JP5159979B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms an image on a recording medium such as printing paper, photosensitive paper, or electrostatic recording paper. In particular, the present invention relates to an image forming apparatus that transfers a toner image from a plurality of image carriers to a rotating body such as an intermediate transfer belt.

  In recent years, color image forming apparatuses employing an electrophotographic system such as a color printer or a color copying machine are required to have high output image quality.

  As an element that determines the quality of the output image, as the recording accuracy represented by the image writing position deviation on the recording medium, the image expansion / contraction, and the overlay accuracy of the toner images that affect the color of the image, Color shift is mentioned.

  Especially in the case of an electrophotographic color image forming apparatus, the quality of the output image is deteriorated due to deterioration in recording accuracy or color variation due to color misregistration due to changes in the environment due to environmental changes or long-term use. I will let you.

  As a cause of these fluctuations, for example, in an image forming apparatus that employs an intermediate transfer belt as an endless belt, speed fluctuation of the intermediate transfer belt can be cited.

  Therefore, for example, the method described in Patent Document 1 is used. Specifically, each color toner patch is formed on the intermediate transfer belt, the position of the toner patch is detected by a registration detection sensor, and the timing of writing each color toner image to the intermediate transfer belt is changed based on the detection result. In order to suppress color misregistration, it has been carried out. Here, the toner patch is an unfixed toner image for color misregistration detection.

Japanese Patent No. 2655603

  However, even when color misregistration correction using a conventional registration detection sensor is performed, the color misregistration when each color toner image is actually transferred to the recording medium cannot be completely eliminated.

  This is mainly because the peripheral surface speed of the belt when the position of the toner patch on the intermediate transfer belt is detected by the registration detection sensor and the peripheral surface speed of the belt during actual image formation are different. The occurrence of the difference in the peripheral surface speed of the intermediate transfer belt will be described below.

  FIG. 11 is a diagram illustrating a load state applied to an intermediate transfer belt unit of a tandem type color image forming apparatus using a general intermediate transfer belt. In FIG. 11, the peripheral speed Vd of the photosensitive drum and the peripheral speed Vb of the intermediate transfer belt are generally set to about 0.5% or less, and the belt peripheral speed Vb is faster in order to improve transfer efficiency. It has become. The belt driving torque T at this time is expressed by the following equation (11), where Tb is a torque for moving only the belt and μF is a frictional force generated by contact between the belt and the drum. Here, μ is the coefficient of friction between the belt and the drum, and the transfer pressure is F.

  T = Tb + μF × 4 (11)

  Next, as shown in FIG. 12, the belt drive torque T where the drum peripheral surface speed Vd is intentionally set higher than the belt peripheral surface speed Vb is expressed by the following equation (12), and the belt drive torque T is rotated to the photosensitive drum. To make it lighter.

  T = Tb−μF × 4 (12)

  The change in the torque T from when the belt is started from the stopped state to when it is stopped again after image formation is defined by the following equations (13) to (17) when the friction coefficient μ between the belt and the drum is defined as the following two. ) Changes in the state of the load torque applied to the belt at this time are shown in FIGS. In each figure, 26 is a photosensitive drum, 54 is a developing roller, 52 is a primary transfer roller, and 30 is an intermediate transfer belt. Y represents yellow, M represents magenta, C represents cyan, and Bk represents black. Here, the friction coefficient μ between the belt and the drum is defined as two, a friction coefficient μ1 when there is no toner between the belt and the drum, and a friction coefficient μ2 when there is toner between the belt and the drum.

T = Tb + μ1F × 4 (13) (see FIG. 13)
T = Tb + (μ1 × 3 + μ2F) (14) (see FIG. 14)
T = Tb + (μ1F × 2 + μ2F × 2) (15) (see FIG. 15)
T = Tb + (μ1F + μ2F × 3) (16) (see FIG. 16)
T = Tb + μ2F × 4 (17) (see FIG. 17)
Thereafter, Expression (16) (see FIG. 18) → Expression (15) (see FIG. 19) → Expression (14) (see FIG. 20) → Expression (13) (see FIG. 13)

  In FIG. 14, when the developing roller 54Y comes into contact with the photosensitive drum 26Y, the toner on the developing roller 54Y adheres to the photosensitive drum as fogging toner regardless of the latent image formation, and then the nip portion between the photosensitive drum and the intermediate transfer belt. Toner enters. Therefore, the presence / absence of toner in the nip portion between the photosensitive drum and the intermediate transfer belt is determined by the contact or separation of the developing roller of the developing unit, not the image forming toner that is formed by the actual latent image.

  The two friction coefficients μ1 and μ2 are generally in a magnitude relationship of (μ1> μ2). The load fluctuation (torque fluctuation) applied to the belt is lightened by the contact of the developing device and becomes heavy when the developing device is separated.

  The torque state of the belt driving when the toner patch on the belt is detected by the registration detection sensor is constant in the state of Expression (17), and the belt peripheral surface speed is also constant. This state is clearly different from the torque fluctuation state during image formation.

  It is known that a belt drive transmission system composed of a gear train that drives a belt elastically deforms in proportion to the stress generated by its load torque, as expressed by Hooke's law, etc. Varies the transmission speed of the drive transmission system = the belt peripheral surface speed.

  That is, the belt peripheral surface speed changes when the current state of each of the equations (13) to (17) shifts to the next state. For example, when the load torque applied to the belt changes from a light state to a heavy state, the belt peripheral surface speed decreases, and conversely, when the belt changes from a heavy state to a light state, the belt peripheral surface speed increases.

  Even if the toner patch on the belt is detected by the registration detection sensor in the state where there is no belt speed fluctuation and correction is made based on the result, the belt peripheral surface speed fluctuates during actual image formation. Deviation (transfer position deviation) occurs.

  There are the following three methods for eliminating the belt speed fluctuation. First, the rigidity of the belt drive transmission system is increased to eliminate elastic deformation, second, the fluctuation of the friction coefficient μ between the belt and the drum is eliminated, and third, image formation is performed after the state of Expression (17) is reached. That is.

  The first method will be described. Generally, the elastic deformation described above can be suppressed by increasing the rigidity of the belt drive transmission system. For example, if the material of the gear, which is an element of the drive transmission system, is changed from a resin such as polyacetal to a metal such as brass, the rigidity can be increased. In our experiment, it has been confirmed that the speed fluctuation can be improved by increasing the rigidity by metallization of the gear. However, the metal gear has too high rigidity, and vibration due to meshing occurs, which causes a problem that the vibration is placed on the image. In addition, the metal gear is not realistic because of the cutting process, resulting in a considerable increase in cost compared to the injection molded resin gear.

  The second method will be described. Theoretically, if the friction coefficients μ1 and μ2 are the same, fluctuations in the friction coefficient μ can be suppressed. However, the surface layer of the current photosensitive drum is smooth and easily adheres to the belt, and a very large frictional force is generated. Although measures such as providing minute irregularities on the surface of the photosensitive drum and reducing the contact area can be considered, image quality is expected to deteriorate, which is not realistic. Further, the fluctuation of friction cannot be reduced to zero because there is not only the presence / absence of toner but also an attractive force due to a transfer bias.

  A third method will be described. The third method is to turn on / off the charging, developing, and transferring steps of the image forming process unit, which is a factor causing load fluctuations, at times other than the time when the visible image is transferred from the photosensitive drum to the intermediate transfer belt. Technically feasible. However, while a high-quality image with suppressed color misregistration can be obtained, charging, development, and transfer processes are performed on and off in the unit other than when a visible image is transferred from the photosensitive drum to the intermediate transfer belt. The process time for development and the like becomes long, and the productivity of the apparatus is lowered. Furthermore, there is a problem that the life of the unit is shortened. In particular, when intermittent recording is performed, this influence cannot be ignored, and the user has to frequently replace the unit, which also causes the running cost to deteriorate.

  Accordingly, an object of the present invention is to suppress a transitional speed fluctuation of a belt that occurs during an image forming operation, and prevent the occurrence of color misregistration without reducing the productivity of the image forming apparatus.

In order to achieve the above object, the present invention provides a first image carrier that carries a toner image , and a toner image that can be brought into contact with and separated from the first image carrier. A first developing means for forming a toner image, a second image carrier carrying a toner image, and a second image carrier capable of contacting and separating from the second image carrier. A transfer unit is formed in contact with the second developing means to be formed and the first and second image carriers, and the toner images formed on the first and second image carriers are transferred. Or a rotating body that conveys a recording medium to which a toner image is transferred, patch detection means that detects a toner patch transferred from the first and second image carriers to the rotating body, and the rotating body. image forming apparatus and a control means for controlling the relative speed of the first, the second image carrier In this case, the image forming apparatus rotates the first image carrier from the first image carrier with the first and second developing units in contact with the first and second image carriers. A first toner patch pattern including a toner patch transferred to a body, a toner patch transferred from the second image carrier to the rotating body, and the first and second image carriers, respectively. A toner patch transferred from the first image carrier to the rotating member in a state where the first and second developing units are in contact with each other, and the first developing unit is separated from the first image carrier. And a second toner patch pattern including a toner patch transferred from the second image carrier to the rotating body in a state where the second developing means is in contact with the second image carrier. The control means can be formed on a rotating body. Depending on the detection results of the toner patch of the second toner patch pattern and the first toner patch pattern by Chi detecting means, controlling the relative velocity of the said rotating body first, the second image carrier It is characterized by doing.
In order to achieve the above object, the present invention provides a first image carrier that carries a toner image, a second image carrier that carries a toner image, and the first and second image carriers. A rotating member that contacts and forms a transfer portion to transfer a toner image formed on the first and second image carriers, or transports a recording medium to which the toner image is transferred, and the first Patch detecting means for detecting a toner patch transferred from the second image carrier to the rotating body, and the rotating body or the first and second image carriers based on the detection result of the patch detecting means. An image forming apparatus comprising: a control unit configured to control a peripheral surface speed of the body; The rotation from the first image carrier in an intervening state The first toner patch to be transferred to the toner image, and the transfer formed by the rotating body and the second image carrier without any toner in the transfer portion formed by the rotating body and the first image carrier. A second toner patch transferred from the second image bearing member to the rotating member in a state where toner is present in the portion, and the control means detects the rotation according to the detection result of the patch detecting means The relative speed of the body and the first and second image carriers is controlled.

  According to the present invention, it is possible to prevent the occurrence of color misregistration without suppressing the transient speed fluctuation of the rotating body that occurs during the image forming operation, and without reducing the productivity of the image forming apparatus. Can be output stably.

Sectional drawing which shows schematic structure of 4 drum type full-color image forming apparatus using an intermediate transfer belt Block diagram showing configuration of image forming apparatus The figure which shows the torque fluctuation of the drive roller of the intermediate transfer belt during the image forming operation Diagram showing color shift behavior on the output image Perspective view of intermediate transfer belt unit Diagram showing the relationship between drum speed and color shift between yellow and black Diagram showing the relationship between drum speed and color shift between yellow and black Flowchart explaining drum speed correction sequence of image forming apparatus State of load torque applied to belt The figure which shows the relationship between the transfer timing of a toner patch, and the load torque curve of a belt State of load torque applied to belt State of load torque applied to belt State of load torque applied to belt State of load torque applied to belt State of load torque applied to belt State of load torque applied to belt State of load torque applied to belt State of load torque applied to belt State of load torque applied to belt State of load torque applied to belt

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in the following embodiments should be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions. Therefore, unless specifically stated otherwise, the scope of the present invention is not intended to be limited thereto.

[First Embodiment]
The image forming apparatus according to the first embodiment will be described. Here, as an example of the image forming apparatus, a four-drum full-color image forming apparatus using an intermediate transfer belt is illustrated as an example of an image forming apparatus employing an electrophotographic system. FIG. 1 is a schematic cross-sectional view showing a schematic configuration of a four-drum full-color image forming apparatus using an intermediate transfer belt.

(Overall configuration of image forming apparatus)
As shown in FIG. 1, the four-drum full-color image forming apparatus 1 is different from the image forming apparatus main body (hereinafter referred to as the apparatus main body) 2 in four color process cartridges PY, PM, and PC of yellow, magenta, cyan, and black. , PBk is configured to be detachable. Further, the apparatus main body 2 is provided with an intermediate transfer belt unit 31 having an intermediate transfer belt 30 as an intermediate transfer body (rotating body) and a fixing device 25.

  Here, each process cartridge P has a memory tag (not shown), and is configured to be able to determine the remaining life and replacement status of the process cartridge through communication with the apparatus main body 2.

  Each process cartridge P has photosensitive drums 26Y, 26M, 26C, and 26Bk as image carriers. Further, each process cartridge P integrally has a primary charger 50 as a charging unit, a developing unit 51 as a developing unit, and a cleaner 53 as a cleaning unit around the photosensitive drum. The process cartridges P are arranged in parallel along the intermediate transfer belt 30.

  In each process cartridge P, the primary charger 50 is disposed on the outer peripheral surface of the photosensitive drum 26 and uniformly charges the surface of the photosensitive drum. Further, the developing device 51 converts the electrostatic latent image of each color on the surface of the photosensitive drum formed by exposure from each laser exposure device (exposure means) 28Y, 28M, 28C, 28Bk to the corresponding color (yellow, magenta). , Cyan, black) toner. The developing roller 54 in the developing device 51 is configured to be separated from the photosensitive drum 26 together with the developing device 51 and stop rotating, thereby preventing the deterioration of the developer. That is, the developing roller 54 is configured to be able to contact or separate from the photosensitive drum 26 together with the developing device 51. The cleaner 53 removes the transfer residual toner adhering to the surface of the photosensitive drum after the toner image is transferred.

  Further, a primary transfer roller 52 that forms a primary transfer portion together with the photosensitive drum 26 is disposed opposite to the position where the intermediate transfer belt 30 is sandwiched with the photosensitive drum 26.

  On the other hand, the intermediate transfer belt unit 31 includes an intermediate transfer belt 30 and three rollers: a driving roller 100 that stretches the intermediate transfer belt 30, a tension roller 105, and a secondary transfer counter roller 108. The intermediate transfer belt 30 is rotated and conveyed by rotating the driving roller 100 by a belt driving motor (not shown).

  The tension roller 105 is configured to be movable in the horizontal direction in FIG. 1 according to the length of the intermediate transfer belt 30.

  Further, in the vicinity of the tension roller 105, two registration detection sensors 90 for detecting toner patches on the intermediate transfer belt 30 are provided at both ends in the roller longitudinal direction. The longitudinal direction is the axial direction of the roller, and is the width direction orthogonal to the belt conveyance direction.

  Further, a secondary transfer roller 27 that forms a secondary transfer portion together with the secondary transfer counter roller 108 is disposed so as to face the intermediate transfer belt 30 between the secondary transfer counter roller 108 and the intermediate transfer belt 30. The secondary transfer roller 27 is held by a transfer conveyance unit 33.

  A feeding unit 3 that feeds the recording medium Q to the secondary transfer unit is disposed below the apparatus main body 2. The feeding unit 3 includes a cassette 20 that stores a plurality of recording media Q, a feeding roller 21, a retard roller pair 22 for preventing double feeding, a pair of conveying rollers 23a and 23b, a pair of registration rollers 24, and the like.

  Discharge roller pairs 61, 62, and 63 are provided in the downstream conveyance path of the fixing device 25.

  Further, the color image forming apparatus 1 is compatible with double-sided printing. After the recording medium on which image formation on the first side has been completed is ejected from the fixing device 25, the switching member 69 is switched to switch the pair of reversing rollers 70. , 71 is conveyed to the recording medium Q. When the rear end of the recording medium exceeds the switching member 72, the switching member 72 is switched, and at the same time, the reverse roller 71 is rotated in the reverse direction to guide the recording medium Q to the double-sided conveyance path 73.

  The double-sided conveyance path roller pair 74, 75, 76 is rotationally driven to re-feed the recording medium Q, thereby enabling printing on the second side.

  Next, the control configuration of the image forming apparatus will be described with reference to FIG. FIG. 2 is a block diagram illustrating a control configuration of the image forming apparatus.

  The apparatus main body 2 shown in FIG. 1 receives RGB image signals from an external host device 10 such as a personal computer that is communicably connected to the apparatus main body, or a document reading unit (not shown) separately provided in the apparatus main body. To do.

  The image processing control unit (control unit) 11 converts the received RGB signal into a CMYK signal, corrects gradation and density, and then generates an exposure signal for the laser exposure unit 28. The image forming control unit 12 controls the image forming operation described below in an integrated manner, and corrects an image forming operation using a registration detection sensor 90 as a patch detection unit and a mark sensor 91 as a mark detection unit. The main body 2 is controlled.

  The image formation control unit 12 includes a CPU 121 that controls processing by the image formation control unit 12, a ROM 122 that stores programs executed by the CPU 121, and a RAM 123 that stores various data during control processing by the CPU 121. ing.

  As shown in FIG. 1, the image forming unit 13 includes a photosensitive drum 26 and a charging unit, a developing unit, a cleaning unit, and an exposure unit that act on the drum, and a plurality (in the rotation direction of the intermediate transfer belt ( There are four) here.

  The belt drive motor 14 is a drive unit that rotationally drives the intermediate transfer belt 30 at a predetermined speed in response to an instruction from the image formation control unit 12.

  The drum drive motor 15 is a drive unit that rotationally drives all the photosensitive drums 26 at a predetermined speed in response to an instruction from the image formation control unit 12.

  The registration detection sensor unit 16 uses the registration detection sensor 90 to detect toner patches on the intermediate transfer belt 30.

  The mark sensor unit 17 uses the mark sensor 91 to detect a position display mark provided on the intermediate transfer belt 30.

(Image forming operation)
Here, an image forming operation of the four-drum full-color image forming apparatus 1 configured as described above will be described with reference to FIG. The image forming apparatus 1 is configured to be able to form an image composed of toners of a plurality of colors (here, four colors) on a recording medium.

  When the image forming operation is started, the recording medium Q in the cassette 20 is first fed by the feeding roller 21 and then separated one by one by the retard roller pair 22, and then the conveying roller pair 23a, 23b, etc. Then, it is conveyed to the registration roller pair 24. Here, the registration roller pair 24 stops rotating at this time, and the skew of the recording medium Q is corrected by the recording medium Q being abutted against the nip of the registration roller pair 24.

  On the other hand, in parallel with the transport operation of the recording medium Q, for example, in the yellow process cartridge PY, the surface of the photosensitive drum 26Y is first uniformly negatively charged by the primary charger 50, and then the image is output by the laser exposure device 28Y. Exposure is performed. As a result, an electrostatic latent image corresponding to the yellow image component of the image signal is formed on the surface of the photosensitive drum 26Y.

  Next, while the developing roller 54 in the developing device 51 is driven to rotate, the developing roller 54 comes into contact with the photosensitive drum 26Y, and the electrostatic latent image is developed by the developing device 51 using negatively charged yellow toner. Is visualized as The yellow toner image thus obtained is primarily transferred onto the intermediate transfer belt 30 by the primary transfer roller 52 supplied with the primary transfer bias.

  Note that after the toner image is transferred, the transfer residual toner adhering to the surface of the photosensitive drum 26Y is removed by the cleaner 53.

  Such a series of toner image forming operations are sequentially performed with predetermined timing in the other process cartridges PM, PC, and PBk. Each color toner image formed on each photoconductive drum 26 is sequentially primary-transferred on the intermediate transfer belt 30 in a primary transfer portion. When the developing operation is finished, the developing roller 54 is sequentially separated from the photosensitive drum and stopped in rotation in order to prevent deterioration of the developer even if the downstream process cartridge is in the primary transfer.

  Next, the four-color toner images transferred superimposed on the intermediate transfer belt 30 in this manner are moved to the secondary transfer portion as the intermediate transfer belt 30 rotates in the arrow direction.

  Further, the recording medium Q whose skew has been corrected by the registration roller pair 24 is sent to the secondary transfer portion in time with the image on the intermediate transfer belt 30.

  Thereafter, the four-color toner images on the intermediate transfer belt 30 are secondarily transferred onto the recording medium Q by the secondary transfer roller 27 in contact with the intermediate transfer belt 30 with the recording medium Q interposed therebetween. Then, the recording medium Q onto which the toner image has been transferred in this manner is conveyed to the fixing device 25, where the toner image is fixed by being heated and pressed, and then is discharged by the discharge roller pair 61, 62, 63. Then, it is discharged and loaded on the upper surface of the apparatus main body.

  The intermediate transfer belt 30 that has finished the secondary transfer is subjected to removal of residual toner remaining on the surface by a belt cleaner (not shown) installed near the drive roller 100.

  Next, fluctuations in the driving torque T of the intermediate transfer belt when the peripheral surface speed Vd of the photosensitive drum and the peripheral surface speed Vb of the intermediate transfer belt are the same will be described.

  FIG. 9 is a load state diagram when the peripheral surface speed Vd of the photosensitive drum and the peripheral surface speed Vb of the intermediate transfer belt are the same. At this time, the driving torque T of the intermediate transfer belt is T = Tb. At this time, since the drum peripheral surface speed Vd and the belt peripheral surface speed Vb are constant, there is no slip, so the belt driving torque T becomes the torque Tb that moves only the belt, and the increase / decrease in torque due to the frictional force is zero. is there.

  The relationship between the peripheral speed difference between the drum peripheral surface speed and the belt peripheral surface speed and the belt driving torque will be described in detail using the results of measurement and verification by an actual image forming apparatus.

  FIG. 3 shows the result of measuring the rotational torque fluctuation of the driving roller 100 when three A3 sheets are continuously printed in the image forming apparatus having the above configuration.

  In the measurement, by changing the steady rotational speed of the photosensitive drum 26, the measurement is performed in a state where there is almost no peripheral speed difference between the case where the peripheral speed difference is intentionally made between the intermediate transfer belt 30 and the photosensitive drum 26. went.

  As can be seen from FIG. 3, in the state where a circumferential speed difference is provided between the photosensitive drum 26 and the intermediate transfer belt 30, a transient torque fluctuation (load fluctuation) occurs during image formation. More specifically, while the developing roller 54 in the developing device 51 is rotationally driven, the torque starts to change from the timing of starting the developing device contact with the yellow photosensitive drum 26Y. After the contact with the photosensitive drum 26, the torque fluctuation is settled. Then, the torque fluctuation starts again from the developing device separation start timing at which the upstream primary transfer of yellow on the upstream side is completed and the developing roller 54 is separated from the photosensitive drum 26Y.

  In the case where the photosensitive drum speed is set to −0.4% with respect to the intermediate transfer belt speed, when the contact of the developing device is started, the driving torque of the intermediate transfer belt decreases. When the photosensitive drum speed is less than the intermediate transfer belt speed, each color toner sequentially reaches the primary transfer nip after contact with the developing device, so that the frictional force between the drum and the belt is weakened and becomes a load on the belt. It was confirmed that the reaction force from the drum was reduced.

  On the other hand, as the primary transfer is completed from the upstream yellow toner at the end of image formation, the toner supply to the primary transfer nip ceases when the developer begins to separate. For this reason, the drum starts to become a driving load of the belt again, and the driving torque of the belt increases.

  Next, when three A3 sheets are continuously output in a state where there is a circumferential speed difference relationship that the photosensitive drum speed is less than the intermediate transfer belt speed, a color shift that is a relative positional shift of yellow with respect to black on the output image is obtained. An example of the measurement result is shown in FIG.

  Here, the vertical axis is positive when yellow is misaligned to the rear end side of the paper with respect to black on the image. Also, paying attention to the color misalignment between black and yellow, the color misregistration taken up here is prominently generated between yellow as the first color and black as the final color in the transfer order for the reasons described later. Because it does.

  As shown in the measurement result of the first sheet in FIG. 4, color misregistration occurs in the first half of the paper transport direction distance of 0 to 200 mm, and one sheet in the second half of the second sheet after 200 mm in the third paper transport direction distance. There is a color shift in the direction opposite to the eyes.

  With respect to the color misalignment of the first sheet, the belt speed during the primary transfer of yellow, which is the first color in the transfer order, gradually decreases with the decrease in the belt driving torque as shown in FIG. Involved in getting faster. On the other hand, with respect to the color misalignment of the third sheet, the belt speed during the primary transfer of the final color, black, gradually decreases as the belt driving torque increases as the developer separation starts as shown in FIG. Being involved.

  With respect to the second sheet on which the primary transfer is performed in a state where there is no torque fluctuation, there is almost no color misregistration. Although not mentioned here, magenta and cyan are also not as prominent as yellow and black, although color misregistration occurs.

  Note that, when three A3 sheets are continuously output in a state where there is a peripheral speed difference relationship that the photosensitive drum speed> the intermediate transfer belt speed, the graph of FIG.

  Further, it is known that the torque fluctuation shown in FIG. 3 becomes more prominent as the difference in the peripheral speed between the drum and the belt becomes larger. The fluctuation in the belt speed due to the torque fluctuation is caused by the belt drive transmission system. This is due to insufficient rigidity.

  Therefore, in the present embodiment, measures are taken to reduce the difference in peripheral speed between the drum and the belt, which is the cause of torque fluctuation.

  Even if the difference in peripheral speed between the drum and the belt is reduced, it is inevitable that the manufacturing cost will increase if the manufacturing tolerances of the parts that determine the respective speeds are tightened. Therefore, in the present embodiment, the color misalignment situation shown in FIG. 4 is detected, the peripheral speed difference between the drum and the belt is estimated based on the detection result, and the peripheral surface of the drum is determined according to the peripheral speed difference. Speed was corrected to take measures to prevent the above color shift.

  The method will be described in detail below.

  In order to reduce the peripheral speed difference between the drum and the belt, the peripheral speed of the belt may be corrected instead of the peripheral speed of the drum, but the peripheral speed difference is corrected by correcting the peripheral speed of the belt. A method for suppressing this will be described in a second embodiment to be described later.

  FIG. 5 is a perspective view showing the configuration of the intermediate transfer belt unit 31.

  The intermediate transfer belt 30 rotates at a peripheral surface speed Vb [mm / s] in the direction of the arrow in the figure. The design value here is Vb = 190.

  The intermediate transfer belt 30 employed in the present embodiment is attached with deviation regulating ribs 301 that prevent meandering of the intermediate transfer belt 30 at both ends of the inner peripheral surface. The deviation regulating rib 301 is regulated by deviation regulating flanges (not shown) installed on both ends of the tension roller 105, thereby preventing the belt from meandering. In addition, a transparent belt reinforcing tape 302 for preventing the intermediate transfer belt 30 from being damaged is attached to both ends of the outer peripheral surface of the intermediate transfer belt 30 one by one. Note that the deviation regulating rib 301 and the belt reinforcing tape 302 do not necessarily have to be at both ends as long as they are attached to the same side.

  The registration detection sensor 90 is a reflection type optical sensor for detecting an unfixed toner patch formed on the intermediate transfer belt 30. Here, one sensor is installed on each of both ends of the tension roller 105 in the longitudinal direction. ing.

  The registration detection sensor 90 is positioned so that the toner patch can be detected at a position where the intermediate transfer belt 30 is wound around the tension roller 105, and is supported so as to move following the movement of the shaft of the tension roller 105. ing.

  Next, the principle of drum speed correction using the registration detection sensor 90 will be described.

(Formation of first toner patch pattern)
Here, a case where there is a peripheral speed difference between the photosensitive drum and the intermediate transfer belt and torque fluctuation as shown in FIG. 10 occurs will be described as an example.

  As shown in FIG. 5, first and second toner patch patterns TP1 and TP2 as shown on the intermediate transfer belt 30 are formed in order at the timing shown in FIG.

  In FIG. 10, TM1 is the timing at which all the developing devices contact with all the photosensitive drums. TM2 is a timing at which the developing device is separated from the photosensitive drum in the yellow image forming portion when the second toner patch pattern is formed.

  Actually, after the toner patch is detected by the registration detection sensor 90 after the toner patch is formed, the toner patch is cleaned by a belt cleaner (not shown) installed in the vicinity of the driving roller 100. Therefore, in practice, although there is no toner patch on the intermediate transfer belt 30 at the position shown in FIG. 5, it is shown in the drawing for convenience of explanation.

  After the image forming operation is started and the developing rollers 54 of the respective colors are sequentially brought into contact with the photosensitive drums 26, the first toner patch pattern TP1 is first formed after a predetermined time (for example, about 5 seconds). First, an image forming operation of the yellow process cartridge PY is started after the predetermined time, and toner patches TY1F and TY1R are formed. The timing for forming the toner patches TY1F and TY1R shown in FIG. 5 on the intermediate transfer belt 30 is the timing of ty1F and R in FIG.

  Next, the toner patches TBk1F, TBk1R and TBk2F, TBk2R are formed by the black process cartridge PBk at positions where the toner patches TY1F, TY1R are sandwiched on the intermediate transfer belt 30 with a predetermined timing. The timings at which the toner patches TBk1F and TBk1R shown in FIG. 5 are formed on the intermediate transfer belt 30 are the timings tbk1F and R in FIG. The timing for forming the toner patches TBk2F and TBk2R on the intermediate transfer belt 30 is the timing of tbk2F and R in FIG.

  As described above, the first toner patch pattern TP1 including the toner patches TY1F, TY1R, TBk1F, TBk1R, TBk2F, and TBk2R is transferred to the intermediate transfer belt 30 with all the developing rollers 54 in contact with all the photosensitive drums 26. Formed on top.

  The toner patches TBk1F and TBk2F are formed on the belt 30 at positions shifted from the toner patch TY1F by the same distance upstream and downstream in the belt rotation direction. Similarly, toner patches TBk1R and TBk2R are also formed on the belt 30 at positions shifted upstream and downstream in the belt rotation direction by the same distance from the toner patch TY1R, respectively.

  Here, the first toner patch pattern TP1 formed on the intermediate transfer belt 30 is formed after all the developing rollers are in contact with all the photosensitive drums as shown in FIG. For this reason, the transient torque fluctuation resulting from the peripheral speed difference between the drum and the belt does not occur. That is, the intermediate transfer belt is formed with no transient speed fluctuation.

(Formation of second toner patch pattern)
Next, after the first toner patch pattern TP1 is formed, the second toner patch pattern TP2 starts to be formed after the intermediate transfer belt 30 has made almost one turn. First, toner patches TY2F and TY2R are formed by the yellow process cartridge PY. The timing for forming the toner patches TY2F, TY2R shown in FIG. 5 on the intermediate transfer belt 30 is the timing of ty2F, R in FIG. Immediately after primary transfer of the toner patches TY2F and TY2R to the intermediate transfer belt 30, the yellow developing device 51 is separated from the photosensitive drum 26.

  Next, the toner patches TBk3F, TBk3R and TBk4F, TBk4R are formed by the black process cartridge PBk at positions where the toner patches TY2F, TY2R are sandwiched on the intermediate transfer belt 30 with a predetermined timing. The timing for forming the toner patches TBk3F and TBk3R shown in FIG. 5 on the intermediate transfer belt 30 is the timing of tbk3F and R in FIG. The timing for forming the toner patches TBk4F and TBk4R on the intermediate transfer belt 30 is the timing of tbk4F and R in FIG. Immediately after primary transfer of the toner patches TBk3F, TBk3R and TBk4F, TBk4R to the intermediate transfer belt 30, the black developing device 51 is separated from the photosensitive drum 26.

  As described above, while the developing rollers 54 are sequentially separated from the photosensitive drums 26, the second toner patch pattern TP2 including the toner patches TY2F, TY2R, TBk3F, TBk3R, TBk4F, and TBk4R is intermediately transferred. It is formed on the belt 30.

  The toner patches TBk3F and TBk4F are formed on the belt 30 at positions shifted from the toner patch TY2F by the same distance upstream and downstream in the belt rotation direction. Similarly, toner patches TBk3R and TBk4R are also formed on the belt 30 at positions shifted upstream and downstream in the belt rotation direction by the same distance from the toner patch TY2R, respectively.

  Of the toner patches formed on the intermediate transfer belt 30, toner patches TBk3F, TBk3R, TBk4F, and TBk4R are formed immediately after the separation start timing of the yellow developer 51 as shown in FIG. In other words, it is formed during a transient torque fluctuation due to the difference in the peripheral speed between the drum and the belt that occurs immediately after the developer separation start timing. Therefore, the intermediate transfer belt speed when the toner patches TY2F and TY2R are formed is different from the intermediate transfer belt speed when the toner patches TBk3F, TBk3R, TBk4F, and TBk4R are formed. That is, the second toner patch pattern is formed during the transient speed fluctuation of the intermediate transfer belt.

  Therefore, the color shift amount (first shift amount) of yellow and black calculated from the toner patches TY1F, TY1R, TBk1F, TBk1R, TBk2F, and TBk2R, and the toner patches TY2F, TY2R, TBk3F, TBk3R, TBk4F, and TBk4R are calculated. The difference in the peripheral speed between the drum and the belt can be estimated from the difference between the color misregistration amounts (second misregistration amounts) of yellow and black.

  To supplement, by performing the above-described image forming operation, it can be seen in the second half of the second to third sheets among the color misregistration situations when the three A3 sheets described in FIG. 4 are continuously passed. The color misregistration state can be reproduced on the intermediate transfer belt 30, and by detecting these toner patches by the registration detection sensor 90, the peripheral speed difference between the drum and the belt can be estimated.

  Here, the interval between the process cartridges P is 95 mm, the outer diameter of each photosensitive drum 26 is 30 mm, the peripheral length of the intermediate transfer belt 30 is 957 mm, and the outer diameter of the driving roller 100 is 30.2 mm.

  The distance TL between the toner patch patterns TP1 and TP2 in FIG. 5 is set to an integer multiple of the interval 95 mm between the process cartridges and a distance 950 mm closest to the peripheral length 957 mm of the intermediate transfer belt 30. Thereby, the color shift component due to the drum cycle and the color shift component due to the belt film thickness unevenness can be canceled, and only the color shift generated due to the peripheral speed difference between the drum and the belt can be detected.

(Calculation of color misregistration amount)
Next, a method for calculating the color misregistration amount by detecting the toner patch formed on the belt by the registration detection sensor 90 will be described. In the following description, the front side of the image forming apparatus is the front side of the image forming apparatus that is the user's operation side, while the rear side of the image forming apparatus is the image forming side that is the back side when viewed from the operation side. It is the back side of the device. Each toner patch name is also given F on the front side and R on the rear side.

  The registration detection sensor 90 sequentially detects the edge of each toner patch of the toner patch pattern conveyed to the reading position, and calculates the center position detection timing of each toner patch from the detection timing.

  The center position detection timing of each toner patch is assumed to be t_toner patch name. For example, the center position detection timing of the toner patch TBK1F is t_TBK1F.

  First, a yellow and black positional shift amount (first shift amount) P_Y1 at a timing not affected by a transient torque fluctuation (belt speed fluctuation) after completion of the developer contact is calculated. Y1F is a positional shift amount on the front side of the image forming apparatus of yellow and black at a timing not affected by a transient torque fluctuation (belt speed fluctuation) after completion of the developing device contact, and a positional shift amount on the rear side. Let Y1R. In this case, in order to average the positional deviation between the front side and the rear side, P_Y1 is defined by the following equation (1).

  P_Y1 = (Y1F + Y1R) / 2 Formula (1)

  Here, Y1F and Y1R are represented by the following formulas (2) and (3), respectively.

Y1F = t_TY1F− (t_TBk1F + t_TBk2F) / 2 Formula (2)
Y1R = t_TY1R− (t_TBk1R + t_TBk2R) / 2 Formula (3)

  For this reason, the first deviation amount P_Y1 is calculated from the equations (1) to (3).

  On the other hand, the yellow and black misregistration amount (second misalignment amount) P_Y2 is calculated at the timing affected by the transient torque fluctuation (belt speed fluctuation) after the start of the developer separation. The yellow and black image forming apparatus front side positional deviation amount at the timing affected by the transient torque fluctuation (belt speed fluctuation) after the start of separation of the developing device is Y2F, and the rear side positional deviation amount is Y2R. And In this case, similarly to P_Y1, P_Y2 was defined by the following equation (4).

  P_Y2 = (Y2F + Y2R) / 2 Formula (4)

  Here, Y2F and Y2R are represented by the following formulas (5) and (6), respectively.

Y2F = t_TY2F− (t_TBk3F + t_TBk4F) / 2 Formula (5)
Y2R = t_TY2R- (t_TBk3R + t_TBk4R) / 2 Formula (6)

  For this reason, the second shift amount P_Y2 is calculated from the equations (4) to (6).

  From the above, the color misregistration amount P (m) generated due to the influence of the transient torque fluctuation (belt speed fluctuation) after completion of the contact of the developing device and after the start of separation is calculated by the following equation (7). .

  P (m) = P_Y2−P_Y1 (7)

  Here, in order to improve the accuracy of the detection result, the color misregistration amount P (m) is detected three times, and the average value is used as the final detected color misregistration amount R. Here, m is a variable representing the number of detections.

  The detection of the color misregistration here detects the color misregistration amount caused by the belt speed variation due to the sequential separation of the developing devices, but similarly occurs due to the belt velocity variation during the sequential contact of the developing devices. The amount of color misregistration may be detected.

  That is, in FIG. 4, each toner patch pattern is formed by changing the image forming sequence so as to detect the color misregistration occurring in the first half of the first sheet and the color misregistration in the second sheet. However, a similar color misregistration amount can be detected.

  Further, the length of the toner patch pattern may be further extended by the distance TL, and both color misregistration amounts generated by the influence of both contact and separation of the developing device may be detected. In this case, although the detection time increases as the toner patch pattern length increases, the S / N for color misregistration detection can be earned.

  FIG. 6 shows the amount of color misregistration actually detected by the method of calculating the amount of color misregistration in this embodiment. In FIG. 6, the vertical axis indicates the amount of color misregistration [μm] between yellow and black, and the horizontal axis indicates the amount of change [%] from the design center value with the design center value of the drum speed being 0. .

  Since the peripheral speed difference between the drum and the belt could not be easily changed experimentally, the result shown in FIG. 6 shows the relationship between the peripheral speed difference and the color shift by changing the speed of the drum drive motor. It is a thing. The effect of changing the primary transfer bias is also shown.

  As can be seen from FIG. 6, as the peripheral speed difference between the belt and the drum increases, the amount of color misregistration between yellow and black increases, and when the primary transfer bias is set higher, the amount of color misregistration also increases. However, it can be seen that the amount of color misregistration reaches its peak when the peripheral speed difference becomes too large.

  Here, since the speed fluctuation ranges in the design of the drum and the belt are about ± 0.1%, the difference in the peripheral speed actually assumed between the drum and the belt is about ± 0.2%. As shown in FIG. 6, in the range of ± 0.2% on the horizontal axis, the relationship between the drum speed change amount and the color misregistration amount is almost linear. Therefore, the peripheral speed difference between the current belt and the drum is estimated based on the detected color misregistration amount, and the peripheral surface speed of the drum for bringing the color misregistration amount close to 0 can be calculated.

  Further, although the amount of color misregistration increases when the primary transfer bias is increased (see FIG. 6), the variation in the detected color misregistration amount for each measurement does not increase even when the primary transfer bias is set high. For this reason, when detecting the amount of color misregistration, the primary transfer bias is set to be higher than that during normal image formation, thereby increasing the detection accuracy of color misregistration. Actually, since the primary transfer bias performs constant current control, the target current value is set to a current value higher than that during normal image formation (here, 25 μA), and a toner patch for detecting a color misregistration amount is used. A pattern was formed. Note that when the primary transfer bias is increased, the amount of color misregistration increases because the electrostatic adsorption force between the primary transfer roller 52 and the photosensitive drum 26 due to the application of the primary transfer bias increases, and the primary transfer nip reaches the primary transfer nip. This is thought to be because the change in friction force due to the presence or absence of toner becomes more prominent. Due to the increase in the attractive force, the load torque fluctuation is expanded. That is, since a larger change can be obtained with the same peripheral speed difference setting, even when the primary transfer bias is low, even a small color shift that cannot be detected by the registration detection sensor can be detected by increasing the primary transfer bias. .

(Drum speed correction)
FIG. 7 shows the relationship between the peripheral speed difference measured by a typical intermediate transfer belt resistance value and a primary transfer target current value (25 μA in this case) and color misregistration within the range of the actually assumed peripheral speed difference. In FIG. 7, the vertical axis represents the amount of color misregistration [μm] between yellow and black, and the horizontal axis represents the amount of change [%] from the design center value, where the design center value of the drum speed is 0. .

  Here, from the slope of the graph shown in FIG. 7, the drum speed correction coefficient D was determined to be 0.002% / μm. Although the correction coefficient D of the drum speed is set as described above from the slope of the graph shown in FIG. 7, this should be set as appropriate according to the apparatus configuration and is not limited to this. . A drum speed correction amount D is calculated by multiplying the detected color misregistration amount R by a drum speed correction coefficient D to calculate a new drum speed Vd. This is expressed by the following equation (9).

  Vd = (100 + D × R) / 100 × Vd_def (9)

  Here, Vd is the calculated photosensitive drum speed [mm / s], and Vd_def is the design center value [mm / s] of the photosensitive drum speed.

  When the image forming apparatus has a plurality of speed modes, speed correction is performed in the same manner. Here, the calculated new drum speed Vd or the correction coefficient D of the drum speed is recorded in the ROM 122 (see FIG. 2) which is a nonvolatile memory.

  If the peripheral speed difference between the drum and the belt is ± 0.2% or more, as can be seen from FIG. 6, the relationship between the peripheral speed difference and the color misregistration amount is out of the linear range. turn into. For this reason, instead of simply correcting the drum speed once, it is necessary to measure the color misregistration amount again after changing the drum speed, and further to correct the drum speed.

  Here, the drum speed correction control is automatically performed when the process cartridge P is replaced and when the new intermediate transfer belt unit is detected. If necessary, the drum speed correction control described above is executed when the drum speed or the intermediate transfer belt speed fluctuates, such as when the environment changes, when the temperature in the apparatus rises, or when the durability progresses. It is also possible to suppress the occurrence of color shift stably.

  The flow of drum speed correction control will be described with reference to the flowchart shown in FIG.

  The variables used in the flowchart shown in FIG. 8 are defined as follows. k is a retry counter when toner patch pattern detection fails (up to 3 times). m is a color misregistration detection number counter (up to 3 times). P (m) is the mth color misregistration amount calculation result [μm]. R is an average value [μm] of the amount of color misregistration detected m times. Vd is a drum speed [mm / s], and Vd_def is a design center value [mm / s] of the drum speed.

  When the process cartridge is replaced or a new intermediate transfer belt unit is detected, a drum speed correction sequence shown in the flowchart of FIG. 8 is executed.

  First, in step S111, each variable is initialized, and the drum speed Vd is set to the design center value.

  Next, the toner patch pattern described above is formed in step S 112, and the toner patch passage timing is sequentially stored in the RAM 123 by the registration detection sensor 90. That is, a first toner patch pattern composed of a plurality of toner patches is formed in a state where no transient load variation occurs, and these are detected, and further, the second toner composed of a plurality of toner patches during the transient load variation. Patch patterns are formed and detected, and stored in the RAM 123. Here, the state in which no transient load fluctuation occurs means that there is no fluctuation in contact or separation between the drum and the developing unit while two types of toner patches for detecting the amount of color misregistration are formed. It is a state. The toner patch pattern is a toner image composed of a first type of toner patch (one yellow patch) and a second type of toner patch (two black patches) formed at a position where the toner patch pattern is sandwiched. On the other hand, during a transient load change is a case where there is a change in contact or separation between the drum and the developing device between the first toner patch and the second toner patch transfer.

  In step S113, it is determined whether the edge of each toner patch of all toner patch patterns has been detected. Here, the criterion is whether or not the number of detected edges is sufficient.

  If it is determined in step S113 that a reading error has occurred, the process proceeds to step S120, and it is determined whether or not the value of the retry counter k has reached 3. If the value of the retry counter k has reached 3 in step S120, that is, if the retry is performed twice due to a reading error, the drum speed correction sequence is terminated. In this case, the drum speed Vd remains set at the design center value Vd_def. If the value of the retry count counter k has not reached 3, the retry count counter k is incremented (processing for incrementing the integer variable value by 1) in step S119, and toner patch pattern formation and detection are performed again. Done.

  If the edge of each toner patch of all toner patch patterns has been detected in step S113, the process proceeds to step S114, and the mth color misregistration detection value P (m) is calculated from the data recorded in the RAM 123. It calculates using (1)-(7). That is, the first misalignment of the two types of toner patches stored in the RAM 123 in a state where no transient load variation has occurred, and the second misalignment of the two types of toner patches during the transient load variation. Calculate the difference from the quantity.

  Next, in step S115, it is determined whether or not the absolute value of the calculated color misregistration amount P (m) is equal to or less than a predetermined value (here, 10 μm). A predetermined value (10 μm here) to be compared is set in advance. If the absolute value of the color misregistration amount P (m) is less than or equal to the predetermined value in step S115, it is determined that there is no peripheral speed difference that causes color misregistration, and the process proceeds to step S121. A process is performed in which it is not necessary to change the drum speed. If it is determined by this processing that the color misregistration is sufficiently small, the correction time is shortened by immediately interrupting the drum speed correction sequence. In addition, since the detection variation of the color misregistration amount for each time was about 30 μm experimentally, if the color misregistration amount is detected to be 10 μm or less, it is detected at the maximum even if the color misregistration detection is finished. The amount of color misregistration is estimated to be 40 μm or less, and in actual image formation, image formation is performed with a lower primary transfer bias than when color misregistration is detected. Can be suppressed. Therefore, in this embodiment, the threshold value for ending the color misregistration detection is set to 10 μm.

  When the absolute value of the color misregistration amount P (m) calculated in step S114 exceeds a predetermined value (here, 10 μm), the process proceeds to step S116, and the value of the color misregistration detection number counter m reaches 3. It is determined whether or not. If the value of the color misregistration detection number counter m has not reached 3 in step S116, the process proceeds to step S122, the color misregistration detection number counter m is incremented, and the mth color misregistration detection from step S112 again. Is executed.

  If the value of the color misregistration detection counter m reaches 3 in step S116, that is, when the detection of the color misregistration amount P (m) is completed three times, the process proceeds to step S117, and the color misregistration detected three times. The average value R of the quantity is calculated.

  Next, in step S118, a new drum speed Vd that suppresses the difference in peripheral speed between the drum and the belt is calculated by the equation (9) using the calculated color misregistration amount R, and the result is stored in the ROM 122 which is a nonvolatile memory. Record. Until the drum speed correction sequence is executed again, the image forming operation is performed at the drum speed Vd.

  By performing drum speed correction control in this way, the peripheral speed difference between the drum and the belt can be kept within a predetermined range (here, within about 0.05%). This suppresses the transient speed fluctuation of the intermediate transfer belt caused by the peripheral speed difference between the drum and the belt, and without reducing the productivity of the image forming apparatus, the leading edge of the first image and the final image It is possible to prevent color misregistration that has occurred at the rear edge of the paper image. For example, it is possible to prevent color misregistration caused by a difference in peripheral speed between the photosensitive drum and the intermediate transfer belt caused by environmental fluctuation, durability fluctuation, temperature rise in the apparatus, and the like, and stably output a high quality image.

[Second Embodiment]
In the above-described embodiment, the description has been given of performing the speed correction of the photosensitive drum 26 in order to suppress the color misregistration generated due to the peripheral speed difference between the photosensitive drum 26 and the intermediate transfer belt 30.

  In this embodiment, the fluctuation range of the steady speed of the intermediate transfer belt (for example, ± 0.2% by design) is larger than the fluctuation range of the steady speed of the photosensitive drum (for example, ± 0.1% by design). In the image forming apparatus, the speed of the intermediate transfer belt 30 is corrected. As a result, the peripheral speed difference between the photosensitive drum and the intermediate transfer belt can be suppressed, the occurrence of color misregistration can be prevented, and the recording accuracy of the output image can be improved. This will be described in detail below.

  The configuration of the image forming apparatus is the same as that of the image forming apparatus shown in FIG.

  In the image forming apparatus shown in FIG. 1, in this embodiment, a rubber layer is provided on the surface layer of the driving roller 100 in order to prevent the intermediate transfer belt 30 from slipping with respect to the driving roller 100. Here, a fixing device 25 serving as a heat source is disposed below the driving roller 100 of the intermediate transfer belt 30. Therefore, by continuously using the image forming apparatus, the driving roller 100 is gradually warmed and its outer diameter changes. When the rubber layer cannot be made very thin in manufacturing, the outer diameter change becomes an outer diameter change that cannot be ignored, and causes the rotational speed of the intermediate transfer belt 30 to change.

  As a result, the superimposed positions of the color toner images that are primarily transferred onto the intermediate transfer belt are shifted, resulting in color misregistration, and due to the difference in peripheral speed between the photosensitive drum and the intermediate transfer belt described in the first embodiment. This will also cause a color shift that degrades the image quality. In addition, the timing at which the toner image primarily transferred onto the intermediate transfer belt 30 reaches the secondary transfer counter roller 108 that is the secondary transfer position also changes. For this reason, the toner image is secondarily transferred from the original transfer position to the recording medium Q re-fed from the registration roller pair 24 at a predetermined timing. Deterioration of recording accuracy such as image writing position misalignment on the recording medium Q must be suppressed particularly from the viewpoint of image quality in an image forming apparatus that supports double-sided printing as in this embodiment.

  Here, in the image forming apparatus having a configuration in which the rotational speed fluctuation range of the intermediate transfer belt 30 is larger than that of the photosensitive drum 26 due to manufacturing tolerances, temperature changes in the apparatus main body, environmental fluctuations, and the like, the speed of the photosensitive drum is large. It is preferable to suppress the color misregistration and the deterioration of the recording accuracy by adjusting the speed of the intermediate transfer belt having a large fluctuation range to the speed of the photosensitive drum instead of adjusting the speed to the speed of the intermediate transfer belt.

  Hereinafter, the speed correction of the intermediate transfer belt 30 will be described.

  Since the color misregistration caused by the peripheral speed difference between the photosensitive drum and the intermediate transfer belt is generated by the relative speed relationship between the two, the torque fluctuation diagram of FIG. 3 shown in the first embodiment and FIG. The color misalignment behavior is the same in this embodiment. By forming a toner patch pattern as shown in FIG. 5 on the intermediate transfer belt, it is possible to similarly detect the occurrence of color misregistration. However, with respect to FIG. 7, the horizontal axis of the graph is the amount of change in the belt speed, and the slope of the graph is also reversed between positive and negative. Therefore, the flowchart for correcting the speed of the intermediate transfer belt 30 in the present embodiment is obtained by replacing the equation in step S118 in FIG. 8 with the following equation (8).

  Vb = (100−D × R) / 100 × Vb_def (8)

  Here, Vb is the calculated intermediate transfer belt speed [mm / s], and Vb_def is the design center value [mm / s] of the intermediate transfer belt speed. D is a correction coefficient for the intermediate transfer belt speed.

  In this way, by correcting the intermediate transfer belt speed, the peripheral speed difference between the photosensitive drum and the intermediate transfer belt is reduced as in the above-described embodiment, and belt speed fluctuation due to transient torque fluctuation is suppressed. The occurrence of color misregistration can be reduced. Further, since the speed of the intermediate transfer belt can be brought close to a drum speed with a small fluctuation in the steady speed, it is possible to suppress the deviation of the intermediate transfer belt speed from the design center value, and the deviation of the recording position on the recording medium (recording accuracy It was also possible to suppress the deterioration of

  The intermediate transfer belt speed is corrected, so that it is possible to considerably suppress the positional deviation of the transferred image on the recording medium, that is, the deterioration of the recording accuracy. In practice, the speed may also deviate from the design center value. Further, the peripheral length of the intermediate transfer belt may change due to the temperature rise or durability deterioration of the apparatus main body. Due to these causes, the recording accuracy is slightly shifted although the intermediate transfer belt speed is corrected.

  Therefore, in this embodiment, a means for preventing the deterioration of the recording accuracy is taken. Hereinafter, this means will be described.

  FIG. 5 is a perspective view showing the configuration of the intermediate transfer belt unit 31 used in the description of the above-described embodiment.

  The intermediate transfer belt 30 rotates at a speed Vb [mm / s] in the direction of the arrow in the figure. The design value here is Vb = 190.

  In the intermediate transfer belt 30 employed in this embodiment, belt-side regulating ribs 301 and belt reinforcing tapes 302 are attached to the both side ends one by one. Further, one position display mark 303 is attached between the belt reinforcing tape 302 and the intermediate transfer belt 30 at the position shown in the figure. Note that the detection waiting time can be reduced by providing a plurality of position display marks.

  The position display mark 303 is a white square PET sheet having a side of about 8 mm and a thickness of 50 μm. The intermediate transfer belt 30 is made of a material PI and has a thickness of about 80 μm. However, the material and thickness of these members do not limit the configuration of the present invention and can be arbitrarily set. The position indication mark may be a mark printed on the intermediate transfer belt or the belt reinforcing tape, or may be a hole provided in the intermediate transfer belt or the belt reinforcing tape.

  The mark sensor 91 is a mark detection means for detecting the upstream end face of the position display mark 303 and is a reflective optical sensor. The mark sensor 91 can detect the arrival of the position display mark 303 from the difference in the amount of diffusely reflected light from the surface of the intermediate transfer belt 30 and the position display mark 303. The mark sensor 91 is positioned so that the position display mark 303 can be detected at a position where the intermediate transfer belt 30 is wound around the tension roller 105, and can move following the movement of the shaft of the tension roller 105. So that it is supported.

  In this embodiment, one rotation cycle of the intermediate transfer belt 30 is measured in advance using the detection result of the position display mark 303. Then, using the measurement result, the timing at which the toner image primarily transferred onto the intermediate transfer belt 30 reaches the secondary transfer counter roller 108 which is the secondary transfer position is predicted, and re-feed from the registration roller pair 24 The refeed timing of the recording medium Q to be controlled is controlled.

  As described above, according to this embodiment, the occurrence of color misregistration can be prevented by the speed correction control of the intermediate transfer belt and the refeed timing control of the recording medium. Furthermore, it is possible to prevent a slight speed deviation from the design center speed of the intermediate transfer belt and a deterioration in recording accuracy due to a change in circumference. Accordingly, it is possible to realize an image forming apparatus that stably provides a high-quality image output.

  Note that the speed correction control of the intermediate transfer belt described in the present embodiment is also effective when executed when the photosensitive drum is replaced or when a new intermediate transfer belt is detected, as in the first embodiment. .

  Furthermore, when the intermediate transfer belt speed deviates from the design center value due to a change in environmental temperature or a change in temperature in the apparatus during continuous paper feeding, a temperature detection means is provided in the apparatus body or in the vicinity of the drive roller. When the temperature rise is detected, the intermediate transfer belt speed correction control may be executed.

  Furthermore, it is possible to correct the speed fluctuation due to the durability factor of the intermediate transfer belt by executing correction control of the intermediate transfer belt speed from the history of the pixel count and the number of sheets passed.

[Other Embodiments]
In the above-described embodiment, the image forming apparatus using the photosensitive drum as the image carrier and the intermediate transfer belt as the intermediate transfer member is illustrated. However, the present invention is not limited to this. For example, an image forming apparatus that employs a photosensitive belt as an image carrier and an intermediate transfer drum as an intermediate transfer member may be used. In this case, it is possible to correct the speed of the photosensitive belt by a similar speed correction sequence.

  In the above-described embodiment, the configuration in which four image forming units having different colors are used as the plurality of image forming units is exemplified. However, the number used is not limited, and may be appropriately set as necessary. good.

  Further, in the embodiment described above, paying attention to the noticeable color misregistration, the plurality of toner patches forming one patch pattern are connected to the most upstream image forming unit and the most downstream in the rotation direction of the intermediate transfer belt. The configuration formed by the image forming unit on the side is illustrated. However, the present invention is not limited to this. For example, among the four image forming units described above, a first image forming unit such as yellow and cyan, and a second image forming unit downstream of the first image forming unit in the rotation direction of the belt are used. Any configuration that forms one patch pattern composed of a plurality of toner patches may be used.

  In the above-described embodiment, as a process cartridge that is detachable from the main body of the image forming apparatus, a process that integrally includes a photosensitive drum and a charging unit, a developing unit, and a cleaning unit as a process unit that acts on the photosensitive drum. A cartridge was illustrated. However, the process cartridge is not limited to this. In addition to the photosensitive drum, a process cartridge that integrally includes any one of a charging unit, a developing unit, and a cleaning unit may be used.

  In the above-described embodiment, the printer is exemplified as the image forming apparatus, but the present invention is not limited to this. For example, the image forming apparatus may be another image forming apparatus such as a copying machine or a facsimile machine, or another image forming apparatus such as a multi-function machine combining these functions. Alternatively, an image forming apparatus that uses a recording medium conveyance body and sequentially superimposes and transfers the toner images of the respective colors onto the recording medium carried on the recording medium conveyance body may be used. The same effect can be obtained by applying the present invention to these image forming apparatuses. Further, in the above-described embodiment, the mode in which the color misregistration is suppressed by suppressing the peripheral speed difference between the drum and the belt has been described, but by using the present invention, the control target value is set to the peripheral speed difference of 0%. For example, by setting the peripheral speed difference between the drum and the belt to a desired value (for example, the peripheral speed of the belt is 0.2% faster than the peripheral speed of the drum), a trade-off between color misregistration and transfer efficiency is achieved. It is also possible to pursue speed settings aimed at achieving both performances.

PY, PM, PC, PBk ... process cartridge Q ... recording medium TBk1F, TBk1R ... toner patch TBk2F, TBk2R ... toner patch TBk3F, TBk3R ... toner patch TBk4F, TBk4R ... toner patch TL ... distance TP1, TP2 ... toner patch pattern TY1F, TY1R ... toner patches TY2F, TY2R ... toner patch 1 ... image forming apparatus 2 ... image forming apparatus main body 10 ... host device 11 ... image processing control section 12 ... image forming control section 13 ... image forming section 14 ... belt drive motor 15 ... drum Drive motor 16 ... registration detection sensor unit 17 ... mark sensor units 26Y, 26M, 26C, 26Bk ... photosensitive drum 27 ... secondary transfer rollers 28Y, 28M, 28C, 28Bk ... laser exposure device 30 ... intermediate transfer belt 31 ... intermediate transfer Copy belt unit 50 ... primary charger 51 ... developer 52 ... primary transfer roller 53 ... cleaner 54 ... developing roller 90 ... registration sensor 91 ... mark sensor 100 ... drive roller 105 ... tension roller 108 ... secondary transfer counter roller 121 ... CPU
122… ROM
123 ... RAM
301 ... Deviation regulating rib 302 ... Belt reinforcing tape 303 ... Position indication mark

Claims (14)

A first image carrier for carrying a toner image;
A first developing unit that is capable of contacting and separating from the first image carrier and forming a toner image on the first image carrier;
A second image carrier for carrying a toner image;
A second developing unit that is capable of contacting and separating from the second image carrier and forming a toner image on the second image carrier;
A transfer portion is formed in contact with the first and second image carriers, and the toner images formed on the first and second image carriers are transferred, or the toner images are transferred. A rotating body for conveying a recording medium;
Patch detecting means for detecting a toner patch transferred from the first and second image carriers to the rotating body;
In the image forming apparatus and a control means for controlling the relative speed of the rotating body and the first, the second image bearing member,
The image forming apparatus transfers the first image carrier to the rotating member in a state where the first and second developing units are in contact with the first and second image carriers, respectively. A first toner patch pattern including a toner patch to be transferred and a toner patch transferred from the second image carrier to the rotating body;
A toner patch transferred from the first image carrier to the rotating body in a state where the first and second developing means are in contact with the first and second image carriers, respectively. The first developing means is separated from the first image carrier and is transferred from the second image carrier to the rotating body with the second developing means in contact with the second image carrier. A second toner patch pattern including a toner patch to be formed on the rotating body,
The control unit is configured to control the rotating body and the first and second image carriers in accordance with detection results of the toner patches of the first toner patch pattern and the second toner patch pattern by the patch detection unit. An image forming apparatus for controlling a relative speed of a body . And a first drive unit that rotates and moves the first image carrier and the second image carrier, and a second drive unit that drives the rotary body. The image forming apparatus according to claim 1, wherein the first driving unit and the second driving unit are controlled . The control unit transfers the toner patch transferred from the first image carrier and the second image carrier in the first toner patch pattern from the detection result of each toner patch of the patch detection unit. A first deviation amount that is a deviation amount of the toner patch, and a toner patch transferred from the first image carrier and a toner transferred from the second image carrier in the second toner patch pattern. The image forming apparatus according to claim 1 , wherein a second shift amount that is a shift amount of the patch is calculated . When the difference between the first deviation amount and the second deviation amount exceeds a predetermined value, the control means determines that the difference between the first deviation amount and the second deviation amount is the predetermined value. The image forming apparatus according to claim 3 , wherein a relative speed between the rotating body and the first and second image carriers is controlled so as to be as follows.   The transfer bias for transferring the toner patch from the image carrier to the rotating body is more than the transfer bias for transferring a toner image from the image carrier to the rotating body when forming an image on a recording medium. The image forming apparatus according to claim 1, wherein the image forming apparatus is set high. 6. The first developing unit is separated from the first image carrier before the second developing unit is separated from the second image carrier. The image forming apparatus according to claim 1. The image forming apparatus according to claim 6 , wherein the first image carrier is disposed upstream of the second image carrier in the moving direction of the rotating body . Has a mark detecting means for detecting the position display mark provided on the rotating body, any one of claims 1 to 7, characterized in that you predict one rotation cycle of the rotating body from the detection result The image forming apparatus described in the item. A first image carrier that carries a toner image, a second image carrier that carries a toner image, and the first and second image carriers are in contact with each other to form a transfer portion. A rotating body for transferring a toner image formed on the second image carrier or conveying a recording medium on which the toner image is transferred; and the first and second image carriers to the rotating body. Patch detecting means for detecting the transferred toner patch; and control means for controlling the peripheral surface speed of the rotating body or the first and second image carriers based on the detection result of the patch detecting means; In an image forming apparatus having
The patch detection means transfers the first image bearing member to the rotating member in a state where toner is interposed in the transfer portions formed by the rotating member and the first and second image bearing members. To the transfer portion formed by the rotating body and the second image carrier without toner intervening in the transfer portion formed by the first toner patch and the rotating body and the first image carrier. Detecting a second toner patch transferred from the second image carrier to the rotating body in a state where toner is interposed, and the control means detects the rotating body and the rotating body according to a detection result of the patch detecting means; An image forming apparatus for controlling a relative speed of the first and second image carriers. And a first drive unit that rotates and moves the first image carrier and the second image carrier, and a second drive unit that drives the rotary body. The image forming apparatus according to claim 9, wherein the first driving unit and the second driving unit are controlled. The control unit calculates a shift amount between the first toner patch and the second toner patch from a detection result of the patch detection unit, and according to the shift amount, the rotating body, the first, and the first 11. The image forming apparatus according to claim 9, wherein the relative speed of the two image carriers is controlled. First developing means that can contact and separate from the first image carrier and forms a toner image on the first image carrier, and can contact and separate from the second image carrier. And a second developing unit that forms a toner image on the second image carrier, and the first developing unit comes into contact with the first image carrier and thereby the first development. The toner adheres to the first image carrier from the means, and the toner attached to the first image carrier reaches the transfer unit by the rotation of the first image carrier. The image forming apparatus according to claim 9, wherein the toner is in an intervening state. The transfer bias for transferring the toner patch from the image carrier to the rotating body is more than the transfer bias for transferring a toner image from the image carrier to the rotating body when forming an image on a recording medium. The image forming apparatus according to claim 8, wherein the image forming apparatus is set high. 14. The method according to claim 8, further comprising mark detection means for detecting a position indication mark provided on the rotating body, and predicting one rotation period of the rotating body from the detection result. The image forming apparatus according to claim 1.

Images