formation on the first photoreceptor to the start of output of the reading result of the test image of the first color by the second image reading means and the actual time from the start of image formation on the first photoreceptor to the start of output of the reading result of the test image of the first color by the second image reading means. The image forming apparatus according to claim 1.

8. The test image of the first color is a solid image, and the test image of the second color is a solid image. The image forming apparatus according to claim 1.

9. The test image of the first color is an image in which the image density varies sinusoidally at a period of a predetermined frequency, and the test image of the second color is a solid image. The image forming apparatus according to claim 1.

10. The adjustment means determines the phase shift amount based on the frequency component of the reading result of the test image of the first color by the first image reading means and the frequency component of the reading result of the test image of the first color by the second image reading means. The image forming apparatus according to claim 9.

Images