Patent Publication Number: US-2013251434-A1

Title: Image forming apparatus with conveyance suspension unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosures herein relate to an image forming apparatus. 
     2. Description of the Related Art 
     An image forming apparatus may use a configuration that transfers a toner image formed on a photo conductor to an intermediate transfer belt. The toner image is then transferred onto a sheet that is placed between the intermediate transfer belt and a transfer roller situated in contact with the intermediate transfer belt. In this manner, a printout is obtained on which the image is printed. 
     At the time of transferring a toner image onto a sheet, the surface speed of the rotating intermediate transfer belt may differ from the speed of the sheet being conveyed. In such a case, the position of the toner image transferred from the intermediate transfer belt onto the sheet may be displaced. This may result in the misalignment of the image and uneven image density, which degrade image quality. Further, accuracy of alignment between a front-side image and a back-side image may be lowered in the case of duplex printing. 
     In consideration of the above, a torque limiter that transmits a drive force smaller than the torque generated by a physical contact between a transfer roller and an intermediate transfer belt is provided along the path that transmits a drive force between a drive unit for driving the transfer roller and the transfer roller (see Japanese Patent Application Publication No. 11-52757, for example). 
     With the configuration described above, the transfer roller rotates to follow the movement of the intermediate transfer belt, so that the surface speed of the intermediate transfer belt and the travel speed of a sheet can be kept equal to each other. This can avoid reduction in image quality caused by positional displacement occurring at the time of image transfer. 
     There is a need to provide an upstream conveyance unit at an upstream position relative to the intermediate transfer belt and the transfer roller to supply a sheet to the intermediate transfer belt and the transfer roller. In this configuration, the sheet is first conveyed by the upstream conveyance unit, and is then held both by the upstream conveyance unit and by the intermediate transfer belt and the transfer roller, followed by being passed to the intermediate transfer belt and the transfer roller. 
     When this happens, a difference between the sheet conveyance speed of the upstream conveyance unit and the sheet conveyance speed of the intermediate transfer belt and the transfer roller creates a pushing force or a pulling force applied by the upstream conveyance unit to the sheet which enters a gap between the intermediate transfer belt and the transfer roller. In this case, therefore, the surface speed of the intermediate transfer belt differs from the travel speed of the sheet, resulting in a positional displacement of an image or the like. Image quality may thus be degraded. Even with the configuration that causes the transfer roller to follow the movement of the intermediate transfer belt, the existence of a difference in conveyance speed caused by the upstream conveyance unit may degrade image quality. 
     Accordingly, there may be a need for an image forming apparatus that can produce a high-quality image by keeping the travel speed of a print medium equal to the surface speed of an image carrier at the time of transferring a toner image to the print medium. 
     SUMMARY OF THE INVENTION 
     It is a general object of at least one embodiment of the present invention to provide an image forming apparatus that substantially obviates one or more problems caused by the limitations and disadvantages of the related art. 
     According to one embodiment, an image forming apparatus includes a drive roller configured to rotate, a driven roller configured to hold and convey a print medium at a gap between the drive roller and the driven roller, an image carrier configured to have a toner image formed on a surface thereof and to rotate, the image carrier being situated downstream relative to the drive roller and the driven roller in a travel direction of the print medium, a transfer roller configured to hold and convey, at a gap between the image carrier and the transfer roller, the print medium passed from the drive roller and the driven roller to transfer the toner image onto a surface of the print medium, and a conveyance suspension unit to stop the drive roller and the driven roller from applying a drive force to the print medium to allow the image carrier and the transfer roller to convey the print medium at a time at which the print medium reaches the gap between the image carrier and the transfer roller and starts to be conveyed by the image carrier and the transfer roller. 
     According to one embodiment, a method of controlling conveyance of a print medium in an image forming apparatus includes conveying a print medium from an upstream conveyance unit to a rotating image carrier having a toner image formed on a surface thereof, controlling the upstream conveyance unit to stop a drive force for conveying the print medium from being applied from the upstream conveyance unit to the print medium at a time at which the print medium comes in contact with the surface of the image carrier and starts to be conveyed by the image carrier, and conveying the print medium by rotation of the rotating image carrier after the stopping of the drive force. 
     According to at least one embodiment, an image forming apparatus is provided that can produce a high-quality image by keeping the travel speed of a print medium equal to the surface speed of an image carrier at the time of transferring a toner image to the print medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and further features of embodiments will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a drawing illustrating an example of the schematic configuration of an image forming apparatus according to a first embodiment; 
         FIG. 2  is a drawing illustrating an example of the schematic configuration of a second transfer unit and an upstream conveyance unit according to the first embodiment; 
         FIG. 3  is a drawing illustrating timing at which an electromagnetic clutch is controlled based on the detection result of a timing sensor according to the first embodiment; 
         FIG. 4  is a drawing illustrating an example of the schematic configuration of a second transfer unit and an upstream conveyance unit according to a second embodiment; 
         FIG. 5  is a drawing illustrating timing at which a transfer roller is separated based on the detection result of the timing sensor according to the second embodiment; and 
         FIG. 6  is a drawing illustrating an example of the schematic configuration of a second transfer unit and an upstream conveyance unit according to a third embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments will be described by referring to the accompanying drawings. In these drawings, the same elements are referred to by the same references, and a description thereof may be omitted. 
     First Embodiment 
     &lt;Configuration of Image Forming Apparatus&gt; 
       FIG. 1  is a drawing illustrating an example of the schematic configuration of an image forming apparatus  100  according to a first embodiment. 
     The image forming apparatus  100  includes a photo conductor  10 , a charging unit  11 , an exposure unit  12 , a developing unit  13 , an intermediate transfer belt  20 , a transfer roller  30 , a fuser unit  40 , an automatic document feeder  50 , and a scanner unit  51 . The image forming apparatus  100  is a multifunction device that prints an image on a sheet P serving as a print medium stored in a sheet tray  60 . 
     When the image forming apparatus  100  is to print an image on a sheet P, the charging unit  11  uniformly charges the surface of the photo conductor  10  that is rotating. A document placed in the automatic document feeder  50  is scanned by the scanner unit  51  to produce image data, based on which the exposure unit  12  exposes the surface of the photo conductor  10  to light to create a latent image. The developing unit  13  that has a developing agent stored therein inclusive of toner develops the latent image on the surface of the photo conductor  10  to create a toner image. The image forming apparatus  100  includes a plurality of photo conductors  10  and a plurality of developing units  13  to create toner images of respective colors, which are then superimposed one over another on the intermediate transfer belt  20  that is rotating. 
     The toner image transferred onto the intermediate transfer belt  20  is further transferred by a second transfer unit situated between the intermediate transfer belt  20  and the transfer roller onto a sheet P supplied from the sheet tray  60 . The sheet P having the toner image transferred thereon is conveyed to the fuser unit  40  by which to be heated and pressed to fuse the toner image before being ejected onto a sheet discharge tray  61 . 
     After transferring the toner images to the intermediate transfer belt  20 , the photo conductors  10  are cleaned by cleaning units  14  which remove remaining toner from the surfaces of the photo conductors  10 . Preparation for a next image forming operation is thus made. 
     &lt;Configuration of Second Transfer Unit and Upstream Conveyance Unit&gt; 
       FIG. 2  is a drawing illustrating an example of the schematic configuration of the second transfer unit and an upstream conveyance unit according to the first embodiment. 
     The sheet P is supplied to a gap between the intermediate transfer belt  20  and the transfer roller  30  at the time at which a toner image  90  on the surface of the rotating intermediate transfer belt  20  reaches the second transfer unit due to the rotation of the intermediate transfer belt  20 . 
     A drive roller  71  is connected to a motor serving as an example of a drive unit via a transmission mechanism, and receives a drive force from the motor  82  to rotate in a direction indicated by an arrow. The transmission mechanism between the drive roller  71  and the motor  82  includes an electromagnetic clutch  81 . The electromagnetic clutch  81  is controlled by a conveyance control unit  79  to transmit or not to transmit a drive force from the motor  82  to the drive roller  71 . 
     A driven roller  72  is situated opposite the drive roller  71 . The driven roller  72  and the rotating drive roller  71  hold and convey the sheet P placed therebetween. The driven roller  72  rotates to follow the movement of the sheet P while the sheet P is conveyed. 
     The intermediate transfer belt  20  is a rotatable endless belt that is stretched between a plurality of rollers. The intermediate transfer belt  20  rotates to move the superimposed toner images  90  transferred from the photo conductors  10 . The intermediate transfer belt  20  is supported from the inner side thereof by a support roller  31  that is situated opposite the transfer roller  30 . The intermediate transfer belt  20  transfers the toner image  90  onto the sheet P that is conveyed to the gap between the support roller  31  and the transfer roller  30 . 
     The transfer roller  30  presses the sheet P against the support roller  31  via the intermediate transfer belt  20  at the time of transferring the toner image  90  onto the sheet P. The transfer roller  30  is rotated by a drive unit (not shown) at the same circumferential speed as the surface speed of the intermediate transfer belt  20 . 
     A torque limiter or the like may be provided in a transmission mechanism situated between the transfer roller  30  and the drive unit for driving the transfer roller  30 , so that the circumferential speed of the transfer roller  30  becomes equal to the surface speed of the intermediate transfer belt  20  at the time of transferring a toner image onto the sheep P. The torque of the torque limiter is limited to smaller than the torque created by a physical contact between the intermediate transfer belt  20  and the transfer roller  30 . With this arrangement, the transfer roller  30  in contact with the intermediate transfer belt  20  rotates to follow the movement of the intermediate transfer belt  20 . At the time of transferring a toner image, thus, the circumferential speed of the transfer roller  30  is kept equal to the surface speed of the intermediate transfer belt  20 . 
     Further, a disconnection mechanism may be provided to separate the transfer roller  30  from the intermediate transfer belt  20  in order to avoid wearing of the surface thereof caused by a physical contact with the intermediate transfer belt  20  when an operation to transfer the toner image  90  onto a sheet P is not being performed. 
     The distance between the set of the drive roller  71  and the driven roller  72  and the set of the intermediate transfer belt  20  and the transfer roller is set smaller than the length in a conveyance direction of the smallest sheet P on which the image forming apparatus  100  prints an image. 
     Further, a timing sensor  80 , which is an example of a print medium detecting unit, is provided between the set of the drive roller  71  and the driven roller  72  and the set of the intermediate transfer belt  20  and the transfer roller  30 . 
     The timing sensor  80  may be a thru-beam or reflective optical sensor, for example. The timing sensor  80  detects the passing of the head end of a sheet P being conveyed between the set of the drive roller  71  and the driven roller  72  and the set of the intermediate transfer belt  20  and the transfer roller  30 . The position of the timing sensor  80  may preferably be set as close to the second transfer unit as possible in order to measure the time at which the sheet P reaches the second transfer unit situated between the intermediate transfer belt  20  and the transfer roller  30 . 
     The conveyance control unit  79  is connected to the timing sensor  80  and to the electromagnetic clutch  81 . The conveyance control unit  79  controls the electromagnetic clutch  81  based on the result of detection made by the timing sensor  80 , thereby transmitting or disconnecting a drive force from the motor  82  to the drive roller  71 . By disconnecting the drive force from the motor  82  to the driver roller  71 , the conveyance control unit  79  serves as a conveyance suspension unit to suspend the supply of a conveyance drive force from the drive roller  71  and the driven roller  72  to a sheet P. The conveyance control unit  79  is implemented by the use of a program that is stored in a memory device such as a ROM or RAM and executed by a CPU of the image forming apparatus  100 . 
     &lt;Sheet Conveyance Control&gt; 
     In the configuration illustrated in  FIG. 2 , the conveyance control unit  79  controls the electromagnetic clutch  81  to stop a drive force from being transmitted from the motor  82  to the drive roller  71  at the time at which the sheet P reaches the gap between the intermediate transfer belt  20  and the transfer roller  30  and starts to be conveyed by the intermediate transfer belt  20  and the transfer roller  30 . Due to the stopping of transmission of the drive force from the motor  82  to the drive roller  71 , the conveyance drive force is stopped from being applied from the drive roller  71  and the driven roller  72  to the sheet P, so that the sheet P is held between and conveyed by the intermediate transfer belt  20  and the transfer roller  30 . When this happens, the drive roller  71  and the driven roller  72  holding the rear end of the sheet P at the gap therebetween are rotated by following the movement of the sheet P that is conveyed by the intermediate transfer belt  20  and the transfer roller  30 . 
     This control serves to eliminate the influence of the drive roller  71  on the travel speed of the sheet P at the time of transferring the toner image  90 , thereby preventing the degradation of image quality such as displacements of image positions caused by a difference between the travel speed of the sheet P and the surface speed of the intermediate transfer belt  20 . 
     The conveyance control unit  79  detects the time at which the sheet P reaches the gap between the intermediate transfer belt  20  and the transfer roller  30 , based on the detection result obtained by the timing sensor  80  detecting the head end of the sheet P, the distance between the timing sensor  80  and the set of the intermediate transfer belt  20  and the transfer roller  30 , and the travel speed of the sheet P. 
     The distance between the timing sensor  80  and the gap between the intermediate transfer belt  20  and the transfer roller  30  is denoted as L, and the travel speed of the sheet P is denoted as v. A time length S 1  between the time at which the timing sensor  80  detects the passing of the head end of the sheet P and the time at which the head end of the sheet P reaches the gap between the intermediate transfer belt  20  and the transfer roller  30  is then represented as follows. 
         S 1 =L/v   (1)
 
     L: distance between the timing sensor  80  and the gap between the intermediate transfer belt  20  and the transfer roller  30   
     v: travel speed of the sheet P 
     With the length of the unprintable head margin of the sheet P in the travel direction of the sheet P being denoted as “a”, a time length S 2  between the time at which the timing sensor  80  detects the passing of the head end of the sheet P and the time at which the unprintable head margin of the sheet P finishes passing through the gap between the intermediate transfer belt  20  and the transfer roller  30  is represented as follows. 
         S 2=( L+a )/ v   (2)
 
     a: length of the unprintable had margin of the sheet P in the travel direction 
     The conveyance control unit  79  controls the electromagnetic clutch  81  to stops a drive force from being transmitted from the motor  82  to the drive roller  71  at the end of a time length T that is between the time length S 1  obtained by the formula (1) and the time length S 2  obtained by the formula (2). 
       FIG. 3  is a drawing illustrating timing at which the electromagnetic clutch  81  is controlled based on the detection result of the timing sensor  80  according to the first embodiment. 
     As illustrated in  FIG. 3 , the conveyance control unit  79  stops a drive force from being transmitted from the motor  82  to the drive roller  71  at the end of the time length T that is between the time length S 1  and the time length S 2 , which are obtained by the formula (1) and the formula (2), respectively. As the drive force is prevented from being transmitted to the drive roller  71 , the intermediate transfer belt  20  and the transfer roller can hold the sheet P placed therebetween without being affected by the drive roller  71  at the time of transferring the toner image  90 . Accordingly, no difference occurs between the travel speed of the sheet P and the surface speed of the intermediate transfer belt  20  at the time of transferring the toner image  90 . A high quality image is thus obtained by transferring the toner image  90  onto the sheet P without creating positional displacements or the like. In the case of duplex printing, high quality images are obtained that are accurately aligned between the front side and the back side. 
     The image forming apparatus  100  according to the first embodiment is configured to transfer a toner image from the intermediate transfer belt  20  to a sheet P. The disclosed technology is also applicable to an image forming apparatus that transfers a toner image formed on the photo conductor  10  serving as an image carrier to a sheet P at a gap between the photo conductor  10  and a transfer roller. In this case also, the electromagnetic clutch  81  may be controlled to stop a drive force from being transmitted to the drive roller  71  of the upstream conveyance unit at the point in time at which the sheet P reaches the gap between the photo conductor and the transfer roller  30  and starts to be conveyed by the photo conductor  10  and the transfer roller  30 . This arrangement keeps the travel speed of the sheet P equal to the circumferential speed of the photo conductor  10  at the time of transferring a toner image. As a result, lowering of image quality caused by the positional misalignment of transferred toner images or the like is prevented, thereby producing a high-quality image. 
     Further, provision may be made such that the electromagnetic clutch  81  is not provided between the drive roller  71  and the motor  82 , and the conveyance control unit  79  switches off the power to the motor  82  at the end of the time length T that is between the time length S 1  and the time length S 2 . In such a case, a DC motor may be used as the motor  82  in order to reduce its effect on the drive roller  71  that rotates by following the movement of the sheet P after the power is switched off. 
     Second Embodiment 
     In the following, a second embodiment will be described with reference to the accompanying drawings. A description will be omitted of the same elements as those of the embodiments already described. The second embodiment differs from the first embodiment in that no electromagnetic clutch is provided between the drive roller  71  and the motor  82 , and a separation mechanism is provided that causes the driven roller  72  to move away from the drive roller  71 . 
       FIG. 4  is a drawing illustrating an example of the schematic configuration of the second transfer unit and an upstream conveyance unit according to the second embodiment. 
     As illustrated in  FIG. 4 , the separation mechanism of the driven roller  72  includes a support member  73 , a spring  76 , and an eccentric cam  75 . 
     The support member  73  supports the rotational shaft of the driven roller  72 , and is rotatable about a pivot point  74 . 
     The spring  76  is in contact with the support member  73  on the side of the pivot point  74  opposite the driven roller  72 . The spring  76  serves as an urging unit to urge the support member  73  such that the driven roller  72  is brought in contact with the drive roller  71  or is pressed against the drive roller  71  with an intervening paper sheet therebetween. 
     The eccentric cam  75  is in contact with the support member  73  on the side of the pivot point  74  opposite the driven roller  72 . The eccentric cam  75  is rotated by a motor (not shown) controlled by the conveyance control unit  79  to move the support member  73  such that the driven roller  72  is moved away from the drive roller  71 . 
     The conveyance control unit  79  is connected to the timing sensor  80  and to the motor that rotates the eccentric cam  75 . When the sheet P reaches the gap between the intermediate transfer belt  20  and the transfer roller  30  and starts to be conveyed by the intermediate transfer belt  20  and the transfer roller  30 , the conveyance control unit  79  causes the eccentric cam  75  to rotate to separate the driven roller  72  from the drive roller  71 . 
       FIG. 5  is a drawing illustrating timing at which the electromagnetic clutch  81  is controlled based on the detection result of the timing sensor  80  according to the second embodiment. 
     The time length S 1  illustrated in  FIG. 5  denotes the length of a period between the time at which the sheet P is detected by the timing sensor  80  and the time at which the sheet P reaches the gap between the intermediate transfer belt  20  and the transfer roller  30 , and is obtained by use of the equation (1). The time length S 2  denotes the length of a period between the time at which the sheet P is detected by the timing sensor  80  and the time at which the unprintable head margin of sheet P finishes passing the gap between the intermediate transfer belt  20  and the transfer roller  30 , and is obtained by use of the equation (2). 
     The conveyance control unit  79  rotates the eccentric cam  75  to rotates the support member  73  at the end of the time length T that is between the time length S 1  and the timing length S 2 , thereby separating the driven roller  72  from the drive roller  71 . Upon the separation of the driven roller  72  from the drive roller  71 , the sheet P is held and carried by the intermediate transfer belt  20  and the transfer roller  30  without being affected by the drive roller  71 . Accordingly, no difference occurs between the travel speed of the sheet P and the surface speed of the intermediate transfer belt  20  at the time of transferring the toner image  90 . A high quality image is thus obtained by transferring the toner image  90  onto the sheet P without creating positional displacements or the like. In the case of duplex printing, high quality images that are accurately aligned between the front side and the back side are obtained. 
     Alternatively, a separation mechanism may be provided that causes the drive roller  71  to move away from the driven roller  72 . In this case, a flexible joint, coupling, or the like may be provided at the rotational shaft of the drive roller  71  to be connected to the shaft or a gear of the motor  82  in order to transmit a drive force from the motor  82  to the drive roller  71  that is moved. 
     Third Embodiment 
     In the following, a third embodiment will be described with reference to the accompanying drawings. A description will be omitted of the same elements as those of the embodiments already described. In the third embodiment, the separation mechanism for causing the driven roller  72  to move away from the drive roller  71  has a different configuration than the configuration used in the second embodiment. 
       FIG. 6  is a drawing illustrating an example of the schematic configuration of the second transfer unit and an upstream conveyance unit according to the third embodiment. 
     As illustrated in  FIG. 6 , the separation mechanism of the driven roller  72  includes the support member  73 , the spring  76 , and a solenoid  77 . 
     The support member  73  supports the rotational shaft of the driven roller  72 , and is rotatable about a pivot point  74 . 
     The spring  76  is in contact with the support member  73  on the side of the pivot point  74  opposite the driven roller  72 . The spring  76  serves as an urging unit to urge the support member  73  such that the driven roller  72  is brought in contact with the drive roller  71  or is pressed against the drive roller  71  with an intervening paper sheet therebetween. 
     The solenoid  77  is controlled by the conveyance control unit  79  to rotate the support member  73 , thereby moving the driven roller  72  away from the drive roller  71 . 
     The conveyance control unit  79  is connected to the timing sensor  80  and to the solenoid  77 . When the sheet P reaches the gap between the intermediate transfer belt  20  and the transfer roller  30  and starts to be conveyed by the intermediate transfer belt  20  and the transfer roller  30 , the conveyance control unit  79  causes the solenoid  77  to rotate the support member  73  to separate the driven roller  72  from the drive roller  71 . 
     As illustrated in  FIG. 5 , the conveyance control unit  79  causes the solenoid  77  to rotate the support member  73  at the end of the time length T that is between the time length S 1  and the timing length S 2 , thereby separating the driven roller  72  from the drive roller  71 . Upon the separation of the driven roller  72  from the drive roller  71 , the sheet P is held and carried by the intermediate transfer belt  20  and the transfer roller  30  without being affected by the drive roller  71 . Accordingly, no difference occurs between the travel speed of the sheet P and the surface speed of the intermediate transfer belt  20  at the time of transferring the toner image  90 . A high quality image is thus obtained by transferring the toner image  90  onto the sheet P without creating positional displacements or the like. In the case of duplex printing, high quality images that are accurately aligned between the front side and the back side are obtained. 
     Alternatively, a separation mechanism may be provided that causes the drive roller  71  to move away from the driven roller  72 . In this case, a flexible joint, coupling, or the like may be provided at the rotational shaft of the drive roller  71  to be connected to the shaft or a gear of the motor  82  in order to transmit a drive force from the motor  82  to the drive roller  71  that is moved. 
     Although the present invention has been described heretofore by referring to one or more embodiments, the present invention is not limited to such embodiments. Various variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority applications No. 2012-067670 filed on Mar. 23, 2012 and No. 2013-046281 filed on Mar. 8, 2013, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.