JP7828550B2 - Image forming apparatus - Google Patents

Description

This invention relates to an image forming apparatus.

Conventionally, an image forming apparatus is known that comprises multiple image forming units, each 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.

Patent Document 1 describes an image forming apparatus that implements image density suppression control by periodically varying the development bias and charging bias, which are imaging conditions for each imaging unit, based on the image density periodic fluctuation acquired by the image density periodic fluctuation acquisition means, where one rotation of the image carrier constitutes one period. Furthermore, Patent Document 1 describes a phase alignment 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.

However, in Patent Document 1, there was a risk that the periodic changes in color of the image obtained by overlaying a low-density image and a high-density image would worsen.

To solve the above problems, the present invention provides an image forming apparatus comprising a plurality of image forming units having an image carrier, a charging member that uniformly charges the image carrier, and a developer carrier, wherein the developer carried by the developer carrier adheres to the image carrier to form a visible image, and an image density periodic variation acquisition means for acquiring image density periodic variations for each of the plurality of image forming units, wherein, based on the image density periodic variation of a predetermined image density for each image forming unit acquired by the image density periodic variation acquisition means, image density suppression control is performed for each image forming unit by periodically varying the image forming conditions to suppress the image density periodic variation. The present invention is characterized by comprising: an image density suppression means; a phase control means that performs phase matching control on the recording material to match the phase of the image density periodic fluctuation of each image-forming unit at a predetermined image density based on the image density periodic fluctuation of each image-forming unit acquired by the image density periodic fluctuation acquisition means; and an adjustment means that increases the amplitude of the charging bias applied to the charging member, which periodically fluctuates in the same phase as the image density periodic fluctuation before the image density suppression control, so that the phase of the image density periodic fluctuation of the low-density image portion after the image density suppression control does not invert with respect to the phase of the image density periodic fluctuation of the high-density image portion.

According to the present invention, it is possible to suppress periodic changes in the color tone of an image created by superimposing a low-density image and a high-density image.

A schematic diagram of an example of an image forming apparatus according to this embodiment. This is an explanatory diagram showing an example of the configuration of the tandem-type image forming section of the image forming apparatus. 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. A block diagram showing an example of the main components of the control system for the image forming apparatus. A flowchart showing an example of image density suppression control in the image forming apparatus. (a) and (b) are diagrams illustrating examples of image patterns used in the first and second controls, respectively. A diagram illustrating the method of applying the image formation conditions in the first and second control systems. (a) is a graph showing the periodic fluctuations in image density of the Y color, the M color, and the C color, and (b) is a graph showing the brightness L ^* , chromaticity a ^* , and b ^* in the sub-scanning direction of a 3C gray image formed by superimposing images of the Y, M, and C colors. A flowchart showing an example of phase alignment control for the image forming apparatus. A sequence diagram showing an example of the rotational drive of the Y, M, and C color photoreceptors. (a) is a graph showing the periodic fluctuations of the image density of the Y color, the M color, and the C color after phase alignment control, and (b) is a graph showing the brightness L*, chromaticity a ^* , and b ^* in the subscan direction of the 3C gray image formed by superimposing the Y ^, M, and C color images after phase alignment control. (a) is a figure showing an example of periodic fluctuations in image density before and after image density suppression control of a high-density image of the Y color, and (b) is a figure showing an example of periodic fluctuations in image density before and after image density suppression control of a low-density image of the Y color. (a) is a graph showing an example of the relationship between the difference value relative to the target toner deposition amount at a predetermined rotation position of the photoreceptor and the image density in the conventional method, and (b) is a graph showing an example of the relationship between the difference value relative to the target toner deposition amount at a predetermined rotation position of the photoreceptor and the image density in this embodiment. A flowchart for checking whether or not there is a phase inversion in the periodic fluctuations of image density in low-density images. A diagram illustrating an example of the procedure for calculating the amplitude of the charge bias.

The embodiments will be described below with reference to the drawings.
Figure 1 is a schematic diagram of an example of an image forming apparatus according to this embodiment. Referring to Figure 1, the image forming apparatus 1 of this embodiment comprises a main unit (printer unit) 100, a paper feeder 200 which is a recording medium supply unit on which the main unit is mounted, and a scanner 300 which is an image reading device mounted on the main unit 100. Furthermore, the image forming apparatus 1 of this embodiment comprises an automatic document feeder (ADF) 400 mounted on the scanner 300.

The main body of the device 100 has an intermediate transfer belt 10, which is an endless belt serving as a surface moving member, in its center. The intermediate transfer belt 10 is stretched across three support rollers 14, 15, and 16, which serve as support rotating bodies, and rotates clockwise in the diagram. 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 diagram to remove residual toner remaining on the intermediate transfer belt 10 after image transfer. Furthermore, a tandem image forming unit 20, which is an image forming means, is positioned opposite each other on the belt portion stretched between the first support roller 14 and the second support roller 15.

As shown in Figure 1, the tandem image forming unit 20 has a configuration in which four image forming units 18Y, 18M, 18C, and 18K (yellow, magenta, cyan, and black) are arranged in a row along the belt movement direction of the belt portion. In this embodiment, the third support roller 16 is used as the drive roller. Furthermore, an exposure device 21 is provided above the tandem image forming unit 20 as an exposure means.

On the opposite side of the tandem image forming unit 20, with the intermediate transfer belt 10 in between, is a secondary transfer device 22, which serves as a second transfer means. In the secondary transfer device 22, an endless belt, the secondary transfer belt 24, which acts as a transfer sheet transport member, is stretched between two rollers 231 and 232. The secondary transfer belt 24 is positioned to be pressed against the third support roller 16 via the intermediate transfer belt 10. This secondary transfer device 22 transfers the toner image on the intermediate transfer belt 10 to the transfer sheet S, which is the transfer material used as a recording medium. As shown in Figure 1, a cleaning device 170 for cleaning the outer surface of the secondary transfer belt 24 may also be provided.

In the diagram of the secondary transfer device 22, a fixing device 25 is provided on the left side for fixing the toner image transferred onto the transfer sheet S. The fixing device 25 consists of a fixing belt 26, which is a heated, endless belt, with a pressure roller 27 pressed against it.

Furthermore, the secondary transfer device 22 is equipped with a sheet transport function that transports the transfer sheet S, after the toner image has been transferred from the intermediate transfer belt 10 to the transfer sheet S, to the fixing device 25. Below the secondary transfer device 22 and the fixing device 25, parallel to the tandem image forming unit 20, is a sheet inversion device 28 that inverts the transfer sheet S to record images on both sides of the transfer sheet S.

When making a copy using the image forming apparatus 1 with the above configuration, the original document is placed on the document glass 30 of the automatic document transporter 400. Alternatively, the automatic document transporter 400 is opened, the original document is placed on the contact glass 32 of the scanner 300, and the automatic document transporter 400 is closed to hold it down. Then, when the start switch on the control panel is pressed, if the original document was placed in the automatic document transporter 400, the original document is transported and moved onto the contact glass 32.

On the other hand, when a document is placed on the contact glass 32, the scanner 300 is immediately driven, causing the first and second travel bodies 33 and 34 to move. The first travel body 33 emits light from the light source, and also reflects the reflected light from the document surface towards the second travel body 34. This light is then reflected by the mirror on the second travel body 34, passes through the imaging lens 35, and enters the image reading sensor 36, where the contents of the document are read.

In parallel with the document scanning described above, the drive motor, which is the driving source, rotates the third support roller 16, which is the drive roller. As a result, the intermediate transfer belt 10 moves clockwise in the diagram, and the remaining two support rollers (driven rollers) 14 and 15 rotate along with this movement.

Furthermore, simultaneously with the document reading and the movement of the intermediate transfer belt 10, the drum-shaped photoreceptors 40Y, 40M, 40C, and 40K, which serve as image carriers, are rotated in each image-forming unit 18. Then, each photoreceptor 40Y, 40M, 40C, and 40K is exposed and developed using color information for yellow, magenta, cyan, and black, respectively, to form a monochrome toner image (visual image).

At positions opposite each photoreceptor 40Y, 40M, 40C, and 40K, across the belt portion between the support rollers 14 and 15 of the intermediate transfer belt 10, primary transfer devices 62Y, 62M, 62C, and 62K, consisting of primary transfer rollers as primary transfer means, are provided. These primary transfer devices 62Y, 62M, 62C, and 62K sequentially transfer the toner images on each photoreceptor 40Y, 40M, 40C, and 40K onto the intermediate transfer belt 10 so that they overlap each other, forming a composite color toner image on the intermediate transfer belt 10.

In parallel with the image formation operation described above, 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 multi-stage paper feed cassettes 44 in the paper bank 43. The fed transfer sheets S are then separated one by one by the separation roller 45 and placed into the paper feed path 46. They are then transported by the transport roller 47 into the paper feed path within the main unit 100 and stopped by the registration roller 49. Alternatively, the paper feed roller 50 is rotated to feed the transfer sheets S from the manual feed tray 51, separated one by one by the separation roller 52 and placed into the manual feed path 53, where they are also stopped by the registration roller 49.

Next, the register roller 49 is rotated in time with the composite color toner image on the intermediate transfer belt 10, feeding the transfer sheet S between the intermediate transfer belt 10 and the secondary transfer device 22. The secondary transfer device 22 then transfers the color toner image onto the transfer sheet S.

After the toner image is transferred, the transfer sheet S is transported by the secondary transfer belt 24 and fed to the fuser unit 25. In the fuser unit 25, heat and pressure are applied by the fuser belt 26 and pressure roller 27 to fix the toner image. After this fixing, the sheet is switched by the switching claw 55 and discharged by the discharge roller 56, stacking on the output tray 57. Alternatively, the sheet is switched by the switching claw 55 and placed in the sheet inversion device 28, where it is inverted and guided back to the transfer position. After recording the image on the reverse side, it is discharged onto the output tray 57 by the discharge roller 56.

Furthermore, after toner image transfer, the intermediate transfer belt 10 is cleaned by the intermediate transfer belt cleaning device 17 to remove any remaining toner, preparing it for further image formation by the tandem image forming unit 20. While the registration roller 49 is generally used in a grounded state, it is also possible to apply a bias to remove paper dust from the sheet.

Furthermore, the main unit 100 of the device is equipped with a toner adhesion amount sensor 310, which is an optical sensor unit composed of an optical sensor or the like, as a density detection means for detecting the density of the toner image formed on the outer circumferential surface of the intermediate transfer belt 10. The toner adhesion amount sensor 310 functions as a density detection means for detecting the density of the toner image on the intermediate transfer belt 10 in order to detect density unevenness in the image by detecting the amount of toner adhered on the intermediate transfer belt 10, and is also called a toner image detection sensor or toner adhesion amount detection sensor. The toner adhesion amount sensor 310 detects the density of the toner image of the correction control image pattern, described later, formed on the surface of the intermediate transfer belt 10 for use in image unevenness correction control. Note that, as shown in Figure 1, an opposing roller 311 may be provided at a position opposite the toner adhesion amount sensor 310 with the intermediate transfer belt 10 in between.

Figure 2 is an explanatory diagram showing an example of the configuration of the tandem-type image forming section of the image forming apparatus 1 according to this embodiment. Since each image forming unit 18 has a similar configuration, the color codes Y, M, C, and K will be omitted as appropriate in the explanation.

The image-forming unit 18, as shown in Figure 2, for example, is equipped with a charging device 60 as a charging means, a potential sensor 70, a developing device 61 as a developing means, a photoreceptor cleaning device 63, a static elimination device, and the like, around a drum-shaped photoreceptor 40.

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

A developing bias is applied to the developing roller 61a, which serves as the developer carrier in the developing device 61. A developing potential, or potential difference, is formed between the electrostatic latent image on the photoreceptor 40 and the developing roller 61a. This developing potential causes the toner on the developing roller 61a to transfer to the electrostatic latent image on the photoreceptor 40, thereby developing the electrostatic latent image and forming a toner image. Furthermore, a toner concentration sensor 312, capable of detecting the toner concentration in the developer, is provided at the bottom of the developer transport section where the developer transport screw 61b is located within the developing device 61.

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.

In the image forming apparatus 1 with the above configuration, the exposure apparatus 21 and the charging apparatuses 60Y, 60M, 60C, and 60K function as latent image forming means for forming electrostatic latent images on the surface of the photoreceptors 40Y, 40M, 40C, and 40K. Furthermore, the exposure apparatus 21, the charging apparatuses 60Y, 60M, 60C, and 60K, and the developing apparatuses 61Y, 61M, 61C, and 61K function as toner image forming means for forming toner images on the surface of the photoreceptors 40Y, 40M, 40C, and 40K.

Furthermore, in this embodiment, the image forming apparatus 1 is equipped with a photointerrupter 71 in the Y, M, and C image forming units 18 as a rotational position detection means capable of detecting the detected portion 71a, which is the rotational reference position of the photoreceptor 40. The photointerrupter 71 optically detects the detected portion 71a provided on the photoreceptor 40. This photointerrupter 71, for example, has a light-emitting element and a light-receiving element arranged facing each other, and the detected portion 71a on the photoreceptor 40 passes between them, blocking the light and thereby detecting the rotational reference position of the photoreceptor. Note that a rotational reference position detection means other than a photointerrupter may be used to detect the rotational reference position of the photoreceptor 40.

Figure 3 is an explanatory diagram illustrating an example configuration of a toner deposition amount sensor 310 as an image density detection means for detecting the density of a toner image in the image forming apparatus 1 according to this embodiment. Figure 3(a) shows an example configuration of a black toner deposition amount sensor 310 (K) suitable for detecting the density of a black toner image, and Figure 3(b) shows an example configuration of a color toner deposition amount sensor 310 (Y, M, C) suitable for detecting the density of color toner images other than black.

As shown in Figure 3(a), the black toner deposition amount sensor 310(K) consists 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 irradiates light onto the intermediate transfer belt 10, and this irradiated light is reflected by the intermediate transfer belt 10. The specular reflection light-receiving element 310b receives the specularly reflected light from this reflected light.

On the other hand, as shown in Figure 3(b), the color toner deposition amount sensor 310 (Y, M, C) is composed of a light-emitting element 310a made of a light-emitting diode (LED) or the like, 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. Similar to the black toner deposition 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, and the diffuse reflection light-receiving element 310c receives the diffusely reflected light.

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. Light-emitting elements and light-receiving elements with different peak wavelengths and peak light-receiving sensitivities may also be used. Furthermore, the black toner deposition amount sensor 310 (K) and the color toner deposition amount sensors 310 (Y, M, C) are arranged with a distance of approximately 5 mm (detection distance) between them and the belt surface of the intermediate transfer belt 10 on which the toner image, which is the object to be detected, is transferred.

Furthermore, in this embodiment, the toner adhesion amount sensor 310 is provided near the intermediate transfer belt 10, and the toner image of a predetermined image pattern formed on each photoreceptor 40Y, 40M, 40C, and 40K is transferred to the intermediate transfer belt 10 to detect the image density of the toner image. The image formation conditions (image creation conditions) are determined based on the detection result of the toner image density (toner adhesion amount) detected on the intermediate transfer belt. While the toner adhesion amount sensor 310 is provided near the intermediate transfer belt 10 in this configuration, the toner adhesion amount sensor 310 may also be provided near each of the photoreceptors 40Y, 40M, 40C, and 40K, or near the transfer transport belt that transports the transfer sheet S. The density of the toner image formed on the photoreceptors 40Y, 40M, 40C, and 40K may be detected directly without going through the intermediate transfer belt 10, or the toner image may be transferred from each photoreceptor to the transfer transport belt for detection.

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.

Figure 4 is a block diagram showing an example of the main components 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 is a control means configured as a computer device such as a microcomputer. The control unit 500 functions as a control means that adjusts the image quality of the output image by controlling the photoreceptor drive motors 72 and the like provided in each image forming unit 18 (Y, M, C, K) according to the input image information. The image quality adjustment control in this embodiment includes controlling each photoreceptor drive motor 72 (Y, M, C) so as to match the image density period fluctuations that occur with the rotation period of the photoreceptor 40 in each image forming unit 18 (Y, M, C).

The control unit 500 includes a CPU (Central Processing Unit) 501. It also includes a ROM (Read Only Memory) 503 and a RAM (Random Access Memory) 504, which are storage means connected to the CPU 501 via a bus line 502, and an I/O interface unit 505. The CPU 501 performs various calculations and drive control of various parts by executing a control program, which is a pre-installed computer program. The ROM 503 stores fixed data such as computer programs and control data. The RAM 504 functions as a work area, etc., that stores various data in a rewritable manner.

The control unit 500 is connected to various sensors of the main unit (printer unit) 100, including the toner deposition amount sensor 310, toner density sensor 312, and potential sensor 70, via the I/O interface unit 505. These sensors send the information they detect to the control unit 500. Furthermore, the control unit 500 is connected to a charge bias setting unit (charge bias power supply) 330, which applies a predetermined charge bias to the charge roller of the charger 60, via the I/O interface unit 505. Additionally, a development bias setting unit (development bias power supply) 340 is connected to the development roller 61a of the development unit 61, which applies a predetermined development bias.

Furthermore, the control unit 500 is connected via the I/O interface unit 505 to a primary transfer bias setting unit (primary transfer bias power supply) 350, which applies a predetermined primary transfer bias to the primary transfer rollers of the primary transfer apparatus 62 (Y, M, C, K). Additionally, an exposure setting unit (light source power supply unit) 360 is connected to the exposure apparatus 21, which applies a predetermined voltage or supplies a predetermined current to the light source.

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 component based on control target values for image formation conditions (e.g., charging bias, development bias, exposure amount, primary transfer bias, etc.).

The ROM 503 or RAM 504 stores, for example, a conversion table containing information regarding the conversion of the output value of the toner deposition amount sensor 310 to the amount of toner deposited per unit area. The ROM 503 or RAM 504 also stores the control target values for the image formation conditions (e.g., charging bias, development bias, exposure amount, primary transfer bias) of each image formation unit 18 (Y, M, C, K) in the image forming apparatus 1.

Furthermore, the control unit 500 may be configured using, for example, an IC (integrated circuit) manufactured for control purposes in the image forming apparatus 1, rather than a computer device such as a microcomputer.

The photoreceptor 40, which serves as the image carrier used in the image forming apparatus 1, is molded in a cylindrical shape. However, due to variations in the parts that occur during molding, it is not a perfect cylinder and has runout. Such runout causes periodic fluctuations in image density during image formation within the image forming apparatus, with one rotation of the photoreceptor representing one period. Therefore, in this embodiment, the development bias and charging bias, which are image formation conditions, are periodically varied according to the rotation period of the photoreceptor to suppress periodic fluctuations in image density with one rotation of the photoreceptor representing one period.

Figure 5 is a flowchart showing an example of image density suppression control that suppresses periodic fluctuations in image density.
Image density suppression control performs a first control that determines a first image-forming condition that reduces periodic fluctuations in image density in high-density image areas, and a second control that determines a second image-forming condition that reduces periodic fluctuations in image density in low-density image areas. This is because periodic fluctuations in image density differ depending on the density of the image being formed (for example, solid images and midtone images). Specifically, in high-density image areas such as solid images, the potential difference between the post-exposure potential and the development bias, i.e., the development potential, is dominant. In contrast, in midtone and low-density image areas, the potential difference between the pre-exposure potential of the photoreceptor and the development bias, i.e., the background potential, is dominant. Therefore, if the development bias is periodically varied to correct density unevenness in high-density image areas, the density unevenness will actually worsen in low-density areas such as midtones. This is because the periodic fluctuation of the background potential caused by the periodic fluctuation of the development bias changes the amount of toner deposited in the midtones and highlight areas. Therefore, in this embodiment, after determining the first imaging condition (development bias) for reducing the periodic fluctuation of image density in the high-density image area in the first control, the second control is performed to determine the second imaging condition (charging bias) for reducing the periodic fluctuation of image density in the low-density image area. This makes it possible to suppress periodic fluctuations in image density in both the high-density and low-density image areas.

In the first control (S1, S2), a high-density image pattern (high-density image pattern) toner image (see Figure 6 below) is first formed on each photoreceptor 40 (Y, M, C, K). Then, the density (amount of toner) of this toner image is detected on the intermediate transfer belt 10 by the toner amount sensor 310. This detection of toner image density (amount of toner) is performed while the rotational position of each photoreceptor 40 (Y, M, C, K) is detected by the photointerrupter 71 (Y, M, C, K).

Next, the toner deposition amount detection signal (toner image density detection signal) detected by the toner deposition amount sensor 310 and the rotation position signal of the photoreceptor 40 detected by the photointerrupter 71 are used to obtain phase information and amplitude information of the image density fluctuation for one rotation period of the photoreceptor. Then, from this phase information and amplitude information, a development bias control table is created to periodically vary the development bias as the first image formation condition. The development bias control table contains the control target values for the development bias at each rotation position of the photoreceptor 40 (Y, M, C, K). The control unit 500 controls the development bias based on this control table, thereby periodically varying the development bias. The development bias control table thus created is stored within the control unit 500.

The second control (S3-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 while periodically varying the development bias, a toner image of a second image pattern (midtone image pattern) of a predetermined density is formed on each photoreceptor 40 (Y, M, C, K). The density (toner amount) of the toner image of the second image pattern formed on each photoreceptor is then detected by the toner amount sensor 310 on the intermediate transfer belt 10. This detection of toner image density (toner amount) is also performed while detecting the rotational position of each photoreceptor 40 (Y, M, C, K) using the photointerrupter 71 (Y, M, C, K).

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

Next, the created charge 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 charge bias control table is corrected based on the density unevenness phase information and density unevenness amplitude information from the first control described above. This corrected charge bias control table is used as the charge bias control table applied during the image forming operation (printing) and is stored in the control unit 500.

Figure 6 illustrates an example of an image pattern used in the first and second controls described in Figure 5.
Figure 6(a) is an explanatory diagram showing an example of an image pattern detected using only a toner deposition amount sensor 310 (central sensor head) positioned in the center of the width direction of the intermediate transfer belt 10. In this example, toner images of single-density, band-shaped 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 deposition amount sensor 310 (central sensor head). The amount of toner deposited (density unevenness of the toner image) of the single-density, band-shaped image patterns 320 (Y, M, C, K) of each color is then detected by the toner deposition amount sensor 310.

The length of each image pattern 320 (Y, M, C, K) in the belt movement direction V is set to be at least one period of each of the circumference Lp of the photoreceptor 40 and the circumference of the developing roller 61a 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 also be at least two periods of each of the circumference Lp of the photoreceptor 40 and the circumference of the developing roller 61a for each color.

Figure 6(b) is an explanatory diagram showing an example of an image pattern detected using multiple toner deposition sensors 310 (sensor heads). In this example, toner images of single-density, band-shaped image patterns 320 (Y, M, C, K) of each color, extending in the belt movement direction V, are formed in the detection area of each of the multiple toner deposition sensors 310 (sensor heads). The density unevenness of the toner images of the single-density, band-shaped image patterns 320 (Y, M, C, K) of each color is then detected by the corresponding toner deposition sensor 310.

In this case as well, similar to the example in Figure 6(a), each image pattern 320 (Y, M, C, K) is a band-shaped pattern of single density, and each color has a length of at least one period of the circumference Lp of the photoreceptor and the circumference of the developing roller. 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 circumference Lp of the photoreceptor 40 and the circumference of the developing roller 61a for each color.

In this embodiment, the first image pattern used for the first control is formed as a high-density band pattern with a high image density. The second image pattern used for the second control is formed as a halftone image pattern, which is a band pattern with a lower image density than the first image pattern.

Here, the first image pattern has an image density of approximately 70% to prevent saturation of the variation on the side where the image density increases. However, a solid image is also acceptable if the variation in image density can be detected. On the other hand, the second image pattern has an image density of 40%.

Regarding image density suppression control, since it is similar to the methods disclosed in, for example, Japanese Patent No. 6115209 and Japanese Unexamined Patent Publication No. 2017-173357, a detailed explanation of the specific methods for acquiring density unevenness phase information and density unevenness amplitude information will be omitted.

Figure 7 illustrates how to apply each control table obtained in the first and second control described in Figure 5.
Figure 7 shows an example of the relationship between the rotation position detection signal 510 and the toner deposition amount detection signal 511 when a predetermined image pattern is formed, and the control table 512 of the imaging conditions determined by the control unit 500 based on these signals 510 and 511. Here, an example of measurement for two cycles of the photoreceptor 40 is shown. The toner deposition amount detection signal 511 fluctuates with the same period as the rotation position detection signal 510, and the control table 512 of the imaging conditions (development bias and charging bias) for the rotation position of the photoreceptor 40 is created so that it is in "opposite phase" (periodic fluctuation with a 180° phase shift) with respect to this toner deposition amount detection signal 511.
As described above, the development bias and charging bias are periodically varied using the created control table 512. This makes it possible to suppress periodic fluctuations (amplitude) in image density that occur during the photoreceptor rotation period.

Furthermore, after the second control, a third control may be performed in which the exposure light amount control data is determined as the control data for the third image formation condition. In the third control, with the development bias control table and the charging bias control table applied, a toner image of the third image pattern (midtone image pattern) is formed on each photoreceptor 40 (Y, M, C, K). Then, in the same manner as described above, the density (toner amount) of the toner image of this third image pattern is detected by the toner amount sensor 310. Next, the toner amount signal (toner image density detection signal) detected by the toner amount sensor 310 and the rotation position signal of the photoreceptor 40 detected by the photointerrupter 71 are used to obtain phase information and amplitude information of the image density fluctuation for one rotation period of the photoreceptor. Then, from this phase information and amplitude information, a third imaging condition is created for each rotational position of the photoreceptor 40 (Y, M, C, K). Specifically, a control table is created to periodically vary the amount of exposure light used to expose the photoreceptor at each rotational position of the photoreceptor 40 (Y, M, C, K). The created control table is then used to periodically vary the amount of exposure light.

The above image density suppression control is performed, for example, immediately after the photoreceptor is set (during initial setup, replacement, attachment/detachment, etc.).
When the photoreceptor 40 is removed from the main body 100 of the image forming apparatus, there is a high possibility that the occurrence of periodic image density fluctuations during the photoreceptor cycle will change. Also, when replacing the photoreceptor, the new photoreceptor will have different flare characteristics than the previously used photoreceptor, resulting in different periodic image density fluctuations for one rotation of the photoreceptor 40. Furthermore, the relationship between the flare characteristics of the photoreceptor 40 and the detected part 71a will also differ, resulting in different phases for the periodic image density fluctuations.

Furthermore, even when simply removing and reattaching the photoreceptor for maintenance, changes in the photoreceptor's mounting position (changes in the misalignment relative to the photoreceptor axis and the rotation axis) may occur. For these reasons, it is preferable to perform image density suppression control immediately after the photoreceptor 40 is set.

Furthermore, it is preferable to perform image density suppression control when environmental conditions within the device change. In particular, when environmental conditions, especially temperature conditions, change, the photoreceptor tube expands and contracts according to its thermal expansion coefficient. Therefore, the external profile of the photoreceptor 40 changes, and the fluctuations in the development gap change, potentially altering the periodic fluctuations of image density. The trigger for performing image density suppression control can be determined, for example, by setting it to a temperature change of N [deg] or more compared to the previous image density suppression control. Alternatively, image density suppression control may also be performed at regular intervals of a certain number of prints.

Even with image density suppression control, it is not possible to completely eliminate periodic fluctuations in image density, and some fluctuations remain. If the phases of the periodic fluctuations in image density for each color differ, periodic fluctuations in color occur in the full-color image, degrading image quality. Such color fluctuations occur in the color areas formed by layering Y, M, and C toners.

Figure 8(a) is a graph showing the periodic fluctuations in image density for the Y color, the M color, and the C color, while Figure 8(b) is a graph showing the brightness L ^* , chromaticity a*, and b ^* in the sub-scanning direction of a 3C gray image formed by superimposing images of the Y, ^M , and C colors.
As shown in Figure 8(a), when a color image is formed with different phases for the periodic fluctuations of the image density of each color, the brightness L ^* , chromaticity a ^* , and b ^* of the image fluctuate periodically, as shown in Figure 8(b). In particular, the periodic fluctuations of chromaticity a ^* and b ^* mean that the hue of the color image fluctuates periodically, and such periodic fluctuations of hue are highly sensitive to the eye and are often pointed out as abnormal images.

Regarding chromaticity a*, the M color fluctuates in the positive direction when the density is high, while the Y and C colors fluctuate in the negative direction when the density is high. Regarding chromaticity b*, the Y color fluctuates in the positive direction when the density is high, while the M and C colors fluctuate in the negative direction when the density is high. For the K color, even if there is a periodic fluctuation in image density with one rotation of the photoreceptor as one period, the chromaticity a ^* and b ^* do not fluctuate periodically.
Therefore, in this embodiment, phase alignment control is implemented to match the phase of periodic fluctuations in image density, thereby suppressing the deterioration of image quality due to changes in color.

Figure 9 is a flowchart showing an example of phase alignment control for residual image density fluctuations of Y, M, and C colors.
In this example, the phase alignment of the image density periodic fluctuations is performed using the Y color as a reference, but the reference color may also be the M color or the C color.
When a print request is received, the control unit 500 first calculates the target phase difference _θYM and the target phase difference _θYC (S11). The target phase difference _θYM is the phase difference between the rotation position signal of the Y-colored photoreceptor 40Y and the rotation position signal of the M-colored photoreceptor 40M when the phases of the periodic fluctuations of the image density of Y, M, and C coincide. The target phase difference _θYC is the phase difference between the rotation position signal of the Y-colored photoreceptor and the rotation position signal of the C-colored photoreceptor when the phases of the periodic fluctuations of the image density of Y, M, and C coincide.

Let _LY be the distance the photoreceptor moves from the development position of the Y color to the primary transfer position, and _LM be the distance the photoreceptor moves from the development position of the M color to the primary transfer position. Let L1 be 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. Let _θY be the phase of the periodic variation of the image density of the Y color, and _θM be the phase of the periodic variation of the image density of the M color. If the rotation speed of the photoreceptor 40 is V, the target phase difference _θYM can be calculated using the following equation (1).
θ _YM = |V(θ _Y −θ _M ) |+(L _Y +L1−L _M )...(1)

Furthermore, let _Lc be the distance the photoreceptor moves from the development position of color C to the primary transfer position, and L2 be the value obtained by dividing the distance from the primary transfer position of color Y to the primary transfer position of color C (drum pitch) by the circumference of the photoreceptor and not divisible. Then, if _θc is the phase of the periodic fluctuation of the image density of color C, the target phase difference _θYc can be calculated using the following formula.
θ _YC = |V(θ _Y -θ _C ) |+(L _Y +L2-L _M )...(2)

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 the Y, M, and C colors, then the target phase difference θ _YM and target phase difference θ _YC can be calculated using only the phases θ _Y , θ _m , and θ _c of the image density periodic fluctuation.
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.

The phases _θY , _θm , and _θc of the periodic fluctuations in image density for the Y, M, and C colors can be obtained using phase information acquired through image density suppression control. Alternatively, a development bias control table and a charging bias control table may be applied to periodically fluctuate the development bias and charging bias to form a toner image of the image pattern on the Y, M, and C colored photoreceptors 40 (Y, M, C), and the phase information and amplitude information of the periodic fluctuations in image density may be reacquired. In this case, a midtone image pattern that is highly visible to the user is preferable. By using a midtone image pattern, it is possible to suppress the color fluctuations of the midtones that are most to be suppressed. Note that the image pattern in this case may also be a high-image-density image pattern. High-image-density images have the advantage of being able to accurately detect the phase information of image density fluctuations because the fluctuations in the amount of toner deposited are large.

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 _YM between the rotational position signal of the M colored photoreceptor and the actual rotational position signal of the Y colored photoreceptor, and the phase difference θR _Yc between the rotational position signal of the C colored photoreceptor and the Y colored photoreceptor position signal are determined (S13).

The phase difference θR _YM can be determined from the time from when the Y-colored photointerrupter 71 detects the detected part 71a, which is the rotation reference position, until the M-colored photointerrupter 71 detects the detected part 71a, and the rotation speed V of the photoreceptor.

The phase difference θR _Yc can be determined from the time elapsed from when the Y-colored photointerrupter 71 detects the detected section 71a, which is the rotation reference position, until the C-colored photointerrupter 71 detects the detected section 71a, and from the rotation speed V of the photoreceptor. Alternatively, the above time measurement may be performed multiple times, and the phase differences θR _YM and θR _YC may be determined from the average value.

Next, the phase shift amount (adjustment amount) Z _YM is calculated by subtracting the measured phase difference θR _YM from the target phase difference θ _YM obtained in Equation 1 above. Similarly, the phase shift amount (adjustment amount) Z _YC is calculated from the target phase difference θ _YC obtained in Equation 2 above and the measured phase difference θR _YC (S14).

Then, based on the calculated phase shift amounts Z _{YM and} Z _YC , the photoreceptor drive motors 72 for Y, M, and C are controlled to rotate the Y, M, and C photoreceptors at a predetermined rotational speed for a predetermined period of time, thereby aligning the phases of the periodic fluctuations of the image density of Y, M, and C (S15).

If the rotation speed of the Y-colored photoreceptor is _VY and the above-specified period is T, then the rotation speed _VM of the M-colored photoreceptor 40M during phase alignment control can be expressed by the following equation (3), and the rotation speed _VC of the C-colored photoreceptor 40C can be expressed by the following equation (4).
V _M = V _Y + (Z _YM /T)...(3)
V _C =V _Y +(Z _YC /T)...(4)

As can be seen from equations 3 and 4, when the calculated phase shift amounts Z _YM and Z _YC are negative, the rotation speed of the Y-colored photoreceptor is reduced, and when the calculated phase shift amounts Z _YM and Z _YC are positive, the rotation speed of the Y-colored photoreceptor is increased.

Figure 10 is a sequence diagram showing an example of the rotational drive of the Y, M, and C color photoreceptors.
As shown in Figure 10, each photoreceptor is driven at a predetermined timing, and the phase difference is measured. In this example, each photoreceptor is driven at a different timing, but they may also be driven simultaneously.

Then, the phase shift amounts Z _{YM and} Z _YC are calculated, and when the Y-colored photointerrupter 71 detects the detected section 71a, which is the rotation reference position, the M-colored photoreceptor and the C-colored photoreceptor are accelerated and decelerated to perform phase alignment control. In the example shown in Figure 10, the rotation speeds of the M-colored and C-colored photoreceptors are accelerated and decelerated so that the phases of the periodic fluctuations of the image density of each color are aligned during the two rotations of the photoreceptors. In the example in Figure 10, the rotation speed of the M-colored photoreceptor is reduced, and the rotation speed of the C-colored photoreceptor is increased to align the phases of the periodic fluctuations of the image density of each color.

In the example shown in Figure 10, the M-color photoreceptor is accelerated and the C-color photoreceptor is decelerated to adjust the phase difference with the Y-color image density periodic fluctuation, which is the reference, to zero. However, it is also possible to adjust the phase difference with the Y-color image density periodic fluctuation to zero by only accelerating or only decelerating. Controlling the phase alignment of image density by only decelerating or only accelerating has the advantage of simplifying the control. Furthermore, controlling the phase alignment of image density by only decelerating allows for the use of an inexpensive motor with low torque as the photoreceptor drive motor 72, which has the advantage of 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.

Furthermore, while the above method uses the Y color as a reference for aligning the phase of the image density periodic fluctuations, the color with the smallest control amount may also be used as the reference. Additionally, the above method accelerates and decelerates the rotation speed of the photoreceptors of colors other than the reference color for a predetermined period relative to the rotation speed of the reference color photoreceptor to match the phases of the image density periodic fluctuations of the Y, M, and C colors. However, the phases of the image density periodic fluctuations of the Y, M, and C colors may also be matched by shifting the drive timing of the photoreceptors relative to the reference color based on the calculated phase shift amount.

Furthermore, if the Y, M, and C color photoreceptors are driven and stopped at the same time, and the rotational positional relationship of each photoreceptor does not change after phase alignment control of the image density periodic fluctuation, then it is not necessary to perform phase alignment control of the image density periodic fluctuation each time printing starts. In such a device, this control should be performed at a timing when the rotational positional relationship of each photoreceptor changes, such as during the first print after the photoreceptors are set in the device body.

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. Therefore, it may be preferable to perform the flow shown in Figure 9 after every specified number of images.

Figure 11(a) is a graph showing the periodic fluctuations in image density of the Y color, M color, and C color after phase alignment control. Figure 11(b) is a graph showing the brightness L ^* , chromaticity a ^* , and b ^* in the subscanning direction of the 3C gray image formed by superimposing the Y, M, and C color images after phase alignment control.
As shown in Figure 11, it can be seen that the fluctuations in chromaticity a ^* and b ^* are suppressed by aligning the phases of the periodic fluctuations of the image density of Y, M, and C.

Furthermore, by aligning the phases of the periodic image density fluctuations of Y, M, and C, the phases of image stretching fluctuations caused by the rotation period of the photoreceptor, which result from fluctuations in the surface velocity of the photoreceptor due to vibration of the photoreceptor, can also be matched for the Y, M, and C colors. This suppresses positional misalignment of the superimposed image of the Y, M, and C colors. Additionally, it eliminates the need to form an image pattern with multiple toner patches at equal intervals and measure the distance between toner patches to detect image stretching fluctuations, thereby reducing toner consumption and device downtime.

Figure 12(a) shows the periodic fluctuation of image density before and after image density suppression control of 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 of the low-density Y-colored image area.
As shown by the solid lines in Figures 12(a) and (b), before image density suppression control, the phases of the periodic image density fluctuations in the high-density image area and the periodic image density fluctuations in the low-density image area coincide. However, as shown by the dashed line in Figure 12(a), after image density suppression control, the phase of the periodic image density fluctuations in the low-density image area is reversed (in opposite phase). On the other hand, in the high-density image area, the phases before and after image density suppression control are in the same phase. Furthermore, for M and C colors, if the phase of the periodic image density fluctuations in the low-density image area is reversed after image density suppression control, there is a risk that the color change of the superimposed image of the low-density and high-density images will be significant.

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.

As shown in Figure 12, after image density suppression control, the phase of the periodic fluctuation of image density in low-density images inverts. Therefore, the image formation conditions are corrected to prevent the phase of the periodic fluctuation of image density in low-density images from inverting. As described above, the periodic fluctuation of image density in low-density images is dominated by the potential difference between the pre-exposure potential of the photoreceptor and the development bias, i.e., the background potential. Therefore, the periodic fluctuation of image density in low-density images is greatly influenced by the charging bias that forms the pre-exposure potential (the change in image density is large in relation to the change in charging bias: high sensitivity). Therefore, the periodic fluctuation of the charging bias is corrected to prevent the phase of the periodic fluctuation of image density in low-density images from inverting.

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 Figure 13, the dashed line represents the image before image density suppression control, the dashed line represents the image after the first control, the solid line represents the image after the second control, and the double-dotted line represents the image after periodic fluctuation correction of the charge bias.
As shown by the solid line in Figure 13, after the second control, the amplitude of the residual image density periodic fluctuations remaining after the control can be suppressed to about 0.002 [mg/ ^cm² ], indicating that the residual image density periodic fluctuations are sufficiently suppressed. However, after the 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 fluctuations in the low-density image is inverted (out of phase) with respect to the phase of the residual image density periodic fluctuations in the high-density image.

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 the high-density image pattern (image density: 70%) is detected, and a development bias control table is created to cancel out this high-density image pattern's image density fluctuation. 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 (opposite phase) relative to the image density periodic fluctuation, 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 in the same phase as the pre-correction image density 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 image density periodic fluctuation to be inverted.

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.

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 pattern (image density: 40%). 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. Consequently, to counteract 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 allows the phase of the image density variation in the low-density image, which had its phase inverted after the first control, to be reversed again. As a result, as shown in Figure 13(b), the phase of the image density periodic variation in the low-density image can be returned to the phase before control. This allows the phase of the image density periodic variation in the low-density image after image density suppression control to match the phase of the image density periodic variation in the high-density image.

The specific adjustment involves correcting the target control value of the charge bias at each rotation position of the photoreceptor in the created charge bias control table, thereby increasing the amplitude of the periodic fluctuation of the charge bias, which is the image formation condition. Furthermore, this adjustment to increase the amplitude of the periodically fluctuating charge bias by a predetermined value is performed only for Y, M, and C colors, where the color (chromaticity a ^* , b ^* ) may fluctuate periodically when low-density and high-density images are superimposed, but it does not need to be performed for K color.

If the periodic fluctuations in image density of a low-density image are large, even if the amplitude of the charging bias that causes the periodic fluctuation is increased, the phase of the periodic fluctuation in the low-density image may not reverse. As a result, the phase of the periodic fluctuation in image density of the low-density image may remain reversed relative to the phase of the periodic fluctuation in image density of the high-density image. Therefore, after increasing the amplitude of the charging bias, it may be advisable to form a low-density image pattern and check whether or not the phase of the periodic fluctuation in image density of the low-density image has reversed.

Figure 14 is an example of a flowchart for checking whether or not there is a phase inversion in the periodic fluctuation of image density in a low-density image.
First, the development bias and charging bias are periodically varied by applying the development bias control table and the charging bias control table to form a solid image pattern and a toner image of a low-density image pattern on the Y, M, C colored photoreceptor 40 (Y, M, C) (S21). The image density of the low-density image pattern is lower than the image density of the second image pattern (midtone image pattern) formed by the second control (for example, image density: 20%).

Next, in the same manner as described above, the phase information of the periodic image density fluctuations of the solid color image patterns (Y, M, and C) and the phase information of the periodic image density fluctuations of the low-density image pattern are obtained (S22). Then, for each color, it is checked whether the phase difference between the periodic image density fluctuations of the solid color image pattern and the periodic image density fluctuations of the low-density image pattern exceeds 90 degrees (S23). If the phase difference is 90 degrees or less (N in S23), it is determined that the phase has not been reversed, and the process ends there.

On the other hand, if the phase difference exceeds 90 degrees (Y in S23), the charge bias control table is recreated (S24). Specifically, the control target values of the charge bias at each rotation position of the photoreceptor in the charge bias control table are corrected so that the amplitude of the periodically fluctuating charge bias increases by a predetermined value.

In Figure 14, the flow terminates after the charge bias control table is recreated. However, after recreating the charge bias control table, the flow may be returned to S21 to check again whether or not the phase inversion has occurred.

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

Furthermore, while the amplitude of the periodically fluctuating charge bias is increased by a predetermined value as described above, the increase in the amplitude of the charge bias may also be calculated.

The calculation procedure will be explained below using Figure 15 as an example, taking the case where the amplitude of the charge bias amplitude is corrected so that the amplitude of the periodic fluctuation of the image density at an image density of 20% becomes "0".
First, a development bias control table is applied to periodically vary only the development bias to form a low-density (image density 20%) image pattern on the Y, M, C colored photoreceptors 40 (Y, M, C). Then, a development bias control table and a charging bias control table are applied to periodically vary the development bias and charging bias to form a toner image of a low-density (for example, image density 20%) image pattern on the Y, M, C colored photoreceptors 40 (Y, M, C).

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)). Furthermore, 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)).

The relationship between the amplitude of the charging bias and the amplitude of the periodic fluctuation of image density is linear. Therefore, from the amplitude of the periodic fluctuation of image density and the amplitude of the charging bias in Figure 15 (2), and the amplitude of the periodic fluctuation of image density and the amplitude of the charging bias in Figure 15 (3), the amplitude of the charging bias at which the amplitude of the periodic fluctuation of image density at 20% is "0" (Figure 15 (4)) can be determined. Since Figure 15 (2) shows the amplitude of the periodic fluctuation of image density at 20% when only the development bias is periodically varied, the amplitude of the charging bias is "0". Therefore, if the amplitude of the periodic fluctuation of image density in Figure 15 (2) is α, the amplitude of the periodic fluctuation of image density in Figure 15 (3) is β, and the amplitude of the charging bias is A1, the amplitude of the charging bias A2 at which the amplitude of the periodic fluctuation of image density at 20% is "0" (Figure 15 (4)) can be determined by the following equation (5).
A2={A1/(α-β)}×α...(5)

The above A1 can be determined from the charge bias control table created in the second control. Then, using the amplitude α of the image density periodic fluctuation in Figure 15 (2), obtained based on a low-density (20% image density) image pattern, and the amplitude β of the image density periodic fluctuation in Figure 15 (3), the charge bias amplitude A2 at which the amplitude of the image density periodic fluctuation at 20% image density becomes "0" (Figure 15 (4)) can be calculated.

Then, based on the calculated amplitude A2 of the charging bias, the target values of the charging bias at each rotation position of the photoreceptor in the charging bias control table are corrected, and the charging bias control table is recreated.

In the above explanation, we used the example of correcting the amplitude of the charge bias so that the amplitude of the periodic fluctuation of image density at an image density of 20% becomes "0". However, in this case, as shown in Figure 15, for image densities less than 20%, the phase of the periodic fluctuation of image density remains inverted compared to high-density images. Therefore, it is preferable to calculate the amplitude of the charge bias such that the amplitude of the periodic fluctuation of image density at an image density of 20% becomes "0" or greater. The amplitude A3 of the charge bias such that the amplitude of the periodic fluctuation of image density at an image density of 20% becomes "γ" can be obtained by the following equation (6).
A3={A1/(α-β)}×(α-γ)...(6)

Furthermore, when correcting the amplitude of the charge bias so that the amplitude of the periodic fluctuation of image density in a low-density image with an image density of 20% becomes "0", the phase of the periodic fluctuation of image density remains inverted relative to the high-density image for image densities below 20%. However, as shown in Figure 15, the amplitude of the periodic fluctuation of low-density images is sufficiently suppressed compared to the case where the amplitude of the periodic fluctuation of midtones (image density: 40%) is set to "0". Therefore, compared to the case where the amplitude of the periodic fluctuation of midtones (image density: 40%) is set to "0", the periodic fluctuation of color when low-density and high-density images are superimposed can be suppressed.

In the above method, the amplitude of the charging bias is increased to re-invert the phase of the periodic image density fluctuations in the low-density image, making it in phase with the pre-correction periodic image density fluctuations. This aligns the phase of the residual periodic image density fluctuations in the low-density image with that of the high-density image. However, for example, the amplitude of the development bias could be increased to overcorrect the periodic image density fluctuations in the high-density image, inverting the phase of the residual periodic image density fluctuations in the high-density image relative to the pre-correction phase, thus making the phases of the residual periodic image density fluctuations in the low-density image and the high-density image in phase.

Up to this point, the explanation has focused on the case where the development gap fluctuates due to rotational runout, which is the rotational fluctuation component of the photoreceptor 40, one of the rotating bodies that form the development gap, and the development roller 61a. However, fluctuations in the development gap can also occur due to rotational runout, which is the rotational fluctuation component of the development roller 61a.
Therefore, the periodic fluctuation of image density during the rotation period of the developing roller 61a may be suppressed, and the phase of the periodic fluctuation of image density during the rotation period of the developing roller 61a may be matched for the Y, M, and C colors. In this case, a rotation position detection means such as a photo interrupter is provided to detect a detected part, which is the rotation reference position provided on the developing roller 61a. Then, phase information and amplitude information of the periodic fluctuation of image density during the rotation period of the developing roller 61a are obtained from the rotation reference position of the developing roller 61a and the toner adhesion amount detection signal that detects the image pattern. Then, using the acquired phase information and amplitude information of the periodic fluctuation of image density during the rotation period of the developing roller 61a, the periodic fluctuation of image density during the rotation period of the developing roller 61a is suppressed, and the phase of the periodic fluctuation of image density is matched. In phase matching, based on the phase information of the periodic fluctuation of image density during the rotation period of the developing roller 61a, the rotation speeds of the M and C color developing rollers are accelerated or decelerated for a predetermined period relative to the rotation speed of the Y color developing roller 61a, which is the reference color.

The above description concerns a full-color image forming apparatus using a four-unit tandem intermediate transfer method, in which the toner images from 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 from each image forming unit 18 are directly and sequentially transferred to the transfer sheet.

The above is just one example; each of the following embodiments produces its own 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 the image density of each imaging unit on the recording material based on the periodic fluctuations of the image density of each imaging 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 imaging conditions so that the phase of the periodic fluctuations of the low-density image portion after image density suppression control does not reverse with respect to the phase of the periodic fluctuations of the high-density image portion for each imaging unit.
In Patent Document 1, for a predetermined image density periodic fluctuation (for example, 70% image density), the development bias is periodically varied by shifting its phase by 180° relative to the periodic fluctuation in the same phase as the image density periodic fluctuation (hereinafter referred to as phase inversion) as an imaging condition that is highly sensitive to high density (the amount of change in image density is large relative to the amount of change in imaging conditions), so that the image density periodic fluctuation becomes 0 (amplitude is 0). Next, for the midtone (image density: 40%) image density periodic fluctuation after periodically varying the development bias as an imaging condition, the charging bias, which is highly sensitive to low image density, is periodically varied by inverting its phase relative to the periodic fluctuation in the same phase as the midtone image density periodic fluctuation, so that the image density periodic fluctuation becomes 0 (amplitude is 0). As a result, as shown in Figure 13(a), both the density flare (amplitude) of the residual image density periodic fluctuation remaining after correction at high density and the density flare (amplitude) of the residual image density periodic fluctuation at low density can be suppressed to the same extent.
However, in the image density suppression control described in Patent Document 1, as shown in Figure 13(a), the phase of the residual image density periodic fluctuation of low-density images, which have a lower image density than the midtones, was inverted relative to the residual image density periodic fluctuation of high-density images. In this state, when the phase of the residual image density periodic fluctuation of low-density images is inverted relative to the phase of the residual image density periodic fluctuation of high-density images, and the phase of the residual image density periodic fluctuation of each imaging unit at a predetermined image density (for example, midtones: 40% image density) is matched by phase matching control, periodic changes in color are suppressed when low-density images are superimposed on each other, and when high-density images are superimposed on each other, but periodic changes in color are worsened when low-density images and high-density images are superimposed on each other.
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, the periodic change in the color tone of the superimposed image of the low-density image can be suppressed.

(Aspect 2)
In embodiment 1, after implementing image density suppression control, it is checked 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. If the phases are inverted, the periodic fluctuation of the image formation conditions is readjusted.
According to this, as explained using Figure 14, for each imaging unit, the phase of the periodic fluctuation of image density in the low-density image area after image density suppression control does not invert with respect to the phase of the periodic fluctuation of image density in the high-density image area.

(Aspect 3)
In embodiment 2, if the phase is not inverted, the adjustment of the periodic fluctuation of the image formation conditions is terminated.

(Aspect 4)
In any of embodiments 1 to 3, the image density suppression means, such as the control unit 500, suppresses the image density periodic fluctuation by periodically varying the charging bias applied to a charging member such as a charging roller, based on the image density periodic fluctuation acquired by the image density periodic fluctuation acquisition means, such as the control unit 500, as an image formation condition, and the adjustment means, such as the control unit 500, increases the amplitude of the periodically 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 was 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.

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

1: Image forming apparatus 18: Image forming unit 21: Exposure apparatus 40: Photoreceptor 60: Charging apparatus 61: Developing apparatus 61a: Developing roller 70: Potential sensor 71: Photointerrupter 71a: Detection unit 72: Photoreceptor drive motor 310: Toner adhesion amount sensor 320: Image pattern 500: Control unit

Japanese Patent Publication No. 2021-182124

Claims (5)

Multiple image-forming units comprising an image carrier, a charging member that uniformly charges the image carrier, and a developer carrier, wherein the developer carried by the developer carrier adheres to the image carrier to form a visible image,
An image forming apparatus comprising an image density periodic variation acquisition means for acquiring image density periodic variations for each of a plurality of imaging units,
Image density suppression means that, based on the image density periodic fluctuation of each imaging unit acquired by the image density periodic fluctuation acquisition means, implements image density suppression control for each imaging unit by periodically varying the imaging conditions to suppress the image density periodic fluctuation,
A phase control means performs phase alignment control on the recording material to match the phases of the image density period fluctuations of each imaging unit based on the image density period fluctuations of each imaging unit acquired by the image density period fluctuation acquisition means.
An image forming apparatus characterized by having, for each imaging unit, an adjustment means for increasing the amplitude of the charging bias applied to the charging member which periodically fluctuates in the same phase as the periodic fluctuation of image density before image density suppression control, so that the phase of the periodic fluctuation of image density in the low-density image area after image density suppression control does not invert with respect to the phase of the periodic fluctuation of image density in the high- density image area. In the image forming apparatus according to claim 1,
An image forming apparatus characterized in that, after the image density suppression control, it is checked whether the phase of the periodic fluctuation of image density in the low image area is inverted with respect to the phase of the periodic fluctuation of image density in the high image area, and if the phase is inverted, the adjustment means is used to increase the amplitude of the charge bias . The image forming apparatus according to claim 2, characterized in that, if the phase is not inverted, the adjustment means does not perform an adjustment to increase the amplitude of the charge bias . In the image forming apparatus according to any one of claims 1 to 3,
The image density suppression means is characterized in that, as an image formation condition, it suppresses the periodic variation in image density by periodically varying the charging bias applied to the charging member based on the periodic variation in image density acquired by the periodic variation acquisition means. In the image forming apparatus according to claim 4,
The image density suppression means is characterized by periodically varying the development bias applied to the developer carrier based on the periodic variation of the image density of the high-density image acquired by the periodic variation of the image density acquisition means, and periodically varying the charging bias based on the periodic variation of the image density of the intermediate density image formed by periodically varying the development bias, thereby suppressing the periodic variation of the image density.