Patent Publication Number: US-2010124450-A1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-294446 filed Nov. 18, 2008. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including: an image carrier that rotates; a developing unit that visualizes an electrostatic latent image carried on the image carrier with a color developer into a developer image; a transfer unit that transfers the developer image carried on the image carrier to a sheet in a predetermined transfer position; a fixing unit that fixes the developer image transferred by the transfer unit to the sheet; a transport unit that transports the sheet to which the color developer image has been fixed by the fixing unit, and returns the sheet to the transfer position for transferring a developer image of another color developer to be superposed on the developer image having been fixed to the sheet; and an adjustment unit that performs adjustment to reduce a time difference between time for returning the sheet by the transport unit and time for an integer number of rotations of the image carrier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a cross-sectional view schematically showing an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a block diagram showing a configuration of a controller and its peripheral units; 
         FIG. 3  is a block diagram showing a structure of an adjustment program executed by a CPU  64  so as to reduce color shifts of respective color developer images overlay-transferred onto a sheet; 
         FIG. 4A  is a graph in an example of execution of the adjustment program indicating a time difference between time for circulation of the sheet in a second transport path and time for an integer number of rotations of an image carrier when sheet contraction does not occur; 
         FIG. 4B  is a graph showing a comparative example to  FIG. 4A ; 
         FIG. 5A  is a graph schematically showing a relation between a phase of the image carrier and a length of a developer image in a sheet transport direction when sheet contraction does not occur; 
         FIG. 5B  is a graph schematically showing the relation between the phase of the image carrier and the length of the developer image in the sheet transport direction when sheet contraction occurs; 
         FIG. 5C  is a graph showing the result of execution of the adjustment program; 
         FIG. 5D  is a comparative example to  FIGS. 5A to 5C ; and 
         FIG. 6  is a flowchart showing an example of the operation (S 10 ) of the image forming apparatus upon execution of the adjustment program by the CPU  64 . 
     
    
    
     DETAILED DESCRIPTION 
     Next, an exemplary embodiment of the present invention will be described based on the drawings. 
       FIG. 1  schematically shows an image forming apparatus  10  according to the exemplary embodiment of the present invention. The image forming apparatus  10  has an image forming apparatus main body  12 . A sheet feed unit  14  which is e.g. a one-stage unit is provided in a lower part of the image forming apparatus main body  12 . 
     The sheet feed unit  14  has a sheet feed cassette  16  containing sheets such as print sheets. A pickup roller  18  to pick up a sheet from the sheet feed cassette  16  is provided in a position above the sheet feed cassette  16 . 
     The pickup roller  18 , driven by a driver (not shown), is rotated so as to supply a sheet toward a first transport path  20 . The first transport path  20  is a sheet path from the pickup roller  18  to a registration roller  22 . The first transport path  20 , with a part of a second transport path  32  to be described later and a third transport path  40 , forms a sheet path to an output port  25 . In the sheet path, an image carrier  26  and a transfer roller  28  having an elastic surface are provided on the upstream side of a fixing device  60 , and the above-described registration roller  22  is provided on the upstream side of the image carrier  26  and the transfer roller  28 . A position in which the image carrier  26  and the transfer roller  28  are in contact with each other is a transfer position T in which a developer image carried on the image carrier  26  is transferred onto a sheet. 
     A first sensor (timing sensor)  23  for control of timing of sheet passage through the registration roller  22  is provided on the upstream side of the registration roller  22 . A second sensor (exposure timing sensor)  24  for control of timing for an exposure device  58  to be described later to write an electrostatic latent image is provided on the downstream side of the registration roller  22 . The first sensor  23  is an optical sensor which, upon detection of a lead edge of a sheet in a transport direction, outputs the result of detection to a controller  62  to be described later. The second sensor  24  is also an optical sensor which, upon detection of the lead edge (and a trail edge) of the sheet in the transport direction, outputs the result of detection to the controller  62 . 
     The second transport path  32  is a circular transport path where the sheet is circulate-transported from the registration roller  22  through the transfer position T and the fixing device  60 , again to the registration roller  22 . Note that the second transport path  32  is provided with e.g. transport rollers  34   a  and  34   b  to transport the sheet passed through the fixing device  60  toward the registration roller  22 . A third sensor (contraction amount detection sensor)  36  for calculation of a sheet contraction amount is provided on the downstream side of the transport roller  34   a . The third sensor  36  is an optical sensor which detects the lead edge and the trail edge of the sheet in the transport direction and outputs the result of detection to the controller  62 . 
     In the second transport path  32 , a switching device  38  is provided on the downstream side of the fixing device  60 , and a third transport path  40 , branched from the second transport path  32  with the switching device  38  and is continued to the output port  25 , is formed. An exit roller  42  is provided in the vicinity of the output port  25  in the third transport path  40 . 
     Accordingly, a sheet picked up with the pickup roller  18  from the sheet feed cassette  16  of the sheet feed unit  14  is guided to the first transport path  20 , and temporarily stopped with the registration roller  22  in accordance with the result of detection by the first sensor  23 . Then the sheet is passed between the image carrier  26  and the transfer roller  28  at predetermined timing, and e.g. a black developer image is transferred onto the sheet. The transferred black developer image is fixed by the fixing device  60  to the sheet, and the sheet is output with the exit roller  42  from the output port  25 . 
     Note that in the case of color printing, the switching device  38  is switched, then the sheet is circulated through the second transport path  32  while passed through the registration roller  22  four times. That is, the sheet is passed through the registration roller  22 , the transfer position T and the fixing device  60  four times and is output from the output port  25 . 
     The image forming apparatus main body  12  has a rotary developing device  44  in e.g. a lower part of an approximately central portion. The rotary developing device  44  has developing units  46   a  to  46   d  respectively containing yellow, magenta, cyan and black color developers. The developing units  46   a  to  46   d  have developing rollers  48   a  to  48   d  and removable developer containers  50   a  to  50   d . The developing units  46   a  to  46   d  supply the developers contained in the developer containers  50   a  to  50   d  to the developing rollers  48   a  to  48   d , and sequentially visualize an electrostatic latent image carried on the image carrier  26  with the respective color developers. 
     A charging device  52  having e.g. a charging roller to uniformly charge the image carrier  26  is provided on the front side of the image carrier  26 . That is, a developing bias is applied to the image carrier  26 . Further, an image carrier cleaner  54  is in contact with the image carrier  26  on the upstream side of the charging device  52  in a rotation direction of the image carrier  26 . The image carrier cleaner  54  scrapes off developer(s) remaining on the image carrier  26  after transfer. Further, a developer collection unit  56  to collect the developer(s) scraped with the image carrier cleaner  54  is removably provided on the front side of the image carrier cleaner  54 . 
     The exposure device  58  to write an electrostatic latent image with a light lay such as a laser beam on the image carrier  26  charged by the charging device  52  is provided between the rotary developing device  44  and the developer collection unit  56 . Further, the above-described transfer roller  28  is positioned on the rear side of the image carrier  26 . The transfer roller  28  sequentially overlay-transfers developer images visualized with the developing units  46   a  to  46   d  on a sheet in the transfer position T. Note that the image carrier  26  is rotated at a constant rotation speed while it transfers a one-color developer image, but in other times, the rotation speed is variably controlled by the controller  62 . 
     The fixing device  60  is provided on the downstream side of the transfer position T. The fixing device  60 , having a heating roller and a pressure roller, fixes the developer image transferred to the sheet with the image carrier  26  and the transfer roller  28  to the sheet with heat and pressure, and transports the sheet. 
     Further, the controller  62  to control the constituent elements of the image forming apparatus  10  is provided in the image forming apparatus main body  12 . 
     As shown in e.g.  FIG. 2 , the controller  62  having a CPU  64 , a memory  66 , a storage device  68  such as a hard disk drive (HDD) and a communication interface (IF)  70  for data transmission/reception, interconnected via a control bus  72 , controls the constituent elements of the image forming apparatus  10 . Note that e.g. the above-described first sensor  23 , the second sensor  24  and the third sensor  36  are also connected to the control bus  72 . 
     The CPU  64  executes predetermined processing based on a program stored in the memory  66  or the storage device  68 , to control the operation of the controller  62 . Further, it may be arranged such that the program is not provided from the memory  66  or the storage device  68  but is stored on a storage medium such as a CD-ROM and is provided from the storage medium. The storage medium may be a magnetic disk, a semiconductor memory or other storage media. The memory  66  holds unused (non-contracted) sizes of sheets in plural different sizes. Further, it may be arranged such that the memory  66  previously holds contracted sheet sizes, each obtained upon every passage of sheet through the fixing device  60  (every fixing of developer image), as a look-up table for sheets in plural different sizes. The communication IF  70  is used for connection with other devices. 
     Next, processing by the controller  62  to reduce color shifts of color developer images overlay-transferred onto a sheet will be described. 
       FIG. 3  is a block diagram showing a structure of an adjustment program  80  executed by the CPU  64  so as to reduce color shifts of respective color developer images overlay-transferred onto a sheet. 
       FIG. 4A  is a graph in an example of execution of the adjustment program  80  indicating a time difference between time for sheet circulation in the second transport path  32  (sheet circulation period) and time for an integer number of rotations of the image carrier  26  when sheet contraction does not occur.  FIG. 4B  is a graph showing a comparative example to  FIG. 4A . 
     As shown in  FIG. 3 , the adjustment program  80  has an adjustment amount setting part  82 , a correction part  84 , a transport roller driver  86 , an image carrier driver  88 , a registration roller driver  90  and a contraction amount calculator  92 . 
     The adjustment amount setting part  82  receives the result of detection by the first sensor  23 , and sets adjustment amounts to adjust e.g. rotation speeds (sheet transport speeds) of the transport rollers  34   a  and  34   b , a rotation speed of the image carrier  26  and rotation start timing of the registration roller  22 , for the transport roller driver  86 , the image carrier driver  88  and the registration roller driver  90  via a correction part  84  to be described later, so as to reduce a time difference between time for sheet circulation in the second transport path  32  and time for an integer number of rotations of the image carrier  26 . 
     For example, as shown in  FIG. 4A , the adjustment amount setting part  82  performs setting for the image carrier driver  88  to rotate the image carrier  26  in a constant period, and performs setting for the transport roller driver  86  and the registration roller driver  90  to adjust the rotation speeds of the transport rollers  34   a  and  34   b  and the rotation start timing of the registration roller  22  such that the time for sheet circulation in the second transport path  32  and the time for the integer number of rotations of the image carrier  26  are the same. Each of the color developer images are transferred onto the sheet from the lead edge to the trail edge in the transport direction. Further, a total peripheral length of the image carrier  26  from a transfer start position of the first color developer image on the surface of the image carrier  26  to a transfer termination position of the first color developer image on the surface of the image carrier  26  after a predetermined number (not limited to an integer number) of rotations corresponds to the length of an unused (non-contracted) sheet in the transport direction. 
     That is, the adjustment amount setting part  82  performs setting (initial settings) for the transport roller driver  86 , the image carrier driver  88  and the registration roller driver  90  so as to, when sheet contraction does not occur (transfer of the first color developer image), start transfer of the first color developer image at timing when a rotation angle of a predetermined first reference position for the lead edge of the sheet in the transport direction on the surface of the image carrier  26  is “0”, then terminate the transfer of the first color developer image at timing when a rotation angle of a second reference position for the trail edge of the sheet in the transport direction is “0”, and start transfer of the second color developer image onto the sheet which has circulated in the second transport path  32  at timing when the rotation angle of the first reference position for the lead edge of the sheet in the transport direction on the surface of the image carrier  26  is again “0” and terminate the transfer of the second color developer image at timing when the rotation angle of the second reference position for the trail edge of the sheet in the transport direction is again “0”. The first reference position and the second reference position may be set in different positions on the surface of the image carrier  26 , or may be set in the same position. In the initial setting (transfer of the first color developer image) for the transport roller driver  86 , the image carrier driver  88  and the registration roller driver  90 , the position on the surface of the image carrier  26  (phase of the image carrier  26 ) for transfer of the developer image to the lead edge of the sheet in the transport direction is approximately the same as the phase of the image carrier  26  upon start of writing of an electrostatic latent image by the exposure device  58 . 
     On the other hand, as shown in the comparative example of  FIG. 4B , when the adjustment program  80  is not executed, a time difference occurs between the time for sheet circulation in the second transport path  32  and the time for the integer number of rotations of the image carrier  26 . That is, as the phase of the image carrier  26  upon start of the transfer of the first color developer image and that upon start of the transfer of the second color developer image are different, a color shift occurs due to eccentricity of the image carrier  26  or the like. 
     When the correction part  84  ( FIG. 3 ) has received a sheet contraction amount from the contraction amount calculator  92  to be described later, the correction part  84  corrects outputs (settings) from the adjustment amount setting part  82  in correspondence with the sheet contraction amount (to be described later using  FIGS. 5A to 5D ), and performs settings after correction for the transport roller driver  86 , the image carrier driver  88  and the registration roller driver  90 . Note that when the correction part  84  has not received a sheet contraction amount from the contraction amount calculator  92 , the correction part  84  outputs the outputs from the adjustment amount setting part  82 , without correction, to the transport roller driver  86 , the image carrier driver  88  and the registration roller driver  90 . That is, the correction part  84  also performs determination as to whether or not the contraction amount calculator  92  has calculated and outputted a contraction amount (whether or not the transfer corresponds to the first color). 
     The transport roller driver  86  adjusts the rotation speeds (sheet transport speeds) of the transport rollers  34   a  and  34   b  and drives the transport rollers in correspondence with the setting received via the correction part  84 . The image carrier driver  88  adjusts the rotation speed of the image carrier  26  and drives the image carrier in correspondence with the setting received via the correction part  84 . Further, the image carrier driver  88  may adjust the phase of the image carrier  26  in correspondence with the result of detection of the lead edge of the sheet in the transport direction by the second sensor  24  ( FIG. 1 ). The registration roller driver  90  adjusts the rotation start timing of the registration roller  22  and drives the registration roller in correspondence with the setting received via the correction part  84 . 
     When the contraction amount calculator  92  has received the result of detection of the lead edge and the trail edge of the sheet in the transport direction from the third sensor  36 , the contraction amount calculator  92  calculates a contraction amount with respect to a predetermined sheet length, and outputs the result of calculation to the correction part  84 . For example, the contraction amount calculator  92  calculates a sheet contraction amount using a size of an unused sheet stored in the memory  66  and the result of detection by the third sensor  36 . Further, the contraction amount calculator  92  may calculate a sheet contraction amount utilizing the result of detection of the lead edge and the trail edge of an unused (non-contracted) sheet in the transport direction by the second sensor  24 . 
       FIGS. 5A to 5D  show the result of execution of the adjustment program  80  by the CPU  64  with a comparative example. 
       FIG. 5A  is a graph schematically showing relation between a phase of the image carrier  26  and a length of a developer image in the sheet transport direction when sheet contraction does not occur.  FIG. 5B  is a graph schematically showing the relation between the phase of the image carrier and the length of the developer image in the sheet transport direction when sheet contraction occurs.  FIG. 5C  is a graph showing the result of execution of the adjustment program  80 .  FIG. 5D  is a comparative example to  FIGS. 5A to 5C . 
     Note that the length of a developer image in the sheet transport direction (developer image forming time) corresponds to the length of a non-contracted sheet in the transport direction for the sake of simplicity of explanation of the sheet contraction, and the total peripheral length of the image carrier  26  when it has been rotated three times (three times the peripheral length of the image carrier  26 ) corresponds to the length of the non-contracted developer image in the sheet transport direction. That is, the figures show a case where the phase of the image carrier  26  (the predetermined first reference position for the lead edge of the sheet in the transport direction on the surface of the image carrier  26 ) upon start of writing of an electrostatic latent image on the image carrier  26  by the exposure device  58  and the phase of the image carrier  26  (the predetermined second reference position for the trail edge of the sheet in the transport direction on the surface of the image carrier  26 ) upon termination of the writing of the electrostatic latent image are the same. 
     Note that the length of the sheet in the transport direction and the length of the developer image in the sheet transport direction are not limited to an integer multiple of the peripheral length of the image carrier  26 . For example, these lengths may be about 1.2 times of the peripheral length of the image carrier  26 , or maybe about 5.8 times the peripheral length of the image carrier  26 . 
     For example, as shown in  FIG. 5A , when the length of the first color developer image in the sheet transport direction corresponds to a peripheral length for three rotations of the image carrier  26 , the first color developer image transferred onto the sheet is fixed by the fixing device  60 , then, when the sheet is contracted, as shown in  FIG. 5B , the length of the developer image in the sheet transport direction is shortened by the sheet contraction amount. 
     Accordingly, the correction part  84  corrects the adjustment amounts set by the adjustment amount setting part  82  such that the difference between the distance between the lead edge of the contracted sheet in the transport direction passing through the transfer position T and the predetermined first reference position for the lead edge of the sheet in the transport direction on the surface of the image carrier  26  and the distance between the trail edge of the sheet in the transport direction passing through the transfer position T and the predetermined second reference position for the trail edge of the sheet in the transport direction on the surface of the image carrier  26  is offset in the sheet transport direction. That is, as shown in  FIG. 5C , the correction part  84  allots about ½ of the shift amount to the lead edge side of the sheet and allots about ½ of the shift amount to the trail edge side of the sheet, and corrects the transfer range for the developer image to the sheet so as to prevent the color shift at an approximately central portion of the sheet. Thus the correction part  84  averages the color shift of the second color developer image with respect to the contracted first color developer image in the sheet transport direction. 
     Note that in  FIG. 5C , in the corrected settings (transfers of the second and subsequent color developer images) for the transport roller driver  86 , the image carrier driver  88  and the registration roller driver  90 , the position on the surface of the image carrier  26  for transfer of developer image to the lead edge of the sheet in the transport direction (the phase of the image carrier  26  to start transfer) is different from the phase of the image carrier  26  to start writing of an electrostatic latent image by the exposure device  58 . 
     On the other hand, as shown in the comparative example of  FIG. 5D , when the phase of the image carrier  26  upon start of writing of an electrostatic latent image by the exposure device  58  on the image carrier  26  and the phase of the image carrier  26  upon termination of the writing of the electrostatic latent image are the same and the transfer of the second color developer image is started from the lead edge of the contracted sheet in the same phase as that for the transfer of the first color developer image, the color shift becomes gradually larger toward the trail edge side of the sheet. 
       FIG. 6  is a flowchart showing an example of the operation (S 10 ) of the image forming apparatus  10  upon execution of the adjustment program  80  by the CPU  64 . 
     As shown in  FIG. 6 , at step S 100 , the first sensor  23  detects the lead edge of the sheet in the transport direction (sheet position) and outputs the result of detection to the controller  62 . 
     As step S 102 , the adjustment amount setting part  82  receives the result of detection from the first sensor  23 , and sets adjustment amounts (initial values) to adjust the rotation speeds of the transport rollers  34   a  and  34   b , the rotation speed of the image carrier  26  and the rotation start timing of the registration roller  22 . 
     At step S 104 , the correction part  84  determines whether or not the contraction amount calculator  92  has calculated and outputted a contract amount (whether or not the transfer of the first color developer image is performed). When it is determined that a sheet contraction amount has been received from the contraction amount calculator  92  (when a contraction amount has been calculated), the process proceeds to step S 106 , and when a sheet contraction amount has not been received from the contraction amount calculator  92  (when no contraction amount has been calculated), the process proceeds to step S 110 . 
     At step S 106 , the contraction amount calculator  92  calculates a sheet contraction amount in correspondence with the result of detection by the third sensor  36 . 
     At step S 108 , the correction part  84  corrects the adjustment amounts set by the adjustment amount setting part  82  in correspondence with the result of calculation by the contraction amount calculator  92 . 
     At step S 110 , the transport roller driver  86 , the image carrier driver  88  and the registration roller driver  90  adjust the rotation speeds of the transport rollers  34   a  and  34   b , the rotation speed of the image carrier  26  and the rotation start timing of the registration roller  22  in correspondence with the adjustment amounts received via the correction part  84 . 
     At step S 112 , the controller  62  determines whether or not transfer of each of the color developer images onto the sheet has been completed. When it is determined that the transfer has not been completed, the process proceeds to step S 100 , and when it is determined that the transfer has been completed, the execution of the adjustment program  80  is terminated. 
     Next, an operation example (color printing) of the image forming apparatus  10  will be described. 
     When an image forming signal is sent, the image carrier  26  is uniformly charged by the charging device  52 , and a light ray corresponding to a yellow image is emitted from the exposure device  58  to the charged image carrier  26  based on the image signal. The light ray from the exposure device  58  exposes the surface of the image carrier  26 , thereby an electrostatic latent image is formed. 
     The electrostatic latent image carried on the image carrier  26  is developed with the yellow developer supplied to the developing roller  48   a  of the developing unit  46   a , and is transferred to a sheet supplied from the sheet feed unit  14 . The sheet to which the yellow developer image has been transferred is guided to the fixing device  60 , and the developer image is fixed to the sheet with the heating roller and the pressure roller. 
     Note that the sheet to which the yellow developer image has been fixed is guided with the switching device  38  toward the transport rollers  34   a  and  34   b . The speed of circulation of the sheet transported with the transport rollers  34   a , and  34   b  in the second transport path  32  is controlled by the controller  62 . 
     The developer remaining on the image carrier  26  is scraped off by the image carrier cleaner  54 , and collected into the developer collection unit  56 . 
     Then the image carrier  26  is uniformly charged again by the charging device  52 , and a light ray corresponding to a magenta image is emitted from the exposure device  58  to the charged image carrier  26  based on the image signal. The light ray from the exposure device  58  exposes the surface of the image carrier  26 , thereby an electrostatic latent image is formed. 
     The electrostatic latent image carried on the image carrier  26  is developed with the magenta developer supplied to the developing roller  48   b  of the developing unit  46   b , then transported with transport rollers  34   a  and  34   b  under the control of the controller  62 , and overlay-transferred to the sheet timing-controlled with the registration roller  22 . 
     The sheet to which the magenta developer image has been transferred is guided to the fixing device  60 , and the developer image is fixed to the sheet with the heating roller and the pressure roller. The sheet to which the magenta developer image has been fixed is guided with the switching device  38  toward the transport rollers  34   a  and  34   b . The developer remaining on the image carrier  26  is scraped off by image carrier cleaner  54  and is collected into the developer collection unit  56 . 
     As the sheet is returned toward the transport rollers  34   a  and  34   b  three times, as in the case of the yellow and magenta developer images, developer images developed with the cyan and the black developers are fixed to the sheet by the fixing device  60 . Thus a color image is formed with overlaid developer images on the sheet. The sheet to which the color image has been fixed is guided with the switching device  38  to the exit roller  42  and output. 
     Note that in the above-described exemplary embodiment, the sheet contraction amount is obtained by calculation in correspondence with the result of detection by the third sensor  36 ; however, the present invention is not limited to this arrangement. For example, it may be arranged such that a previously measured sheet contraction is stored by condition (including the number of fixings, a sheet size, an environmental condition and the like) in storage device  68  or stored as a look-up table in the memory  66 , then the CPU  64  as a contraction amount acquisition unit obtains the contraction amount stored in the storage device  68  or the memory  66 , and the correction part  84  corrects the outputs (settings) from the adjustment amount setting part  82  in correspondence with the sheet contraction amount. Further, it maybe arranged such that the adjustment amount setting part  82  sets the adjustment amounts such that the time for sheet circulation in the second transport path  32  and the time for the integer number of rotations of the image carrier  26  are the same by changing the rotation speed (rotation period) of the image carrier  26  when a developer image is not transferred. 
     The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.