JP5958089B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus in which a transfer unit is pressed against an image carrier carrying a toner image, a sheet is passed between the image carrier and the transfer unit, and a toner image on the image carrier is transferred to the sheet. About.

  In recent years, image forming apparatuses having a plurality of functions such as printers, facsimiles, copiers, and multifunction machines have been widely used. In this image forming apparatus, a latent image is formed on a photoconductor based on image data, the latent image is developed with a developer, and then transferred to a sheet directly or via an intermediate transfer unit. When transferring an image, a transfer portion composed of a transfer roller and a transfer belt is pressed against an image carrier composed of a photosensitive member and an intermediate transfer portion, and a sheet is inserted into the pressure contact portion (also referred to as a transfer nip portion). The toner image is transferred to the paper.

  The transfer unit can be driven by the image carrier by being pressed against the image carrier that is driven to rotate. However, if a load is applied to the transfer unit, the follower becomes difficult and the transfer unit is driven to rotate. In some cases, a transfer unit driving unit is required. For example, when a cleaning unit is provided to remove a toner image attached to the transfer unit, a blade or the like is pressed against the surface of the transfer unit such as a transfer roller or a transfer belt. Take it. For this reason, the image forming apparatus is provided with a transfer unit driving unit for driving the transfer unit.

  When the image carrier and the transfer unit are individually rotated in this way, it is necessary to prevent the rotation of the transfer unit from affecting the rotation of the image carrier to impair image formation accuracy. In Patent Document 1, the driving force applied to the transfer unit is controlled in accordance with at least one of the usage history of the cleaning member and the amount of moisture in the air, thereby reducing fluctuations in the load on the image carrier due to the rotation of the transfer unit. doing. In Patent Document 2, the control of the intermediate transfer unit is prevented from failing by detecting the torque command value of the intermediate transfer unit and changing the speed of the transfer unit when the command value exceeds a set lower limit value. ing.

  However, when the sheet is passed through the pressure contact portion while the transfer portion is rotationally driven while being pressed against the image carrier (here, the intermediate transfer belt), the rotation diameter of the transfer portion changes by the thickness of the sheet. . As a result, when the transfer unit is controlled at a constant speed so as to rotate at a constant speed, the torque applied to the image carrier changes in the paper passage cycle, resulting in a speed change in the image carrier, There is a problem in that image formation accuracy is impaired by causing color misregistration or the like.

  In order to cope with such a torque change of the transfer unit or the like, the transfer unit and the image carrier are arranged such that the rotation speed of the transfer unit and the image carrier is constant when the transfer unit and the image carrier are separated from each other. The image is controlled at a constant speed by feedback control, and the transfer portion is controlled at a constant speed according to the constant-speed driving torque when the transfer portion and the image carrier are pressed against each other and the constant speed control detected when the transfer portion is separated. A forming apparatus has been proposed.

  However, when constant torque control is performed in a state where the fixing unit is in pressure contact with the intermediate transfer belt, the transfer load of the transfer unit changes with time, and the change in the magnification of the image is caused by the change in the transfer load. There is a problem that it occurs. FIG. 7A shows the change in transfer load on the transfer portion side, and FIG. 7B shows the image magnification. The vertical axis shown in FIG. 7A is the transfer load, and the horizontal axis is time. In FIG. 7B, the vertical axis represents image magnification, and the horizontal axis represents time. In FIG. 4B, the “□” mark represents the average speed of the transfer unit (drive unit) when each sheet is passed. The same meaning is used in the following drawings.

  As shown in FIG. 7A, the transfer load accompanying the transfer of the first image, the transfer of the second image, the transfer of the third image, the transfer of the fourth image, etc. Gradually decreases. This change in the transfer load itself was assumed to be sufficiently slow and over time due to changes in the material of the transfer part and the cleaning part (gradual change throughout life). However, in practice, it has been found that the transfer load is a phenomenon that varies greatly depending on the cleaning configuration (such as lubricant application) of the transfer portion and changes at the level of several tens of sheets in a short period of time. As the transfer load is reduced in this manner, as shown in FIG. 7B, the rotation speed of the transfer portion is increased, and as a result, the image magnification tends to increase (extend).

  As a solution to such a problem, a method has been proposed in which the average speed of the transfer section when each sheet is passed is compared and this difference is fed back to the torque command value of the next sheet. FIG. 8 is a diagram for explaining a method of feeding back this difference to the torque command value of the next sheet. FIG. 8A shows the change in transfer load on the transfer unit side, the vertical axis is the transfer load, and the horizontal axis is the elapsed time. FIG. 8B shows the torque command value (PWM) of the transfer portion, the vertical axis is the torque command value, and the horizontal axis is the elapsed time. FIG. 8C shows the speed of the drive unit that drives the transfer unit, the vertical axis is the speed, and the horizontal axis is the elapsed time. FIG. 8D shows the image magnification transferred onto the paper, the vertical axis is the image magnification, and the horizontal axis is the elapsed time.

  When the sheet is passed with the transfer portion pressed, the transfer load of the transfer portion gradually decreases due to the influence of the lubricant in the cleaning portion (FIG. 8A). The drive unit that drives the transfer unit is driven based on the torque command value in the constant speed control detected when the transfer unit is separated (FIG. 8B).

  At this time, an average speed Va (hereinafter also referred to as a reference speed) Va at the time of passing the first sheet is measured (FIG. 8C), and a measurement value obtained by this measurement is used as a reference value. Remember. Next, the average speed Vb of the transfer portion when the second sheet is passed is measured (FIG. 8C), and the measured value of the second sheet is compared with the reference value of the first sheet. The difference G1 is feedback-controlled to the torque command value for the third sheet of the next sheet. By the feedback of this difference G1, the command value of the third sheet is lowered from the torque command value of the second sheet (FIG. 8B), and the average speed Vc of the drive unit is returned to the reference speed. Can do. As a result, the image magnification of the third image can be maintained at substantially the same magnification as the image magnification of the first image (FIG. 8D). Such feedback control is also performed on the third and subsequent sheets.

JP 2008-304552 A JP 2009-009103 A

  However, the above-described method of comparing the average speed of the transfer section when each sheet passes and feeding back the difference to the torque command value of the next sheet has the following problems. In other words, if the above method is employed when a tray change is performed by changing the paper during a print job in which image forming operations are continuously performed, there is a problem that the image magnification changes with the paper after the tray change. Below, the case where it changes from thin paper to thick paper by tray change is demonstrated.

  Specifically, as shown in FIG. 8, the fifth sheet after the tray change is driven based on the torque command value of the fourth sheet before the tray change. The average speed (rotational speed) Vd of the part becomes slower. At this time, the back side (transfer portion side) of the fifth sheet is conveyed at the same slow speed as the transfer portion as the load increases due to the thickness of the sheet, but the front side (intermediate transfer belt side) is the intermediate transfer. It is conveyed at the same speed as the belt. For this reason, the fifth sheet after the tray change is transported at the same speed as the intermediate transfer belt on the front side, so that a normal magnification can be obtained (A in FIG. 8D).

  On the other hand, for the sixth sheet after the tray change, the measured value of the average speed Vd of the driving unit for the fifth sheet is compared with the reference value of the first sheet before the tray change, The difference G2 is fed back to the torque command value for the sixth sheet. For this reason, the average speed Ve of the transfer portion of the sixth sheet is returned to the reference speed, but the speed on the front side of the paper also increases, so that slippage or the like occurs between the intermediate transfer belt and the tray after the tray change. There is a problem that the image magnification of the sixth sheet increases (B in FIG. 8D).

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to normally maintain the image magnification of the entire job even when the paper changes due to a tray change at the time of execution of the job. An object is to provide an image forming apparatus.

The image forming apparatus according to the present invention solves the above-described problems, and can support and hold an image carrier that carries a toner image and the image carrier, and can be carried on the image carrier at the time of pressure contact. A transfer unit that transfers the toner image to a sheet, a drive unit that rotationally drives the transfer unit based on a command value, and a constant speed control that rotates the transfer unit at a constant speed by setting a command value in the drive unit. And a control unit that performs constant torque control for rotationally driving the transfer unit with a constant torque, and the control unit detects when performing constant speed control of the drive unit in a state where the transfer unit is separated from the image carrier. According to the constant speed driving torque, the constant torque control of the driving unit is performed in a state where the transfer unit is pressed against the image carrier, and it is obtained when the first sheet is passed through the transfer unit in the constant torque control. the average speed of the drive unit as a reference value that is, the reference value and the second and subsequent Calculating a difference between the measured value of the average speed of the drive unit obtained when the fed to the transfer section constant sheet of paper, is calculated from the difference of the command value of the sheet conveyed next to the predetermined sheet of paper When the tray change is performed during continuous operation, the average speed of the drive unit obtained when the first sheet after the tray change is passed through the transfer unit is used as a reference value. The command value for the next sheet is calculated from the difference from the measured value of the average speed of the drive unit obtained when the second and subsequent sheets after the change are passed through the transfer unit.

  In the present invention, the continuous operation means a state in which a job is executed in a state where the transfer unit is pressed against the image carrier. The tray change includes, for example, changing from a tray in which thin paper is set to a tray in which thick paper is set, or changing to another tray when the paper in the currently fed tray runs out. .

  According to the present invention, when the tray change is performed in a state where the transfer unit is pressed against the image carrier, the average speed of the driving unit obtained when the first sheet after the tray change is passed is newly set. Therefore, even when the paper is changed due to a tray change, it is possible to prevent a change in image magnification.

1 is a schematic diagram illustrating a configuration example of an image forming apparatus according to a first embodiment of the present invention (part 1); FIG. 3 is a schematic diagram illustrating a configuration example of an image forming apparatus (part 2). 1 is a diagram illustrating a block configuration example of an image forming apparatus. It is a figure which shows the example of control of the transfer part in the case of performing a tray change at the time of constant torque control. FIG. 10 is a diagram illustrating a control example of a transfer unit when performing a tray change during constant torque control of an image forming apparatus according to a second embodiment of the present invention. 6 is a flowchart illustrating an operation example of the image forming apparatus when a tray change is performed during constant torque control. It is a figure which shows the example of a relationship between the transfer load in the past, and image magnification. It is a figure which shows the example of control of the transfer part at the time of the conventional constant torque control.

Hereinafter, the best mode for carrying out the invention (hereinafter referred to as an embodiment) will be described.
<First Embodiment>
[Configuration example of image forming apparatus]
First, a configuration example of the image forming apparatus 100 according to the first embodiment will be described. FIG. 1 schematically illustrates an example of the configuration of the image forming apparatus 100 when the transfer unit is separated, and FIG. 2 schematically illustrates an example of the configuration of the image forming apparatus 100 when the transfer unit is pressed. In addition, the dimension ratio of drawing is expanded on account of description and may differ from an actual ratio.

  As shown in FIGS. 1 and 2, the image forming apparatus 100 includes an image forming unit 102, an intermediate transfer belt 104, an image carrier driving roller 110, an image carrier driven roller 106, a transfer roller 120, and a transfer unit driving roller 126. A transfer unit driven roller 128, a transfer unit driving belt 130, a cleaning unit 140, and a transfer unit press-contact / separation mechanism 160 are provided. The image carrier driving roller 110, the transfer roller 120, the transfer unit driving roller 126, the transfer unit driven roller 128, the transfer unit driving belt 130, the cleaning unit 140, and the like constitute an example of the transfer unit.

  The image forming unit 102 includes, for example, a photosensitive drum, an optical writing unit, a developing unit, a charging unit, and the like (not shown). The charging unit uniformly charges the photosensitive drum to a predetermined potential. The optical writing unit forms an electrostatic latent image based on the image information on the photosensitive drum. The developing unit develops the electrostatic latent image (toner image) formed on the photosensitive drum. The photoconductor drum is an example of an image carrier, and transfers the toner image carried on the photoconductor drum to the intermediate transfer belt 104.

  The intermediate transfer belt 104 is an example of an image carrier, and is an endless belt, and is stretched by an image carrier driving roller 110, an image carrier driven roller 106, and a driven roller (not shown). The image carrier driving roller 110 is rotationally driven by a motor, which will be described later, and transports the intermediate transfer belt 104 in the transport direction of the paper P. The image carrier driven roller 106 rotates following the rotation of the intermediate transfer belt 104.

  The transfer roller 120 is disposed to face the image carrier driven roller 106. The transfer unit driving belt 130 is an endless belt, and is stretched by the transfer roller 120, the transfer unit driving roller 126, and the transfer unit driven roller 128. The transfer unit driving roller 126 is rotationally driven by a motor to be described later, and conveys the transfer unit driving belt 130 in the paper conveyance direction.

  The cleaning unit 140 is provided below the transfer roller 120 and has a cleaning blade 142. The cleaning blade 142 abuts on the transfer unit driving belt 130 to remove the toner image attached to the surface of the transfer unit driving belt 130. A lubricant is supplied to the cleaning unit 140 in order to prevent damage due to a toner image at the time of cleaning and wax contained therein.

  As shown in FIGS. 1 and 2, the transfer unit press-contact / separation mechanism 160 is provided in the periphery of the transfer roller 120, and the transfer roller 120 and the transfer unit are arranged so that the transfer roller 120 presses and separates from the intermediate transfer belt 104. The driving roller 126, the transfer unit driven roller 128, and the cleaning unit 140 are integrally moved toward and away from the intermediate transfer belt 104. As the transfer part press-contacting / separating mechanism 160, a known configuration can be adopted, and the configuration is not particularly limited in the present invention.

  The large-capacity paper feeder 200 is connected to the upstream side of the image forming apparatus 100 in the paper conveyance direction, and has a plurality of paper feed trays 202 and 204. In each of the paper feed trays 202 and 204, paper P such as thin paper or thick paper is set, paper P having different surface characteristics is set, or paper P of the same size or different size is set. The large-capacity paper feeder 200 takes out the corresponding paper P from each of the paper feed trays 202 and 204 and feeds it to the transfer unit, and also performs a tray change based on a change instruction of the paper P, and feeds the paper after the tray change. Paper P is fed from the tray to the transfer unit. Note that the number of paper feed trays is not limited to two. In addition to the large-capacity paper feeding device 200, the paper P can be fed from a paper feeding unit (not shown) in the image forming apparatus 100.

[Block Configuration Example of Image Forming Apparatus]
Next, an example of a block configuration of the image forming apparatus 100 will be described. FIG. 3 shows an example of a block configuration of the image forming apparatus 100. As shown in FIG. 3, the image forming apparatus 100 includes a control unit 150 for controlling the operation of the entire apparatus. The control unit 150 includes a CPU (Central Processing Unit) 152. The CPU 152 reads out and executes a program related to constant speed control, constant torque control, image formation, and the like from the storage unit 170, and controls constant speed control, constant torque control, and the like by controlling operations of a transfer unit drive motor 122 and the like described later. Do.

  The control unit 150 is connected to an image carrier driving motor 112, a transfer unit driving motor 122, a transfer unit press-contact / separation motor 162, and a storage unit 170. The image carrier drive motor 112 is composed of, for example, a DC brushless motor, and an image carrier drive roller 110 is connected to the drive shaft via a drive force transmission mechanism 114. The image carrier driving motor 112 is driven based on a torque command value supplied from the control unit 150, and rotates the image carrier driving roller 110 by this driving. The torque command value is a PWM signal that controls the speed and torque of the image carrier drive motor 112.

  A rotation sensor (not shown) is attached to the image carrier driving motor 112. The rotation sensor detects the rotation of the image carrier driving motor 112 and feeds back speed information obtained by this detection to the control unit 150. Note that a known sensor such as a Hall element can be used as the rotation sensor, and the present invention is not limited to a specific sensor.

  The transfer unit drive motor 122 is composed of, for example, a DC brushless motor, and a transfer unit drive roller 126 is connected to the drive shaft via a drive force transmission mechanism 124. The transfer unit driving motor 122 is driven based on the torque command value supplied from the control unit 150, and rotates the transfer unit driving roller 126 by this driving. The torque command value is a PWM signal that controls the speed and torque of the transfer unit driving motor 122. The transfer unit driving motor 122 constitutes an example of a driving unit.

  A rotation sensor (not shown) is attached to the transfer unit drive motor 122. The rotation sensor detects the rotation of the transfer unit driving motor 122 and feeds back speed information obtained by this detection to the control unit 150. Note that a known sensor such as a Hall element can be used as the rotation sensor, and the present invention is not limited to a specific sensor.

  The transfer unit pressure contact / separation motor 162 is formed of, for example, a DC brushless motor, and a transfer unit pressure contact / separation mechanism 160 is connected to a drive shaft of the transfer unit pressure contact / separation mechanism 160 via a driving force transmission mechanism 164. The transfer unit press-contact / separation motor 162 operates the transfer unit press-contact / separation mechanism 160 based on the operation command value supplied from the control unit 150, thereby pressing the transfer roller 120 against the intermediate transfer belt 104 or moving the transfer roller 120. It is separated from the intermediate transfer belt 104. A position detection sensor (not shown) is attached to the transfer unit pressure contact / separation mechanism 160. The position detection sensor detects the press contact and separation of the transfer roller 120 and the like, and supplies the control unit 150 with the press contact separation information obtained by this detection.

  The storage unit 170 includes, for example, a ROM (Read Only Memory), a RAM (Random Access Memory), and the like, and stores a program for executing constant speed control, constant torque control, image forming operation, and the like. The storage unit 170 stores speed information when performing constant speed control, a torque command value when performing constant torque control, and the like.

  The large-capacity paper feeding device 200 is connected to the control unit 150 of the image forming apparatus 100 via a communication unit (not shown), and takes out the paper P based on instruction information supplied from the control unit 150 of the image forming apparatus 100. Paper is fed to the image forming apparatus 100 or the paper feed tray is changed.

[Example of basic operation of image forming apparatus]
Next, an example of a basic operation of the image forming apparatus 100 will be described with reference to FIGS. First, an operation example on the intermediate transfer belt 104 side will be described. When the job is started, the control unit 150 supplies a torque command value including a PWM signal to the image carrier driving motor 112 to rotate the image carrier driving roller 110 at a constant speed. The control unit 150 generates a torque command value based on information regarding the torque command value stored in the storage unit 170.

  The rotation of the image carrier driving motor 112 is detected by a rotation sensor (not shown), and the detection result is fed back to the control unit 150 as speed information. The control unit 150 determines whether or not the image carrier driving motor 112 is within the speed range set based on the speed information, and maintains the current torque command value when it is within the set range. If it falls below the set range, an increased torque command value is generated to drive and control the image carrier drive motor 112. When the value exceeds the set range, a reduced torque command value is generated and the image carrier drive motor 112 is driven and controlled so that the speed is within the set range. Thereby, the intermediate transfer belt 104 can be controlled at a constant speed so as to rotate at a constant speed.

  Next, an operation example on the transfer roller 120 side will be described. The transfer roller 120 is controlled to rotate differently when it is in pressure contact with the intermediate transfer belt 104 and when it is separated. When the control unit 150 detects that the transfer roller 120 is separated from the intermediate transfer belt 104, the control unit 150 supplies a torque command value including a PWM signal to the transfer unit drive motor 122, thereby causing the transfer unit drive roller 126 to move. Rotate at a constant speed. The control unit 150 generates a torque command value based on information regarding the torque command value stored in the storage unit 170. Whether the transfer roller 120 is in the separated or pressure contact state with respect to the intermediate transfer belt 104 is determined by, for example, a sensor that detects the position of the transfer roller 120 or a member that moves with the transfer roller 120 in accordance with the pressure contact / separation operation. The separation and pressure contact state of the transfer roller 120 can be determined based on the detection result of the sensor.

  The rotation of the transfer unit driving motor 122 is detected by a rotation sensor (not shown), and the detection result is fed back to the control unit 150 as speed information. The control unit 150 determines whether or not the transfer unit driving motor 122 is within the speed range set based on the speed information. If the transfer unit driving motor 122 is within the set range, the current torque command value is maintained. If it falls below the set range, an increased torque command value is generated and the transfer unit drive motor 122 is driven and controlled. When the value exceeds the set range, a reduced torque command value is generated and the transfer unit drive motor 122 is driven and controlled to have a speed within the set range. Thereby, constant speed control can be performed so that the transfer roller 120 rotates at a constant speed.

  The transfer roller 120 driven by the transfer unit driving roller 126 via the transfer unit driving belt 130 may be set at a constant speed so as to be the same speed as the intermediate transfer belt 104, or the intermediate transfer belt. It may be set to be faster than the rotational speed of 104 by a predetermined value.

  When the transfer roller 120 is controlled at a constant speed, the control unit 150 detects the drive torque in the transfer unit drive motor 122 as a constant-speed drive torque. A torque detector can be used to detect the constant speed driving torque. For example, by connecting a torque detector between the transfer unit driving motor 122 and the transfer unit driving roller 126, the constant-speed driving torque is detected based on the screw amount. In the case where the PWM signal is used as described above, torque detection can be performed by analyzing the PWM signal itself as a torque command value in the constant speed control. In torque detection, it is preferable to employ a value with small fluctuation variation, such as by employing an average value within a predetermined time. Further, the detection time of the constant speed driving torque can be arbitrarily set as long as it is within the detectable time, and it is not necessary to perform detection over the entire detectable time.

  Next, in a state where the transfer roller 120 is in pressure contact with the intermediate transfer belt 104, the control unit 150 determines the constant speed driving torque of the transfer unit driving motor 122 detected during the constant speed control of the transfer roller 120. The drive torque control at a constant speed of the transfer unit drive motor 122 is performed. The torque value at this time may be the same as the torque value of the constant speed driving torque detected during the constant speed control, but taking into account the fluctuation of the driving torque when the transfer unit driving motor 122 is rotating. The constant torque control is preferably performed based on a torque value larger than the constant speed driving torque. As the magnitude, it is sufficient that the fluctuation range of the driving torque is within the range. The fluctuation range of the driving torque can be grasped by collecting the operation data of the transfer unit driving motor 122 in advance.

  In addition to the above, when the transfer roller 120 is controlled at a constant speed, by setting the speed of the transfer roller 120 to be larger than the speed of the intermediate transfer belt 104, the constant-speed driving torque detected at this time is set. The torque value during constant torque control can be determined. At this time, the detected driving torque at a constant speed is a torque value larger than the torque value detected when the transfer roller 120 is controlled at a constant speed at the same speed as the intermediate transfer belt 104. Thereby, the constant torque control can be performed based on a torque value larger than a torque value detected when the transfer roller 120 is controlled at a constant speed at the same speed as the intermediate transfer belt 104. If the fluctuation range of the drive torque can be accommodated within the difference between the two torque values, the intermediate transfer belt 104 can be prevented from changing the torque even if the drive torque of the transfer unit drive motor 122 is changed. From this point of view, the rotation speed of the transfer roller when the constant speed control is performed at a speed higher than that of the intermediate transfer belt 104 may be determined.

  Switching from separation of the transfer portion to press contact can be performed, for example, with the start of image formation. Also, switching from pressure contact to separation can be performed with the end of the job and the reservation job. Therefore, the torque detection of the transfer unit driving motor 122 and the constant torque control of the transfer unit driving motor 122 according to the torque detection can be performed, for example, every time from the end to the start of a series of jobs. It is possible to perform rotation control with an appropriate torque command value while adjusting the command value.

[Example of operation of image forming apparatus when tray change is performed]
Next, an example of the operation of the image forming apparatus 100 when performing a tray change during a continuous operation of image formation (a state where the transfer unit is pressed against the image carrier) will be described. FIG. 4 shows an example of control on the transfer unit side of the image forming apparatus 100 when tray change is performed during continuous image formation. Specifically, FIG. 4A shows a change in transfer load, the vertical axis in FIG. 4A is the transfer load, and the horizontal axis is time. 4B shows the torque command value (PWM signal) of the transfer unit drive motor 122, the vertical axis of FIG. 4B is the torque command value, and the horizontal axis is time. 4C shows the speed of the transfer unit driving motor 122, the vertical axis of FIG. 4C is the speed, and the horizontal axis is time. FIG. 4D shows the image magnification transferred onto the paper P, the vertical axis of FIG. 4D is the image magnification, and the horizontal axis is time.

  As shown in FIG. 4A, it is assumed that the transfer load gradually decreases as time passes due to the image forming operation due to the influence of load fluctuations such as the lubricant in the cleaning unit 140.

  When the image forming operation is started, the control unit 150 controls the transfer unit press-contact / separation mechanism 160 to press the transfer roller 120 against the intermediate transfer belt 104, and then performs constant torque control on the transfer unit drive motor 122. I do. The torque command value is, for example, a torque value of constant speed driving torque detected by constant speed control when the transfer roller 120 is separated (FIG. 4B).

  When the image forming operation is started, the speed (number of rotations) of the transfer unit driving motor 122 when each sheet P is passed is measured. The control unit 150 calculates an average speed V1 (hereinafter also referred to as a reference speed) from the measured speed of the transfer unit driving motor 122 when the first sheet P is passed (FIG. 4C). The calculated average speed V1 is stored in the storage unit 170 as a reference value. Subsequently, the control unit 150 calculates the average speed V2 from the speed of the transfer unit driving motor 122 when the second sheet P is passed (FIG. 4C).

  The control unit 150 reads the reference value of the transfer unit driving motor 122 at the time of passing the first sheet from the storage unit 170, and the read reference value and the calculated transfer unit driving motor 122 at the time of passing the second sheet. The difference X1 from the measured value of the average speed V2 is calculated, and this difference X1 is fed back to the torque command value for the third sheet as the next sheet. As a result, the torque command value for the third sheet decreases according to the difference X1, and the speed of the transfer unit drive motor 122 for the next sheet can be returned to the reference speed (FIGS. 4B and 4C).

  Subsequently, the control unit 150 calculates an average speed V3 of the transfer unit driving motor 122 when the third sheet P is passed. The control unit 150 calculates the measured value of the average speed V3 of the transfer unit driving motor 122 at the time of passing the third sheet and the transfer unit driving motor 122 at the time of passing the first sheet stored in the storage unit 170. A difference X2 from the reference value is calculated, and this difference X2 is fed back to the torque command value of the fourth sheet P, which is the next sheet (FIGS. 4B and 4C). In this example, such feedback control is repeatedly executed until a tray change is performed.

  When the fourth sheet P is passed, a tray change is performed with the transfer roller 120 in pressure contact (during continuous operation). For example, a tray change is performed from a paper feed tray on which thin paper of the large capacity paper feeder 200 is set to a paper feed tray on which thick paper is set. When the tray change is completed, the control unit 150 determines whether or not the paper P fed from the paper feed tray is the first paper P after the tray change. In this example, the fifth sheet P from the start of the job becomes the first sheet P after the tray change.

  When it is determined that the fifth sheet P is the first (first) sheet P after the tray change, the average speed V4 of the transfer unit driving motor 122 during the passage of the fifth sheet P is set to The calculated average speed V4 is stored again in the storage unit 170 as a reference value (FIG. 4C). That is, the first stored reference value before the tray change is updated to the reference value of the average speed of the first sheet after the tray change. After the tray change, since the reference value is newly reset, feedback control to the torque command value for the next sheet is not performed. For this reason, the fifth torque command value is used as it is as the sixth torque command value.

  The torque command value for the fifth sheet is the same as that for the fourth sheet, but since the sheet P is changed to a thick sheet, the average speed (number of rotations) V4 of the transfer unit driving motor 122 is the roller diameter or the like. Due to the change, it becomes slower than the case where the thin paper P before the tray change is used (FIG. 4C). As a result, the transfer roller 120 side of the paper P passing between the transfer roller 120 and the intermediate transfer belt 104 becomes slower as the speed of the transfer unit drive motor 122 decreases, but the opposite intermediate transfer belt 104 side. Is conveyed at the same speed as the intermediate transfer belt 104. Therefore, the image magnification can be maintained within a normal range (C in FIG. 4D).

  When the passing of the fifth sheet P is completed, the sixth sheet P is continuously passed. The control unit 150 calculates the average speed V5 of the transfer unit driving motor 122 when the sixth sheet P is passed (FIG. 4C). The control unit 150 calculates a difference X3 between the calculated measurement value of the average speed V5 of the sixth transfer unit drive motor 122 and the reference value of the fifth transfer unit drive motor 122 stored in the storage unit 170. The difference X3 is calculated and fed back to the torque command value for the seventh sheet as the next sheet. Thereby, the torque command value for the seventh sheet decreases according to the difference X3, and the speed of the transfer unit driving motor 122 for the next sheet can be returned to the reference speed (FIGS. 4B and 4C).

  For the sixth sheet P, the average speed V5 of the transfer unit drive motor 122 is substantially the same as the average speed V4 of the transfer unit drive motor 122 for the fifth sheet P (FIG. 4C). Therefore, the speed of the fifth sheet and the front and back surfaces can be made the same, so that the image magnification can be maintained normally even after the tray change (C in FIG. 4D).

  For the seventh and subsequent sheets P, until the next tray change is performed, the first sheet P after the tray change, that is, when the fifth sheet P is passed through from the start of the job is passed. Using the reference value of the average speed V4 of the transfer unit driving motor 122, feedback control of the torque command value of the next sheet is performed. In the seventh sheet P, the average speed of the transfer unit drive motor 122 is substantially the same as the average speed V4 of the transfer unit drive motor 122 of the fifth sheet P (FIG. 4C). Even after the change, the image magnification can be maintained within the normal range (C in FIG. 4D).

  As described above, in the first embodiment, when the tray change is performed by changing the sheet during the continuous operation, the transfer unit driving motor at the time of passing the first first sheet P after the tray change is performed. The average speed of 122 is used as a reference value. Thus, for example, even when the average speed of the transfer unit drive motor 122 is reduced by changing the paper P from thin paper to thick paper by a tray change, the paper passing speed of the paper P after the tray change is used as a reference. Thus, the feedback control of the torque value is performed, so that it is possible to prevent the speed change of the transfer portion due to the change of the thick paper from being fed back, and the speed of the surface of the paper P relative to the intermediate transfer belt 104 can be maintained. As a result, it is possible to reliably prevent a change in image magnification after the tray change.

  In addition, according to the first embodiment, not only a long-term change in the environment / load over time of the load applied to the transfer unit such as the transfer roller 120 but also a short-term change at the beginning of the printing operation. In addition, the torque relationship with the intermediate transfer belt 104 is properly maintained, and unnecessary load fluctuations on the intermediate transfer belt 104 are minimized to prevent deterioration of image quality, while minimizing changes in image magnification caused by short-term load fluctuations. Can be

<Second Embodiment>
In the second embodiment, when a predetermined number of tray changes are made, the transfer portion is separated and subjected to constant speed control, and then pressed to perform constant torque control and reset the transfer portion speed. However, this is different from the first embodiment. The configuration of the other image forming apparatuses and the operation including constant speed control and constant torque control are the same as those in the first embodiment. Description is omitted.

[Explanation of image magnification change phenomenon]
As described in the first embodiment, the tray change is performed during the continuous operation, and the average speed of the transfer unit driving motor 122 at the time of passing the first sheet P after the tray change is used as the reference speed. When storing is repeated (absolute value is not compensated), a phenomenon in which the image magnification gradually changes may occur.

FIG. 5 is a diagram for explaining a phenomenon of a change in image magnification that occurs when a tray change is repeatedly performed. FIG. 5A shows the variation of the load torque, the vertical axis is the load torque, and the horizontal axis is the time. FIG. 5B shows the torque command value of the transfer unit driving motor 122, the vertical axis is the torque command value, and the horizontal axis is time. FIG. 5C shows the speed of the transfer unit drive motor 122, the vertical axis is the speed, and the horizontal axis is the time. Note that FIG. 5 is illustrated under the following conditions, but this example is set for convenience and is not limited to this example.
(1) The paper thickness is the same before and after the tray change (transfer speed = image magnification)
(2) Assuming that the space between sheets (running of the transfer section) becomes longer than usual due to restrictions such as the number of circulating tray changes,
a) When running between sheets, the load torque increases due to load fluctuations in the cleaning unit.
b) The load torque decreases when the paper passes after idle running, and if the torque command value is fixed, the speed of the first sheet is smaller than the second sheet.
c) From the above relation a) and b), after the tray change, the torque command value is lowered with respect to the previous state after the second sheet.
(3) Perform tray change every 2 pages

  The above phenomenon will be described in detail below. When the first sheet P and the second sheet P are passed after the pressure contact of the transfer roller 120, the load torque gradually decreases due to a decrease in the amount of the lubricant (FIG. 5A). On the other hand, the speed of the transfer roller 120 increases as the load torque decreases (FIG. 5C). At this time, the speed of the transfer roller 120 when the second sheet is passed is faster than the speed of the transfer roller 120 when the first sheet is passed.

  The control unit 150 uses the average speed (also referred to as a reference speed) VA of the transfer unit driving motor 122 measured when the first sheet P is passed, as a reference value, and the reference value and the second sheet P A difference Z1 from the measured value of the average speed VB of the transfer unit driving motor 122 measured at the time of passing the sheet is calculated, and this difference Z1 is fed back to the torque command value of the third sheet (FIG. 5B). , FIG. 5 (C)). As a result, the torque command value for the third sheet decreases by the difference Z1.

  When the second sheet P is passed, a tray change is performed. Since the period during which the transfer roller 120 is idle between the sheets on which the tray change is performed becomes longer, the load torque of the transfer roller 120 increases due to the influence of the above-described lubricant or the like (FIG. 5A). The speed of the transfer unit drive motor 122 gradually decreases due to the lowered torque command value and the increase of the load torque (FIG. 5C).

  When the tray change is completed, the passing of the third and subsequent sheets of paper P is started. The average speed VC of the transfer section drive motor 122 when the third sheet P passes after the tray change is equal to the average speed VA of the transfer section drive motor 122 when the first sheet P passes before the tray change. Is slower than

  When the third and fourth sheets P are passed, the load torque gradually decreases due to a decrease in the amount of the lubricant (FIG. 5A). On the other hand, the speed of the transfer unit driving motor 122 increases as the load torque decreases (FIG. 5C). At this time, since the average speeds VC and VD of the transfer unit driving motor 122 are slower than the average speed VA before the tray change in the images transferred to the third and fourth sheets P, the sheets are correspondingly reduced. Since the conveyance speed of P also becomes slow, it becomes an image contracted from the image transferred onto the first and second sheets of paper P.

  The control unit 150 uses the average speed VC of the transfer unit driving motor 122 measured during the passing of the third sheet P after the tray change as a reference value, and the reference value and the fourth sheet P pass through. A difference Z2 from the measured value of the average speed VD of the transfer unit driving motor 122 measured in the paper is calculated, and this difference Z2 is fed back to the torque command value of the fifth sheet (FIG. 5B). , FIG. 5 (C)). As a result, the torque command value for the fifth sheet decreases by the difference Z2.

  When the fourth sheet P is passed, a tray change is performed. Since the period during which the transfer roller 120 rotates idly becomes longer between the sheets on which the tray change is performed, the load torque of the transfer unit driving motor 122 increases due to the influence of the above-described lubricant or the like (FIG. 5A). ). The speed of the transfer unit drive motor 122 gradually decreases due to the lowered torque command value and the increase of the load torque (FIG. 5C).

  When the tray change is completed, the passage of the fifth and subsequent sheets of paper P is started. The average speed VE of the transfer unit drive motor 122 when the fifth sheet P after the tray change is passed is the average speed VC of the transfer unit drive motor 122 when the third sheet P before the tray change is passed. Is slower than

  When the fifth and sixth sheets P are passed, the load torque gradually decreases due to a decrease in the amount of the lubricant (FIG. 5A). On the other hand, the speed of the transfer unit driving motor 122 increases as the load torque decreases (FIG. 5C). At this time, since the average speeds VE and VF of the transfer unit driving motor 122 are slower than the average speed VC before the tray change in the images transferred to the fifth and sixth sheets P, the corresponding sheets Since the conveyance speed of P also becomes slow, the image is further shrunk from the image transferred to the third and fourth sheets P.

  As described above, when the control for re-storing the average speed of the first sheet at which the speed after the tray change is reduced as the reference value is repeated for each tray change, a phenomenon in which the image magnification gradually changes may occur. is there. Therefore, in the second embodiment, after a predetermined number of tray changes, the absolute value is compensated for “constant speed control when the transfer roller 120 is released, and a torque command corresponding to the load torque at that time. The control is returned to “fixed value” and the reference speed of the transfer section drive motor 122 is reset. In addition, although FIG. 5 demonstrated the example in case the fluctuation | variation of load torque rises as a whole with progress of time, the fluctuation | variation of load torque is not limited to this.

[Operation example of image forming apparatus]
Next, an example of the operation of the image forming apparatus 100 according to the second embodiment will be described. FIG. 6 is a flowchart illustrating an example of the operation of the image forming apparatus 100 including a case where a tray change is performed. The operations from step S100 to S150 are the same as the operations of the image forming apparatus 100 described in the first embodiment, and therefore will be described in a simplified manner.

  As shown in FIG. 6, in step S <b> 100, the control unit 150 causes the rotation speeds of the transfer roller 120 and the intermediate transfer belt 104 to be constant with the transfer roller 120 and the intermediate transfer belt 104 being separated. The image carrier driving motor 112 and the transfer unit driving motor 122 are controlled at a constant speed by feedback.

  In step S <b> 110, the control unit 150 presses the transfer roller 120 against the intermediate transfer belt 104 by controlling the transfer unit press-contact / separation mechanism 160.

  In step S120, when the pressure contact of the transfer roller 120 is completed, the control unit 150 continuously performs constant speed control of the image carrier driving motor 112 on the intermediate transfer belt 104 side, and controls the transfer unit driving motor 122 on the transfer roller 120 side. On the other hand, constant torque control is performed according to the constant speed driving torque detected before the pressure contact of the transfer roller 120.

  In step S <b> 130, the control unit 150 determines whether or not the sheet P to be passed between the intermediate transfer belt 104 and the transfer roller 120 is the first sheet. The first sheet P includes the sheet P that is first passed by the start of the job and the sheet P that is first passed after the tray change. For example, the control unit 150 determines whether or not the sheet P currently being passed is based on a count result of a sensor or the like installed at a predetermined position. The control unit 150 proceeds to step S140 when determining that the sheet P is the first sheet P, and proceeds to step S150 when determining that the sheet P is the second and subsequent sheets P.

  In step S140, the speed of the transfer unit driving motor 122 when the first sheet P is passed is measured. The control unit 150 calculates the average speed of the transfer unit driving motor 122 from the measured speed, and stores the average speed in the storage unit 170 as a reference value.

  On the other hand, in step S150, the speed of the transfer unit driving motor 122 when the second and subsequent sheets P are passed is measured. The control unit 150 calculates the average speed of the transfer unit driving motor 122 from the measured speed, the measured value of the calculated average speed, and the reference value of the first sheet P stored in the storage unit 170. To calculate the difference. After calculating the difference, the control unit 150 feeds back the difference to the torque command value and updates the torque command value.

  In step S160, the control unit 150 determines whether the job is finished. When the control unit 150 determines that all jobs have been completed, the series of image forming operations ends. On the other hand, if it is determined that all jobs have not been completed, the process proceeds to step S170.

  In step S170, the control unit 150 determines whether or not to change the tray based on the content of the job. Examples of the case where the tray is changed include a case where the type of paper is changed from thin paper to thick paper, or a case where the paper P in the paper feeding tray that is currently feeding is exhausted and the paper feeding tray is changed to another paper feeding tray. Can be mentioned. If it is determined that the tray change is to be performed, the control unit 150 proceeds to step S180. On the other hand, if it is determined that the tray change is not performed, the process returns to step S130 to determine whether or not the sheet P after the tray change is the first sheet P.

  In step S180, when performing the tray change, the control unit 150 adds +1 to the tray change counter value, and updates the counter value before the addition to the counter value after the addition. This counter value can be stored in advance in the storage unit 170, for example.

  In step S190, control unit 150 determines whether or not the counter value after the addition has reached a preset reference count value. For example, the control unit 150 reads a reference counter value stored in the storage unit 170 and compares the reference counter value with the added counter value. If the counter value after the addition reaches the reference counter value as a result of the comparison, the speed of the transfer unit driving motor 122 decreases due to multiple tray changes, and a change in image magnification occurs or is possible. It is determined that the property is high, and the process proceeds to step S200. On the other hand, if the control unit 150 determines that the counter value after addition is less than the reference count value, the speed change of the transfer unit driving motor 122 due to the tray change is within an allowable range, or there is no problem. It returns to step S130, and it is determined whether or not the paper P after the tray change is the first paper P.

  In step S <b> 200, when the number of tray changes reaches the reference counter value, the control unit 150 controls the transfer unit press-contact / separation mechanism 160 to separate the transfer roller 120 from the intermediate transfer belt 104. Further, the control unit 150 clears the counter value for tray change. When the counter value is cleared, the process returns to step S100, and constant speed control of the image carrier driving motor 112 and the transfer unit driving motor 122 is performed in a state where the transfer roller 120 and the intermediate transfer belt 104 are separated. Subsequently, the job is restarted, and with the transfer roller 120 pressed against the intermediate transfer belt 104, a constant torque according to the constant speed driving torque detected before the transfer roller 120 is pressed against the transfer unit drive motor 122. Take control. Thereby, the reduced speed of the transfer unit driving motor 122 can be reset to the speed at the constant speed control. In the second embodiment, such a series of operations is repeatedly executed.

  As described above, in the second embodiment, the transfer roller 120 is once separated when a predetermined number of tray changes are performed in response to a gradual change in the speed of the transfer roller 120 due to repeated tray changes. Thus, the constant speed control is performed, and the constant torque control is performed according to the driving torque at the time of the constant speed control after the pressure contact. Thereby, the speed and torque command value of the transfer unit driving motor 122 can be reset, and the change range of the image magnification throughout the job can be kept within a predetermined range.

  It should be noted that the technical scope of the present invention is not limited to the above-described embodiments, and includes those in which various modifications are made to the above-described embodiments without departing from the spirit of the present invention. Although the above embodiment has been described on the premise of the intermediate transfer belt, the present invention may be an image forming apparatus that does not perform intermediate transfer, in which a transfer portion is pressed against a photoconductor as an image carrier. The same effect can be obtained.

  Further, as an example of the tray change, the example of the thin paper P to the thick paper P has been described. However, the present invention can be applied to the case where the thick paper P is changed to the thin paper P.

100 Image forming apparatus 104 Intermediate transfer belt (image carrier)
120 Transfer roller (transfer section)
122 Transfer unit drive motor (drive unit)
124 Drive force transmission mechanism (drive unit)
126 Transfer unit drive roller (drive unit)
128 Transfer unit driven roller (drive unit)
130 Transfer unit drive belt (drive unit)
140 Cleaning unit 150 Control unit

Claims (2)

An image carrier for carrying a toner image;
A transfer unit that is capable of being pressed against and separated from the image carrier, and that transfers a toner image carried on the image carrier to the paper during the pressure contact;
A drive unit that rotationally drives the transfer unit based on a command value;
A constant speed control for setting a command value in the drive section and driving the transfer section to rotate at a constant speed; and a control section for performing constant torque control for rotating the transfer section at a constant torque;
The controller is
A state in which the transfer unit is pressed against the image carrier in accordance with a constant speed driving torque detected when performing constant speed control of the drive unit in a state where the transfer unit is separated from the image carrier. To perform constant torque control of the drive unit,
In the constant torque control, an average speed of the driving unit obtained when the first sheet is passed through the transfer unit is used as a reference value, and the reference value and a second and subsequent predetermined sheets are used as the reference value. Calculating a difference from the measured value of the average speed of the driving unit obtained when the sheet passes through the transfer unit, calculating a command value of a sheet conveyed next to the predetermined sheet of paper from the difference ,
When the tray change is performed during the continuous operation, the average speed of the drive unit obtained when the first sheet after the tray change is passed through the transfer unit is set as the reference value, and the reference value and A command value for the next sheet is calculated from a difference from an average speed measurement value of the drive unit obtained when the second and subsequent sheets after the tray change are passed through the transfer unit. apparatus. The controller is
When a tray change is performed a predetermined number of times, the transfer unit is separated and the drive of the transfer unit is controlled at a constant speed, and then the constant speed detected when the transfer unit is pressed again and the constant speed control is performed. The image forming apparatus according to claim 1, wherein constant torque control of the driving unit is performed in accordance with speed driving torque.

Images