Patent Publication Number: US-7717422-B2

Title: Sheet processing apparatus and sheet processing method

Description:
This is a continuation of U.S. patent application Ser. No. 10/793,416 filed Mar. 4, 2004. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sheet processing apparatus and a sheet processing method which stack sheets which have been conveyed. 
   2. Description of the Related Art 
   A conventional sheet processing apparatus conveys sheet by sheet sheets discharged from an image forming apparatus to an intermediate processing tray to stack the sheets on the same, and carries out predetermined post processing such as stapling set in advance on an operating screen of the image forming apparatus, on a bundle of sheets stacked on the intermediate processing tray. After discharging the post processed bundle of sheets from the intermediate processing tray, the sheet processing apparatus waits for the next sheets conveyed from the image forming apparatus. 
   A sheet processing apparatus has been disclosed by Japanese Laid-Open Patent Publication (Kokai) No. 2001-97631, which is provided with a sheet buffer mechanism that operates to keep constant intervals between sheets discharged from an image forming apparatus to thereby secure a proper time period for carrying out post processing and sheet bundle discharge processing. 
   However, the conventional sheet processing apparatus have the following problem, which has been desired to be solved. That is, to secure the time period for processing a bundle of sheets processed on the intermediate processing tray, it is necessary to keep sheets discharged from the image forming apparatus in a waiting position until the processing of the preceding bundle of sheets is completed, leading to degraded processing efficiency. 
   To provide the sheet buffer mechanism in the sheet processing apparatus, a space for provision of the mechanism is required, leading to an increased size of the apparatus and increased costs. Therefore, the sheet processing apparatus with the sheet buffer mechanism is not suitable for use as a sheet processing apparatus attached to an image forming apparatus which has a relatively low processing speed. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a sheet processing apparatus and a sheet processing method which are capable of securing a sufficient sheet bundle processing time period even with a short sheet conveying path to thereby maintain required capability of processing sheets conveyed at constant intervals. 
   To attain the above object, in a first aspect of the present invention, there is provided a sheet processing apparatus comprising a conveying device that conveys sheets, and a controller that controls the conveying device to increase a conveyance speed of a sheet being conveyed by the conveying device in first timing so that a distance between the sheet and a succeeding sheet being conveyed by the conveying device is increased, when a sheet preceding the sheet being conveyed by the conveying device is a last sheet of a set of sheets to be processed by the sheet processing apparatus, the controller controls the conveying device to increase the conveyance speed of the sheet being conveyed by the conveying device in second timing later than the first timing. 
   According to the above first aspect of the present invention, it is possible to secure a sufficient sheet bundle processing time period even with a short sheet conveying path to thereby maintain required capability of processing sheets conveyed at constant intervals. 
   Preferably, the conveying device conveys sheets received from an image forming apparatus that forms images on the sheets, and discharges the sheets to a first stacking position, and the sheet processing apparatus further comprises a sheet processing device that aligns the sheets discharged to the first stacking position and discharges in a bundle the sheets stacked at the first stacking position to a second stacking position. 
   More preferably, the sheet processing device has a stopper and causes the sheets discharged to the first stacking position to abut on the stopper for aligning the sheets by moving the sheets in a direction opposite to a direction in which the sheets are discharged to the first stacking position. 
   Also more preferably, the controller controls the conveying device to decrease the conveyance speed of the sheet being conveyed by the conveying device until the sheet is discharged to the first stacking position, after the conveyance speed of the sheet is increased in the first timing or in the second timing. 
   Also more preferably, an interval between the sheet, of which the conveyance speed has been increased in the first timing, and the succeeding sheet is set to at least an interval which enables an operation of aligning the sheet being conveyed by the conveying device to be performed by the sheet processing device. 
   Further preferably, an interval between the sheet, of which the conveyance speed has been increased in the second timing, and the preceding sheet is set to at least an interval which enables a bundle of sheets including the preceding sheet to be discharged by the sheet processing device. 
   To attain the above object, in a second aspect of the present invention, there is provided a sheet processing apparatus comprising a sheet conveying device that conveys sheets externally handed over thereto, a sheet stacking device that stacks the sheets conveyed by the sheet conveying device, a bundle discharging device that discharges the stacked sheets in a bundle, a conveyance speed changing device that changes a conveyance speed of each of the sheets being conveyed by the sheet conveying device from a handover speed at which the sheets are handed over to the sheet conveying device to a separating speed set higher than the handover speed, for separating each of the handed-over sheets from a succeeding sheet of the sheets being conveyed by the sheet conveying device, and a changeover timing controller that controls timing in which the conveyance speed is changed from the handover speed to the separating speed. 
   Preferably, the changeover timing controller sets in advance first changeover timing applied at a first conveyance speed changing position, and second changeover timing applied at a second conveyance speed changing position downstream of the first conveyance speed changing position, and wherein the conveyance speed changing device changes the conveyance speed of a first sheet of a first bundle of sheets that is conveyed after a second bundle of sheets preceding the first bundle of sheets has been discharged by the bundle discharging device, in the second changeover timing, and changes the conveyance speed of a second sheet of the first bundle of sheets in the first changeover timing. 
   More preferably, the second changeover timing is set to timing earlier than timing in which the first sheet of the first bundle of sheets interferes with the second sheet of the first bundle of sheets. 
   Also preferably, the sheet processing apparatus comprises a bundle processing device that processes a bundle of sheets stacked on the sheet stacking device, and the bundle discharging device discharges the processed bundle of sheets. 
   To attain the above object, in a third aspect of the present invention, there is provided a sheet processing method comprising a sheet conveying step of conveying sheets externally handed over, a sheet stacking step of stacking the sheets conveyed in the sheet conveying step, a bundle discharging step of discharging the stacked sheets in a bundle, a conveyance speed changing step of changing a conveyance speed of each of the sheets being conveyed in the sheet conveying step from a handover speed at which the sheets are handed over to a separating speed set higher than the handover speed, for separating each of the handed-over sheets from a succeeding sheet of the sheets being conveyed in the sheet conveying step, and a changeover timing controlling step of controlling timing in which the conveyance speed is changed from the handover speed to the separating speed. 
   The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view showing the construction of an image processing apparatus provided with a sheet processing apparatus according to an embodiment of the present invention; 
       FIG. 2  is a front view showing the construction of the sheet processing apparatus in  FIG. 1 ; 
       FIG. 3  is a plan view showing the construction of the sheet processing apparatus in  FIG. 2 ; 
       FIGS. 4A to 4C  are views useful in explaining the operation of a swinging roller in the sheet processing apparatus; 
       FIGS. 5A and 5B  are views useful in explaining the operation of a return belt in the sheet processing apparatus; 
       FIGS. 6A to 6C  are views useful in explaining a bundle discharging operation carried out by the swinging roller; 
       FIGS. 7A to 7C  are views useful in explaining an operation of discharging a bundle of sheets on a processing tray of the sheet processing apparatus onto a stack tray, and aligning/stacking the bundle of sheets on the stack tray; 
       FIGS. 8A to 8F  are views useful in explaining an operation of discharging sheets, carried out by a discharging section of the sheet processing apparatus; 
       FIG. 9  is a timing chart showing changes in driving voltage for a sheet discharging motor of the sheet processing apparatus; 
       FIGS. 10A to 10C  are views useful in explaining an operation of discharging sheets and an operation of discharging a bundle of sheets, carried out by the sheet processing apparatus; 
       FIG. 11  is a block diagram showing the construction of a controller which controls the overall operation of the image forming apparatus; 
       FIG. 12  is a block diagram showing a sheet processing apparatus controller of the controller in  FIG. 11 ; 
       FIG. 13  is a flow chart showing a process for aligning/stacking discharged sheets, carried out by a controller of the sheet processing apparatus; and 
       FIG. 14  is a flow chart showing a process for changing an accelerating position for sheets, carried out by the controller of the sheet processing apparatus. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described in detail with reference to the accompanying drawings showing a preferred embodiment thereof. 
     FIG. 1  is a sectional view showing the construction of an image forming apparatus provided with a sheet processing apparatus according to an embodiment of the present invention. The sheet processing apparatus according to the present embodiment is provided in an image forming apparatus. In  FIG. 1 , reference numeral  200  denotes an image forming apparatus main body. An original reader  150  is provided on an upper side of the image forming apparatus main body  200 , and an automatic original reader  100  is mounted on top of the original reader  150 . Further, the sheet processing apparatus  500  according to the present embodiment, which is stored in a housing of the image forming apparatus, is provided in an upper part of the image forming apparatus main body  200  and below the original reader  150 . 
   The automatic original reader  100  separates originals, which are set on an original tray  101  with upper surfaces thereof facing upward, from the top, feeds the originals one by one in sequence from the top page to the left as viewed in  FIG. 1 , conveys each original onto a platen glass  102  via a curved path, not shown, and reads and discharges each original onto a discharged sheet tray  112 . 
   Light from a lamp of a scanner unit  104  is irradiated upon the original placed on the platen glass  102 , and light reflected from the original is led to an image sensor  109  via optical elements such as mirrors  105  and  106  and a lens  107 , so that the original is read. Image data of the original read by the image sensor  109  is subjected to image processing, and then transmitted to an exposure controller  202 . The exposure controller  202  emits laser light based on the image data. 
   The laser light is reflected by a rotating polygon mirror, and is reflected again by a reflex mirror and irradiated upon a photosensitive drum  203  with the surface thereof uniformly electrified. The irradiation of the laser light forms an electrostatic latent image on the photosensitive drum  203 . The electrostatic latent image on the photosensitive drum  203  is developed by a developing device  205 , and then transferred as a toner image on a sheet S such as thick paper and an OHP sheet. 
   Sheets S are selectively let out from a sheet cassette  231 ,  233 , or  234  by a pick-up roller  238 , and separated sheet by sheet by a separating section  237  and then fed. The sheet S is corrected for skew by a pair of pre-resist rollers, and led to a transfer position in synchronism with rotation of the photosensitive drum  203 . This causes the toner image formed on the photosensitive drum  203  to be transferred onto the sheet S via a transfer belt  211 . 
   The sheet S is then led to a pair of fixing rollers  206 , and is heated and pressurized by the fixing rollers  206 , so that the transferred toner image is fixed on the sheet S. A fixing upper separation claw and a fixing lower separation claw are disposed in abutment on the respective fixing rollers  206  to separate the sheet S from the fixing rollers  206 . The separated sheet S is conveyed by a pair of main body side discharging rollers  207  to the sheet processing apparatus  500  provided in the housing of the image forming apparatus. 
     FIG. 2  is a front view showing the construction of the sheet processing apparatus  500  provided in the image forming apparatus in  FIG. 1 .  FIG. 3  is a plan view showing the construction of the sheet processing apparatus  500 . The sheet processing apparatus  500  includes a processing tray  540  located at a relatively upstream location and on which sheets S with images formed thereon discharged from the image forming apparatus main body  200  are temporarily stacked, and a stack tray  504  which is located downstream of the processing tray  540  and disposed substantially horizontally and on which sheets S discharged from the processing tray  540  are stacked. 
   The sheet S discharged by the main body side discharging rollers  207  of the image forming apparatus main body  200  is discharged toward the stack tray  504  by a discharging section  508  on the sheet processing apparatus  500  side, which is comprised of a discharging roller  508   a  and a discharging roller  508   b  following the discharging roller  508   a . On this occasion, in timing in which the trailing end of the sheet S passes the discharging section  508 , the trailing end of the sheet S is dropped onto the processing tray  540  by a swinging roller  550 , into a state sandwiched between the swinging roller  550  and a following roller  571 . The operation of the swinging roller  550  will be described later in further detail. 
   The sheets S thus discharged onto the processing tray  540  in the sheet processing device  500  are subjected to post processing such as stapling and aligning on the processing tray  540 , and then stacked on the stack tray  504 . Examples of post processing modes executed on the processing tray  540  are a sort mode in which sheets S are sorted into a plurality of groups, and a stapling mode in which a plurality of sheets are stapled by a stapler unit  510 . The post processing mode is selected and set by an operator via an operating section  380  (refer to  FIG. 11 ) before a post processing job is started. In the stapling mode, it is possible to designate one-point stapling, two-point stapling, stapling position, or the like. The stapler unit  510  moves to a stapling position according to settings as to sheet size, stapling position, and so forth. 
   The operation of the discharging section will be described in detail later. 
     FIGS. 4A to 4C  are views useful in explaining the operation of the swinging roller  550  of the sheet processing apparatus  500 . The swinging roller  550  is attached to a swinging arm  551  which is capable of vertically swinging about a swinging roller shaft  552 . Power is transmitted from a swinging arm driving motor  643  (refer to  FIG. 12 ) to a swinging arm shaft  553  which pivotably supports a swinging cam  554 . In response to rotation of the swinging arm driving motor  643 , the swinging arm  551  vertically swings about the swinging roller shaft  552  in unison with the swinging cam  554 . Further, a swinging arm tension spring  555  for assisting in upward swinging of the swinging arm  551  is attached to the swinging arm  551 . 
   The swinging roller  550  is connected to the swinging roller driving motor  643  (refer to  FIG. 12 ) via a swinging roller driving belt  556  (refer to  FIG. 3 ), a swinging roller following pulley  557 , and the swinging roller shaft  552 . When the swinging roller driving motor  642  is rotated in response to a driving signal transmitted from a CPU  611 , described later with reference to  FIG. 12 , via a roller driving motor driver  622 , torque of the swinging roller driving motor  642  is transmitted to the swinging roller  550 , so that the swinging roller  550  is rotated. 
   As shown in  FIG. 4A , the home position of the swigging roller  550  is set at an upper location so as not to abut on the sheet S discharged onto the processing tray  540  by the discharging section  508 . When the sheet S is discharged from the discharging section  508 , and the arm  551  rotates counterclockwise about the swinging roller shaft  552  in response to driving energy from the swinging arm driving motor  643 , the swinging roller  550  moves down to press the trailing end of the discharged sheet S and drops it toward the processing tray  540 , as shown in  FIG. 4B . At the same time, the swinging roller  550  forms a nip with the following roller  571 , and rotates counterclockwise in response to driving energy from the swinging roller driving motor  642 , so that as shown in  FIG. 4C , the sheet S is pulled along a lower guide  561  in a direction opposite to the direction in which the sheet S has been conveyed until the trailing end of the sheet S dropped onto the processing tray  540  abuts on a return belt  560 . Thereafter, the swinging roller  550  moves up to the home position shown in  FIG. 4A , and prepares for discharge of the next sheet S. Details of the sheet discharging operations by the swinging arm  551  and the discharging roller  508   a  will be described later. 
     FIGS. 5A and 5B  are views useful in explaining the operation of the return belt  560  in the sheet processing apparatus  500 . The return belt  560  is supported by a discharging roller shaft  509 , and is comprised of the discharging roller  508  pivotably supported by the discharging roller shaft  509 , a return belt pulley  564  supported by a housing  563 , and a belt member  565  wound around the discharging roller  508   a  and the return belt pulley  563  (refer to  FIG. 2 ). The return belt  560  is at least one sheet feeding rotary member, and is normally disposed at such a location as to come into contact with the sheet S on the processing tray  540  so as to urge the sheet S against a sheet trailing stopper  562 . 
   As shown in  FIG. 5A , when the discharging roller shaft  509  rotates counterclockwise, the belt member  565  conveys the sheet S in a direction toward the sheet trailing stopper  562 . Further, as shown in  FIG. 5B , the return belt  560  swings in such a manner as to go away from the sheets S stacked on the processing tray  540  with an increase in the thickness of the sheet stack. 
   The sheets S thus pressed counterclockwise by the swinging roller  550  and the return belt  560  are received by the sheet trailing end stopper  562  located at an end of the processing tray  540 , and aligned sheet by sheet in the sheet conveying direction. 
   A front aligning plate  541  and a rear aligning plate  542  (refer to  FIG. 3 ), which are moveable in parallel with the discharging roller shaft  509  are provided on the sheet processing tray  540 . The front aligning plate and the rear aligning plate  542  are driven by a front aligning motor  646  (refer  FIG. 12 ) and a rear aligning plate  647  (refer  FIG. 12 ), respectively. 
   While the sheet processing apparatus  500  is not in operation, the front aligning plate  541  and the rear aligning plate  542  are waiting at respective positions which are detected by a front alighting home position sensor  530  (refer to  FIG. 12 ) and a rear aligning home position sensor  531  (refer to  FIG. 12 ), respectively. These positions are called “aligning home positions (reference positions)”, and are set at such positions such that the sheet S being conveyed does not come into contact with the front aligning plate  541  and the rear aligning plate  542 . 
   The front aligning plate  541  and the rear aligning plate  542  move to respective waiting positions suitable for the size of the sheets S before the sheets S are conveyed from the image forming apparatus. After the sheets S are aligned in the sheet conveying direction as above, the front aligning plate  541  and the rear aligning plate  542  are moved to respective sheet aligning positions in the post-processing mode set before the start of a job, so that the sheets S are aligned in the direction of the width thereof, i.e. in a direction perpendicular to the sheet conveying direction. 
   For example, in the case where the sheets S of the Nth copy are aligned in the direction of the width thereof in the sort mode, the front aligning plate  541  is caused to wait at the reference position, and the rear aligning plate  542  is moved from the waiting position to the sheet aligning position, so that the sheets S are aligned at edges thereof at an inner side with respect to the sheet surface of  FIGS. 5A and 5B . Then, as described later, the aligned sheets S are discharged to the stack tray  540 . 
   In the case where the sheets S of the N+1th copy are aligned, the rear aligning plate  542  is caused to wait at the reference position, and the front aligning plate  541  is moved from a waiting position to an aligning position, so that the sheets S are aligned at edges thereof at an outer side with respect to the sheet surface of  FIGS. 5A and 5B  thereof. Then, the aligned sheets S are discharged to the stack tray  540  as above. As a result, the sheets S can be stacked on the stack tray  540  while they are sorted each time a bundle of sheets S is discharged. It should be noted that the sheets S may be aligned at middle positions thereof, and in this case, both the front aligning plate  541  and the rear aligning plate  542  are moved from respective waiting positions to aligning positions suitable for the middle positions. 
   Also, when the stapling mode is selected, the sheets S are aligned in the direction of the width thereof at a position suitable for a set stapling position, and then they are stapled. On this occasion, the stapler unit  510  is driven by a staple clinch motor  648  (refer to  FIG. 12 ) to staple the sheets S. The stapler unit  510  is driven by a staple slide motor  649  (refer to  FIG. 12 ) to freely move in a direction vertical to the sheet surface of  FIGS. 5A and 5B  (i.e. in a direction vertical to the sheet conveying direction). Upon the start of a job, the stapler unit  510  moves to an actual stapling position which has been determined in dependence on a stapling position and a sheet size designated before the start of the job. Then, the stapler unit  510  staples a bundle of the sheets S aligned in the direction of the width thereof as above. 
     FIGS. 6A to 6C  are views showing how a bundle of sheets S is discharged by the swinging roller  550 . After a bundle of sheets S is aligned in the sheet conveying direction and in the direction of the width thereof, and stapled as above, the swinging roller  550  is driven by the swinging arm driving motor  643  to move down about the swinging roller shaft  552  until it abuts on the bundle of sheets S as shown in  FIG. 6A . Then, the swinging roller  550  forms a nip with the following roller  571 , and rotates clockwise to convey the bundle of sheets S until the trailing end of the bundle of the sheets S reaches a position in the vicinity of an upper end of a trailing end aligning wall member  570  (refer to  FIG. 6B ), and then stops the bundle of sheets S. 
   Then, the swinging roller  550  comes apart from the bundle of sheets S and returns to its home position (refer to  FIG. 6C ). At the same time, a cam  572  located below the trailing end aligning wall  570  rotates about a cam swinging rotary shaft  573 , the trailing end aligning wall  570  swings about a swinging shaft  570   a  in a direction away from the bundle of sheets S. The operations of the trailing end aligning wall member  570  and the cam  572  will be described later. 
     FIGS. 7A to 7C  are views showing how a bundle of sheets S on the sheet processing tray  540  of the sheet processing apparatus  500  is discharged onto the stack tray  504 , and aligned and stacked on the stack tray  540 . The rear end aligning wall member  570  can swing about the swinging rotary shaft  570   a , and has one end  570   b  thereof urged by a spring  512 . The one end  570   b  is in abutment on the cam  572  that is rotatable about the cam swinging rotary shaft  573 ; when the cam  572  lying at its home position (refer to  FIG. 6A ) rotates, the rear end aligning wall member  570  swings in a direction opposite to the direction in which the bundle of sheets S is conveyed. 
   The trailing end of the discharged bundle of sheets S comes into abutment on the upper end of the trailing end aligning wall member  570  (refer to  FIG. 6B ), the trailing end aligning wall member  570  is moved back upstream in the sheet conveying direction, so that the trailing end of the bundle of sheets S comes into abutment on an inclined surface of the trailing end aligning wall member  570  (refer to  FIGS. 6C and 7A ). 
   When the moved-back trailing end aligning wall member  570  returns to its home position (refer to  FIG. 6A ) while swinging about the swinging rotary shaft  570   a , the trailing end aligning wall member  570  urgingly moves the trailing end of the bundle of sheets S in a horizontal direction while aligning the bundle of sheets S at the rear edge thereof, so that the bundle of sheets S is stacked on the stack tray  504  (refer to  FIGS. 7B and 7C ). 
   The bundle of sheets S stacked on the stack tray  504  is pulled back toward the trailing end aligning wall member  570  and pressed at the top thereof by a sheet returning member  583 . The sheet returning member  583  is a puddle-like member freely rotatable about a puddle rotary shaft  590  provided on the rear end aligning wall member  570 . The sheet returning member (puddle)  583  makes one rotation counterclockwise each time a bundle of sheets S is discharged onto the stack tray  504  by the swinging roller  550 , and pulls back the discharged bundle of sheets S toward the trailing end aligning wall member  570  to press the trailing end of the bundle of sheets S. 
   Here, the sheet returning member  583  is kept in a state shown in  FIGS. 6A and 6B  to press the bundle of sheets S except when it carries out the sheet bundle pulling-back operation. The position of the sheet returning member  583  is detected by a puddle home position sensor, not shown. The stack tray  504  is configured to be moved up and down by a driving mechanism, not shown, so as to keep the height of stacked bundle of sheets S constant. 
   It should be noted that although in the present embodiment, the stack tray  504  has its sheet stacking surface lying on a substantially horizontal plane, the sheet stacking surface may be inclined. Even if the sheet stacking surface of the stack tray  504  is inclined, the trailing end aligning wall member  570  is capable of operating effectively. Further, if the sheet stacking surface of the stack tray  540  is inclined downward toward the trailing end aligning wall member  570  (in the present embodiment, the angle of inclination is set to 18°), the interference of the trailing end of a bundle of sheets S stacked on the stack tray  540  with the following bundle of sheets S discharged from the processing tray  540  can be easily avoided, and the sheet processing apparatus can be reduced in size. 
     FIGS. 8A to 8F  and  FIG. 9  are views useful in explaining an operation of discharging sheets, carried out by the discharging section  508  of the sheet processing apparatus. In the figures, a sheet S 1  is the last sheet of a sheet bundle of an Nth copy, and a sheet S 2  is the top sheet of a sheet bundle of an (N+1)th copy. In the sheet processing apparatus  500 , a relay roller  591  is rotated at a speed equal to the discharging speed of the image forming apparatus main body  200  to receive the sheet S 1  discharged from the image forming apparatus main body  200  (refer to  FIG. 8A ). Namely, the relay roller  591  and the discharging roller  508   a  are driven at a speed equal to the discharging speed of sheets discharged from the discharging rollers  207  on the image forming apparatus main body  200  side by a sheet discharging motor  641  (refer to  FIG. 12 ). 
   When the leading end of the sheet has reached a predetermined position (first accelerating position  596  or second accelerating position  597 ) between the relay roller  591  and the discharging roller  508   a , the leading sheet is accelerated to become more distant from the following sheet to thereby widen the interval or distance between the sheets. The first accelerating position  596  is a reference position at which sheets of a sheet bundle other than the top sheet are accelerated, and the second accelerating position  597  is a reference position at which the top sheet of the sheet bundle is accelerated. The first accelerating position  596  is set upstream of the second accelerating position  597  in the sheet conveying direction. The first and second accelerating positions  596  and  597  are set at locations intermediate between the relay roller  591  and the discharging roller  508   a . When the leading end of the sheet S 1  as the last sheet of the Nth copy sheet bundle has reached the first accelerating position  596  (refer to  FIG. 8B ), the sheet discharging motor  641  is controlled such that the conveyance speed of the sheet S 1  is accelerated from a handover speed ν 1  to a separating speed ν 2 , then the sheet S 1  is conveyed at the separating speed ν 2  for a predetermined time period, and then the conveyance speed of the sheet S 1  is decelerated to the speed ν 1  (refer to  FIG. 10 ) until it is discharged onto the processing tray  540 . By this acceleration control, the interval between the sheet S 1  as the last sheet of the Nth copy sheet bundle and the sheet S 2  as the top sheet of the (N+1)th copy sheet bundle becomes widened (refer to  FIG. 8C ), to thereby secure the time period for the pulling-back processing on the sheet S 1  on the processing tray  540 , described earlier with reference to  FIGS. 4A to 4C . 
   Thereafter, the sheet S 2  as the top sheet of the (N+1)th copy sheet bundle discharged from the image forming apparatus main body  200  is handed over to the relay roller  591  in the sheet processing apparatus  500 , and then, as is the case with the last sheet of the Nth sheet bundle, the sheet S 2  is conveyed at the handover speed ν 1  equal to the discharging speed of the image forming apparatus main body  200  (refer to  FIG. 8D ). When the leading end of the sheet S 2  as the top sheet of the (N+1)th copy sheet bundle has reached the second accelerating position  597  (refer to  FIG. 8E ), the conveyance speed of the sheet S 2  is accelerated from the handover speed ν 1  to the separating speed ν 2 , then the sheet S 2  is conveyed at the separating speed ν 2  for a predetermined time period by the sheet discharging motor  641 , and then the conveyance speed of the sheet S 2  is decelerated to the speed ν 1  (refer to  FIG. 10 ) until it is discharged onto the processing tray  540 . By this acceleration control, the interval between the sheet S 2  as the top sheet of the (N+1)th copy sheet bundle and a sheet S 3  as the second sheet of the (N+1)th copy sheet bundle becomes widened (refer to  FIG. 8F ), to thereby secure the time period for the pulling-back processing on the sheet S 2  on the processing tray  540 , described earlier with reference to  FIGS. 4A to 4C . 
   By thus setting the second accelerating position  597  downstream of the first accelerating position  596  in the sheet conveying direction, that is, the speed increasing timing for the sheet S 2  as the top sheet of a succeeding or (N+1)th copy sheet bundle is set later than that for another sheet (e.g. the sheet S 1 ) of a preceding or Nth copy sheet bundle, a sufficient time period for processing the Nth copy sheet bundle can be secured. The time period for processing a sheet bundle means a time period required for an operation of stapling a sheet bundle on the processing tray  540  by the stapler unit  510 , an operation of discharging a sheet bundle from the processing tray  540 , described earlier with reference to  FIGS. 6A to 6C  and  FIGS. 7A to 7C , and an operation of aligning and stacking sheets on the stack tray  504 . 
   Speed control for the sheet S 3  as the second sheet of the (N+1)th copy sheet bundle is carried out in the same manner as that for the sheet S 1  as the last sheet of the Nth copy sheet bundle. Here, the second accelerating position  597  as the accelerating position for the top sheet (sheet S 2 ) of the (N+1)th copy sheet bundle is set at such a location as does not affect the conveyance of the second sheet (sheet S 3 ) of the (N+1)th copy sheet bundle. That is, if the acceleration of the second sheet S 3  of the (N+1)th copy sheet bundle is too high compared with the acceleration of the top sheet S 2  of the (N+1)th copy sheet bundle, there is a possibility that the leading end of the second sheet S 3  of the (N+1)th copy sheet bundle collides with the trailing end of the sheet S 2  of the (N+1)th copy sheet bundle, and therefore, the second accelerating position  597  is set at a location where the above inconvenience cannot occur. 
   The first acceleration position  596  and the second accelerating position  597  may be each set at a location corresponding to a time period over which the number of pulses of the sheet discharging motor  641  reaches a predetermined count or a timer count reaches a predetermined time period after the leading end of a sheet is detected by a sensor (e.g. the handover detecting sensor  520 ) provided at a predetermined location. It should be noted that the handover detecting sensor  520  is disposed at a suitable location upstream of the relay roller  591  in the sheet conveying direction. 
   Next, the speed control of the sheet discharging motor  641  will be briefly described.  FIG. 9  is a timing chart showing changes in driving voltage for the sheet discharging motor  641 . Here, the driving voltage for the sheet discharging motor  641  corresponds to the conveyance speed ν at which sheets are conveyed by the sheet discharging motor  641 . 
   The sheet S 1  as the last sheet of the Nth copy sheet bundle discharged from the image forming apparatus main body  200  is handed over to the relay roller  591  in the sheet processing apparatus  500 , and conveyed at the conveyance speed (handover speed) ν 1 . When the leading end of the sheet S 1  has reached the first accelerating position  596  by the relay roller  591  (after the lapse of a time period t 1 ), the conveyance speed of the sheet S 1  is accelerated from the handover speed ν 1  to the separating speed ν 2 . 
   Next, the top sheet S 2  of the (N+1)th copy sheet bundle discharged from the image forming apparatus main body  200  is handed over to the relay roller  591  in the sheet processing apparatus  500 , and conveyed at the conveyance speed (handover speed) ν 1 . When the leading end of the sheet S 2  has reached the second accelerating position  597  by the relay roller  591  (after the lapse of a time period t 2 ), the conveyance speed of the sheet S 2  is accelerated from the handover speed ν 1  to the separating speed ν 2 . 
   Here, since the first accelerating position  596  is set upstream of the second accelerating position  597  in the sheet conveying direction, the time period t 1  from the time point the sheet discharging motor  641  is started for the last sheet S 1  of the Nth copy sheet bundle to the time point the leading end of the sheet S 1  reaches the first accelerating point  596  and the time period t 2  from the time point the sheet discharging motor  641  is started for the top sheet S 2  of the (N+1)th copy sheet bundle to the time point the leading end of the sheet S 2  reaches the second accelerating point  597  are in the relationship of t 1 &lt;t 2 . Namely, the accelerating position is determined such that the operating time periods T 1  and T 2  of the sheet discharging motor  641  satisfy the relationship of T 1 &lt;T 2 . Here, the operating time periods T 1 , T 2  represent a time period from completion of acceleration of the sheet S 2  to completion of acceleration of the sheet S 3 , and a time period from completion of acceleration of the sheet S 1  to completion of acceleration of the sheet S 2 , respectively. 
   Next, the second sheet S 3  of the (N+1)th copy sheet bundle discharged from the image forming apparatus main body  200  is handed over to the relay roller  591  in the sheet processing apparatus  500 , and conveyed at the conveyance speed (handover speed) ν 1 . When the leading end of the sheet S 3  has reached the first accelerating position  596  by the relay roller  591 , the conveyance speed of the sheet S 3  is accelerated from the handover speed ν 1  to the separating speed ν 2 . 
   By thus changing the accelerating position for the top sheet of the (N+1)th copy sheet bundle from the first accelerating position to the second accelerating position, a sufficient time period for processing the Nth copy sheet bundle can be secured. Here, the time period for processing a sheet bundle refers to a time period required for the stapling operation and the bundle discharging operation, described earlier. The process for changing the accelerating position will be described later. 
   Next, a description will be given of an operation of discharging sheets handed over from the image forming apparatus  200  and an operation of discharging a bundle of sheets subjected to post processing on the processing tray  540 .  FIGS. 10A to 10C  are views useful in explaining the operation of discharging the sheets and the operation of discharging the bundle of sheets. When the leading end of the last sheet S 1  of the Nth copy sheet bundle has reached the first accelerating position  596 , the sheet S 1  is accelerated as described before (refer to  FIG. 10A ), to be discharged onto the processing tray  540 . The sheet S 1  discharged onto the processing tray  540  is sandwiched between the lowered swinging roller  550  attached to one end of the swinging arm  551  and the following roller  571  by the action of the swinging arm  551 , and pushed in the direction opposite to the sheet conveying direction by the swinging roller  550  which is then rotated backward (refer to  FIG. 10B ). 
   The sheet bundle stacked on the processing tray  540  is subjected to post processing set in advance on the operating screen, not shown, of the image forming apparatus body  200 , and then the sheet bundle is pushed out of the processing tray  540  and discharged onto the stack tray  504  by the swinging roller  550  which is then rotated forward. 
   At this time, the top sheet S 2  of the (N+1)th copy sheet bundle has not yet reached the second accelerating position  597 , and the above sheet bundle discharging operation is completed before the sheet S 2  reaches the second accelerating position  597  (refer to  FIG. 10C ). After this, when the top sheet S 2  of the (N+1)th copy sheet bundle has reached the second accelerating position  597 , the sheet S 2  is accelerated. By this time, the sheet bundle discharging operation has already been completed, and hence the swinging arm  551  starts to be moved upward so that the sheet S 2  can enter the processing tray  540 . After completion of the upward movement of the swinging arm  551 , the sheet S 2  can smoothly enter the processing tray  540 . This sheet aligning and stacking operation will be described later. 
     FIG. 11  is a block diagram showing the construction of a controller which controls the overall operation of the image forming apparatus. The controller is comprised of a CPU circuit section  350 , the operating section  380 , a sheet processing apparatus controller  600 , an original feeder controller  360 , an image reader controller  370 , an image signal controller  330 , and a printer controller  340 . An external computer  310  is connected to the image signal controller  330  via an external interface (I/F)  320 . 
   The CPU circuit section  350  has a CPU  351 , a ROM  352 , and a RAM  353  incorporated therein. The CPU  351  executes control programs stored in the ROM  352  to collectively control component parts of the controller. The RAM  353  temporarily stores control data, and serves as a working area for arithmetic operations when the CPU  351  executes the control programs. 
   The original feeder controller  360  controls the automatic original feeder  100  in accordance with instructions from the CPU circuit section  350 . The image reader controller  370  controls the scanner unit  104 , the image sensor  109 , and so forth, and transfers an analog image signal output from the image sensor  109  to the image signal controller  330 . 
   The image signal controller  330  converts the analog image signal output from the image sensor  109  into a digital signal, performs various kinds of processing on the digital signal, converts the resulting digital signal into a video signal, and outputs the video signal to the printer controller  340 . Also, the image signal controller  330  performs various kinds of processing on a digital image signal input from the computer  310  via the external I/F  320 , converts the resulting digital image signal into a video signal, and outputs the video signal to the printer controller  340 . The operation of the image signal controller  330  is controlled by the CPU circuit section  350 . 
   The printer controller  340  drives the laser scanner unit (exposure controller)  202  according to the input video signal. The operating section  380  includes a plurality of keys for setting various functions relating to image formation, a display for displaying setting information, and so forth, and outputs a key signal corresponding to operation of each key to the CPU circuit section  350  and displays the corresponding information on the display according to the key signal from the CPU circuit section  350 . 
   The sheet processing apparatus controller  600  is provided in the sheet processing apparatus  500 , for exchanging information with the CPU circuit section  350  to control the overall operation of the sheet processing apparatus  500  as described later. 
     FIG. 12  is a block diagram showing the construction of the sheet processing apparatus controller  600 . 
   The sheet processing apparatus controller  600  includes a CPU circuit section  610  to which various drivers and various sensors are connected. The CPU circuit section  610  is comprised of a CPU  611 , a ROM  612 , and a RAM  613 . The CPU  611  executes control programs stored in the ROM  612  to control the sheet processing apparatus  500 . Further, the CPU circuit section  610  communicates with the CPU circuit section  350  in the main body of the image forming apparatus via a communication IC  614  to exchange data with the same, and controls the sheet processing apparatus  500  in accordance with instructions from the CPU circuit section  350 . 
   To control the sheet processing apparatus  500 , the CPU circuit section  610  captures detection signals from various sensors. Examples of the sensors include a handover detecting sensor  520 , an inlet sensor  521 , a swinging home position sensor  522 , a swinging pendulum home position sensor  523 , a tray detecting sensor  524 , a sheet surface detecting sensor  525 , a return belt moving-back sensor  526 , a staple slide home position sensor  527 , a staple clinch home positions sensor  528 , a processing tray sheet detecting sensor  529 , a front aligning home position sensor  530 , a rear aligning home position sensor  531 , a puddle home position sensor  532 , a stack tray sheet detecting sensor  533 , a stack tray encoder clock sensor  534 , a sheet surface detecting upper sensor  535 , a sheet surface detecting lower sensor  536 , a tray upper limit sensor  537 , a tray lower limit sensor  538 , a front cover opening/closing detecting sensor  539 , and a sheet detecting sensor  595 . 
   Further, various motor drivers  621  to  630  are connected to the CPU circuit  610 ; the motor drivers  621  to  630  drive corresponding respective motors according to signals from the CPU circuit section  610 . Examples of the motors include a sheet discharging motor  641 , the swinging roller driving motor  642 , the swinging arm driving motor  643 , a trailing end aligning wall driving motor  644 , a puddle motor  645 , the front aligning motor  646 , the rear aligning motor  647 , the staple clinch motor  648 , the staple slide motor  649 , and a stack tray motor  650 . 
   The sheet discharging motor  641  drives a pair of inlet conveying rollers (discharging section  508 ) and the discharging roller  508   a  constituting the return belt  560 . The swinging roller driving motor  642  is attached to an end of the swinging arm  551 , for driving the swing roller  550  which pulls back a sheet conveyed by the pair of inlet conveying rollers, and discharges a bundle of sheets processed on the processing tray  540  onto the stack tray  504 . The swinging arm driving motor  643  drives the swinging arm  551  to swing vertically so as to catch the trailing end of sheets discharged onto the processing tray  540 . 
   The rear end aligning wall driving motor  644  drives the rear end aligning wall member  570  which aligns a bundle of sheets, which has been discharged onto the stack tray  504 , at the rear edge thereof. The puddle motor  645  drives the sheet returning member  583  which presses the trailing end of a bundle of sheets stacked on the stack tray  504 . The front aligning motor  646  and the rear aligning motor  647  drive the front aligning plate  541  and the rear aligning plate  542  which align sheets stacked on the processing tray  540  in the direction perpendicular to the sheet conveying direction. 
   The staple clinch motor  648  drives the stapler unit  510  to staple sheets. The staple slide motor  649  moves the stapler unit  510  forward and backward. The stack tray motor  650  moves the stack tray  504  in a vertical direction. 
   Here, the sheet discharging motor  641 , swinging roller driving motor  642 , swinging arm driving motor  643 , trailing end aligning wall driving motor  644 , puddle motor  645 , front aligning motor  646 , rear aligning motor  647 , and staple slide motor  649  are implemented by stepping motors that are each capable of rotating pairs of rollers driven by the respective motors at a constant speed or different speeds by controlling the excitation pulse rate. 
   Further, the sheet discharging motor  641 , swinging roller driving motor  642 , swinging arm driving motor  643 , front aligning motor  646 , rear aligning motor  647 , and staple slide motor  649  are capable of being driven to rotate in forward and backward rotational directions by the sheet discharging motor driver  621 , swinging roller driving motor driver  622 , swinging arm driving motor driver  623 , front aligning motor driver  626 , rear aligning motor driver  627 , and staple slide motor driver  629 , respectively. The staple clinch motor  648  and the stack tray motors  650  are each implemented by a DC motor. 
     FIG. 13  is a flow chart showing a process for aligning/stacking discharged sheets. A program for implementing the process is stored in the ROM  612  of the sheet processing apparatus controller  600 , and is executed by the CPU  611 . 
   It is awaited that the trailing end of a sheet discharged by the discharging roller  508   a  at the discharging speed ν is detected by the sheet detecting sensor  595  provided upstream of the discharging roller  508   a  (step S 11 ). When the trailing end of the sheet is detected by the sheet detecting sensor  595 , an operation of moving down the swinging arm  551  from a waiting position to a sandwiching position is started (step S 12 ). 
   When the trailing end of the sheet discharged by the discharging roller  508   a  is sandwiched at the sandwiching position between the swinging roller  550  attached to the end of the swinging arm  551  and the following roller  571 , the sheet is aligned on the processing tray  540  such that the trailing end of the sheet is pushed back to the sheet trailing end stopper  562  by counterclockwise torque from the swinging roller  550  (step S 13 ). 
   It is then determined whether the aligned sheet is the last sheet of a bundle or not (step S 14 ). If the aligned sheet is the last sheet of a bundle, predetermined processing is performed on the processing tray  540 , and then the bundle of sheets is discharged by clockwise torque from the swinging roller  550  (step S 15 ). It is determined whether the bundle of sheets has been completely discharged or not (step S 16 ), and if the bundle of sheets has been completely discharged, an operation of moving up the swinging arm  551  from the sandwiching position to the waiting position is started (step S 20 ). The process then returns to the step S 11 . The upward movement of the swinging arm  551  in the step S 20  is intended to prevent the leading end of the succeeding sheet from interfering with the swinging arm  551 . The timing in which the upward movement is started is determined based on a return time period required for the swinging arm  551  to return from the sandwiching position to the waiting position and a moving time period after discharge of a sheet and before the leading end of the succeeding sheet reaches the detecting position of the sheet detecting sensor  595 . 
   On the other hand, if it is determined in the step S 14  that the aligned sheet is not the last sheet of a bundle, it is determined whether the leading end of the succeeding sheet discharged by the discharging roller  508   a  has been detected by the sheet detecting sensor  595  or not (step S 17 ). If the leading end of the succeeding sheet has not been detected, the processing of the step S 17  is repeated until the leading end of the succeeding sheet is detected. If the leading end of the succeeding sheet is detected, the process proceeds to the step S 20  to start the upward movement of the swinging arm  551 . 
   On the other hand, if it is determined in the step S 16  that the bundle of sheets has not been completely discharged, it is determined in a step S 18  whether the sheet is being fed after completion of the discharge of the bundle of sheets or not. If the sheet is being fed, it is determined whether the leading end of the succeeding sheet has been detected by the sheet detecting sensor  595  or not (step S 19 ). If the leading end of the succeeding sheet has been detected, the process proceeds to the step S 20  to start the upward movement of the swinging arm  551 . 
   On the other hand, if the sheet is not being fed after completion of the discharge of the bundle of sheets in the step S 18  or if the leading end of the succeeding sheet has not been detected in the step S 19 , the process returns to the step S 16 , wherein the swinging arm  551  is caused to wait at the sandwiching position until the discharge of the sheet bundle is completed. 
     FIG. 14  is a flow chart showing a process for changing the accelerating position for sheets. A program for implementing the process is stored in the ROM  612  of the sheet processing apparatus controller  600 , and is executed by the CPU  611 . 
   When a sheet on which an image has been formed by the image forming apparatus main body  200  is conveyed toward (handed over to) the sheet processing apparatus  500 , the process waits until a sensor (e.g. the handover detecting sensor  520 ) provided at a predetermined position inside the image forming apparatus main body  200  detects the leading end of the sheet, and when the leading end of the sheet has been detected, the sheet discharging motor  641  is activated so as to set the conveyance speed of the sheet to the handover speed (step S 21 ). 
   It is determined whether the sheet being conveyed by the sheet discharging motor  641  is the top sheet of a bundle of sheets or not (step S 22 ), and if it is the top sheet of a bundle of sheets, the process waits until the leading end of the sheet reaches the second accelerating position  597  (step S 23 ). Here, the determination as to whether the leading end of the sheet has reached the second accelerating position  597  or not may be carried out by a sheet detecting sensor, not shown, provided at the second accelerating position  597  directly detecting the leading end of the sheet, or alternatively, the determination may be carried out by determining whether the number of pulses of the sheet discharging motor  641  has reached a predetermined count or a timer has counted a predetermined time period after the leading end of the sheet was detected by a sensor (e.g. the handover detecting sensor  520 ) provided at a predetermined position. If it is determined that the leading end of the sheet has reached the second accelerating position  597 , the acceleration control using the sheet discharging motor  641  is started to accelerate the conveyance speed of the sheet to the separating speed (step S 24 ). After the conveyance speed of the sheet has been accelerated to the separating speed in the step S 24 , the sheet is conveyed at the separating speed for a predetermined time period, and then the conveyance speed of the sheet is decelerated until the sheet is discharged onto the processing tray  540 . Then, the present process is terminated. 
   On the other hand, if it is determined in the step S 22  that the sheet being conveyed by the sheet discharging motor  641  is not the top sheet of a bundle of sheets, the process waits until the leading end of the sheet reaches the first accelerating position  596  (step S 25 ). The determination as to whether the leading end of the sheet has reached the first accelerating position  596  or not is carried out in the same manner as the determination as to whether the leading end of the sheet has reached the second accelerating position  597  or not, described above. When it is determined that the leading end of the sheet has reached the first accelerating position  596 , the acceleration control using the sheet discharging motor  641  is started to accelerate the conveyance speed of the sheet to the separating speed (step S 24 ). After the conveyance speed of the sheet has been accelerated to the separating speed in the step S 24 , the sheet is conveyed at the separating speed for the predetermined time period, and then the conveyance speed of the sheet is decelerated until the sheet is discharged onto the processing tray  540 . Then, the present process is terminated. 
   Although in the above step S 22  it is determined whether the sheet being conveyed is the top sheet of a bundle of sheets or not, alternatively the process may be configured such that, for the first bundle of sheets to be processed, the determination of the step S 22  is not used, and the process jumps from the step S 21  to the step S 25 , and for the second and subsequent bundles of sheets, the determination of the step S 22  is used. 
   According to the present embodiment, as described above, the transfer of the first (top) sheet of a bundle of sheets conveyed after discharge to the processing tray  540  is delayed. Consequently, even if the conveyance path is short, a sufficient time period for processing a bundle of sheets on the processing tray  540  can be secured to thereby maintain required capability of processing sheets conveyed at constant intervals. As a result, the processing efficiency of the sheet processing apparatus can be improved. 
   It should be understood that the present invention is not limited to the embodiment described above, but various changes in or to the above described embodiment may be possible without departing from the spirits of the present invention, including changes as described below. 
   For example, in the above described embodiment, the conveyance speed of sheets is changed from the handover speed to the separating speed suddenly or at a time, it may be changed progressively or in multiple steps.