Patent Publication Number: US-2011064427-A1

Title: Sheet processing device, image forming system, and sheet conveying method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2009-213472 filed in Japan on Sep. 15, 2009. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing device that receives a sheet-type recording medium (hereinafter, “sheet”) and processes the sheet, an image forming system, such as a copying machine, a printer, a facsimile machine, and a digital MFP, that includes the sheet processing device, and a sheet conveying method used in the sheet processing device or the image forming system. 
     2. Description of the Related Art 
     Together with the various needs for image forming apparatuses and high functional image forming apparatuses, sheet processing devices have been developed that can staple and punch copies printed by an image forming apparatus. As the number of available functions increases, the size of the sheet processing device increases and the installation area and the electric-power consumption also increase. This brings the need for space-saving and power-saving image forming apparatuses and space-saving and power-saving sheet processing devices. 
     A space-saving sheet processing device has limited functions and is designed to use small components arranged closely together so as to reduce the size of the sheet processing device. Moreover, although most sheet processing devices operate as external devices of an image forming apparatus, inner finishers have been developed that are arranged within an inner space of an image forming apparatus that includes a reading unit, a printing unit, and a paper feeding unit. 
     As the size of a sheet processing device decreases, it is more difficult have a space available to remove jams in the device/apparatus. Therefore, various technologies have been suggested that enable easy jam recovery. 
     For example, Japanese Patent Application Laid-open 2006-240759 discloses a sheet processing device that includes a sheet processing unit that is slidable with respect to the image forming apparatus. When a jam occurs in the sheet processing unit, the user slides the sheet processing unit, thereby making a space for removing the jam. 
     Japanese Patent Application Laid-open H5-221582 is well known as a technology that enables easy jam recovery. A sheet processing device disclosed in Japanese Patent Application Laid-open H5-221582 includes a first conveying path A, a second conveying path B, and a switching claw that switches between the first conveying path A and the second conveying path B. A sheet coming from an external device is conveyed inside along either the first conveying path A or the second conveying path B. If a conveyer error occurs when the sheet is conveyed inside along the first conveying path A, the switching claw switches to the second conveying path B so that subsequent sheets are conveyed along the second conveying path B. With this configuration, user actions necessary for jam recovery are reduced to the minimum. 
     The sheet processing device disclosed in Japanese Patent Application Laid-open 2006-240759 needs a slide rail and a mechanism that separates the sheet processing device into parts and connects the parts; therefore, the sheet processing device includes additional components unrelated to the main function and has a complicated mechanism, which increases manufacturing costs. 
     The sheet post-processing device disclosed in Japanese Patent Application Laid-open H5-221582 needs at least two conveying paths; therefore, this cannot be realized in a small sheet processing device that has only one conveying path. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a device/apparatus that is small but allows, without increasing the manufacturing costs due to additional components and due to having a complicated configuration, a user to easily remove a sheet remaining inside the device/apparatus. 
     According to an aspect of the present invention, there is provided a sheet processing device including: a conveying unit that conveys a sheet coming from a preceding device; a sheet processing unit that receives the sheet from the conveying unit and processes the sheet in a predetermined manner; a sheet detecting unit that detects a position of the sheet being conveyed; a stacking unit that stacks thereon the sheet or a sheet set including the sheet that has been processed by the sheet processing unit; a sheet-error detecting unit that detects a conveyance error of the sheet being conveyed in accordance with information received from the sheet detecting unit; a load-error detecting unit that detects an error of a load for driving the sheet processing unit and the stacking unit; a jam recovery door that is used to remove any sheet that causes a jam and any remaining sheet from the sheet processing device; a door detecting unit that detects an open/close state of the jam recovery door; and a control unit, wherein when an error is detected by the sheet-error detecting unit and/or the load-error detecting unit or when the jam recovery door is open, if the sheet being conveyed is detected by the sheet detecting unit, the control unit stops a load for driving the conveying unit after the sheet is conveyed a predetermined distance. 
     According to another aspect of the present invention, there is provided an image forming system including: an image forming apparatus that forms a visible image on a sheet; and the sheet processing device defined above. 
     According to still another aspect of the present invention, there is provided a sheet conveying method used in a sheet processing device that includes a conveying unit that conveys a sheet coming from a preceding device; a sheet processing unit that receives the sheet from the conveying unit and processes the sheet in a predetermined manner; wherein, when any of a sheet conveyance error, a load error, and an open state of a jam recovery door is detected, if a sheet being conveyed is detected by a sheet detecting unit, stopping a load for driving the conveying unit after the sheet is conveyed a predetermined distance. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the configuration of an image forming system according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of the configuration of a sheet processing device; 
         FIG. 3  is a block diagram of the configuration of a control system according to the present embodiment; 
         FIG. 4  is a schematic diagram that explains a paper-jam recovery method performed by the sheet processing device according to the present embodiment; 
         FIG. 5  is an enlarged perspective view of a relevant portion of the sheet processing device when staples are filled; 
         FIG. 6  is a schematic diagram of an example illustrating a sheet remaining in a sheet discharging path of the sheet processing device; 
         FIG. 7  is a schematic diagram of an example illustrating a sheet remaining at a removable position in the sheet discharging path; 
         FIG. 8  is a flowchart of a control process according to the present embodiment that is performed when a paper jam or a motor error occurs when the system is running or when a certain door opens during sheet conveyance; and 
         FIG. 9  is a flowchart of another control process according to the present embodiment that is performed when a paper jam or a motor error occurs when the system is running or when a certain door opens during sheet conveyance. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is made to provide an image forming system that includes a small sheet processing device that discharges, for example, copies to an inner space of the image forming system. The image forming system enables easy jam recovery without increasing the manufacturing costs due to additional components or due to having a complicated mechanism. Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
     In the following embodiments, a conveying unit corresponds to a pair of entrance rollers  201  and a pair of discharging rollers  206 ; a sheet processing unit corresponds to a pair of shift discharging rollers  204 , a trailing-edge referential fence  220 , a pair of jogger fences (alignment plates)  212 , and a stapler  215 ; a sheet detecting unit corresponds to an entrance sensor  207 , a sheet presence/absence sensor  213 , and a sheet-set discharging sensor  210 ; a stacking unit corresponds to a discharge tray  203 ; a sheer-error detecting unit corresponds to a sheet-error detecting unit  406 ; a load-error detecting unit corresponds to a load-error detecting unit  407 ; a jam recovery door corresponds to a jam recovery door  180 ; a door detecting unit corresponds to a door SW  490 ; a load corresponds to an entrance motor  216 ; a control unit corresponds to a CPU  401 ; a furthest-upstream conveying roller corresponds to the entrance rollers  201 ; a furthest-downstream conveying roller corresponds to the discharging rollers  206 ; an image forming apparatus corresponds to an image forming apparatus  100 ; and a sheet processing device corresponds to a sheet processing device  200 . 
     General Configuration 
       FIG. 1  is a schematic diagram of the configuration of an image forming system according to an embodiment of the present invention. As shown in  FIG. 1 , the image forming system according to the present embodiment includes an image forming apparatus  100 , a sheet processing device  200 , and an image reading device  300 . 
     The image forming apparatus  100  is a tandem-type color image forming apparatus that uses an intermediate transfer mechanism. As shown in  FIG. 1 , the image forming apparatus  100  includes an image forming unit  110  that includes image forming stations  111  for four different colors in the middle; an optical writing unit  113  that is adjacent to the bottom section of the image forming unit  110 ; a paper feeding unit  120  that is below the optical writing unit  113 ; a sheet conveying path (vertically conveying path)  130  through which, after a sheet is picked up by the paper feeding unit  120 , the sheet is conveyed to an intermediate transferring unit  140  and a fixing unit  150 ; a sheet discharging path  160  through which the sheet with an image fixed thereto is conveyed toward the sheet processing device  200 ; and a duplex-printing conveying path  170  that is used to reverse a single-side printed sheet so that another image is formed on the other side of the sheet. 
     The image forming unit  110  includes drum-shaped photosensitive elements for Y, M, C, and K of the image forming stations. The image forming unit  110  includes, around the outer circumference of each photosensitive element are a charging unit, a developing unit, a primary transferring unit, a cleaning unit, and a neutralizing unit. The image forming unit  110  further includes an intermediate transfer belt  112  that receives images from the photosensitive elements by the primary transferring units and the optical writing unit  113  that writes different color images to the photosensitive elements. The optical writing unit  113  is arranged below the image forming stations  111 . The intermediate transfer belt  112  is arranged above the image forming stations  111 . 
     The intermediate transfer belt  112  is rotatably supported by a plurality of supporting rollers that includes a supporting roller  114 . The supporting roller  114  faces a secondary transfer roller  115  across the intermediate transfer belt  112  in the secondary transferring unit  140  so that an image is transferred from the intermediate transfer belt  112  onto a sheet during secondary transfer. The reference numeral  116  is a detachable toner container. Image forming processes by tandem-type color image forming apparatuses that use an intermediate transfer mechanism is well known. Such an image forming process is not within a scope of the invention; therefore, any image forming processes will not be described in detail below. 
     The paper feeding unit  120  includes a paper feed tray  121 , a pick-up roller  122 , and a sheet conveying roller  123 . The paper feeding unit  120  feeds a sheet from the paper feed tray  121  and conveys the sheet upward along the vertically conveying path  130 . An image is transferred by the secondary transferring unit  140  onto the sheet coming from the paper feeding unit  120 . The sheet is then conveyed to the fixing unit  150 . The fixing unit  150  includes a fixing roller  151  and a pressing roller. When the sheet is passing between the fixing roller and the pressing roller, heat and pressure is applied to the sheet and thus the toner image is fixed to the sheet. 
     The fixing unit  150  includes the fixing roller  151 . Downstream of the fixing unit  150  are the sheet discharging path  160  and the duplex-printing conveying path  170 , both paths being bifurcated from the same path by operation of a bifurcating claw  161 . Either path is selected depending on whether the sheet is to be conveyed to the sheet processing device  200  or the duplex-printing conveying path  170 . A bifurcating-point conveying roller  162  is arranged immediately upstream of the bifurcating claw  161  in the sheet conveying direction. The bifurcating-point conveying roller  162  applies to the sheet a power to go forward. 
     The sheet processing device  200  is in an inner space of the image forming apparatus  100 . The sheet processing device  200  processes an image formed sheet that is received from the image forming apparatus  100  in a predetermined manner and stacks the processed sheet on a discharge tray  203  that is arranged furthest downstream. The sheet processing device  200  will be described in detail later. 
     The image reading device  300  is a well-known reading device that scans an original that is placed on an exposure glass with light, thereby reading an image from the original. The configuration and the function of the image reading device  300  are well-known and not directly related to the scope of the present invention; therefore, the image reading device  300  will not be described in detail below. 
     The image forming apparatus  100  creates image data in accordance with data containing the image of the original scanned by the image reading device  300  or data containing an image to be printed received from an external. PC. The optical writing unit  113  writes an image to each photosensitive element with light in accordance with the created image data. The image forming stations sequentially transfer the different four color images onto the intermediate transfer belt  112  in a superimposed manner and thus a color image is formed on the intermediate transfer belt  112 . A sheet is conveyed from the paper feed tray  121  in accordance with the image formation. The sheet stops at a registration-roller position (not shown) immediately before the intermediate transferring unit  140  and then goes forward at appropriate timing with respect to the leading edge of the image that is formed on the intermediate transfer belt  112 . The sheet receives the image by the intermediate transferring unit  140  during secondary transfer and the sheet with the image is conveyed to the fixing unit  150 . 
     After the image is fixed to the sheet by the fixing unit  150 , if the single-printing mode is selected or if duplex-printing mode is selected but images are already formed on the both surfaces, by switching operation of the bifurcating claw  161 , the sheet is conveyed to the sheet discharging path  160 . If the duplex-printing mode is selected, the sheet is conveyed to the duplex-printing conveying path  170 . If the sheet is conveyed to the duplex-printing conveying path  170 , the sheet is reversed so that the upper side faces downward and then the sheet is conveyed again to the intermediate transferring unit  140 . After that, an image is formed on the opposite surface and the sheet is then conveyed to the sheet discharging path  160 . After the sheet is conveyed to the sheet discharging path  160 , the sheet is conveyed to the sheet processing device  200 . The sheet is subjected to predetermined sheet processing by the sheet processing device  200  or passes through the sheet processing device  200  without being subjected to any post-processing. The processed or unprocessed sheet is then discharged onto the discharge tray  203 . 
     Sheet Processing Device 
       FIG. 2  is a schematic diagram of the configuration of the sheet processing device  200 . 
     As shown in  FIG. 2 , the sheet processing device  200  includes a pair of entrance rollers  201 , a sheet discharging path  202 , a pair of shift discharging rollers  204 , a staple tray  219 , a tapping roller  211 , a reverse roller  214 , a trailing-edge referential fence  220 , a pair of jogger fences (alignment plates)  212 , a pair of discharging rollers  206 , and the discharge tray  203 . These units are arranged in this order with the entrance rollers  201  being furthest upstream in the sheet conveying direction. 
     A sheet receiving unit of the sheet processing device  200  includes the entrance rollers  201  that receives a sheet from the image forming apparatus  100  through the sheet discharging path  160 ; the sheet discharging path  202  through which the received sheet is conveyed to the shift discharging rollers  204 ; and the shift discharging rollers  204  that corresponds to the sheet processing unit that shifts the sheet and discharges the sheet onto the discharge tray  203 . By rotation of the entrance rollers  201  and the shift discharging rollers  204  driven by an entrance motor  216 , the sheet is conveyed along the sheet discharging path  202 . 
     Near the sheet discharging path  202  is an entrance sensor  207  that detects the leading edge and the trailing edge of a sheet. The operational timing is determined in accordance with the detected timing of the leading edge and the trailing edge of a sheet with respect to the number of driving steps of the entrance motor  216  and the number of driving steps of a later-described discharging motor  217 . Both of the entrance motor  216  and the discharging motor  217  are stepping motors. 
     Both the entrance rollers  201  and the shift discharging rollers  204  that are arranged along the sheet discharging path  202  together work as a conveying unit. 
     Sheets are discharged in different manners depending on modes. In a shift mode, sheets are shifted and then discharged. In a staple mode, sheets are stapled and the stapled sheet set is then discharged. In the following, the sheet discharging manner is described in each mode. 
     Shift Mode Operation 
     In the shift mode, sheets are discharged in a sorted manner that a set of a predetermined number of sheets is stacked at a discharge position that is displaced from a discharge position of another set of sheets in a direction perpendicular to the sheet conveying direction. 
     The shift discharging rollers  204  are arranged at the downstream end of the sheet discharging path  202 . The shift conveying rollers  204  are moved, by a shift motor (not shown), back and forth in the direction perpendicular to the sheet conveying direction. In the shift mode, sheets are discharged onto the discharge tray  203  in a sorted manner by moving, when the last sheet of each set of sheets is discharged, in the direction perpendicular to the sheet conveying direction so that the next set is stacked a certain distance displaced from the position of the current set. As a result, sets of sheets are stacked alternately at the different positions and thus sheets are sorted. 
     Downstream of the shift discharging rollers  204  are a sheet guiding plate  205  and the discharging rollers  206 . The discharging rollers  206  are driven by the discharging motor  217 . The sheet guiding plate  205  is movable driven up and down by a stepping motor (not shown). The sheet is conveyed to the discharge tray  203 , nipped between the discharging rollers  206  and a driven roller attached to the sheet guiding plate  205 , and then stacked on the discharge tray  203 . 
     A sheet holder  209  is at a portion of the discharge tray  203  that is attached to the main body of the sheet processing device  200 . The sheet holder  209  holds and release sheets that are stacked on the discharge tray  203  by ON/OFF of a solenoid  218 . 
     The solenoid  218  is switched ON in accordance with sheet conveyance and, in turn, the sheet holder  209  releases the sheets stacked. When the sheet has passed through the discharging rollers  206 , the solenoid  218  is switched OFF and, in turn, the sheet holder  209  holds the sheets stacked. 
     The discharge tray  203  has an unmovable tray section  208   a  and a movable tray section  208   b  that is upstream of the unmovable tray section  208   a  in the sheet conveying direction. The movable tray section  208   b  is moved up and down by a tray DC motor  221   a  and a cam/link mechanism. The movable tray section  208   b  has an upstream rotatable end and is supported via a supporting shaft  221   c  swingably by the unmovable tray section  208   a . The movable tray section  208   b  is connected to an operational end of a cam/link mechanism  221   b . With this configuration, when the tray DC motor  221   a  rotates, the movable tray section  208   b  swings around the supporting shaft  221   c.    
     When a predetermined number of sheets are discharged, in response to an instruction received from a later-described control unit, the tray DC motor  221   a  rotates and, in turn, a free end, namely the upstream rotatable end of the movable tray section  208   b  moves down. The sheet holder  209  has a surface-of-sheet-on-tray sensor (not shown). If the surface-of-sheet-on-tray sensor is OFF in a situation that the sheet holder  209  holds the sheets, the discharge tray  203  moves up until the surface-of-sheet-on-tray sensor is switched ON. If the surface-of-sheet-on-tray sensor is ON, the discharge tray  203  moves down until the surface-of-sheet-on-tray sensor is switched OFF and then moves up until the surface-of-sheet-on-tray sensor is switched ON, again. With this configuration, the height of the discharge tray  203  with the sheets thereon is maintained at the same level. 
     As described above, in accordance with the conditions of the sheets stacked on the discharge tray  203 , the free end of the movable tray section  208   b  moves up and down so as to maintain the distance between the nip between the discharging rollers  206  and a sheet stacking section of the movable tray section  208   b  to the same value, which maintains the angle between a sheet discharged from the discharging rollers  206  and the movable tray section  208   b  to be the fixed angle, enables accurate alignment of sheets stacked on the discharge tray  203 , and enables stack of a large number of sheets on the discharge tray  203 . 
     The above operation is repeated and thus sheets are stacked on the discharge tray  203  in a sorted manner. 
     Staple Mode Operation 
     In the staple mode, a set of sheets is stapled by a stapler and the stapled sheet set is then discharged. 
     Between the shift discharging rollers  204 , which is at the downstream end of the sheet discharging path  202 , and the sheet guiding plate  205 , which is immediately upstream of the discharge tray  203 , is the tapping roller  211  that is moved up and down by a stepping motor (not shown). The tapping roller  211  includes a lever section that moves up and down and a roller section. The roller section is rotated by the discharging motor  217  in the direction opposite to the sheet conveying direction. 
     In the staple mode, when the trailing edge of the sheet has passed through the shift discharging rollers  204 , the tapping roller  211  moves down and presses, at the roller section, the staple tray  219  that is a sheet stacking unit. The roller section rotates so that the sheet reverses until the trailing edge of the sheet abuts against the trailing-edge referential fence  220 . It is noted that the roller section of the tapping roller  211  is rotated by the discharging motor  217 , i.e., the tapping roller  211  and the discharging rollers  206  are rotated by the same motor. The reverse roller  214  is above the trailing-edge referential fence  220 . The reverse roller  214  is an auxiliary for reversing the sheet. The reverse roller  214  aligns the sheets in the sheet conveying direction. 
     The sheets are aligned by abutting against the trailing-edge referential fence  220  with respect to the trailing-edge referential fence  220 . 
     When the sheet reverse operation is completed, the sheets are aligned by the jogger fences  212 , which are arranged on the staple tray  219 , in the direction perpendicular to the sheet conveying direction. One of the jogger fences  212  is an unmovable member and the other is a movable member. The movable member moves in the direction perpendicular to the sheet conveying direction so as to hold the edges of the sheets with the unmovable member. The jogger fences  212  come into contact with the edges of the sheets, thereby aligning the sheets with respect to a referential position. 
     During the alignment, a corner of the trailing edge of the sheets is inserted to a staple position of a stapler  215 . The stapler  215  corresponds to the sheet processing unit. After receiving a predetermined number of sheets, reversing the sheets, and aligning the sheets, the sheets are stapled. Therefore, in the present embodiment, the trailing-edge referential fence  220  and the jogger fences  212  work together as the sheet processing unit. 
     After the stapling, as indicated by in the dot line of  FIG. 2 , the sheet guiding plate  205  moves down and holds the sheet set with the discharging rollers  206  using a driven roller attached to the sheet guiding plate  205 . By driving of the discharging motor  217 , the sheet set is discharged onto the discharge tray  203 . After the elapse of a predetermined number of steps since the start of sheet-set discharging, the solenoid  218  is switched ON and, in turn, the sheet holder  209  releases the sheet set, and the discharge tray  203  further moves down by a certain distance. 
     After that, when the trailing edge of the sheet set has passed by a sheet-set discharging sensor  210 , the sheet guiding plate  205  moves up and then the discharging motor  217  stops so as to receive the next sheet. At the same time, the solenoid  218  is switched OFF to hold sheets. The reference numeral  213  is a sheet presence/absence sensor that is used to determine whether one or more sheets are present on the staple tray  219 . This sensor is used at a power-on and during a jam recovery to determine whether any sheet is present in the sheet processing device  200 . 
     Control Device 
       FIG. 3  is a block diagram of the configuration of a control system according to the present embodiment. 
     The image forming apparatus  100  is controlled by an image-forming-apparatus control unit  410 . The image-forming-apparatus control unit  410  includes, as shown in  FIG. 3 , a CPU  411 , a ROM  412 , a RAM  413 , a nonvolatile RAM  414 , a serial I/F  415 , a timer  416 , etc. Control program codes are stored in the ROM  412 . The CPU  411  loads a program code on the RAM  413 , stores data necessary for a control process in the RAM  413 , and performs the control process in accordance with the program code while using the RAM as a work area. 
     The image-forming-apparatus control unit  410  is connected to motors that are used for the photosensitive elements or the like of the image forming unit  110 , DC loads  450  and AC loads  470  that include motors and clutches that are used for the paper feeding unit  120 , the sheet conveying path  130 , and the duplex-printing conveying path  170 , and sensors  460  that includes a temperature sensor for detecting the temperature of the fixing roller. 
     The image-forming-apparatus control unit  410  is also connected to the image reading device  300  and an operation display unit  440 . The units of the image forming apparatus  100  are under the control of the image-forming-apparatus control unit  410 . 
     The image-forming-apparatus control unit  410  receives various information, such as information whether a certain cover (not shown) is open and consumables including toners is accessible and information whether a door/cover SW  480  ( FIG. 3 ) indicates that a jam recovery door  180  ( FIG. 4 ) is open for jam recovery maintenance. Although the door/cover SW  480  is not shown in  FIG. 4 , the door/cover SW  480  operates in accordance with the open/close state of the jam recovery door  180 . 
     The sheet processing device  200  is controlled by a sheet-processing-device control unit  400 . The sheet-processing-device control unit  400  includes a CPU  401 , a ROM  402 , a RAM  403 , and a serial I/F  404 , a timer  405 , etc. Control program codes are stored in the ROM  402 . The CPU  401  loads a program code on the RAM  403 , stores data necessary for a control process in the RAM  403 , and performs the control process in accordance with the program code while using the RAM as a work area. In this manner, the CPU  401  controls DC loads  420  that include the entrance motor  216 , the discharging motor  217 , the tray DC motor  221   a , and the solenoid  218 . 
     The image forming apparatus  100  and the sheet processing device  200  send/receive sheet conveyance commands to/from each other via the serial I/F  415  and the serial I/F  404 . The sheet processing device  200  performs sheet conveyance control and post-processing in response to the commands using sheet position information received from sensors  430  that include the entrance sensor  207 , the sheet presence/absence sensor  213 , and the sheet-set discharging sensor  210 . 
     The post-processing-device control unit  400  receives information whether a staple-cartridge replacement door  230  ( FIG. 5 ) is open for filling staples in the stapler  215 . The sheet-processing-device control unit  400  further includes a sheet-error detecting unit  406  and a load-error detecting unit  407 . The sheet-error detecting unit  406  detects a later-described paper jam, i.e., an error during sheet conveyance. The load-error detecting unit  407  detects a motor error, i.e., an error of any load that is used for driving a sheet processing unit and a stacking unit. 
     Paper Jam and Motor Error 
     When a sheet cannot move forward any more due to a curled portion or a folded portion of the sheet and a device failure and cannot move to the entrance sensor  207  even after a predetermined number of steps of the entrance motor  216 , or when a sheet stops at the entrance sensor  207 , the sheet-processing-device control unit  400  determines that a paper jam occurs. When a paper jam occurs, the loads including the entrance motor  216  in the sheet processing device  200  stop to stop the sheet conveyance and the post-processing process. Information indicative of occurrence of the paper jam is sent from the sheet-processing-device control unit  400  to the image-forming-apparatus control unit  410  via the serial I/F  404  and the serial I/F  415 . 
     Any motors, for example, the tapping roller  211  other than the entrance motor  216  and the discharging motor  217  that are used for sheet conveyance have home position sensors (not shown). Using information indicative of ON/OFF of the home position sensor and the number of steps of the stepping motor, the stand-by position of the tapping roller  211 , the number of driving steps counted from the stand-by position during moving up/down operation, etc., are managed. If the home position sensor is not switched ON or OFF, because of a component failure, still after the tapping roller  211  is driven a predetermined number of steps, the sheet-processing-device control unit  400  determines that a motor error occurs. 
     When a motor error is detected, in the same manner as in the process in response to a paper jam, the loads in the sheet processing device  200  stop to stop the sheet conveyance and the post-processing process and information indicative of occurrence of the motor error is sent from the sheet-processing-device control unit  400  to the image-forming-apparatus control unit  410  via the serial I/F  404  and the serial I/F  415 . 
       FIG. 4  is a schematic diagram of the image forming system according to the present embodiment that explains, especially, a paper-jam recovery method performed by the sheet processing device  200 . 
     If a paper jam or a motor error occurs near the sheet discharging path  160  of the image forming apparatus  100  or the sheet discharging path  202  of the sheet processing device  200  and motion stops with a sheet remaining inside the sheet processing device  200 , the user opens the jam recovery door  180 , accesses the sheet (P 1 ) (A 1 ), and removes the sheet. If the user cannot access the sheet from the jam recovery door  180 , the user accesses the sheet (P 2 ) (A 2 ) from the discharge tray  203  of the sheet processing device  200  and removes the sheet. It is noted that information indicative of open/close of the jam recovery door  180  is sent from the image-forming-apparatus control unit  410  to the sheet post-processing-device control unit  400  via the serial I/F  415  and the serial I/F  404 . 
     After the sheet is removed, the user closes the jam recovery door  180 . After the jam recovery door  180  is closed, if no more sheets remain inside the sheet processing device  200 , information about the jam and the motor error is deleted and a certain process is performed for receiving subsequent sheets. Even if the sheet is conveyed appropriately and the motors operate correctly, when the user opens the jam recovery door  180  or the staple-cartridge replacement door  230  when the system is running, in order to prevent an accident caused by access by the user to the units behind the door, the loads in the sheet processing device  200  stop. As shown in  FIG. 5  that illustrates an enlarged perspective view of a relevant portion of the sheet processing device  200 , when the stapler  215  has no staples, the user opens the staple-cartridge replacement door  230 , removes a staple cartridge  231 , and fills staples in the staple cartridge  231 . 
     The sheet processing device  200  is arranged in an inner space between the image forming apparatus  100 , which includes the image forming unit  110  and the paper feeding unit  120 , and the image reading device  300 . The upper section of the sheet processing device  200  is adjacent to the image reading device  300 , making no space. There are no doors, covers, and similar from which the user can directly access, from above or from the side, the sheet discharging path  202  along which the entrance rollers and the shift discharging rollers  204  are arranged. Therefore, if, because of paper jam or a motor error or because of either door being open during sheet conveyance, motion stops with a sheet remaining inside the sheet processing device  200  in the same manner as described above, depending on the position of the sheet, there is a possibility that it is difficult or impossible to access the sheet from the jam recovery door  180  or the discharge tray  203  of the sheet processing device  200 . 
       FIG. 6  is a schematic diagram of an example illustrating a sheet remaining in the sheet discharging path  202 . 
       FIG. 6  illustrates the situation where a sheet P 3  stops with the leading edge being near the sheet guiding plate  205  and the trailing edge being near the entrance rollers  201 , regardless of the reason for stopping the sheet. If the sheet stops at the above position, it is impossible to access the sheet from the jam recovery door  180  of the image forming apparatus  100  and it is difficult to access the sheet from the discharge tray  203  of the sheet processing device  200  because the way is blocked by the sheet guiding plate  205 . Moreover, as described above, the upper section of the sheet processing device  200  is adjacent to the image reading device  300  with no space and there are no jam recovery doors, covers, and similar. 
     Therefore, in the present, embodiment, the following process is performed to prevent, even when a paper jam or a motor error occurs when the system is running or the door opens during sheet conveyance, a sheet from stopping at the position as shown in  FIG. 6 .  FIG. 8  is a flowchart of the control process. 
     When a paper jam, a motor error, or an open door occurs in the sheet processing device  200  when the system is running (Step S 100 ), the sheet-processing-device control unit  400  stops all the loads in the sheet processing device  200  except for the entrance motor  216  (Step S 101 ). 
     After that, the sheet-processing-device control unit  400  determines the entrance sensor  207  that is arranged at the sheet discharging path  202  detects any sheet (Step S 102 ). If no sheet is detected, the entrance motor  216  stops in the same manner as the other loads stop (Step S 105 ). On the other hand, if the entrance sensor  207  detects any sheet, the entrance motor  216  continues driving (Step S 103 ). When the number of steps of the entrance motor  216  that continues driving increases to a predetermined number or more (Step S 104 ), the entrance motor  216  stops (Step S 105 ). Therefore, the user can access the sheet P 3  from the discharge tray  203 .  FIG. 7  is a schematic diagram of an example illustrating a sheet remaining at a removable position in the sheet discharging path  202 . 
       FIG. 9  is a flowchart of another control process that is performed when a paper jam or a motor error occurs when the system is running or when the door opens during sheet conveyance. 
     When a paper jam, a motor error, or an open door occurs in the sheet processing device  200  when the system is running (Step S 100 ), the sheet-processing-device control unit  400  stops all the loads in the sheet processing device  200  except for the entrance motor  216  (Step S 101 ). 
     After that, the sheet-processing-device control unit  400  determines whether the entrance sensor  207  that is arranged at the sheet discharging path  202  detects any sheet (Step S 102 ). If no sheet is detected, the entrance motor  216  stops in the same manner as the other loads stop (Step S 105 ). 
     If the entrance sensor  207  detects any sheet (Yes at Step S 102 ), the sheet-processing-device control unit  400  determines whether the length of the sheet being conveyed is a predetermined length or shorter (Step S 106 ). If the length of the sheet is longer than the predetermined length, the entrance motor  216  stops (Step S 105 ). 
     On the other hand, if the entrance sensor  207  detects any sheet and the length of the detected sheet is the predetermined length or shorter, the entrance motor  216  continues driving (Step S 103 ). When the number of steps of the entrance motor  216  that continues driving increases to a predetermined number or more (Step S 104 ), the entrance motor  216  stops (Step S 105 ). 
     The predetermined length used in the determination at Step S 106  is equal to the distance between the furthest-upstream roller, i.e., the nip section of the entrance rollers  201  and the furthest-downstream roller, i.e., the nip section of the discharging rollers  206 . Alternatively, the predetermined length used in the determination at Step S 106  can be the length of a half-letter-size sheet in the longitudinal direction. 
     The conveying speed at sheet receiving time is needed to be equal to the speed at which the sheet is conveyed from the sheet discharging path  160  of the image forming apparatus  100 ; therefore, the entrance motor  216  is driven at the driving speed equal to the linear speed at which the sheet is discharged out of the main body. The linear speed control information is sent from the sheet-processing-device control unit  400 . After that, when the trailing edge of the sheet has passed through the main-body furthest-downstream roller, namely bifurcating-point conveying roller  162 , in order to increase the time allowed for post-processing with maintaining a certain distance away from the next sheet, the sheet conveying speed is increased. In the shift mode, when the sheet is further conveyed and the trailing edge of the sheet has passed through the shift discharging rollers  204 , in order to prevent the sheet from going out to the discharge tray  203 , the driving speed of the entrance motor  216  is decreased. After that, when the sheet is further conveyed and the trailing edge of the sheet has passed through the discharging rollers  206 , the driving speed increases to the linear speed at which the sheet is discharged from the main body so that the next sheet is received smoothly. 
     In the staple mode, after the trailing edge of the sheet passes through the shift discharging rollers  204 , the sheet is conveyed backward and then stacked on the staple tray  219 ; therefore, before the trailing edge of the sheet has passed through the shift discharging rollers  204 , the driving speed of the entrance motor  216  is decreased to a certain linear speed at which the sheet is discharged onto the staple tray. 
     If, in the flowchart of either  FIG. 8  or  9 , it is determined the entrance motor  216  continues driving (Step S 103 ), the entrance motor  216  is driven at the speed equal to the latest speed. 
     As described above, the present embodiment brings the following effects: 
     1) It is unnecessary to add doors, covers, and similar from which a user accesses the sheet conveying path inside the sheet processing device and a mechanism for sliding the sheet processing device; therefore, any unnecessary increase in the manufacturing costs is suppressed.
 
2) When an error is detected or when a door opens, if the sheet detecting unit detects a sheet remaining and the length of the detected sheet in the sheet conveying direction is the predetermined length or shorter, the load for driving the conveying unit continues driving so as to convey the sheet a predetermined distance and then the load stops. With this configuration, even a paper jam caused by a short sheet is recoverable.
 
3) When a sheet stops at a position detectable by the sheet detecting unit, if the sheet is long enough for easy access from the jam recovery door of the image forming apparatus or the discharge tray of the post-processing device, the load for driving the conveying unit stops immediately. With this configuration, unnecessary electric-power consumption is suppressed.
 
4) When a sheet stops with the leading-edge section at a position detectable by the sheet detecting unit, if the sheet is long enough that the trailing-edge section is still at the fixing roller of the image forming apparatus, the load for driving the conveying unit stops immediately. With this configuration, damage to the fixing roller is prevented that will be caused by the force generated when the load for driving the conveying keeps on driving and the trailing-edge section is pulled out of the fixing roller.
 
5) The predetermined distance is equal to either the distance between the furthest-upstream conveying roller and the furthest-downstream conveying roller or the length of a half-letter-size sheet in the longitudinal direction; therefore, the sheet is conveyed to a position that allows the user to easily access to the sheet. The user can thus remove the sheet easily.
 
6) As for the load for driving the conveying unit, the driving speed for conveying the sheet the predetermined distance is equal to the latest driving speed; therefore, a complicated control that changes the conveying speed in a complicated manner is not needed for the sheet processing device.
 
     According to the present invention, a device/apparatus is provided that is small but allows, without increasing the manufacturing costs due to additional components and due to having a complicated configuration, a user to easily remove a sheet remaining inside the device/apparatus. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.