Patent Publication Number: US-7216865-B2

Title: Sheet conveying device for an image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sheet conveying device and more particularly to a sheet conveying device including a unique mechanism for switching a sheet conveying path, a sheet processing apparatus including the sheet conveying device, an image forming apparatus including the sheet processing apparatus, an image forming system including the sheet processing apparatus, a computer program for controlling the sheet conveying device or the sheet processing apparatus, a computer program for executing a sheet processing method with a computer, a recording medium storing such a computer program such that a computer can read it out, and a sheet processing method. 
   2. Description of the Background Art 
   Japanese Patent Laid-Open Publication Nos. 7-315668 and 2000-53302, for example, each disclose a sheet conveying device in which path selectors are positioned in parallel in a direction of sheet conveyance. This configuration minimizes the widthwise dimension of the path selectors for thereby reducing the overall size of the sheet conveying device. 
   Particularly, in the sheet conveying device taught in the above Laid-Open Publication No. 7-315668, two path selectors do not pivot independently of each other, but pivot at the same time as each other. Such path selectors, however, occupy a great exclusive area when pivoting and cannot pivot at the same time unless use is made of solenoids having great power. 
   On the other hand, the sheet conveying device taught in Laid-Open Publication No. 2000-53302 includes path selectors respectively positioned at a first and a second branch portion and interconnected by a first, a second and a third link member and solenoids that control the links to switch a sheet path. Further, a third path selector is positioned at the second branch portion and driven independently of the second path selector about its own fulcrum. This configuration has a problem that when the edge of the upper path selector contacts the upper surface of the lower path selector when selecting an upward path, the above edge and the edge of the lower path selector are apart from each other by a great distance. As a result, it is likely that the leading edge of a sheet being conveyed abuts against the upper surface of the lower path selector and is steed downward thereby instead of being steered upward by the edge portion of the upper path selector, resulting in a jam. 
   As stated above, arranging path selectors in parallel is one of effective implementations for reducing the overall size of a sheet processing apparatus. However, a problem with the conventional technologies is that a particular solenoid or drive source must be assigned to each of two path selectors arranged in parallel and rotatable independently of each other, increasing the cost of the sheet processing apparatus. Moreover, the solenoids each being assigned to a particular path selector obstruct the reduction of the size, particularly width, of the sheet processing apparatus. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a sheet conveying device, a sheet processing apparatus and an image forming apparatus each being small size and low cost. 
   It is another object of the present invention to provide a sheet conveying device, a sheet processing apparatus and an image forming apparatus each being capable of surely effecting, e.g., three-way or similar sheet conveyance control and shift control even when reduced in size and cost. 
   In accordance with the present invention, a sheet conveying device includes a conveying member for conveying a sheet, a switching mechanism for switching the direction of conveyance of the sheet being conveyed by the sheet conveying member, and a shifting mechanism for shifting the sheet passed through the switching mechanism and nipped by the conveying member in a direction perpendicular to the direction of conveyance. The switching mechanism and shifting mechanism share a single drive source. 
   A sheet processing apparatus, an image forming apparatus and an image forming system each using the above sheet conveying device are also disclosed. 
   Further, in accordance with the present invention, a sheet processing method capable of dealing with a shift mode, a staple mode and a proof mode begins with the step of determining which of the shift mode, staple mode and proof mode is selected. If the shift mode is selected, a motor configured to move a shift roller pair in a direction perpendicular to the direction of sheet conveyance is rotated by a preselected amount when the shift roller pair is conveying a sheet in a preselected direction. Further, if the staple mode is selected, the motor is rotated in a direction opposite to the preselected direction for thereby actuating a switching mechanism configured to switch a path selector to a position for steering a sheet to a path that extends to a staple tray. On the other hand, if the proof mode is selected, the motor is rotated in the direction opposite to the preselected direction for thereby actuating the switching mechanism configured to switch the path selector to a position for steering a sheet to a path that extends to a proof tray. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and features of the present invention will become more apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is an isometric view showing an image forming system embodying the present invention and generally made up of an image forming apparatus and a sheet processing apparatus; 
       FIG. 2  is a view showing various devices arranged in the image forming system of  FIG. 1 ; 
       FIG. 3  is a fragmentary view showing path selectors unique to the illustrative embodiment in a shift mode condition; 
       FIGS. 4 and 5  are views similar to  FIG. 3 , showing the path selectors in a proof mode condition and a staple mode condition, respectively; 
       FIG. 6  is a view demonstrating the operation of the path selectors of the illustrative embodiment that share a single fulcrum; 
       FIG. 7  is a view showing conventional path selectors each having a respective fulcrum; 
       FIG. 8  is a schematic block diagram showing a control system included in the illustrative embodiment; 
       FIG. 9  is a perspective view showing arrangements for switching the path selectors of the illustrative embodiment and causing a shift roller pair to slide; 
       FIG. 10  is a fragmentary enlarged view of a drive section included in the arrangements of  FIG. 9 ; 
       FIG. 11  is a fragmentary enlarged view of a path selector drive mechanism also included in the arrangements of  FIG. 9 ; 
       FIG. 12  is a fragmentary perspective view showing part of the drive mechanism of  FIG. 11  associated with a pivot cam; 
       FIG. 13  is a front view showing the condition of the mechanism associated with the pivot cam; 
       FIG. 14  is a view similar to  FIG. 12 , showing a condition for steering a sheet toward a staple tray; 
       FIG. 15  is a front view showing the condition of  FIG. 14 ; 
       FIG. 16  is a view also similar to  FIG. 12 , showing a condition for steering a sheet toward a proof tray; 
       FIG. 17  is a front view showing the condition of  FIG. 16 ; 
       FIGS. 18 and 19  are flowcharts demonstrating a specific control procedure available with the illustrative embodiment; and 
       FIG. 20  is a flowchart showing an initialization subroutine included in the control procedure in detail. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1  of the drawings, an image forming system embodying the present invention is shown. As shown, the image forming system is generally made up of an image forming apparatus (printer hereinafter) PR and a sheet finishing apparatus (sheet finisher hereinafter) FR. As shown in  FIG. 2 , the printer PR is selectively operable as a printer or a copier with an image reading section  51 , an image writing section  52 , a sheet feeding section  53  and a document feeding section  54 . The printer PR may additionally be configured to operate as a facsimile apparatus or may even be implemented as a digital MFP (Multi Function Peripheral) having all of such different functions, as desired. While the printer PR and sheet finisher FR are shown as being separate from each other in  FIG. 2 , they may, of course, be constructed integrally with each other. 
   The image reading section  51 , implemented as a conventional scanner, optically scans a document in the main scanning direction while being moved in the subscanning direction to thereby read the document. The sheet feeding section, or ADF (Automatic Document Feeder) as often referred to,  54  conveys the above document to a glass platen included in the image reading section  51 . The image writing section  52  is constituted by conventional optics including a laser diode, a polygonal mirror and an f θ lens and optically writes a latent image representative of the document on the surface of a photoconductive element. The latent image thus formed on the photoconductive element is developed by toner and then transferred to a sheet or recording medium as a toner image. Subsequently, the toner image is fixed on the sheet by a fixing unit and then transferred to the sheet finisher FR by an outlet roller pair  55 . 
   In the illustrative embodiment, the sheet feeding section  53  includes four sheet cassettes arranged one above the other. A vertical sheet path  56  adjoins the right side of the sheet cassettes, as viewed in  FIG. 2 , where sheets are expected to be paid out. A sheet paid out from any one of the sheet cassettes is conveyed to the image writing section  52  via the vertical sheet path  56 . 
   The sheet, carrying the toner image thereon, is transferred from the printer PR to the sheet finisher FR in a direction indicated by an arrow M in  FIG. 2 . The sheet finisher FR includes an inlet roller pair  1  arranged to receive and convey the above sheet driven out of the printer PR. A punch unit  4  is positioned downstream of the inlet roller pair  1  in the direction of sheet conveyance in order to punch the sheet. A roller pair  6  for conveyance is positioned downstream of the punch unit  4  in the direction of sheet conveyance. 
   A conveying unit  5  is arranged beneath the punch unit  4  perpendicularly to the direction of sheet conveyance in order to convey chad produced from the sheet by the punch unit  4  to a hopper  3 . More specifically, the conveying unit  5  conveys the chad toward an operation side OP, see  FIG. 1 , where the operator of the image forming system is expected to input desired processing meant for the sheet finisher FR or the printer PR on a control panel  57 , see  FIG. 1 , replace toner or remove a jamming sheet. The hopper  3  is mounted on the inner surface of a front cover  14 , see  FIG. 1 , which the operator opens to replace toner or deal with a jam. The front cover  14  forms part of the casing of the sheet finisher PR at the operation side OP. 
   A first and a second path selector  20  and  21 , respectively, are located downstream of the roller pair  6  and cooperate to steer the sheet punched by the punch unit  4  toward a shift tray  9  via a sorting, stapling or similar processing station or simply steer it toward a proof tray  22 . 
   More specifically, in the illustrative embodiment, a particular path is assigned to each of a sort mode, a staple mode and a proof mode. In the sort mode, the first and second path selectors  20  and  21  are respectively so positioned as to block a path terminating at the proof tray  22  and a path including a roller pair  10  while unblocking a path including a roller pair  7 . As a result, the sheet is driven out to the shift tray  9 , which has a shifting function, by an outlet roller pair  8  via the roller pair  7 . The shifting function is assigned to the roller pair  7  capable of moving back and forth in the direction perpendicular to the direction of sheet conveyance volume by volume to thereby sort consecutive volumes on the shift tray  9 . In this sense, the roller pair  7  will be referred to as a shift roller pair hereinafter. 
   In the staple mode, the second path selector  21  unblocks the path including the roller pair  10  and blocks the path terminating at the shift tray  9 . At the same time, the first path selector  20  blocks the path terminating at the proof tray  22 . In this condition, the sheet is routed through a staple roller pair  11  to a staple tray  12 . Every time such a sheet is driven out to the staple tray  12  by the staple roller pair  11 , a knock roller knocks down the sheet toward an end fence. Subsequently, jogger fences jog the edges of the sheet in the direction perpendicular to the direction of sheet conveyance. As soon as a preselected number of sheets, constituting a single volume, are sequentially stacked on the staple tray  12  in the manner described above, a stapler  13  staples the end portion of the sheet stack, i.e., the trailing end in the illustrative embodiment in the direction of sheet conveyance. Thereafter, a belt conveyor lifts the sheet stack thus stapled toward the outlet roller pair  8 . As a result, the sheet stack is driven out to the shift tray  9  by the outlet roller pair  8 . 
   Further, in the proof mode, the first path selector  20  is pivoted to unblock the path terminating at the proof tray  22 , while blocking the path terminating at the shift tray  9 . At the same time, the second path selector  21  blocks the path including the roller pair  10 . As a result, the sheet being driven by the roller pair  6  is steered toward the proof tray  22 . 
   As stated above, in the illustrative embodiment, the punch unit  4  and hopper  3  are positioned upstream of all sheet finishing stations. Basically, therefore, the punch unit  4  can punch any sheet introduced into the sheet finisher FR. Sheets thus punched may be simply stacked on the proof tray  22  or driven out to and sorted on the shift tray  9  or driven out to the shift tray  9  via the stapler  13 . 
   While the printer PR of the illustrative embodiment is assumed to form an image corresponding to an image optically read by the image reading unit  51 , the printer PR can, of course, form an image in accordance with image data directly received from a data processing apparatus or indirectly received via a network or even facsimile data. In the illustrative embodiment, the operation timing of the punch unit  4  and the operation timings of the first and second path selectors  20  and  21  are set in accordance with the timing at which an inlet sensor  2  senses the leading edge or the trailing edge of a sheet. 
   The paths included in the sheet finisher FR will be described more specifically with reference to  FIGS. 3 through 5 . As shown, an inlet path PS downstream of the punch unit  4 ,  FIG. 2 , branches into an upward or upper path PS 1 , a straight or middle path PS 2  and a downward or lower path PS 3 . The upward path PS 1  terminates at the proof tray  22 ,  FIG. 2 , while the straight path PS 2  and downward path PS 3  both terminate at the shift tray  9 ,  FIG. 2 . It should be noted that the three paths PS 1  through PS 3  branch off in three directions at the same position, implementing a three-way sheet conveyance. 
   Sheets that do not have to be finished are simply stacked on the proof gray  22 . On the other hand, sheets, sorted by being shifted in the direction perpendicular to the direction of sheet conveyance volume by volume, are stacked on the shift tray  9 . The shift tray  9  is moved up and down by a motor under the control of a control mechanism, although shown or described specifically. 
   The shift roller pair  7  and outlet roller pair  8  mentioned earlier are sequentially arranged on the straight path PS 2  and configured to convey a sheet introduced into the path PS 2  to the shift tray  9 . The roller pair  10 , staple roller pair  11  and staple unit  12  also mentioned earlier are sequentially arranged on the downstream path PS 3 . 
   The first path selector  20  selectively steers a sheet toward the proof tray  22  in the proof mode or steers it toward the shift tray  9  via the shift roller pair  7  in the shift mode. The second path selector  21  selectively steers the sheet toward the shift tray  9  via the shift roller pair  7  or steers it toward the staple tray  12  via the roller pair  11  in the staple mode. 
   More specifically, as shown in  FIG. 3 , in the shift mode, the two path selectors  20  and  21  are held in their initial positions for allowing a sheet to advance straight toward the shift roller  7  from a direction A to a direction B. At this instant, the, shift roller pair  7 , preceding the outlet roller pair  8 , is moved in the direction perpendicular to the direction of sheet conveyance to thereby shift the sheet in the above direction by a preselected amount. 
   As shown in  FIG. 4 , in the proof mode, the path selector  20  is caused to pivot on a fulcrum or shaft  23  clockwise to a position for steering a sheet, which is fed in the direction A, toward the proof tray  22  in a direction C. 
   Further, as shown in  FIG. 5 , in the staple mode, the other path selector  21  is caused to pivot on the same fulcrum  23  counterclockwise to a position for steering the sheet fed in the direction A toward the staple tray  12  in a direction D. 
     FIG. 6  shows in an enlarged view the configuration of the two path selectors  20  and  21  unique to the illustrative embodiment in that they share the same fulcrum or axis of rotation  23 . As shown, when the path selector  20 , for example, is pivoted on the shaft  23  clockwise, the edge  20 -B of the path selector  20  adjoins the edge  21 -Bb of the path selector  21  at a distance L 1 . The distance L 1  is small enough for the leading edge Pa of a sheet P, which is being conveyed along the path PS, to surely abut against a slant  20 -C included in the path selector  20  even if the leading edge Pa is bent downward. The sheet P can therefore be surely steered upward by the above slant  20 -C into the upward path PS 1 . 
   By contrast, as shown in  FIG. 7 , assume that the path selectors  20  and  21  are positioned parallel to each other, but respectively pivotable on different fulcrums or shafts  39  and  40 . Then, when the path selector  20 , for example, is pivoted on the shaft  39  clockwise, a distance L 2  between the locus of rotation of the leading edge  20 -B of the path selector  20  and the locus of rotation of the leading edge  21 -B of the path selector  21  is far greater than the distance L 1  shown in  FIG. 1 . Consequently, if the leading edge Pa of the sheet P being conveyed along the path PS is bent downward, it fails to abut against the slant  20 -C of the path selector  20 , but abuts against the edge  20 -B of the path selector  20  and brings about a jam. 
   On the other hand, as shown in  FIG. 5 , assume that the path selector  21 , held in the position shown in  FIG. 3 , is pivoted on the shared fulcrum  23  counterclockwise in order to guide a sheet into the downward path PS 3 , as indicated by an arrow D. Then, the relation between the two path selectors  20  and  21  shown in  FIG. 6  is inverted, i.e., the edge  21 -B of the path selector adjoins the edge  20 -B of the path selector  20  at the distance L 1 . It follows that the leading edge of the sheet can surely abut against the slant  21 -C of the path selector  21  and can therefore be surely guided into the downward path PS 3  thereby. 
   As stated above, in the illustrative embodiment, the first and second path selectors  20  and  21  share a single fulcrum or axis of rotation  23  positioned between them. This reduces positional deviation between the edges  20 -B and  21 -B of the path selectors  20  and  21 , respectively, when the path selector  20  or  21  is pivoted on the shared fulcrum  23 . 
   Hereinafter will be described a drive mechanism for operating the path selectors  20  and  21  and a slide mechanism for causing, by using the force of the drive mechanism, the shift roller pair  7  to slide in the direction perpendicular to the direction of sheet conveyance. 
     FIG. 9  shows the general configuration of the drive mechanism and slide mechanism mentioned above while  FIG. 10  shows the drive mechanism in a fragmentary enlarged view. As shown, the shift roller pair  7  conveys a sheet, not shown, by being rotated by a pulley  25 , which is, in turn, rotated by a stepping motor, not shown, via a timing belt. A shaft  7 -A, supporting the shift rollers  7 , and the pulley  25  are engaged with each other in such a manner as to rotate integrally with each other. More specifically, the engaging portions of the shaft  7 -A and pulley  25  are generally D-shaped in cross section and abut against each other at the straight portion of letter D. 
   As shown in  FIG. 9 , a cam  27  and a link  26  cooperate to move the shaft or slide shaft  7 -A and therefore the shift roller  7  mounted thereon back and forth in a direction indicated by a double-headed arrow. More specifically, when a stepping motor  29  is rotated in one direction, the output torque of the stepping motor  29  is transferred to the cam  27  via a drive gear  29 -A and a driven gear  29 -B meshing with each other, so that the cam  27  is caused to rotate. A pin or cam pin  27 -B is studded on one axial end of the cam  27  and movably received in a slot  26 -A formed in the link  26  perpendicularly to the axial direction of the slide shaft  7 -A. In this configuration, when the cam  27  is rotated via the above gearing, the cam pin  27 -B studded on the cam  27  is angularly moved with the result that the link  26  with the slot  26 -A is moved back and forth in the direction perpendicular to the axial direction of the slide shaft  7 -A. It is to be noted that the length of the slot  26 -A is great enough to allow the cam pin  27 -B to move in the up-and-down direction as viewed in  FIG. 9 . 
   The link  26  is integrally mounted on the slide shaft  7 -A having a D-shaped cross-section mentioned earlier. Therefore, when the link  26  is linearly moved back and forth in accordance with the rotation of the cam  27 , it causes the shift roller pair  7  to slide back and forth via the slide shaft  7 -A in the direction indicated by the arrow in  FIG. 9 . To shift a sheet, the slide shaft  7 -A, supporting the shift roller pair  7 , is caused to slide in one direction when a sheet is passing through the shift roller pair  7 , i.e., when a sheet is being nipped by a drive roller and a driven roller that constitute the shift roller pair  7 . Subsequently, after the above sheet has moved away from the shift roller pair  7 , but before the next sheet arrives at the roller pair  7 , the shift roller pair  7  is caused to slide in the other direction in order to shift the next sheet in the same manner as it shifted the previous sheet. 
   In the illustrative embodiment, the sliding movement of the shift roller pair  7  stated above is implemented by the rotation of the stepping motor  29  effected in one direction. More specifically, the cam  27  geared to the stepping motor  29  causes the shift roller pair  7  to slide when rotated by 180° and then returns it when rotated by another 180°. Such control over the 180°—or half-rotation of the cam  27  is controlled on the basis of the number of drive pulses input to the stepping motor  29 . 
   An HP (Home Position) sensor  28  is responsive to the home position of the cam  27 , so that the angular position of the cam  27  is determined in accordance with the output of the HP sensor  28 . More specifically, the cam  27  is determined to have reached its home position when an interrupter, protruding radially outward from the cam  27 , interrupts the optical path of the HP sensor  28 . 
   The shifting operation described above is effected volume by volume so as to sort consecutive sheets on the shift tray  9  while conveying the sheets. As for a volume, assume that ten volumes of identical booklets, for example, should be produced by copying or printing by a single job. Then, a single volume refers to each of ten volumes to be sequentially sorted on the shift tray  9 . 
   Reference will be made to  FIGS. 11 and 12  for describing the drive mechanism for driving the path selectors  20  and  21  and also including the stepping motor  29 . As shown, a gear  30  is operatively connected to the cam  27  via a one-way clutch  31  and held in mesh with the driven gear  29 -B. The one-way clutch  31  is press-fitted in the gear  30  and so configured as to transfer the output torque of the stepping motor  29  to the gear  30  only when rotated in a preselected direction. More specifically, in the illustrative embodiment, the one-way clutch  31  transfers the output torque of the stepping motor  29  to the gear  30  when the stepping motor  29  is rotated in the direction (opposite direction hereinafter) opposite to the previously mentioned direction (one direction hereinafter) in which the motor  29  is rotated for driving the shift roller pair  7 . 
   A worm  32  is fixedly mounted on a drive shaft that drives the gear  30 . A worm wheel  33  is held in mesh with the worm  32  while a pivot cam  33 -A is rotatable integrally, coaxially with the worm wheel  33 . A spring, not shown, constantly biases the worm  32  toward the gear  30  in order to maintain the worm  32  in mesh with the worm wheel  33 . As shown in  FIG. 11 , the pivot cam  33 -A coaxial with the worm wheel  33  has a sectorial cross-section and selectively contacts either one of cam surfaces  20 -A and  21 -A included in the path selectors  20  and  21 , respectively, thereby causing the path selector  20  or  21  to pivot in a preselected angular range. An interrupter  33 -B is mounted on one end of the pivot cam  33 -A. An HP sensor  36  determines that the pivot cam  33 -A is in its home position on sensing the interrupter  33 -B. The output of the HP sensor  36  is used to control the angular position of the pivot cam  33 -A. 
     FIG. 13  shows the path selectors  20  and  21 , which basically move in the manner stated with reference to  FIGS. 3 through 6 , held in a default condition specifically. As shown, the cam surfaces  20 -A and  21 -A mentioned earlier are respectively positioned on one side face of the path selector  20  and one side surface of the path selector  21 . In the default condition, the downstream ends of the cam surfaces  20 -A and  21 -A are open by the same angle as each other with respect to the direction of sheet conveyance indicated by an arrow in  FIG. 13 . The pivot cam  33 -A is provided with a profile configured to selectively slide on the cam surface  20 -A or  21 -A for thereby angularly moving the path elector  20  or  21 . The path selectors  20  and  21  are pivotable about the shared fulcrum or shaft  23 , as stated previously. 
   In the general configuration of the drive mechanism described above, when the stepping motor  29  is rotated in the opposite direction mentioned earlier, the output torque of the stepping motor  29  is transferred to the gear  30  via the cam  27 , causing the gear  30  to rotate together with the one-way clutch  31 . At this instant, because the one-way clutch  31  is configured to act on a shaft over which it is coupled in a locking direction, the worm  32  rotates together with the shaft  30 -A of the gear  30  to thereby cause the worm wheel  33  to rotate. As a result, the pivot cam  33 -A rotatable integrally with the worm wheel  33  and the cam surface  20 -A or  21 -A of the path selector  20  or  21 , respectively, contact each other, switching the position of the path selector  20  or  21 , as will be described more specifically later. 
   As stated above, in the illustrative embodiment, the operation for switching the path selector  20  or  21  is effected when the one-way clutch  31  press-fitted in the gear  30  acts in the locking direction. On the other hand, the operation for moving the shift roller pair  7  back and forth in the axial direction of the slide shaft  7 -A is effected when the one-way clutch  31  acts in the unlocking direction. It follows that the path selector switching operation is not effected when the shift roller sliding operation is under way. In the shift mode in which consecutive sheets are conveyed via the shift roller pair  7 , the path selectors  20  and  21  are held in the default condition shown in  FIG. 3  or  13  so as not to obstruct the conveyance. More specifically, as shown in  FIG. 11 , such a default condition is implemented by springs  34  and  35  constantly biasing the path selectors  20  and  21 , respectively. 
   When the one-way clutch  31  acts in the locking direction, the path selector  20  or  21  is switched in position, as stated above. At this instant, the shift roller pair  7  is caused to slide at the same time because the cam  27  rotates integrally with the driven gear  29 -B. However, so long as the path selector  20  or  21  is switched to a position shown in  FIG. 16  or  14 , respectively, a sheet can be successfully conveyed because it is prevented from reaching the shift roller pair  7  via the gap between the path selectors  20  and  21 . If such a slide of the shift roller pair  7  is undesirable from a noise and vibration standpoint, then a one-way clutch, not shown, similar to the one-way clutch  31  assigned to the path selectors  20  and  21  may be mounted on the drive shaft of the driven gear  29 -B and cam  27  and so configured as to interrupt drive transmission when the stepping motor  29  is rotated in the direction for driving the path selector  20  or  21 . 
   Further, the one-way clutch assigned to the shifting operation makes it possible to reverse the rotation of the stepping motor  29  and therefore to start switching the path selector  20  or  21  only if the shifting operation has completed, i.e., even if a sheet has not moved away from the shift roller pair  7 . This successfully enhances the productivity of the apparatus. 
   The operation for switching the path selectors  20  and  21  will be described more specifically hereinafter. 
     FIGS. 12 and 13  show the path selectors  20  and  21  in the default condition mentioned earlier. As shown, a preselected small gap exists between the pivot cam  33 A and the cam surface  20 -A of the first path selector  20  while the spring  35  maintains the second path selector  21  in the default position. Further, the interrupter  33 -B movable integrally with the worm wheel  33  is held in the position where it interrupts the optical path of the HP sensor  36 . Therefore, the default condition is set up in the shift mode for causing the path selectors  20  and  21  to guide a sheet toward the shift tray  9  via the shift roller pair  7 . 
   Assume that the stepping motor  29  is rotated in the direction for switching the path selector  20  or  21  held in the default condition. Then, the pivot cam  33 -A is rotated clockwise, as viewed in  FIG. 13 , via the drive transmission including the gears  29 -B and  30 , worm  32  and worm wheel  33 . As a result, the pivot cam  33 -A abuts against the cam surface  21 -A of the path selector  21  and causes it pivot clockwise on the shaft  23 , i.e., pushes it down. Therefore, the path selector  21  is also turned clockwise, as viewed in  FIG. 13 .  FIGS. 14 and 15  show the resulting condition in which the path, terminating at the staple tray  12 , is unblocked while the paths, terminating at the shift tray  9  and proof tray  29 , respectively, are blocked. This condition corresponds to the condition shown in  FIG. 5 , i.e., the staple mode in which the path selector  21  steers a sheet toward the staple tray  12 . 
   Assume that the pivot cam  33 -A is further rotated clockwise from the condition of  FIGS. 14 and 15  in which the second path selector  21  is pivoted by the maximum angle. Then, the pivot cam  33 -A leaves the dead point of the cam surface  21 -A of the path selector  20  with the result that the cam surface  21 -A starts turning counterclockwise about the shaft  23 . Subsequently, the pivot cam  33 -A starts contacting the cam surface  20 -A of the first path selector  20  and causes the cam surface  20 -A to turn counterclockwise about the shaft  23  in  FIG. 15 , i.e., pushes it up while leaving the cam surface  21 -A itself. Consequently, the first path selector  20  is also caused to turn counterclockwise, as viewed in  FIG. 15 .  FIGS. 16  and  17  show the resulting condition in which the path, terminating at the proof tray  29 , is unblocked while the paths, respectively terminating at the shift tray  9  and staple tray  12 , are blocked. This condition corresponds to the condition shown in  FIG. 4 , i.e., the proof mode in which the first path selector  20  steers a sheet toward the proof tray  29 . 
   Subsequently, when the pivot cam  33 -A is further rotated until it leaves the cam surface  20 -A, the force of the pivot cam  20 -A, acting on the first path selector  20 , is canceled. As a result, the first and second path selectors  20  and  21  both are returned to their default positions by the action of the springs  34  and  35 , respectively. Further, as soon as the interrupter  33 -B of the pivot cam  33 -A, rotating in the above direction, interrupts the optical path of the HP sensor  36 , the stepping motor  29  is deenergized so as to restore the default condition shown in  FIG. 13 . 
   In the illustrative embodiment, the pivot cam  33 -A is driven via the one-way clutch  31  and therefore rotatable in only one direction, as stated previously. It follows that to define the transition from the default condition of  FIG. 13  to the staple mode condition of  FIG. 16  or the proof mode condition of  FIG. 17 , the rotation of the stepping motor  29  is controlled on the basis of the profile of the pivot cam  33 -A, the configuration and angle of each of the cam surfaces  20 -A and  21 -A, and the number of pulses counted from the home position sensed by the HP sensor  36 . 
   A control system included in the illustrative embodiment will be described with reference to  FIG. 8 . As shown, a controller or control unit  350  is implemented by a microcomputer including a CPU (Central Processing Unit)  360  and an I/O (Input/Output) interface  370 . The CPU  360  receives via the I/O interface  370  the outputs of switches arranged on the control panel of the printer PR, the outputs of various sensors arranged in the sheet finisher FR and including the inlet sensor  2  and a discharge sensor, not shown, responsive to the level or height of the top sheet on the shift tray  9 . 
   The CPU  360  controls, in accordance with the outputs of the above switches and sensors, various operations including the up-and-down movement of a punch included in the punch unit  4 , the operation of the conveying unit  5 , the jogging or positioning operation effected on the staple tray  12  perpendicularly to the direction of sheet conveyance, the stapling operation of the staple unit  13 , the discharge of a stapled sheet stack, the up-and-down movement and shift of the shift tray  9 , and the operation of the knock roller that knocks down a sheet toward the rear fence mentioned earlier. Further, the CPU  360  counts drive pulses input to a staple conveyance motor, not shown, for driving the staple roller pair  11  and controls the knock roller and jogging operation in accordance with the count of the drive pulses. 
   It is to be noted that the CPU  360  controls the sheet finisher FR by executing a program stored in a ROM (Read Only Memory), not shown, while using a RAM (Random Access Memory), not shown, as a work area. 
   A specific procedure for controlling the drive mechanism included in the illustrative embodiment will be described hereinafter with reference to  FIGS. 18 and 19 . The procedure to be described is executed by the CPU  360 ,  FIG. 8 , in accordance with a program stored in the ROM not shown. Alternatively, a program for executing the procedure may be downloaded from a server to an HDD (Hard Disk Drive) via a network or may be read out of a CD-ROM (Compact Disk ROM), SD (Secure Digital) memory card or similar recording medium by a medium drive, in which case version-up is available. 
   Briefly, as shown in  FIGS. 18 and 19 , the CPU  360  executes particular control in each of the shift mode (step S 1 ), staple mode (step S 2 ) and proof mode (step S 3 ) and finally ends the procedure by performing an initialization subroutine (step S 5 ). 
   More specifically, as shown in  FIG. 18 , the CPU  360  first determines whether or not the shift mode is selected (step S 1 ). If the answer of the step Si is positive (Y), meaning that the shift mode is selected, then the first and second path selectors  20  and  21  are expected to be held in the default positions shown in  FIGS. 12 and 13 . Therefore, as shown in  FIG. 3 , the path, terminating at the shift tray  9 , is blocked while the paths, respectively terminating at the staple tray  12  and proof tray  29 , are blocked. In this case, the procedure is transferred from the step S 1  to a step S 101  shown in  FIG. 19 , as indicated by a connector {circle around (1)}. 
   In the step S 101 , to cause the shift roller pair  7  to shift consecutive sheets volume by volume, the CPU  360  determines whether or not a volume to deal with is an odd volume, i.e., a 2(N−1) volume. If the answer of the step S 101  is Y, the CPU  360  determines whether or not the trailing edge of a sheet has moved away from the roller pair  6  to see if the sheet can be shifted or not. For this purpose, in the illustrative embodiment, the CPU  360  determines whether or not a preselected period of time t 1  elapses from the time when the trailing edge of the sheet moves away from the inlet sensor  2  to the time when it moves away from the roller pair  6  (step S 102 ). If the answer of the step S 102  is Y, the CPU  360  causes the stepping motor  29  to rotate in the forward direction (step S 103 ). It is to be noted that in the illustrative embodiment the forward direction refers to the direction for shifting the shift roller pair  7 . The step S 103  is followed by a step S 104 . 
   As for the step S 104 , assume that the shift roller pair  7  is movable between a first position or initial or leftmost position, as viewed in  FIGS. 9 and 10 , and a second position or rightmost position, and that in the first position the cam pin  27 -B,  FIGS. 9 and 10 , is also positioned at the leftmost position while, in the second position set up when the cam  27  is rotated by 180° from the initial position, the cam pin  27 -B is located at the rightmost position. Then, in the step S 104 , the CPU  360  determines whether or not the shift roller pair  7  has moved from the first position to the second position on the basis of the rotation angle of the cam  27  from the home position, i.e., the number of drive steps of the stepping motor  29 . 
   If the answer of the step S 104  is Y, meaning that the shift roller pair  7  has reached the second position, the CPU  360  deenergizes the stepping motor  29  (step S 105 ). Subsequently, the CPU  360  determines whether or not a preselected period of time t 2  elapses from the time when the trailing edge of the sheet moves away from the inlet sensor to the time when it moves away from the shift roller pair  7  (step S 106 ), thereby determining whether or not the sheet has moved away from the shift roller pair  7 . On the elapse of the period of time t 2  (Y, step S 106 ), the CPU  360  determines whether or not the sheet thus shifted is the last sheet of the odd or 2(N−1) volume (step S 107 ). If the answer of the step S 107  is Y, the procedure is transferred to the step S 4 ,  FIG. 18 , as indicated by a connector  02 . 
   If the answer of the step S 107  is negative (N) the CPU  360  causes the stepping motor  29  to rotate in the forward direction to thereby return the shift roller pair  7  from the second position to the first stated mentioned earlier (step S 108 ). The CPU  360  then determines whether or not the shift roller pair  7  has reached the first position (step S 109 ) and then deenergizes, if the answer of the step S 109  is Y, the stepping motor  29  (step S 110 ). The step S 110  is also followed by the step S 4 ,  FIG. 18 . 
   In the step S 4  following the step S 107  or S 110 ,  FIG. 19 , the CPU  360  determines whether or not the sheet shifted is the last sheet of the volume and the last sheet of the job at the same time, i.e., whether or not the job has ended. If the answer of the step S 4  is Y, the CPU  360  ends the procedure after initialization (step S 5 ). However, if the answer of the step S 4  is N, meaning that the sheet shifted is not the last sheet of the job or the last sheet of the volume, the procedure returns to the step Si because the job has not ended. 
   The distance between the first and second positions of the shift roller pair  7  is two times as great as the distance between the cam pin  27 -B and the center of the cam  27 . In the illustrative embodiment, this distance is selected to be  15  mm although it can be freely selected at the design stage on the basis of the distance between the cam  27 -B and the center of the cam  27 . 
   On the other hand, if the answer of the step S 101  is N, meaning that the volume to deal with is an even volume, the CPU  360  determines whether or not the preselected period of time t 1  has elapsed as in the step S 102  (step S 111 ), thereby determining whether or not the trailing edge of the sheet has moved away from the roller pair  6 . If the answer of the step S 111  is Y, the CPU  360  causes the stepping motor  29  to rotate in the forward direction (step S 112 ). At this instant, if the sheet being conveyed is the last sheet, as determined in the step S 107 , the shift roller pair  7  has been located at the second position in the step S 104 , so that the stepping motor  29  moves the cam pin  27 -B and therefore the shift roller pair  7  from the second position to the first position. Therefore, the CPU  360  determines whether or not the shift roller  7  has returned from the second position to the first position (step S 113 ) and then deenergizes, if the answer of the step S 113  is Y, the stepping motor  29  (step S 114 ). 
   After the step S 114 , the CPU  360  determines whether or not the preselected period of time t 2  has elapsed as in the step S 106  (step S 115 ), thereby determining whether or not the trailing edge of the sheet has moved away from the shift roller pair  7 . As a result, the sheet is shifted from the first position to the second position. At this instant, the amount of shift is 30 mm because the distance between the first and second positions is 15 mm, as stated earlier. Consequently, consecutive volumes are sequentially stacked on the shift tray  9  while being shifted from each other by 30 mm. 
   If the answer of the step S 115  is Y, the CPU  360  determines whether or not the sheet shifted is the last sheet of the even volume or  2 N volume (step S 116 ). If the answer of the step S 116  is Y, the procedure returns to the step S 4 ,  FIG. 18 . If the answer of the step S 116  is N, the CPU  360  causes the stepping motor  29  to rotate in the forward direction for thereby moving the shift roller pair  7  to the second position (step S 117 ). The CPU  360  then determines whether or not the shift roller pair  7  has reached the second position (step S 118 ) and then deenergizes, if the answer of the step S 118  is Y, the stepping motor  29  (step S 119 ). The step S 119  is also followed by the step S 4 . In the step S 4 , the CPU  360  makes decision similar to the decision stated earlier in relation to the odd volume and then returns to the step S 8  if the even volume has not been fully processed. 
   Referring again to  FIG. 18 , if the shift mode is not selected (N, step S 1 ), the CPU  360  determines whether or not the staple mode is selected (step S 2 ). If the answer of the step S 2  is Y, the CPU  360  determines whether or not a sheet being conveyed is the first sheet to be dealt with in the staple mode (step S 201 ). If the answer of the step S 201  is Y, the CPU  360  causes the stepping motor  29  to rotate in the reverse direction in order to guide the sheet toward the staple tray  12  (step S 202 ). As a result, the path selector  21  is angularly moved from the default position shown in FIGS.  12  and  13  (sometimes referred to as a first path selector position hereinafter) toward the position shown in  FIGS. 14 and 15  (sometimes referred to as a second path selector position hereinafter). Subsequently, the CPU  360  determines whether or not the second path selector  21  has reached the second path selector position (step S 203 ). If the answer of the step S 203  is Y, the CPU  360  deenergizes the stepping motor  29  (step S 204 ) and then waits for the entry of a sheet and the end of the job (step S 4 ). 
   On the other hand, if the answer of the step S 201  is N, meaning that the sheet being conveyed is the second or successive sheet, the CPU  360  simply waits for the entry of the sheet and the end of the job with the path selector  21  remaining in the second path selector position (step S 4 ). On the end of the job, the CPU  360  executes the initialization (step S 5 ) and then ends the procedure. 
   If the staple mode is not selected (N, step S 2 ) the CPU  360  determines whether or not the proof mode is selected (step S 3 ). If the answer of the step S 3  is Y, the CPU  360  determines whether or not a sheet being conveyed is the first sheet to be dealt with in the proof mode (step S 301 ). If the answer of the step S 301  is Y, the CPU  360  causes the stepping motor  29  to rotate in the reverse direction in order to guide the sheet toward the proof tray  22  (step S 302 ). As a result, the path selector  20  is angularly moved to the position shown in  FIGS. 16 and 17  (sometimes referred to as a third path selector position hereinafter). Subsequently, the CPU  360  determines whether or not the path selector  20  has reached the third path selector position (step S 303 ). If the answer of the step S 303  is Y, the CPU  360  deenergizes the stepping motor  29  (step S 304 ) and then waits for the entry of a sheet and the end of the job (step S 4 ). 
   On the other hand, if the answer of the step S 301  is N, meaning that the sheet being conveyed is the second or successive sheet, the CPU  360  simply waits for the entry of the sheet and the end of the job with the path selector  20  remaining in the third path selector position (step S 4 ). On the end of the job, the CPU  360  executes the initialization (step S 5 ) and then ends the procedure. 
     FIG. 20  demonstrates the initialization subroutine executed in the step S 5  in detail. As shown, the CPU  360  first causes the stepping motor  29  to rotate in the reverse direction (step S 501 ) and then determines whether or not the HP sensor  36  has sensed the home position of the pivot cam  33 -A (step S 502 ). If the answer of the step S 502  is Y, the CPU  360  deenergizes the stepping motor  29  (step S 503 ) and then causes it to rotate in the forward direction (step S 504 ). Subsequently, the CPU  360  determines whether or not the HP sensor  28  has sensed the home position of the pivot cam  27  (step S 505 ). If the answer of the step S 505  is Y, the CPU  360  deenergizes the stepping motor  29  (step S 506 ). Consequently, the two path selectors  20  and  21 , cams  33 -A and  27  and shift roller pair  7  each are returned to the respective home position and prepared for the next operation thereby. 
   It should be noted that the position of the stepping motor  29  is indefinite in a power-down condition. Therefore, when the entire system is initialized in the event of power-up, the subroutine shown in  FIG. 20  is also executed in order to bring the stepping motor  29  and cams  27  and  33 -A to their default positions. 
   As stated above, in the illustrative embodiment, the stepping motor  29 , which is a drive source assigned to the shift mechanism, is used to move the path selectors  20  and  21 , but the one way-clutch  31  prevents the path selectors  20  and  21  from moving when the shifting operation is under way. When the edges of the path selectors  20  and  21  are spaced apart from each other, a sheet conveyed to the path selectors  20  and  21  is driven out to the shift tray  9  via the shift roller pair  7  and outlet roller pair  8 . 
   Assume that the path selector  20  or  21  is angularly moved when the sheet, passing through the shift roller pair  7 , is shifted in the direction perpendicular to the direction of conveyance. Then, it is likely that the leading edge of the next sheet is caught by the path selector  20  or  21  or that the edge of the path selector  20  or  21  contacts a sheet passing through the gap between the path selectors  20  and  21 , resulting in a jam. The illustrative embodiment obviates this kind of jam by preventing the path selectors  20  and  21  from moving when the shifting operation is under way, as stated above, thereby insuring stable sheet conveyance. 
   Although the stepping motor  29  is shared by both of the shift mechanism and path selector switching mechanism, productivity in an interrupt mode is enhanced because when one mechanism is operating, the other mechanism does not operate. 
   In the illustrative embodiment, when a sheet is conveyed to the shift roller pair  7 , the stepping motor  29 , driving the shift roller pair  7 , is rotated in the reverse direction just after the shift of the sheet so as to switch the path selector  20  or  21 , thereby allowing the above sheet to be steered to another path. This unique arrangement is achievable because the shift roller pair  7  and path selectors  20  and  21  are driven by the forward and reverse rotation of a single motor and because such forward and reverse rotation effect the above drive independently of each other. 
   Controlling the path selector positions with the number of pulses from a home position, the illustrative embodiment can recognize a plurality of positions by use of a single home position sensor. In addition, using a motor for path selector switching in place of conventional DC solenoids, the illustrative embodiment is capable of moving the path selectors slowly with a minimum of noise. 
   Further, when one of the two path selectors is in movement, the other path selector is surely held in a halt. This prevents the path selectors from hitting against each other for thereby maintaining the switching operation stable. 
   Moreover, the two path selectors are shaped symmetrically to each other with respect to the fulcrum or shaft  23  while the pivot cam  33 -A is positioned between the cam surfaces  20 -A and  21 -A of the path selectors. More specifically, the cam surfaces  20 -A and  21 -A are inclined toward the pivot cam  33 -A while parting from each other and move the path selectors  20  and  21 , respectively, in contact with the pivot cam  33 -A. The cam surfaces  20 -A and  21 -A thus inclined relative to the cam  33 -A exert a minimum of force on the cam  33 -A. 
   In summary, it will be seen that the present invention provides a sheet conveying device in which a switching mechanism shares a single drive source with a shifting mechanism to thereby obviate the need for solenoids. The conveying device is therefore small size and low cost. Further, the shifting operation and switching operation can be performed independently of each other by using the reversible rotation of the drive source. This not only reduces the size and cost of the device, but also realizes sure control over sheet conveyance, e.g., three-way sheet conveyance and shift. 
   Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.