Patent Publication Number: US-6219503-B1

Title: Sheet processing apparatus and method with multi-mode sheet conveying

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus, and more particularly, to a sheet processing apparatus which is used in an image forming apparatus, such as a copier, a laser-beam printer or the like, and which performs processing, such as sorting, binding and the like, for sheets discharged from the image forming apparatus. 
     2. Description of the Related Art 
     In some apparatuses for conveying, mounting and sorting sheets discharged from an image forming apparatus or the like, in order to prevent degradation in the processing capability due to sheet processing at the downstream side requiring a relatively long processing time, such as a bundle-binding operation, a portion where a sheet is temporarily stopped is provided at a conveying portion, in order to temporarily interrupt conveyance of the sheet. Thereafter, a plurality of sheets are conveyed together with another sheet conveyed from the upstream side, in order to secure a time for sheet processing at the downstream side and prevent a decrease in the overall efficiency of sheet processing. In some of such apparatuses, when the system enters a non-operating state due to a sheet jam or the like, in order to prevent occurrence of curl of a sheet to a degree more than necessary and prevent the sheet from remaining within the apparatus from the viewpoint of maintenance of the system, the sheet is automatically discharged when the sheet is present at a temporarily stopped portion. 
     However, an apparatus in which a sheet is automatically discharged when the system is operating and the sheet is present at a temporarily stopped retracted portion is not present. 
     In the above-described situation, problems may arise, such as a failure in sheet conveyance or sheet alignment due to simultaneous conveyance of sheets having different conveying speeds in a superposed state. 
     Furthermore, when there is a sheet which is temporarily stopped in order to perform the above-described processing, there is a problem such that another sheet cannot be conveyed to the conveying path for the stopped sheet or to another portion requiring driving for a portion for conveying the stopped sheet. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a sheet processing apparatus and an image forming apparatus including the same, and more particularly, to provide a sheet processing apparatus in which, when sheets having different conveying conditions are included in sheet conveying means, an excellent sheet conveying property, and an excellent sheet aligning property after sheet discharge are realized by preventing simultaneous conveyance of a plurality of sheets from the sheet conveying means. 
     In accordance with these objects, there is provided a sheet processing apparatus comprising sheet conveying means for conveying a sheet and placing a sheet in a waiting state by temporarily stopping conveyance of the sheet, the sheet conveying means operable in one of at least a first mode, a second mode and a third mode, and sheet-conveying-condition detection means for detecting a conveying condition for each sheet. In the first mode, a sheet is conveyed by driving the sheet conveying means without placing the sheet in a waiting state. In the second mode, a sheet is conveyed by driving the sheet conveying means and putting the sheet conveying means in the waiting state, thereafter releasing the waiting state of the sheet conveying means in accordance with conveyance of a succeeding sheet, and conveying preceding and succeeding sheets in a superposed state. In the third mode, a sheet is conveyed by driving the sheet conveying means and putting the sheet conveying means in the waiting state, thereafter releasing the waiting state of the sheet conveying means and conveying the preceding sheet separately from the succeeding sheet. Control means switch the sheet conveying means from the second mode to the third mode when the sheet-conveying-condition detection means has detected that a sheet having a different sheet conveying condition is included within a plurality of sheets to be discharged from the sheet conveying means in a superposed state when in the second mode. 
     According to another aspect of the present invention, an image forming apparatus includes the above-described sheet processing apparatus, a main body having therein image forming means for forming an image on a sheet, and discharging means of a main body for discharging the sheet having the image formed thereon by the image forming means to the sheet processing apparatus. 
     According to still another aspect of the present invention, a method for controlling an image forming apparatus in which a preceding sheet is temporarily stopped in a waiting state in a sheet conveying path and is then conveyed together with a succeeding sheet upon arrival of the succeeding sheet includes the steps of detecting whether or not a sheet conveying condition for the succeeding sheet is the same as a sheet conveying condition for the preceding sheet, and restarting conveyance of the preceding sheet in a waiting state separately from the succeeding sheet when a result of the detection is negative. 
     According to yet another aspect of the present invention, an image forming apparatus has image forming means for forming an image on a sheet, sheet conveying means for conveying a sheet and placing the sheet in a waiting state by temporarily stopping conveyance of the sheet having the image formed thereon, the sheet conveying means operable in one of at least first mode, a second mode and a third mode, and sheet-conveying-condition detection means for detecting a conveying condition for each sheet. In the first mode, a sheet is conveyed by driving the sheet conveying means without placing the sheet in a waiting state. In the second mode, a sheet is conveyed by driving the sheet conveying means without placing the sheet in a waiting state. In the second mode, a sheet is conveyed by driving the sheet driving means and putting the sheet conveying means in the waiting state, thereafter releasing the waiting state of the sheet conveying means in accordance with conveyance of a succeeding sheet, and conveying preceding and succeeding sheets in a superposed state. In the third mode, a sheet is conveyed by driving the sheet conveying means and putting the sheet conveying means in the waiting state, thereafter releasing the waiting state of the sheet conveying means, and conveying the preceding sheet separately from the succeeding sheet. Control means switch the sheet conveying means from the second mode to the third mode when the sheet-conveying-condition detection means has detected that a sheet having a different sheet conveying condition is included within a plurality of sheets to be discharged from the sheet conveying means in a superposed state in the second mode. 
     According to the above-described configuration, when simultaneously conveying a plurality of sheets using a sheet temporarily awaited by the sheet conveying means, if a different condition with respect to a “sheet conveying condition” serving as one of conditions relating to the sheet conveyance, such as the speed of a sheet, and the like, an operation of conveying one or a plurality of sheets in a waiting state is resumed, and simultaneous conveyance of a plurality of sheets having different conveying conditions is prohibited, so that an excellent sheet conveying property and an excellent sheet aligning property are realized. 
     When there is a sheet which is temporarily awaited, conveyance of a different sheet at the conveying path or at another portion requiring driving for a portion for conveying the awaited sheet is detected. An operation of conveying one or a plurality of sheets in a waiting state is resumed, and conveyance of the other sheet to the conveying path or the other portion is allowed in the above-described manner, so that sheet processing having a high degree of freedom is realized. 
     As described above, according to the present invention, since simultaneous discharge of sheets having different conveying conditions at a discharging portion is prohibited, it is possible to realize an excellent sheet conveying property and improvement in a sheet aligning property after sheet discharge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional front view of a sheet processing apparatus according to an embodiment of the present invention and a main body of an image forming apparatus including the sheet processing apparatus; 
     FIGS. 2A-2H are diagrams illustrating images formed in the main body of the image forming apparatus shown in FIG. 1; 
     FIG. 3 is a block diagram illustrating the configuration of control in the sheet processing apparatus shown in FIG. 1; 
     FIG. 4 is a block diagram illustrating the configuration of an image-signal processing unit, shown in FIG. 3, of the sheet processing apparatus; 
     FIG. 5 is a longitudinal sectional front view illustrating the configuration of the sheet processing apparatus shown in FIG. 1; 
     FIG. 6 is a flowchart illustrating mode determination processing of the sheet processing apparatus shown in FIG. 1; 
     FIG. 7 is a flowchart illustrating non-sorting processing of the sheet processing apparatus shown in FIG. 1; 
     FIG. 8 is a flowchart illustrating sorting processing of the sheet processing apparatus shown in FIG. 1; 
     FIG. 9 is a flowchart illustrating stapling sorting processing of the sheet processing apparatus shown in FIG. 1; 
     FIG. 10 is a flowchart illustrating a sheet sorting sequence of the sheet processing apparatus shown in FIG. 1; 
     FIGS. 11 and 12 are flowcharts illustrating sheet-attribute determination processing of the sheet processing apparatus shown in FIG. 1; 
     FIG. 13 is a flowchart illustrating bundle-discharge-operation determination processing of the sheet processing apparatus shown in FIG. 1; 
     FIG. 14 is a flowchart illustrating stapling processing of the sheet processing apparatus shown in FIG. 1; 
     FIG. 15 is a flowchart illustrating sheet-conveying-condition determination processing of the sheet processing apparatus shown in FIG. 1; and 
     FIG. 16 is a flowchart illustrating clearing of sheet conveying conditions of the sheet processing apparatus shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A sheet processing apparatus according to a preferred embodiment of the present invention is mounted in an image forming apparatus, and processes sheets discharged from the image forming apparatus. 
     Image Forming Apparatus 
     First, a main body of the image forming apparatus will be described. FIG. 1 is a diagram illustrating the configuration of the image forming apparatus and the sheet processing apparatus. 
     The image-forming apparatus includes an image reader  200  and a printer  300 , and an original-feeding device  100  is mounted on the image reader  200 . The original-feeding device  100  sequentially feeds set originals to the left starting from the leading page, conveys each of the fed originals from the left to the right on platen glass  102  via a curved path, and then discharges the original onto a discharged-sheet tray  112 . At that time, a scanner unit  104  is held at a predetermined position. The original is read by being passed from the left to the right (as viewed in FIG. 1) on the scanner unit  104 . This reading method is termed “original-passing reading”. As the original passes, light from a lamp  103  of the scanner unit  104  is projected onto the original, and reflected light from the original is guided to an image sensor  109  via mirrors  105 ,  106  and  107 , and a lens  108 . It is also possible to read the original by stopping the original on the platen glass  102  after conveying it by the original-feeding device  100 , and then moving the scanner unit  104  from the left to the right. This reading method is termed “original-fixing reading”. 
     When reading an original without using the original-feeding device  100 , the user sets the original on the platen glass  102  after raising the original-feeding device  100 . In this case, original-fixing reading is performed. 
     The image of the original read by the image sensor  109  is transmitted to an exposure control unit  110  after being subjected to image processing. The exposure control unit  110  outputs a laser beam corresponding to an image signal representing the image. This laser beam is projected onto a photosensitive drum (image forming means)  111 , and an electrostatic latent image is formed on the photosensitive drum  111 . The electrostatic latent image on the photosensitive drum  111  is developed by a developing unit  113  using a developer. The developer on the photosensitive drum  111  is transferred onto a sheet fed from one of cassettes  114  and  115 , a manual-insertion sheet feeding unit  125  and a duplex conveying path  124  at a transfer unit  116 . 
     The sheet having the developer transferred thereto is subjected to developer fixing processing at a fixing unit  117 . The sheet passing through the fixing unit  117  is first guided to a path  122  by a flapper  121 . After the trailing edge of the sheet has passed flapper  121 , the sheet is subjected to switchback, and is guided to discharging rollers  118  of the main body (discharging means of the main body) by the flapper  121 . The sheet is thereby discharged from the printer  300  by the discharging rollers  118  in a state in which the surface having the developer transferred thereto is placed downward (a face-down state). Such a discharging operation is termed “reversal sheet discharging”. By discharging sheets in the face-down state, a correct order of pages can be obtained when sequentially forming images from the leading page, as, for example, when using the original-feeding device  100 , or when printing images output from a computer. 
     When forming an image on a hard sheet, such as an OHP (overhead projector) sheet or the like, from the manual-insertion sheet feeding unit  125 , the sheet is discharged from the discharging rollers  118  in a state in which the surface having the developer transferred thereto is placed upward (a face-up state) without being guided to the path  122 . 
     When forming images on both surfaces of the sheet, the sheet is directly guided from the fixing unit  117  to the discharging rollers  118 . The sheet is subjected to switchback immediately after the trailing edge of the sheet has passed flapper  121 , and is guided to the duplex conveying path  124  by the flapper  121 . 
     As described above, in original-fixing reading, as shown in FIG. 2A, the original is read from the right end to the left end of the original in a sub-scanning direction, and an image is sequentially formed from the right end of the original. In a main scanning direction, an image read in the direction of an arrow within the image sensor  109  is formed in the direction of an arrow near the exposure control unit  110 . By transmitting the image thus read to the exposure control unit  110  without being modified, the image of the original is reproduced without producing an mirror image. 
     In the original-passing reading, however, as shown in FIG. 2B, the original is read from the left end to the right end of the original. Hence, the sub-scanning direction is inverse to that in the original-fixing reading. If the read image is transferred to the exposure control unit  110  without being modified, a mirror image of the original image is formed. Accordingly, mirror-image processing is performed by inverting the main scanning direction, in order to obtain the correct image from the formed mirror image. By transmitting the corrected image to the exposure control unit  110 , a correct image as shown in FIG. 2B is formed. When performing reversal sheet discharge in the above-described manner, the trailing-edge side of the sheet becomes the left end of the image. Accordingly, by binding the trailing-edge side, the left-end side of the image is bound. Mirror-image processing may also be performed by inverting the sub-scanning direction. However, taking into consideration that mirror-image processing cannot be performed until image reading for one page is completed, and the left-end side of the image is bound by binding the trailing-edge side of the sheet after reversal sheet discharge, it is preferable to invert the main scanning direction. 
     The sheet discharged from the discharging rollers  118  is fed to a folding device  400 . The folding device  400  performs processing of folding the sheet in the form of Z. When folding processing is assigned for an A3-size or B4-size sheet, folding processing is performed by the folding device  400 . Other sheets are directly fed to a finisher  500 . The finisher  500  performs binding processing, punching processing or the like. 
     FIG. 3 is a block diagram illustrating control for the above-described apparatus. A CPU (central processing unit) circuit unit  150  includes a CPU, and controls an original-feeding-device control-unit  101 , an image-reader control unit  201 , an image-signal control unit  202 , a printer control unit  301 , a folding-device control unit  401 , a finisher control unit  501  and an external I/F (inferface)  207  in accordance with programs stored in a ROM (read-only memory)  151 , and is set through an operation unit  1 . The original-feeding-device control unit  101 , the image-reader control unit  201 , the printer control unit  301 , the folding-device control unit  201 , the printer control unit  301 , the folding-device control unit  401  and the finisher control unit  501  control the original-feeding device  100 , the image reader  200 , the printer  300 , the folding device  400  and the finisher  500 , respectively. A RAM (random access memory)  152  is used as a region for temporarily holding control data, and an operational region for calculations relating to control. The external I/F  207  operates as an interface with a computer  208 , and develops printing data into an image and outputs the developed image to the image-signal control unit  202 . An image read by the image sensor  109  is transmitted from the image-reader control unit  201  to the image-signal control unit  202 . An image transmitted from the image-signal control unit  202  to the printer control unit  301  is input to the exposure control unit  110 . 
     FIG. 4 illustrates the configuration of the image-signal control unit  202 . An image processing unit  203  performs image correction processing, and editing processing corresponding to setting through the operation unit  1 . A line memory  204  performs the above-described mirror-image processing of inverting the main scanning direction. An image from the line memory  204  is output to the printer control unit  301  via a page memory  205 . A hard disk  206  is used if necessary, for example, when exchanging the order of pages. 
     As described above, an image is formed on a fed sheet S and is discharged onto the sheet processing apparatus  1  of the embodiment. 
     Sheet Processing Device 
     The sheet processing apparatus of the embodiment will now be described with reference to FIG.  5 . 
     In FIG. 5, there are shown the folding device  400  and the finisher  500 . 
     First, the configuration of the folding device  400  will be described. The sheet discharged from the image forming apparatus is conveyed by being guided in a folding-conveyance horizontal path  401 . At that time, first, an on/off state of the sheet is detected by a folding-conveyance-horizontal-path sensor  430 . When not performing folding for the sheet, a folding-path selection flapper  410  is made in an off-state, and the sheet is directly conveyed to the finisher  500 . When performing folding for the sheet, the folding-path selection flapper  410  is made in an on-state, and the sheet is conveyed to a folding-conveyance folding path  402 . By making the folding-path selection flapper  410  in an off-state after performing folding processing, the sheet is conveyed to the finisher  500 . Within the folding-conveyance folding path  402 , a folding-conveyance folding-path sensor  431  is used for detecting the timing of sheet conveyance, a sheet jam, and the like. 
     Next, the finisher  500  will be described. 
     The sheet conveyed to the finisher  500  is first conveyed to a saddle selection flapper unit  515 . When the saddle selection flapper unit  515  is in an on-state, the sheet is fed to a “saddle conveyance path  524 ”, serving as a conveyance path for binding processing. The binding processing will be described later. 
     In FIG. 5, there are also shown conveying rollers  503 , an inlet-sheet-conveying-path sensor  531 , and a punching unit  550  for punching the conveyed sheet. A large conveying roller (sheet conveying means)  505  conveys the sheet by pressing the sheet with a pressing roller contacting the large conveying roller  505 . Discharging rollers  509  discharge the sheet onto a sample tray  701  using a conveying path  521 . A switching flapper  511  performs switching between a sample path  521  and a sorting path  522 . A switching flapper  510  performs switching between the sorting path  522  and a buffer path  523  for temporarily storing the sheet. An intermediate tray (hereinafter termed a “processing tray”)  630  temporarily accumulates sheets in order to align and staple the sheets. Discharging rollers  507  discharge the sheet onto the processing tray  630 . A bundle discharging roller (feeding means)  680  conveys a bundle of sheets on the processing tray  630  and discharges the bundle onto a stack tray  700 . A stapler  601  staples the bundle of sheets on the processing tray  630 , and can move in a direction substantially orthogonal to the sheet conveying direction. By moving along an end portion of the bundle of sheets, the stapler  601  can staple the bundle, for example, at two portions of the end portion. 
     Members  640  provided at the front side and the rear side of the apparatus align a direction substantially orthogonal to the conveying direction of the sheets discharged onto the processing tray  630 . By adjusting the aligning position of the sheets, offset (at the front side or the rear side) on the processing tray  630  can be provided. 
     Next, a description will be provided of the stack tray  700  and a sample tray  701 . 
     Each of these trays has an independent motor so as to be able to perform independent movement in a vertical direction. 
     Next, the punching unit  550  will be described. 
     The punching unit  550  includes punching means and lateral-registration detection means. The punching means includes a punch and a dice meshed with each other which can be rotated by driving of a punch driving motor. During a punching operation, after detecting the trailing edge of the sheet, the punch driving moter is driven at a predetermined timing in order to punch the sheet being conveyed. At that time, by making the rotational speed of the punch and the die to be the same as the rotational speed of the above-described conveying rollers, the sheet being conveyed can be punched. There is also provided a sensor (not shown) for detecting a rear end portion of the sheet being conveyed. By moving the entire punching unit  550  in a direction orthogonal to the sheet conveying direction, stopping the lateral movement of the punching unit  550  when detecting the end portion of the sheet, and punching the sheet at that position, alignment of the sheet in a lateral direction can be effected. 
     The entire movement of the apparatus will now be described with reference to flowcharts. 
     Processing of Determining the Operational Mode 
     Processing of determining the mode of an operation will be described with reference to the flowchart shown in FIG.  6 . 
     First, in step S 1 , it is determined if the finisher  500  has started. When a start key for a copying operation has been depressed on an operation unit (not shown) on the main body of the image forming apparatus, and a signal for starting the operation of the finisher  500  has been input from the main body of the image forming apparatus to a CPU of the finisher  500  via a communication IC (integrated circuit), start of the finisher  500  enters an on-state, and an inlet motor, a buffer motor and a sheet discharging motor start driving (step S 2 ). If the starting signal is not input, the finisher  500  continues in a waiting state. 
     Then, in step S 3 , the operational mode is determined. The process proceeds to non-sorting processing (see FIG.  7 ), sorting processing (see FIG.  8 ), and stapling sorting processing (see FIG. 9) if the operational mode is a non-sorting mode, a sorting mode and a stapling sorting mode, respectively. 
     Upon completion of one of the above-described sets of processing, the driving of the inlet motor, the buffer motor and the sheet-discharging motor is stopped (step S 7 ), and the finisher  500  returns to the waiting state. 
     Non-sorting Processing 
     The non-sorting processing will be described with referece to the flowchart shown in FIG.  7 . 
     When the process has proceeded to the non-sorting processing as a result of the above-described processing of determining the operational mode, the non-sorting path (sample path)  521  is selected by driving the flapper  511  in order to guide the sheet onto the sample tray  701  (step S 101 ). At that time, since binding is not performed, a flapper  515  is operated so as to pass the sheet to a conveying path  520 . 
     In step S 102 , it is determined if the finisher  500  has been started. If the result of the determination in step S 102  is affirmative, the sheet discharged from the main body of the image forming apparatus is conveyed to the sheet path within the finisher  500 . The conveyed sheet is further conveyed by the inlet motor. In step S 103 , it is determined if the leading edge of the sheet has been detected by a path sensor  531  provided within the sheet path. If the result of the determination in step S 103  is affirmative, the process proceeds to step S 104 , where it is determined if the sheet has been further conveyed and the trailing edge of the sheet has passed through the path sensor  531  to turn off the path sensor  531 . 
     If the result of the determination in step S 104  is affirmative, the process returns to step S 101  and then proceeds to step S 102 . If the result of the determination in step S 102  is negative, the process proceeds to step S 105 , where it is determined if all sheets has been discharged onto the sample tray  701 . If the result of the determination in step S 105  is affirmative, the process proceeds to step S 106 , where the operation of the flapper  511  is released, and the non-sorting processing is terminated. 
     Sorting Processing 
     The sorting processing will be described with reference to the flowchart shown in FIG.  8 . 
     When the process proceeds to the sorting processing in the above-described operational-mode determination processing, in order to guide the sheet to the processing tray  630 , the sorting path  522  is selected by driving the flapper  511  (step S 201 ). At that time, since binding is not performed, the flapper  515  is operated so as to pass the sheet to the conveying path  520 . 
     In step S 202 , it is determined if the finisher  500  has been started. If the result of the determination in step S 202  is affirmative, the sheet discharged from the main body of the image forming apparatus is conveyed to the sheet path within the finisher  500 . 
     The conveyed sheet is further conveyed by the inlet motor. In step S 203 , it is determined if the leading edge of the sheet has been detected by the path sensor  531 . If the result of the determination in step S 203  is affirmative, the process proceeds to step S 204 , where a sheet sorting sequence is started. The process then proceeds to step S 205 , where it is determined if the sheet has been further conveyed and the trailing edge of the sheet has passed through the path sensor  531  to turn off the path sensor  531 . 
     If the result of the determination in step S 205  is affirmative, the process returns to step S 201  and then proceeds to step S 202 . If the result of the determination in step S 202  is negative, the process proceeds to step S 206 , where it is determined if all sheets has been discharged onto the processing tray  630 . If the result of the determination in step S 206  is affirmative, the process proceeds to step S 207 , where the operation of the flapper  511  is released, and the sorting processing is terminated. 
     Stapling Sorting Processing 
     The stapling sorting processing will be described with reference to the flowchart shown in FIG.  9 . 
     When the process proceeds to the stapling sorting processing in the above-described operational-mode determination processing, in order to guide the sheet to the processing tray  630 , the sorting path  522  is selected by driving the flapper  511  (step S 301 ). At that time, since binding is not performed, the flapper  515  is operated so as to pass the sheet to the conveying path  520 . 
     In step S 302 , it is determined if the finisher  500  has been started. If the result of the determination in step S 302  is affirmative, the sheet discharged from the main body of the image forming apparatus is conveyed to the sheet path within the finisher  500 . The conveyed sheet is further conveyed by the inlet motor. In step S 303 , it is determined if the leading edge of the sheet has been detected by the path sensor  531 . If the result of the determination in step S 303  is affirmative, the process proceeds to step S 304 , where a sheet sorting sequence is started. 
     The process then proceeds to step S 305 , where it is determined if the sheet has been further conveyed and the trailing edge of the sheet has passed through the path sensor  531  to turn off the path sensor  531 . If the result of the determination in step S 305  is affirmative, the process returns to step S 301  and then proceeds to step S 302 . If the result of the determination in step S 302  is negative, the process proceeds to step S 306 , where it is determined if all sheets have been discharged onto the processing tray  630 . If the result of the determination in step S 306  is affirmative, the process proceeds to step S 307 , where the operation of the flapper  511  is released, and the stapling sorting processing is terminated. 
     Sheet Sorting Sequence Processing 
     Next, a description will be provided of the processing of a sheet sorting sequence with reference to the flowchart shown in FIG.  10 . 
     This processing is started from the sorting processing or the stapling sorting processing, and is allocated to each conveyed sheet. The program for this processing is processed according to multitasking. 
     First, in step S 420 , sheet-attribute determination processing is performed in the finisher  500 . Although the details of this processing will be described later, this is processing of determining the attribute of the conveyed sheet, such as whether the sheet is “a sheet to be wound” or “a sheet to be discharged as a bundle” after accummulating sheets as a bundle on the processing tray  630 . 
     Then, in step S 401 , the sheet is conveyed by 50 mm. The process then proceeds to step S 402 , where the buffer motor is started. The sheet sorting sequence is started when the path sensor  531  has been turned on. The buffer motor is started when the sheet has been conveyed by 50 mm to the downstream side from the position where the leading edge of the sheet has turned on the path sensor  531 . This operation is performed for the subsequent sheet conveyance and in order to provide a timing for restarting a “wound sheet” (to be described later) which is stopped in a state of being wound around a buffer roller. According to this operation, it is possible to convey the sheet together with the “wound sheet” in a superposed state. Although in this case, 50 mm is selected as the condition for providing the timing, any other value may be selected. The process then proceeds to step S 403 , where the sheet is further conveyed by 150 mm. 
     The process then proceeds to step S 405 , where it is determined if the sheet is assigned as a wound sheet. If the result of the determination in step S 405  is affirmative, the process proceeds to step S 406 . If the result of the determination in step S 405  is negative, the process proceeds to step S 430 . 
     Processing starting from step S 406  is an operation for the “wound sheet”. First, in step S 406 , the buffer path  523  is selected by driving the flapper  510 . It is thereby possible to guide the sheet to the conveying path (buffer path)  523  for winding the sheet around the buffer roller. When a path sensor  532  provided in the buffer path  523  is turned on, control of stopping the buffer motor is started (steps S 407  and S 408 ). As a result, the leading edge of the sheet stops after passing through the path sensor  532 . No problem will arise if this amount of overrun is taken into consideration when performing the above-described winding control. 
     Two types of approaches can be considered after stopping the buffer roller. 
     In the first approach, the wound sheet waits in a state of being wound around the buffer roller until the succeeding sheet causes the buffer roller to restart, and is then discharged together with the succeeding sheet. 
     In the second approach, the buffer roller is automatically restarted by some event, and the sheet is discharged. 
     In this embodiment, a “winding discharging flag” shown in the flowchart of FIG. 10 indicates automatic sheet discharge caused by an “event”. 
     In the first approach, since this flag is not set, the process proceeds from step S 409  to step S 410 , and monitoring is performed until completion of sheet discharge. 
     Upon completion of sheet discharge onto the processing tray  630  after restarting the buffer roller, the value of a discharge counter is incremented by one, and the process is terminated (steps S 410  and S 416 ). 
     The second approach will be described later. 
     If the result of the determination in step S 405  is negative, the process proceeds to step S 430 , where sheet conveying conditions are cleared. This processing is briefly shown in FIG.  16 . In this processsing, data stored in the buffer storage indicating the sheet conveying conditions is cleared. This data is used in sheet-attribute determination processing. 
     In step S 411 , the sorting path  522  is selected by driving the flapper  510 . The sheet is thereby guided to the path  522 , serving as a discharging path to the processing tray  630 , instead of being guided to the above-described buffer path. After confirming completion of sheet discharge onto the processing tray  630  (step S 412 ), the value of the discharge counter is incremented by one (step S 413 ), an aligning operation is performed (step S 414 ), bundle-discharge-operation determination processing (to be described later) is performed (step S 415 ), and the process is terminated. 
     In the above-described operation of discharging the sheet onto the processing tray  630 , alignment of the sheet conveying direction is performed by performing an operation of aligning the sheet in a direction substantially orthogonal to the sheet conveying direction simultaneously with the sheet discharge and rotating a paddle. A detailed description of this operation will be omitted. 
     A description will now be provided of the above-described second approach after winding the sheet around the buffer roller and stopping the buffer motor. In this case, since a “winding discharging flag” indicating sheet discharge from the buffer roller is set, the process proceeds from step S 409  to step S 417 , where the “winding discharging flag” is reset, and driving of the buffer roller for sheet discharge is started (step S 418 ). Thereafter, as in the above-described case of “not a wound sheet”, the flapper is switched, and the processing of discharging the sheet onto the processing tray  630  is performed (steps S 430 , S 411 , S 412 , S 413 , S 414  and S 415 ). 
     By thus performing control so as to allow automatic sheet discharging processing, it is possible to prevent inferior sheet alignability on the processing tray  630  which may occur, for example, when misalignment in the conveying speed or the like is present between a plurality of sheets which are simultaneously discharged. 
     Sheet-attribute Determination Processing 
     Next, the above-described sheet-attribute determination processing will be described in detail with reference to the flowchart shown in FIG.  11 . 
     First, in step S 501 , the value of a buffer-portion pass counter is incremented by one. Then, in step S 502 , it is determined if the current sheet is the last sheet of a bundle. The bundle is a unit of sorting in the sorting mode, and a unit of stapling in the stapling sorting mode. If the result of the determination in step S 502  is affirmative, the process proceeds to step S 515  in FIG.  12 . If the result of the determination in step S 502  is negative, the process proceeds to step S 503 . In step S 503 , it is determined if the size of the sheet is a size capable of being wound. If the result of the determination in step S 503  is affirmative, the process proceeds to step S 504 . If the result of the determination in step S 503  is negative, the process proceeds to step S 513 . In step S 504 , it is determined if the value of a “winding counter” indicating the number of sheets capable of being wound equals 0. If the result of the determination in step S 504  is negative, the process proceeds to step S 505 , where the value of the counter is decremented by one. Then, the process proceeds to step S 506 , where sheet-conveying-condition determination processing (by sheet-conveying-condition detection means) is performed. In this processing, it is determined if the current sheet is to be assigned as a “wound sheet”. This processing will be described later. Then, the process is terminated. 
     The object of providing a wound sheet is to give time allowance at processing at the downstream side by temporarily holding the conveyed sheet and discharging the sheet simultaneously with the succeeding sheet, and thereby to improve productivity. If the result of the determination in step S 504  is affirmative, the process proceeds to step S 507 , where it is determined if the operational mode is the sorting mode. If the result of the determination in step S 507  is negative, i.e., if the operational mode is the stapling mode, the process proceeds to A as indicated in the flowchart, and the process is terminated. If the result of the determination in step S 507  is affirmative, determination processes in steps S 508 , S 509  and S 512  are performed. When “the value of the buffer-portion passage counter is 4” and the sheet is “the second sheet before the last sheet of the bundle” in these processes, the process proceeds to step S 510 , where the sheet is assigned as a “bundle discharge sheet” indicating discharge of a sheet bundle from the processing tray  630 . The same processing is also performed when the “the value of the buffer passage counter is 5”. In cases other than the above-described cases, the processing is terminated. 
     In step S 513 , it is determined if the operational mode is the sorting mode. If the result of the determination in step S 513  is affirmative, the process proceeds to step S 514 . If the result of the determination in step S 513  is negative, i.e., if the operational mode is the stapling mode, the process is terminated. In step S 514 , it is determined if the value of the buffer-portion passage counter is 3. If the result of the determination in step S 514  is negative, the process is terminated. If the result of the determination in step S 514  is affirmative, the process of steps S 510  and S 511  are performed. In steps S 510  and S 511 , processing of assigning the sheet as a “bundle-discharge sheet” indicating discharge of a bundle of sheets, and processing of performing setting in counters in accordance with the assignment (clearing of the buffer-portion passage counter and setting in the winding counter) are performed. The assignment of a “bundle-discharge sheet” indicates start of an operation of discharging a bundle from the processing tray  630  to the stack tray  700  when the sheet is discharged/mounted on the processing tray  630 , and is used in the flowchart of “bundle-discharge-operation determination processing” (to be described later). 
     In steps S 515 , S 516  and S 517 , setting of the direction of offset on the processing tray  630  is performed. Namely, “aligning-position information” indicating the direction of offset when this processing is executed is set as alignment-offset information of the sheet. The aligning-position information” indicating the direction of offset is inverted by the last sheet, and “mounting in a shifted state” on the pocessing tray  630  is realized. 
     Next, sheet-conveying-condition determination processing (S 506 ) will be described with reference to FIG.  15 . 
     In step S 801 , it is determined if a sheet set as a “wound sheet” is present before the current sheet. In this embodiment, this process is performed in order to determine if conveying conditions for a plurality of sheets to be wound are identical. If the result of the determination in step S 801  is negative, the conveying condition of the sheet is set in a buffer storage (step S 802 ), and the sheet is assigned as the first sheet to be wound (step S 803 ). 
     If the result of the determination in step S 802  is affirmative, it is then determined if the “sheet conveying condition” of the preceding sheet set as the wound sheet equals the conveying condition of the current sheet stored in the buffer storage (step S 804 ). If the result of the determination in step S 804  is affirmative, processing of winding the current sheet is performed (step S 803 ). If the result of the determimation in step S 804  is negative, a “winding discharging flag” instructing discharge of the preceding wound sheet from the buffer roller is set (step S 805 ). Then, setting of data relating to winding is performed (step S 806 ). Thus, winding processing is not performed until a condition allowing subsequent winding is set. 
     As described above, in this embodiment, the “sheet conveying speed” is set as the sheet conveying condition. Namely, if a plurality of different sheets having different conveying speeds are conveyed, inferior conveyance of sheets and inferior alignment on the processing tray will occur. 
     The processing of determining/setting the attribute of the sheet (whether winding control is to be performed or bundle discharge is to be performed) is completed in the above-described manner. 
     In the foregoing description, detection of the difference in the sheet conveying speed has been set as means for detecting the sheet conveying condition in the sheet-conveying-condition determination processing performed in step S 505  of FIG. 11 or in FIG.  15 . However, detection of the difference in the size of the conveyed sheet, detection of stop of a sheet due to an event at a conveying path other than the large conveying roller  505  which serves as sheet conveying means, detection of the trays  700  and  701  where the sheet is to be discharged, detection of the difference in the material of the conveyed sheet, detection of the difference in the job of the sheet, detection of the difference in the mass of the conveyed sheet, or detection that the conveyed sheet has a binding mode may also be used as the sheet-conveying-condition detection means. 
     Means for detecting that the conveyed sheet is one of an OHP sheet, an inserter sheet, a cover, a synthetic sheet and a back cover of bound sheets, and a sheet for an interrupt operation may be used as the above-described sheet-conveying-condition detection means. 
     Bundle-discharging-operation Determination Processing 
     Bundle-discharging-operation determination processing will be described with reference to the flowchart shown in FIG.  13 . 
     When the process proceeds to bundle-discharging operation determination processing in the above-described sheet sorting sequence, it is then determined if the operational mode is the stapling sorting mode (step S 601 ). If the result of the determination in step S 601  is negative, it is then determined if the sheet discharged onto the processing tray  630  is a bundle-discharge sheet (step S 602 ). If the result of the determination in step S 602  is negative, the bundle-discharging-operation determination processing is terminated, and the process returns to the above-described sheet sorting sequence. 
     If the result of the determination in step S 602  is affirmative, a bundle-discharge upper roller is brought in contact with the sheet bundle on the processing tray  630  by operating a swinging guide (step S 605 ). Then, after awaiting attenuation of bounding of the bundle-discharge upper roller, the bundle of sheets on the processing tray  630  is discharged onto the stack tray  700  by driving the bundle-discharge upper roller by a predetermined amount while controlling the speed of the bundle discharging roller  680  (step S 606 ). 
     Then, the stack tray  700  is raised, and an operation of mounting the bundle on the stack tray  700  is completed (step S 607 ). Then, the discharge counter is set to 0 (step S 608 ), and the bundle-discharging-operation determination processing is terminated. 
     If the result of the determination in step S 601  is affirmative, it is then determined if the sheet discharged onto the processing tray  630  is a bundle-discharge sheet (step S 603 ). If the result of the determination in step S 603  is negative, the bundle-discharge-operation determination processing is terminated, and the process returns to the above-described sheet sorting sequence. 
     If the result of the determination in step S 603  is affirmative, the process proceeds to a stapling processing sequence (step S 604 ). Upon completion of the processing of stapling the sheet bundle on the processing tray  630 , the process proceeds to step S 605 , where an operation of lowering the swinging guide is performed, and the above-described bundle discharging operation (from step S 605  to step S 608 ) is performed, and the bundle-discharging-operation determination processing is terminated. The process then proceeds to the sheet sorting sequence. 
     Stapling Processing 
     Stapling processing will be described with reference to FIG.  14 . 
     When the process enters the stapling processing mode, the stapler  601  is moved by a predetermined amount to a stapling position (step S 701 ), the bundle on the processing tray  630  is aligned using the aligning means  640  comprising the front aligning member and the rear aligning member (step S 702 ), and a stapling operation is performed (step S 703 ). 
     Then, it is determined if stapling is to be performed in a two-position stapling mode (step S 704 ). If the result of the determination in step S 704  is negative, alignment of the bundle by the aligning means comprising the front aligning member and the rear aligning member is released (step S 707 ), and the stapling processing is terminated. If the result of the determination in step S 704  is affirmative, the stapler  601  is moved by a predetermined amount to a second stapling position (step S 705 ), a stapling operation on the second position is performed (step S 706 ), alignment of the bundle by the aligning means  640  comprising the front aligning member and the rear aligning member is released (step S 707 ), and the stapling processing is terminated. 
     Binding Processing 
     The sheet is conveyed to the saddle conveyance path  524 , processing, such as center saddle stitch and the like, is performed, and the bundle of bound sheets is discharged onto and mounted on a saddle discharge tray  702  by saddle discharging rollers  508 . 
     The individual components shown in outline or designated by blocks in the drawings are all well known in the sheet processing apparatus and image forming apparatus arts and their specific construction and operation are not critical to the operation or the best mode for carrying out the invention. 
     While the present invention has been described with respect to what is presently considered to be the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.