JP5594590B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus for transferring a visible image carried on each of a plurality of image carriers on a surface of an endless belt member or a recording member held on the surface.

  In this type of image forming apparatus, if the transfer positions from the respective image carriers to the belt member (or the recording member on the belt) are irregularly shifted, an overlay shift is caused. The main cause of the misalignment is the eccentricity of the driving roller that moves the belt member as the belt member is rotated while being stretched inside the loop of the belt member. Specifically, when the drive roller is eccentric, a speed fluctuation with a characteristic of drawing a sine curve for one cycle per one rotation of the roller occurs in the belt member. Due to this speed fluctuation, the transfer positions of the visible images from the respective image carriers to the belt member (or the recording member on the belt) are shifted from each other, thereby causing an overlay shift.

  Therefore, in the image forming apparatus described in Patent Document 1, the intermediate transfer belt as a belt member is moved at a target speed as follows. That is, this image forming apparatus includes four photoconductors for individually forming yellow (Y), magenta (M), cyan (C), and black (K) toner images as a plurality of image carriers. I have. Further, an intermediate transfer belt as a belt member to which the toner images of the respective colors are transferred in a superimposed manner, a speed detection means for detecting the moving speed of the intermediate transfer belt, and the like are also provided. Then, based on the result of detecting the speed fluctuation of the belt by the speed detecting means, the speed fluctuation of the intermediate transfer belt is adjusted by finely adjusting the driving speed of the belt driving motor which is the driving source of the intermediate transfer belt and the driving roller. Is supposed to suppress.

  In the configuration in which the speed fluctuation of the intermediate transfer belt is suppressed in this way, the present inventors use a belt drive motor as a drive source for any one of the four photosensitive members from the viewpoint of cost reduction. We are considering sharing. More specifically, we are considering sharing a belt drive motor as a drive source for a K photoconductor that drives in either a monochrome mode for forming a monochrome image or a color mode for forming a color image. . However, in such a configuration, if the driving speed of the belt driving motor is finely adjusted to follow the speed fluctuation of the intermediate transfer belt, the driving speed of the K photoconductor is changed to change the linear speed of the K photoconductor. I will let you. This change causes a transfer position shift of the K toner image, and causes a shift in overlay between the K toner image and the other color toner images.

  The present invention has been made in view of the above background, and an object thereof is to provide the following image forming apparatus. That is, while reducing the cost by sharing the belt drive motor as a drive source for any one image carrier, a visible image formed by the image carrier and a visible image formed on another image carrier Is an image forming apparatus that can reduce misalignment with the image forming apparatus.

In order to achieve the above-mentioned object, the invention of claim 1 is one of a plurality of stretching members while an endless belt member is stretched by a plurality of stretching members disposed in the loop. A belt unit that moves endlessly in accordance with the rotational drive of the drive rotator, a belt drive motor that moves the belt member endlessly by exerting a driving force for rotationally driving the drive rotator, and movement of the belt member A belt speed detecting means for detecting the speed, a plurality of image carriers that carry a visible image on its endlessly moving surface, and a toner image carried on the surface of each image carrier on the surface of the belt member or The belt member is moved endlessly at a predetermined target belt speed on the basis of the detection result of the transfer means for transferring and superimposing on the recording member held on the surface and the belt speed detection means. In the image forming apparatus and a fine adjustment to the control unit the driving speed of the drive motor so, among the plurality of image bearing members, as a drive source for what Re one of the image bearing member, sharing the belt drive motor The other image carrier is driven by an image carrier drive motor different from the belt drive motor, and the arrangement pitch along the moving direction of the belt member in the plurality of image carriers is set to the drive rotator. An image detection means for detecting a visible image transferred to the surface of the belt member is provided, and the belt drive motor and the image are set based on a detection result by the belt speed detection means. The control means is configured to perform the same speed fine adjustment process with respect to the carrier driving motor, and each image carrier is assigned to each image carrier based on pre-stored image information. Formed predetermined After the reference visible images are transferred to different positions on the surface of the belt member, the reference visible images are detected based on the timing difference detected by the image detection means on the belt member. A relative position shift amount of the first image, and based on the grasp result, a visible image on another image carrier relative to a visible image on any one of the plurality of image carriers. Is set to a value that can keep the relative positional deviation amount to ½ dot or less, and the visible image formation start timing with respect to the other image carrier is set, and the relative that remains at the visible image formation start timing after setting is set. And a linear velocity difference based on the positional deviation amount is provided in the image carrier driven by the belt driving motor and the image carrier driven by the image carrier driving motor. As above A process for individually setting the target drive speed of the belt drive motor and the target drive speed of the image carrier drive motor is performed each time a predetermined timing arrives, and an image based on an instruction command from the operator is formed. In this case, the belt driving motor and the image carrier driving motor are each subjected to processing for finely adjusting the driving speed based on the detection result by the belt speed detecting means with reference to individual target driving speeds. As described above, the control means is configured .

In these inventions, the belt drive motor can be shared as a drive source for at least one of the image carriers, thereby reducing costs.
Further, by reducing the belt member speed fluctuation by finely adjusting the belt drive motor driving speed in accordance with the belt member speed fluctuation, the visual image overlay deviation due to the belt member speed fluctuation is reduced. . At this time, fine adjustment of the driving speed is performed on the image carrier driving motor in the same manner as the belt driving motor. As a result, the same speed fluctuations are generated for all the image carriers. These speed fluctuations are mainly due to the eccentricity of the drive rotator that drives the belt member, and therefore each has a characteristic of drawing a sine curve for one cycle per rotation of the drive rotator. Moreover, the phases of the respective speed fluctuations are synchronized with each other. For example, when the speed variation of a certain image carrier reaches a peak on the sine curve, the speed variation of another image carrier also reaches the peak on the sine curve. At this time, the visible image transferred from one of the two adjacent image carriers to the belt member (or the recording member on the belt member) is then transferred to the other image carrier as the belt member moves. Enter the opposite position (transfer position). The time required to enter the transfer position facing the latter image carrier after passing through the transfer position facing the former image carrier is an integral multiple of the period of the sine curve described above. This is because the arrangement pitch of the image carriers is set to an integral multiple of the circumferential length of the drive rotator. For this reason, even when the aforementioned visible image enters the latter transfer position, the speed fluctuation of the image carrier reaches the peak on the sine curve. In this way, the visible images transferred to the belt member at the peak of the sine curve are synchronized at each transfer position. Since these visible images have the same positional shift amount due to the speed fluctuation of the image carrier, there is almost no overlay shift. Similarly, the visible images transferred onto the belt member when the valley side peak of the sine curve is approaching are synchronized with each other, or when the peak-to-peak center of the sine curve is reaching the center of the belt member. The transferred visible images are synchronized with each other. That is, visible images that are displaced by the same displacement amount are synchronized at each transfer position, so that there is almost no occurrence of overlay displacement. Therefore, it is possible to reduce the misalignment between the visible image formed by the image carrier driven by the driving force of the belt drive motor and the visible image formed by another image carrier.

1 is a schematic configuration diagram showing a printer according to a reference form. FIG. 3 is an enlarged configuration diagram illustrating a process unit for Y of the printer. FIG. 3 is an enlarged configuration diagram illustrating four photoconductor gears in the printer. FIG. 4 is an enlarged configuration diagram showing four photoconductor gears from the opposite side to FIG. 3. FIG. 2 is a block diagram showing a part of an electric circuit of the printer. FIG. 3 is a schematic diagram for explaining the arrangement pitch of photosensitive members of the printer. FIG. 2 is a schematic diagram illustrating a transfer unit, photoconductors of each color, and gears in the printer. FIG. 2 is a schematic diagram illustrating a drive control unit in the printer and various devices electrically connected thereto. 6 is a graph showing a change with time in a speed fluctuation amount of a photoreceptor. The enlarged schematic diagram for demonstrating the relationship between an optical writing position and a nip center position. 6 is a graph showing the relationship between the number of continuous output sheets and the positional deviation amount of a toner image. FIG. 3 is an enlarged perspective view showing a part of an intermediate transfer belt of the printer according to the embodiment together with an optical sensor unit. FIG. 3 is an enlarged schematic view showing a chevron patch formed on the intermediate transfer belt. FIG. 3 is a schematic diagram showing a relative positional relationship of optical scanning start positions in the sub scanning direction with respect to Y, C, M, and K photoconductors of the printer. FIG. 4 is a diagram illustrating a relationship between positional deviation amounts of respective color toner images in the printer. FIG. 6 is a diagram showing a relationship between positional deviation amounts of color toner images when the phase of the maximum eccentric portion of each color photoconductor gear is not matched based on the primary transfer nip of each color. FIG. 6 is a diagram illustrating a positional shift relationship of each color toner image when the arrangement pitch L of each photoconductor is not set to an integral multiple of the circumference of the drive roller.

Before describing an embodiment of an electrophotographic printer (hereinafter simply referred to as a printer) as an image forming apparatus to which the present invention is applied, a printer according to a reference embodiment will be described .
First, the basic configuration of the printer according to the reference embodiment will be described. FIG. 1 is a schematic configuration diagram illustrating a printer according to a reference embodiment. In this figure, this printer includes four process units 6Y, 6C, 6M, and 6K for forming yellow, cyan, magenta, and black (hereinafter referred to as Y, C, M, and K) toner images. Yes. These use Y, C, M, and K toners of different colors as image forming materials, but the other configurations are the same and are replaced when the lifetime is reached. Taking a process unit 6Y for generating a Y toner image as an example, as shown in FIG. 2, a drum-shaped photosensitive member 1Y as an image carrier, a drum cleaning device 2Y, a charge eliminating device (not shown), a charging device 4Y, A developing unit 5Y and the like are provided. The process unit 6Y can be attached to and detached from the printer body, so that consumable parts can be replaced at a time.

  The charging device 4Y uniformly charges the surface of the photoreceptor 1Y that is driven to rotate clockwise in the drawing by a driving unit (not shown). The uniformly charged surface of the photoreceptor 1 </ b> Y is exposed and scanned by the laser beam L to carry a Y electrostatic latent image. The electrostatic latent image of Y is developed into a Y toner image by a developing device 5Y using a Y developer containing Y toner and a magnetic carrier. Then, intermediate transfer is performed on an intermediate transfer belt 8 as a belt member described later. The drum cleaning device 2Y removes the toner remaining on the surface of the photoreceptor 1Y after the intermediate transfer process. The static eliminator neutralizes residual charges on the photoreceptor 1Y after cleaning. By this charge removal, the surface of the photoreceptor 1Y is initialized and prepared for the next image formation. Similarly, in the other color process units (6C, M, K), (C, M, K) toner images are formed on the photoreceptors (1C, M, K), and the intermediate transfer belt 8 is subjected to an intermediate process. Transcribed.

  The developing device 5Y has a developing roll 51Y disposed so as to be partially exposed from the opening of the casing. Further, it also includes two conveying screws 55Y, a doctor blade 52Y, a toner density sensor (hereinafter referred to as T sensor) 56Y, and the like that are arranged in parallel to each other.

  In the casing of the developing device 5Y, a Y developer (not shown) including a magnetic carrier and Y toner is accommodated. The Y developer is frictionally charged while being agitated and conveyed by the two conveying screws 55Y, and then carried on the surface of the developing roll 51Y. Then, after the layer thickness is regulated by the doctor blade 52Y, the layer is transported to the developing region facing the Y photoreceptor 1Y, where Y toner is attached to the electrostatic latent image on the photoreceptor 1Y. This adhesion forms a Y toner image on the photoreceptor 1Y. In the developing unit 5Y, the Y developer that has consumed Y toner by the development is returned into the casing as the developing roll 51Y rotates.

  A partition wall is provided between the two transport screws 55Y. By this partition wall, the first supply unit 53Y that accommodates the developing roll 51Y, the right conveyance screw 55Y in the drawing, and the like, and the second supply unit 54Y that accommodates the left conveyance screw 55Y in the drawing are separated in the casing. . The right conveying screw 55Y in the drawing is driven to rotate by a driving means (not shown), and supplies the Y developer in the first supply unit 53Y to the developing roll 51Y while being conveyed from the near side to the far side in the drawing. The Y developer conveyed to the vicinity of the end of the first supply unit 53Y by the right conveyance screw 55Y in the drawing enters the second supply unit 54Y through an opening (not shown) provided in the partition wall. In the second supply unit 54Y, the left conveyance screw 55Y in the drawing is driven to rotate by a driving means (not shown), and the Y developer sent from the first supply unit 53Y is the right conveyance screw 55Y in the drawing. Transport in the reverse direction. The Y developer transported to the vicinity of the end of the second supply unit 54Y by the transport screw 55Y on the left side in the drawing passes through the other opening (not shown) provided in the partition wall, and the first supply unit. Return to 53Y.

  The T sensor 56Y including a magnetic permeability sensor is provided on the bottom wall of the second supply unit 54Y, and outputs a voltage having a value corresponding to the magnetic permeability of the Y developer passing therethrough. Since the magnetic permeability of the two-component developer containing toner and magnetic carrier shows a good correlation with the toner concentration, the T sensor 56Y outputs a voltage corresponding to the Y toner concentration. This output voltage value is sent to a control unit (not shown). This control unit includes a RAM that stores a Vtref for Y that is a target value of an output voltage from the T sensor 56Y. The RAM also stores data for C Vtref, M Vtref, and K Vtref, which are target values of output voltages from a T sensor (not shown) mounted in another developing device. The Y Vtref is used for driving control of a Y toner conveying device to be described later. Specifically, the control unit drives and controls a Y toner conveying device (not shown) so that the value of the output voltage from the T sensor 56Y is close to the Y Vtref, and the Y toner in the second supply unit 54Y. To replenish. By this replenishment, the Y toner concentration in the Y developer in the developing device 5Y is maintained within a predetermined range. The same toner replenishment control using the C, M, and K toner conveying devices is performed for the developing units of other process units.

  In FIG. 1 shown above, an optical writing unit 7 as optical writing means is disposed below the process units 6Y, 6C, 6M, and 6K in the drawing. The optical writing unit 7 irradiates the respective photosensitive members in the process units 6Y, 6C, 6M, and 6K with the laser light L emitted based on the image information and exposes it. By this exposure, electrostatic latent images for Y, C, M, and K are formed on the photoreceptors 1Y, 1C, 1M, and 1K. The optical writing unit 7 irradiates the photosensitive member through a plurality of optical lenses and mirrors while scanning the laser light L emitted from the light source with a polygon mirror rotated by a motor.

  On the lower side of the optical writing unit 7 in the figure, a paper storage means having a paper storage cassette 26, a paper feed roller 27 incorporated therein, and the like is disposed. The paper storage cassette 26 stores a plurality of recording sheets P, which are sheet-like recording bodies, and a sheet feeding roller 27 is brought into contact with each uppermost recording sheet P. When the paper feeding roller 27 is rotated counterclockwise in the drawing by a driving means (not shown), the uppermost recording sheet P is sent out toward the paper feeding path 70.

  A registration roller pair 28 is disposed near the end of the paper feed path 70. The registration roller pair 28 rotates both rollers so as to sandwich the recording sheet P. However, the registration roller pair 28 temporarily stops the rotation as soon as it is sandwiched. Then, the recording sheet P is sent out toward a later-described secondary transfer nip at an appropriate timing.

  Above the process units 6Y, 6C, 6M, and 6K, a transfer unit 15 that moves the intermediate transfer belt 8 endlessly while being stretched is disposed. The transfer unit 15 as a transfer unit includes a secondary transfer bias roller 19 and a belt cleaning device 10 in addition to the intermediate transfer belt 8. Also provided are four primary transfer bias rollers 9Y, 9C, 9M, 9K, a driving roller 12, a cleaning backup roller 13, a driven roller 14, a tension roller 11, and the like. The intermediate transfer belt 8 is endlessly moved counterclockwise in the figure by the rotational drive of the driving roller 12 while being stretched around these rollers. The primary transfer bias rollers 9Y, 9C, 9M, and 9K hold the intermediate transfer belt 8 that is moved endlessly in this manner between the photoreceptors 1Y, 1C, 1M, and 1K to form primary transfer nips. Yes. In these methods, a transfer bias having a polarity opposite to that of toner (for example, plus) is applied to the back surface (loop inner peripheral surface) of the intermediate transfer belt 8. All the rollers except the primary transfer bias rollers 9Y, C, M, and K are electrically grounded. The intermediate transfer belt 8 that is an endless moving body sequentially passes through the primary transfer nips for Y, C, M, and K along with the endless movement, and Y on the photoreceptors 1Y, 1C, 1M, and 1K. , C, M, K toner images are superimposed and primarily transferred. As a result, a four-color superimposed toner image (hereinafter referred to as a four-color toner image) is formed on the intermediate transfer belt 8.

  A drive roller 12 as a drive rotator sandwiches the intermediate transfer belt 8 with the secondary transfer roller 19 to form a secondary transfer nip. The visible four-color toner image formed on the intermediate transfer belt 8 is transferred to the recording sheet P at the secondary transfer nip. Then, combined with the white color of the recording sheet P, a full color toner image is obtained. Untransferred toner that has not been transferred to the recording sheet P adheres to the intermediate transfer belt 8 after passing through the secondary transfer nip. This is cleaned by the belt cleaning device 10. The recording sheet P on which the four-color toner images are secondarily transferred at the secondary transfer nip is sent to the fixing device 20 via the post-transfer conveyance path 71.

  The fixing device 20 forms a fixing nip by a fixing roller 20a having a heat source such as a halogen lamp inside, and a pressure roller 20b that rotates while contacting the roller with a predetermined pressure. The recording sheet P fed into the fixing device 20 is sandwiched between the fixing nips so that the unfixed toner image carrying surface is in close contact with the fixing roller 20a. Then, the toner in the toner image is softened by the influence of heating and pressurization, and the full color image is fixed.

  The recording sheet P on which the full-color image is fixed in the fixing device 20 exits the fixing device 20 and then reaches a branch point between the paper discharge path 72 and the pre-reversal conveyance path 73. A first switching claw 75 is swingably disposed at this branch point, and the path of the recording sheet P is switched by the swing. Specifically, the path of the recording sheet P is set to the direction toward the paper discharge path 72 by moving the tip of the claw in a direction to approach the pre-reverse feed path 73. Further, by moving the tip of the claw away from the pre-reversing conveyance path 73, the path of the recording sheet P is changed to the direction toward the pre-reversing conveyance path 73.

  When the path to the paper discharge path 72 is selected by the first switching claw 75, the recording sheet P is disposed outside the apparatus after passing through the paper discharge roller pair 100 from the paper discharge path 72. Are stacked on a stack 50a provided on the upper surface of the printer housing. On the other hand, when the first switching claw 75 selects the course toward the conveyance path 73 before reversal, the recording sheet P enters the nip of the reversing roller pair 21 via the conveyance path 73 before reversal. The reversing roller pair 21 conveys the recording sheet P sandwiched between the rollers toward the stack portion 50a, and reversely rotates the roller immediately before the trailing end of the recording sheet P enters the nip. Due to this reverse rotation, the recording sheet P is conveyed in the opposite direction, and the rear end side of the recording sheet P enters the reverse conveying path 74.

  The reverse conveyance path 74 has a shape extending while curving from the upper side to the lower side in the vertical direction, and the first reverse conveyance roller pair 22, the second reverse conveyance roller pair 23, and the third reverse conveyance in the path. A roller pair 24 is provided. The recording sheet P is conveyed while sequentially passing through the nips of these roller pairs, so that the recording sheet P is turned upside down. The recording sheet P that has been turned upside down is returned to the paper feed path 70 and then reaches the secondary transfer nip again. Then, this time, the image transfer surface enters the secondary transfer nip while bringing the non-image carrying surface into close contact with the intermediate transfer belt 8, and the second four-color toner image of the intermediate transfer belt is collectively transferred to the non-image carrying surface. The Thereafter, the sheet is stacked on the stack unit 50a outside the apparatus via the post-transfer conveyance path 71, the fixing device 20, the paper discharge path 72, and the paper discharge roller pair 100. A full color image is formed on both sides of the recording sheet P by such reverse conveyance.

  A bottle support portion 31 is disposed between the transfer unit 15 and the stack portion 50a located above the transfer unit 15. The bottle support portion 31 is equipped with toner bottles 32Y, 32C, 32M, and 32K serving as toner storage portions for storing Y, C, M, and K toners. The toner bottles 32Y, 32C, 32M, and 32K are arranged so as to be arranged at an angle slightly inclined from the horizontal, and the arrangement positions are higher in the order of Y, C, M, and K. The Y, C, M, and K toners in the toner bottles 32Y, 32C, 32M, and 32K are appropriately replenished to the developing units of the process units 6Y, 6C, 6M, and 6K, respectively, by a toner conveyance device described later. These toner bottles 32Y, 32C, 32M, and 32K are detachable from the printer main body independently of the process units 6Y, 6C, 6M, and 6K.

  In this printer, the contact state between the photosensitive member and the intermediate transfer belt 8 is different between a monochrome mode for forming a monochrome image and a color mode for forming a color image. Specifically, among the four primary transfer bias rollers 9Y, 9C, 9M, and 9K in the transfer unit 15, the K primary transfer bias roller 9K is illustrated separately from the other primary transfer bias rollers. Not supported by a dedicated bracket. The three primary transfer bias rollers 9Y, 9C, 9M for Y, C, M are supported by a common moving bracket (not shown). The moving bracket can be moved in a direction approaching the Y, C, M photoconductors 1Y, C, M and a direction away from the photoconductors 1Y, C, M by driving a solenoid (not shown). is there. When the moving bracket is moved away from the photoreceptors 1Y, 1C, 1M, the tension posture of the intermediate transfer belt 8 changes, and the intermediate transfer belt 8 is moved to the three photoreceptors 1Y, 1C for Y, C, and M. , M. However, the K photoconductor 1K and the intermediate transfer belt 8 remain in contact with each other. In the monochrome mode, the image forming operation is performed with only the K photoconductor 1K in contact with the intermediate transfer belt 8 as described above. At this time, among the four photosensitive members, only the K photosensitive member 1K is rotationally driven, and the driving of the Y, C, and M photosensitive members 1Y, C, and M is stopped.

  When the moving bracket described above is moved in a direction approaching the three photoconductors 1Y, 1C, 1M, the tension posture of the intermediate transfer belt 8 changes and has been separated from the 3 photoconductors 1Y, 1C, 1M until then. Further, the intermediate transfer belt 8 comes into contact with the three photoreceptors 1Y, 1C, 1M. At this time, the photoconductor 1K for K and the intermediate transfer belt 8 remain in contact with each other. In the color mode, the image forming operation is performed in a state where all of the four photoconductors 1Y, 1C, 1M, and 1K are in contact with the intermediate transfer belt 8 in this way. In such a configuration, the moving bracket, the solenoid described above, and the like function as contact / separation means for contacting / separating the photosensitive member and the intermediate transfer belt 8.

  FIG. 3 is an enlarged configuration diagram showing four photoconductor gears. In the figure, photoconductors 1Y, 1C, 1M, and 1K are supported by bearings (not shown) so as to be rotatable around rotation shafts 201Y, C, M, and K provided at the rotation centers thereof. Photoconductor gears 202Y, C, M, and K having a diameter much larger than that of the photoconductors 1Y, 1C, 1M, and 1K are fixed to one end portions of the rotary shafts 201Y, C, M, and K. A K driving gear 95 fixed to the motor shaft of the K photoconductor motor 90K is engaged with the K photoconductor gear 202K. The photoconductor 1K for K is rotated by the rotation driving force of the K photoconductor motor 90K transmitted by this meshing. On the other hand, a color driving gear 96 is disposed between the M photoconductor gear 202M and the C photoconductor gear 202C so as to mesh with these photoconductor gears. The color driving gear 96 is fixed to the motor shaft of the color photoconductor motor 90YCM, and transmits the driving force of the color photoconductor motor 90YCM to the M photoconductor gear 202M and the C photoconductor gear 202C. . Accordingly, the M photoconductor 1M and the C photoconductor 1C are driven to rotate. An idler gear 97 is disposed between the Y photoconductor gear 202Y and the M photoconductor gear 202M so as to mesh with these photoconductor gears. As a result, the driving force of the color photoconductor motor 90YCM is transmitted to the Y photoconductor 1Y through the color driving gear 96, the M photoconductor gear 202M, the idler gear 97, and the Y photoconductor gear 202Y sequentially. The

  With this configuration, the three photoconductors 1Y, 1C, and M other than K are rotationally driven by one color photoconductor motor 90YCM. The photoconductor gears 202Y, C, and M respectively corresponding to the three photoconductors 1Y, 1C, and 1M are primary transfer nips for Y, C, and M, and the toner image position shift caused by the eccentricity of the photoconductor gear is detected. The gear meshing position is adjusted during assembly so as to synchronize with each other at the primary transfer nips for Y, C, and M. Specifically, each of the photoconductor gears 202Y, 202C, 202M has a slight eccentricity due to the limit of processing accuracy. Due to this eccentricity, each of the photoreceptors 1Y, 1C, and 1M causes slight linear velocity fluctuations in a characteristic that draws a sine curve for one cycle per rotation. The toner image that is primarily transferred from the photosensitive member to the intermediate transfer belt 8 when the peak of the sine curve reaches its peak has the largest amount of positional deviation in the belt movement direction in one cycle. As described above, when the toner image shifted to the maximum in the belt moving direction enters the adjacent primary transfer nip as the intermediate transfer belt 8 moves, the linear speed fluctuation amount of the adjacent photoconductor is a sine curve. Try to reach the peak of the mountain side. As a result, the toner images that are displaced to the maximum in the belt movement direction overlap each other, so that the overlay deviation hardly occurs. The example in which the toner image transferred when the peak side of the sine curve is approaching is synchronized with the primary transfer nip for Y, C, and M has been described. The toner image transferred when greeted and the toner image transferred when greeted at the center of the peak-to-peak are also synchronized at each primary transfer nip. In this way, the meshing of the gears is adjusted so that the toner images having the same positional deviation amount are superimposed on each other in the primary transfer nips for Y, C, and M.

  The K photoconductor 1K is rotationally driven by a K photoconductor motor 90K which is a drive source different from other photoconductors. The reason why the driving source is different for the K photoconductor 1K is that the demand for monochrome printing is higher than that for color printing. At the time of high-demand monochrome printing, by driving only the K photoconductor 1K, the consumption of other photoconductors 1Y, 1C, 1M, and motors can be suppressed, and energy can be saved. is there. In monochrome printing, only the K photoconductor 1K is driven in this way. At this time, the transfer unit 15 shown in FIG. 1 is for K of the four photoconductors 1Y, 1C, 1M, and 1K. The intermediate transfer belt 8 is brought into contact with only the photosensitive member 1K.

  In monochrome printing, only the K photoconductor 1K is rotationally driven in this manner, and therefore the phase of the maximum eccentricity between the K photoconductor gear 202K and the other photoconductor gears 202Y, 202C, and 202M. The relationship is always different. Therefore, in this printer, at the start of the printing operation, phase alignment control is performed to adjust the rotational phase between the K photoconductor gear 202K and the other photoconductor gears 202Y, 202C, and 202M to have a predetermined relationship. It is like that.

  FIG. 4 is an enlarged configuration diagram showing the four photoconductor gears from the opposite side to FIG. In the drawing, a K rotating disk 203K is fixed to the end of the rotating shaft 201K of the K photosensitive member 1K opposite to the photosensitive member gear 202K. The K rotating disk 203K is integrally formed with a partly larger diameter, and this is a K gear sensor 91K composed of a transmissive photosensor when the K photoconductor gear 202K is at a predetermined rotational position. Detected by.

  On the other hand, a color rotating disk 203YCM is fixed to the end of the rotating shaft 201C of the C photoconductor 1C opposite to the photoconductor gear 202C. The color rotating disk 203YCM is also integrally formed with a large-diameter portion 204YCM having a partially increased diameter, which is such that the Y, C, and M photoconductor gears 202Y, C, and M are at predetermined rotational positions, respectively. Is detected by a color gear sensor 91YCM composed of a transmissive photosensor.

  FIG. 5 is a block diagram showing a part of the electric circuit of the printer. In the figure, a bus 94 includes a process unit 6Y, C, M, K, an optical writing unit 7, a paper feed cassette 26, a registration motor 92, a data input port 68, a transfer unit 15, an operation display unit 93, and a main control unit 150. Y, C, M, K process units 9Y, C, M, K, a motor control unit 190, and the like are connected. Further, the K photoconductor motor 90K, the color photoconductor motor 90YCM, the K gear sensor 91K, the color gear sensor 91YCM, the driven encoder 171 and the like are connected to the motor control unit 190. The registration motor 92 is a drive source for the registration roller pair 28 described above. The data input port 68 receives image information sent from an external personal computer (not shown) or the like. The main control unit 150 controls the entire printer, and includes a CPU 1a, a RAM 1a as information storage means, a ROM 1b, and the like. The operation display unit 93 includes a touch panel, a liquid crystal panel, and a plurality of touch keys. The operation display unit 93 displays various information under the control of the main control unit 150, and controls input information from the operator. Or sent to the unit 150.

  The printer has a motor control unit 190 in addition to the main control unit 150. The motor control unit 190 controls driving of the K photoconductor motor 90K and the color photoconductor motor 90YCM.

  In the printer having the above-described configuration, each of the photoreceptors 1Y, 1C, 1M, and 1K, which are image carriers, can be visualized by combining the four process units 6Y, 6C, 6M, and 6K with the optical writing unit 7. The visible image forming means for forming the toner image is configured.

FIG. 6 is a schematic diagram for explaining the arrangement pitch of the photoconductors. In this printer, the Y, C, M, and K photoconductors 1Y, 1C, 1M, and 1K are arranged at an arrangement pitch of L in the horizontal direction along the belt moving direction. Further, as the driving roller 12 that moves the intermediate transfer belt 8 endlessly with its own rotational driving, a roller portion whose circumferential length (π × diameter D) is an integral number of the pitch L is used. . More specifically, a pitch that is half the pitch L is used.

  FIG. 7 is a schematic diagram showing the transfer unit 15, the photoconductors 1Y, 1C, 1M, and 1K for each color, and the respective gears supported in the printer main body. In the drawing, a driving roller gear 158 is engaged with a photoconductor gear 202K of a photoconductor 1K for K through a first relay gear 156 and a second relay gear 157. The driving roller gear 158 is fixed to a shaft member of the driving roller 12 that moves the intermediate transfer belt 8 endlessly, and transmits a rotational driving force to the driving roller 12. That is, in this printer, the K photoconductor motor is used as a drive source for the K photoconductor 1K and also functions as a belt drive motor.

  FIG. 8 is a schematic diagram showing the drive control unit 190 and various devices electrically connected thereto. In the drawing, the rotational angular velocity of the driven roller 14 is applied to the rotating shaft member of the driven roller 14 that rotates while the belt is stretched inside the loop of the intermediate transfer belt 8 while the belt is endlessly moved. A driven encoder 171 consisting of a rotary encoder to be detected is fixed. The rotational angular velocity of the driven roller 14 and the linear velocity are correlated, and the linear velocity of the driven roller 14 and the linear velocity of the intermediate transfer belt 8 are the same. It functions as a belt speed detecting means for detecting the moving speed. Instead of this, an optical sensor that optically detects each mark of a scale having a plurality of marks arranged at a predetermined pitch in the circumferential direction on the belt surface may be used as the speed detection means.

  The detection result of the rotational angular velocity by the driven encoder 171 is input to the calculation unit 193 of the drive control unit 190. If the drive roller 12 is decentered, the running speed of the intermediate transfer belt 8 changes in speed so as to draw a sine curve for one cycle per rotation of the drive roller. The center of the peak-to-peak peak of the sine curve is the target belt speed. Therefore, when detecting a deviation from the target belt speed based on the signal from the driven encoder 171, the calculation unit 183 calculates a driving speed fluctuation amount that causes a speed fluctuation in the opposite direction and the same deviation amount. However, a signal for causing the drive speed fluctuation amount is output to the first motor driver 191. Then, the first motor driver 191 changes the motor driving speed to bring the belt speed closer to the target. At this time, the calculation unit 193 also outputs the same signal output to the first motor driver 191 to the second motor driver 192, and similarly controls the motor driving speed for the color photoconductor motor 90YCM. Change. As a result, the same speed fluctuation occurs in all of the four photoconductors 1Y, 1C, 1M, and 1K.

  FIG. 9 is a graph showing the speed fluctuation of the photoconductor generated by finely adjusting the driving speed of the K photoconductor motor (90K) and the color photoconductor motor (90YCM) in accordance with the belt speed fluctuation. The speed fluctuation of the intermediate transfer belt (8) is mainly caused by the eccentricity of the drive roller (12). Due to this eccentricity, the intermediate transfer belt (8) undergoes a speed fluctuation with a characteristic of drawing a sine curve for one cycle per one rotation of the driving roller. When the driving speed of the color photoconductor motor is finely adjusted so as to cancel out this speed fluctuation, the speed fluctuation of the intermediate transfer belt is reduced, but the sine curve speed fluctuation as shown in the figure is shown in the K photoconductor. Occur. One cycle of this sine curve is also the same as one rotation cycle of the driving roller. In this printer, the color photoconductor motor is finely adjusted at the same drive speed as that of the K photoconductor motor. Therefore, the Y, C, and M photoconductors also vary in speed as shown in the figure. To do. The speed fluctuations of the four photoconductors have the same amplitude and phase phases. In the illustrated sine curve-shaped speed fluctuation, the center of the peak-to-peak peak of the sine curve indicates that the speed fluctuation amount is zero and the linear velocity of the photoconductor is a value as designed.

  In FIG. 8 shown above, the intermediate transfer belt 8 passes through the transfer nips for Y, C, M, and K in that order. Therefore, the primary transfer of the toner image is performed in the order of Y, C, M, and K. At time t1 in FIG. 9, the speed fluctuation amount of the photosensitive member reaches a peak on the mountain side (plus side). At this time, in the transfer nip for Y in FIG. 8, the Y toner image on the photoreceptor 1Y is primarily transferred with the largest positional deviation in the belt moving direction. The time required for this Y toner image to enter the adjacent primary transfer nip for C after passing through the primary transfer nip for Y is the same as the two cycles (2T) of the sine curve in FIG. is there. This is because the arrangement pitch L of the photosensitive members is set to be twice the circumferential length (π × diameter D) of the driving roller 12. For this reason, even when the Y toner image on the intermediate transfer belt 8 enters the primary transfer nip for C, the speed fluctuation amount of the photosensitive member reaches the peak on the peak side of the sine curve as shown at time t4 in FIG. ing. Then, the C toner image is primarily transferred with the largest positional deviation in the belt moving direction onto the Y toner image that has been primarily transferred with the largest positional deviation in the belt moving direction. Since the positional deviation amounts of the toner images are the same, there is almost no misalignment between the toner images. Even when the YC superimposed toner image obtained in this way enters the M primary transfer nip or the K primary transfer nip on the downstream side, the speed fluctuation amount of the photosensitive member has a sine curve. The mountain side is at its peak. Therefore, the M toner image and the K toner image are primarily transferred onto the YC superimposed toner image described above without deviation.

  At time t2 in FIG. 9, the speed fluctuation amount of the photosensitive member reaches a peak on the valley side (minus side). At this time, at the transfer nip for Y, the Y toner image on the photoreceptor 1Y is primarily transferred with the largest positional deviation in the direction opposite to the belt moving direction. Even at the time t5 when the Y toner image enters the adjacent C primary transfer nip, the speed fluctuation amount of the photosensitive member reaches the peak on the valley side of the sine curve. Similarly, the downstream side M primary transfer nip and the primary transfer nip for K reach the peak on the valley side. That is, the toner images that are primarily transferred with the largest positional deviation in the direction opposite to the belt moving direction are superimposed on each other at the Y, C, M, and K transfer nips. Therefore, the overlay deviation hardly occurs.

  At time t0 in FIG. 9, the speed fluctuation amount of the photosensitive member is zero. At this time, in the Y transfer nip, the Y toner image on the photoconductor 1Y is primarily transferred to the normal position without being displaced. At the time t3 when the Y toner image enters the adjacent C primary transfer nip, the speed fluctuation amount of the photosensitive member becomes zero. Similarly, the speed fluctuation amount becomes zero at the downstream primary transfer nip for M and the primary transfer nip for K. That is, the toner images that are primarily transferred without positional deviation are superposed at the Y, C, M, and K transfer nips. Almost no misalignment occurs.

  As described above, in this printer, toner images that are displaced by the same displacement amount are synchronized at each primary transfer nip, so that there is almost no occurrence of overlay displacement. Therefore, the K toner image formed by the K photoconductor 1K driven by the driving force of the K photoconductor motor (90K) as the belt drive motor, and the Y, C, M toner images formed by other photoconductors. Can be reduced.

  When the speed fluctuation occurs in the photoconductor, the electrostatic latent image written at the optical writing position on the photoconductor is deformed. Therefore, in the printer, the deformation is offset by the linear velocity between the belt and the photosensitive member at the transfer nip. Specifically, in this printer, the time from when the surface of the photosensitive member enters the optical writing position by the laser beam L until it reaches the center of the primary transfer nip is made equal to one cycle of the drive roller 12. ing. Specifically, in FIG. 10, P <b> 1 indicates the optical writing position of the photosensitive member by the laser light L. P2 indicates the center position in the direction of movement of the photoreceptor surface in the primary transfer nip (hereinafter referred to as the nip center position). It is assumed that a positive peak of the photoreceptor speed fluctuation is reached at a certain time point at the optical writing position P1. At this time, since the linear velocity of the photoconductor reaches its maximum on the plus side, the length of the latent image written at the optical writing position P1 is stretched in the photoconductor surface movement direction more than usual. In this way, the latent image portion having a shape expanded more than usual reaches the nip center position P2 when one rotation cycle of the driving roller 12 has passed thereafter. At this time, the linear velocity of the photosensitive member also reaches a maximum on the plus side and becomes maximum. As a result, the linear velocity of the photosensitive member becomes faster than the linear velocity of the intermediate transfer belt 8, and the toner image on the surface of the photosensitive member is primarily transferred onto the surface of the intermediate transfer belt 8 in a contracted state. The Since the contraction rate at this time is the same as the expansion rate of the latent image at the previous optical writing position P1, the expansion at the optical writing position P1 is offset by the contraction at the nip center position P2. As a result, the toner image on the belt has a normal shape. Although the latent image portion written when the photosensitive member reaches the peak speed at the optical writing position P1 has been described, the other latent image portions are similarly expanded and contracted at the optical writing position P1 at the nip center position P2. It is offset by the expansion and contraction.

  Up to now, the description has been made focusing only on the positional deviation of each color toner image due to the eccentricity of the driving roller 12, but in actuality, as described above, the position of each color toner image due to the eccentricity of each photoconductor gear. Deviation also occurs. However, in this printer, as already described, since the phase matching process is performed to match the phase of the maximum eccentricity portion of each photoconductor gear, the positional shift caused by the eccentricity of each photoconductor gear is the same position. Deviation amounts of toner images are synchronized at the primary transfer nip of each color. That is, in practice, as shown in FIG. 15, the amount of positional deviation that occurs in one rotation period of the driving roller due to the eccentricity of the driving roller 12 and the rotation period of the photosensitive member due to the eccentricity of the photosensitive member gear. A positional deviation amount is generated by superimposing the generated positional deviation amount. In each primary transfer nip, toner images with the same positional deviation after superimposition are synchronized with each other. For example, in the superimposed waveform shown in FIG. 15, the toner images transferred at the first peak on the peak side are synchronized with each other at the primary transfer nip for Y, C, M, and K. Become.

  In the case where the three colors Y, C, and M are driven by individual photoconductor motors, the following may be performed. That is, Y, C, and M photoconductor gears 202Y, C, and M are attached to large-diameter portions (corresponding to 204YCM), and the large-diameter portions are individually detected for Y, C, and M, respectively. A gear sensor (equivalent to 91YCM) is provided. Then, at the end of the print job or at the start, the rotation postures of the photoconductor gears of the respective colors are adjusted so that the timing at which the gear sensor detects the large diameter portion in each of the colors Y, C, and M has a predetermined relationship. To do. As a result, the toner images having the same positional deviation amount due to the eccentricity of the photoreceptor gear in the three-color primary transfer nips are synchronized with each other.

  For reference, FIG. 16 shows the relationship of the amount of misregistration of each color toner image when the phase of the maximum eccentric portion of each color photoconductor gear is not matched based on the primary transfer nip of each color. In this example, since the arrangement pitch L of each photoconductor is an integral multiple of the circumference of the drive roller 12, toner images having the same amount of positional displacement can be obtained with respect to the positional shift caused by the eccentricity of the drive roller 12. Each primary transfer nip can be synchronized. However, with respect to the positional deviation caused by the eccentricity of the photoconductor gear, the toner images having the same positional deviation amount cannot be synchronized at each primary transfer nip. For this reason, the same misalignment amount resulting from the eccentricity of the drive roller 12 and the actual misregistration amount due to the eccentricity of the photoconductor gear are synchronized at each primary transfer nip. I can't let you. For this reason, misalignment occurs.

  For reference, FIG. 17 shows the relationship of the positional deviation of each color toner image when the arrangement pitch L of each photoconductor is not set to an integral multiple of the circumference of the drive roller 12. In this example, a process of matching the phase of the maximum eccentric portion of the photoconductor gear between Y, C, M, and K with respect to the primary transfer nip of each color is performed. For this reason, with respect to the positional deviation caused by the eccentricity of the photoconductor gear, the toner images having the same positional deviation amount can be synchronized with each other at each primary transfer nip. However, with respect to misalignment caused by the eccentricity of the drive roller 12, toner images having the same misalignment amount cannot be synchronized at each primary transfer nip. For this reason, the same misalignment amount resulting from the eccentricity of the drive roller 12 and the actual misregistration amount due to the eccentricity of the photoconductor gear are synchronized at each primary transfer nip. I can't let you. For this reason, misalignment occurs.

Next, a description will be given of a printer according to the embodiment. Incidentally, unless otherwise noted below, the configuration of the printer according to the embodiment is similar to the reference embodiment.
The misalignment of the toner images of the respective colors occurs due to the sub-scanning resist misalignment of the respective color toner images in addition to the speed fluctuation of the intermediate transfer belt 8 and the photosensitive member. The sub-scanning registration deviation is a phenomenon in which the normal transfer position of the toner image is totally displaced in the sub-scanning direction, which is the moving direction of the intermediate transfer belt 8. The main cause of the sub-scanning registration deviation is that the components of the optical writing unit 7 such as a reflection mirror and a lens expand and contract with temperature changes. During a continuous printing operation in which images are continuously formed on a plurality of recording papers, the temperature of the optical writing unit 7 continues to rise, so that the amount of color misregistration increases as the continuous operation time increases.

Therefore, the main control unit 150 of the printer according to the embodiment performs the following writing position correction process every time a predetermined number of prints are performed. In other words, after the toner images formed on the photoreceptors of the respective colors are transferred side by side on the belt, the amount of misregistration of the toner images of each color is detected based on the timing at which the toner images are detected by an optical sensor as a displacement detection means. To do. Then, the sub-scanning registration shift amount is reduced by correcting the latent image writing start timing based on the detection result. As a result, as shown in FIG. 11, in the continuous print mode, the color misregistration amount that gradually increases with the increase in the number of continuous prints is periodically reduced to almost zero by periodically performing the writing position correction process. Can be reset to

  FIG. 12 is an enlarged perspective view showing a part of the intermediate transfer belt 8 together with an optical sensor unit 236 as a positional deviation amount detecting means. As shown in the figure, the optical sensor unit 236 is opposed to a portion where the intermediate transfer belt 8 is wound around the driving roller 12 with a predetermined gap therebetween. The main control unit 150 performs a writing position correction process immediately after a power switch (not shown) is turned on or whenever a predetermined number of prints are performed. In this writing position correction process, a color misregistration detection image made up of a plurality of toner images called chevron patches PV is formed on one end, the center, and the other end of the intermediate transfer belt 8 in the width direction. The optical sensor unit 236 includes a first optical sensor 237 that faces one end in the width direction of the intermediate transfer belt 8, a second optical sensor 238 that faces the center, and a third optical sensor 239 that faces the other end. It has. The first optical sensor 237 passes the light emitted from the light emitting means through the condenser lens, reflects the light on the surface of the intermediate transfer belt 8, receives the reflected light by the light receiving means, and determines the voltage according to the amount of light received. Is output. When the toner image in the chevron patch PV formed at one end of the intermediate transfer belt 8 passes immediately below the first optical sensor 237, the amount of light received by the light receiving means of the first optical sensor 237 changes greatly. Accordingly, the main control unit 150 can detect each toner image in the chevron patch PV formed at one end portion in the width direction of the intermediate transfer belt 8. Similarly, each toner image in the chevron patch PV formed at the center of the intermediate transfer belt 8 can be detected based on the output from the second optical sensor 238. Further, each toner image in the chevron patch PV formed on the other end of the intermediate transfer belt 8 can be detected based on the output from the third optical sensor 239. Based on the detection timing, it is possible to detect the positional deviation amount of each toner image. As described above, the first optical sensor 237, the second optical sensor 238, and the third optical sensor 239 each function as a positional deviation amount detection unit in combination with the main control unit 150. As the light emitting means, an LED or the like having an amount of light that can generate reflected light necessary for detecting a toner image is used. As the light receiving means, a CCD in which a large number of light receiving elements are arranged in a straight line is used.

  The main control unit 150 detects each toner image in the chevron patch PV, so that the position in the main scanning direction, the position in the sub-scanning direction (belt moving direction), the magnification error in the main scanning direction, the main scanning in each toner image. Each skew from the direction is detected. The main scanning direction here refers to the direction in which the laser light is phased on the surface of the photosensitive member as it is reflected by the polygon mirror. As shown in FIG. 13, the chevron patch is a posture in which the toner images of Y, C, M, and K are inclined by about 45 [°] from the main scanning direction at a predetermined pitch in the belt moving direction that is the sub scanning direction. It is a line pattern group arranged. For such Y, C, M toner images in the chevron patch PV, the difference in detection time from the K toner image is read. In this figure, the vertical direction of the paper surface corresponds to the main scanning direction, and after the Y, C, M, and K toner images are arranged in order from the left, the postures are different from those by 90 [°]. , Y toner images are further arranged. Based on the difference between the actual measurement value and the theoretical value for the detection time differences tyk, tck, and tmk with respect to K as the reference color, the shift amount in the sub-scanning direction of each color toner image, that is, the registration shift amount is obtained. Then, based on the amount of registration deviation, the optical writing start timing with respect to the photoconductor or the inclination of the reflection mirror is corrected to reduce registration deviation of each color toner image. Further, the inclination (skew) of each color toner image from the main scanning direction is obtained based on the difference in the amount of deviation in the sub-scanning direction between both ends of the belt. Based on the result, surface tilt correction of the reflecting mirror is performed to reduce skew of each color toner image. As described above, the process of correcting the optical writing start timing and the like based on the timing of detecting each toner image in the chevron patch PV, which is a color misregistration detection image, reduces the registration shift and the skew shift. It is a correction process.

  FIG. 14 is a schematic diagram showing the relative positional relationship of the optical scanning start position in the sub-scanning direction with respect to the photoreceptors 1Y, 1C, 1M, and 1K. In the figure, the alphabets Y, C, M, and K surrounded by circles indicate the center positions of the first laser spots with respect to the photoreceptors 1Y, 1C, 1M, and 1K, respectively. Further, in the figure, C surrounded by a dotted line ◯ indicates the planned irradiation center position of the first dot laser spot on the C photoconductor 1C. At the same time, L1 indicates the length of one dot in the sub-scanning direction. Needless to say, (L1 / 2) indicates half the length of L1.

  In the same figure, if the center position of the laser spot of the first dot with respect to the C photoconductor 1C is the position of C surrounded by a dotted line ◯ in the figure, the C toner image and the K toner image are in the sub-scanning direction. Overlay deviation of (L1 / 2) or more, that is, half or more of the size of one dot occurs. In order to eliminate such a shift in the sub-scanning direction, it is not possible to simply shift the irradiation time of the laser beam by that amount. Since the laser beam moves in the main scanning direction, if the irradiation is started slightly shifted from the scheduled irradiation start timing, optical writing corresponding to the end is performed from the center rather than the end in the main scanning direction. Because it will end up. In order to shift the irradiation start timing, it is necessary to shift at least a timing of one dot (one line). Here, it is assumed that the irradiation start timing for the C photoconductor 1C is delayed by one dot from the original schedule. Then, the optical writing start position for the C photoconductor 1C is shifted from the position C surrounded by a dotted line ◯ in the drawing to the position C surrounded by a solid line ◯. The overlay deviation between the C toner image and the K toner image is shorter than (L1 / 2). That is, the overlay deviation is reduced. By shifting the irradiation start timing in this way, the overlay deviation amount can be kept to (L1 / 2) or less without causing a linear velocity difference between the photoconductors. Similarly, in the relationship between the M toner image and the K toner image, and the C toner image and the K toner image, the overlay deviation amount is kept to (L1 / 2) or less without causing a linear velocity difference between the photoconductors. be able to.

  Therefore, the main control unit 150 causes the overlay deviation from K (L1 / 2) for each of the three colors Y, C, and M based on the detection timing of each reference toner image in the above-described writing position correction processing. ) The following processing is performed for the result of the above calculation. That is, first, as shown in FIG. 14, the irradiation start timing is set to be advanced or delayed by one dot from the scheduled irradiation start timing. As a result, the amount of misalignment that should have occurred at a length of (L1 / 2) or more without changing the driving speed of the photosensitive member is set to be able to stay below (L1 / 2).

  However, even if such a setting is made, an overlay error occurs with a length of (L1 / 2) or less. Therefore, the setting of the driving speed of the color photoconductor motor 90YCM is changed for the overlay deviation amount of the length of (L1 / 2) or less, and the Y, C, M photoconductors 1Y, C, M, A linear speed difference is given to the driving speed (target value) of the K photoconductor 1K. The linear velocity difference is set to a value that can minimize the amount of deviation between the three color dots Y, C, and M and the K dot.

  In such a configuration, the overlay deviation is suppressed only by the linear velocity of the photoconductor while reducing the linear velocity difference as compared with the case where the overlay deviation of (L1 / 2) or more is suppressed only by the linear velocity difference of the photoconductor. In the same manner as described above, overlay deviation can be suppressed.

  The motor control unit 190 basically outputs reference clock pulses of reference values to the first motor driver 191 and the second motor driver 192, respectively. When each motor driver drives the motor at a driving speed opposite to the reference clock pulse, the photoconductor rotates at the reference rotation speed. However, when a speed change instruction signal corresponding to the belt speed fluctuation is sent from the calculation unit 193, the drive speed is increased or decreased by the corresponding amount. The motor control unit 190 outputs an output to the second motor driver 192 when a signal related to the linear velocity difference of the photosensitive member obtained by the writing position correction process described above is sent from the main control unit 150. The reference clock pulse is changed from the reference value by adding or subtracting an amount corresponding to the linear velocity difference from the reference value. As a result, the Y, C, and M photoconductors rotate at the linear velocity average value (the center value of the peak-to-peak in the graph of FIG. 9) with the above-described linear velocity difference. The second motor driver 192 uses the linear speed average value as a reference target speed.

  In such a configuration, the photosensitive member for K and the photosensitive member for Y, C, and M are provided with a linear velocity difference to reduce the sub-scanning resist deviation to an accuracy of less than 1/2, and the speed variation of the photosensitive member. It is possible to reduce the overlay deviation caused by.

1Y, C, M, K: photoconductor (image carrier)
8: Intermediate transfer belt (belt member)
12: Drive roller (drive rotator)
15: Transfer unit (transfer means)
90K: K photoconductor motor (belt drive motor)
90YCM: color photoconductor motor (image carrier drive motor)
150: Main control unit (part of control means)
190: Motor control unit (part of control means)
171: Driven encoder (belt speed detection means)

JP 2008-112132 A

Claims (1)

A belt unit that moves an endless belt member endlessly in accordance with the rotational drive of a drive rotating body that is one of a plurality of stretching members, while the endless belt member is stretched by a plurality of stretching members disposed in the loop; ,
A belt drive motor that moves the belt member endlessly by exerting a driving force for rotationally driving the drive rotor;
Belt speed detecting means for detecting the moving speed of the belt member;
A plurality of image carriers that carry visible images on their endlessly moving surfaces;
Transfer means for transferring a toner image carried on the surface of each image carrier in a superimposed manner on the surface of the belt member or a recording member held on the surface;
An image forming apparatus comprising: a control unit that finely adjusts a driving speed of the driving motor so as to move the belt member endlessly at a predetermined target belt speed based on a detection result by the belt speed detecting unit;
Wherein among the plurality of image bearing members, as a drive source for what Re one of the image bearing member, sharing the belt drive motor,
The other image carrier is driven by an image carrier drive motor different from the belt drive motor,
The arrangement pitch along the moving direction of the belt member in the plurality of image carriers is set to an integral multiple of the circumferential length of the drive rotator,
An image detecting means for detecting a visible image transferred on the surface of the belt member is provided,
Based on the detection result by the belt speed detection means, the control means is configured to perform the same fine speed adjustment processing on the belt drive motor and the image carrier drive motor ,
In addition, after transferring predetermined reference visible images formed on the respective image carriers on the basis of image information stored in advance to different positions on the surface of the belt member, those on the belt member are transferred. Based on the difference in timing at which a reference visible image is detected by the image detection means, the relative positional deviation amount of each reference visible image is grasped, and based on the grasping result, among the plurality of image carriers. The other image carrier is set to a value capable of keeping the relative positional shift amount of the visible image on the other image carrier relative to the visible image on any one of the image carriers to ½ dot or less. The relative image shift amount that remains at the image formation start timing after the setting is calculated, and the linear velocity difference based on the image shift amount is calculated as the belt drive motor. Image carrier driven by A predetermined process for individually setting the target drive speed of the belt drive motor and the target drive speed of the image carrier drive motor so as to be provided on the image carrier driven by the image carrier drive motor. When the timing is reached, and when forming an image based on the operator's instruction command, the belt drive motor and the image carrier drive motor are each based on the individual target drive speed, An image forming apparatus , wherein the control unit is configured to perform a process of finely adjusting a driving speed based on a detection result by the belt speed detecting unit .

Images