Patent Publication Number: US-2023150787-A1

Title: Sheet stacker and image forming system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2021-184888, filed on Nov. 12, 2021, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure relate to a sheet stacker and an image forming system. 
     Related Art 
     A sheet stacker that includes a sheet stacking portion, a pair of sheet restrictors, and a position detector is known. Sheets are stacked on the sheet stacking portion, and the sheet stacking portion moves up and down. The pair of sheet restrictors is movable in a width direction of the sheets to regulate a position of the sheets stacked on the sheet stacking portion in the width direction. The position detector is disposed between the pair of sheet restrictors to detect a position of the sheet restrictor in the width direction. 
     SUMMARY 
     Embodiments of the present disclosure describe an improved sheet stacker that includes a sheet stacking portion, a pair of sheet restrictors, a sheet feeder, and a position detector. A bundle of sheets is stacked on the sheet stacking portion in a stacking direction, and the sheet stacking portion moves in the stacking direction. The pair of sheet restrictors move in a width direction of the bundle of sheets orthogonal to the stacking direction to regulate a position of the bundle of sheets in the width direction. The sheet feeder feeds a top sheet of the bundle of sheets at a sheet feeding position in a sheet feeding direction orthogonal to the stacking direction and the width direction. The sheet feeding position is disposed at a top of the bundle of sheets in the stacking direction. The position detector is disposed between the pair of sheet restrictors in the width direction and above the sheet feeding position in the stacking direction. The position detector detects a position of the pair of sheet restrictors in the width direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG.  1    is a schematic view of an image forming system according to an embodiment of the present disclosure; 
         FIGS.  2 A and  2 B  are schematic views of a sheet feeding tray of a sheet feeding device of the image forming system in  FIG.  1   ; 
         FIG.  3    is a schematic view of the sheet feeding tray, illustrating an example of movement of a guide frame and an end fence when a sheet is set on the sheet feeding tray; 
         FIGS.  4 A and  4 B  are schematic views of a fixing mechanism of the sheet feeding tray; 
         FIG.  5    is a schematic view of the sheet feeding tray, illustrating detection of a position of a side fence in a width direction by a rangefinder of the sheet feeding tray; 
         FIGS.  6 A and  6 B  are schematic views of the sheet feeding tray on which sheets having the maximum width are stacked; 
         FIGS.  7 A and  7 B  are schematic views of the sheet feeding tray on which sheets having the minimum width are stacked; 
         FIG.  8    is a flowchart of a control for determining the positional deviation of the side fence; and 
         FIG.  9    is a schematic view of the sheet feeding tray in which the side fence does not regulate the position of a bundle of sheets in the width direction. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result. 
     Referring now to the drawings, embodiments of the present disclosure are described below. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     A description is given below of an image forming system according to an embodiment of the present disclosure. In the present embodiment, the image forming system includes a sheet feeding device that is a sheet stacker including a sheet feeder.  FIG.  1    is a schematic view of an image forming system  200  according to the present embodiment. The image forming system  200  includes a sheet feeding device  210 , a pretreatment liquid application device  220 , an inkjet printer  230 , a drying device  240 , and a sheet output device  250 . 
     The sheet feeding device  210  supplies a sheet  8  as a recording medium such as cut paper to the pretreatment liquid application device  220  disposed downstream from the sheet feeding device  210  in a conveyance path of the sheet  8 . The sheet feeding device  210  as a sheet stacker includes a sheet feeding tray  100  and a controller  150 , which are described later. The pretreatment liquid application device  220  applies a pretreatment liquid to the sheet  8 . The pretreatment liquid prevents blurring and bleed-through of inkjet ink for printing. The pretreatment liquid application device  220  includes a sheet reverse path to reverse the sheet  8 . The pretreatment liquid is applied to a front side of the sheet  8 . Thereafter, the sheet  8  is reversed in the sheet reverse path, and the pretreatment liquid is also applied to a back side of the sheet  8  in double-sided printing. 
     The inkjet printer  230  as an image forming device discharges ink droplets onto the front side of the sheet  8  to which the pretreatment liquid has been applied by the pretreatment liquid application device  220  to form a desired image. The drying device  240  includes a dryer that dries the image on the front side of the sheet  8  formed by the inkjet printer  230 . In double-sided printing of the sheet  8 , the sheet  8  is reversed in a path from the drying device  240  back to the inkjet printer  230 . Then, the inkjet printer  230  discharges ink droplets to a front side (i.e., the back side before reversed) of the reversed sheet  8  to form a desired image, and the drying device  240  dries the image on the front side (i.e., the back side before reversed) of the sheet  8 . Thereafter, the sheet  8  is ejected to the sheet output device  250 . 
       FIGS.  2 A and  2 B  are schematic views of the sheet feeding tray  100  of the sheet feeding device  210 .  FIG.  2 A  is a front view of the sheet feeding tray  100 , and the sheet  8  is fed from right to left in  FIG.  2 A .  FIG.  2 B  is a right side view of the sheet feeding tray  100 . The left-right direction in  FIG.  2 B  is the front-back direction of the sheet feeding device  210  and a width direction of the sheet  8 . 
     The sheet feeding tray  100  includes a bottom portion  6 , a left side plate  10  disposed on the downstream side in a sheet feeding direction, a front side plate  30  disposed on the front side of the sheet feeding device  210 , a back side plate  31  disposed on the back side of the sheet feeding device  210 , which construct a device body of the sheet feeding device  210 . Thus, the sheet feeding tray  100  has a box shape in which an upper portion and a right side are opened. The sheet feeding device  210  further includes a lift table  4  as a sheet stacking portion that moves up and down (i.e., a stacking direction) by a motor  22 . Position sensors  20  and  21  are disposed above and below a range where the lift table  4  is movable in the vertical direction, respectively. When the lift table  4  moves upward, the position sensor  20  detects the upper surface of the lift table  4 , and the lift table  4  stops at a predetermined position based on the detection result of the position sensor  20 . Similarly, when the lift table  4  moves downward, the position sensor  21  detects the lower surface of the lift table  4 , and the lift table  4  stops at a predetermined position based on the detection result of the position sensor  21 . 
     Side fences  25   a  and  25   b  as a pair of sheet restrictors are disposed on both sides of the lift table  4  in the width direction of the sheet  8  (the front-back direction of the sheet feeding device  210 ) orthogonal to the stacking direction, and the positions of the pair of side fences  25   a  and  25   b  are moved in accordance with the sheet  8  set on the lift table  4 . The pair of side fences  25   a  and  25   b  move in opposite directions in the width direction of the sheet  8  in conjunction with each other by a connector  26 . Each of the side fences  25   a  and  25   b  regulates the position of the edge of the sheet  8  in the width direction. 
     The sheet feeding tray  100  further includes a fixing mechanism  40  as a fixing unit that locks the side fences  25   a  and  25   b  to immobilize the side fences  25   a  and  25   b  in the width direction. The fixing mechanism  40  includes a slide plate  41  attached to the side fence  25   b  on the right side in  FIG.  2 B  (i.e., the back side of the sheet feeding device  210 ), and a fixing plate  42  secured to the back side plate  31 . When the slide plate  41  is secured to the fixing plate  42 , the side fences  25   a  and  25   b  are locked so as not to move in the width direction. 
     An end fence  5  is disposed at a position corresponding to a trailing end of the sheet  8 . The end fence  5  serves as a trailing-end restrictor that is movable to regulate the position of the trailing end of the sheet  8 . The end fence  5  regulates the position of the trailing end of the sheet  8  while a leading end of the sheet  8  contacts the left side plate  10  that is disposed on the downstream side of the sheet feeding tray  100  in the sheet feeding direction. The end fence  5  is held by a guide rail  16  serving as a stay disposed at an upper portion of the sheet feeding tray  100 . The end fence  5  is movable along the guide rail  16  in a longitudinal direction of the guide rail  16  (i.e., the sheet feeding direction). 
     The guide rail  16  is attached to a guide frame  15  as a frame. The guide frame  15  has a frame shape that is along an edge of the box-shaped sheet feeding tray  100 . Specifically, the guide rail  16  is attached to the center in the width direction of an upstream side support portion of the guide frame  15 . The upstream side support portion extends in the sheet width direction on the upstream side of the guide frame  15  in the sheet feeding direction. The guide frame  15  is hinged on (swingably attached to) the back side plate  31  via a plurality of rotating hinges  17  which is rotatable. The end fence  5  moves along the guide rail  16  in the sheet feeding direction and is secured at a desired position in the longitudinal direction of the guide rail  16 . The end fence  5  is secured at the desired position in response to various sizes of the sheet  8 . The end fence  5  includes a rotating hinge  18 , which is rotatable, therein. 
     A rangefinder  50  as a position detector is attached to the guide rail  16  to measure a distance to detect the position of the side fence  25   b  in the width direction. In the present embodiment, an infrared rangefinder is used as the rangefinder  50 . The rangefinder  50  emits, but not limited to, infrared light and may emit visible light or ultraviolet light. The rangefinder  50  may use radio waves such as millimeter waves or ultrasonic waves. The rangefinder  50  detects the position of the side fence  25   b  to detect a width of the sheet  8  set on the sheet feeding tray  100  based on the detected position. 
     As the lift table  4  moves upward, the position sensor  20  detects that a top sheet  8  of a bundle of sheets  8  stacked on the lift table  4  reaches a sheet feeding position, and the lift table  4  stops at the position in response to the detection of the position sensor  20 . A plurality of pickup belts  3  is faces the downstream side portion of the lift table  4  in the sheet feeding direction. The pickup belt  3  has a plurality of suction holes, and the top sheet  8  at the sheet feeding position is attracted onto the pickup belt  3  by the suction fan. Thus, when sheets  8  are stacked on the lift table  4 , the sheets  8  are fed one by one in the sheet feeding direction orthogonal to the stacking direction and the width direction by the pickup belt  3 . The pickup belt  3 , the suction fan, and the driver of the pickup belt  3  construct a sheet feeder. 
       FIG.  3    is a schematic view of the sheet feeding tray  100 , illustrating an example of movement of the guide frame  15  and the end fence  5  when the sheet  8  is set on the lift table  4 . The lift table  4  is lowered by the motor  22 , the position sensor  21  detects the position of the lift table  4 , and then the lift table  4  is stopped. The sheet feeding tray  100  is pulled out from the sheet feeding device  210 , and the guide frame  15  is swung upward around the plurality of rotating hinges  17 . At that time, the guide rail  16  secured to the guide frame  15  is also lifted together. The end fence  5  coupled to the guide rail  16  is moved and retracted in the vertical direction while being swung around the rotating hinge  18 , so that the end fence  5  does not hinder the sheet  8  from being set on the lift table  4 . The rotating hinge  17  applies torque to the guide frame  15  against only a direction in which the guide frame  15  is swung downward. Accordingly, when an operator sets the sheet  8  on the lift table  4 , the guide frame  15  is not automatically swung downward even if the operator does not hold the guide frame  15  by hand. 
     The side fences  25   a  and  25   b  are moved in response to the width of the sheet  8  set on the lift table  4 , and the guide frame  15  is swung downward around the rotating hinge  17 , thereby changing the angle of the guide frame  15 . The end fence  5  keeps a vertical posture around the rotating hinge  18  under gravity. Accordingly, when the guide frame  15  is swung downward, the end fence  5  returns to a predetermined position. Then, the end fence  5  and the side fences  25   a  and  25   b  are moved to suitable positions for the length and width of the sheet  8  set on the lift table  4 . 
       FIGS.  4 A and  4 B  are schematic views of the fixing mechanism  40 .  FIG.  4 A  illustrates the fixing mechanism  40  when sheets having a maximum size in the width direction that can be stacked on the sheet feeding tray  100  are set.  FIG.  4 B  illustrates the fixing mechanism  40  when sheets having a minimum size in the width direction that can be stacked on the sheet feeding tray  100  are set. 
     The fixing mechanism  40  includes the slide plate  41  as a second component attached to the side fence  25   b  on the back side of the sheet feeding device  210 , and the fixing plate  42  as a first component secured to the back side plate  31  which is a side face of the device body of the sheet feeding device  210 . 
     The slide plate  41  is formed of a metal sheet and secured to an upper portion of the side fence  25   b  by screws  45 . Two slotted holes  43  extend in the width direction at a predetermined interval in the sheet feeding direction. As illustrated in  FIGS.  4 A and  4 B , the slotted hole  43  is longer than a movement range of the side fence  25   b  in the width direction, in which the side fence  25   b  is movable from a position for regulating the sheet having the maximum width to a position for regulating the sheet having the minimum width. 
     The fixing plate  42  is formed of a sheet metal and secured to the back side plate  31  by screws  46 . A back side portion (on the right side in  FIGS.  4 A and  4 B ) of the slide plate  41  is placed on the fixing plate  42 . Two screw holes into which screws  48  as fastening members are screwed are disposed in the vicinity of the front end (on the left side in  FIGS.  4 A and  4 B ) of the fixing plate  42  at a predetermined interval in the sheet feeding direction. Each screw  48  passes through the slotted hole  43  of the slide plate  41  and is screwed into the screw hole of the fixing plate  42 . 
     As each screw  48  is loosened, the side fence  25   b  can move in the width direction. When the side fence  25   b  slides in the width direction, the slide plate  41  moves in the width direction relative to the fixing plate  42 . After the side fences  25   a  and  25   b  contact the edge of the stacked sheets  8  in the width direction to regulate the position of the sheets  8  in the width direction, the screws  48  are tightened to fasten the slide plate  41  to the fixing plate  42 , thereby immobilizing the side fences  25   b  in the width direction. In the present embodiment, the pair of side fences  25   a  and  25   b  are moved in conjunction with each other by the connector  26 . Accordingly, when one side fence  25   b  is locked, the other side fence  25   a  is also locked so as not to move. 
     An end portion of the slide plate  41  adjacent to the side fence  25   b  is bent downward by 90 degrees, and the bent face serves as a detection portion  47  detectable by the rangefinder  50  to detect the position of the side fence  25   b . The slide plate  41  is formed of the sheet metal that is a material on which infrared light is sufficiently reflected. Accordingly, with the detection portion  47  of the slide plate  41 , the rangefinder  50  can reliably detect the position of the side fence  25   b.    
       FIG.  5    is a schematic view of the sheet feeding tray  100 , illustrating detection of the position of the side fence  25   b  in the width direction by the rangefinder  50 . The pair of side fences  25   a  and  25   b  are moved by the same amount in opposite directions in the width direction by the connector  26 . Accordingly, the side fence  25   a  on the left side in  FIG.  5    (i.e., the front side of the sheet feeding device  210 ) and the side fence  25   b  on the right side in  FIG.  5    (i.e., the back side of the sheet feeding device  210 ) are constantly positioned away from a center O 1  of the sheet feeding tray  100  by the same distance L in the width direction. 
     The rangefinder  50  is attached to a right side face of the guide rail  16  as a stay in  FIG.  5   , and is disposed at a position shifted from the center O 1  by a distance b (mm) toward the right in  FIG.  5    in the width direction. Therefore, the distance L is obtained by adding the distance b to a distance a (mm) that is the length from the rangefinder  50  to the side fence  25   b  measured by the rangefinder  50  (i.e., L=a+b). The distance b (mm) from the center O 1  to the rangefinder  50  is constant. The distance b (mm) is measured in advance and stored in a nonvolatile memory of the sheet feeding device  210 . Using the distance a (mm) measured by the rangefinder  50  and the distances b (mm) from the center O 1  to the rangefinder  50  stored in the nonvolatile memory, the distance L (i.e., positions of the side fences  25   a  and  25   b  relative to the center O 1 ) is obtained. Accordingly, the width of the sheets  8  set on the sheet feeding tray  100  is equal to 2L calculated based on the distance L (i.e., positions of the side fences  25   a  and  25   b ). 
     In the present embodiment, the rangefinder  50  is secured to the guide rail  16  disposed at the center of the upper portion of the sheet feeding tray  100  in the width direction. Thus, as compared with the case in which the rangefinder  50  is disposed at one end of the sheet feeding tray  100  in the width direction, the rangefinder  50  measures a shorter distance from the rangefinder  50  to the side fence  25   b . Accordingly, the distance a from the rangefinder  50  to the side fence b can be detected with high accuracy even if an inexpensive rangefinder having a short measurement range is used. 
     In the present embodiment, the pair of side fences  25   a  and  25   b  are moved by the same distance in opposite directions in the width direction by the connector  26 . Thus, as described with reference to  FIG.  5   , by measuring the distance a from the one side fence  25   b  by the rangefinder  50 , the width of the sheet  8 , i.e., 2L=2 (a+b), set on the sheet feeding tray  100  can be detected. As a result, the measurement range of the rangefinder  50  can be shortened as compared with a comparative configuration in which the rangefinder measures the distance between the side fences  25   a  and  25   b  to detect the width of the sheet  8 . Therefore, the width of the sheet  8  can be detected by an inexpensive rangefinder having a short measurement range. 
       FIGS.  6 A and  6 B  are schematic views of the sheet feeding tray  100  on which a bundle of sheets  8  having the maximum width are stacked.  FIG.  6 A  illustrates a state in which the maximum number of sheets  8  that can be stacked on the sheet feeding tray  100  are set on the lift table  4 , and  FIG.  6 B  illustrates a state in which the several sheets  8  remain on the lift table  4 . 
     When the bundle of sheets  8  having the maximum width is set on the sheet feeding tray  100 , the lift table  4  is lowered to a position at which the position sensor  21  detects the lift table  4 . The screws  48  of the fixing mechanism  40  illustrated in  FIG.  4    are loosened, and the side fences  25   a  and  25   b  are moved to the end portions in the width direction. After the bundle of sheets  8  is set on the sheet feeding tray  100 , the side fences  25   a  and  25   b  are slid in the width direction and brought into contact with an edge of the bundle of sheets  8  in the width direction stacked on the lift table  4 . After the side fences  25   a  and  25   b  contact the edge of the sheets  8  to regulate the position of the sheets  8  in the width direction, the screws  48  are tightened to lock the side fences  25   a  and  25   b . At that time, the slide plate  41  is fastened to the fixing plate  42  as illustrated in  FIG.  4 A . The end fence  5  moves in the sheet feeding direction and contacts the trailing end of the sheets  8  to regulate the position of the trailing end of the sheets  8 . 
     The rangefinder  50  measures the distance a from the rangefinder  50  to the detection portion  47  of the slide plate  41  to detect that the width of the sheets  8  stacked on the lift table  4  is the maximum width based on the measured distance a. 
     The lift table  4  is raised based on the detection result of the position sensor  20 . Specifically, when the position sensor  20  does not detect the upper surface of the bundle of sheets  8  stacked on the lift table  4 , the lift table  4  starts moving upward, and when the position sensor  20  detects the upper surface of the bundle of sheet  8  stacked on the lift table  4 , the lift table  4  stops moving upward. 
     At the sheet feeding position at which the position sensor  20  detects the upper surface of the bundle of sheets  8 , the top sheet  8  of the bundle of sheets  8  is floated by the suction force of the suction fan and attracted onto the pickup belt  3  illustrated in  FIG.  2   . Then, the sheet  8  attracted onto the pickup belt  3  by the suction force of the suction fan is fed to a guide plate  7  by the pickup belt  3 . 
       FIGS.  7 A and  7 B  are schematic views of the sheet feeding tray  100  on which a bundle of sheets  8  having the minimum width are stacked.  FIG.  7 A  illustrates a state in which the maximum number of sheets  8  that can be stacked on the sheet feeding tray  100  are set on the lift table  4 , and  FIG.  7 B  illustrates a state in which the several sheets  8  remain on the lift table  4 . 
     When the bundle of sheets  8  having the minimum width is set on the sheet feeding tray  100 , the lift table  4  is lowered to a position at which the position sensor  21  detects the lift table  4 , and the bundle of sheets  8  having the minimum width is set on the sheet feeding tray  100 . After the bundle of sheets  8  having the minimum width is set on the sheet feeding tray  100 , the side fences  25   a  and  25   b , which are slidable by loosening the screws  48 , are slid in the width direction and brought into contact with an edge of the bundle of sheets  8  in the width direction stacked on the lift table  4  to regulate the position of the sheets  8  in the width direction. Then, the screws  48  are tightened to lock the side fences  25   a  and  25   b . At that time, the slide plate  41  is fastened to the fixing plate  42  as illustrated in  FIG.  4 B . The end fence  5  moves in the sheet feeding direction and contacts the trailing end of the sheets  8  to regulate the position of the trailing end of the sheets  8 . 
     The rangefinder  50  measures the distance a from the rangefinder  50  to the detection portion  47  of the slide plate  41  to detect that the width of the sheets  8  stacked on the lift table  4  is the minimum width based on the measured distance a. 
     The lift table  4  is raised based on the detection result of the position sensor  20 . Specifically, when the position sensor  20  does not detect the upper surface of the bundle of sheets  8  stacked on the lift table  4 , the lift table  4  starts moving upward, and when the position sensor  20  detects the upper surface of the bundle of sheet  8  stacked on the lift table  4 , the lift table  4  stops moving upward. 
     If the rangefinder  50  is disposed on the bottom portion  6  of the sheet feeding tray  100 , the rangefinder  50  is preferably positioned so as to avoid the connector  26  disposed above the bottom portion  6 . Thus, an arrangement position of the rangefinder  50  may be limited. Further, waste such as paper powder from the sheets  8  set on the lift table  4 , dust, or dirt accumulates on the bottom portion  6  of the sheet feeding tray  100  over time. If such waste accumulates on the optical axis of the rangefinder  50 , the light of the rangefinder  50  may be blocked by the waste, and the distance a to the side fence  25   b  may not be measured. 
     To avoid the measurement failure of the rangefinder  50  due to the accumulating waste, a mount on which the rangefinder  50  is mounted may be installed on the bottom portion  6 , and the rangefinder  50  may be disposed at a position one step higher than the bottom portion  6 . In such a configuration, the lowermost position of the lift table  4  may be changed upward to prevent the lift table  4  from colliding with the rangefinder  50 . As a result, the maximum number of sheets  8  that can be stacked on the lift table  4  may be reduced. 
     On the other hand, in the present embodiment, the rangefinder  50  is secured to the guide rail  16  disposed at the upper portion of the sheet feeding tray  100 . Thus, the rangefinder  50  can be positioned above the sheet feeding position which is the highest position of the sheets  8  stacked on the lift table  4  in the height direction. As a result, the rangefinder  50  can be constantly positioned above the sheets  8  stacked on the lift table  4 . Therefore, as illustrated in  FIGS.  6 A to  7 B , the rangefinder  50  does not interfere with the sheets  8  stacked on the lift table  4  in any situation. Further, the rangefinder  50  can reliably measure the distance a to the side fence  25   b  above the sheets  8  stacked on the lift table  4  without the light of the rangefinder  50  being blocked by the sheets  8  stacked on the lift table  4 . 
     The pickup belt  3  is disposed on the downstream side in the sheet feeding direction above the sheet feeding position, and a certain space is disposed on the upstream side of the pickup belt  3  above the sheet feeding position. Therefore, the rangefinder  50  can be disposed in the certain space without upsizing the sheet feeding tray  100  in the vertical direction. 
     Unlike the bottom portion  6 , waste, such as paper powder or dust, adhering to the bundle of sheets  8  does not fall on the upper surface of the bundle of sheets  8  stacked on the lift table  4 . Accordingly, the waste is less likely to accumulate on the upper surface of the bundle of sheets  8  than on the bottom portion  6 . Even if waste adheres to the upper surface of the bundle of sheets  8 , when the top sheet  8  of the bundle of sheets  8  is fed, the waste adhering to the upper surface of the bundle of sheets  8  is conveyed together with the top sheet  8 . Thus, unlike the bottom portion  6 , almost no dust accumulates on the upper surface of the bundle of sheets  8 . Therefore, the light of the rangefinder  50  is not blocked by the accumulating waste, and the rangefinder  50  can reliably measure the distance a to the side fence  25   b.    
       FIG.  8    is a flowchart of a control for determining the positional deviation of the side fences  25   a  and  25   b . When the sheet feeding tray  100  is pushed and installed into the device body of the sheet feeding device  210  (Yes in S 1 ), the rangefinder  50  measures the distance a from the rangefinder  50  to the side fence  25   b , and the controller  150  of the sheet feeding device  210  calculates the width of the sheet  8 , i.e., 2L=2 (a+b) based on the distance a measured by the rangefinder  50  (S 2 ). 
     Next, the controller  150  compares the calculated width of the sheet  8  with an input sheet width input to the controller  150 , which is designated by an operator with a control panel of the image forming system  200  illustrated in  FIG.  1   , a personal computer (PC) connected to the image forming system  200 , or the like. 
     As illustrated in  FIG.  9   , after the operator sets the bundle of sheets  8 , the operator may install the sheet feeding tray  100  into the device body of the sheet feeding device  210  without the side fences  25   a  and  25   b  being in contact with the edge of the sheets  8  in the width direction. In this case, the difference between the input sheet width designated by the operator and the width of the sheet  8  calculated based on the distance a from the rangefinder  50  to the side fence  25   b  measured by the rangefinder  50  is equal to or greater than a threshold (No in S 3 ). 
     If the sheet feeding device  210  starts to feed the sheet  8  with the difference of the widths being equal to or greater than the threshold, that is, in the state in which the side fences  25   a  and  25   b  do not regulate the position of the sheets  8  stacked on the lift table  4  in the width direction, the sheet  8  may be fed with a deviation from a predetermined position in the width direction. As a result, an image to be printed on the sheet  8  may be misaligned from a proper position (i.e., a print misalignment occurs). Alternatively, a conveyance failure may occur, and the printing operation of the image forming system  200  may stop. 
     Therefore, in the present embodiment, when the difference is equal to or greater than the threshold (No in S 3 ), the controller  150 , for example, displays a size mismatch information on a display unit such as the control panel or a monitor of the PC to indicate, to the operator, that the width of the sheet  8  calculated based on the distance a measured by the rangefinder  50  is different from the input sheet width designated by the operator (i.e., the difference of the widths is equal to or greater than the threshold) (S 5 ). 
     When the size mismatch information is indicated to the operator, the operator can recognize that the side fences  25   a  and  25   b  do not regulate the sheet  8  in the width direction, and the operator pulls out the sheet feeding tray  100  again. Then, the operator moves the side fences  25   a  and  25   b  in the width direction and brings the side fences  25   a  and  25   b  into contact with the edge the sheets  8  stacked on the lift table  4  in the width direction to regulate the position of the sheets  8  in the width direction. Accordingly, the conveyance failure and the print misalignment can be prevented. 
     Preferably, in addition to indicating the size mismatch information, the controller  150  inhibits the sheet feeding device  210  from feeding the sheet  8  in the state in which the side fences  25   a  and  25   b  do not regulate the position of the sheets  8  and the image forming system  200  from performing the printing operation. Accordingly, the conveyance failure and the print misalignment can be more reliably prevented. 
     On the other hand, when the difference between the input sheet width designated by the operator and the width of the sheet  8  calculated based on the distance a to the side fence  25   b  measured by the rangefinder  50  is less than the threshold (Yes in S 3 ), the side fences  25   a  and  25   b  regulate the position of the sheets  8  stacked on the lift table  4 . At that time, the sheet feeding device  210  is ready to feed the sheet  8  (S 4 ). 
     The embodiments described above are just examples, and the various aspects of the present disclosure attain respective effects as follows. 
     Aspect 1 
     A sheet stacker such as the sheet feeding device  210  includes a sheet stacking portion such as the lift table  4 , a pair of sheet restrictors such as the pair of the side fences  25   a  and  25   b , a sheet feeder (in the present embodiment, constructed of the pickup belt  3 , the suction fan, and the driver of the pickup belt  3 ), and a position detector such as the rangefinder  50 . A bundle of sheets is stacked on the sheet stacking portion in a stacking direction, and the sheet stacking portion moves in the stacking direction. The pair of sheet restrictors move in a width direction of the bundle of sheets orthogonal to the stacking direction to regulate a position of the bundle of sheets in the width direction. The sheet feeder feeds a top sheet of the bundle of sheets at a sheet feeding position in a sheet feeding direction orthogonal to the stacking direction and the width direction. The sheet feeding position is disposed at a top of the bundle of sheets in the stacking direction. The position detector is disposed between the pair of sheet restrictors in the width direction and above the sheet feeding position in the stacking direction. The position detector detects a position of the pair of sheet restrictors in the width direction. 
     With this configuration, since the position detector is disposed above the sheet feeding position, the position detector can more reliably detect the position of the sheet restrictor than a position detector that is disposed on the bottom portion of the sheet stacker. That is, unlike the bottom portion, waste adhering to the bundle of sheets such as paper dust does not fall on the upper surface of the bundle of sheets. Accordingly, the waste is less likely to accumulate on the upper surface of the bundle of sheets than on the bottom portion. Even if waste adheres to the upper surface of the bundle of sheets, when the top sheet of the bundle of sheets is fed, the waste adhering to the upper surface of the bundle of sheets is conveyed together with the top sheet. Therefore, unlike the bottom portion, almost no waste accumulates on the upper surface of the bundle of sheets  8 . Therefore, the waste does not hinder the position detector from detecting the sheet restrictor. As a result, the position detector can more reliably detect the position of the sheet restrictor than a position detector that is disposed on the bottom portion of the sheet stacker. 
     Aspect 2 
     In Aspect 1, the sheet stacker further includes a stay such as the guide rail  16 . The stay is disposed at a center of the pair of sheets restrictors in the width direction and above the sheet feeding position. The position detector such as the rangefinder  50  is attached to the stay. 
     With this configuration, the position detector such as the rangefinder  50  can be disposed between the pair of sheet restrictors and above the sheets stacked on the sheet stacking portion. 
     Aspect 3 
     In Aspect 2, the sheet stacker further includes a device body, a trailing-end restrictor such as the end fence  5 , and a frame such as the guide frame  15 . The trailing-end restrictor regulates the position of the trailing end of the bundle of sheets stacked on the sheet stacking portion such as the lift table  4  in the sheet feeding direction. The frame is swingably attached the device body. The stay such as the guide rail  16  is attached to the frame. The trailing-end restrictor is attached to the stay, and is slidable in the sheet feeding direction. 
     With this configuration, as described in the above embodiment, since the frame such as the guide frame  15  is swingable, the trailing-end restrictor such as the end fence  5  can be retreated from the center of the device body in the width direction together with the stay such as the guide rail  16 . As a result, when the sheet is set on the sheet stacking portion such as the lift table  4 , the trailing-end restrictor does not become an obstacle, thereby facilitating sheet setting. 
     Aspect 4 
     In any one of Aspects 1 to 3, one of the pair of sheet restrictors such as the side fence  25   b  moves in a first direction and another of the pair of sheet restrictors such as the side fence  25   a  moves in a second direction opposite to the first direction by the same distance in conjunction with each other in the width direction. The position detector such as the rangefinder  50  detects the position of the one of the pair of sheet restrictors. 
     With this configuration, as described in the above embodiment, since one sheet restrictor and the other sheet restrictor are linked with each other and move by the same distance in opposite directions in the width direction, the position of the other sheet restrictor can also be determined by detecting the position of the one sheet restrictor. As a result, the distance from the one sheet restrictor to the other sheet restrictor can be calculated based on the position of the one sheet restrictor, and the width of the sheets stacked on the sheet stacking portion such as the lift table  4  can be calculated. 
     Aspect 5 
     In any one of Aspects 1 to 4, the position detector is a rangefinder such as the rangefinder  50  that measures a distance. 
     With this configuration, the position of the sheet restrictor in the width direction can be determined based on the distance from the position detector such as the rangefinder  50  to the sheet restrictor such as the side fence  25   b  measured by the position detector. 
     Aspect 6 
     In any one of the Aspects 1 to 5, the sheet stacker further includes a fixing unit such as the fixing mechanism  40  that locks the pair of sheet restrictors such as the side fences  25   a  and  25   b  to fix the position of the pair of sheet restrictors in the width direction. The fixing unit includes a detection portion such as the detection portion  47  detectable by the position detector such as the rangefinder  50 . 
     With this configuration, the fixing unit can lock the pair of sheet restrictors such as the side fences  25   a  and  25   b  so as to regulate the position of the sheets stacked on the sheet stacking portion such as the lift table  4  in the width direction. The position of the side fence  25   b  can be detected by detecting the detection portion  47  of the fixing unit. 
     Aspect 7 
     In the Aspect 6, the sheet stacker further includes a device body having a side face. The fixing unit such as the fixing mechanism  40  further includes a first component such as the fixing plate  42 , a second component such as the slide plate  41 , and a fastening member such as the screws  48 . The first component is secured to the side face of the device body. The second component is secured to an upper end of one of the pair of sheet restrictors such as the side fence  25   b  and slidably attached to the first component in the width direction. The second component includes the detection portion such as the detection portion  47 . The fastening member fastens the second component to the first component. 
     With this configuration, since the second component such as the slide plate  41  is fastened to the first component such as the fixing plate  42  with the fastening member such as the screws  48 , the sheet restrictor such as the side fence  25   b  can be locked to immobilize the pair of sheet restrictors in the width direction. When the screws  48  are loosened, the second component can be moved relative to the first component to move the sheet restrictor in the width direction. 
     Since the second component attached to the sheet restrictor includes the detection portion, the position detector such as the rangefinder  50  can detect the position of the sheet restrictor by detecting the detection portion. 
     Aspect 8 
     In any one of Aspects 1 to 7, the sheet stacker further include circuitry such as the controller  150 . The circuitry calculates a width of the bundle of sheets based on the position of the pair of sheet restrictors such as the side fence  25   b  detected by the position detector such as the rangefinder  50 , compares the width of the bundle of sheets with an input sheet width input to the circuitry to obtain a difference between the width of the bundle of sheets and the input sheet width, and indicates that the difference is equal to or greater than a threshold. 
     With this configuration, as described in the above embodiment, the sheet stacked on the sheet stacking portion such as the lift table  4  is prevented from being fed without being regulated in the width direction by the pair of sheet restrictors such as the side fences  25   a  and  25   b . As a result, the sheet can be prevented from being conveyed with a deviation from a predetermined position in the width direction to prevent the print misalignment and the conveyance failure. 
     Aspect 9 
     An image forming system includes the sheet stacker according to any one of Aspects 1 to 8, to feed a sheet with the sheet feeder and an image forming device such as the inkjet printer  230  to form an image on the sheet fed by the sheet feeder of the sheet stacker. 
     With this configuration, the sheet stacked on the sheet stacking portion can be fed to the image forming device to form an image on the sheet. 
     As described above, according to the present disclosure, the position detector can reliably detect the position of the sheet restrictor. 
     The above-described embodiments are illustrative and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present invention. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above. 
     The functionality of the elements disclosed herein may be implemented using circuitry or processing circuitry which includes general purpose processors, special purpose processors, integrated circuits, application specific integrated circuits (ASICs), digital signal processors (DSPs), field programmable gate arrays (FPGAs), conventional circuitry and/or combinations thereof which are configured or programmed to perform the disclosed functionality. Processors are considered processing circuitry or circuitry as they include transistors and other circuitry therein. In the disclosure, the circuitry, units, or means are hardware that carry out or are programmed to perform the recited functionality. The hardware may be any hardware disclosed herein or otherwise known which is programmed or configured to carry out the recited functionality. When the hardware is a processor which may be considered a type of circuitry, the circuitry, means, or units are a combination of hardware and software, the software being used to configure the hardware and/or processor.