Image forming device

Abstract

To provide an image forming apparatus that controls a surface speed difference in a normal imaging operation.SOLUTION: An image forming apparatus transfers a toner image formed on a photoconductor drum 101 to an intermediate transfer body 130. The photoconductor drum 101 is controlled and driven by a photoconductor drum drive control unit 134. The intermediate transfer body 130 is controlled and driven by an intermediate transfer body drive control unit 131. A drive load detection unit 135 detects drive torque Ta when the photoconductor drum 101 and the intermediate transfer body 130 are brought into contact with each other, and drive torque Tb when the photoconductor drum 101 and the intermediate transfer body 130 are separated from each other. A controller 1001 determines a setting value of the rotation speed of the photoconductor drum 101 on the basis of the drive torque Ta and the drive torque Tb.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] An image forming apparatus forms an image by first transferring an image formed on a photosensitive drum, which is a drum-shaped photosensitive member, to an intermediate transfer member, and then transferring the image from the intermediate transfer member to a sheet. The intermediate transfer member is, for example, an endless belt-like member. Both the photosensitive drum and the intermediate transfer member rotate when the image is transferred. The photosensitive drum and the intermediate transfer member generally rotate at a constant speed because their respective surface speeds affect the image ultimately formed on the sheet. The rotational speeds of the photosensitive drum and the intermediate transfer member affect image quality as follows:

[0003] For example, uneven rotation speeds of the photosensitive drum and intermediate transfer body cause the image to expand or contract. This appears on the sheet as uneven image density variations known as banding. Furthermore, in tandem image forming devices, speed fluctuations of multiple photosensitive drums result in deviations from the intended image formation and transfer positions, and speed fluctuations of the intermediate transfer body result in deviations in the transfer position. In this case, the image formed on the sheet will be shifted from the intended transfer position for each color, resulting in a condition known as "color misalignment." Color misalignment of 100 μm or more becomes visible to the naked eye. Therefore, the required accuracy for misalignment of the transfer position must be less than 100 μm.

[0004] Drive devices that rotate photosensitive drums and intermediate transfer bodies often employ PLL (Phase Locked Loop) control using brushless DC motors. In PLL control, the FG signal, which indicates the rotational phase of the brushless DC motor, is used as a rotational speed signal that indicates the rotational speed, and the rotational speed signal is synchronized with an externally supplied clock signal. This allows for synchronization of a stable, constant-period clock signal with the rotational distance per period, thereby achieving a constant rotational speed. Furthermore, such drive devices are commonly used due to the widespread use of general-purpose driver ICs (Integrated Circuits).

[0005] Even when such a drive device controls the rotation at a constant speed, the surface speed of the photosensitive drum is not always constant at the target speed due to, for example, tolerances in the diameter of the photosensitive drum, and also due to tolerances in the diameter of the drive roller that drives the intermediate transfer body and the thickness of the intermediate transfer body.

[0006] The target surface speeds of the photosensitive drum and intermediate transfer body will now be described. When the surface speeds of the photosensitive drum and intermediate transfer body are made equal and the difference in surface speed is set to zero, the image is transferred from the photosensitive drum to the intermediate transfer body without the surfaces rubbing against each other. This improves the reproducibility of fine lines in the image. Therefore, it is preferable to set the target surface speeds of the photosensitive drum and intermediate transfer body so that the difference in surface speed between the photosensitive drum and intermediate transfer body is as small as possible.

[0007] When attempting to set the surface speed difference between the photosensitive drum and the intermediate transfer body to zero, the magnitude relationship can periodically change due to tolerances such as eccentricity and roundness of the photosensitive drum and drive roller. Furthermore, backlash in gears and couplings can cause discontinuity in the transmission of rotational force from the drive device. These phenomena can cause image degradation. Therefore, it is best to set the surface speed difference to a level that does not change the magnitude relationship between the surface speeds. Conventionally, the surface speed difference is set so that the intermediate transfer body is about 0.2% faster than the photosensitive drum in terms of the motor's target speed. This value is the minimum value at which the magnitude relationship between the surface speeds does not change, even with dimensional tolerances.

[0008] However, the difference in surface speeds generates friction between the photosensitive drum and the intermediate transfer body. This friction acts as a load that acts as a disturbance. When toner (image) is interposed between the photosensitive drum and the intermediate transfer body while friction is applied, a momentary load fluctuation occurs. If the load fluctuation causes speed fluctuations in the intermediate transfer body, color shift occurs, and if speed fluctuations in the photosensitive drum occur, shock occurs, both of which result in image defects. Such image defects become more severe the greater the difference in surface speeds. Therefore, it is necessary to reduce the difference in surface speeds. The surface speeds of the photosensitive drum and the intermediate transfer body can be measured by marking the photosensitive drum and the intermediate transfer body at equal intervals to detect the surface speed and reading the passing time of the markings with an optical sensor, or by using a Doppler velocimeter. While it is possible to control the difference in surface speeds to reduce it based on the measured surface speeds, this method significantly increases costs.

[0009] Patent Document 1 discloses an image forming apparatus that compares the load between a photosensitive drum and an intermediate transfer body when there is toner and when there is not, and sets a target speed in a direction that reduces the difference in load. An image is formed on the photosensitive drum by the toner being attached to the photosensitive drum by a developing sleeve. The rotational load of the motor that drives the photosensitive drum is measured when the developing sleeve is driven and when it is not driven. The image forming apparatus adjusts the speed of the photosensitive drum motor so that the rotational load when the developing sleeve is not driven approaches the rotational load when the developing sleeve is driven.

[0010] When the developing sleeve is driven, toner adheres to the photosensitive drum. This toner acts as a roller by entering between the photosensitive drum and the intermediate transfer body. Because the toner acts as a roller to reduce frictional force, the motor load is simulated as if there was no difference in surface speed between the photosensitive drum and the intermediate transfer body. When the developing sleeve is not driven, the actual frictional force occurs between the photosensitive drum and the intermediate transfer body due to the difference in surface speed. Since a surface speed difference occurs when the difference in rotational load is not zero, the target rotational speed of the photosensitive drum is changed in the direction that reduces the difference in rotational load, thereby reducing the difference in surface speed.

[0011] Patent Document 2 discloses a configuration for detecting the frictional force between the photosensitive drum and the intermediate transfer body by using the difference in drive load between a state in which the photosensitive drum and the intermediate transfer body are in contact with each other and a state in which they are separated from each other. This configuration makes it possible to detect the frictional force without disturbances caused by the presence or absence of rotation of the developing sleeve. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 2018-10097 [Patent Document 2] Patent Publication No. 2021-85945 Summary of the Invention [Problem to be solved by the invention]

[0013] In Patent Document 1, the photosensitive drums must be rotated with the developing sleeves not rotating. In Patent Document 2, when detecting the frictional forces between the intermediate transfer body and all the photosensitive drums, the intermediate transfer body or the photosensitive drums must be rotated with the intermediate transfer body and all the photosensitive drums spaced apart. These operations are unnecessary for normal image formation, and may cause downtime depending on the timing of control execution.

[0014] In addition, some tandem image forming apparatuses are configured to reduce the number of drive motors by using the same drive motor for the intermediate transfer body and some of the photosensitive drums. In this configuration, in order to reduce toner consumption and wear on the photosensitive drums, images are created using only the intermediate transfer body and the black photosensitive drum during monochrome jobs, and the chromatic color photosensitive drums are separated from the intermediate transfer body and do not operate. Therefore, the black photosensitive drum and the intermediate transfer body are driven by the same drive motor, while the chromatic color photosensitive drums are driven by different drive motors.

[0015] Because the intermediate transfer body and the black photosensitive drum are driven by the same drive motor, the difference in their surface speeds varies depending on the machine and the state of the machine when creating an image. Because the intermediate transfer body and the chromatic color photosensitive drums are driven by separate drive motors, the difference in their surface speeds can be adjusted by changing the target speed of each drive motor.

[0016] Because the proportion of monochrome products produced is higher than the proportion of color products produced, the black photosensitive drum is often located at the most downstream position in the rotational direction of the intermediate transfer body and close to the secondary transfer unit. In this case, the position of the intermediate transfer body upstream of the secondary transfer unit is governed by the frictional force it receives from the black photosensitive drum. For example, if the frictional force the intermediate transfer body receives from the black photosensitive drum is small, the intermediate transfer body may slacken upstream of the secondary transfer unit, making it impossible to maintain an appropriate gap between the intermediate transfer body and the sheet, which can lead to discharge phenomena. For this reason, it is preferable that the frictional force generated at the interface between the intermediate transfer body and the black photosensitive drum, i.e., the surface speed difference, be at least a certain level.

[0017] On the other hand, the frictional force generated at the interface between the intermediate transfer body and the chromatic color photosensitive drums, which are arranged upstream of the black photosensitive drum in the rotation direction of the intermediate transfer body, does not fulfill such a function. Therefore, it is preferable to set the speed so as to reduce the difference in surface speed within a range in which the magnitude relationship between the surface speeds is not reversed.

[0018] SUMMARY OF THE INVENTION In view of the above problems, it is a primary object of the present invention to provide an image forming apparatus that controls the surface speed difference during normal image forming operations. [Means for solving the problem]

[0019] The image forming apparatus of the present invention includes a first photosensitive member on which a color image is formed, a second photosensitive member on which a black image is formed, an intermediate transfer belt that is wound around a plurality of rollers including a drive roller and onto which the color image and the black image are transferred, a transfer unit that transfers the image on the intermediate transfer belt to a sheet, a first drive source that drives to rotate the first photosensitive member, a second drive source that drives to rotate the second photosensitive member and the drive roller, a state control unit that controls a contact state between the first photosensitive member and the intermediate transfer belt and a contact state between the second photosensitive member and the intermediate transfer belt, a determination unit that determines a target rotation speed of the first drive source, and a rotation control unit that controls a rotation speed of the first drive source. and a drive control means for controlling the first drive source so that a speed becomes the target rotation speed determined by the determination means, wherein the state control means controls the first drive source to a first state in which the first photosensitive member and the intermediate transfer belt are in contact and the second photosensitive member and the intermediate transfer belt are in contact when a mixed-color image is formed on a sheet, and the state control means controls the first drive source to a second state in which the first photosensitive member and the intermediate transfer belt are not in contact and the second photosensitive member and the intermediate transfer belt are in contact when a black monochromatic image is formed on a sheet, and the determination means determines first load information relating to a load of the first drive source in the first state and second load information relating to the load of the first drive source in the second state. and the first load information and the second load information are stored. Based on this, the first driving source The aforementioned Determine the target rotation speed The first driving source rotates the first photosensitive member based on the determined target rotation speed, and the determining unit adjusts the color misregistration while the first driving source rotates based on the target rotation speed, and when the color misregistration adjustment is successful, erases the stored first load information and the stored second load information. It is characterized by: [Effects of the Invention]

[0020] According to the present invention, it is possible to control the surface speed difference during normal image formation, thereby suppressing downtime and setting the speed difference between the intermediate transfer body and the photosensitive drum to an appropriate relationship, thereby maintaining good image quality. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a diagram illustrating the configuration of an image forming apparatus. [Figure 2] FIG. [Figure 3] FIG. 4 is an explanatory diagram of potentials during development processing. [Figure 4] 4A to 4C are explanatory diagrams illustrating a state in which the photosensitive drum and the intermediate transfer member are in contact with each other. [Figure 5] FIG. [Figure 6] FIG. [Figure 7] FIG. [Figure 8] FIG. [Figure 9] Graph showing rotation load. [Figure 10] FIG. [Figure 11] 10 is a flowchart showing a color misregistration adjustment process. [Figure 12] 10 is a flowchart showing a color misregistration adjustment process. [Figure 13] 1A and 1B are explanatory diagrams of contact / separation between a photosensitive drum and an intermediate transfer body. [Figure 14] 4A and 4B are diagrams illustrating changes in drive torque Ta and drive torque Tb. DETAILED DESCRIPTION OF THE INVENTION

[0022] The following description of the preferred embodiments of the present invention will be made with reference to the accompanying drawings. The dimensions, materials, shapes, and relative positions of the components described in the preferred embodiments may be changed as appropriate depending on the configuration of the device to which the invention is applied and various conditions, and the scope of the present invention is not limited to the following preferred embodiments.

[0023] (Image forming device) 1 is a diagram showing the configuration of an image forming apparatus according to this embodiment. The image forming apparatus 1 according to this embodiment is a color image forming apparatus that employs an electrophotographic system. Because the image forming apparatus 1 is excellent in adaptability to a wide variety of sheets and print productivity, it employs an intermediate transfer tandem system in which four color image forming units 100Y, 100M, 100C, and 100K are arranged in the rotation direction of an intermediate transfer member 130.

[0024] Image forming apparatus 1 of this embodiment is configured to include a printer, a scanner, and an operation panel 330. The scanner is provided on top of the printer. The scanner includes a platen glass 55 on which an original is placed, an ADF (Auto Document Feeder) having an original tray 152 on which the original is placed, and a reading sensor 233. The scanner can read images of the original placed on platen glass 55 and the original transported by the ADF using reading sensor 233. The reading sensor 233 transmits image data representing the read original image to the printer. The printer realizes a copying function by forming an image on a sheet based on the image data obtained from the scanner.

[0025] The operation panel 330 is a user interface and includes various keys and a touch panel as input devices, and a display as an output device. A user inputs instructions and setting values ​​through the input device. The output device displays an input screen for the image forming apparatus 1, a screen showing the operating status, and the like.

[0026] The printer includes image forming units 100Y, 100M, 100C, and 100K, an intermediate transfer member 130, an exposure unit 103, a secondary transfer unit 118, a fixing unit 170, a paper feed cassette 111, and a sheet transport mechanism. The sheet transport mechanism includes, in this order from the upstream side in the sheet transport direction, a paper feed roller 113, a separation roller 114, a registration roller 116, and a discharge roller 139. The secondary transfer unit 118 is provided downstream of the registration roller 116, and the fixing unit 170 is provided downstream of the secondary transfer unit 118.

[0027] Image forming unit 100Y is used to form yellow images. Image forming unit 100M is used to form magenta images. Image forming unit 100C is used to form cyan images. Image forming unit 100K is used to form black images. Image forming units 100Y, 100M, 100C, and 100K have the same configuration and only differ in the colors of the images they form. In the following description, the suffixes Y(y), M(m), C(c), and K(k) will be omitted when there is no need to distinguish between colors.

[0028] The image forming unit 100 includes a photosensitive drum 101, a charger 102, a developing unit 104, and a drum cleaner 107. The photosensitive drum 101 is a drum-shaped photosensitive element having a charged layer on its surface, and serves as an image carrier for carrying an image (toner image). The photosensitive drum 101 is rotatable clockwise around its drum axis. The charger 102 uniformly charges the surface of the rotating photosensitive drum 101. The exposure unit 103 scans the charged surface of the photosensitive drum 101 with a laser beam modulated based on image data. This forms an electrostatic latent image on the surface of the photosensitive drum 101. The laser beam scans the photosensitive drum 101 in the drum axis direction. Therefore, the drum axis direction of the photosensitive drum 101 is the main scanning direction, and the rotation direction of the photosensitive drum 101 is the sub-scanning direction. The sub-scanning direction is the same as the rotation direction of an intermediate transfer body 130 (described later) and the sheet transport direction.

[0029] The exposure unit 103 includes four exposure devices 103y, 103m, 103c, and 103k to output four laser beams corresponding to the colors of the image to be formed. The exposure unit 103 (exposure devices 103y, 103m, 103c, and 103k) modulates the laser beam for each color according to image data for each color component. As a result, the electrostatic latent images formed on the photosensitive drums 101 of the image forming sections 100Y, 100M, 100C, and 100K are images corresponding to each color.

[0030] The developing unit 104 develops the electrostatic latent image formed on the surface of the photosensitive drum 101 to form a toner image. The developing unit 104 performs development using a two-component developer containing non-magnetic toner and a low-magnetization, high-resistivity carrier. The non-magnetic toner is composed of an appropriate amount of a binder resin such as a styrene resin or polyester resin, a colorant such as carbon black, dye, or pigment, a release agent such as wax, and a charge control agent. Such toner can be produced by conventional methods such as pulverization or polymerization. The toner is charged by friction between the carrier and toner in the developing unit 104. A development bias is applied to the charged toner, creating a potential difference between the surface of the photosensitive drum 101 and the toner, causing it to adhere to the photosensitive drum 101. This develops the electrostatic latent image on the surface of the photosensitive drum 101 and makes it visible. In this embodiment, a negatively charged toner is used.

[0031] The toner image formed on the photosensitive drum 101 is transferred to the intermediate transfer body 130. For this purpose, a primary transfer roller 105 is provided at a position facing the photosensitive drum 101 with the intermediate transfer body 130 sandwiched therebetween. The intermediate transfer body 130 is an endless belt-like transfer body that is wound around multiple rollers including a drive roller 106 and driven to rotate counterclockwise in the figure by the drive roller 106. Toner images are sequentially transferred and superimposed from the image forming units 100Y, 100M, 100C, and 100K at a timing according to the rotation speed of the intermediate transfer body 130. As a result, the intermediate transfer body 130 carries a full-color toner image. Any toner remaining on the photosensitive drum 101 after transfer is removed by a drum cleaner 107.

[0032] The toner image formed on the intermediate transfer body 130 is transported to a secondary transfer unit 118 by the rotation of the intermediate transfer body 130. The secondary transfer unit 118 includes a drive roller 106. The secondary transfer unit 118 transfers the toner image on the intermediate transfer body 130 onto a sheet fed by a transport mechanism. A belt cleaner 108 is provided near the intermediate transfer body 130. The belt cleaner 108 removes toner remaining on the intermediate transfer body 130 after transfer.

[0033] Sheets are fed from a paper feed cassette 111 by a paper feed roller 113. A separation roller 114 separates the sheets fed by the paper feed roller 113 one by one and conveys them to a conveyance path. The leading edge of the sheet conveyed by the separation roller 114 in the conveyance direction abuts against a registration roller 116 that is stopped. After the leading edge abuts, the sheet is conveyed a predetermined distance, forming a loop and correcting any skew with respect to the conveyance direction. The registration roller 116 conveys the sheet to a secondary transfer unit 118 in accordance with the timing at which the toner image formed on the intermediate transfer body 130 is conveyed to the secondary transfer unit 118. This causes the toner image to be transferred to a predetermined position on the sheet.

[0034] The sheet onto which the toner image has been transferred is transported from secondary transfer unit 118 to fuser 170. The fuser 170 fuses the toner image onto the sheet. For example, fuser 170 fuses the toner image onto the sheet by applying heat and pressure to the sheet onto which the toner image has been transferred. The sheet onto which the toner image has been fused is discharged from fuser 170 to discharge tray 140 via discharge rollers 139. When double-sided printing is performed, the sheet on which an image has been printed on its first side is transported to registration rollers 116 via double-sided printing path 180. At this time, the sheet is flipped over so that the printing side on which the image is to be printed is turned from the first side to the second side.

[0035] An image detection sensor 1004 is provided downstream of the image forming unit 100K in the rotation direction of the intermediate transfer body 130. The image detection sensor 1004 is used to detect (read) a detection image for detecting color shift and image density formed on the intermediate transfer body 130.

[0036] (Development and transfer processing) Fig. 2 is a diagram illustrating the operation of the image forming unit 100. Fig. 3 is a diagram illustrating the potential during the development process.

[0037] The surface of the photosensitive drum 101 is charged to a negative charging potential Vd by the charger 102. The potential (exposed portion potential) VL of the portion of the photosensitive drum 101 where the electrostatic latent image is formed is neutralized from the charging potential Vd to 0 [V]. The charging potential Vd is, for example, −700 [V], and the exposed portion potential VL is, for example, −200 [V].

[0038] The developing device 104 transports developer containing negatively triboelectrically charged toner to the vicinity of the photosensitive drum 101 by the developing sleeve 109. A developing bias potential Vdc applied to the developing sleeve 109 during development is a potential between the charging potential Vd and the exposed portion potential VL, e.g., −550 V. The negative developing bias potential Vdc causes the negatively charged toner on the developing sleeve 109 to fly to the exposed portion potential VL, which is relatively closer to a positive potential than the charging potential Vd or the developing bias potential Vdc on the surface of the photosensitive drum 101. As a result, an amount of toner corresponding to the developed latent image potential Vcont, which is the difference between the developing bias potential Vdc and the exposed portion potential VL, adheres to the photosensitive drum 101. The density of the toner image depends on the amount of toner adhering to the photosensitive drum 101. Therefore, the image density can be adjusted by adjusting the developed latent image potential Vcont.

[0039] The negative toner that has flown onto the photosensitive drum 101 is transferred to the intermediate transfer body 130 by the pressure and electric field between the primary transfer roller 105 and the intermediate transfer body 130. At this time, a primary transfer bias potential Vtr1 of a polarity opposite to that of the toner is applied to the primary transfer roller 105. For example, the primary transfer bias potential Vtr1 is +1500 [V].

[0040] The photosensitive drum 101 is driven to rotate by a photosensitive drum drive control unit 134. The load on the photosensitive drum 101 is detected by a drive load detection unit 135. The developing sleeve 109 is driven to rotate by a developing sleeve drive control unit 132. The intermediate transfer body 130 rotates when the drive roller 106 is driven to rotate by an intermediate transfer body drive control unit 131. The primary transfer roller 105 can be moved by a transfer state switching unit 133 in a direction to contact the photosensitive drum 101 and a direction to move away from the photosensitive drum 101. The operations of the intermediate transfer body drive control unit 131, the photosensitive drum drive control unit 134, the developing sleeve drive control unit 132, and the transfer state switching unit 133 are controlled by a controller 1001.

[0041] (Image formation sequence) The image forming apparatus 1 performs an image sequence during image formation, which can be broadly divided into a pre-rotation operation, an image creation operation, an inter-sheet operation, and a post-rotation operation. The pre-rotation operation is an operation performed to stabilize the drive states and bias voltage states of each part in order to form an image. The image creation operation is an operation to develop an electrostatic latent image formed on the photosensitive drum 101 with toner to make it visible. The inter-sheet operation is an operation performed between sheets on which an image is formed, and does not involve image creation. The post-rotation operation is an operation performed to stop the drive states and bias voltages of each part in order to form an image.

[0042] 4 is an explanatory diagram of the contact state between the photosensitive drum 101 and the intermediate transfer body 130. The photosensitive drum 101 and the intermediate transfer body 130 are switched between a contact state and a separated state by a transfer state switching unit 133 depending on the operating state.

[0043] In the pre-rotation operation, the photosensitive drum 101 and the intermediate transfer body 130 necessary for image formation are brought into contact with each other. FIG. 4A shows a state in which the photosensitive drum 101k corresponding to black is in contact with the intermediate transfer body 130, and the photosensitive drums 101y, 101m, and 101c of the other chromatic colors are separated from the intermediate transfer body 130. The photosensitive drum 101k and the intermediate transfer body 130 are brought into contact with each other by the transfer state switching unit 133 causing the primary transfer roller 105k to contact the intermediate transfer body 130. The photosensitive drums 101y, 101m, and 101c are separated from the intermediate transfer body 130 by the transfer state switching unit 133 causing the primary transfer rollers 105y, 105m, and 105c to separate from the intermediate transfer body 130. In this embodiment, the state shown in FIG. 4A is the home position during standby.

[0044] When forming an image in monochrome mode, the transfer state switching unit 133 does not operate, and image formation is performed in the state shown in FIG. 4(a). When forming an image in full-color mode, the transfer state switching unit 133 moves the primary transfer rollers 105y, 105m, and 105c from the state shown in FIG. 4(a) toward the photosensitive drums 101y, 101m, and 101c. As a result, as shown in FIG. 4(b), all of the photosensitive drums 101y, 101m, 101c, and 101k come into contact with the intermediate transfer body 130. In this way, the contact state between the chromatic color photosensitive drums 101y, 101m, and 101c and the intermediate transfer body 130 is switched between monochrome mode and full-color mode. This makes it possible to suppress deterioration over time of the surfaces of the photosensitive drums 101y, 101m, and 101c.

[0045] Next, the photosensitive drum 101 and the intermediate transfer member 130 are rotated. Because the photosensitive drum 101 and the intermediate transfer member 130 have large inertia, it takes time from the start of rotation until they reach their target speed and stabilize at a constant speed. The driving method for the photosensitive drum 101 and the intermediate transfer member 130 will be described later. Once the photosensitive drum 101 and the intermediate transfer member 130 begin rotating at a constant speed, a charging bias is applied to the charger 102. The primary transfer bias is applied to the primary transfer roller 105 after the charged area on the surface of the photosensitive drum 101 passes a position facing the primary transfer roller 105. The developing sleeve 109 is rotated and a developing bias is applied so that the electrostatic latent image formed on the photosensitive drum 101 reaches a predetermined rotation speed and a predetermined developing bias potential before it is transported to a position facing the developing sleeve 109. However, to prevent deterioration of the developer, the developing sleeve 109 is rotated and the developing bias is applied as late as possible.

[0046] In the image forming operation, the photosensitive drum 101, the surface of which is charged, is irradiated with laser light from the exposure unit 103 at the timing of the start of scanning determined by a color misregistration adjustment mode, which will be described later. The electrostatic latent image formed on the photosensitive drum 101 by the irradiation of the laser light is developed by the developing device 104. The toner image formed on the photosensitive drum 101 by the development is transferred to the intermediate transfer body 130 by the primary transfer roller 105. In this way, in the image forming operation, processes from the formation of the toner image to the transfer to the intermediate transfer body 130 are carried out.

[0047] During the sheet interval operation, the exposure unit 103 stops exposing the laser beam, but the other components maintain the state they were in during the image forming operation. In the post-rotation operation, the following operations are stopped in this order: laser light exposure by the exposure unit 103, rotational driving of the developing sleeve 109, application of the developing bias, application of the primary transfer bias, and application of the charging bias. Then, the rotational driving of the photosensitive drum 101 and the intermediate transfer body 130 is stopped. When an image is formed in full-color mode, the transfer state switching unit 133 finally moves the primary transfer rollers 105y, 105m, and 105c in a direction away from the photosensitive drums 101y, 101m, and 101c. As a result, the primary transfer rollers 105y, 105m, and 105c move from the state shown in FIG. 4(b), and the photosensitive drums 101y, 101m, and 101c are separated from the intermediate transfer body 130 as shown in FIG. 4(a). In addition, when obtaining the torque used to control the speed of the photosensitive drum 101, when the main body is not used for a long period of time, or when the photosensitive drum 101 or the intermediate transfer body 130 is replaced, all of the photosensitive drums 101 and the intermediate transfer body 130 are separated as shown in Figure 4(c).

[0048] (Color misregistration adjustment) During color misregistration adjustment, a detection image, which is a toner image for color misregistration adjustment, is formed on the intermediate transfer body 130. The image detection sensor 1004 reads (detects) the detection image formed on the intermediate transfer body 130. The amount of color misregistration is detected based on the reading result (detection result). The start timing (image writing start position) of scanning (exposing) the photosensitive drum 101 by the exposure unit 103 is adjusted based on the amount of color misregistration. Such color misregistration adjustment is performed in the color misregistration adjustment mode. The color misregistration adjustment mode is performed in response to a user instruction via the operation panel 330, or at a predetermined timing set in advance, such as when the image forming apparatus 1 is started or after a predetermined number of prints have been made. The color misregistration adjustment mode corrects misalignment of the image forming position and changes over time that occur due to manufacturing variations in the image forming apparatus 1 and changes in the internal temperature.

[0049] When the color misregistration adjustment mode is initiated, the intermediate transfer body 130 starts to be driven, and the formation of a detection image begins. FIG. 5 is an explanatory diagram of the detection images formed on the intermediate transfer body 130 in the color misregistration adjustment mode. A set of four detection images 702y, 702m, 702c, and 702k for each color is formed consecutively on the intermediate transfer body 130. Each of the detection images 702y, 702m, 702c, and 702k is formed at a reading position (detection position) by the image detection sensor 1004. In the figure, the dashed two-dot line indicates the reading position (detection position) of the image detection sensor 1004. The image detection sensor 1004 is, for example, an optical sensor.

[0050] The relative positional relationship between the detection images 702y, 702m, 702c, and 702k of each color is detected based on the time it takes for the detection images 702y, 702m, 702c, and 702k of each color to pass through the reading position of the image detection sensor 1004. The detection images 702y, 702m, 702c, and 702k are L-shaped, and one image passes through the reading position of the image detection sensor 1004 twice.

[0051] When the detection images 702y, 702m, 702c, and 702k pass the reading position of the image detection sensor 1004, the time interval between two readings of one image is detected. This time interval is used to detect the amount of color shift in the direction perpendicular to the rotation direction of the intermediate transfer body 130, i.e., the main scanning direction. For example, the time interval between the detection images 702y is Lys, and the time interval between the detection images 702m is Lms. As the detection image 702m shifts in the main scanning direction, the time interval Lms becomes shorter than the time interval Lys. In this way, the amount of color shift in the main scanning direction is detected based on the relative length of the time intervals between the detection images 702y, 702m, 702c, and 702k passing the reading position.

[0052] Furthermore, the amount of color misregistration in the rotational direction of the intermediate transfer body 130, i.e., the sub-scanning direction, is detected based on the positions of the detection images 702y, 702m, 702c, and 702k. The positions of the detection images 702y, 702m, 702c, and 702k are expressed as the average value of the time required for two detections. The relative positions of the detection images 702y and 702m are expressed as a time interval Lym, which represents the distance between their respective positions. The difference between this time interval Lym and the time interval when no color misregistration occurs between the detection images 702y and 702m represents the amount of color misregistration in the sub-scanning direction between the detection images 702y and 702m.

[0053] In this way, the amount of color shift is calculated from the relative positional relationship between each of the detection images 702y, 702m, 702c, and 702k. The amount of color shift is calculated multiple times from multiple sets of detection images 702y, 702m, 702c, and 702k. The average value of the color shift amounts calculated multiple times is used for color shift adjustment. By averaging the results of multiple calculations, minute variations caused by various disturbances in the formation positions of each set of detection images 702y, 702m, 702c, and 702k are absorbed.

[0054] That is, the series of processes of forming, reading (detecting), and calculating the amount of color misregistration of the detection images 702y, 702m, 702c, and 702k are repeated until the predetermined number of detection images 702y, 702m, 702c, and 702k have been read. When the series of processes is completed, the average value of the amount of color misregistration is calculated. A correction value for correcting the color misregistration is derived from the calculated average value. Based on this correction value, the timing at which the exposure unit 103 starts exposing the photosensitive drum 101 to laser light is determined.

[0055] (Photosensitive drum rotation speed control) FIG. 6 is an explanatory diagram of a chromatic color drum drive unit that controls the drive of the chromatic color photosensitive drums 101y, 101m, and 101c. The drum shafts of the photosensitive drums 101y, 101m, and 101c are rotated by the driving force output from the chromatic color drive motor 31, which is transmitted via a motor gear 32, a drum reduction gear 33, and a drum drive gear 34. Three drum drive gears 34 are provided corresponding to the photosensitive drums 101y, 101m, and 101c, respectively. The drum reduction gears 33 are provided between the drum drive gears 34. The end faces of each drum drive gear 34 and the photosensitive drums 101y, 101m, and 101c are connected by claws such as couplings. Each drum drive gear 34 is drivingly connected by an idler gear, and the rotation of the chromatic color drive motor 31 is transmitted to each of the photosensitive drums 101y, 101m, and 101c.

[0056] The chromatic color drive motor 31 is drive-controlled by a photosensitive drum drive control unit 134. In the photosensitive drum drive control unit 134, a target speed value (target rotation speed) for the rotation speed of the chromatic color drive motor 31 is set by a drive speed setting unit 1002. The rotation speeds of the photosensitive drums 101y, 101m, and 101c are determined by the target rotation speed.

[0057] In this embodiment, the chromatic color drive motor 31 is a DC brushless motor that outputs an FG signal to the photosensitive drum drive control unit 134. The photosensitive drum drive control unit 134 compares the rotation speed of the chromatic color drive motor 31, indicated by the FG signal acquired from the chromatic color drive motor 31, with a target rotation speed. Based on the comparison result, the photosensitive drum drive control unit 134 generates a drive signal to the chromatic color drive motor 31 so that the rotation speed of the chromatic color drive motor 31 corresponds to the target rotation speed, and transmits the drive signal to the chromatic color drive motor 31. This feedback controls the chromatic color drive motor 31. The drive signal is, for example, a pulse width modulation (PWM) signal. The rotation speed of the chromatic color drive motor 31 is controlled according to the duty ratio of the drive signal. The drive signal is also input to a drive load detection unit 135. The drive load detection unit 135 averages the drive signal over a predetermined period of time and detects the duty ratio as the rotation load. The drive speed setting unit 1002 sets a target rotation speed in accordance with the rotation load detected by the drive load detection unit 135 .

[0058] 7 is an explanatory diagram of an intermediate transfer body drive unit that controls the drive of the black photosensitive drum 101k and the intermediate transfer body 130. The photosensitive drum 101k is driven by an intermediate transfer body drive motor 1300 that is different from the chromatic color drive motor 31 that drives the chromatic color photosensitive drums 101y, 101m, and 101c. The drum shaft of the photosensitive drum 101k is rotated by the driving force output from the intermediate transfer body drive motor 1300 that is transmitted via a motor gear 32, a drum reduction gear 33, and a drum drive gear 34. The drum drive gear 34 and the end face of the photosensitive drum 101k are connected by a claw such as a coupling. When the diameter of the black photosensitive drum 101k is different from that of the other photosensitive drums 101y, 101m, and 101c, the gear ratios of the motor gear 32, the drum reduction gear 33, and the drum drive gear 34 differ between the photosensitive drum 101k and the photosensitive drums 101y, 101m, and 101c.

[0059] As described above, the intermediate transfer body 130 is driven to rotate by the drive roller 106. The drive roller 106 is driven to rotate by the driving force output from the intermediate transfer body drive motor 1300, which is transmitted via the motor gear 32, the intermediate transfer body reduction gear 36, the intermediate transfer body idler gear 37, and the intermediate transfer body drive gear 38. The end faces of the intermediate transfer body drive gear 38 and the drive roller 106 are connected by a claw such as a coupling.

[0060] The intermediate transfer member drive motor 1300 is drive-controlled by an intermediate transfer member drive controller 131. In the intermediate transfer member drive controller 131, a target speed value (target rotation speed) for the rotation speed of the intermediate transfer member drive motor 1300 is set by a drive speed setting unit 1002. The rotation speeds of the intermediate transfer member 130 and the photosensitive drum 101k are determined by the target rotation speed. The intermediate transfer member drive motor 1300 in this embodiment is a DC brushless motor, similar to the chromatic color drive motor 31, and outputs an FG signal to the intermediate transfer member drive controller 131. The control of the rotation speed of the intermediate transfer member drive motor 1300 and the setting of the target rotation speed are performed by a drive load training unit 35 and a drive speed setting unit 1002, similar to the chromatic color drive motor 31.

[0061] The difference in surface speed between the photosensitive drum 101k and the intermediate transfer body 130 can be set to a target surface speed difference by appropriately setting the reduction ratio between the drum drive train and the intermediate transfer body drive train, and the diameter difference between the drive roller 106 and the photosensitive drum 101k. The drum drive train includes a drum reduction gear 33 and a drum drive gear 34. The intermediate transfer body drive train includes an intermediate transfer body reduction gear 36, an intermediate transfer body idler gear 37, and an intermediate transfer body drive gear 38. Note that the measures shown in this embodiment can be implemented even when the photosensitive drum 101k and the intermediate transfer body 130 are not driven by the same drive source.

[0062] (controller) FIG. 8 is an explanatory diagram of a controller 1001 that controls the operation of the image forming apparatus 1. The controller 1001 is built into the image forming apparatus 1. The controller 1001 includes a CPU (Central Processing Unit) 1000. The CPU 1000 controls the overall operation of the image forming apparatus 1 by executing a predetermined computer program. The controller 1001 includes a color misregistration adjustment control unit 1003, a drive speed setting unit 1002, an intermediate transfer body control unit 1005, a development control unit 1006, image formation control units 1100Y, 1100M, 1100C, and 1100K, and a drive load detection unit 135. The components of the controller 1001, excluding the CPU 1000, may be implemented by hardware or may be implemented by software when the CPU 1000 executes a computer program. The color misregistration adjustment control unit 1003, the drive speed setting unit 1002, the intermediate transfer body control unit 1005, and the development control unit 1006 operate in accordance with instructions from the CPU 1000. The CPU 1000 also controls the operations of the transfer state switching unit 133 and the developing device drive motor 1320 .

[0063] The color misregistration adjustment control unit 1003 is connected to the image formation control units 1100Y, 1100M, 1100C, and 1100K, the drive speed setting unit 1002, and the image detection sensor 1004. The color misregistration adjustment control unit 1003 obtains the detection image read result (detection result) by the image detection sensor 1004 and calculates the amount of color misregistration based on this read result. The image formation control units 1100Y, 1100M, 1100C, and 1100K control the operation of the corresponding image forming units 100Y, 100M, 100C, and 100K. The color misregistration adjustment control unit 1003 notifies the image formation control units 1100Y, 1100M, 1100C, and 1100K of the amount of color misregistration. The image formation control units 1100Y, 1100M, 1100C, and 1100K perform color misregistration adjustment based on the notified amount of color misregistration. Specifically, the image formation control units 1100Y, 1100M, 1100C, and 1100K determine the timing of starting exposure based on the amount of color shift, and perform exposure control of the exposure units 103 (exposure devices 103y, 103m, 103c, and 103k) of the corresponding image forming units 100.

[0064] The drive speed setting unit 1002 is connected to the photosensitive drum drive control unit 134, the intermediate transfer body drive control unit 131, and the drive load detection unit 135. These components perform the above-mentioned "photosensitive drum rotation speed control." The drive load detection unit 135 detects the drive loads of the photosensitive drum drive control unit 134 and the intermediate transfer body drive control unit 131 and notifies the drive speed setting unit 1002 of the detection result.

[0065] The intermediate transfer body control unit 1005 is connected to the intermediate transfer body drive control unit 131 and the transfer state control unit 136. The intermediate transfer body drive control unit 131 is connected to the intermediate transfer body drive motor 1300. The intermediate transfer body drive motor 1300 is a drive source that drives the drive roller 106 to rotate. The intermediate transfer body control unit 1005 controls the drive of the intermediate transfer body drive motor 1300 via the intermediate transfer body drive control unit 131. The intermediate transfer body control unit 1005 instructs the intermediate transfer body drive control unit 131 of a target rotation speed of the intermediate transfer body drive motor 1300. The intermediate transfer body drive control unit 131 controls the intermediate transfer body drive motor 1300 so that the rotation speed of the intermediate transfer body drive motor 1300 becomes the target rotation speed of the intermediate transfer body drive motor 1300. The transfer state control unit 136 is connected to the transfer state switching unit 133. The transfer state switching unit 133 controls the primary transfer roller 105 as described in FIG. 4.

[0066] The development control unit 1006 is connected to a development sleeve drive control unit 132. The development sleeve drive control unit 132 is connected to a development device drive motor 1320. The development device drive motor 1320 is a drive source that drives the development sleeve 109 to rotate. The development control unit 1006 instructs the development sleeve drive control unit 132 to rotate the development sleeve 109. In response to this instruction, the development sleeve drive control unit 132 controls the development device drive motor 1320 to rotate the development sleeve 109.

[0067] (Drive control) The rotational speed control of the photosensitive drum 101 by the controller 1001 having such a configuration will be described below.

[0068] The intermediate transfer body 130 is driven by an intermediate transfer body drive motor 1300, and has the characteristic that the surface friction coefficient μb increases in accordance with the number of sheets on which images have been formed. The rotation speeds of the photosensitive drum 101 and the intermediate transfer body 130 are initially set so that the surface speed of the photosensitive drum 101 is slightly slower than the surface speed of the intermediate transfer body 130.

[0069] When image formation is performed under these settings, the photosensitive drum 101 is rotated by the intermediate transfer body 130 due to frictional forces caused by differences in the surface speed of the primary transfer roller 105 and electrical attraction forces between the photosensitive drum 101 and the intermediate transfer body 130 caused by various bias voltages. This reduces the rotational load on the chromatic color drive motor 31 for the chromatic color photosensitive drums 101y, 101m, and 101c. This frictional force and electrical attraction force are collectively referred to as "tangential force." Figure 9 is a graph showing the rotational load on the chromatic color drive motor 31 for the chromatic color photosensitive drums 101y, 101m, and 101c due to wear of the intermediate transfer body 130. This graph illustrates the reduction in the rotational load on the chromatic color drive motor 31 due to tangential force.

[0070] The tangential force has the property that it increases as the difference in surface speed increases, and once the difference in surface speed exceeds a certain level, it does not increase any further. When toner is interposed between the photosensitive drums 101y, 101m, and 101c and the intermediate transfer body 130 when the tangential force is large, the toner acts as a roller. This causes the tangential force to suddenly decrease, resulting in image defects such as color shift and shock. In this embodiment, the surface speeds are changed by controlling the rotational speeds of the chromatic color photosensitive drums 101y, 101m, and 101c, thereby reducing the difference in the two surface speeds, suppressing an increase in the tangential force, and preventing image defects caused by a momentary decrease in the tangential force.

[0071] (Color shift due to rear rotation) FIG. 10 is an explanatory diagram of a color misregistration phenomenon that occurs due to the influence of tangential forces between the intermediate transfer member 130 and the chromatic color photosensitive drums 101y, 101m, and 101c.

[0072] During normal image formation, in order to prevent developer degradation and toner consumption, the image forming unit 100, after completing transfer of a toner image to the intermediate transfer body 130, reduces the drive (development drive) of the development sleeve 109 and the charging bias. In the image forming unit 100 where the development drive and charging bias are reduced, the toner present at the interface between the intermediate transfer body 130 and the photosensitive drum 101 is removed while maintaining the difference in surface speed between the intermediate transfer body 130 and the photosensitive drum 101, resulting in a change in the tangential force before and after the reduction. If the surface speed of the photosensitive drum 101 is set slightly slower than that of the intermediate transfer body 130, the frictional force that the intermediate transfer body 130 receives from the photosensitive drum 101 increases as the toner that acted as a roller to reduce friction is removed. As a result, the traveling speed of the intermediate transfer body 130 slows, and the time required for the intermediate transfer body 130 to move between each image forming unit 100 increases. This causes color misalignment in the sub-scanning direction relative to the color upstream in the rotation direction of the intermediate transfer body 130. Specifically, the image of the color downstream in the rotation direction of the intermediate transfer body 130 is shifted to the color upstream in the rotation direction, that is, toward the leading edge of the sheet.

[0073] Figure 10 shows the waveforms of the color shift amount in the sub-scanning direction for images of magenta (M), cyan (C), and black (K) relative to an image of the reference color Y (yellow) when five sheets are fed consecutively in a high-temperature environment. In these graphs, the shift toward the rear end of the sheet (recording material) is represented as the positive direction on the vertical axis.

[0074] 10, it can be seen that the fifth sheet, which is the final sheet, differs from the first four sheets in that it exhibits color shift in the negative direction in the order of K, C, and M. This shows that the thermal expansion of the drive roller 106 increases the speed difference between the intermediate transfer body 130 and the photosensitive drum 101, and color shift occurs sequentially from the time when the photosensitive drum 101y, which is the first to fall during the post-rotation operation, falls. The reason why the color shift begins in the order of K, C, and M, which are located downstream in the rotation direction of the intermediate transfer body 130, is because the transfer timing is slow and the affected area on the sheet is wide.

[0075] There is also a slight variation in the waveforms for the second to fourth sheets compared to the first sheet, but this is due to the force that the intermediate transfer body 130 receives when the sheet is pushed into the secondary transfer section 118. Normally, the same trend as for the second to fourth sheets should be observed for the fifth sheet.

[0076] As described above, it is known that the frictional force caused by the speed difference between the intermediate transfer body 130 and the photosensitive drums 101y, 101m, and 101c has an adverse effect on the image. This is not only due to the thermal expansion of the drive roller 106, as given as an example, but also to differences between machines due to dimensional variations and changes in conditions such as wear, so speed control on the machine itself is necessary.

[0077] 11 and 12 are flowcharts showing a color misregistration adjustment process based on the measurement results of the driving torque of the photosensitive drum 101. This process starts when the image forming apparatus 1 is started.

[0078] First, the CPU 1000 checks the state of a backup data clear flag (S301). The backup data clear flag will be described later. If the backup data clear flag is off (S301: N), the CPU 1000 determines whether a job has been submitted (S303). If a job has been submitted (S303: Y), the CPU 1000 determines whether the job is a color job that instructs printing of a color image (S304).

[0079] If it is a color job (S304: Y), the CPU 1000 operates the transfer state switching unit 133 to switch from the monochrome mode (FIG. 4A) at the home position to the full color mode (FIG. 4B). This causes the photosensitive drums 101y, 101m, and 101c to enter a contact state in which they contact the intermediate transfer body 130 (S305). When the transition to the contact state occurs, the CPU 1000 starts the job operation (S306).

[0080] After starting the job, CPU 1000 determines whether it is time to perform automatic color misregistration adjustment by pre-rotation operation, inter-sheet operation, and post-rotation operation (S307). If it is not time to perform adjustment (S307: N), CPU 1000 determines whether image formation has ended (S308). If it has not ended (S308: N), CPU 1000 returns to the process of S307 and determines again whether it is time to perform automatic color misregistration adjustment.

[0081] If image formation has ended (S308: Y), the CPU 1000 checks the data acquisition flag (S309). The conditions under which the data acquisition flag is turned on will be described later. If the data acquisition flag is off (S309: N), the CPU 1000 returns to the process of S301. If the data acquisition flag is on (S309: Y), the CPU 1000 acquires information about the drive load of the chromatic color drive motor 31 when the photosensitive drums 101y, 101m, and 101c are in contact with the intermediate transfer body 130 after image formation has ended (S310). The information about the drive load acquired by the CPU 1000 is the drive torque Ta detected by the drive load detection unit 135. The CPU 1000 backs up (stores) the acquired drive load (drive torque Ta) in a predetermined memory and ends the post-rotation operation (job) (S311).

[0082] Thereafter, the CPU 1000 operates the transfer state switching unit 133 again to separate the primary transfer rollers 105y, 105m, and 105c from the photosensitive drums 101y, 101m, and 101c, thereby separating the photosensitive drums 101y, 101m, and 101c from the intermediate transfer body 130 (S312). As the photosensitive drums 101y, 101m, and 101c are released, they transition to the home position (monochrome mode) shown in FIG. 4(a).

[0083] If the job is not a color job (S304: N), the CPU 1000 starts the job operation in monochrome mode (S313). The CPU 1000 checks whether it is time to perform automatic color misregistration adjustment, as with a color job (S314). If it is not time to perform adjustment (S314: N), the CPU 1000 determines whether image formation has ended (S315). If it has not ended (S315: N), the CPU 1000 returns to the process of S314 and checks again whether it is time to perform automatic color misregistration adjustment.

[0084] If image formation is complete (S315: Y), the CPU 1000 checks the data acquisition flag (S316). If the data acquisition flag is off (S316: N), the CPU 1000 returns to the process of S301. If the data acquisition flag is on (S316: Y), after image formation is complete, the CPU 1000 drives the chromatic color drive motor 31 in a monochrome mode in which the photosensitive drums 101y, 101m, and 101c are separated from the intermediate transfer body 130 (S317). The CPU 1000 acquires information about the drive load of the chromatic color drive motor 31 in the monochrome mode (S318). The information about the drive load acquired by the CPU 1000 is the drive torque Tb detected by the drive load detection unit 135. The CPU 1000 backs up (saves) the acquired drive load (drive torque Tb) in memory and ends the post-rotation operation (S319).

[0085] When measuring the drive load in monochrome mode, the development drive and application of the charging bias are performed in the same manner as when measuring in full-color mode. This allows the states of the photosensitive drums 101y, 101m, and 101c and the intermediate transfer body 130, other than the contact / separation states, to be consistent between the process in S310 and the process in S318.

[0086] Figure 13 is an explanatory diagram of contact / separation between the photosensitive drum 101 and the intermediate transfer body 130. Figure 13(a) shows the contact state, and Figure 13(b) shows the separation state. The photosensitive drums 101 in Figure 13 are chromatic color photosensitive drums 101y, 101m, and 101c.

[0087] As shown in FIG. 13(a), when the photosensitive drum 101 contacts the intermediate transfer body 130 and there is a difference in the surface speed between the photosensitive drum 101 and the intermediate transfer body 130, a frictional force is generated between the two contact surfaces. This frictional force affects the rotational load (drive load) of the chromatic color drive motor 31 that drives the photosensitive drum 101. If there is no difference in the surface speed, no frictional force is generated between the two contact surfaces. In this case, the rotational load of the chromatic color drive motor 31 is not affected by the intermediate transfer body 130.

[0088] The ideal state in which image defects are unlikely to occur is one in which there is no speed difference between the two contact surfaces and the intermediate transfer body 130 does not affect the rotational load of the photosensitive drum 101. To simulate this state, as shown in FIG. 13(b), the transfer state switching unit 133 moves the primary transfer roller 105 to separate the intermediate transfer body 130 and the photosensitive drum 101. This suppresses the effect of the intermediate transfer body 130 on the photosensitive drum 101.

[0089] By detecting the rotational load in this state, it is possible to obtain the rotational load of the photosensitive drum 101 in an ideal state. At this time, the developer on the surface of the developing sleeve 109 is magnetically raised while the carrier holds the toner, and is in contact with the surface of the photosensitive drum 101. If the developing sleeve 109 is rotated in this state, the surface speed of the developing sleeve 109 is faster than the surface speed of the photosensitive drum 101, so the developer influences in a direction that assists the rotational load of the photosensitive drum 101. Furthermore, if the developing sleeve 109 is not rotated, the developer influences in a direction that puts a burden on the rotational load of the photosensitive drum 101.

[0090] Therefore, by maintaining the same rotational state of the developing sleeve 109 when the drive torque Ta is acquired and when the drive torque Tb is acquired, the influence of disturbances can be eliminated. Figure 14 is an explanatory diagram of changes in drive torque Ta and drive torque Tb. Figure 14(a) shows the drive torque Ta and drive torque Tb when the speed setting value of the photosensitive drum 101 is changed when the intermediate transfer body 130 is in the initial state. Figure 14(b) shows the drive torque Ta and drive torque Tb when the speed setting value of the photosensitive drum 101 is changed when the intermediate transfer body 130 has changed over time.

[0091] Compared to the initial state, the driving torque Ta is significantly different after aging. The area around 0.1% where the driving torque Ta intersects with the driving torque Tb, which is the ideal state, is considered to be the speed setting value where the difference in surface speed between the photosensitive drum 101 and the intermediate transfer body 130 actually becomes zero.

[0092] Furthermore, when the driving torque Ta is smaller than the driving torque Tb, friction occurs when the intermediate transfer body 130 and the photosensitive drum 101 come into contact and rotate, and it is thought that the photosensitive drum 101 is pulled by the intermediate transfer body 130, thereby reducing the rotational load. Therefore, in order to make the surface speed difference zero, it is necessary to increase the driving motor speed of the photosensitive drum 101. Conversely, when the driving torque Ta is larger than the driving torque Tb, the photosensitive drum 101 pulls the intermediate transfer body 130, so it is necessary to decrease the driving motor speed of the photosensitive drum 101.

[0093] Therefore, as shown in the flowchart of FIG. 11, the CPU 1000 repeats the processes from S301 onward until a predetermined number of drive loads (drive torques) have been backed up (S320: N). In other words, the drive loads (drive torques Ta, Tb) are acquired during the execution of jobs that are executed at different times. When the CPU 1000 has backed up a predetermined number of drive loads (drive torques) (S320: Y), it determines a new speed setting value for the drive motor of the photosensitive drum 101 in a direction that reduces the surface speed difference based on the predetermined number of drive loads (S321). The new speed setting value is determined, for example, based on the difference between the average values ​​of the predetermined number of drive torques Tb and the drive torque Ta.

[0094] If the backup data clear flag is on in the process of S301 (S301: Y), the CPU 1000 erases the predetermined number of backed-up drive loads (S302) and returns to the process of S301. The backup data clear flag is turned on when the photosensitive drum 101 or the unit including the drive roller 106 of the intermediate transfer body 130 is replaced, and the component tolerances, etc., change, causing the relationship between the surface speeds to change.

[0095] In this embodiment, the drive torque Ta is measured for a full-color job and the drive torque Tb is measured for a monochrome job. Therefore, depending on the user's usage, there may be an interval between the timing of each measurement. In such a case, for example, the variation in the internal temperature at the time of data acquisition may exceed a predetermined temperature. Therefore, if it is determined that the difference in the thermal expansion / contraction state of the drive roller 106 under multiple conditions cannot be ignored, the backup data clear flag is turned on and the backed-up drive load is deleted. In this case, a temperature sensor is provided to measure the temperature inside the image forming apparatus 1. If the internal temperature measured by the temperature sensor is equal to or higher than a predetermined temperature, the CPU 1000 deletes the backed-up drive load.

[0096] Specifically, the thermal expansion coefficient of the drive roller 106 in this embodiment is approximately 1 μm / ° C., and the surface speed effect calculated from the nominal dimensions is approximately 0.007% / ° C. If the target surface speed difference range determined from the torque difference is ±0.10%, if the temperature variation between the data used is kept to approximately 5° C., the surface speed variation will be approximately 0.035%, and it will be possible to determine the threshold temperature taking other variations into consideration, for example.

[0097] When the speed setting value of the chromatic color drive motor 31 that drives the photosensitive drums 101y, 101m, and 101c is changed, if an image is output as is, the time from exposure to primary transfer will change, causing the position of the leading edge of the image to change. Therefore, if a new speed setting value has been determined at the start of the color misregistration adjustment mode, which is the color misregistration adjustment control, adjustment is performed using the determined setting value as the new speed.

[0098] Explaining this with reference to the flowcharts of FIGS. 11 and 12, if no job has been submitted in the processing of S303 (S303: N), CPU 1000 determines whether or not to perform automatic color shift adjustment (S322). If not (S322: N), CPU 1000 returns to the processing of S301. If color shift adjustment is to be performed (S322: Y), CPU 1000 determines whether or not a new speed setting value has been determined (S323). Also, if it is time to perform automatic color shift adjustment in the processing of S307 and S314 (S307: Y, S314: Y), CPU 1000 determines whether or not a new speed setting value has been determined (S323).

[0099] If the new speed setting value has not been determined (S323: N), the CPU 1000 performs color misregistration adjustment without changing the speed setting (S324) and returns to the process of S301. If the new speed setting value has been determined (S323: Y), the CPU 1000 changes the new speed to the setting value and performs color misregistration adjustment (S325, S326).

[0100] If the color misregistration adjustment fails (S327: N), the CPU 1000 restores the speed setting (S328) and returns to the processing of S301. If the color misregistration adjustment is successful (S327: Y), the CPU 1000 erases the backed-up differential data and finally determines the speed (S329). If the color misregistration adjustment is a manual adjustment in accordance with a user instruction (S330: Y), the CPU 1000 simply ends the processing. This processing is performed if no job was submitted in the processing of S303. If the color misregistration adjustment is an automatic adjustment during the job (S330: N), the CPU 1000 completes the remaining operations of the job and ends the process (S331). This process is performed when it is time to perform automatic color misregistration adjustment in the processes of S307 and S314. By the above operation, it is possible to suppress the occurrence of color misregistration even if the rotation speeds of the photosensitive drums 101y, 101m, and 101c are changed.

[0101] As described above, the image forming apparatus 1 of this embodiment changes the target speed of the chromatic color drive motor 31 so that the surface speeds of the photosensitive drums 101y, 101m, and 101c match the surface speed of the intermediate transfer body 130. This makes it possible to maintain an appropriate difference in surface speed, preventing color registration and banding and maintaining high-quality images. Note that in this embodiment, the target rotation speed of the chromatic color drive motor 31, which rotates the multiple photosensitive drums 101y, 101m, and 101c, is changed by detecting the rotation load of the chromatic color drive motor 31. This embodiment can also be applied to cases where the target rotation speed of a drive motor that drives a single photosensitive drum 101 is changed.

[0102] The data acquisition flag will now be described. According to this embodiment, the drive load can be detected during any normal post-rotation operation, so the data acquisition flag may be set to ON at all times. However, since there is no need to change the speed very frequently and the chromatic color drive motor 31, which is not required in normal monochrome mode, must be driven, it is preferable to change the speed at a timing and frequency that is considered necessary.

[0103] In this embodiment, because the speed setting change and color misregistration adjustment must be performed together, the frequency of the speed setting change cannot exceed the frequency of the color misregistration adjustment. Also, because the color misregistration adjustment requires a certain amount of downtime, it is unacceptable to increase the frequency of the color misregistration adjustment in order to change the speed.

[0104] Color misregistration adjustment prevents deviations in the laser beam irradiation position due to thermal deformation of the housing caused by changes in the ambient temperature around the exposure unit 103. To achieve this, the execution conditions are defined by the change in sensor temperature since the previous execution and the cumulative number of sheets passed. The execution conditions for speed change and color misregistration adjustment have something in common in that a speed change is required when conditions change due to temperature changes or wear and tear. Therefore, if the conditions for turning on the data acquisition flag are the same as for color misregistration adjustment, or the same conditions but with a small threshold, sufficient backup data can be accumulated when color misregistration adjustment is required.

[0105] In the above embodiment, when detecting the drive load during normal image creation operations with different image creation modes, if the printer is used in a biased manner, such as performing only monochrome jobs, the frequency of load detection and, ultimately, the opportunity to change the speed will be lost. However, even when only monochrome jobs are performed, there are opportunities to switch to full-color mode for image creation. For example, to supply toner as a lubricant for the cleaners of the chromatic photosensitive drums, this can be addressed by switching to full-color mode during the pre- or post-monochrome job rotation and creating color images. By detecting the load as in the above embodiment during such periodic full-color mode image creation, control is possible.

Claims

1. a first photoreceptor on which a color image is formed; a second photoreceptor on which a black image is formed; an intermediate transfer belt that is wound around a plurality of rollers including a drive roller and onto which the color image and the black image are transferred; a transfer means for transferring an image on the intermediate transfer belt to a sheet; a first driving source that drives the first photosensitive member to rotate; a second drive source that drives the second photosensitive member and the drive roller to rotate; a state control unit that controls a contact state between the first photosensitive member and the intermediate transfer belt and a contact state between the second photosensitive member and the intermediate transfer belt; a determining means for determining a target rotation speed of the first driving source; a drive control means for controlling the first drive source so that the rotation speed of the first drive source becomes the target rotation speed determined by the determination means, the state control unit controls the first photosensitive member and the intermediate transfer belt to a first state in which the first photosensitive member and the intermediate transfer belt are in contact with each other and the second photosensitive member and the intermediate transfer belt are in contact with each other when a mixed-color image is formed on a sheet; the state control means controls the first photosensitive member and the intermediate transfer belt to a second state in which the first photosensitive member and the intermediate transfer belt are not in contact with each other and the second photosensitive member and the intermediate transfer belt are in contact with each other when a black monochrome image is formed on a sheet; the determining means stores first load information relating to a load on the first driving source in the first state and second load information relating to a load on the first driving source in the second state, and determines the target rotation speed of the first driving source based on the first load information and the second load information; the first driving source rotates the first photosensitive member based on the determined target rotation speed; the determining unit performs color misregistration adjustment while the first driving source is rotating based on the target rotation speed, and when the color misregistration adjustment is successful, erases the stored first load information and the stored second load information. Image forming device.

2. the determining means determines the target rotation speed based on a predetermined number of the first load information and a predetermined number of the second load information, 2. The image forming apparatus according to claim 1.

3. The determination means determines the target rotation speed based on a difference between an average value of a predetermined number of pieces of first load information and an average value of a predetermined number of pieces of second load information.

3. The image forming apparatus according to claim 2.

4. the determining unit determines the target rotation speed in a direction that reduces a difference in surface speed between the first photosensitive member and the intermediate transfer belt.

4. The image forming apparatus according to claim 3.

5. the determining unit stores the first load information and the second load information, and when the first photosensitive member or the intermediate transfer belt is replaced, the determining unit erases the stored first load information and the stored second load information.

5. The image forming apparatus according to claim 4.

6. Further comprising a measuring means for measuring an internal temperature, the determining means erases the stored first load information and the stored second load information if the internal temperature measured by the measuring means is equal to or higher than a predetermined temperature.

6. The image forming apparatus according to claim 5.

7. the determining unit acquires the first load information and the second load information during execution of jobs that are executed at different times.

2. The image forming apparatus according to claim 1.

Images