Patent Publication Number: US-11040844-B2

Title: Relay conveyance device and image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims the benefit of priority from Japanese Patent application No. 2018-055046 filed on Mar. 22, 2018, which is incorporated by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates to a relay conveyance device configured to convey a sheet from an image forming apparatus to a discharge device and an image forming system including the relay conveyance device. 
     An image forming system is known, which includes an image forming apparatus, a discharge device (for example, a post-processing device) and a relay conveyance device. The image forming apparatus forms an image on a sheet as a recording medium. To the discharge device, the sheet on which the image is formed by the image forming apparatus is discharged. The relay conveyance device relays the sheet from the image forming apparatus to the discharge device. 
     In the image forming system, since the sheets are continuously conveyed along a conveyance path, conveyance members are configured such that a sheet interval is maintained at a suitable distance. If the sheet interval is too short, a conveyance failure, such as a paper jamming, may occur; if the sheet interval is too long, a productivity (a number of the printed sheet per unit time) may de deteriorated. 
     For example, an image forming apparatus has a technique such that when two sheets are continuously conveyed, a feeding start timing at which the sheet is fed from a sheet feeding cassette is controlled based on a sheet interval time from a first sheet passing period to a second sheet detection period. 
     In the above technique, the feeding start timing is controlled based on the sheet interval time; however, a control for the interval of the sheets already fed and conveyed along the conveyance path is not particularly described. Accordingly, it is difficult to reduce a variation of the sheet interval during the conveyance, which is caused by external factor such as deterioration of the conveyance member, by applying the above technique. 
     SUMMARY 
     In accordance with an aspect of the present disclosure, a relay conveyance device includes a relay conveyance path, a plurality of conveyance members, at least one detection member and a control device. The relay conveyance path is arranged between an image forming apparatus forming an image on a sheet and a discharge device discharging the sheet on which the image is formed by the image forming apparatus. The sheet is conveyed along the relay conveyance path from the image forming apparatus to the discharge device. The plurality of conveyance members is provided along the relay conveyance path and configured to convey the sheet from an upstream side to a downstream side of the relay conveyance path. The detection member is provided along the relay conveyance path and configured to detect the sheet conveyed along the relay conveyance path by the conveyance members. The control device is configured to decrease or increase a conveyance speed of the conveyance members. The control device executes a correction processing such that, in a state where a first sheet and a second sheet positioned at the upstream side from the first sheet are conveyed along the relay conveyance path, a sheet interval from a detection end of the first sheet by the detection member to a detection start of the second sheet by the detection member is compared with a predetermined target value, when the sheet interval is smaller than the target value, the conveyance speed of the conveyance member is decreased so as to widen the sheet interval, and when the sheet interval is larger than the target value, the conveyance speed of the conveyance member is increased so as to shorten the sheet interval. 
     In accordance with an aspect of the present disclosure, an image forming system includes an image forming apparatus, a discharge device and a relay conveyance device. The image forming apparatus is configured to form an image on a sheet. The discharge device is configured to discharge the sheet on which the image is formed by the image forming apparatus. The relay conveyance device is configured to relay the sheet from the image forming apparatus to the discharge device. The relay conveyance device includes a relay conveyance path, a plurality of conveyance members, at least one detection member and a control device. The relay conveyance path is arranged between an image forming apparatus forming an image on a sheet and a discharge device discharging the sheet on which the image is formed by the image forming apparatus. The sheet is conveyed along the relay conveyance path from the image forming apparatus to the discharge device. The plurality of conveyance members is provided along the relay conveyance path and configured to convey the sheet from an upstream side to a downstream side of the relay conveyance path. The detection member is provided along the relay conveyance path and configured to detect the sheet conveyed along the relay conveyance path by the conveyance members. The control device is configured to decrease or increase a conveyance speed of the conveyance members. The control device executes a correction processing such that, in a state where a first sheet and a second sheet positioned at the upstream side from the first sheet are conveyed along the relay conveyance path, a sheet interval from a detection end of the first sheet by the detection member to a detection start of the second sheet by the detection member is compared with a predetermined target value, when the sheet interval is smaller than the target value, the conveyance speed of the conveyance member is decreased so as to widen the sheet interval, and when the sheet interval is larger than the target value, the conveyance speed of the conveyance member is increased so as to shorten the sheet interval. 
     The above and other objects, features, and advantages of the present disclosure will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present disclosure is shown byway of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing an image forming system according to one embodiment of the present disclosure. 
         FIG. 2  is a block diagram showing a control device provided in a relay conveyance device according to the embodiment of the present disclosure. 
         FIG. 3  is a schematic view conceptionally showing the image forming system and a sheet conveyance according to the embodiment of the present disclosure. 
         FIG. 4  is a view explaining a sheet interval in the relay conveyance device according to the embodiment of the present disclosure. 
         FIG. 5  is a view explaining the sheet interval in the relay conveyance device according to the embodiment of the present disclosure. 
         FIG. 6  is a flow chart showing a correction processing according to the embodiment of the present disclosure. 
         FIG. 7  is a view explaining the correction processing in the relay conveyance device according to the embodiment of the present disclosure. 
         FIG. 8  is a view explaining the correction processing in the relay conveyance device according to the embodiment of the present disclosure. 
         FIG. 9  is a view explaining the correction processing in the relay conveyance device according to the embodiment of the present disclosure. 
         FIG. 10  is a view explaining the correction processing in the relay conveyance device according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, with reference to the attached drawings, an image forming system according to an embodiment will be described. Arrows L, R, U and Lo suitably marked in each figure respectively indicate a left side, a right side, an upper side and a lower side of the image forming system. Hereinafter, “an upstream (side)” and “a downstream (side)” described in below respectively indicate “an upstream (side)” and “a downstream (side)” in a conveyance direction of a sheet in the image forming system. 
     1. Configuration of the Image Forming System 
     With reference to  FIG. 1 , an image forming system  1  according to the embodiment of the present disclosure will be described. In the following description, a near side of a paper surface of  FIG. 1  is defined to be a front side of the image forming system  1 , for convenience of explanation. 
     As shown in  FIG. 1 , the image forming system  1  is provided with an image forming apparatus  2 , a post-processing device  3  (a discharge device) and a relay conveyance device  4 . The image forming apparatus  2  forms an image on a sheet S. The post-processing device  3  discharges the sheet on which the image is formed by the image forming apparatus  2 . The relay conveyance device  4  relays the sheet from the image forming apparatus  2  to the post-processing device  3 . 
     1-1. Configuration of the Image Forming Apparatus 
     As shown in  FIG. 1 , the image forming apparatus  2  includes a box-shaped apparatus main body  20 , a plurality of sheet feeding parts  21  stored in a lower portion of the apparatus main body  20  and an image forming part  22  stored in an upper portion of the apparatus main body  20 . Inside the apparatus main body  20  of the image forming apparatus  2 , an upstream side conveyance path X along which the sheet S is conveyed is provided. At a downstream end portion of the upstream side conveyance path X, an upstream side discharge port  24  is provided. The upstream side discharge port  24  is opened to an upper portion of a left side face (a face at a side of the relay conveyance device  4 ) of the apparatus main body  20 . 
     The sheet feeding parts  21  of the image forming apparatus  2  are arranged at an upstream end portion of the upstream side conveyance path X. The sheet feeding parts  21  are aligned in the upper-and-lower direction. Each sheet feeding part  21  includes a sheet feeding cassette  25  storing the sheet S and a sheet feeding mechanism  26  arranged at a right upper side of the sheet feeding cassette  25 . The sheet S is made of paper, synthetic resin or fabric. 
     The image forming part  22  of the image forming apparatus  2  is arranged at a downstream portion of the upstream side conveyance path X. The image forming part  22  adopts an inkjet type image forming method. The image forming part  22  includes a conveyance belt  27  and four recording heads  28  arranged above the conveyance belt  27 . The conveyance belt  27  is wound around a plurality of rollers  29  and supported by the rollers  29  in a rotatable manner. The recording heads  28  are configured to be capable of ejecting inks of different colors. 
     It is obviously understood that the image forming part  22  adopts an electrophotographic type image forming method instead of the inkjet type image forming method. 
     The image forming apparatus  2  includes a control device (not shown) and a touch panel  23  on which information is displayed for a user and through which an operation instruction from the user is received. 
     1-2. Operation of the Image Forming Apparatus 
     The sheet feeding mechanism  26  in each sheet feeding part  21  feeds the sheet S from the sheet feeding cassette  25  to the upstream side conveyance path X. The sheet S fed to the upstream side conveyance path X is conveyed to the downstream side along the upstream side conveyance path X, and then enters the image forming part  22 . The sheet S entered the image forming part  22  is adsorbed on an upper face of the conveyance belt  27 , and conveyed to the downstream side as the conveyance belt  27  is rotated. Each recording head  28  ejects the ink downward to the sheet S adsorbed on the upper face of the conveyance belt  27 . This forms an image on the sheet S. The sheet S on which the image is formed is further conveyed to the downstream side along the upstream side conveyance path X and then discharged from the upstream side conveyance path X through the upstream side discharge port  24 . 
     1-3. Configuration of the Post-Processing Device 
     As shown in  FIG. 1 , the post-processing device  3  includes a casing  30 , a plurality of discharge trays  31  to  33  protruding from a left side face of the casing  30  and a plurality of post-processing mechanisms  34  to  36  stored in the casing  30 . The post-processing device  3  is a device called a finisher, conventionally. 
     Inside the casing  30  of the post-processing device  3 , a downstream side conveyance path Y along which the sheet S is conveyed is provided. The downstream side conveyance path Y includes a first path Y 1 , a second path Y 2  branched from an upstream portion of the first path Y 1  and a third path Y 3  branched from a midstream portion of the first path Y 1 . At an upstream end portion of the first path Y 1 , a downstream side introduction port  38  is provided. The downstream side introduction port  38  is opened to an upper portion of a right side face (a face at a side of the relay conveyance device  4 ) of the casing  30 . 
     The discharge trays  31  to  33  of the post-processing device  3  include the first discharge tray  31  arranged at a downstream end portion of the first path Y 1 , the second discharge tray  32  arranged at a downstream end portion of the second path Y 2  and the third discharge tray  33  arranged at a downstream end portion of the third path Y 3 . 
     The post-processing mechanisms  34  to  36  of the post-processing device  3  include the punching mechanism  34  arranged at the upstream portion of the first path Y 1 , the staple mechanism  35  arranged at a branch portion of the first path Y 1  and the third path Y 3  and the sheet folding mechanism  36  arranged at a downstream portion of the first path Y 1 . 
     1-4. Operation of the Post-Processing Device 
     The sheet S on which the image is formed by the image forming apparatus  2  is discharged to the post-processing device  3  through the relay conveyance device  4  (described later in detail), and then introduced to the first path Y 1  through the downstream side introduction port  38 . The sheet S introduced to the first path Y 1  enters the punching mechanism  34 . The punching mechanism  34  subjects a punching processing to the sheet S if needed. The first sheet S passed through the punching mechanism  34  enters the second path Y 2 , and then is discharged to the second discharge tray  32  from the downstream end portion of the second path Y 2 . The second sheet S passed through the punching mechanism  34  is further conveyed to the downstream side along the first path Y 1  and then enters the staple mechanism  35 . The staple mechanism  35  subjects a staple processing to the sheet S if needed. 
     The first sheet S passed through the staple mechanism  35  enters the third path Y 3 , and then is discharged to the third discharge tray  33  from the downstream end portion of the third path Y 3 . The second sheet S passed through the staple mechanism  35  is further conveyed to the downstream side along the first path Y 1  and then enters the sheet folding mechanism  36 . The sheet folding mechanism  36  subjects a sheet folding processing to the sheet S if needed. The sheet S passed through the sheet folding mechanism  36  is discharged to the first discharge tray  31  from the downstream end portion of the first path Y 1 . 
     1-5. Configuration of the Relay Conveyance Device 
     As shown in  FIG. 1 , the relay conveyance device  4  is provided separately from the image forming apparatus  2  and the post-processing device  3 . The relay conveyance device  4  is detachably coupled to the image forming apparatus  2  and the post-processing device  3 . The relay conveyance device  4  is a device called a bridge unit, conventionally. 
     The relay conveyance device  4  includes a casing  40 , first and second inversion units  41  and  42 , a curl correction unit  43 , first and second correction units  44  and  45  and a height adjustment unit  47 . The first and second inversion units  41  and  42  are respectively stored in an upper space and a right side space of the casing  40 . The curl correction unit  43  is stored in a right lower space of the casing  40 . The first and second correction units  44  and  45  are stored in a lower space of the casing  40 . The height adjustment units  47  are attached to four corners of a bottom wall of the casing  40 . The units  41  to  45  are arranged in the order of the inversion units  41  and  42 , the curl correction unit  43  and the correction units  44  and  45  from the upstream side to the downstream. 
     Inside the casing  40  of the relay conveyance device  4 , a relay conveyance path Z along which the sheet S is conveyed is provided. The relay conveyance path Z includes a main path Z 1 , a first sub-path Z 2 , a second sub-path Z 3  and an escape path Z 4 . In  FIG. 1 , the main path Z 1  is shown by a wide line, and the first sub-path Z 2 , the second sub-path Z 3  and the escape path Z 4  are shown by dotted lines. 
     The main path Z 1  of the relay conveyance path Z is arranged between the upstream side conveyance path X of the image forming apparatus  2  and the downstream side conveyance path Y of the post-processing device  3 , and relays the sheet S from the upstream side conveyance path X to the downstream side conveyance path Y. 
     At an upstream end portion of the main path Z 1 , a relay introduction port  50  is provided. The relay introduction port  50  is opened to an upper portion of a right side face (a face at a side of the image forming apparatus  2 ) of the casing  40 , and faces the upstream side discharge port  24  of the image forming apparatus  2 . At a downstream end portion of the main path Z 1 , a relay discharge port  51  is provided. The relay discharge port  51  is opened to an upper portion of a left side face (a face at a side of the post-processing device  3 ) of the casing  40 , and faces the downstream side introduction port  38  of the post-processing device  3 . 
     The main path Z 1  of the relay conveyance path Z is branched at an upstream side brunch point UB into a first inversion path I 1  and a second inversion path I 2 . The first inversion path I 1  and the second inversion path I 2  are joined at an upstream side join point UJ provided at the downstream side of the upstream side brunch point UB. The main path Z 1  is branched at a downstream side brunch point DB provided at the downstream side of the upstream side join point UJ into a first correction path C 1  and a second correction path C 2 . The first correction path C 1  and the second correction path C 2  are joined at a downstream side join point DJ provided at the downstream side of the downstream side brunch point DB. 
     The first sub-path Z 2  of the relay conveyance path Z is branched from the first inversion path I 1  of the main path Z and joined to the downstream end portion of the main path Z 1 . The first sub-path Z 2  is provided at the upper space of an inner space of the casing  40 . 
     The second sub-path Z 3  of the relay conveyance path Z is branched from the first inversion path I 1  of the main path Z. At a downstream end portion of the second sub-path Z 3 , a support tray  52  is arranged. The support tray  52  is provided on an upper face of the casing  40 . 
     The escape path Z 4  of the relay conveyance path Z is branched from the main path Z 1  at the downstream side of the downstream side join point DJ. At a downstream end portion of the escape path Z 4 , an escape tray  53  is arranged. The escape tray  53  is provided in the inner space of the casing  40 . 
     The first inversion unit  41  of the relay conveyance device  4  is arranged on the first inversion path I 1  of the main path Z 1 . The second inversion unit  42  of the relay conveyance device  4  is arranged on the second inversion path I 2  of the main path Z 1 . That is, the first and second inversion units  41  and  42  are arranged in parallel on the main path Z 1 . 
     Each of the first and second inversion units  41  and  42  includes an inversion area  55  and two pairs of inversion rollers  56  arranged at an upstream portion of the inversion area  55 . The upstream portion of the inversion area  55  of the first inversion unit  41  is overlapped with an upstream portion of the first sub-path Z 2 . That is, the inversion area  55  of the first inversion unit  41  is partially shared with the first sub-path Z 2 . A reference W in  FIG. 1  shows a share portion of the inversion area  55  of the first inversion unit  41  and the first sub-path Z 2  (hereinafter, called “a share portion W”). At the downstream side (the left side) of the share portion W, a first brunch claw  58  is provided, and at the upstream side (the right side) of the share portion W, a second brunch claw  59  is provided. 
     The curl correction unit  43  of the relay conveyance device  4  is arranged between the upstream side join point UJ and the downstream side brunch point DB of the main path Z 1 . The curl correction unit  43  is arranged at the downstream side of the first and second inversion units  41  and  42 . The curl correction unit  43  includes a first curl correction mechanism  61 , a second curl correction mechanism  62  provided at the downstream side of the first curl correction mechanism  61  and a third curl correction mechanism  63  provided at the downstream side of the second curl correction mechanism  62 . 
     The first correction unit  44  of the relay conveyance device  4  is arranged on the first correction path C 1  of the main path Z 1 , and the second correction unit  45  is arranged on the second correction path C 2  of the main path Z 1 . That is, the first and second correction units  44  and  45  are arranged in parallel on the main path Z 1 . The first and second correction units  44  and  45  are aligned in the upper-and-lower direction. The first and second correction units  44  and  45  are arranged at the downstream side of the first and second inversion units  41  and  42  and the curl correction unit  43 . 
     Each of the first and second correction units  44  and  45  includes three pairs of switch rollers  65  and a pair of correction rollers  66  arranged at the downstream side of the three pairs of switch rollers  65 . The three pairs of switch rollers  65  and the pair of correction rollers  66  are arranged at intervals in the left-and-right direction (a horizontal direction). An upper roller of the switch rollers is configured to be movable in the upper-and-lower direction between a nip position (refer to a solid line in  FIG. 1 ) and a nip release position (refer to a dotted line in  FIG. 1 ). In the nip position, the sheet S entered each of the correction paths C 1  and C 2  is put between the upper roller and a lower roller of the switch rollers  65 . In the nip release position, the sheet S entered each of the correction paths C 1  and C 2  is released from the upper roller and the lower roller of the switch rollers  65 . The pair of correction rollers  66  is configured to be movable in the front-and-rear direction (the direction perpendicular to the conveyance direction of the sheet S). 
     Hereinafter, the roller, which is provided along the relay conveyance path Z and conveys the sheet S from the upstream side to the downstream side along the relay conveyance path Z, is sometimes called a conveyance roller CR generally, and the conveyance roller CR includes the pairs of inversion rollers  56 , the pairs of switch rollers  65 , the pairs of correction rollers  66 , the pair of discharge rollers  69  and the pair of conveyance rollers  70  which are described above. The conveyance roller CR is typically composed of a pair of rollers; however, the conveyance roller CR may include a single roller which is not a pair of rollers, such as the roller of the third curl correction mechanism  63 . 
     Each of the height adjustment unit  47  of the relay conveyance device  4  includes a caster  71 , a supporting member  72 , a bolt  73  and a nut  74 . The caster  71  is placed on a placement surface Q on which the image forming system  1  is placed. The supporting member  72  supports the caster  71  in a rotatable manner. The bolt  73  is fixed to the supporting member  72 , and the nut  74  is engaged with the bolt  73 . The bolt  73  is penetrated through the bottom wall of the casing  40 . The nut  74  comes into contact with a lower face of the bottom wall of the casing  40 . 
     The relay conveyance device  4  includes a plurality of sheet detection sensors SD (a detection member). The sheet detection sensor SD is a device which is provided on the relay conveyance path Z and detects the sheet S conveyed along the relay conveyance path Z by the conveyance roller CR. The sheet detection sensor SD is an infrared sensor, for example. The sheet detection sensor SD is switched into an ON state when the sheet S exists within a detection area on the relay conveyance path Z (the sheet S is passing through the detection area) and into an OFF state when the sheet S does not exist within the detection area. An output of each sheet detection sensor SD is inputted to a relay control device  80  described later. 
     As shown in  FIG. 2 , the relay conveyance device  4  includes the relay control device  80  (a control device). The relay control device  80  is a device constructed by a microcomputer, and includes an arithmetic processing part  81  and a storage part  82 . The arithmetic processing part  81  includes a microprocessor as a CPU (central processing unit), and the storage part  82  includes a ROM (read only memory) and RAM (random access memory). The ROM is a readable recording memory storing a program used for boot processing and control of the relay conveyance device  4  and the others. A target value T of a sheet interval IN described later is also stored in the ROM of the storage part  82 . The RAM is a readable and writable recording medium, serves as a main storage device and stores written information. The storage part  82  may include an auxiliary storage device, such as a flash memory. 
     The arithmetic processing part  81  executes a predetermined processing referring to the information stored in the RAM according to the program stored in the ROM. The arithmetic processing part  81  logically builds various function blocks achieved by the processing according the program. The arithmetic processing part  81  writes various information obtained by the processing in the storage part  82 . 
     The relay control device  80  is electrically connected to each part of the relay conveyance device  4 , such as the conveyance roller CR and the sheet detection sensor SD. The relay control device  80  is electrically connected to the control device (not shown) of the image forming apparatus  2  and the control device (not shown) of the post-processing device  3 . 
     1-6. Operation of the Relay Conveyance Device 
     The sheet S on which the image is formed is discharged through the upstream side discharge port  24  from the upstream side conveyance path X of the image forming apparatus  2 , and then introduced into the main path Z 1  through the relay introduction port  50 . The sheets S are continuously introduced into the main path Z 1  at predetermined intervals. 
     The first sheet S of the sheets S introduced into the main path Z 1  continuously enters the first inversion path I 1 . The first inversion unit  41  inverts the sheet S entered the first inversion path I 1  as follows. 
     First, each pair of inversion rollers  56  of the first inversion unit  41  is rotated in one direction to introduce the sheet S into the inversion area  55  of the first inversion unit  41 . At this time, the first brunch claw  58  directs a read portion of the sheet S to the downstream portion (the downstream portion from the share portion W) of the inversion area  55  of the first inversion unit  41 . Then, each pair of inversion rollers  56  of the first inversion unit  41  is rotated in the counter direction to the one direction to reverse the conveyance direction of the sheet S. This inverts the sheet S. The read portion of the sheet S whose conveyance direction is reversed is directed to the downstream side of the first inversion path I 1  by the second brunch claw  59  and guided to the upstream side join point UJ. 
     The second sheet S of the sheets introduced into the main path Z 1  continuously enters the second inversion path I 2 . The second inversion unit  42  inverts the sheet S entered the second inversion path I 2 . The operation to invert the sheet S by the second inversion unit  42  is the same as the operation to invert the sheet S by the first inversion unit  41 , and its explanation is omitted. 
     At the upstream side brunch point UB of the main path Z 1 , an upstream side brunch guide (not shown) is provided. By the upstream side brunch guide, the sheets S introduced into the main path Z 1  continuously at predetermined intervals are guided to the first inversion path I 1  and to the second inversion path I 2  alternately. Thereby, the sheets S are introduced into the first inversion unit  41  and into the second inversion unit  42  alternately. 
     A configuration that the sheets S introduced into the main path Z 1  continuously at predetermined intervals are continuously introduced into one of the first inversion path I 1  and the second inversion path I 2  may be adopted. For example, a configuration that when one print job instructs a printing for the plurality of sheets S, the sheets S are introduced into one of the first inversion path I 1  and the second inversion path I 2  may be adopted. 
     The sheet S which is inverted by the first inversion unit  41  or the second inversion unit  42  as described above is passed through the upstream side join point UJ, and then enters the curl correction unit  43 . The curl correction unit  43  corrects the curl of the sheet S. 
     The first sheet S of the sheets S whose curl is corrected enters the first correction path C 1 . The first correction unit  44  corrects the position of the sheet S entered the first correction path C 1  in the front-and-rear direction (the near-and-far direction of  FIG. 1 ) as follows. 
     First, a sheet front-and-rear position sensor (not shown) detects the position of the sheet S in the front-and-rear direction. Next, in a state where the sheet S is put between the switch rollers  65  and between the correction rollers  66  of each pair of the first correction unit  44 , the rotation of the switch rollers  65  and the correction rollers  66  of each pair of the first correction unit  44  is temporality stopped. Then, the upper roller of the switch rollers  65  of each pair of the first correction unit  44  is moved from the nip position (refer the solid line in  FIG. 1 ) to the nip-release position (refer to the dotted line in  FIG. 1 ). 
     Next, the pair of correction rollers  66  of the first correction unit  44  is moved based on a detection result of the above sheet front-and-rear position sensor in the front-and-rear direction with the sheet S put between the rollers  66 . For example, in a case where the above sheet front-and-rear position detection sensor detects that the sheet S is displaced from a reference position forward by 1 mm, the pair of correction rollers  66  of the first correction unit  44  moves rearward by 1 mm. In contrast, in a case where the above sheet front-and-rear position detection sensor detects that the sheet S is displaced from the reference position rearward by 1 mm, the pair of correction rollers  66  of the first correction unit  44  moves forward by 1 mm. Thereby, the position of the sheet S is corrected in the front-and-rear direction. 
     After the position of the sheet S is corrected in the front-and-rear direction, the upper roller of the switch rollers  65  of each pair of the first correction unit  44  is moved from the nip-release position (refer to the dotted line in  FIG. 1 ) to the nip position (refer to the solid line in  FIG. 1 ). Then, the rotation of the rollers  65  and  66  of each pair of the first correction unit  44  is restarted. 
     The second sheet S of the sheets S whose curl is corrected enters the second correction path C 2 . The second correction unit  45  corrects the position of the sheet S entered the second correction path C 2  in the front-and-rear direction. The operation to correct the position of the sheet S in the front-and-rear direction by the second correction unit  45  is the same as the operation to correct the position of the sheet S in the front-and-rear direction by the first correction unit  44 , and its explanation is omitted. 
     At the downstream side brunch point DB of the main path Z 1 , a downstream side brunch guide (not shown) is provided. By the downstream side brunch guide, the sheets S introduced into the main path Z 1  continuously at predetermined intervals are guided to the first correction path C 1  and to the second correction path C 2  alternately. Thereby, the sheets S are introduced to the first correction unit  44  and to the second correction unit  45  alternately. 
     A configuration that the sheets S are continuously introduced into one of the first correction path C 1  and the second correction path C 2  may be adopted. For example, a configuration that when one print job instructs a printing for the plurality of sheets S, the sheets S are introduced into one of the first correction path C 1  and the second correction path C 2  may be adopted. 
     The sheet S whose position is corrected in the front-and-rear direction by the first correction unit  44  or the second correction unit  45  is discharged through the relay discharge port  51  from the main path Z 1 , and enters the downstream side conveyance path Y through the downstream side introduction port  38  of the post-processing device  3 . 
     2. Variation of the Sheet Interval 
       FIG. 3  is a view schematically showing the image forming system  1  of the present embodiment. As explained again, the image forming system  1  includes the image forming apparatus  2  which forms the image on the sheet S, the post-processing device  3  (the discharge device) which discharges the sheet S on which the image is formed by the image forming apparatus  2  and the relay conveyance device  4  which relays the sheet S from the image forming apparatus  2  to the post-processing device  3 . The sheet S is fed within the image forming apparatus  2 , is discharged after the image is formed on the sheet S and then enters the relay conveyance device  4 . The sheet S is conveyed along the relay conveyance path Z within the relay conveyance device  3 , discharged and then enters the post-processing device  3 . The sheet S is subjected to the post-processing, and then discharged from the post-processing device  3 . 
       FIG. 4  and  FIG. 5  are views explaining the sheet interval IN. The sheets S fed continuously are conveyed along the relay conveyance path Z at the sheet intervals IN. The sheet interval IN is preferably close to a predetermined target value T; however, the sheet interval IN is practically varied due to slipping of the sheet S caused by the abrasion of the conveyance roller CR (the conveyance member). The sheet interval IN may be presented by a difference in time (a time difference) or a difference in space (a distance) in the relay control device  80 . 
     For example, as shown in  FIG. 4 , in a case where the sheet interval IN is shorter than the target value T, the post-processing by the post-processing device  3  cannot be performed in time, and the succeeding sheet S may enter the post-processing device  3  during the post-processing for the preceding sheet S by the post-processing device  3 . As a result, a sheet jamming may occur. 
     On the other hand, as shown in  FIG. 5 , in a case where the sheet interval IN is longer than the target value T, the post-processing is performed without problems; however, the productivity of the printing processing may not be sufficient for the original performance of the image forming system  1 . 
     Accordingly, in the present embodiment, the correction to bring the sheet interval IN close to the target value T is executed such that the sheet interval IN is specified based on the output from the sheet detection sensor SD and the rotation speed (a conveyance speed) of the conveyance roller CR is controlled (decreased or increased) based on the specified sheet interval IN. 
     3. Correction Processing for the Sheet Interval 
       FIG. 6  is a flowchart showing the correction processing for the sheet interval IN in the relay conveyance device  4  according to the present embodiment. When the relay conveyance device  4  receives the sheet S from the image forming apparatus  2 , the conveyance processing for the sheet S and the correction processing for the sheet interval IN are executed. 
     In the following example, of the sheets S conveyed along the relay conveyance path Z continuously, by focusing attention on the continuous first sheet S 1  and second sheet S 2  shown in  FIG. 7  and  FIG. 8 , the correction processing will be described. The second sheet S 2  is a sheet conveyed along the relay conveyance path Z at the upstream side of the first sheet S 1 . In the following example, the correction processing is explained by focusing attention on one of the sheet detection sensors SD; however, the correction processing is executed for each of the other sheet detection sensors SD in the same manner. 
     When the correction processing is started, the arithmetic processing part  81  of the relay control device  80  acquires the sheet interval IN (step S 101 ). More specifically, the arithmetic processing part  81  acquires a period from a detection end time at which the detection of the first sheet S 1  by the sheet detection sensor SD is ended ( FIG. 7 ) to a detection start time at which the detection of the second sheet S 2  by the sheet detection sensor SD is started ( FIG. 8 ) as the sheet interval IN, and stores it in the storage part  82 . Alternatively, the arithmetic processing part  81  may acquire a distance from a tail end of the first sheet S 1  to a lead end of the second sheet S 2  as the sheet interval IN based on the period from the detection end time of the first sheet S 1  to the detection start time of the second sheet S 2  and the conveyance speed of the sheet S. 
     Next, the arithmetic processing part  81  of the relay control device  80  compares the sheet interval IN acquired in step S 101  with the target value T stored in the storage part  82  (step S 102 ). In a case where a difference Δ between the sheet interval IN and the target value T is larger than a threshold value Th (S 102 : YES), the arithmetic processing part  81  judges that it is necessary to correct the sheet interval IN, and proceeds the processing to step S 103 . On the other hand, in a case where the difference Δ is less than the threshold value Th (S 102 : NO), the arithmetic processing part  81  judges that it is not necessary to correct the sheet interval IN, and the correction processing for one sheet detection sensor SD and for the first sheet S 1  and the second sheet S 2  is finished. 
     In step S 103 , in a case where the sheet interval IN is smaller than the target value T (S 103 : NO), the arithmetic processing part  81  of the relay control device  80  proceeds the processing to step S 104 . In a case where the sheet interval IN is larger than the target value T (S 103 : YES), the arithmetic processing part  81  of the relay control device  80  proceeds the processing to step S 105 . In step S 104 , the arithmetic processing part  81  of the relay control device  80  decreases the rotation speed of the conveyance roller CR so as to widen the sheet interval IN. On the other hand, in step S 105 , the arithmetic processing part  81  of the relay control device  80  increases the rotation speed of the conveyance roller CR so as to shorten the sheet interval IN. 
     Next, the arithmetic processing part  81  of the relay control device  80  counts up a number of rotation speed decreasing or rotation speed increasing (a number of correction processing) for the conveyance roller CR whose rotation speed is decreased or increased in step S 104  or step S 105  (step S 106 ). Then, the arithmetic processing part  81  judges whether the number of rotation speed decreasing or rotation speed decreasing (the number of correction processing) is over a specific number or not, based on the result of the count-up in step  106  (step S 107 ). 
     In a case where the number of correction processing is larger than the specific number (S 107 : YES), the arithmetic processing part  81  of the relay control device  80  notifies the image forming apparatus  2  so as to display an alert message for the corresponding conveyance roller CR (step S 108 ). When receiving the above notification, the control device of the image forming apparatus  2  displays the alert message prompting a replacement of the conveyance roller CR on the touch panel  23  of the image forming apparatus  2 . 
     That is, the relay control device  80  counts the number of rotation speed decreasing or rotation speed increasing (the number of correction processing) for each conveyance roller CR, and then notifies the image forming apparatus  2  so as to display the alert message for the conveyance roller CR whose counted number is over the specific number. In the present embodiment, the specific number is 40 or 50, for example. 
     In a case where the number of correction processing is less than the specific number (S 107 : NO), or, after the step S 108  is completed, the arithmetic processing part  81  of the relay control device  80  finishes the correction processing for one sheet detection sensor SD and for the first sheet S 1  and the second sheet S 2 . 
     In the above steps S 104  and S 105 , the conveyance roller CR whose rotation speed is decreased or increased by the relay control device  80  is the conveyance roller CR which conveys the second sheet S 2  at the time when the step S 104  or S 105  is executed, for example. 
     Alternatively, the relay control device  80  may decrease or decrease the rotation speed of the conveyance roller CR arranged at the downstream side of the conveyance roller CR which conveys the second sheet S, in step S 104  or S 105 . 
     In the above correction processing, after the conveyance roller CR whose rotation speed is decreased or increased finishes the conveyance of the second sheet S 2 , the relay control device  80  preferably controls the conveyance roller CR so as to restore its rotation speed to the original speed before the decreasing or increasing of the rotation speed. 
     Additionally, in the above correction processing, the relay control device  80  preferably decreases or increases the rotation speed of the plurality of conveyance rollers CR conveying the second sheet S 2  sequentially until the difference Δ between the sheet interval IN and the target value T is less than the threshold value Th. 
     4. Correction Processing for Each Conveyance Section as a Unit 
     Although the correction processing is executed for each conveyance roller CR in the above embodiment, the correction processing may be executed for each of the conveyance sections P into which the relay conveyance path Z is divided. The relay conveyance path Z is divided into the conveyance sections P, and each conveyance section P is provided with the plurality of conveyance rollers CR. 
     With reference to  FIG. 1  again, an example of the conveyance section P will be described. The relay conveyance path Z is divided into four conveyance sections P, for example. The first section P 1  includes the portion of the relay conveyance pats Z from the relay introduction port  50  to the upstream side join point UJ. The second conveyance section P 2  includes the portion of the relay conveyance path Z from the downstream side of the upstream side join point UJ to the upstream side of the downstream side brunch point DB. The third conveyance path P 3  includes the portion of the relay conveyance path Z from the downstream side brunch point DB to the downstream side join point DJ. The fourth conveyance path P 4  includes the portion of the relay conveyance path Z from the downstream side of the downstream side join point DJ to the relay discharge port  51 . Because the present embodiment supposes a configuration that the sheet S is conveyed to one of the inversion units  41  and  42  and one of the correction units  44  and  45 , the relay conveyance path Z within each conveyance section P is single (not parallel). 
     With reference to  FIG. 9  and  FIG. 10 , the correction processing for each conveyance section P as a unit will be described. The basic operation is the same as that shown by the flowchart in  FIG. 6 . In step S 101 , the arithmetic processing part  81  acquires the period from the detection end time of the first sheet S 1  by the sheet detection sensor SD within the first conveyance section P 1  ( FIG. 9 ) to the detection start time of the second sheet S 2  by the sheet detection sensor SD within the first conveyance section P 1  ( FIG. 10 ) as the sheet interval IN, and then stores it in the storage part  82 . In a case where the difference Δ between the sheet interval IN and the target value T is larger than the threshold value Th (S 102 : YES), the arithmetic processing part  81  proceeds the processing to step S 103 . 
     In a case where the sheet interval IN is smaller than the target value T (S 103 : NO), the arithmetic processing part  81  decreases the rotation speed of the plurality of conveyance rollers CR within the second conveyance section P 2  so as to widen the sheet interval IN (step S 104 ). In a case where the sheet interval IN is larger than the target value T (S 103 : YES), the arithmetic processing part  81  increases the rotation speed of the plurality of conveyance rollers CR within the second conveyance section P 2  so as to shorten the sheet interval IN (step S 105 ). 
     Preferably, in step S 106 , the arithmetic processing part  81  of the relay control device  80  may count up the number of rotation speed increasing or rotation speed decreasing (the number of correction processing) for the conveyance section P provided with the conveyance roller CR whose rotation speed is increased or decreased in step S 104  or step S 105 . Then, the arithmetic processing part  81  judges whether the number of rotation speed increasing or rotation speed decreasing caused by the conveyance section P is over the specific number or not, based on the result of the count-up in step S 106  (step S 107 ). In a case where the number of rotation speed increasing or rotation speed decreasing is over the specific number (S 107 : YES), the arithmetic processing part  81  of the relay control device  80  notifies the image forming apparatus  2  to display an alert message for the corresponding conveyance section P (step S 108 ). When receiving the notification, the control device of the image forming apparatus  2  displays the alert message showing frequent occurrences of the correction caused by the conveyance section P, on the touch panel  23  of the image forming apparatus  2 . 
     5. Technical Effect of the Present Embodiment 
     In the above described embodiment, the relay conveyance device  4  arranged between the image forming apparatus  1  and the post-processing device  3  includes the relay conveyance path Z, the conveyance rollers CR, the sheet detection sensors SD and the relay control device  80 . The sheet S is conveyed along the relay conveyance path Z from the image forming apparatus  1  to the post-processing device  3 . The conveyance rollers CR are provided along the relay conveyance path Z and convey the sheet S from an upstream side to a downstream side of the relay conveyance path Z. The sheet detection sensor SD is provided along the relay conveyance path Z and detects the sheet S conveyed along the relay conveyance path Z by the conveyance rollers CR. In a state where the first sheet S 1  and the second sheet S 2  positioned at the upstream side from the first sheet S 1  are conveyed along the relay conveyance path Z, the relay control device  80  compares the sheet interval IN from a detection end of the first sheet S 1  by the sheet detection sensor SD to a detection start of the second sheet S 2  by the detection sensor SD with the predetermined target value T. Then, when the sheet interval IN is smaller than the target value T, the relay control device  80  decreases the conveyance speed of the conveyance roller CR so as to widen the sheet interval IN, and when the sheet interval IN is larger than the target value T, the relay conveyance device  80  increases the conveyance speed of the conveyance roller CR so as to shorten the sheet interval IN. According to the above configuration, the increasing and decreasing of the conveyance speed of the conveyance roller CR is controlled based on the sheet interval IN acquired during the conveyance of the sheet S so that it becomes possible to suitably correct the variation in the sheet interval IN in the relay conveyance device  4  during the conveyance of the sheet S. 
     Additionally, the above embodiment has a configuration that the conveyance roller CR whose conveyance speed is decreased or increased by the relay control device  80  so as to widen or shorten the sheet interval IN is the conveyance roller CR which conveys the second sheet S 2 . According to the configuration, the conveyance speed of the conveyance roller CR which just conveys the second sheet S 2  is increased or decreased so that it becomes possible to correct the sheet interval IN effectively and immediately. 
     Additionally, the above embodiment has a configuration that the conveyance roller CR whose conveyance speed is decreased or increased by the relay control device  80  so as to widen or shorten the sheet interval IN is the conveyance roller CR arranged at the downstream side of the conveyance roller CR which conveys the second sheet S 2 . According to the configuration, because the conveyance speed of the conveyance roller CR which is to convey the second sheet S 2  in future is increased or decreased, it becomes possible to correct the sheet interval IN more surely (for example, even if there is a time lag between the correction processing by the relay control device  80  and the practical increasing or decreasing of the conveyance speed of the conveyance roller CR). 
     Additionally, the above embodiment has a configuration that when the conveyance roller CR whose conveyance speed is decreased or increased finishes the conveyance of the second sheet S 2 , the relay control device  80  controls the conveyance roller CR so as to restore the conveyance speed to an original speed before the decreasing or increasing of the conveyance speed. According to the configuration, because the conveyance speed of the conveyance roller CR is returned to the original speed after the correction of the variation of the specific sheet interval IN, it becomes possible to keep the constant control performance of the relay control device  80 . 
     Additionally, the above embodiment has a configuration that the relay control device  80  sequentially decreases or increases the conveyance speed of the conveyance rollers CR which convey the second sheet S 2  until a difference Δ between the sheet interval IN and the target value T is less than the predetermined threshold value Th. According to the configuration, because the conveyance speed correction of the conveyance roller CR is continuously executed until the sheet interval IN between the first sheet S 1  and the second sheet S 2  becomes small sufficiently, it becomes possible to correct the variation in the sheet interval IN more surely. 
     Additionally, the above embodiment has a configuration that the relay control device  80  counts a number of the increasing or decreasing the conveyance speed for each conveyance roller CR, and notifies the image forming apparatus  2  so as to display an alert message for the conveyance roller CR of which the counted number is over the specific number. According to the configuration, because the alert is displayed on the conveyance roller CR of which the number (the number of the correction) of the increasing or decreasing the conveyance speed is large, that is, the conveyance roller CR which is assumed to be deteriorated, a user can recognize the conveyance roller CR to be replaced. Accordingly, it becomes possible to improve a maintenance performance of the relay conveyance device  4 . 
     Additionally, the above embodiment has a configuration that the relay conveyance path Z is divided into a plurality of conveyance sections P, when the sheet interval IN is smaller than the target value T, the relay control device  80  decreases the conveyance speed of the conveyance roller CR provided in the conveyance section P so as to widen the sheet interval IN, and when the sheet interval IN is larger than the target value T, the relay control device  80  increases the conveyance speed of the conveyance roller CR provided in the conveyance section P so as to shorten the sheet interval IN. According to the above configuration, because the conveyance roller CR is controlled for each conveyance section P, it becomes possible to achieve a simple control compared with a case where each conveyance roller CR is controlled individually. 
     According to the configuration of the above embodiment, the image forming system  1  including the image forming apparatus  2  and the post-processing device  4  can be achieved. 
     6. Modified Example 
     The inversion unit according to the above embodiment makes the sheet introduced in the inversion area invert the conveyance direction so as to invert the sheet. On the other hand, in the other embodiments, the sheet may be inverted by circulating the sheet along a loop-shaped inversion path. The inversion unit may have any configuration such that the sheet is inverted. 
     The correction unit according to the above embodiment makes the pairs of correction rollers between which the sheet is put move so as to correct the position of the sheet in the direction perpendicular to the conveyance direction of the sheet. On the other hand, in the other embodiments, the correction unit may have a pair of cursors contacting with both side edges of the sheet, and correct the position of the sheet in the direction perpendicular to the conveyance direction of the sheet. The correction unit may have any configuration such that the position of the sheet can be corrected in the direction perpendicular to the conveyance direction of the sheet. 
     In the above embodiments, the post-processing device (so-called finisher) including the post-processing mechanism is used as the discharge device. On the other hand, in the other embodiments, a sheet stacking device (so-called stacker) not including the post-processing mechanism may be used as the discharge device.