Patent Publication Number: US-7722041-B2

Title: Sheet processing apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sheet processing apparatus for receiving sheets, which are discharged from an image forming apparatus such as a copying machine, a printer, a facsimile, and a composite machine, or discharged from other business machines, and stacking the sheets on ascendable/descendable sheet stacking means. 
   2. Description of the Related Art 
   A sheet processing apparatus for receiving sheets discharged from an image forming apparatus such as a copying machine, a printer, and a facsimile to perform processes such as alignment, sorting, stacking, stapling, bookbinding, punching, and checking has been put into practical use. In addition, in some types of image forming apparatuses, such the sheet processing apparatus is built therein or connected thereto as a so-called purchase option. 
   Some types of sheet processing apparatuses are provided with an ascendable/descendable stacking tray, and are capable of continuously stacking a large amount of sheets by allowing the stacking tray to descend in accordance with a stacking process of sheets or sheet bundles. 
   A sheet processing apparatus disclosed JP H10-198101 A is provided with an ascendable/descendable stacking tray having a plurality of stages, and is capable of stacking sheets after positioning a designated stage of the stacking tray having the plurality of stages at a sheet receiving position. In addition, an arithmetic control device of the sheet processing apparatus estimates a total estimated time for sheet processing prior to performing the sheet processing, and notifies an image forming apparatus of a sheet discharge interval (i.e., time) calculated on a predetermined safety time or a safety factor in addition to the total estimated time. Further, the arithmetic control device of the image forming apparatus allows sheets to undergo image formation at the sheet discharge intervals and allows the image forming apparatus to discharge the sheets. 
   The total estimated time in the sheet processing apparatus is constituted of individual estimated times for respective operations from sheet reception to completion of height adjustment of a stacking tray on which sheets are already stacked, such as a sheet transporting time, a staple process time, and a stacking tray-moving time. In many cases, a length of each of the individual estimated times is 1 second or shorter. Meanwhile, the individual estimated time for an operation of switching a stacking tray of a discharge destination is incomparably long, that is, from 10 seconds to 30 seconds. Accordingly, when the operation of switching the stacking tray is frequently performed, a waiting time for processing of the image forming apparatus increases, thereby remarkably lowering a process speed. As a result, both the sheet processing apparatus and the image forming apparatus cannot utilize potential processing abilities of those. 
   SUMMARY OF THE INVENTION 
   As described above, a time required for a switching operation of a stacking tray constitutes a greater portion of a total time required for an entirety of sheet processing. As a result, if the switching time of the stacking tray can be shortened, it is possible to increase the number of sheets to be processed per minute to a large extent by shortening a sheet discharge interval time. However, when a high output and high speed are realized in a motor so as to increase an ascending/descending speed of the stacking tray, a power consumption is increased, and it becomes difficult to reduce the sheet processing apparatus in size and weight. In addition, manufacturing costs of parts are also increased. Further, a mechanical strength of a supporting mechanism and a drive mechanism must be increased, and a mechanical damage or a damage of a sheet caused in case of a possible accident becomes serious. 
   A sheet processing apparatus disclosed in JP 10-198101 A adds a predetermined safety time to a time which is preset in accordance with a moving distance, a moving direction, and a stacking amount of a stacking tray, thereby calculating an estimated time for a switching operation of sheet stacking means. Meanwhile, the safety time is set to expect an individual difference or temporal change of the motor or mechanism. When an excessive time is set by considering a worst case, an estimated process time for sheet processing becomes unnecessarily long, which lowers the process speed of the image forming apparatus. 
   In other words, the moving time of the stacking tray greatly varies depending on the individual difference, the temporal change, a difference of a drive voltage, or the like of a motor or a mechanism. However, when an excessive safety time or safety factor is expected so as to allow such the uncertainties, the estimated process time for the sheet processing is excessively increased in many cases. On the other hand, when the safety time or safety factor is expected to be small, in a case where an actual moving time is increased in rare cases due to the temporal change such as deterioration, abrasion, a rust of a permanent magnet provided to the motor, a sheet discharge interval of the image forming apparatus becomes excessively small. As a result, the sheet processing apparatus is more likely to cause paper jamming, which increase a possibility of causing a damage of a sheet or a mechanical trouble. 
   According to a first aspect of the present invention, there is provided a sheet processing apparatus, including: a stacking unit that is capable of stacking discharged sheets; a drive mechanism that is capable of moving the stacking unit; a storage device that holds data on a moving time of the stacking unit in a rewritable manner; an arithmetic unit that calculates an estimated value of the moving time of the stacking unit based on the data; and a correcting unit that corrects the data held in the storage device by using an actual measurement value of the moving time of the stacking unit. 
   According to the present invention, the sheet processing apparatus sets a small safety time or safety factor while absorbing an individual difference, a temporal change, or the like of a motor or a mechanism for driving a sheet stacking unit, thereby making it possible to shorten an estimated time for a switching operation. As a result, it is possible to perform the sheet processing at high speed by fully utilizing the potential processing ability of the sheet processing apparatus without changing the motor or the mechanism. 
   Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an explanatory view of an internal structure of a copying machine which is an example of an image forming apparatus provided with a sheet processing apparatus according to an embodiment of the present invention. 
       FIG. 2  is a partially enlarged view of the sheet processing apparatus according to the embodiment. 
       FIG. 3  is a block diagram of a control system of the sheet processing apparatus. 
       FIG. 4  is a flowchart of a control for calculation of a sheet discharge interval to be transmitted. 
       FIG. 5  is a flowchart of calculation of a sheet processing time. 
       FIG. 6  is a flowchart of calculation of a tray switching time. 
       FIG. 7  is a flowchart of a sheet stacking control. 
       FIG. 8  is a flowchart of a control for calculating a tray moving speed. 
       FIG. 9  is an explanatory diagram of a data structure of tray moving speeds. 
       FIG. 10  is a partially enlarged view of a sheet processing apparatus according to another embodiment of the present invention. 
       FIG. 11  is a flowchart of calculation of a tray switching time. 
       FIGS. 12A and 12B  are explanatory diagrams of data structures of tray moving speeds. 
   

   DESCRIPTION OF THE EMBODIMENTS 
   A description is given as to a sheet processing apparatus  1  according to an embodiment of the present invention, and a copying machine E according to one mode of an image forming apparatus provided with the sheet processing apparatus  1  with reference to the attached drawings. Note that the sheet processing apparatus of the present invention is not limited to a staple process according to this embodiment, but may have a structure in which sheets are merely stacked on sheet stacking means, may be additionally provided with a structure for performing another process such as a punching process, or may be carried out with a structure for performing only the other processes or with another structure for performing the same processes. In addition, the image forming apparatus of the present invention is not limited to the copying machine E according to this embodiment, but may be carried out in a facsimile, a printer, various printing machines, or the like. 
   In addition, the sheet processing apparatus  1  according to this embodiment may be connected to a printing apparatus or the like other than an apparatus main body  100  of the copying machine E. The sheet processing apparatus  1  according to this embodiment may be constituted of another housing which can be separated from the apparatus main body  100 , or may be incorporated into a housing of the apparatus main body  100  in an inseparable manner. 
   &lt;Image Forming Apparatus&gt; 
     FIG. 1  shows an internal structure of a copying machine which is an example of an image forming apparatus provided with a sheet processing apparatus according to this embodiment. The copying machine E is provided with an apparatus main body  100  serving as image forming means, and a sheet processing apparatus, for example, a sheet processing apparatus  1 . 
   As shown in  FIG. 1 , the sheet processing apparatus  1  is arranged beside the apparatus main body  100  of the copying machine E. In the sheet processing apparatus  1 , discharged sheets on which images are formed in the apparatus main body  100  are received, and the sheets can be stacked on a ascendable/descendable tray unit  26  (see  FIG. 2 ). At an upper portion of the apparatus main body  100 , there is arranged an automatic document feed (hereinafter, referred to as “ADF”)  300  for automatically feeding documents. At a lower portion of the apparatus main body  100 , there are arranged feed cassettes  200  loaded with plural kinds of sheets with different sizes. 
   In the automatic document feed  300 , take-out rollers  304  and  305  take out one by one a plurality of read documents  302  stacked on each other on a feed tray  301 . Transport rollers  306  and  308  transport the sheets to pass through a read position  307 , and discharge and stack the sheets onto a discharge tray  309 . An image formed on a lower surface of the read document  302  is read through a scanning unit  106  which is allowed to stop immediately below the read position  307 . 
   The scanning unit  106  is mounted with an illuminating device (not shown) having a long length in a depth direction and a mirror (not shown) integrated with each other, and is movable from left to right in  FIG. 1 . In a case where the automatic document feed  300  is opened to a rear side to place documents one by one on a glass plate of the apparatus main body  100 , the scanning unit  106  is moved from left to right in  FIG. 1 , thereby reading the image. 
   In any case, the image formed on a surface of the document illuminated by the illuminating device of the scanning unit  106  is relayed by mirrors  103  and  104  from the mirror of the scanning unit  106 , and is cast on a light receiving element (not shown) of a reading part  105  by an optical system (not shown). A linear image is read from the cast image by the light receiving element. An image signal of the read linear image is amplified in a signal processing circuit (not shown), is subjected to digital processing or the like, and is converted into image data to be temporarily stored. 
   The apparatus main body  100  transfers a toner image formed on a photosensitive drum  108  onto a sheet taken out from one of the feed cassettes  200 , thereby performing image formation. A light source  122  is controlled by the image signal produced from the image data, and outputs a light beam corresponding to the image signal. A scanning part  124  produces a scanning beam by rotating a scanning mirror  123  in synchronization with an output of the light source  122 . The scanning beam is applied onto the photosensitive drum  108  which is uniformly charged, thereby forming an electrostatic latent image on a surface of the photosensitive drum  108 . 
   The electrostatic latent image formed on the surface of the photosensitive drum  108  is developed by allowing toner supplied from a developing unit  109  to adhere thereto, thereby obtaining a toner image. 
   On the other hand, a sheet on which an image is to be formed is taken out of a corresponding feed cassette  200  in advance by take-out rollers  201 , transport rollers  118  and  202 , and the like. Then, the sheet is allowed to abut against a registration roller pair  107  which are stopped to stand by. 
   The registration roller pair  107  is driven in synchronization with the rotation of the photosensitive drum  108 , and performs registration such that a leading end of the toner image is aligned with a leading edge of the sheet transported between the photosensitive drum  108  and a transferring part  110 . The transferring part  110  charges the sheet and transfers the toner image on the surface of the photosensitive drum  108  to the sheet. The sheet having the toner image transferred thereon is transported to a fixing part  112  by a transport belt  111 , and then the toner image is fixed on the surface of the sheet by being heated and pressurized in the fixing part  112 . 
   In a case of a one-side printing, the sheet having the toner image fixed thereon passes through transporting rollers  113  and a flapper  114  to be discharged from a discharge roller pair  115  directly to the sheet processing apparatus  1 . Meanwhile, in a case of a two-side printing, the flapper  114  is switched while a trailing edge of the sheet is nipped by the discharge roller pair  115 , thereby switching back the sheet to a reverse path  116 . Then, transport rollers  117  and  118  transport the sheet, and the sheet is allowed to abut against the registration roller pair  107  to stand by while rear and front surfaces of the sheet are reversed. In a similar manner as in the one-side printing, the sheet which is on standby is transported between the photosensitive drum  108  on which the toner image is formed, and the transferring part  110 , thereby transferring the toner image also onto a rear surface of the sheet. The fixing part  112  fixes the toner image on the sheet, and the sheet is discharged to the sheet processing apparatus  1 . 
   The apparatus main body  100  activates the photosensitive drum  108  at an interval of a sheet processing time which is notified from the sheet processing apparatus  1 , and controls respective parts, such as the light source  122 , of the apparatus main body  100  to perform image formation and sheet discharge. The sheet processing apparatus  1  reads out settings of process contents transmitted from the apparatus main body  100 , calculates the sheet processing time required for the process for each sheet, and transmits the calculated sheet processing time to the apparatus main body  100 . 
   &lt;Sheet Processing Apparatus&gt; 
     FIG. 2  is a partially enlarged view of the sheet processing apparatus according to this embodiment. The sheet processing apparatus  1  includes: sheet stacking means, for example, the tray unit  26 ; arithmetic means, for example, a CPU  600 ; storage means, for example, a flash memory  51 ; correcting means, for example, the CPU  600 ; load detecting means, for example, the CPU  600 ; direction control means, for example, the CPU  600 ; distance detecting means, for example, a shift clock sensor  49 ; time detecting means, for example, the CPU  600 ; image forming means, for example, the apparatus main body  100 ; control means, for example, a control part  150 ; and an image forming apparatus, for example, the copying machine E. 
   Sheets discharged from the discharge roller pair  115  of the apparatus main body  100  are subjected to the set process in the sheet processing apparatus  1  as shown in  FIG. 2 , and then discharged onto a designated first tray  23  (or second tray  24 ) of the tray unit  26  to be stacked thereon. 
   A flapper  3  switches a sheet transport path between a third transport path  7  for guiding sheets into a folding device  70  arranged at a lower portion of the sheet processing apparatus  1 , and a first transport path  6  for guiding sheets into a staple tray  38 . A flapper  4  switches a sheet transport path between a second transport path  8  provided with a buffer roller  9 , and the first transport path  6 . When the sheets are stacked on the staple tray  38 , an upstream side end portion of the flapper  3  is positioned downward, and an upstream side end portion of the flapper  4  is positioned upward. As a result, the sheets are transported into the first transport path  6  through a roller pair  5 . When the sheets are transported into the folding device  70 , the upstream side end portion of the flapper  3  is positioned upward. In order to transport the sheets to the buffer roller  9  for buffering, the upstream side end portion of the flapper  4  is positioned downward. 
   The staple tray  38  temporarily stacks the sheets before being stacked on the tray unit  26  to perform various processes. The sheets discharged onto the staple tray  38  by a first discharge roller  17  and a holding-down roller  18  are transported in a lower right direction by a discharge alignment belt  19 , and then trailing edges of the sheets are allowed to abut against a bumping plate  20 . 
   The discharge alignment belt  19  rotates by being nipped between the first discharge roller  17  and the holding-down roller  18 , and is provided with an endless rib (not shown) in the vicinity of a central part of an inner side of the belt as a prevention against deviation of the belt. The bumping plate  20  is movable from a home position at which the bumping plate  20  subsequently aligns the trailing edges of the sheets, to an evacuating position (indicated by the broken line) at which the bumping plate  20  does not interrupt movement of a stapler unit  400 . Side edges of the sheets abutted against the bumping plate  20  are aligned by moving a lateral shifting guide  21  to the inner side while the discharge alignment belt  19  is deformed upward to cancel the abutment. 
   With respect to the sheet bundle obtained on the staple tray  38  by being repeatedly subjected to the stacking and aligning operations, it is possible to perform a staple process in the trailing edge portion thereof by activating the stapler unit  400 . The sheet bundle subjected to the staple process is nipped between a moving discharge roller  33  and a discharge roller  32  by rotating a rotating guide  31  provided on an outlet side, and is transported in a left direction of  FIG. 2  to be discharged onto the first tray  23  (or the second tray  24 ) of the tray unit  26 . 
   The rotating guide  31  holds the moving discharge roller  33  in a rotatable manner. When the sheets are stacked on the staple tray  38 , the rotating guide  31  causes the moving discharge roller  33  to rotate upward to be spaced apart from the discharge roller  32 . Meanwhile, in a case where the sheets are not stacked on the staple tray  38 , in other words, in a case where the sheets are discharged to the tray unit  26  one by one without forming a sheet bundle, the rotating guide  31  is caused to rotate downward, thereby bringing the moving discharge roller  33  into press-contact with the discharge roller  32 . Thus, the sheets are discharged from a nip between the first discharge roller  17  and the holding-down roller  18  to a nip between the moving discharge roller  33  and the discharge roller  32 . 
   The buffer roller  9  performs buffering by nipping the sheets between peripheral parts such as a buffer roller  15  and the buffer roller  9 . When the stapler unit  400  is activated to perform stapling, an upper end side of a flapper  39  is rotated leftward, and two, three, and more sheets are overlapped on each other to be wound around the buffer roller  9 . As a result, it is possible to continuously receive sheets from the apparatus main body  100  (see  FIG. 1 ) even during the movement and activation of stapler unit  400 . The sheets held, i.e., buffered, on the buffer roller  9  are rotated around the buffer roller  9  once while the upper end side of the flapper  39  is rotated rightward. Thus, the sheets are discharged to the nip between the first discharge roller  17  and the holding-down roller  18 , thereby being stacked on the staple tray  38  as a sheet bundle. 
   The stapler unit  400  includes a stapler for performing a stapling operation with respect to the sheet bundle stacked on the staple tray  38 , and is capable of performing one-position stitch on a front side, two-position stitch, and one-position stitch on a trailing side. 
   A sheet detecting sensor  10  detects a leading edge and a trailing edge of the sheet entering the third transport path  7 . A sheet detecting sensor  12   a  detects the leading edge and the trailing edge of the sheet entering the first transport path  6 . A sheet detecting sensor  13  detects the trailing edge of the sheet discharged to the nip between the moving discharge roller  33  and the discharge roller  32 . A sheet detecting sensor  12   b  detects the sheet entering a second transport path  8 , and the detection result is utilized for a rotational control of the buffer roller  9 . 
   The tray unit  26  is an ascendable/descendable stack table unit which is mounted with the lower first tray  23  and the upper second tray  24  fixed thereto. The tray unit  26  is driven by a shift motor  601  (see  FIG. 3 ) having a drive mechanism (not shown) built in the housing of the sheet processing apparatus  1 . A rack gear provided to the tray unit  26  is engaged with an ascendable/descendable gear of the drive mechanism to be rotated, thereby moving the tray unit  26  in a vertical direction. 
   A distance sensor  60  arranged at the upper portion of the housing of the sheet processing apparatus  1  includes an infrared light emitting element and light receiving sensor, and detects a reflected right from an object in front of the distance sensor  60  to generate an output according to the distance. The distance sensor  60  detects a distance from the distance sensor  60  to an uppermost surface of the sheets (or a sheet bundle) stacked on the first tray  23  (or the second tray  24 ). 
   &lt;Control of Sheet Processing Apparatus&gt; 
   The sheet processing apparatus  1  is controlled by the CPU  600  serving as a microcomputer system as shown in  FIG. 3 . The CPU  600  has a ROM, a RAM, a serial interface circuit, and the like (not shown) built therein. A processing program corresponding to a control procedure of the flowcharts shown in  FIGS. 4 to 8  is stored in the ROM. The processing program read out from the ROM is held in the RAM, and working data, input data, communication data, calculation results, and the like which are subsequently generated in control processes are written/deleted every control process. The serial interface circuit performs transmitting/receiving the control data to/from the control part  150  of the apparatus main body  100  (see  FIG. 1 ), and is also capable of communicating bi-directionally with other computers and a facsimile reception part (not shown). 
   The flash memory  51  connected to the CPU  600  holds data on moving speeds shown in  FIG. 9  in a nonvolatile and rewritable manner. The data on the moving speeds held in the flash memory  51  is initially set to have a sufficient margin. In addition, when the parts of the tray unit  26  are replaced, the data on the moving speed is reset to the initial setting. 
   After that, the actual measurement values of the moving speeds are calculated every ascending/descending of the tray unit  26  by using the measured ascending/descending distance and ascending/descending time, and the data on the moving speeds is corrected every ascending/descending of the tray unit  26  based on the actual measurement values. As a result, the data on the moving speeds is replaced with numerical values having less allowance and corresponding to the actual measurement values. Time required for replacement of stack trays which is calculated based on the corrected data on the moving speeds becomes a shortened time while securing sufficient safety factors. 
   An input side of the CPU  600  is electrically connected not only to the distance sensor  60 , but also to a buffer sensor  11 , an entrance sensor  12   a , an up-cover sensor  40 , a discharge motor clock sensor  41 , a staple tray sensor  43 , a first gridiron sensor  44 , a second gridiron sensor  45 , a discharge sensor  46 , a staple movement HP sensor  47 , a shift clock sensor  49 , an up-limit sensor  53 , a door open/close detecting SW  54 , a joint SW sensor  55 , and other sensors, respectively. 
   The buffer sensor  11  detects that the sheet is on standby at the buffer roller  9 . The entrance sensor  12   a  detects the sheet discharged from the apparatus main body  100  enters the sheet processing apparatus  1 . The up-cover sensor  40  detects that an upper cover of the sheet processing apparatus  1  is opened. The discharge motor clock sensor  41  outputs each pulse of a predetermined rotational angle of a discharge motor  35   a  to provide information on a malfunction caused when the sheet is discharged from the inside of the sheet processing unit  1  to the tray unit  26 , or information on a speed control. 
   An alignment HP sensor  42  detects a home position of the bumping plate when stapling is performed. The staple tray sensor  43  detects presence or absence of the sheet on the staple tray  38 . The first gridiron sensor  44  and the second gridiron sensor  45  detect positions of an upper gridiron guide  27  and a lower gridiron guide  30  (see  FIG. 2 ) which form upper and lower wall surfaces of an outlet  50 , respectively. The discharge sensor  46  detects that the sheet is discharged from the inside of the sheet processing unit  1  onto the first tray  23  (or the second tray  24 ). 
   The staple movement HP sensor  47  detects that the stapler unit  400  capable of moving inside the sheet processing apparatus  1  is positioned at the home position. The shift clock sensor  49  detects a movement of the ascendable/descendable tray unit  26 , or a malfunction or the like caused in the shift motor  601  serving as the drive source of the tray unit  26  to notify such the information of the CPU  600 . The up-limit sensor  53  detects an upper limit of the movable tray unit  26 . The door open/close detecting SW  54  detects opening/closing of a door of the sheet processing apparatus  1 . The joint SW sensor  55  detects that the sheet processing apparatus  1  is connected to the apparatus main body  100 . 
   On the other hand, an output side of the CPU  600  is electrically connected not only with the shift motor  601 , but also a transport motor M 230 , a discharge motor  35   a , an alignment motor M 250 , a staple portion moving motor (i.e., pulse motor)  452 , a staple motor  406   a , a staple motor  406   b , an entrance solenoid SL 290 , an outlet solenoid SL 300 , a switching solenoid SL 310 , display means  650 , and other motors and actuators, through drivers D 1 , D 2 , D 3 , D 4 , D 5   a , D 5   b , D 7 , D 8 , D 9 , and D 11 , respectively. 
   The transport motor M 230  drives the roller pair  5  and the first discharge roller  18  to transport the sheet provided within the sheet processing apparatus  1 . The discharge motor  35   a  drives the discharge roller  32  to discharge the sheet (or sheet bundle) to the tray unit  26 . The alignment motor M 250  drives the width shifting guide  21  to align the sheets. The staple portion moving motor  452  moves the stapler unit  400 . The staple motor  406   a  causes the staple portion for stapling the sheet bundle to perform a stapling operation. 
   The staple motor  406   b  causes the staple portion for stapling the sheet bundle to perform the stapling operation. The entrance solenoid SL 290  drives the flapper  3  to switch a transport path for the sheets discharged from the copying apparatus main body  100 . The outlet solenoid SL 300  drives the flapper  4  to switch an outlet for the sheets discharged from the inside of the sheet processing apparatus  1 . The switching solenoid L 310  drives the flapper  39  to switch the transport path for the sheets within the sheet processing apparatus  1 . The display means  650  draws an attention of an operator when over-stacking or the like of the sheets is detected in measurement of a sheet stacking surface distance. 
   The CPU  600  reads out the processing program from the ROM, and causes the PAM to hold the read processing program. According to the processing program, the CPU  600  refers to outputs of those sensors to perform a necessary calculation process based on the control data sent from the control part  150  of the apparatus main body  100 . Further, the CPU  600  controls the various motors, solenoids, and the like, thereby performing control for each part of the sheet processing apparatus  1 . 
   The CPU  600  detects an output of the distance sensor  60 , and measures a distance from the distance sensor  60  to the uppermost surface of the sheets (or sheet bundle) stacked on the first tray  23  (or the second tray  24 ). Further, the CPU  600  causes the tray unit  26  to descend every time a predetermined number of sheets are stacked on the first tray  23  (or the second tray  24 ), or every time a sheet bundle is stacked thereon, and causes the tray unit  26  to stop at a position where the same distance as that obtained before the sheets are stacked is measured. As a result, the first tray  23  (or the second tray  24 ) is caused to descend by a thickness of the stacked sheets (or sheet bundle), thereby maintaining the height of the uppermost surface (i.e., stacking surface) of the stacked sheets to be substantially constant. 
   The CPU  600  corrects the data on the moving speed Which is stored in the flash memory  51  every time the tray unit  26  is caused to ascend or descend for replacement of trays. Then, the CPU  600  gradually decreases a margin of the moving time as an estimated time, shortens a sheet processing time (i.e., an estimated time required for replacement of trays) which is to be transmitted to the control part  150  of the apparatus main body  100 , and shortens a sheet discharge interval of the apparatus main body  100 . As a result, the process speed of the sheet processing apparatus  1  is gradually increased. 
   &lt;Calculation of Sheet Processing Time&gt; 
   In the apparatus main body  100  shown in  FIG. 1 , image formation is started and a signal for a stacking probe (i.e., pre-registration-on request) is transmitted from the control part  150  to the CPU  600  shown in  FIG. 3  at a timing when sheets are taken out from one of the feed cassettes  200 . 
   Then, as shown in  FIG. 4 , the CPU  600  extracts settings for the process contents from the communication data transmitted from the control part  150  to store the extracted settings in the RAM provided inside the CPU  600 . The CPU  600  acquires a sheet size (S 111 ), checks which of the first tray  23  or the second tray  24  a sheet is to be discharged onto (S 112 ), checks that a process mode is set at simple stacking, a staple process, or the like (S 113 ), and calculates the sheet processing time (i.e., estimated time) for the sheet on which an image is to be formed, based on the contents of the settings (S 114 ). Then, the calculated sheet processing time is set in a RAM for transmission (S 115 ) and the calculated sheet processing time is sent back to the control part  150  together with a response signal (S 116 ). As a result, the control part  150  waits only for the sheet processing time designated by the CPU  600  to start the subsequent image formation and sheet discharge process. 
   In Step S 114 , a sheet processing time calculation process shown in  FIG. 5  is invoked. As shown in  FIG. 5 , a transport time is first set as a process time required for sheets to pass through the sheet transport path (S 121 ). After that, the CPU  600  checks whether or not it is necessary to perform switching from the first tray  23  to the second tray  24 , or the second tray  24  to the first tray  23  (S 122 ). In a case where the switching is to be performed (Yes in Step S 122 ), the CPU  600  calculates the process time by performing the tray switching time calculation process shown in  FIG. 6  (S 123 ), and adds the acquired process time to the process time set in Step S 121  (S 124 ). In a case where the switching is not to be performed (No in Step S 122 ), the acquired process time is not added thereto. 
   After that, the CPU  600  checks whether or not alignment is to be performed (S 125 ). In a case where the alignment is to be performed, a predetermined alignment time is added to the process time (S 126 ). Then, the CPU  600  checks whether or not a staple process is to be performed (S 127 ), and in a case where the staple process is to be performed, a predetermined staple time is added to the process time (S 128 ). After that, the CPU  600  checks whether or not a discharge of a sheet bundle is to be performed (S 129 ). In a case where the discharge of the sheet bundle is to be performed, a predetermined sheet bundle discharge time is added to the process time (S 131 ). 
   Finally, the CPU  600  takes into account an error as a sheet processing time, and adds a predetermined margin time without conditions (S 132 ) to determine the sheet processing time. The control part  150  of the apparatus main body  100  determines a discharge timing of a transfer sheet based on the determined sheet processing time, thereby controlling the time interval between the discharge of a preceding sheet and that of a subsequent sheet. 
   As shown in  FIG. 6 , in the tray switching time calculation process of Step S 123 , the CPU  600  first adds the number of stacked sheets on the first tray  23  to the number of stacked sheets on the second tray  24 , thereby calculating a total number of stacked sheets in the tray unit  26  (S 141 ). Then, the CPU  600  selects a value of a tray speed in accordance with a moving direction (i.e., ascending or descending) of the tray unit  26  and the number of stacked sheets in the tray unit  26 , from a table containing tray speeds shown in  FIG. 9  which is recorded in a rewritable nonvolatile memory (S 142 ). Next, the CPU  600  calculates the moving distance from a current position of the tray unit  26  to a position after switching of the trays of the tray unit  26  (S 143 ), thereby obtaining the estimated time for movement of the tray based on the moving distance and the tray speed (S 144 ). 
   When the sheet processing time is transmitted to the control part  150  in Step S 116  shown in  FIG. 4 , a process starting signal (i.e., registration-on request) is transmitted from the control part  150  to the CPU  600 , and then the CPU  600  starts a stacking process shown in  FIG. 7 . As shown in  FIG. 7 , prior to arrival of the sheet, the CPU  600  first checks whether or not movement of the tray is to be performed (S 151 ). In a case where the movement of the tray is necessary, the tray unit  26  is caused to move (S 152 ). Determination as to whether or not the movement of the tray is necessary is executed by comparing a tray which is currently provided at the outlet with the discharge tray checked in the above-mentioned pre-registration-on requesting process shown in  FIG. 5 . 
   The CPU  600  waits for the arrival of the sheet discharged from the apparatus main body  100 , and then executes various processes, sheet transportation, a sheet stacking process, or the like in the sheet processing apparatus  1  (S 153 ). 
   In a case where the movement of the tray is performed in Step S 152 , a learning process of the tray speed shown in  FIG. 8  is carried out. As shown in  FIG. 8 , the CPU  600  first starts time counting, that is, counting of clock pulses (S 161 ), and starts measuring the moving distance, that is, counting output pulses of the shift clock sensor  49  (S 162 ). Then, the CPU  600  outputs a moving direction signal (S 163 ) and initiates the shift motor  601 . As a result, the tray unit  26  starts moving. 
   Then, the CPU  600  refers to the output of the distance sensor  60 , and determines whether or not the stacking surface of the designated first tray  23  (or second tray  24 ) has reached a predetermined stacking height (S 165 ). When the first tray  23  (or second tray  24 ) reaches a target position in accordance with the movement of the tray unit  26  (YES in Step S 165 ), the CPU  600  turns off a motor driving signal of the shift motor  601  (S 166 ). 
   Then, the CPU  600  divides the moving distance (which is converted from pulse-count) which is obtained when the first tray  23  (or second tray  24 ) reaches the target position by a time count value obtain at the time, thereby calculating the actual measurement value of the moving speed in ascending or descending of the tray unit  26  at the time. The CPU  600  adds a predetermined safety time (i.e., margin) to the calculated value, and corrects the data on the moving speed which is recorded in the flash memory  51  (S 167 ). In other words, the data on a corresponding range of the number of stacked sheets and on the moving direction among the data on the moving speeds shown in  FIG. 9  is replaced with a value obtained by adding the margin to the actual measurement value at the time. 
   It should be noted that instead of simply replacing the data with the value, it is possible to execute calculation of the moving average in which the preceding data is replaced with a value obtained by averaging the pieces of data of the past several times of measurements in the same classification. Alternatively, instead of calculating the moving speed, it is possible to record the actual measurement value of the moving time in the flash memory  51  to perform control of correcting the data for every ascend or descend of the tray unit  26 . Also, it is possible to perform the control by storing another process time as constants on the apparatus main body  100  side, performing calculation in which only an estimated time for replacement of trays is selected from the flash memory  51 , and transmitting only the selected data from the sheet processing apparatus  1  to the apparatus main body  100 . Further, in the table shown in  FIG. 9 , it is possible to periodically correct the data in a frame, in which frequency of writing data is small, by interpolation calculation or extrapolation calculation by using data with high frequency of being written. When data greatly different from the preceding data is calculated by the calculation of the actual measurement or actual measurement values, the replacement of the data may be cancelled. 
   After that, the CPU  600  stops a timer (S 168 ) and the measuring of the distance (S 169 ) as an end process, thereby completing this process. 
   As shown in  FIG. 9 , the flash memory  51  holds a speed learning table in a nonvolatile manner. The speed learning table is defined as a two-dimensional arrangement of the number of stacked sheets and the moving direction, and the number of stacked sheets is learned individually per 100 sheets. This table learns by storing the moving speeds during the process of the flowchart shown in  FIG. 8 , and refers to a learning value to be used for calculation of the moving time of the tray in the process of the flowchart shown in  FIG. 6 . 
   In the sheet processing apparatus according this embodiment, the actual measurement value of the moving time (i.e., switching time) of the tray is calculated in accordance with the distance and time each detected during the movement of the tray. Based on the thus obtained actual measurement value, it is possible to obtain an optimized time, which is more approximate to the actual operating time, by the subsequent calculation of the sheet processing time. As a result, the productivity of the sheet processing apparatus having a plurality of stack trays, and that of the image forming system can be enhanced. 
   Another Embodiment 
   Another embodiment will be described with reference to  FIGS. 10 to 12 .  FIG. 10  is a partially enlarged view of a sheet processing apparatus according to another embodiment of the present invention.  FIG. 11  is a flowchart of a calculation of a tray switching time.  FIGS. 12A and 12B  are explanatory diagrams of a data structure of the tray moving speeds. A sheet processing apparatus  1 B according to another embodiment is provided with a first tray  23 B and a second tray  24 B, both of which are independently provided in an ascendable/descendable manner, the first tray  23 B being provided below the second tray  24 B. The first tray  23 B and the second tray  24 B are provided in place of the tray unit  26  of the sheet processing apparatus  1  according to the embodiment described with reference to  FIGS. 1 to 9 , in which an interval between two trays is fixed. Structures of the sheet processing apparatus  1 B other than the above-mentioned difference are similar to those of the sheet processing apparatus  1 . Accordingly, the same reference symbols are given to the same parts, and detailed descriptions thereof will be omitted. 
   The CPU  600  measures an ascending/descending time and ascending/descending distance for each of the first tray  23 B and the second tray  24 B, and corrects two process time tables as shown in  FIGS. 12A and 12B  for each of the actual ascending/descending movements, thereby having the tables held in a nonvolatile memory. When calculating a sheet processing time to be transmitted to the control part  150  of the apparatus main body  100 , the CPU  600  calculates the tray switching time by the process of the flowchart shown in  FIG. 11 . 
   As shown in  FIG. 10 , in the sheet processing apparatus  1 B, the first tray  23 B and the second tray  24 B are independently supported in an ascendable/descendable manner. The first tray  23 B and the second tray  24 B each have a shift motor (not shown) and a drive mechanism built therein, and a rack provided to the sheet processing apparatus  1 B is engaged with pinion gears of the respective driving mechanisms. As a result, the first tray  23 B and the second tray  24 B are independently driven to ascend or descend. 
   Accordingly, in the sheet processing apparatus  1 B, when a discharge destination is changed from the first tray  23 B to the second tray  24 B, the first tray  23 B is caused to descend to a position indicated by the broken line to stand by, while the second tray  24 B is caused to descend to a height position at which a lower stacking surface of a sheet outlet  50  reaches a predetermined height. Meanwhile, when the discharge destination is changed from the second tray  24 B to the first tray  23 B, the second tray  24 B is caused to ascend to the above of the sheet outlet  50  to stand by, while the first tray  23 B is caused to ascend until reaching right below the sheet outlet  50  from the standby position indicated by the broken line. 
   When calculating the sheet processing time to be transmitted to the control part  150  of the apparatus main body  100  prior to the sheet processing (Step S 114  of  FIG. 4 ), in a case where switching between the first tray  23 B and the second tray  24 B is necessary for the designated process (YES in Step S 122  of  FIG. 5 ), the CPU  600  calculates a time required for switching between the first tray  23 B and the second tray  24 B (S 123 ). 
   When calculating the time for switching between the first tray  23 B and the second tray  24 B, the CPU  600  first calculates a moving time of the first tray  23 B as shown in  FIG. 11  (S 241 ). In other words, the moving distance of the first tray  23 B is calculated based on a current height position of the first tray  23 B and a height position thereof after the first tray  23 B is moved. In addition, based on the current number of stacked sheets on the first tray  23 B and the moving direction of the first tray  23 B, the moving speed is selected from the table shown in  FIG. 12A . Then, the calculated moving distance is divided by the selected moving speed, thereby calculating the moving time of the first tray  23 B. 
   Next, the moving time of the second tray  24 B is calculated (S 242 ). In other words, the moving distance of the second tray  24 B is calculated based on a current height position of the second tray  24 B and a height position thereof after the second tray  24 B is moved. In addition, based on the current number of stacked sheets on the second tray  24 B and the moving direction of the second tray  24 B, the moving speed is selected from the table shown in  FIG. 12B . Then, the calculated moving distance is divided by the selected moving speed, thereby calculating the moving time of the second tray  24 B. 
   Next, the moving time of the first tray  23 B is compared with that of the second tray  24 B (S 243 ), and when the moving time of the first tray  23 B is longer than that of the second tray  24 B (YES in Step S 243 ), the moving time of the first tray  23 B is selected (S 244 ). On the other hand, when the moving time of the second tray  24 B is longer than that of the first tray  23 B (NO in Step S 243 ), the moving time of the second tray  24 B is selected (S 245 ). 
   The table of  FIG. 12A  showing moving times of the first tray  23 B is replaced with latest moving speeds calculated by the actual measurement of the moving distance and moving time every time the first tray  23 B is moved by the actual switching of the tray, in accordance with the process of the flowchart shown in  FIG. 8 . 
   The table of  FIG. 12B  showing moving times of the second tray  24 B is also replaced with latest moving speeds calculated by the actual measurement of the moving distance and moving time every time the second tray  24 B is moved by the actual switching of the tray, in accordance with the process of the flowchart shown in  FIG. 8 . 
   In the sheet processing apparatus  1 B, the CPU  600  acquires the speed of the first tray  23 B by referring to the table in terms of the number of stacked sheets and moving direction of the first tray  23 B, multiplies an inverse of the acquired speed by the moving distance, and substitutes the calculation result into the moving time of the first tray  23 B which is a work variable. In a similar manner, the CPU  600  acquires the speed of the second tray  24 B by referring to the table in terms of the number of stacked sheets and moving direction of the second tray  24 B, multiplies an inverse of the acquired speed by the moving distance, and substitutes the calculation result into the moving time of the second tray  24 B which is a work variable. Then, the CPU  600  determines a magnitude correlation between both of the work variables acquired herein, selects a time longer than the other, and substitutes the longer time into a variable tray moving time. As a result, an optimal value of the tray moving time is obtained based on the data in the same condition as learned before, and a moving time of the first tray or the second tray which is longer than the other is selected, thereby making it possible to obtain a safe tray switching time. 
   The object of the present invention is also achieved by providing a storage medium, which stores a program of software for realizing the functions according to the embodiments, to a system or an apparatus, and reading out program codes stored in the storage medium to execute the program. In this case, the program codes and the storage medium in which the program codes are stored constitute the present invention. 
   It should be noted that the present invention is not limited to the above-mentioned embodiments, and may be carried out by being appropriately modified without departing from the spirit of the present invention. For example, the image forming apparatus is not limited to that employing the electrophotographic process, and various types of apparatuses capable of printing images, such as those employing an ink jet system, a thermal system, and the like may be adopted. 
   While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
   This application claims the benefit of Japanese Patent Application No. 2005-278866, filed Sep. 26, 2005, which is hereby incorporated by reference herein in its entirety.