JP2013156398A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize stable rotation of a photoconductor drum even when a sheet such as cardboard is used in an image forming apparatus in which an image is transferred from a photoconductor drum to a sheet.SOLUTION: When cardboard is used for a sheet P on which a toner image is transferred, conveyance speed of the sheet P is controlled. A conveyance belt 180 for conveying the sheet P decreases the conveyance speed of the sheet P when the leading edge of the sheet P reaches a deceleration position P5 on the upstream side of a transfer position TP. The conveyance belt 180 subsequently restores the conveyance speed of the sheet P at an acceleration point P6. The leading edge of the sheet P contacts the peripheral surface of a photoconductor drum 121 at the reduced conveyance speed.

Description

  The present invention relates to an image forming apparatus in which an image is transferred from a photosensitive drum to a sheet.

  2. Description of the Related Art Conventionally, in an image forming apparatus that forms an image on a sheet, a technique described in Patent Document 1 is known as a technique for controlling the conveyance speed of a conveyed sheet. In this technique, the image forming apparatus includes a paper feed roller and a registration roller. The leading edge of the sheet sent out by the paper feeding roller comes into contact with the registration roller in a state where the rotation is stopped, so that the inclination of the paper due to the oblique feeding of the paper is corrected.

  In the technique described in Patent Document 1, the sheet conveyance speed is controlled to be reduced immediately before contacting the registration roller in order to reduce noise generated when the sheet contacts the registration roller.

JP 2009-249099 A

  In recent years, with the development of a sheet conveying technique in an image forming apparatus and a transfer technique for transferring an image to a sheet, thick paper having a thickness of 0.10 mm or more is used as the sheet in the image forming apparatus. In the technique described in Patent Document 1, even when such thick paper is used, abnormal noise generated when contacting the registration roller is suppressed as much as possible.

  On the other hand, when such thick paper is further fed from the registration roller and enters a transfer nip formed between the photoconductor and the transfer unit, the rotation of the photoconductor drum may be temporarily hindered. . In this case, image quality defects occur in the toner image forming process at the development position and the toner image transfer process to the sheet.

  The present invention has been made in view of the above problems, and in an image forming apparatus in which an image is transferred from a photosensitive drum to a sheet, a stable photosensitive drum even when cardboard or the like is used as a sheet. The purpose is to realize the rotation of.

  An image forming apparatus according to one aspect of the present invention is arranged to be rotationally driven and carry a toner drum carrying a toner image on a peripheral surface thereof and to be opposed to the photosensitive drum at a transfer position, and to place the toner image on a sheet. A transfer roller to be transferred, a sheet conveyance path that extends so as to pass through the transfer position, and is disposed upstream of the transfer position of the sheet conveyance path in the sheet conveyance direction. A conveyance unit that conveys the sheet toward the transfer position; and a conveyance speed control unit that controls a conveyance speed at which the conveyance unit conveys the sheet. The conveyance speed control unit includes the conveyance unit, After the sheet is conveyed at the first speed, immediately before the leading edge of the sheet reaches the transfer position, the conveyance is performed so that the sheet is conveyed at a second speed slower than the first speed. And controlling the conveying speed of the unit.

  According to this configuration, the sheet is conveyed by the conveying unit toward the transfer position at the first speed. The conveyance speed control unit controls the conveyance speed of the conveyance unit at a second speed slower than the first speed immediately before the leading edge of the sheet reaches the transfer position. Therefore, even when thick paper or the like is used as the sheet, the leading edge of the sheet enters the transfer position at the second speed before the toner is transferred to the sheet at the transfer position. For this reason, even when thick paper or the like is used as a sheet, even when the leading end of the sheet comes into contact with the peripheral surface of the photosensitive drum, the rotation of the photosensitive drum is hardly hindered. As a result, a stable toner image is formed on the peripheral surface of the photosensitive drum.

  In the above configuration, the second speed is preferably 50% to 90% of the first speed.

  According to this configuration, the conveyance speed control means has a conveyance speed of the conveyance means such that the leading edge of the sheet contacts the peripheral surface of the photosensitive drum at a speed of 50% to 90% of the first speed. To control. Therefore, even when thick paper or the like is used as a sheet, rotation of the photosensitive drum is difficult to be inhibited when the leading end of the sheet contacts the peripheral surface of the photosensitive drum.

  In the above configuration, the transport unit is an endless transport belt that is rotationally driven and carries a sheet on a surface thereof, and the transfer roller is in contact with a rotation inner peripheral surface of the transport belt, and the photosensitive drum Preferably, the transport speed control means controls the transport speed to the first speed after controlling the transport speed to the second speed.

  According to this configuration, the conveyance belt as the conveyance unit carries the sheet on the surface and conveys the sheet to the transfer position. Further, a transfer roller is brought into contact with the rotation inner peripheral surface of the transport belt. The conveyance belt conveys the sheet at the second speed immediately before reaching the transfer position, and then passes the transfer position at the first speed. Therefore, the sheet is stably conveyed to the transfer position by the conveyance belt, and the rotation of the photosensitive drum is difficult to be inhibited when the leading end of the sheet contacts the peripheral surface of the photosensitive drum.

  In the above configuration, it is preferable that the conveying unit is a registration roller that conveys the sheet toward the transfer position in accordance with a timing at which the toner image is transferred to the sheet.

  According to this configuration, the sheet is conveyed toward the transfer position by the registration roller. At this time, the registration roller conveys the sheet at the second speed immediately after reaching the transfer position after conveying the sheet at the first speed. Then, the sheet is transferred from the registration roller to the transfer roller, and passed through the transfer position while the toner image is transferred. Therefore, in the configuration including the registration roller and the transfer roller, the rotation of the photosensitive drum is stably maintained.

  In the above configuration, the conveyance speed control unit includes a detection unit that is disposed opposite to the sheet conveyance path on the upstream side in the sheet conveyance direction from the transfer position and detects a position of a leading end portion of the sheet. Preferably, the conveying speed of the conveying means is changed from the first speed to the second speed according to the detection result of the leading edge of the sheet by the detecting means.

  According to this configuration, the detection unit is disposed opposite to the sheet conveyance path on the upstream side in the sheet conveyance direction from the transfer position, and detects the position of the leading edge of the sheet. The transport speed control means changes the transport speed of the transport means from the first speed to the second speed according to the detection result of the detection means. Therefore, the time when the leading edge of the sheet reaches the transfer position is predicted, and the sheet conveyance speed can be accurately changed to the second speed.

  In the above configuration, the sheet information acquisition unit that acquires the sheet information of the sheet conveyed through the sheet conveyance path is provided, and the conveyance speed control unit is based on the sheet information acquired by the sheet information acquisition unit. It is preferable to control the transport speed of the transport means.

  According to this configuration, the conveyance speed control unit controls the conveyance speed of the conveyance unit based on the sheet information acquired by the sheet information acquisition unit. Therefore, the conveyance speed of the conveyance unit is controlled according to the sheet characteristics, and the rotation of the photosensitive drum is stably maintained.

  In the above configuration, it is preferable that the transport speed control unit controls the transport speed so that the second speed becomes slower as the thickness of the sheet increases.

  According to this configuration, the transport speed control unit controls the transport speed of the transport unit so that the second speed becomes slower as the thickness of the sheet increases. For this reason, even when the sheet is thick, rotation of the photosensitive drum is difficult to be inhibited when the leading end of the sheet comes into contact with the peripheral surface of the photosensitive drum immediately before the transfer position.

  In the above configuration, it is preferable that the image forming apparatus further includes an input unit for inputting image formation information, and the sheet information acquisition unit acquires the sheet information from the image formation information input to the input unit.

  According to this configuration, the sheet information acquisition unit acquires the sheet information from the image formation information input to the input unit. Therefore, it is possible to control the transport speed of the transport unit using the image formation information.

  In the above configuration, the sheet conveyance path further includes a conveyance roller that is arranged in the sheet conveyance path and conveys the sheet, and a torque detection unit that detects a rotational torque of the conveyance roller, and the sheet information acquisition unit includes the torque It is preferable to acquire the sheet information based on the rotational torque detected by the detecting means.

  According to this configuration, the sheet information acquisition unit acquires the sheet information based on the rotational torque detected by the torque detection unit. Therefore, the conveyance speed of the conveyance unit is controlled according to the characteristics of the sheet actually conveyed on the sheet conveyance path, and the rotation of the photosensitive drum is stably maintained.

  In the above configuration, it is preferable that the photosensitive drum is rotationally driven so that the peripheral surface of the photosensitive drum passes through the transfer position at a slower speed than the peripheral surface of the transfer roller.

  According to this configuration, the toner image is transferred to the sheet in a state where there is a speed difference between the photosensitive drum and the transfer roller. Therefore, the hollowing out phenomenon in which the central portion of the toner image nipped at the transfer position is difficult to be transferred is suppressed.

  According to the present invention, in an image forming apparatus in which an image is transferred from a photosensitive drum to a sheet, stable rotation of the photosensitive drum can be realized even when a sheet such as cardboard is used. In particular, when the sheet enters the transfer nip portion, the rotational speed of the photosensitive drum is unlikely to fluctuate. For this reason, the formation of the toner image on the photosensitive drum at the development position is stably realized.

1 is a perspective view of an image forming apparatus according to an embodiment of the present invention. 1 is an internal cross-sectional view of an image forming apparatus according to an embodiment of the present invention. FIG. 3 is an enlarged view of a sheet conveyance path of the image forming apparatus according to the embodiment of the present invention. FIG. 3 is a diagram illustrating a structure around a transfer position in the image forming apparatus according to the embodiment of the present invention. 1 is a block diagram illustrating an electrical structure of an image forming apparatus according to an embodiment of the present invention. 4 is a flowchart illustrating drive control of the image forming apparatus according to the embodiment of the present invention. It is a figure which shows the change of the conveyance speed of the sheet | seat which concerns on embodiment of this invention. FIG. 6 is an enlarged view of a sheet conveyance path of an image forming apparatus according to another embodiment of the present invention. It is a block diagram which shows the electrical structure of the image forming apparatus which concerns on other embodiment of this invention.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of an image forming apparatus 1 according to an embodiment of the present invention. FIG. 2 schematically shows an internal structure of image forming apparatus 1 shown in FIG. FIG. 3 is an enlarged view of the periphery of the transfer position TP inside the image forming apparatus 1. In FIG. 2, the upper housing 22 of FIG. 1 does not appear. The image forming apparatus 1 shown in FIGS. 1 and 2 is a so-called monochrome printer, but in other embodiments, the image forming apparatus is a color printer, a facsimile machine, a multifunction machine having these functions, or a toner image. Other devices for forming the sheet may be used. Note that the terms used in the following description, such as “up”, “down”, “front”, “rear”, “left” and “right”, are merely for the purpose of clarifying the description. There is no limitation on the principle of the image forming apparatus. Further, in the following description, the term “sheet” means copy paper, coated paper, OHP sheet, cardboard, postcard, tracing paper, other sheet material subjected to image forming processing, or any processing other than image forming processing. Means the sheet material to receive.

  The image forming apparatus 1 includes a main housing 2 having a substantially rectangular parallelepiped shape. The main housing 2 connects a substantially rectangular parallelepiped lower housing 21, a substantially rectangular parallelepiped upper housing 22 disposed above the lower housing 21, and the lower housing 21 and the upper housing 22. And a connecting housing 23. The connection housing 23 extends along the right edge and the back edge of the main housing 2. The sheet subjected to the printing process is discharged into a discharge space 24 surrounded by the lower casing 21, the upper casing 22, and the connecting casing 23.

  The operation unit 221 protruding in the front direction of the upper housing 22 includes, for example, an LCD touch panel 222. The operation unit 221 is formed so as to be able to input information related to the image forming process. For example, the user can input the number of sheets to be printed, the print density, and the like through the LCD touch panel 222. Housed in the upper housing 22 are mainly an apparatus for reading an image of a document and an electronic circuit that controls the entire image forming apparatus 1.

  A pressing cover 223 disposed on the upper housing 22 is used for pressing the document. The presser cover 223 is attached to the upper housing 22 so as to be rotatable up and down. The user rotates the holding cover 223 upward to place the document on the upper housing 22. Thereafter, the user can operate the operation unit 221 to read an image of the document on a device provided in the upper housing 22.

  A manual feed tray 240 is disposed on the right side surface of the lower housing 21. The manual feed tray 240 can be rotated up and down on the upper end 240B side with the lower end 240A (FIG. 2) as a fulcrum. The user can place a sheet on the manual feed tray 240 when the manual feed tray 240 is rotated downward and the manual feed tray 240 is in a position protruding to the right of the lower housing 21. The sheet on the manual feed tray 240 is drawn into the lower housing 21 based on an instruction input by the user through the operation unit 221, is subjected to image forming processing, and is discharged into the discharge space 24.

  With reference to FIG. 2, the image forming apparatus 1 includes a cassette 110, a paper feeding unit 11, a second paper feeding roller 114, an intermediate roller 115, a registration roller pair 116, and an image forming unit 120. The paper feed unit 11 includes a pickup roller 112 and a first paper feed roller 113. The sheet feeding unit 11 sends the sheet P to the sheet conveyance path PP. The sheet conveyance path PP is disposed so as to pass from the sheet feeding unit 11 through the intermediate roller pair 115 and the registration roller pair 116 to the transfer position TP (FIG. 3) disposed in the image forming unit 120. It is a transport path.

  The cassette 110 accommodates the sheet P inside. The cassette 110 can be pulled out from the lower housing 21 in the front direction (the front side in FIG. 2). The sheet P accommodated in the cassette 110 is sent upward in the lower housing 21. Thereafter, the sheet P is subjected to an image forming process in the lower housing 21 based on an instruction input by the user through the operation unit 221, and is discharged to the discharge space 24. The cassette 110 includes a lift plate 111 that supports the sheet P. The lift plate 111 is inclined so as to push up the leading edge of the sheet P upward.

  The pickup roller 112 is disposed on the leading edge of the sheet P pushed upward by the lift plate 111. When the pickup roller 112 rotates, the sheet P is pulled out from the cassette 110.

  The first paper feed roller 113 is disposed downstream of the pickup roller 112 in the sheet conveyance direction. The first paper feed roller 113 sends the sheet P further downstream. The second paper feed roller 114 is disposed inside the manual feed tray 240. The second paper feed roller 114 pulls the sheet P on the manual feed tray 240 into the lower housing 21. The user can selectively use the sheet P stored in the cassette 110 or the sheet P placed on the manual feed tray 240.

  The intermediate roller pair 115 (conveying roller) is disposed on the downstream side in the sheet conveying direction of the first paper feeding roller 113 and the second paper feeding roller 114. The intermediate roller pair 115 conveys the sheet P sent out by the first paper feed roller 113 and the second paper feed roller 114 further downstream.

  The registration roller pair 116 defines the position of the sheet in a direction orthogonal to the sheet conveyance direction. Thereby, the position of the image formed on the sheet P is adjusted. The registration roller pair 116 forms a nip portion RP (FIG. 3) between the rollers. The registration roller pair 116 conveys the sheet P to the image forming unit 120 in accordance with the timing at which the toner image is transferred to the sheet P in the image forming unit 120. The registration roller pair 116 has a function of correcting skew (skew) of the sheet P.

  2 and 3, the image forming unit 120 includes a photosensitive drum 121, a charger 122, an exposure device 123, a developing device 124, a toner container 125, a transfer roller 126, and a conveyance belt 180. And a cleaning device 127.

  The photosensitive drum 121 has a substantially cylindrical shape. The photosensitive drum 121 has an electrostatic latent image formed on its peripheral surface and carries a toner image corresponding to the electrostatic latent image.

  The charger 122 is applied with a predetermined voltage and charges the peripheral surface of the photosensitive drum 121 substantially uniformly. The exposure device 123 irradiates the peripheral surface of the photosensitive drum 121 charged by the charger 122 with laser light. The laser light is emitted in accordance with image data output from an external device (not shown) such as a personal computer that is communicably connected to the image forming apparatus 1. As a result, an electrostatic latent image corresponding to the image data is formed on the peripheral surface of the photosensitive drum 121.

  The developing device 124 supplies toner to the peripheral surface of the photosensitive drum 121 on which the electrostatic latent image is formed. The toner container 125 supplies toner to the developing device 124. The toner container 125 supplies toner to the developing device 124 sequentially or as necessary. When the developing device 124 supplies toner to the photosensitive drum 121, the electrostatic latent image formed on the peripheral surface of the photosensitive drum 121 is developed (visualized). As a result, a toner image is formed on the peripheral surface of the photosensitive drum 121. The developing device includes a developing roller 124A (FIG. 3) that carries toner on its peripheral surface. The developing roller 124A is disposed to face the photosensitive drum 121 at the developing position DP. The developing roller 124 </ b> A is rotated and supplies toner to the photosensitive drum 121.

  The transfer roller 126 is disposed to face the peripheral surface of the photosensitive drum 121 at the transfer position TP (FIG. 3). The transfer roller 126 is disposed so as to be in contact with the inner peripheral surface of the conveyance belt 180 and to face the photosensitive drum 121.

  The conveyor belt 180 is an endless belt. The conveyance belt 180 is driven to rotate, carries the sheet P on its surface, and conveys the sheet P to the transfer position TP. The conveyor belt 180 transmits a rotational driving force to the transfer roller 126 via the inner peripheral surface. The conveyance belt 180 is stretched by a stretching roller 182 and a driving roller 181 on the upstream side and the downstream side in the sheet conveyance direction from the transfer roller 126. The driving roller 181 receives a rotational driving force from a third driving unit 513 described later, and rotates the conveyance belt 180. The transfer roller 126 and the stretching roller 182 rotate along with the conveyance belt 180 and rotate at the same speed as the conveyance belt 180. When the sheet P passes the transfer position TP, the toner image formed on the peripheral surface of the photosensitive drum 121 is transferred to the sheet P.

  The cleaning device 127 removes the toner remaining on the peripheral surface of the photosensitive drum 121 after the toner image is transferred to the sheet P. The peripheral surface of the photosensitive drum 121 cleaned by the cleaning device 127 passes again below the charger 122 and is uniformly charged. Thereafter, the above-described toner image is newly formed.

  The image forming apparatus 1 further includes a fixing device 130 (FIG. 2) that fixes the toner image on the sheet P on the downstream side in the transport direction from the image forming unit 120. The fixing device 130 includes a heating roller 131 that melts the toner on the sheet P, and a pressure roller 132 that causes the sheet P to adhere to the heating roller 131. When the sheet P passes between the heating roller 131 and the pressure roller 132, the toner image is fixed on the sheet P.

  The image forming apparatus 1 further includes a transport roller pair 133 disposed downstream of the fixing device 130 and a discharge roller pair 134 disposed downstream of the transport roller pair 133. The sheet P is discharged from the lower housing 21 by the conveyance roller pair 133 and the discharge roller pair 134. The sheets P discharged from the lower housing 21 are stacked on the upper wall 210.

  Further, the image forming apparatus 1 includes a first sensor 510 and a second sensor 511. The first sensor 510 and the second sensor 511 detect the position of the sheet P in the sheet conveyance path PP. As the first sensor 510 and the second sensor 511, a known optical sensor or the like is used. That is, in the sheet conveyance path PP, when there is no sheet P at a position where the first sensor 510 and the second sensor 511 face each other, the first sensor 510 and the second sensor 511 output a LOW signal of 0V. On the other hand, when the sheet P exists at a position where the first sensor 510 and the second sensor 511 face each other, the first sensor 510 and the second sensor 511 output a HIGH signal of + 5V. Thus, the position of the sheet P is detected by the first sensor 510 and the second sensor 511 outputting a HIGH signal and a LOW signal according to the presence or absence of the sheet.

  The first sensor 510 is disposed at a position immediately upstream of the intermediate roller pair 115 in the sheet conveyance path PP. The first sensor 510 is used to control the timing at which the intermediate roller pair 115 is rotationally driven. The second sensor 511 is disposed at a position immediately upstream of the registration roller pair 116 in the sheet conveyance path PP. The second sensor 511 is used to control the timing at which the registration roller pair 116 is rotationally driven. The second sensor 511 is used to predict the time when the leading edge of the sheet P reaches a position immediately upstream of the transfer position TP.

<Problems when entering the transfer position of the sheet P>
Next, a problem when the sheet P is conveyed to the transfer position TP will be described. FIG. 4 is a schematic diagram when the sheet P is conveyed to the transfer position TP. The photosensitive drum 121 and the developing roller 124A are rotationally driven in the directions of arrows A1 and A2, respectively. A transfer position TP is formed at a position where the photosensitive drum 121 and the transfer roller 126 face each other. When the driving roller 181 is rotationally driven in the direction of arrow A7, the conveyance belt 180, the transfer roller 126, and the stretching roller 182 are rotated in the directions of arrows A3, A5, and A6, respectively. An electrostatic latent image (not shown) is formed on the photosensitive drum 121 in accordance with the timing at which the sheet P is conveyed to the transfer position TP. The electrostatic latent image is visualized as a toner image TI by the developing roller 124A.

  Here, the leading end portion of the sheet P conveyed toward the transfer position TP contacts the circumferential surface of the photosensitive drum 121 before reaching the transfer position TP. When relatively thin paper such as plain paper is used as the sheet P, the plain paper easily enters the transfer position TP by the rotational force of the photosensitive drum 121. However, in the case of thick paper having a thickness of 0.10 mm or more, as shown in FIG. 4, the leading end portion of the sheet P contacts the peripheral surface of the photosensitive drum 121, so that the direction of the arrow A <b> 1 of the photosensitive drum 121 is reached. Rotation is temporarily prevented. At this time, a toner image TI transferred to the sheet P is formed at the development position DP. As shown in the enlarged view of FIG. 4, when the rotation of the photosensitive drum 121 is temporarily hindered, the toner image TI formed on the peripheral surface of the photosensitive drum 121 is carried on the developing roller 124A. The toner layer (not shown) is scraped off, and an image defect CT occurs. If the peripheral surface of the photoconductive drum 121 facing the development position DP is a non-image portion, the toner layer on the developing roller 124A is removed when the rotation of the photoconductive drum 121 is temporarily hindered. More than usual, it contacts the peripheral surface of the photosensitive drum 121. For this reason, toner fog occurs on the photosensitive drum 121.

  As described above, in the configuration in which the toner image is transferred from the photosensitive drum 121 to the sheet P, when the relatively thick sheet P is transported to the transfer position TP, the position is upstream of the transfer position TP in the sheet transport direction. The leading end of the sheet P is in contact with the peripheral surface of the photosensitive drum 121. At this time, there arises a problem that the rotation of the photosensitive drum 121 becomes unstable temporarily. In order to solve the problems as described above, in this embodiment, the conveyance speed of the sheet P when entering the transfer position TP is suitably controlled.

  Next, the main electrical configuration of the image forming apparatus 1 according to the present embodiment will be described. The image forming apparatus 1 includes a control unit 500 that comprehensively controls the operation of each unit of the image forming apparatus 1. FIG. 5 is a functional block diagram of the control unit 500. The controller 500 includes a CPU (Central Processing Unit), a ROM (Read Only Memory) that stores a control program, a RAM (Random Access Memory) that is used as a work area of the CPU, and the like. In addition to the first sensor 510 and the second sensor 511, the control unit 500 includes an application unit 525, a drum driving unit 520, a first driving unit 521, a second driving unit 522, a third driving unit 523, a torque. A sensor 509 and the like are electrically connected. The control unit 500 includes an overall control unit 501, a speed control unit 502, a time measurement unit 503, a storage unit 504, and a sheet information acquisition unit 505.

  The application unit 525 includes a DC power supply and an AC power supply, and applies a developing bias to the developing roller 124A based on a control signal from the overall control unit 501.

  The drum driving unit 520 includes a motor and a gear mechanism that transmits torque thereof, and rotates the photosensitive drum 121 in accordance with a control signal from the overall control unit 501. The drum driving unit 520 causes the photosensitive drum 121 to rotate at a transfer position TP so that the circumferential surface of the photosensitive drum 121 rotates at a slightly slower speed than the circumferential surfaces of the transport belt 180 and the transfer roller 126. Rotate. Specifically, the peripheral surface of the photosensitive drum 121 is rotated at a speed that is 1 to 3% slower than the peripheral surfaces of the conveyance belt 180 and the transfer roller 126. Accordingly, the toner image is transferred to the sheet P in a state where a speed difference is generated between the photosensitive drum 121 and the transfer roller 126. Therefore, the hollowing out phenomenon in which the central portion of the toner image nipped at the transfer position TP is difficult to be transferred is suppressed. Further, the rotational driving force transmitted to the photosensitive drum 121 is transmitted to the developing roller 124A by a gear train (not shown), and the developing roller 124A is rotationally driven.

  The first drive unit 521 includes a motor and a gear mechanism that transmits torque thereof, and rotates the intermediate roller pair 115 according to a control signal from the overall control unit 501.

  The second drive unit 522 includes a motor and a gear mechanism that transmits torque thereof, and rotates the registration roller pair 116 according to a control signal from the overall control unit 501.

  The third drive unit 523 includes a motor and a gear mechanism that transmits the torque thereof, and rotates the drive roller 181 in accordance with a control signal from the overall control unit 501. When the driving roller 181 is rotationally driven, the conveying belt 180 is rotated.

  The torque sensor 509 is connected to a drive shaft (not shown) of the intermediate roller pair 115. The torque sensor 509 detects the torque fluctuation of the intermediate roller pair 115 at the timing when the sheet P conveyed on the sheet conveyance path PP is inserted into the nip portion of the intermediate roller pair 115. The torque sensor 509 outputs an electrical signal corresponding to the torque fluctuation to the control unit 500. The electrical signal detected by the torque sensor 509 is acquired by the sheet information acquisition unit 505 and converted into paper thickness information of the sheet P.

  The control unit 500 functions to include an overall control unit 501, a speed control unit 502, a time measurement unit 503, a storage unit 504, and a sheet information acquisition unit 505 by the CPU executing a control program stored in the ROM. To do.

  The overall control unit 501 controls the drum driving unit 520, the first driving unit 521, the second driving unit 522, and the third driving unit 523 to respectively control the photosensitive drum 121, the developing roller 124A, the intermediate roller pair 115, and the resist. The roller pair 116 and the driving roller 181 are driven to rotate. The overall control unit 501 also controls the applying unit 525 to apply a developing bias to the developing roller 124A.

  The speed control unit 502 controls the third driving unit 523 to control the rotation speed of the driving roller 181. The speed control unit 502 refers to the first count time TC1 counted by the time measuring unit 503, and at the timing when the leading end of the conveyed sheet P approaches the peripheral surface of the photosensitive drum 121, Reduce the speed. As a result, the conveyance speed V of the sheet P by the conveyance belt 180 is reduced. The speed control unit 502 further increases the rotational speed of the driving roller 181 at the timing when the leading end of the sheet P approaches the transfer position TP. Thereby, the conveyance speed V of the sheet P is increased.

  The time measuring unit 503 starts counting the first count time TC1 from the timing when the second sensor 511 detects the leading end of the sheet P. The first count time TC1 counted by the time measuring unit 503 is used to determine the rotation start timing of the registration roller pair 116 and the deceleration and acceleration timings of the conveyance belt 180. The first count time TC1 is stored in the storage unit 504.

  The storage unit 504 stores the first count time TC1 counted by the time measuring unit 503. The storage unit 504 stores time thresholds T0, T1, and T2 for comparison with the first count time TC1 in advance. The time threshold T0 is used when the timing at which the registration roller pair 116 is rotated is controlled. The time threshold T1 is used when the timing at which the conveyance belt 180 is decelerated is controlled. The time threshold T2 is used when the timing at which the conveyor belt 180 is accelerated is controlled. Further, the storage unit 504 stores in advance the deceleration speeds V1 and V2 of the conveyance belt 180 corresponding to the type of the sheet P.

  The sheet information acquisition unit 505 acquires the electrical signal output from the torque sensor 509 according to the torque fluctuation of the intermediate roller pair 115. Further, the sheet information acquisition unit 505 derives the paper thickness information of the sheet P according to the level of the electric signal. The paper thickness information is referred to by the speed control unit 502, and the conveyance speed of the sheet P is controlled according to the paper thickness information.

<Conveyance control of sheet P>
Next, the control of the conveying operation of the sheet P according to the present embodiment will be described in detail. FIG. 6 is a flowchart in the conveying operation of the sheet P. 3, 5, and 6, when image forming operation of image forming apparatus 1 is started by control unit 500, sheet feeding unit 11 feeds sheet P from cassette 110 (step) S001). At this time, the paper feeding unit 11 feeds the sheet P at the transport speed V0 (first speed).

  Eventually, the first sensor 510 detects the leading edge of the sheet P fed by the paper feeding unit 11 (YES in step S002). At this time, the output signal of the first sensor 510 is switched from the LOW signal to the HIGH signal. When a HIGH signal is output from the first sensor 510, the overall control unit 501 controls the first drive unit 521 to rotate the intermediate roller pair 115 (step S003). The intermediate roller pair 115 conveys the sheet P at the conveyance speed V0. As a result, the sheet P is further conveyed toward the registration roller pair 116.

  When the intermediate roller pair 115 is rotationally driven and the sheet P passes through the nip portion of the intermediate roller pair 115, the torque sensor 509 detects the torque fluctuation of the intermediate roller pair 115 (step S004). The torque sensor 509 outputs an electrical signal corresponding to the torque fluctuation amount to the control unit 500. The electrical signal is acquired by the sheet information acquisition unit 505.

  When the sheet P is conveyed further downstream in the sheet conveyance path PP by the intermediate roller pair 115, the second sensor 511 detects the leading end of the sheet P (YES in step S005). At this time, the output signal of the second sensor 511 is switched from the LOW signal to the HIGH signal. When the HIGH signal is output from the second sensor 511, the time measuring unit 503 starts counting the first count time TC1 (step S006). The time measuring unit 503 counts the first count time TC1 until the first count time TC1 reaches the time threshold value T0 stored in the storage unit 504 in advance. This time threshold value T0 is calculated in advance as T0 = T + ΔT. The time T is a value obtained by dividing the distance from the position where the second sensor 511 is opposed to the nip portion RP of the registration roller pair 116 in the sheet conveyance path PP by the conveyance speed V0 of the sheet P. That is, the time T corresponds to the time for the leading end of the sheet P to move from the second sensor 511 to the nip portion RP of the registration roller pair 116.

  On the other hand, ΔT is a preset delay time. The delay time corresponds to the time until the registration roller pair 116 is rotationally driven after the leading edge of the sheet P reaches the nip portion RP. The delay time ΔT is set to correct skew (skew) of the sheet P in the registration roller pair 116. When the sheet P is conveyed along the sheet conveyance path PP while being inclined with respect to the sheet conveyance direction, a toner image inclined with respect to the sheet P is transferred at the transfer position TP. For this reason, the inclination of the sheet P is corrected in the registration roller pair 116. That is, by setting the delay time ΔT, the sheet P conveyed on the sheet conveyance path PP comes into contact with the registration roller pair 116 in a state where the rotation is stopped. At this time, at the leading edge of the inclined sheet P, the region conveyed earlier in the direction orthogonal to the sheet conveying direction has contacted the registration roller pair 116 and stopped and then conveyed with a delay. The area contacts the registration roller pair 116. In the sheet P in which the leading edge is stopped, the center portion in the sheet conveying direction is bent. Then, when the registration roller pair 116 is driven to rotate, the sheet P starts to be conveyed again with the leading edge of the sheet P aligned in a direction orthogonal to the sheet conveying direction. As a result, the inclination of the sheet P is corrected.

  Thus, when the time measurement unit 503 continues to count the first count time TC1 and the first count time TC1 reaches the time threshold value T0 (YES in step S007), the overall control unit 501 includes the second drive unit 522. And the registration roller pair 116 is rotationally driven (step S008). Thereby, the sheet P is further conveyed toward the transfer position TP. Further, the overall control unit 501 controls the third drive unit 523 to rotate the drive roller 181. At this time, the peripheral surfaces of the registration roller pair 116 and the conveyance belt 180 are rotated at the conveyance speed V0 (first speed). The sheet P is delivered from the registration roller pair 116 to the peripheral surface of the conveyance belt 180 with the same conveyance speed V0. Note that the counting of the first count time TC1 by the time measuring unit 503 is continued.

  When the registration roller pair 116 and the conveyance belt 180 are rotationally driven, the speed control unit 502 acquires paper thickness information from the sheet information acquisition unit 505. At this time, the sheet information acquisition unit 505 acquires the sheet thickness information of the sheet P based on the torque fluctuation amount acquired from the torque sensor 509 in step S004. Table 1 shows the relationship between the torque fluctuation amount of the intermediate roller pair 115 and the paper thickness information acquired by the sheet information acquisition unit 505.

  The sheet information acquisition unit 505 determines that the paper thickness is 0.10 mm or more when the torque fluctuation amount of the intermediate roller pair 115 acquired from the torque sensor 509 is X (%) or more (YES in step S009). To do. The sheet information acquisition unit 505 further determines that the paper thickness is 0.20 mm or more when the torque fluctuation amount is Y (%) or more (YES in Step S010). When the torque fluctuation amount is less than Y (%) (NO in step S010), the sheet information acquisition unit 505 determines that the paper thickness is 0.10 mm or more and less than 0.20 mm. In step S009, if the torque fluctuation amount is less than X (NO in step S009), the sheet information acquisition unit 505 determines that the paper thickness is less than 0.10 mm.

  Hereinafter, a case where the sheet P has a thickness of 0.20 mm or more (YES in step S010) will be described. The speed control unit 502 refers to the deceleration speed V2 (second speed) and the time thresholds T12 and T22 when the paper thickness is 0.20 mm or more among the stored information stored in advance in the storage unit 504. Table 2 shows the stored information stored in the storage unit 504 in advance.

  In Table 2, the deceleration speeds V1 and V2 are expressed as a ratio to the transport speed V0. Further, the time thresholds T1 (T11, T12) and T2 (T21, T22) are obtained by setting the reference time obtained by dividing the distance from the nip portion RP of the registration roller pair 116 to the transfer position TP by the transport speed V0 as 100. Expressed as a ratio to time.

  When the first count time TC1 counted by the time measuring unit 503 reaches the time threshold value T12 (YES in step S011), the speed control unit 502 controls the third driving unit 523 to rotate the rotational speed of the driving roller 181. Is decelerated (step S012). At this time, the driving roller 181 is decelerated so that the conveyance speed of the sheet P by the conveyance belt 180 becomes V2 (= 0.6 × V0).

  Furthermore, when the first count time TC1 counted by the time measuring unit 503 reaches the time threshold value T22 (YES in step S013), the speed control unit 502 controls the third driving unit 523 to control the driving roller 181. The rotational speed is increased (step S014). At this time, the driving roller 181 is increased so that the conveying speed of the sheet P by the conveying belt 180 returns to V0.

  With reference to FIGS. 4 and 7, the state of the leading end of the sheet P when the conveying speed of the conveying belt 180 is decelerated and increased will be described. FIG. 7 shows a change in the conveyance speed of the leading end portion of the sheet P when the sheet conveyance path PP is conveyed. The horizontal axis schematically represents the position of the leading end portion of the sheet P on the sheet conveyance path PP, and the positions P1 to P7 are also shown in FIG. The sheet P conveyed by the sheet feeding unit 11 at the conveyance speed V0 is temporarily stopped at the nip portion RP (P4) of the registration roller pair 116 and then conveyed again at the conveyance speed V0. Then, at the deceleration position P51 corresponding to the time threshold T12, the sheet P is decelerated to the conveyance speed V2. As shown in FIG. 4, the deceleration position P <b> 51 is set on the upstream side in the sheet conveyance direction from the position where the leading end of the sheet P is in contact with the peripheral surface of the photosensitive drum 121. Accordingly, the sheet P enters the transfer position TP while being in contact with the peripheral surface of the photosensitive drum 121 while being decelerated to the conveyance speed V2. For this reason, the impact when the leading end of the sheet P contacts the peripheral surface of the photosensitive drum 121 is weakened. As a result, the rotation of the photosensitive drum 121 is stably maintained. In FIG. 7, Vpr is the speed of the peripheral surface of the photosensitive drum 121 rotated with a speed difference with respect to the transport belt 180.

  The sheet P is again accelerated to the conveyance speed V0 at the acceleration position P61 corresponding to the time threshold value T22. The acceleration position P61 is set upstream of the transfer position TP in the sheet conveyance direction. Accordingly, the toner image is transferred to the sheet P in a state where the conveyance speed of the sheet P is returned to V0.

  Similarly, in the case where it is determined in step S010 in FIG. 6 that the sheet P has a paper thickness of 0.10 mm or more and less than 0.20 mm (NO in step S010), the speed control unit 502 is stored in the storage unit 504 in advance. Of the stored information, the deceleration speed V1 (second speed) and time threshold values T11 and T21 when the paper thickness is 0.10 mm or more and less than 0.20 mm are referred to (Table 2).

  When the first count time TC1 counted by the time measuring unit 503 reaches the time threshold value T11 (YES in step S015), the speed control unit 502 controls the third driving unit 523 to control the driving roller 181. The rotational speed is decelerated (step S016). At this time, the driving roller 181 is decelerated so that the conveyance speed of the sheet P by the conveyance belt 180 becomes V1 (= 0.8 × V0).

  Furthermore, when the first count time TC1 counted by the time measuring unit 503 reaches the time threshold value T21 (YES in step S017), the speed control unit 502 controls the third drive unit 523 to control the driving roller 181. The rotational speed is increased (step S018). At this time, the driving roller 181 is increased so that the conveying speed of the sheet P by the conveying belt 180 returns to V0.

  Referring to FIG. 7, when the paper thickness of sheet P is 0.10 mm or more and less than 0.20 mm, sheet P is decelerated to conveyance speed V1 at deceleration position P52 corresponding to time threshold T11. The deceleration position P52 is set downstream of the deceleration position P51 in the sheet conveying direction. This is because the leading end of the sheet P is in contact with the circumferential surface of the photosensitive drum 121 at a position closer to the transfer position TP than when the sheet P has a sheet thickness of 0.20 mm or more. Then, the sheet P enters the transfer position TP while being in contact with the peripheral surface of the photosensitive drum 121 while being decelerated to the conveyance speed V1. Even in this case, the impact when the leading end of the sheet P contacts the peripheral surface of the photosensitive drum 121 is weakened. As a result, the rotation of the photosensitive drum 121 is stably maintained.

  The sheet P decelerated to the conveyance speed V1 is again accelerated to the conveyance speed V0 at the acceleration position P62 corresponding to the time threshold value T21. The acceleration position P62 is set downstream of the acceleration position P61 in the sheet conveyance direction. In other words, the acceleration position P61 is set upstream of the acceleration position P62 in the sheet conveyance direction. This is because the thicker sheet P requires a transport force when entering the transfer position TP, and therefore it is preferable to bring the transport speed V of the sheet P closer to the transport speed V0 at an early stage. Thus, even when the sheet P has a paper thickness of 0.10 mm or more and less than 0.20 mm, the toner image is transferred to the sheet P with the sheet P transport speed returned to V0.

  If the sheet P has a thickness of less than 0.10 mm (NO in step S009), the speed control unit 502 does not decelerate the conveyance speed V of the conveyance belt 180. Accordingly, the sheet P enters the transfer position TP while being conveyed from the registration roller pair 116 to the conveyance belt 180 at the conveyance speed V0, and the toner image is transferred.

  As described above, according to the present embodiment, when the sheet P enters the transfer position TP, the conveyance speed of the sheet P when the leading end portion of the sheet P contacts the peripheral surface of the photosensitive drum 121 is suitably reduced. For this reason, the impact when the leading end of the sheet P abuts on the circumferential surface of the photosensitive drum 121 is weakened, and fluctuations in the rotation of the photosensitive drum 121 are suppressed. As a result, the toner image formed on the peripheral surface of the photosensitive drum 121 at the development position DP is not disturbed. In particular, in the present embodiment, in order to suppress the hollowing out phenomenon at the transfer position TP, the photosensitive drum 121 is rotated so that the peripheral surface of the photosensitive drum 121 rotates slower than the peripheral surfaces of the conveyance belt 180 and the transfer roller 126. 121 is rotationally driven. In such a case, the circumferential surface of the photosensitive drum 121 is dragged by the circumferential surface of the transport belt 180 at the transfer position TP. For this reason, the rotation of the photosensitive drum 121 tends to become unstable. Even in such a case, as described above, since the conveyance speed of the sheet P when contacting the peripheral surface of the photosensitive drum 121 is suitably reduced, the rotation of the photosensitive drum 121 is fluctuated. Is suppressed.

  When the conveyance speed V of the sheet P is decelerated, the deceleration speed is 50% to 90% with respect to the conveyance speed V0 of the sheet P when the toner image is transferred at the transfer position TP. Preferably there is. In this case, the impact when the leading end of the sheet P contacts the peripheral surface of the photosensitive drum 121 is effectively attenuated, and when the sheet P enters the transfer position TP, the conveyance speed of the sheet P is It will be restored promptly.

  Further, in the present embodiment, the sheet information of the sheet P is acquired by the sheet information acquisition unit 505. Then, according to the paper thickness information of the sheet P, the conveyance speed of the sheet P when the sheet P contacts the peripheral surface of the photosensitive drum 121 is set. At this time, the thicker the sheet P used, the lower the sheet P is decelerated to a lower conveyance speed. This is because the thicker the sheet P, the more easily the rotation of the photosensitive drum is prevented when contacting the peripheral surface of the photosensitive drum 121. Therefore, even when thick paper is used as the sheet P, the rotation of the photosensitive drum 121 is stably maintained.

  The image forming apparatus according to the embodiment of the present invention has been described above. However, the present invention is not limited to this, and for example, the following modified embodiment can be taken.

  (1) In the above embodiment, the rotation speed of the driving roller 181 is controlled before the leading end of the sheet P contacts the circumferential surface of the photosensitive drum 121, and the conveyance speed of the sheet P by the conveyance belt 180 is reduced. It was. However, the mode of reducing the conveyance speed of the sheet P is not limited to this. For example, when the first count time TC1 reaches the time threshold value T1, in addition to the conveyance belt 180, the rotation speed of the registration roller pair 116 may be simultaneously reduced. As described above, the conveyance speed of the conveyance belt 180 is reduced from V0 to V2 while the leading end portion of the sheet P is held on the conveyance belt 180. In this case, if the rear end portion of the sheet P is still sandwiched between the registration roller pair 116, the rear end portion of the sheet P is transported at the transport speed V0. For this reason, slight bending occurs in the central portion of the sheet P in the sheet conveyance direction. On the other hand, when the rotation speed of the registration roller pair 116 is simultaneously reduced in addition to the conveyance belt 180, the bending that occurs in the central portion of the sheet P can be eliminated. Even in such a case, the rotation of the photosensitive drum 121 is stably maintained when the leading end of the sheet P contacts the peripheral surface of the photosensitive drum 121.

  (2) Further, in the above-described embodiment, the transport belt 180 is disposed as the transport unit that transports the sheet P toward the transfer position TP. However, the present invention is not limited to this. is not. For example, the registration roller pair 116A may directly convey the sheet P to the transfer position TP. FIG. 8 is a schematic configuration diagram of the periphery of the sheet conveyance path PP when the conveyance belt 180 is not provided. In this case, the leading end of the sheet P is detected by the second sensor 511, and the second count time TC2 is counted. When the second count time TC2 reaches the above-described time threshold value T0, the registration roller pair 116A is rotationally driven. Furthermore, when the leading end of the sheet P approaches the peripheral surface of the photosensitive drum 121, the second count time TC2 reaches the time threshold value T1 (T11, T12). At this time, the registration roller pair 116A is decelerated so that the conveyance speed of the sheet P becomes V2 (or V1). Even in such a case, when the leading end of the sheet P contacts the peripheral surface of the photosensitive drum 121, the rotation of the photosensitive drum 121 is stably maintained.

  (3) Further, in the above-described embodiment, the torque sensor 509 is disposed in the intermediate roller pair 115 in order to acquire the paper thickness information of the sheet P. However, the present invention is limited to this. It is not a thing. FIG. 9 is an electrical block diagram illustrating a mode in which the sheet information acquisition unit 505 acquires sheet information from the operation unit 221 instead of the torque sensor 509. That is, in FIG. 1, the paper type information of the sheet P to be used is input from the operation unit 221 by the user. At this time, the sheet information acquisition unit 505 acquires the paper thickness information of the sheet P from the paper type information. Then, the speed control unit 502 can control the rotation speed of the driving roller 181 based on the paper thickness information of the sheet P, and can reduce the transport speed V of the sheet P by the transport belt 180. Further, the information on the sheet P for controlling the conveyance speed V is not limited to the paper thickness. For example, the hardness, material, and surface property of the sheet P may be used. This is because even if the sheet is thin, when the sheet is a hard sheet made of a resin material or a sheet with rough surface properties, the rotation of the photosensitive drum is more hindered when contacting the peripheral surface of the photosensitive drum 121. It is because it is easy to be done.

  (4) In the above embodiment, when the sheet P is conveyed to the deceleration position exemplified by P51 and P52 in FIG. 7, the sheet P is a sheet when the toner image is transferred at the transfer position TP. Although the aspect conveyed by the conveyance speed V0 of P was demonstrated, it is not limited to this. When the sheet P is conveyed to the deceleration position P51 (P52), the sheet P may be conveyed at a speed higher than the conveyance speed V0 at the transfer position. In this case, the time until the leading end of the sheet P approaches the peripheral surface of the photosensitive drum 121 can be shortened.

11 Feeder 112 Pickup roller 113 First feed roller 115 Intermediate roller pair (conveyance roller)
116 Registration roller pair (Registration roller)
121 Photosensitive drum 124A Developing roller 126 Transfer roller 130 Fixing device 180 Conveying belt (conveying means)
181 Driving roller 182 Stretching roller 221 Operation unit (input unit)
500 Control unit 501 Overall control unit 502 Speed control unit (conveyance speed control means)
503 Time measurement unit 504 Storage unit 505 Sheet information acquisition unit (sheet information acquisition unit)
509 Torque sensor (torque acquisition means)
510 1st sensor 511 2nd sensor (detection means)
520 Drum drive unit 521 First drive unit 522 Second drive unit 523 Third drive unit 525 Application unit

Claims (10)

A photosensitive drum that is rotationally driven and carries a toner image on its peripheral surface;
A transfer roller disposed at a transfer position opposite to the photosensitive drum and transferring the toner image to a sheet;
A sheet conveyance path that extends so as to pass through the transfer position and that conveys the sheet;
A transport unit disposed upstream of the transfer position in the sheet transport path with respect to the sheet transport direction, and transports the sheet toward the transfer position;
A conveyance speed control means for controlling a conveyance speed at which the conveyance means conveys the sheet;
With
The conveyance speed control unit is configured to output the sheet slower than the first speed immediately after the conveyance unit conveys the sheet at the first speed and immediately before the leading end of the sheet reaches the transfer position. An image forming apparatus, wherein the conveying speed of the conveying means is controlled so as to convey at a speed of 2.   The image forming apparatus according to claim 1, wherein the second speed is 50% to 90% of the first speed. The transport means is an endless transport belt that is rotationally driven and carries a sheet on its surface,
The transfer roller is disposed so as to be in contact with the rotation inner peripheral surface of the transport belt and to face the photosensitive drum,
The image forming apparatus according to claim 1, wherein the transport speed control unit controls the transport speed to the first speed after controlling the transport speed to the second speed.   The image forming apparatus according to claim 1, wherein the conveying unit is a registration roller that conveys the sheet toward the transfer position in accordance with a timing at which the toner image is transferred to the sheet. apparatus. It is disposed opposite to the sheet conveyance path on the upstream side in the sheet conveyance direction from the transfer position, and has a detection unit that detects the position of the leading edge of the sheet,
The conveyance speed control means changes the conveyance speed of the conveyance means from the first speed to the second speed according to a detection result of the leading edge of the sheet by the detection means. Item 5. The image forming apparatus according to any one of Items 1 to 4. Having sheet information acquisition means for acquiring sheet information of a sheet conveyed through the sheet conveyance path;
The said conveyance speed control means controls the conveyance speed of the said conveyance means based on the said sheet information acquired by the said sheet information acquisition means, The any one of Claim 1 thru | or 5 characterized by the above-mentioned. Image forming apparatus.   The image forming apparatus according to claim 6, wherein the conveyance speed control unit controls the conveyance speed so that the second speed is slower as the thickness of the sheet is larger. It further has an input unit for inputting image formation information,
The image forming apparatus according to claim 6, wherein the sheet information acquisition unit acquires the sheet information from the image formation information input to the input unit. A conveying roller disposed in the sheet conveying path and conveying the sheet;
Torque detecting means for detecting rotational torque of the transport roller, and
The image forming apparatus according to claim 6, wherein the sheet information acquisition unit acquires the sheet information based on the rotational torque detected by the torque detection unit.   10. The photosensitive drum is driven to rotate so that the peripheral surface of the photosensitive drum passes through the transfer position at a slower speed than the peripheral surface of the transfer roller. The image forming apparatus according to claim 1.

Images