Image forming device

Abstract

To provide an image forming apparatus that can prevent a periodical change of the hue of an image obtained by overlapping a low density image and a high density image with each other.SOLUTION: An image forming apparatus, based on image density periodical change of imaging units acquired by image density periodical change acquisition means, executes, for the image forming units, image density prevention control of preventing the image density periodical change by periodically changing an image forming condition. After the image density prevention control, the image forming apparatus adjusts the periodical change of the image forming condition so as to prevent inversion of the phase of the image density periodical change of a low density image part after the image density prevention control, with respect to the phase of the image density periodical change of a high density image part.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] Conventionally, an image forming apparatus is known that comprises multiple image forming units having an image carrier and a developer carrier, which attach the developer carried by the developer carrier to the image carrier to form a visible image, and an image density periodic variation acquisition means for acquiring the image density periodic variation for each of the multiple image forming units.

[0003] Patent Document 1 describes an image forming apparatus that implements image density suppression control to suppress image density periodic fluctuations by periodically varying the development bias and charging bias as imaging conditions for each imaging unit, based on the image density periodic fluctuations acquired by the image density periodic fluctuation acquisition means, where one rotation of the image carrier constitutes one period. Furthermore, Patent Document 1 also describes a phase matching control that matches the phase of the image density periodic fluctuations of the midtone images of each imaging unit that remain uncorrected by the image density suppression control, thereby suppressing periodic changes in the color tone of the image when the visible images formed by each imaging unit are superimposed to form a full-color image. Summary of the Invention [Problem to be solved by the invention]

[0004] However, in Patent Document 1, there is a risk that the periodic change in color tone of an image in which a low-density image and a high-density image are superimposed may become worse. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention provides an image forming apparatus including a plurality of imaging units each having an image carrier, a charging member for uniformly charging the image carrier, and a developer carrier, and each imaging unit forming a visible image by causing a developer carried by the developer carrier to adhere to the image carrier, and an image density cyclical fluctuation acquisition means for acquiring cyclic image density fluctuations for each of the plurality of imaging units, the image density suppression means performing image density suppression control for each imaging unit by cyclically varying image forming conditions to suppress the cyclic image density fluctuations based on the cyclic image density fluctuations of a predetermined image density of each imaging unit acquired by the cyclic image density fluctuation acquisition means, a phase control means performing phase matching control for matching the phases of the cyclic image density fluctuations of the predetermined image density of each imaging unit on a recording material based on the cyclic image density fluctuations of each imaging unit acquired by the cyclic image density fluctuation acquisition means, and a phase control means for controlling the phase of the cyclic image density fluctuations of a low-density image portion after the image density suppression control so that the phase of the cyclic image density fluctuations of a high-density image portion is not inverted with respect to the phase of the cyclic image density fluctuations of a high-density image portion after the image density suppression control for each imaging unit. The amplitude of the charging bias applied to the charging member, which is periodically varied in phase with the periodic variation of the image density before the image density suppression control, is increased. It is characterized by having an adjustment means. [Effects of the Invention]

[0006] According to the present invention, it is possible to suppress periodic changes in the color tone of an image obtained by superimposing a low-density image and a high-density image. [Brief explanation of the drawings]

[0007] [Figure 1] A schematic diagram of an example of an image forming apparatus according to this embodiment. [Figure 2] This is an explanatory diagram showing an example of the configuration of the tandem-type image forming section of the image forming apparatus. [Figure 3] This diagram illustrates an example configuration of a toner deposition amount sensor that detects the density of the toner image in the image forming apparatus. [Figure 4] A block diagram showing an example of the main components of the control system for the image forming apparatus. [Figure 5] A flowchart showing an example of image density suppression control in the image forming apparatus. [Figure 6]5A and 5B are diagrams illustrating examples of image patterns used in the first control and the second control, respectively. [Figure 7] 5A and 5B are diagrams for explaining a method of applying image forming conditions in the first control and the second control. [Figure 8] (a) is a graph showing the periodic fluctuations in image density of Y color, M color, and C color, and (b) is a graph showing the lightness L*, chromaticity a*, and b* in the sub-scanning direction of a 3C gray image formed by superimposing images of Y, M, and C colors. [Figure 9] 4 is a flowchart showing an example of phase alignment control of the image forming apparatus. [Figure 10] FIG. 10 is a sequence diagram showing an example of rotational driving of photoconductors for Y, M, and C colors. [Figure 11] (a) is a graph showing the periodic fluctuations in image density of Y color, M color, and C color after phase alignment control, and (b) is a graph showing the lightness L*, chromaticity a*, and b* in the sub-scanning direction of a 3C gray image formed by superimposing images of Y, M, and C colors after phase alignment control. [Figure 12] (a) is a diagram showing an example of the periodic fluctuation in image density before and after image density suppression control of a high-density image of Y color, and (b) is a diagram showing an example of the periodic fluctuation in image density before and after image density suppression control of a low-density image part of Y color. [Figure 13] (a) is a graph showing an example of the relationship between the difference value from the target toner adhesion amount at a predetermined rotational position of a conventional photosensitive body and image density, and (b) is a graph showing an example of the relationship between the difference value from the target toner adhesion amount at a predetermined rotational position of a photosensitive body in this embodiment and image density. [Figure 14] 10 is a flowchart for checking whether or not there is a phase inversion of the periodic image density fluctuation of a low-density image. [Figure 15] FIG. 6 is a diagram illustrating an example of a procedure for calculating the amplitude of a charging bias. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, an embodiment will be described with reference to the drawings. Fig. 1 is a schematic diagram of an example of an image forming apparatus according to this embodiment. Referring to Fig. 1, the image forming apparatus 1 of this embodiment includes an apparatus main body (printer unit) 100, a paper feeder 200 serving as a recording medium supply unit on which the apparatus main body is placed, and a scanner 300 serving as an image reading device attached to the apparatus main body 100. Furthermore, the image forming apparatus 1 of this embodiment includes an automatic document feeder (ADF) 400 attached to the scanner 300.

[0009] An intermediate transfer belt 10, which is an endless belt serving as a surface moving member, is provided in the center of the device main body 100. The intermediate transfer belt 10 is stretched over three support rollers 14, 15, and 16 serving as supporting rotors, and rotates clockwise in the drawing. Of these three support rollers, an intermediate transfer belt cleaning device 17 is provided on the left side of the second support roller 15 in the drawing to remove residual toner remaining on the intermediate transfer belt 10 after image transfer. In addition, a tandem image forming unit 20, which serves as an image forming means, is disposed opposite the belt portion stretched between the first support roller 14 and the second support roller 15 of the three support rollers.

[0010] 1, the tandem image forming section 20 has four imaging units 18Y, 18M, 18C, and 18K for yellow (Y), magenta (M), cyan (C), and black (K) arranged in a line along the belt movement direction of the belt portion. In this embodiment, the third support roller 16 is used as a drive roller. Also, an exposure device 21 is provided above the tandem image forming section 20 as an exposure means.

[0011] A secondary transfer device 22 serving as a second transfer means is provided on the opposite side of the tandem image forming unit 20 with the intermediate transfer belt 10 in between. In the secondary transfer device 22, a secondary transfer belt 24, which is an endless belt serving as a transfer sheet transport member, is stretched between two rollers 231 and 232. The secondary transfer belt 24 is provided so as to be pressed against a third support roller 16 via the intermediate transfer belt 10. The secondary transfer device 22 transfers the toner image on the intermediate transfer belt 10 to a transfer sheet S, which is a transfer material serving as a recording medium. As shown in FIG. 1, a cleaning device 170 may be provided to clean the outer peripheral surface of the secondary transfer belt 24.

[0012] On the left side of the secondary transfer device 22 in the drawing, there is provided a fixing device 25 that fixes the toner image transferred onto the transfer sheet S. The fixing device 25 has a configuration in which a pressure roller 27 is pressed against a fixing belt 26, which is an endless belt that is heated.

[0013] The secondary transfer device 22 also has a sheet conveying function of conveying the transfer sheet S, after the toner image has been transferred from the intermediate transfer belt 10, to the fixing device 25. Further, below the secondary transfer device 22 and the fixing device 25, a sheet reversing device 28 is provided parallel to the tandem image forming section 20, which reverses the transfer sheet S so that images can be recorded on both sides of the transfer sheet S.

[0014] When making a copy using the image forming apparatus 1 configured as described above, an original is set on the platen 30 of the automatic document feeder 400. Alternatively, the automatic document feeder 400 is opened and the original is set on the contact glass 32 of the scanner 300, and then the automatic document feeder 400 is closed and held down by the platen 300. After that, when the start switch on the operation unit is pressed, the original is conveyed and moved onto the contact glass 32 when the original is set on the automatic document feeder 400.

[0015] On the other hand, when an original is set on the contact glass 32, the scanner 300 is immediately driven to move the first carriage 33 and the second carriage 34. Then, the first carriage 33 emits light from a light source, and the light reflected from the surface of the original is further reflected toward the second carriage 34, reflected by a mirror of the second carriage 34, passes through an imaging lens 35, and enters the image reading sensor 36, where the contents of the original are read.

[0016] In parallel with the document reading, the third support roller 16, which is a drive roller, is rotated by a drive motor, which is a drive source. As a result, the intermediate transfer belt 10 moves clockwise in the drawing, and the remaining two support rollers (driven rollers) 14 and 15 rotate together with this movement.

[0017] Simultaneously with the document reading and the movement of the intermediate transfer belt 10, drum-shaped photoconductors 40Y, 40M, 40C, and 40K serving as image carriers are rotated in the individual image forming units 18. Then, each of the photoconductors 40Y, 40M, 40C, and 40K is exposed and developed using color-specific information for yellow, magenta, cyan, and black, respectively, to form a monochrome toner image (visible image).

[0018] Primary transfer devices 62Y, 62M, 62C, and 62K, each consisting of a primary transfer roller, are provided as primary transfer means at positions facing each of the photoconductors 40Y, 40M, 40C, and 40K, with the belt portion of the intermediate transfer belt 10 sandwiched between the support rollers 14 and 15. The primary transfer devices 62Y, 62M, 62C, and 62K sequentially transfer the toner images on each of the photoconductors 40Y, 40M, 40C, and 40K onto the intermediate transfer belt 10 so that they are superimposed on each other, thereby forming a composite color toner image on the intermediate transfer belt 10.

[0019] In parallel with the image forming operation, one of the paper feed rollers 42 of the paper feed device 200 is selected and rotated to feed a transfer sheet S from one of the paper feed cassettes 44 provided in multiple stages in a paper bank 43. The fed transfer sheets S are then separated one by one by a separation roller 45 and placed in a paper feed path 46, transported by a transport roller 47 and guided to a paper feed path inside the device main body 100, where they are stopped by hitting registration rollers 49. Alternatively, the paper feed roller 50 is rotated to feed a transfer sheet S on a manual feed tray 51, separated one by one by a separation roller 52 and placed in a manual feed path 53, where they are also stopped by hitting registration rollers 49.

[0020] Next, the registration roller 49 is rotated in synchronization with the composite color toner image on the intermediate transfer belt 10, the transfer sheet S is fed between the intermediate transfer belt 10 and the secondary transfer device 22, and the color toner image is transferred onto the transfer sheet S by the secondary transfer device 22.

[0021] After the toner image has been transferred, the transfer sheet S is transported by secondary transfer belt 24 and sent to fixing device 25, where the transferred toner image is fixed by applying heat and pressure using fixing belt 26 and pressure roller 27. After this fixing, the sheet is switched by switching claw 55, discharged by discharge rollers 56, and stacked on discharge tray 57. Alternatively, the sheet is switched by switching claw 55 and placed in sheet reversing device 28, where it is reversed and guided to the transfer position again, an image is recorded on the back side as well, and then discharged by discharge rollers 56 onto discharge tray 57.

[0022] After the toner image is transferred, the intermediate transfer belt 10 is cleaned by an intermediate transfer belt cleaning device 17 to remove any residual toner remaining on the intermediate transfer belt 10, in preparation for next image formation by the tandem image forming unit 20. Here, the registration rollers 49 are generally grounded in most cases, but it is also possible to apply a bias to them to remove paper dust from the sheet.

[0023] The device main body 100 also includes a toner adhesion amount sensor 310, which serves as a density detection sensor and is an optical sensor unit including an optical sensor, as a density detection unit that detects the density of the toner image formed on the outer peripheral surface of the intermediate transfer belt 10. The toner adhesion amount sensor 310 functions as a density detection unit that detects the density of the toner image on the intermediate transfer belt 10 to detect density unevenness in the image by detecting the amount of toner adhesion on the intermediate transfer belt 10, and is also referred to as a toner image detection sensor or a toner adhesion amount detection sensor. The toner adhesion amount sensor 310 detects the density of the toner image of a correction control image pattern, which will be described later, formed on the surface of the intermediate transfer belt 10 and is used for correcting image unevenness. Note that, as shown in FIG. 1, an opposing roller 311 may be provided opposite the toner adhesion amount sensor 310 across the intermediate transfer belt 10.

[0024] 2 is an explanatory diagram showing an example of the configuration of a tandem image forming section of the image forming apparatus 1 according to this embodiment. Note that, since each imaging unit 18 has the same configuration, the color codes Y, M, C, and K will be omitted as appropriate in the description.

[0025] As shown in FIG. 2, the imaging unit 18 includes, for example, a charging device 60 as a charging means, a potential sensor 70, a developing device 61 as a developing means, a photosensitive body cleaning device 63, a charge removal device, and the like, around the drum-shaped photosensitive body 40.

[0026] During image formation, the photoconductor 40 is rotated in the direction of arrow A by a drive motor serving as an image carrier rotation drive means. After the surface of the photoconductor 40 is uniformly charged by the charging device 60, it is exposed to writing exposure L from the exposure device 21, which is controlled based on an image signal of an original or the like from the scanner 300, to form an electrostatic latent image. A color image signal based on image data from the scanner 300 is subjected to image processing such as color conversion processing in an image processing unit, and is output to the exposure device 21 as image signals for each color of K, Y, M, and C. The exposure device 21 converts the image signal from the image processing unit into an optical signal, and scans and exposes the surface of the uniformly charged photoconductor 40 based on this optical signal, thereby forming an electrostatic latent image.

[0027] A development bias is applied to the development roller 61a serving as a developer carrier of the development device 61, and a development potential, which is a potential difference, is formed between the electrostatic latent image on the photosensitive member 40 and the development roller 61a. This development potential causes the toner on the development roller 61a to transfer from the development roller 61a to the electrostatic latent image on the photosensitive member 40, thereby developing the electrostatic latent image and forming a toner image. In addition, a toner concentration sensor 312 capable of detecting the toner concentration in the developer is provided at the bottom of the developer transport section in which the developer transport screw 61b in the development device 61 is disposed.

[0028] The toner image formed on the photoreceptor 40 is first transferred onto the intermediate transfer belt 10 by the primary transfer device 62. After the toner image transfer, the photoreceptor 40 is cleaned of residual toner by the photoreceptor cleaning device 63 and then discharged by the static elimination device to prepare it for the next image formation.

[0029] In the image forming apparatus 1 configured as described above, the exposure device 21 and the charging devices 60Y, 60M, 60C, and 60K function as latent image forming means that form electrostatic latent images on the surfaces of the photoreceptors 40Y, 40M, 40C, and 40K. In addition, the exposure device 21, the charging devices 60Y, 60M, 60C, and 60K, and the developing devices 61Y, 61M, 61C, and 61K function as toner image forming means that form toner images on the surfaces of the photoreceptors 40Y, 40M, 40C, and 40K.

[0030] Furthermore, in the image forming apparatus 1 of this embodiment, the Y, M, and C imaging units 18 are each provided with a photointerrupter 71 as rotational position detection means capable of detecting a detectable portion 71a, which is the rotational reference position of the photoconductor 40. The photointerrupter 71 optically detects the detectable portion 71a provided on the photoconductor 40. The photointerrupter 71 has, for example, a light-emitting element and a light-receiving element arranged opposite each other, and detects the rotational reference position of the photoconductor when the detectable portion 71a provided on the photoconductor 40 passes between them and blocks the light. Note that the rotational reference position detection means for detecting the rotational reference position of the photoconductor 40 may be something other than a photointerrupter.

[0031] 3A and 3B are explanatory diagrams illustrating an example of the configuration of a toner adhesion amount sensor 310 serving as an image density detection unit that detects the density of a toner image in the image forming apparatus 1 according to this embodiment. Fig. 3A shows an example of the configuration of a black toner adhesion amount sensor 310(K) suitable for detecting the density of a black toner image, and Fig. 3B shows an example of the configuration of a color toner adhesion amount sensor 310(Y, M, C) suitable for detecting the density of a color toner image other than black.

[0032] 3(a), the black toner adhesion amount sensor 310(K) is composed of a light-emitting element 310a such as a light-emitting diode (LED) and a specular reflection light-receiving element 310b that receives specularly reflected light. The light-emitting element 310a emits light onto the intermediate transfer belt 10, and this emitted light is reflected by the intermediate transfer belt 10. The specular reflection light-receiving element 310b receives the specularly reflected light from the reflected light.

[0033] 3(b), the color toner adhesion amount sensor 310 (Y, M, C) is composed of a light-emitting element 310a such as a light-emitting diode (LED), a specular reflection light-receiving element 310b that receives specularly reflected light, and a diffuse reflection light-receiving element 310c that receives diffusely reflected light. As with the black toner adhesion amount sensor, the light-emitting element 310a irradiates light onto the intermediate transfer belt 10, and this irradiated light is reflected by the surface of the intermediate transfer belt 10. The specular reflection light-receiving element 310b receives the specularly reflected light of the reflected light, and the diffuse reflection light-receiving element 310c receives the diffusely reflected light of the reflected light.

[0034] In this embodiment, a GaAs infrared light-emitting diode with a peak wavelength of 950 nm is used as the light-emitting element, and a Si phototransistor with a peak light-receiving sensitivity of 800 nm is used as the light-receiving element. The light-emitting element and the light-receiving element may have different peak wavelengths and peak light-receiving sensitivities. The black toner adhesion amount sensor 310(K) and the color toner adhesion amount sensors 310(Y, M, C) are disposed at a distance (detection distance) of, for example, about 5 mm from the belt surface of the intermediate transfer belt 10 onto which the toner image, which is the detection target, is transferred.

[0035] In this embodiment, the toner adhesion sensor 310 is provided near the intermediate transfer belt 10, and the toner images of a predetermined image pattern formed on each of the photoreceptors 40Y, 40M, 40C, and 40K are transferred to the intermediate transfer belt 10 to detect the image density of the toner images. Image formation conditions (image creation conditions) are determined based on the detection results of the toner image density (toner adhesion amount) detected on the intermediate transfer belt. While the toner adhesion sensor 310 is provided near the intermediate transfer belt 10 in this manner, the toner adhesion sensor 310 may also be provided near each of the photoreceptors 40Y, 40M, 40C, and 40K or near the transfer conveyor belt that conveys the transfer sheet S. The densities of the toner images formed on the photoreceptors 40Y, 40M, 40C, and 40K may be detected directly without using the intermediate transfer belt 10, or by transferring the toner images from each photoreceptor to the transfer conveyor belt.

[0036] The outputs from the black toner deposition amount sensor 310(K) and the color toner deposition amount sensors 310(Y,M,C) are converted into toner deposition amounts by a deposition amount conversion algorithm. The deposition amount conversion algorithm can be the same as that used in conventional technology.

[0037] FIG. 4 is a block diagram showing an example of the main configuration of the control system of the image forming apparatus 1 according to this embodiment. The image forming apparatus 1 includes a control unit 500, which serves as a control means implemented by a computer device such as a microcomputer. The control unit 500 functions as a control means for performing image quality adjustment control, which adjusts the image quality of an output image by controlling the photoconductor drive motors 72 provided in each of the imaging units 18 (Y, M, C, K) in accordance with input image information. The image quality adjustment control of this embodiment includes at least controlling the photoconductor drive motors 72 (Y, M, C) so as to match the periodic fluctuations in image density that occur with the rotation period of the photoconductor 40 in each of the imaging units 18 (Y, M, C).

[0038] The control unit 500 includes a CPU (Central Processing Unit) 501. The control unit 500 also includes a ROM (Read Only Memory) 503 and a RAM (Random Access Memory) 504 as storage means connected to the CPU 501 via a bus line 502, and an I / O interface unit 505. The CPU 501 executes a control program, which is a pre-installed computer program, to perform various calculations and drive control of each unit. The ROM 503 pre-stores fixed data such as computer programs and control data. The RAM 504 functions as a work area or the like for rewritably storing various data.

[0039] The control unit 500 is connected to various sensors of the apparatus main body (printer unit) 100, such as the toner adhesion amount sensor 310, toner concentration sensor 312, and potential sensor 70, via an I / O interface unit 505. The various sensors, such as the toner adhesion amount sensor 310, toner concentration sensor 312, and potential sensor 70, send information detected by each sensor to the control unit 500. The control unit 500 is also connected to a charging bias setting unit (charging bias power supply) 330, which applies a predetermined charging bias to the charging roller of the charging device 60, via the I / O interface unit 505. The control unit 500 is also connected to a developing bias setting unit (developing bias power supply) 340, which applies a predetermined developing bias to the developing roller 61a of the developing device 61.

[0040] The control unit 500 is also connected to a primary transfer bias setting unit (primary transfer bias power supply) 350 that applies a predetermined primary transfer bias to the primary transfer rollers of the primary transfer devices 62 (Y, M, C, K) via an I / O interface unit 505. Furthermore, an exposure setting unit (light source power supply unit) 360 that applies a predetermined voltage to the light source of the exposure device 21 or supplies a predetermined current thereto is also connected.

[0041] Furthermore, the control unit 500 is connected to the paper feeder 200, scanner 300, and automatic document transporter 400 via the I / O interface unit 505. The control unit 500 controls each unit based on control target values ​​for image formation conditions (e.g., charging bias, development bias, exposure amount, primary transfer bias, etc.).

[0042] The ROM 503 or the RAM 504 stores, for example, a conversion table that stores information on converting the output value of the toner adhesion amount sensor 310 into the amount of toner adhesion per unit area. The ROM 503 or the RAM 504 also stores control target values ​​for the image forming conditions (e.g., charging bias, developing bias, exposure amount, primary transfer bias) of each of the imaging units 18 (Y, M, C, K) in the image forming apparatus 1.

[0043] The control unit 500 may be configured using, for example, an IC or the like as a semiconductor circuit element produced for control in the image forming apparatus 1, instead of a computer device such as a microcomputer.

[0044] The photoreceptor 40, which serves as an image carrier used in the image forming apparatus 1, is molded in a cylindrical shape, but due to variations in parts that occur during molding, it is not a perfect cylinder and has some deviations. Such deviations in parts cause cyclic fluctuations in image density, with one rotation of the photoreceptor being one cycle, during image formation in the image forming apparatus. Therefore, in this embodiment, the development bias and charging bias, which are image creation conditions, are cyclically varied with the rotation cycle of the photoreceptor to suppress cyclic fluctuations in image density, with one rotation of the photoreceptor being one cycle.

[0045] Figure 5 is a flowchart showing an example of image density suppression control that suppresses periodic fluctuations in image density. The image density suppression control executes two control operations: a first control that determines first image-forming conditions that reduce cyclical fluctuations in image density in high-density image areas, and a second control that determines second image-forming conditions that reduce cyclical fluctuations in image density in low-density image areas. This is because cyclical fluctuations in image density differ depending on the density of the image being formed (e.g., solid image or halftone image). Specifically, in high-density image areas such as solid images, the potential difference between the post-exposure potential and the developing bias, i.e., the development potential, is dominant. In contrast, in halftone image areas and low-density image areas, the potential difference between the pre-exposure potential of the photoconductor and the developing bias, i.e., the background potential, is dominant. Therefore, if the developing bias is cyclically varied to correct density unevenness in high-density image areas, density unevenness in low-density areas such as halftones will actually worsen. This is because the amount of toner adhesion in halftone and highlight areas changes due to the cyclical fluctuations in the background potential caused by the cyclical fluctuations in the developing bias. Therefore, in this embodiment, after determining the first image forming condition (development bias) for reducing the cyclic fluctuation of image density in the high-density image portion by the first control, the second image forming condition (charging bias) for reducing the cyclic fluctuation of image density in the low-density image portion is determined by the second control, thereby suppressing the cyclic fluctuation of image density in the high-density image portion and the low-density image portion.

[0046] In the first control (S1, S2), first, a toner image (see FIG. 6 described later) of a high-density image pattern (high-density image pattern) is formed on each photoconductor 40 (Y, M, C, K). Then, the density (toner adhesion amount) of this toner image is detected on the intermediate transfer belt 10 by the toner adhesion amount sensor 310. The toner image density (toner adhesion amount) is detected while the rotational position of each photoconductor 40 (Y, M, C, K) is detected by the photointerrupter 71 (Y, M, C, K).

[0047] Next, the toner adhesion amount detection signal (toner image density detection signal) detected by the toner adhesion amount sensor 310 and the rotational position signal of the photoconductor 40 detected by the photointerrupter 71 are used to obtain phase information and amplitude information of the image density fluctuation during one rotation period of the photoconductor. Then, from this phase information and amplitude information, a development bias control table for cyclically varying the development bias as a first image formation condition is created. The development bias control table contains control target values ​​for the development bias at each rotational position of the photoconductor 40 (Y, M, C, K), and the control unit 500 controls the development bias based on this control table to cyclically vary the development bias. The development bias control table created in this manner is stored in the control unit 500.

[0048] The second control (S3 to S6) is executed following the first control (S1, S2). In the second control, the development bias control table obtained in the first control is applied, and a toner image of a second image pattern (halftone image pattern) of a predetermined density is formed on each photoconductor 40 (Y, M, C, K) while cyclically varying the development bias. Then, the density (toner adhesion amount) of the toner image of the second image pattern formed on each photoconductor is detected on the intermediate transfer belt 10 by the toner adhesion amount sensor 310. This toner image density (toner adhesion amount) is also detected while the rotational position of each photoconductor 40 (Y, M, C, K) is detected by the photointerrupter 71 (Y, M, C, K).

[0049] Next, the toner adhesion amount signal (toner image density detection signal) detected by the toner adhesion amount sensor 310 and the rotational position signal of the photoconductor 40 detected by the photointerrupter 71 are used to obtain phase information and amplitude information of the image density fluctuation during one rotation period of the photoconductor. Then, from this phase information and amplitude information, a charge bias control table for cyclically varying the charge bias as a second image creation condition is created. The charge bias control table contains control target values ​​for the charge bias at each rotational position of the photoconductor 40 (Y, M, C, K), and the control unit 500 controls the charge bias based on this control table to cyclically vary the charge bias.

[0050] Next, the created charging bias control table is corrected. This correction is performed according to the density unevenness detected in the toner image of the first image pattern in the first control. Specifically, the created charging bias control table is corrected based on the density unevenness phase information and density unevenness amplitude information in the first control. This corrected charging bias control table is used as the charging bias control table to be applied during image formation (printing) and is stored in the control unit 500.

[0051] FIG. 6 is a diagram illustrating an example of an image pattern used in the first control and the second control described with reference to FIG. 6(a) is an explanatory diagram showing an example of an image pattern detected using only the toner adhesion amount sensor 310 (center sensor head) disposed at the center in the width direction of the intermediate transfer belt 10. In this example, toner images of belt-shaped, single-density image patterns 320 (Y, M, C, K) of each color extending in the belt movement direction V are sequentially formed in the detection area of ​​the toner adhesion amount sensor 310 (center sensor head). Then, the toner adhesion amount (density unevenness of the toner image) of the belt-shaped, single-density image patterns 320 (Y, M, C, K) of each color is detected by the toner adhesion amount sensor 310.

[0052] The length of each image pattern 320 (Y, M, C, K) in the belt movement direction V is set to at least one cycle of the circumferential length Lp of the photoconductor 40 and one cycle of the circumferential length of the developing roller 61 a for each color in order to calculate the variation in image density unevenness information described later. The length of each image pattern 320 (Y, M, C, K) in the belt movement direction V may be at least two cycles of the circumferential length Lp of the photoconductor 40 and one cycle of the circumferential length of the developing roller 61 a for each color.

[0053] 6(b) is an explanatory diagram showing an example of an image pattern detected using a plurality of toner adhesion amount sensors 310 (sensor heads). In this example, a toner image of a band-shaped, single-density image pattern 320 (Y, M, C, K) of each color extending in the belt movement direction V is formed in the detection area of ​​each of the plurality of toner adhesion amount sensors 310 (sensor heads). Then, density unevenness of the toner image of the band-shaped, single-density image pattern 320 (Y, M, C, K) of each color is detected by the corresponding toner adhesion amount sensor 310.

[0054] 6(a), each image pattern 320 (Y, M, C, K) is a belt-shaped pattern of a single density, and has a length of at least one period of the circumferential length Lp of the photoconductor 40 and one period of the circumferential length of the developing roller for each color. Furthermore, the length of each image pattern 320 (Y, M, C, K) in the belt movement direction V may also be at least two periods of the circumferential length Lp of the photoconductor 40 and one period of the circumferential length of the developing roller 61a for each color.

[0055] In this embodiment, the first image pattern used for the first control is formed as a high-image-density band pattern, and the second image pattern used for the second control is formed as a halftone image pattern, which is a band pattern using halftones, so as to form a halftone portion having a lower image density than the first image pattern.

[0056] Here, the first image pattern is an image pattern with an image density of about 70% so that fluctuations in the image density do not saturate, but a solid image may be used as long as fluctuations in image density can be detected, while the second image pattern is an image pattern with an image density of 40%.

[0057] Note that the image density suppression control is similar to the methods disclosed in, for example, Japanese Patent No. 6115209 and Japanese Patent Application Laid-Open No. 2017-173357, and therefore detailed explanations of the specific methods for acquiring density unevenness phase information and density unevenness amplitude information will be omitted.

[0058] FIG. 7 is a diagram illustrating a method of applying the control tables obtained in the first control and the second control described with reference to FIG. 7 shows an example of the relationship between a rotational position detection signal 510 and a toner adhesion amount detection signal 511 when a predetermined image pattern is formed, and a control table 512 of image creation conditions determined by the control unit 500 based on these signals 510 and 511. Here, an example of measurement for two cycles of the photoconductor 40 is shown. The toner adhesion amount detection signal 511 fluctuates with the same period as the rotational position detection signal 510, and a control table 512 of image creation conditions (development bias and charging bias) for the rotational position of the photoconductor 40 is created so that the toner adhesion amount detection signal 511 is in "opposite phase" with the toner adhesion amount detection signal 511 (periodic fluctuation shifted by 180°). As described above, the developing bias and the charging bias are periodically varied using the created control table 512. This makes it possible to suppress the periodic fluctuation (amplitude) of the image density that occurs with the rotation period of the photosensitive member.

[0059] Alternatively, after the second control, a third control may be performed in which the control data for the exposure light amount is determined as the control data for the third image forming condition. In the third control, a toner image of a third image pattern (halftone image pattern) is formed on each photoconductor 40 (Y, M, C, K) with the development bias control table and the charging bias control table applied. Then, as described above, the toner adhesion amount sensor 310 detects the density (toner adhesion amount) of the toner image of this third image pattern. Next, using the toner adhesion amount signal (toner image density detection signal) detected by the toner adhesion amount sensor 310 and the rotational position signal of the photoconductor 40 detected by the photointerrupter 71, phase information and amplitude information of the image density fluctuation during one rotation period of the photoconductor are obtained. Then, from this phase information and amplitude information, a third image forming condition for each rotational position of the photoconductor 40 (Y, M, C, K), specifically, a control table for periodically varying the exposure light amount for exposing the photoconductor at each rotational position of the photoconductor 40 (Y, M, C, K), is created. Then, the amount of exposure light is periodically varied using the created control table.

[0060] The image density suppression control is performed, for example, immediately after the photosensitive member is set (when initially set, when replaced, when removed, etc.). When the photoconductor 40 is removed from the main body 100 of the image forming apparatus, there is a high possibility that the occurrence of cyclical image density fluctuations in the photoconductor cycle will change. Furthermore, when the photoconductor is replaced, the runout characteristics of the new photoconductor will be different from those of the photoconductor that was previously used, and the cyclical image density fluctuations in one rotation cycle of the photoconductor 40 will be different. Furthermore, the relationship between the runout characteristics of the photoconductor 40 and the detected portion 71a will also be different, and the phase of the cyclical image density fluctuation will also be different.

[0061] Furthermore, even when the photoconductor is simply detached for maintenance, the attachment state of the photoconductor may change (change in the misalignment of the photoconductor shaft with respect to the rotation axis direction).For the above reasons, it is preferable to execute image density suppression control immediately after the photoconductor 40 is set.

[0062] It is also preferable to execute image density suppression control when environmental conditions within the device change. When environmental conditions, particularly temperature conditions, change, the photoconductor tube expands and contracts according to its thermal expansion coefficient. This changes the external profile of the photoconductor 40, which in turn changes the fluctuation state of the development gap, potentially changing the periodic fluctuation of image density. The trigger for executing image density suppression control may be determined, for example, when there is a temperature change of N degrees or more compared to the previous image density suppression control. Image density suppression control may also be executed at intervals of printing a fixed number of sheets.

[0063] Even when image density suppression control is implemented, it is not possible to completely eliminate cyclical image density fluctuations, and some cyclical image density fluctuations remain. If the phases of the cyclical image density fluctuations of each color are different from each other, cyclical color fluctuations occur in the full-color image, degrading image quality. Such color fluctuations occur in the color area formed by overlapping Y, M, and C color toners.

[0064] Figure 8(a) is a graph showing the periodic fluctuations in image density for the Y color, the M color, and the C color, and Figure 8(b) shows the brightness L in the sub-scanning direction of a 3C grayscale image formed by superimposing Y, M, and C color images. * , chromaticity a * , b * This is a graph showing [the relationship between the graph and the data]. When a color image is formed in a state where the phases of the image density periodic fluctuations of the colors are different from each other as shown in FIG. 8(a), the image brightness L * , chromaticity a * , b * It fluctuates periodically. In particular, chromaticity a * , b * This periodic fluctuation means that the color tones of the color image fluctuate periodically. Such periodic color fluctuations are highly sensitive to visual cues and are often pointed out as abnormal images.

[0065] Regarding chromaticity a*, M color fluctuates in the positive direction when the density is high, and Y and C colors fluctuate in the negative direction when the density is high. Regarding chromaticity b*, Y color fluctuates in the positive direction when the density is high, and M and C colors fluctuate in the negative direction when the density is high. Regarding K color, even if there is a cyclic fluctuation in image density, with one rotation of the photoconductor being one cycle, chromaticity a * , b * does not fluctuate periodically. Therefore, in this embodiment, phase matching control is performed to match the phase of the periodic fluctuation in image density, thereby suppressing the deterioration of image quality due to changes in color tone.

[0066] FIG. 9 is a flowchart showing an example of phase matching control for residual image density fluctuations of Y, M, and C colors. In this example, phase alignment of the periodic fluctuation in image density is performed using Y color as a reference, but the reference color may be M color or C color. When a print request is made, the control unit 500 first calculates the target phase difference θ YM and the target phase difference θ YC (S11) The target phase difference θ YM This is the phase difference between the rotational position signal of the Y-colored photoreceptor 40Y and the rotational position signal of the M-colored photoreceptor 40M when the phases of the periodic fluctuations of the Y, M, and C image densities coincide. Target phase difference θ YC is the phase difference between the rotational position signal of the photoconductor for Y color and the rotational position signal of the photoconductor for C color when the phases of the periodic fluctuations in image density of Y, M, and C match.

[0067] The photoconductor movement distance from the Y color development position to the primary transfer position is L Y , the photoconductor movement distance from the M color development position to the primary transfer position is L M Let's assume that L1 is the value obtained by dividing the distance from the primary transfer position of the Y color to the primary transfer position of the M color (drum pitch) by the circumference of the photoreceptor, and if it is not divisible, then L1 is defined as the value. Let θ be the phase of the periodic variation in the image density of the Y color. Y , the phase of the periodic fluctuation of the image density of M color is θ M If the rotation speed of the photosensitive member 40 is V, the target phase difference θ YM can be calculated using the following formula (1): θYM =|V(θ Y -θ M )|+(L Y +L1-L M )···(1)

[0068] Furthermore, the photoreceptor movement distance from the development position of color C to the primary transfer position is L c The distance from the primary transfer position of the Y color to the primary transfer position of the C color (drum pitch) is divided by the circumference of the photoreceptor, and the remaining value L2 is defined as the fractional value. Then, the phase of the periodic variation of the image density of the C color is defined as θ. c Then, the target phase difference θ Yc It can be calculated using the following formula. θ YC =|V(θ Y -θ C )|+(L Y +L2-L M )···(2)

[0069] If the pitch between drums is an integer multiple of the circumference of the photoreceptor, and the distance the photoreceptor travels from the development position to the primary transfer position is the same for Y, M, and C colors, then the target phase difference θ YM and the target phase difference θ YC is the phase θ of the image density periodic fluctuation Y , θ m , θ c It can be calculated using only this method. Furthermore, if the values ​​calculated using equations (1) and (2) exceed the circumference of the photoreceptor, the circumference of the photoreceptor is subtracted to make the calculated value less than or equal to the circumference of the photoreceptor.

[0070] Phase θ of the periodic variation in image density of Y, M, and C colors Y , θ m , θ cThe phase information acquired by the image density suppression control can be used. Alternatively, the development bias control table and the charging bias control table can be used to periodically vary the development bias and charging bias to form toner images of an image pattern on the Y, M, and C photoconductors 40 (Y, M, C), and then the phase information and amplitude information of the periodic fluctuations in image density can be reacquired. The image pattern in this case is preferably a halftone image pattern that is highly visible to the user. By using a halftone image pattern, it is possible to suppress color fluctuations in the halftones that are most desired to be suppressed. The image pattern in this case may also be a high-image-density image pattern. A high-image-density image has the advantage that the phase information of the image density fluctuations can be accurately detected because the amount of toner adhesion is large.

[0071] Next, the photoreceptor drive motors 72 (Y, M, C) are controlled to drive the Y, M, and C colored photoreceptors at the same rotational speed (S12). Then, the phase difference θR between the rotational position signal of the M colored photoreceptor and the actual rotational position signal of the Y colored photoreceptor is calculated. YM and the phase difference θR between the photoconductor position signal for Y color and the photoconductor rotation position signal for C color. Yc (S13).

[0072] Phase difference θR YM This can be determined from the time elapsed from when the Y-colored photointerrupter 71 detects the detection unit 71a, which is the rotation reference position, until the M-colored photointerrupter 71 detects the detection unit 71a, and from the rotation speed V of the photoreceptor.

[0073] Phase difference θR Yc This can be determined from the time it takes for the Y-colored photointerrupter 71 to detect the detection unit 71a, which is the rotation reference position, to be detected by the C-colored photointerrupter 71, and from the rotation speed V of the photoreceptor. Furthermore, by performing the above time measurement multiple times and taking the average value, the phase difference θR can be calculated. YM , θR YC You may ask for:

[0074] Next, the target phase difference θ calculated using the above formula 1 YM From the measured phase difference θRYM Subtracting this gives the phase shift amount (adjustment amount) Z YM Similarly, the target phase difference θ calculated by the above formula 2 is calculated. YC And the measured phase difference θR YC Therefore, the phase shift amount (adjustment amount) Z YC Calculate (S14).

[0075] Then, the calculated phase shift amount Z YM、 Z YC Based on this, the photoconductor drive motors 72 for each of Y, M, and C are controlled to rotate the Y, M, and C photoconductors at a predetermined rotation speed for a predetermined period of time, thereby matching the phases of the periodic fluctuations in image density of Y, M, and C (S15).

[0076] The rotation speed of the Y-colored photoreceptor is V Y , where the above-mentioned specified period is T, the rotation speed V of the M-color photoconductor 40M during phase alignment control M can be expressed by the following formula (3), and the rotation speed V of the C-color photoconductor 40C is C can be expressed by the following equation (4). V M =V Y +(Z YM / T)···(3) V C =V Y +(Z YC / T)···(4)

[0077] As can be seen from Equation 3 and Equation 4, the calculated phase shift amount Z YM , Z YC However, when it is negative, the rotation speed of the Y-color photoconductor is slowed down, and the calculated phase shift amount Z YM , Z YC When is positive, the rotation speed of the Y-color photoconductor is increased.

[0078] FIG. 10 is a sequence diagram showing an example of rotational driving of the photosensitive members for Y, M, and C colors. As shown in Fig. 10, each photoconductor is driven at a predetermined timing to measure the phase difference deviation. In this example, each photoconductor is driven at a different timing, but each photoconductor may be driven simultaneously.

[0079] And the phase shift amount Z YM、 Z YC is calculated, and at the timing when the Y photointerrupter 71 detects the detection target portion 71a, which is the rotation reference position, the M photoconductor and the C photoconductor are accelerated or decelerated to perform phase alignment control. In the example shown in Fig. 10, the rotation speeds of the M and C photoconductors are accelerated or decelerated so that the phases of the periodic fluctuations in image density of each color are aligned while the photoconductor makes two revolutions. In the example of Fig. 10, the rotation speed of the M color is decelerated and the rotation speed of the C color is increased to align the phases of the periodic fluctuations in image density of each color.

[0080] In the example of FIG. 10 , the speed of the M photoconductor is increased and the speed of the C photoconductor is decreased so that the phase difference with the cyclic fluctuation of the Y image density, which is the reference, becomes zero. However, adjustment may be made so that the phase difference with the cyclic fluctuation of the Y image density becomes zero by only increasing or decreasing the speed. In this way, performing phase matching control of the image density by only decreasing or increasing the speed has the advantage of simplifying the control. Furthermore, performing phase matching control of the image density by only decreasing the speed has the advantage of allowing the use of an inexpensive motor with low torque as the photoconductor drive motor 72, thereby reducing the cost of the device. On the other hand, controlling the phase alignment of image density solely by increasing the speed has the advantage of shortening the control time required for phase alignment compared to controlling the phase alignment of image density solely by decreasing the speed.

[0081] Furthermore, in the above description, the phase of the cyclical image density fluctuation is aligned using Y as the reference, but the color with the smallest control amount may also be used as the reference. Furthermore, in the above description, the photoconductors for colors other than the reference color are accelerated or decelerated for a predetermined period of time relative to the rotational speed of the photoconductor for the reference color to align the phases of the cyclical image density fluctuations of Y, M, and C. However, the phases of the cyclical image density fluctuations of Y, M, and C may also be aligned by shifting the drive timing of the photoconductor relative to the reference color based on the calculated phase shift amount.

[0082] In an apparatus in which the Y, M, and C photoconductors are driven and stopped at the same timing so that the relative rotational positions of the photoconductors do not change after the phase alignment control of the cyclical image density fluctuations, it is not necessary to perform the phase alignment control of the cyclical image density fluctuations each time printing starts. In such an apparatus, the phase alignment control can be performed when the relative rotational positions of the photoconductors change, such as during the first printing operation after the photoconductors are installed in the apparatus body.

[0083] Furthermore, even if each photoreceptor is driven and stopped at the same time, it is difficult to drive and stop them precisely at the same time, and there is a risk that the relative positions will gradually become distorted. For this reason, the flow shown in Figure 9 may be performed every specified number of images.

[0084] Figure 11(a) is a graph showing the periodic fluctuations of image density for the Y color, M color, and C color after phase alignment control. Figure 11(b) shows the brightness L in the sub-scan direction of the 3C gray image formed by superimposing the Y, M, and C color images after phase alignment control. * , chromaticity a * , b * This is a graph showing [the relationship between the graph and the data]. As shown in Figure 11, by matching the phases of the periodic fluctuations in image density of Y, M, and C, the chromaticity a * , b * It can be seen that the fluctuations in

[0085] Furthermore, by aligning the phases of the periodic fluctuations in image density for Y, M, and C, it is possible to align the phases of the image expansion / contraction fluctuations with the rotational period of the photoconductor, which are caused by fluctuations in the surface speed of the photoconductor due to vibration of the photoconductor. This also reduces misalignment of the superimposed images of Y, M, and C. It also eliminates the need to form an image pattern with multiple toner patches formed at equal intervals and measure the distance between the toner patches to detect image expansion / contraction fluctuations, thereby reducing toner consumption and device downtime.

[0086] Figure 12(a) shows the periodic fluctuation of image density before and after image density suppression control in the high-density Y-colored image area, and Figure 12(b) shows the periodic fluctuation of image density before and after image density suppression control in the low-density Y-colored image area. As shown by the solid lines in Figures 12(a) and 12(b), before image density suppression control, the phases of the image density periodic fluctuations in high-density image portions and the image density periodic fluctuations in low-density image portions are the same. However, as shown by the dashed lines in Figure 12(a), after image density suppression control, the phases of the image density periodic fluctuations in low-density image portions are inverted (opposite phase). On the other hand, in high-density image portions, the phases after image density suppression control are the same as those before image density suppression control. Furthermore, for M and C colors, if the phases of the image density periodic fluctuations in low-density image portions are inverted after image density suppression control, there is a risk of significant changes in the color tone of the superimposed image of low-density and high-density images.

[0087] Therefore, in this embodiment, in image density suppression control, the periodic fluctuations of the image formation conditions are corrected so that the phase of the periodic fluctuation of the image density of the low-density image does not invert (become out of phase) with respect to the phase of the periodic fluctuation of the image density of the high-density image.

[0088] As shown in Figure 12, the phase of the cyclical image density fluctuation of low-density images is inverted after image density suppression control, so the image creation conditions are corrected to prevent the phase of the cyclical image density fluctuation of low-density images from being inverted. As described above, the cyclical image density fluctuation of low-density images is dominated by the potential difference between the pre-exposure potential of the photosensitive member and the development bias, i.e., the background potential. Therefore, the cyclical image density fluctuation of low-density images is largely influenced by the charging bias that forms the pre-exposure potential (the amount of change in image density relative to the amount of change in charging bias is large: high sensitivity). Therefore, the cyclical change in the charging bias is corrected to prevent the phase of the cyclical image density fluctuation of low-density images from being inverted.

[0089] Figure 13(a) is a graph showing an example of the relationship between the difference value relative to the target toner deposition amount at a predetermined rotational position of the photoreceptor in the conventional method, and the image density. Figure 13(b) is a graph showing an example of the relationship between the difference value relative to the target toner deposition amount at the position where the periodic fluctuation of the image density of the photoreceptor peaks, and the image density in this embodiment. In FIG. 13, the dashed line indicates the state before the image density suppression control, the broken line indicates the state after the first control, the solid line indicates the state after the second control, and the two-dot chain line indicates the state after the correction of the cyclic fluctuation of the charging bias. As shown by the solid line in FIG. 13, after the second control, the amplitude of the periodic fluctuation of the residual image density remaining after the control was reduced to 0.002 [mg / cm 2 It can be seen that the fluctuations can be suppressed to a certain extent, and that the residual image density periodic fluctuations can be sufficiently suppressed. However, after suppression control, the low-density image shows a negative fluctuation relative to the target image density, while the high-density image shows a positive fluctuation relative to the target image density. This indicates that the phase of the residual image density periodic fluctuation in the low-density image is inverted (out of phase) with respect to the phase of the residual image density periodic fluctuation in the high-density image.

[0090] As shown by the dashed line in Figure 13, before image density suppression control, the amplitude of image density fluctuations increases as the image density increases. In the first control, the periodic fluctuation of image density in a high-density image pattern (image density: 70%) is detected, and a development bias control table is created to cancel out the image density fluctuations of this high-density image pattern. Therefore, as shown by the dashed line in Figure 13, when the development bias control table is applied and the development bias is periodically varied with phase inversion (in opposite phase) relative to the periodic fluctuation of image density, the target toner deposition amount is achieved at an image density of 70%. For images with an image density higher than 70%, the periodic fluctuation of the development bias cannot completely correct the image density, and a periodic fluctuation of image density in the same phase as the pre-correction periodic fluctuation remains. On the other hand, at image densities below 70%, the periodic fluctuation of the development bias overcorrects the image density, causing the phase of the periodic fluctuation of image density to be inverted.

[0091] In the second control, the image density fluctuation of the midtone (image density: 40%) image pattern formed while periodically varying the development bias is detected, and a charge bias control table is created to cancel out the image density fluctuation of this midtone image pattern. Therefore, as shown by the solid line in Figure 13, when the development bias and charge bias are periodically varied by applying the development bias control table and the charge bias control table, the target toner deposition amount is achieved at an image density of 40%. Below an image density of 40%, the correction by periodic variation of the charge bias is not completely correct, and the phase is inverted relative to the periodic variation of the image density before correction. As a result, the phase of the residual image density periodic variation of the low-density image under image density suppression control is inverted (out of phase) relative to the phase of the residual image density periodic variation of the high-density image. In this way, because the phase of the residual image density periodic variation of the low-density image under image density suppression control is inverted (out of phase) relative to the phase of the residual image density periodic variation of the high-density image, there is a risk that the periodic variation of color will worsen when an image with an image density of less than 40% and an image with an image density of 40% or more are superimposed.

[0092] In the second control, as described above, a development bias control table is applied to periodically vary the development bias and create a midtone (image density: 40%) image pattern. Therefore, as is clear from the dashed line in Figure 13, the phase of the image density variation obtained by detecting this image pattern is inverted (out of phase) with respect to the phase of the image density before control. Accordingly, to cancel out the image density variation obtained by detecting this midtone image pattern, the amplitude of the charging bias, which periodically varies in phase with the pre-correction image density periodic variation, is increased by a predetermined value to apply overcorrection. This makes it possible to reverse the phase of the image density variation of the low-density image, which had its phase inverted after the first control. As a result, as shown in Figure 13(b), the phase of the image density periodic variation of the low-density image can be returned to the phase before control. This makes it possible to match the phase of the image density periodic variation of the low-density image after image density suppression control with the phase of the image density periodic variation of the high-density image.

[0093] Specifically, the amplitude of the cyclic fluctuation of the charging bias, which is an image forming condition, is increased by correcting the control target value of the charging bias at each rotation position of the photosensitive member in the created charging bias control table. Also, the adjustment to increase the amplitude of the cyclically fluctuating charging bias by a predetermined value is performed to improve the color tone (chromaticity a) when a low-density image and a high-density image are superimposed. * , b * ) may be periodically changed, but may not be changed for the K color.

[0094] When the fluctuation of the image density periodic fluctuation of the low-density image is large, even if the amplitude of the periodically fluctuating charging bias is increased, the phase of the periodic fluctuation of the low-density image may not be re-inverted. As a result, the phase of the periodic fluctuation of the image density of the low-density image may remain inverted relative to the phase of the periodic fluctuation of the image density of the high-density image. Therefore, after increasing the amplitude of the charging bias, a low-density image pattern may be formed to check whether the phase of the periodic fluctuation of the image density of the low-density image has been inverted.

[0095] FIG. 14 is an example of a flowchart for checking whether or not there is a phase inversion of the periodic fluctuation in image density of a low-density image. First, the developing bias control table and the charging bias control table are applied to periodically vary the developing bias and the charging bias, and toner images of a solid image pattern and a low-density image pattern are formed on the photoconductors 40 (Y, M, C) of Y, M, and C colors (S21). The image density of the low-density image pattern is set lower (for example, image density: 20%) than the image density of the second image pattern (halftone image pattern) formed by the second control.

[0096] Next, in the same manner as described above, phase information of the image density periodic fluctuations of the solid image patterns of Y, M, and C colors and phase information of the image density periodic fluctuations of the low-density image patterns are obtained (S22). Then, for each color, it is confirmed whether the phase difference between the image density periodic fluctuations of the solid image patterns and the low-density image patterns exceeds 90 degrees (S23). If the phase difference is 90 degrees or less (N in S23), it is determined that the phase is not inverted, and the flow ends.

[0097] On the other hand, if the phase difference exceeds 90 degrees (Y in S23), the charging bias control table is recreated (S24). Specifically, the control target value of the charging bias at each rotational position of the photosensitive member in the charging bias control table is corrected so that the amplitude of the periodically fluctuating charging bias increases by a predetermined value.

[0098] In FIG. 14, the flow is ended after the charging bias control table is recreated, but after the charging bias control table is recreated, the flow may be returned to S21 to check again whether or not the phase is inverted.

[0099] Furthermore, the phase of the cyclical fluctuation in image density of a high-density image does not undergo phase inversion, as is clear from Figures 12 and 13. Therefore, a solid image pattern may not be formed, and the phase difference may be calculated using the phase information of the cyclical fluctuation in image density of a high-density image pattern obtained by the first control.

[0100] Furthermore, in the above description, the amplitude of the periodically fluctuating charging bias is increased by a predetermined value, but the increase in the amplitude of the charging bias may be calculated by calculation.

[0101] Hereinafter, the calculation procedure will be described with reference to FIG. 15, taking as an example the case where the amplitude of the charging bias is corrected so that the amplitude of the periodic fluctuation in image density at an image density of 20% becomes "0". First, the developing bias control table is applied to periodically vary only the developing bias, thereby forming a low-density (20% image density) image pattern on the Y, M, and C photoconductors 40 (Y, M, C). Then, the developing bias control table and the charging bias control table are applied to periodically vary the developing bias and the charging bias, thereby forming a toner image of a low-density (e.g., 20% image density) image pattern on the Y, M, and C photoconductors 40 (Y, M, C).

[0102] Then, in the same manner as described above, the amplitude α of the periodic variation in image density of the low-density image pattern formed by periodically varying only the development bias is obtained (Figure 15 (2)). In addition, the amplitude β of the periodic variation in image density of the low-density image pattern formed by periodically varying both the development bias and the charging bias is obtained (Figure 15 (3)).

[0103] The relationship between the amplitude of the charging bias and the amplitude of the cyclical image density fluctuation is linear. Therefore, the amplitude of the charging bias at which the amplitude of the cyclical image density fluctuation at 20% is "0" ((4) of FIG. 15) can be calculated from the amplitude of the cyclical image density fluctuation and the amplitude of the charging bias in FIG. 15(2) and the amplitude of the cyclical image density fluctuation and the amplitude of the charging bias in FIG. 15(3). Since FIG. 15(2) is the amplitude of the cyclical image density fluctuation at 20% when only the developing bias is cyclically varied, the amplitude of the charging bias is "0." Therefore, if the amplitude of the cyclical image density fluctuation in FIG. 15(2) is α, the amplitude of the cyclical image density fluctuation in FIG. 15(3) is β, and the amplitude of the charging bias is A1, the amplitude A2 of the charging bias at which the amplitude of the cyclical image density fluctuation at 20% is "0" ((4) of FIG. 15) can be calculated using the following equation (5): A2={A1 / (α-β)}×α···(5)

[0104] The above A1 can be determined from the charging bias control table created in the second control. Then, using the amplitude α of the cyclical fluctuation of image density in (2) of Fig. 15 obtained based on the low-density (image density 20%) image pattern and the amplitude β of the cyclical fluctuation of image density in (3) of Fig. 15, it is possible to calculate the amplitude A2 of the charging bias at which the amplitude of the cyclical fluctuation of image density at 20% becomes "0" ((4) of Fig. 15).

[0105] Then, based on the calculated amplitude A2 of the charging bias, the control target value of the charging bias at each rotational position of the photosensitive member in the charging bias control table is corrected to recreate the charging bias control table.

[0106] In the above, an example was explained in which the amplitude of the charging bias is corrected so that the amplitude of the cyclical fluctuation in image density at an image density of 20% becomes "0." However, in this case, as shown in FIG. 15, for image densities less than 20%, the phase of the cyclical fluctuation in image density remains inverted relative to that of a high-density image. Therefore, it is preferable to calculate the amplitude of the charging bias so that the amplitude of the cyclical fluctuation in image density at an image density of 20% becomes "0" or more. The amplitude A3 of the charging bias so that the amplitude of the cyclical fluctuation in image density at an image density of 20% becomes "γ" can be calculated using the following equation (6): A3={A1 / (α-β)}×(α-γ)···(6)

[0107] When the amplitude of the charging bias is corrected so that the amplitude of the periodic image density fluctuation of a low-density image with an image density of 20% becomes "0," the phase of the periodic image density fluctuation remains inverted relative to the high-density image for image densities less than 20%, but as shown in Fig. 15, the amplitude of the periodic image density fluctuation of the low-density image is sufficiently suppressed compared to when the amplitude of the periodic image density fluctuation of the intermediate tone (image density: 40%) is set to "0." Therefore, the periodic color fluctuation when a low-density image and a high-density image are superimposed can be suppressed compared to when the amplitude of the periodic image density fluctuation of the intermediate tone (image density: 40%) is set to "0."

[0108] In the above, the amplitude of the charging bias is increased to re-invert the phase of the cyclical image density fluctuation of the low-density image and make it the same phase as the cyclical image density fluctuation before correction, thereby making the phase of the cyclical image density fluctuation of the low-density image coincide with the phase of the cyclical image density fluctuation of the high-density image. However, for example, the amplitude of the developing bias may be increased to over-correct the cyclical image density fluctuation of the high-density image and invert the phase of the cyclical image density fluctuation of the high-density image with respect to the phase of the cyclical image density fluctuation before correction, thereby making the phase of the cyclical image density fluctuation of the low-density image coincide with the phase of the cyclical image density fluctuation of the high-density image.

[0109] In the above explanation, we have described a case where fluctuations in the development gap occur due to rotational runout, which is a rotational fluctuation component of the photosensitive member 40, which is one of the rotating bodies that form the development gap: the photosensitive member 40 and the developing roller 61a. Fluctuations in the development gap also occur due to rotational runout, which is a rotational fluctuation component of the developing roller 61a. Therefore, the periodic fluctuation in image density during the rotation period of the developing roller 61a can be suppressed and the phase of the periodic fluctuation in image density during the rotation period of the developing roller 61a can be matched for the colors Y, M, and C. In this case, a rotational position detection unit, such as a photointerrupter, is provided to detect a detection target, which is the rotation reference position of the developing roller 61a. Then, phase information and amplitude information of the periodic fluctuation in image density during the rotation period of the developing roller 61a are obtained from the rotation reference position of the developing roller 61a and a toner adhesion amount detection signal that detects the image pattern. The obtained phase information and amplitude information of the periodic fluctuation in image density during the rotation period of the developing roller 61a are used to suppress the periodic fluctuation in image density during the rotation period of the developing roller 61a and match the phase of the periodic fluctuation in image density. In phase matching, the rotational speed of the developing roller 61a for the M and C colors is accelerated or decelerated for a predetermined period of time relative to the rotational speed of the reference color, Y, based on the phase information of the periodic fluctuation in image density during the rotation period of the developing roller 61a.

[0110] In the above description, we have explained a full-color image forming apparatus using a four-unit tandem intermediate transfer method, in which the toner images of each image forming unit 18 are sequentially transferred to the surface of an intermediate transfer belt 10, which is a surface moving member, and then the toner images are transferred all at once to a transfer sheet S, which is a recording material. However, the present invention can also be applied to a full-color image forming apparatus using a four-unit tandem direct transfer method, in which the toner images of each image forming unit 18 are directly and sequentially transferred to the transfer sheet.

[0111] The above description is merely an example, and each of the following aspects provides unique effects. (Aspect 1) In an image forming apparatus comprising a plurality of image forming units 18 having an image carrier such as a photoreceptor 40, a charging member such as a charging roller of a charging device 60 that uniformly charges the image carrier, and a developer carrier such as a developing roller 61a, which form a visible image by attaching the developer carried by the developer carrier to the image carrier, and an image density periodic variation acquisition means such as a control unit 500 that acquires the periodic variation of image density for each of the plurality of image forming units, the image forming conditions (in this embodiment, the development bias and the charging bias) are set for each image forming unit based on the image density periodic variation of each image forming unit acquired by the image density periodic variation acquisition means. The system includes image density suppression means such as a control unit 500 that performs image density suppression control to suppress periodic fluctuations in image density by periodically fluctuating the as (amount), phase control means such as a control unit 500 that performs phase matching control to match the phases of the periodic fluctuations of each image-forming unit on the recording material based on the periodic fluctuations of the image density of each image-forming unit acquired by the image density periodic fluctuation acquisition means, and adjustment means such as a control unit 500 that adjusts the periodic fluctuations of the image-forming conditions so that the phase of the periodic fluctuations of the low-density image portion after image density suppression control does not invert with respect to the phase of the periodic fluctuations of the high-density image portion for each image-forming unit. In Patent Document 1, for a predetermined image density (e.g., 70% image density), the development bias is cyclically varied by 180° out of phase with the cyclic variation in image density (hereinafter referred to as "phase inversion") as an image-making condition that is highly sensitive to high densities (i.e., the amount of change in image density is large relative to the amount of change in image-making conditions) so that the cyclic variation in image density is zero (amplitude is zero). Next, for the cyclic variation in image density of a halftone (image density: 40%) after cyclically varying the development bias as an image-making condition, the charging bias is cyclically varied by inverting its phase with the cyclic variation in image density of a halftone, so that the cyclic variation in image density is zero (amplitude is zero). As a result, as shown in Figure 13(a), the density fluctuation (amplitude) of the cyclic variation in image density remaining after correction at high densities and the density fluctuation (amplitude) of the cyclic variation in image density remaining at low densities can be suppressed to the same level. However, in the image density suppression control of Patent Document 1, the periodic fluctuation of the residual image density of low-density images, which have image densities lower than halftones, is inverted in phase with respect to the periodic fluctuation of the residual image density of high-density images, as shown in Figure 13(a). In this way, when the phase of the periodic fluctuation of the residual image density of low-density images is inverted with respect to the phase of the periodic fluctuation of the residual image density of high-density images, and the phase of the periodic fluctuation of the residual image density of each imaging unit for a predetermined image density (for example, halftones: image density 40%) is matched by phase matching control, the periodic change in color is suppressed when low-density images and high-density images are superimposed, but the periodic change in color worsens when low-density images and high-density images are superimposed. This is because, when superimposing low-density images or high-density images, the phases of the periodic fluctuations in image density of one image and the other image coincide. As a result, the periodic fluctuations in the image density difference between the two images are kept small, and periodic changes in color are suppressed. However, when superimposing a low-density image and a high-density image, the phases of the periodic fluctuations in image density of the other image are inverted relative to the periodic fluctuations of the one image. Therefore, the periodic fluctuations in the image density difference between the two images become larger, and these periodic fluctuations in image density difference appear as periodic changes in the color of the image. Therefore, in Embodiment 1, in image density suppression control, the periodic variation of the image-making conditions is adjusted so that the phase of the periodic variation of image density of the low-density image is inverted relative to the phase of the periodic variation of image density of the high-density image. For example, as an image-making condition, a development bias that is highly sensitive to the high-density image is periodically varied to suppress the periodic variation of the image of the high-density image, while as an image-making condition, a charging bias that is highly sensitive to the low-density image is periodically varied to match the phase of the periodic variation of image density of the low-density image. As a result, when the phase of residual periodic variation of image density that could not be corrected by the image density suppression control is matched by phase matching control, periodic changes in the color tone of the superimposed image of the low-density and high-density images can be suppressed.

[0112] (Aspect 2) In aspect 1, after image density suppression control is performed, it is confirmed whether the phase of the periodic fluctuation of image density in the low image area is inverted relative to the phase of the periodic fluctuation of image density in the high image area, and if the phase is inverted, the periodic fluctuation of the image creation conditions is readjusted. As a result, as explained using Figure 14, it is possible to prevent the phase of the periodic fluctuation in image density of the low-density image portion after image density suppression control for each imaging unit from being inverted relative to the phase of the periodic fluctuation in image density of the high-density image portion.

[0113] (Aspect 3) In the second aspect, if the phase is not inverted, the adjustment of the periodic fluctuation of the image forming conditions is terminated.

[0114] (Aspect 4) In any of aspects 1 to 3, the image density suppression means such as the control unit 500 suppresses the cyclical image density fluctuation by cyclically varying the charging bias applied to a charging member such as a charging roller based on the cyclical image density fluctuation acquired by the cyclical image density fluctuation acquisition means such as the control unit 500 as an image creation condition, and the adjustment means such as the control unit 500 increases the amplitude of the cyclically varying charging bias. According to this, as explained using Figure 13, the phase of the periodic image density fluctuations of the low-density image, which were inverted relative to the periodic image density fluctuations of the high-density image, can be inverted to match the phase of the periodic image density fluctuations of the high-density image.

[0115] (Aspect 5) In aspect 4, an image density suppression means such as the control unit 500 periodically varies the development bias applied to a developer carrier such as the development roller 61a based on the periodic image density fluctuation of a high-density image acquired by an image density periodic fluctuation acquisition means such as the control unit 500, and periodically varies the charging bias based on the periodic image density fluctuation of a medium-density image created by periodically varying the development bias, thereby suppressing the periodic image density fluctuation. According to this, as described in the embodiment, periodic fluctuations in image density can be effectively suppressed. [Explanation of symbols]

[0116] 1: Image forming device 18: Imaging unit 21: Exposure equipment 40: Photoreceptor 60: Charging device 61: Developing device 61a: developing roller 70: Potential sensor 71: Photo Interrupter 71a: Detected unit 72: Photoconductor drive motor 310: Toner deposition amount sensor 320: Image pattern 500: Control section [Prior art documents] [Patent documents]

[0117] [Patent Document 1] Patent Publication No. 2021-182124

Claims

1. a plurality of image forming units each having an image carrier, a charging member for uniformly charging the image carrier, and a developer carrier, and each forming a visible image by causing a developer carried by the developer carrier to adhere to the image carrier; an image forming apparatus including an image density periodic fluctuation acquisition unit that acquires image density periodic fluctuations for each of a plurality of image forming units, an image density suppression unit that performs image density suppression control for suppressing the cyclical image density fluctuation by periodically varying the image forming conditions for each image forming unit based on the cyclical image density fluctuation of each image forming unit acquired by the cyclical image density fluctuation acquisition unit; a phase control unit that performs phase matching control to match the phases of the image density periodic fluctuations of the image forming units on the recording material based on the image density periodic fluctuations of the image forming units acquired by the image density periodic fluctuation acquisition unit; and adjusting means for increasing the amplitude of the charging bias applied to the charging member, which is periodically varied in phase with the cyclic variation of image density before the image density suppression control, so that the phase of the cyclic variation of image density in a low-density image portion after the image density suppression control is not inverted relative to the phase of the cyclic variation of image density in a high-density image portion for each imaging unit.

2. 2. The image forming apparatus according to claim 1, After the image density suppression control, it is confirmed whether the phase of the periodic fluctuation of the image density of the low image portion is inverted relative to the phase of the periodic fluctuation of the image density of the high image portion, and if the phase is inverted, the adjustment means adjusts the amplitude of the charging bias to increase.

3. 3. The image forming apparatus according to claim 2, wherein when the phase is not inverted, the adjusting means does not adjust the amplitude of the charging bias to increase.

4. 4. The image forming apparatus according to claim 1, The image forming apparatus is characterized in that the image density suppression means suppresses the cyclical image density fluctuation by cyclically varying the charging bias applied to the charging member based on the cyclical image density fluctuation acquired by the cyclical image density fluctuation acquisition means as the image creation condition.

5. 5. The image forming apparatus according to claim 4, The image forming apparatus is characterized in that the image density suppression means periodically varies the developing bias applied to the developer carrier based on the periodic fluctuation of the image density of the high-density image acquired by the image density periodic fluctuation acquisition means, and periodically varies the charging bias based on the periodic fluctuation of the image density of the medium-density image created by periodically varying the developing bias, thereby suppressing the periodic fluctuation of the image density.

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