Patent Publication Number: US-8528900-B2

Title: Sheet loading unit and sheet handling apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, based upon, and claims the benefit of priority from Japanese Patent Application No. P2011-045424 filed Mar. 2, 2011, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Exemplary embodiments described herein relate generally to a sheet loading unit that feeds stacked sheets (or sheet-like articles) such as mail items one by one, and a sheet handling apparatus including such a sheet loading unit. 
     BACKGROUND 
     Sheet handling apparatuses such as mail item handling machines that handle mail items such as postcards and letters include, for example, a loading unit, a recognition device (OCR), a stacking device, a rejection stacking device, a switchback device, a conveyance path connecting the devices, gates that distribute conveyed sheets to the respective devices, and so on. A plurality of sheets set in a supply unit of the loading unit are separated and taken out one by one by the loading unit, and conveyed to the recognition device. The recognition device recognizes the sheet, and determines a destination of the sheet (for example, the stacking device or the stacking device) and a conveyance route (for example, whether to cause the sheet to pass through the switchback device so as to be turned over). After that, the sheet is conveyed to the determined device via the conveyance path and a gate mechanism, and various types of processing are performed in that apparatus. 
     As the loading unit of such a sheet handling apparatus, a type of loading unit that takes out sheets by suction using a negative pressure has been proposed. This loading unit includes an air suction structure that picks up a sheet by suction using a perforated belt and an air chamber, and a separation roller that picks up a second sheet by suction and separates the second sheet from the first sheet so as to prevent the feeding of two sheets at once. The loading unit also includes an optical sensor for detecting whether there is a sheet in the supply unit that supplies sheets or whether the number of sheets in the supply unit is few or many. If the sensor determines that there is no sheet, the supply unit is operated to deliver the next sheet to a feeding position. 
     Using the sheet loading unit described above, in the case where the sheet has a low light reflectivity (for example, the sheet is black), the optical sensor cannot detect light reflected from the sheet, and thus it is difficult to accurately detect the presence of the sheet. Accordingly, the supply unit pushes the sheet at a speed that is higher than the ideal speed, and the sheets tend to jam in the feeding position. In this case, it may not be possible to feed the front most sheet in a stable manner, or there may be skewing or overlapped feeding of the sheets. 
     There is a need in the field for a sheet loading unit that can suppress anomalies in the sheet supply operation and feed sheets in a stable manner, and a sheet handling apparatus including such a sheet loading unit. Further, there is a need for a sheet loading unit that can detect sheets that have low light reflectivities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically showing a mail item handling apparatus according to an embodiment of the present disclosure; 
         FIG. 2  is a plan view of a loading unit of the sheet handling apparatus; 
         FIG. 3  is a perspective view of a sub-loading unit of the loading unit; 
         FIG. 4  is a perspective view of a take-out belt and a guide of the loading unit; 
         FIG. 5  is a perspective view of the guide; 
         FIG. 6  is a perspective view of a suction mechanism of the loading unit; 
         FIG. 7  is a block diagram showing a control unit of the loading unit and various types of sensors; 
         FIG. 8  is a flowchart illustrating mail item supply operations performed by the loading unit; 
         FIGS. 9(   a ) and  9 ( b ) are diagrams schematically showing detection signal information of a first letter sensor and a second letter sensor of the loading unit. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. 
       FIG. 1  is a block diagram schematically showing a sheet handling apparatus  100  including a sheet loading unit  1  according to an embodiment of the present disclosure. The sheet handling apparatus  100  includes, in addition to the loading unit  1 , a recognition unit  102 , a rejection unit  104 , a switchback unit  106 , and a stacking unit  108 . The handling apparatus  100  of the present embodiment handles sheets such as mail items, but the types of items that can be handled by the handling apparatus  100  are not limited to mail items. 
     A plurality of mail items such as postcards and letters are set in the loading unit  1  in a stacked manner and taken out to a conveyance path  101  by the loading unit performing operations as described later. In the conveyance path  101 , a plurality of sets of conveyance belts (not shown) are provided so as to sandwich the conveyance path  101 . A mail item is sandwiched between the conveyance belts and conveyed. 
     The mail item taken out to the conveyance path  101  is fed to the recognition unit  102 , where a variety of information is read from the mail item. The recognition unit  102  determines information such as the conveyance orientation and sorting destination of the mail item based on the read information. The recognition unit  102  determines the sorting destination by reading address information such as the postal code and address written on the mail item. 
     After the mail item has passed through the recognition unit  102 , the conveyance direction of the mail item is branched by a gate G 1 . Specifically, a mail item that has been recognized by the recognition unit  102  as a mail item to be rejected is conveyed to the rejection unit  104  via the gate G 1 , and stacked in the rejection unit. The other mail items are conveyed to the stacking unit  108  via the gate G 1  and stacked in the stacking unit  108 . 
     At this time, if the recognition unit  102  determines that the conveyance direction of the mail item needs to be reversed, the mail item is fed to the switchback unit  106  via the gate G 1  and a gate G 2 , and its conveyance direction is reversed by the switchback unit  106 . The mail items whose conveyance direction does not need to be reversed are conveyed to the stacking unit  108  while bypassing the switchback unit  106  via the gate G 2 . 
     The mail items that have been delivered to the stacking unit  108  via the conveyance path  101  are sorted and stacked in sorter/stacker pockets (not shown) in accordance with the result of determination by the recognition unit  102 . The mail items sorted and stacked in each sorter/stacker pocket are stacked such that the leading or trailing edges are aligned. 
     The sheet loading unit  1  will be described next in detail.  FIG. 2  is a plan view of the loading unit  1 . The loading unit  1  includes: a loading unit (supply unit)  51  in which a plurality of stacked mail items P are set such that the mail items are substantially vertically upright with respect to the horizontal plane; a supply mechanism  2  that moves the loaded mail items P in the stacking direction so as to supply the frontmost mail item P in the moving direction to a feeding position  87 , which will be described later; a feeding mechanism  56  that feeds the mail item P supplied to the feeding position  87  in the surface direction of the mail item P, (or in other words, in a direction substantially perpendicular to the moving direction in this example) such that the mail item P is taken out to a conveyance path  10 , which will be described later; a suction mechanism  53  that draws, by suction, the frontmost mail item P of the mail items P loaded into the loading unit  51  toward the feeding position  87 ; a separation mechanism  54  that separates a second and subsequent mail item P conveyed following the mail item P taken out from the feeding position  87 , from the first mail item P; an assistance mechanism  55  that assists the mail item P feeding operation by causing a negative pressure to act on the mail item P supplied to the feeding position  87  on an upstream side of the feeding mechanism  56  and rotating in both forward and reverse directions; and a conveyance mechanism  58  that pulls out the mail item P that has passed through the separation mechanism  54  at a speed slightly higher than a feeding speed and conveys the mail item P toward the downstream side. 
     The loading unit  1  includes two sensors  57   a  and  57   b  that detect the passage of the mail item P that has been fed from the feeding position  87  located at an end of the loading unit  51  to the conveyance path  10 , and a plurality of conveyance guides  84 . The sensors  57   a  and  57   b  each include a light emitting portion and a light receiving portion that are provided so as to sandwich the conveyance path  10  through which the mail item P passes, and sequentially detect the passage of the mail item P by the optical axis being blocked by the mail item P. Each conveyance guide  84  guides the movement or conveyance of the mail item P by bringing an edge or surface of the mail item P into contact with the conveyance guide  84 . 
     As shown in  FIGS. 2 and 3 , a plurality of stacked mail items P are placed in the loading unit  51  in an upright manner. In a bottom wall  51   a  of the loading unit  51 , a main belt  126  that causes the lower edges of the mail items P to abut the main belt  126  to convey the mail item P in the stacking direction (in the direction indicated by the arrow F in the drawings) and a pair of sub-belts  125  that adjusts the orientation (inclination) of the mail item P are provided, and the belts  126  and  125  are configured to be capable of being driven independently. The main belt  126  extends substantially across the entire length of the loading unit  51  along the conveyance direction F. The sub-belts  125  are provided on both sides of the main belt  126  in the vicinity of the feeding position  87 . 
     A backup plate  9  is disposed at a position that comes into surface contact with the rearmost mail item P in the moving direction of the plurality of mail items P. The backup plate  9  is, for example, simply connected to the main belt  126 , and moves in the direction indicated by the arrow F in synchronization with the main belt  126  to press the mail items P in the feeding position direction, and thereby supplies the frontmost mail item P in the moving direction to the feeding position  87 . A driving motor  90  drives the main belt, which together with a sub-belt function as the supply mechanism  2 . 
     One of the conveyance guides  84  is provided at a position defining one side of the loading unit  51  along the direction indicated by the arrow F and guides the edge of each mail item P. The other conveyance guides  84  are arranged along the feeding position  87  at an end of the loading unit  51 , and function to stop and position the frontmost mail item P in the moving direction that has been supplied in the direction indicated by the arrow F at the feeding position  87 , as well as guide the mail item P by coming into contact with one side of the mail item P taken out from the feeding position  87 . 
     As shown in  FIG. 2 , the feeding mechanism  56  includes a chamber  52 , a guide  60  and a vacuum pump  61  (or an equivalent). The vacuum pump  61  is connected with the inside of the chamber  52  via a pipe  62 . The feeding mechanism  56  also includes an feeding belt  79  at least a portion of which corresponds to a predetermined region and runs in the direction indicated by the arrow D 1  in the drawing (in the feeding direction of the mail item P) along the feeding position  87 , and a motor  81  that drives the feeding belt  79 . The feeding belt  79  is provided in a tensioned manner by being looped over a plurality of rollers  80  such that at least a part of the feeding belt  79  runs in the direction indicated by the arrow D 1  in  FIG. 2  along the feeding position  87  and the conveyance path  10  continuously extending from the feeding position  87 . 
     The guide  60  is disposed at a position opposed to the feeding position  87  inside the feeding belt  79  with the belt interposed between the guide  60  and the feeding portion  87 . The chamber  52  is disposed on the back side of the guide  60 , specifically, at a position opposed to the feeding position  87  with the feeding belt  79  and the guide  60  interposed between the chamber  52  and the feeding portion  87 . As shown in  FIG. 4 , the feeding belt  79  has many suction holes  79   a . Also, the guide  60  includes, as shown in  FIG. 5 , a plurality of elongated slits  60   a  extending along the running direction D 1  of the feeding belt  79 . 
     As shown in  FIG. 2 , when the interior of the chamber  52  is brought into a vacuum by operating the vacuum pump  61 , a negative pressure (indicated by the arrow S 1  in the figure) acts on the mail item P that has been supplied to the feeding position  87  via an opening (not shown) of the chamber  52  that is opposed to the guide  60 , and the mail item P is attached by suction to the surface of the feeding belt  79  and taken out to the conveyance path  10  from the feeding position  87  along with the running of the feeding belt  79 . 
     At this time, the suction force of the vacuum pump  61  is greater than at least the friction force between the first mail item P and the second mail item P. The feeding mechanism  56  feeds the mail items P at the feeding position  87  one by one to the conveyance path  10 . However, in the case where a plurality of overlapping mail items P are fed to the conveyance path  10 , the overlapping mail items P are separated one by one by the separation mechanism  54 , which will be described later. 
     The suction mechanism  53  includes a chamber  63  disposed on the back side, with respect to the feeding position  87  and the conveyance guide  84 . A blower  65  (or an equivalent) for drawing air into the chamber  63  is connected to the inside of the chamber  63  via a pipe  66 . The chamber  63  is disposed adjacent to the feeding position  87 , between the feeding mechanism  56  and the assistance mechanism  55 , such that its opening (not shown) is opposed to the back surface of the conveyance guide  84 . As shown in  FIG. 6 , the conveyance guide  84  has a plurality of long holes  84   a  having a width that matches the width of the opening of the chamber  63 . The long holes  84   a  are disposed within the opening of the chamber  63 . 
     As shown in  FIGS. 2 and 6 , when the air in the chamber  63  is drawn by operating the blower  65 , an air flow is generated in the direction indicated by the arrow B 1  in the drawings via the long holes  84   a  of the conveyance guide  84 , and the mail item P in the loading unit  51  that is closest to the feeding position  87  is drawn by suction toward the feeding position  87 . After the mail item P drawn to the feeding position  87  has been fed toward the feeding position, the next mail item P is drawn by suction toward the feeding position  87 . In other words, with the suction mechanism  53 , the mail item P that is to be taken out next can be quickly supplied to the feeding position  87 . Accordingly, even if the supply force of the supply mechanism  2  in the direction indicated by the arrow F is reduced, at least the first mail item P can be quickly supplied to the feeding position  87  in a constant and stable manner. Consequently, it is possible to speed up the mail item P feeding operation described above, which is performed by the feeding mechanism  56 . 
     As shown in  FIG. 2 , the separation mechanism  54  is provided opposite to the feeding mechanism  56  and the conveyance path  10  extending toward a downstream side (upward in  FIG. 2 ) of the feeding position  87 . The separation mechanism  54  applies a separation torque that acts in the direction opposite to the feeding direction of the mail item P while causing a negative pressure to act on the mail item P, which is conveyed via the conveyance path  10 . Specifically, even if the second and subsequent mail items P (there are cases where three or more overlapping mail items P are taken out) are conveyed following the mail item P taken out from the feeding position  87 , by operating the separation mechanism  54 , the second and subsequent mail items P are stopped or conveyed in the opposite direction by the negative pressure and separation torque described above, and the second and subsequent mail items P are thereby separated from the first mail item P. 
     More specifically, the separation mechanism  54  includes a separation roller  68  provided along the mail item P feeding direction D 1  so as to be capable of rotation in both forward and reverse directions. The separation roller  68  is formed of a substantially cylindrical rigid body made of a metal material or the like. The outer surface of the separation roller  68  is positioned at a position at which it is exposed to the conveyance path  10 . The separation roller  68  is attached so as to be capable of rotation about a rotation shaft fixedly attached with respect to the conveyance path  10 , and may be rotatable about a cylindrical body  67  including a chamber  64 . The separation roller  68  has many suction holes extending therethrough so as to provide communication between its inner surface and outer surface of the separation roller  68 . The cylindrical body  67  includes the chamber  64  for generating negative pressure, and it is positioned and fixedly provided such that an opening of the chamber  64  faces the conveyance path  10 . 
     The separation mechanism  54  includes an AC servo motor  69  that rotates the separation roller  68  in both forward and reverse directions at the desired torque, and an endless timing belt  70  for transmitting the driving force generated by the motor  69  to the separation roller  68 . The timing belt  70  is provided in a tensioned manner by being looped over a pulley fixed to the rotation shaft of the motor  69  and a pulley (not shown) fixed to the rotation shaft of the separation roller  68 . Furthermore, the separation mechanism  54  includes a vacuum pump  71 . This vacuum pump is connected to the chamber  64  of the cylindrical body  67  via a pipe  72 . 
     When the interior of the chamber  64  is evacuated by operating the vacuum pump  71 , a negative pressure acts on the surface of the mail item P passing through the conveyance path  10  via the opening of the chamber  64  and those suction holes of the separation roller  68  that are opposed to the opening of the chamber, and the mail item P is attached by suction to the outer surface of the separation roller  68 . At this time, when the separation roller  68  is rotating, the conveyance force that acts along the rotation direction of the separation roller  68  is also applied to the mail item P attached by suction to the outer surface of the separation roller  68 . 
     Meanwhile, the AC servo motor  69  drives the separation roller  68  such that a predetermined separation torque in the direction D 2  opposite to the feeding direction D 1  is constantly applied to the separation roller  68 . The separation torque is set so that, in the case where one mail item P is conveyed via the conveyance path  10 , the separation roller  68  that has drawn by suction the single mail item P can rotate along the feeding direction D 1  together with the mail item P, and in the case where a plurality of overlapping mail items P are taken out to the conveyance path  10 , the second and subsequent mail items P can be separated from the first mail item P by stopping the second and subsequent mail items P on the separation roller  68  side or conveying them in the opposite direction. 
     In a state in which one mail item P is properly taken out from the feeding position  87  and conveyed via the conveyance path  10 , the conveyance force in the forward direction (in the direction indicated by the arrow D 1 ) applied to the mail item P by the feeding mechanism  56  is larger than the conveyance force in the opposite direction applied to the mail item P by the separation roller  68  to which the separation torque in the opposite direction D 2  is applied. Therefore, the mail item P is conveyed in the forward direction D 1  while the separation roller  68  rotates in the forward direction D 1  together with the mail item P, or stops or rotates in the direction opposite to the feeding direction. 
     When the separation roller  68  rotates in the opposite direction D 2 , a possibility arises that if a predetermined separation torque is continuously applied, the rotation speed will gradually increase and adversely affect the feeding of the mail item P. For this reason, in the present embodiment, an upper limit is set to the reverse speed of the separation roller  68 . Specifically, an upper limit speed having an absolute value smaller than that of the feeding speed of the mail item P is set. 
     As shown in  FIG. 2 , the assistance mechanism  55  disposed below the suction mechanism  53  in the figure, or in other words, on an upstream side of the feeding mechanism  56  along the mail item P feeding direction D 1  has substantially the same structure as that of the separation mechanism  54  described above. Specifically, the assistance mechanism  55  has an assistance roller  75  provided along the mail item P feeding direction D 1  so as to be capable of rotation in both forward direction and reverse direction D 2 . 
     The assistance roller  75  is attached to a rotation shaft fixedly provided in opposed relationship to the feeding position  87 , or in other words, to a cylindrical body  74  so as to be capable of rotation, and has many suction holes extending therethrough so as to provide communication between its inner surface and outer surface of the assistance roller  75 . The assistance roller  75  is formed of a substantially cylindrical rigid body made of a metal material or its equivalent. The outer surface of the assistance roller  75  is positioned at a position at which it is exposed to the feeding position  87 . The cylindrical body  74  includes a chamber  73  for generating negative pressure, and is positioned and fixedly provided such that an opening of the chamber  73  faces the feeding position  87 . 
     The assistance mechanism  55  includes an AC servo motor  88  for rotating the assistance roller  75  in both forward and reverse directions at the desired torque, and an endless timing belt  76  for transmitting the driving force generated by the motor  88  to the assistance roller  75 . The assistance mechanism  55  includes a vacuum pump  77  connected, via a pipe  78 , to the chamber of the cylindrical body  74  to which the assistance roller  75  is attached so as to be capable of rotation. 
     The assistance mechanism  55  supports the mail item P feeding operation and the separation operation by rotating the assistance roller  75  in both forward and reverse directions at the desired speed and stopping the assistance roller  75 , and turning on or off the negative pressure of the vacuum pump  77 . 
     As shown in  FIG. 2 , the conveyance mechanism  58  that conveys the mail item P fed by the feeding mechanism  56  to a downstream side includes a plurality of conveyance rollers  22  and  83 , a tension roller  26 , conveyance belts  20 ,  82  and  85 , and a tension mechanism  21 . The conveyance roller  83  is disposed on a downstream side of the separation roller  68 , and is adjacent to the conveyance path  10 . The conveyance belt  82  is looped over the conveyance roller  83  and another conveyance roller (not shown). The conveyance belt  20  is looped over the tension roller  26  and one of the conveyance rollers  22 . The conveyance belt  20  defines the conveyance path  10  together with the conveyance roller  83 , and is in contact with the conveyance belt  82 . 
     The tension mechanism  21  includes a tension arm  24  with its center portion being pivotally supported by a pivot  25 . The tension roller  26  is pivotally supported at an end of the tension arm  24 . A tension spring  27  is provided at the other end of the tension arm  24 . The tension arm  24  is thereby biased in a counterclockwise direction about the pivot  25 , and is in resilient contact with a stopper  29 . The tension roller  26  and the conveyance belt  20  are thereby biased in the conveyance path  10  direction, and the conveyance belt  20 , while tension is applied, is in contact with the conveyance belt  82 . Furthermore, the conveyance belt  85  is looped over the other conveyance roller  22  and another conveyance roller (not shown). The conveyance belt  85  is in contact with the conveyance belt  82 . A driving belt  23  that synchronously rotates the two conveyance rollers  22  is looped over the two conveyance rollers  22 . The mail item P is sandwiched between the conveyance belt  82  and the conveyance belts  20  and  85 , and is conveyed by these conveyance belts. 
     As shown in  FIGS. 2 ,  3  and  7 , the loading unit  1  includes a thickness detector  120  that detects the thickness of the mail item P that has been fed, and a count sensor (counter)  121  that counts the number of mail items P that have been fed. The thickness detector  120  and the count sensor  121  are provided at the conveyance path  10  on a downstream side of the sensors  57   a  and  57   b . The loading unit  1  also includes, at the feeding position  87  and on a slightly upstream side of the feeding position  87 , a plurality of sensors that detect whether there is a mail item P in the loading unit  51  or whether the number of mail items P in the loading unit  51  is few or many, such as a first letter sensor  122 , a second letter sensor  127 , and a pushing force detection sensor  123  that detects the pushing force of the supply mechanism  2  to push the mail item P, in particular, the pushing force that acts on the frontmost mail item P. The thickness detector  120  may be configured to also function as the count sensor  121 . 
     The sensors  57   a  and  57   b , the thickness detector  120 , the count sensor  121 , the first letter sensor  122  and the second letter sensor  127  are connected to a control unit  200  of the loading unit  1 , and they send output detection signals to the control unit  200 . The control unit  200  is connected to a driver  202  that drives the vacuum pumps  61 ,  71  and  77 , a driver  204  that drives the blower  65 , a driver  206  that drives the AC servo motors  69 ,  81  and  88 , and a driver  107  that drives the driving motor  90  of the supply mechanism  2 . The control unit  200  drives each driver in response to a detection signal from the sensor. 
     The pushing force detection sensor  123  can be, for example, a pressure sensor, a sensor that uses a lever and a spring to detect the amount by which the lever is pushed, or the like, and determines how much the mail item P is pushed to the feeding position  87 . For example, if it is determined from a detection signal from the pushing force detection sensor  123  that the mail item P is not pushed, or in other words, the detected pushing force is less than a reference value, then the control unit  200  operates the supply mechanism  2  and prompts an operation such as feeding the mail item P forward. If, on the other hand, it is determined that the mail item P is excessively pushed, or in other words, the detected pushing force is higher than a reference value, then the control unit  200  operates the supply mechanism  2  and prompts an operation such as feeding in reverse of the mail item P. The pushing force detection sensor  123  may be a sensor that measures the pushing force itself, or a sensor that only detects the presence of the mail item P. 
     As shown in  FIGS. 2 and 3 , the first letter sensor  122  and the second letter sensor  127  are sensors that detect from different directions whether there is a mail item P at the feeding position  87  or on a slightly upstream side of the feeding position  87 , and at least one of them is an optical sensor. In the present embodiment, the first letter sensor  122  is a transmissive optical sensor that detects transmitted light, and is provided so as to emit detection light in the surface direction of the stacked mail items P in the vicinity of the feeding position  87  and detect the transmitted detection light. The first letter sensor  122  detects whether there is a mail item P on the optical axis of the first letter sensor  122 , or whether the number of mail items P on the optical axis is few or many. For example, if there is a mail item P at the feeding position  87 , the detection light from the first letter sensor  122  impinges on an edge of the mail item P and is blocked, and the first letter sensor becomes dark (off). The first letter sensor  122  thereby outputs mail item presence information (dark). If, on the other hand, there is no mail item P at the feeding position  87 , the detection light passes through the feeding position and is detected by the first letter sensor  122 . The first letter sensor  122  thereby outputs mail item absence information (bright). 
     The second letter sensor  127  is a reflective optical sensor that detects the light reflected from the mail item P. The second letter sensor  127  is provided, for example, in the vicinity of the front wall of the loading unit  51 , and is provided so as to emit detection light toward the mail item P at the feeding position  87  in a direction that intersects with the mail item surface and detect the light reflected feeding position  87 , the detection light from the second letter sensor  127  impinges on the surface of the mail item P and is reflected, and the second letter sensor  127  detects the reflected light and becomes bright (on). The second letter sensor  127  thereby outputs mail item presence information (bright). If, on the other hand, there is no mail item P at the feeding position  87 , the second letter sensor  127  does not receive reflected light and becomes dark (off). The second letter sensor  127  thereby outputs mail item absence information (dark). 
     As shown in  FIG. 8 , in a normal or regular mode, the control unit  200  controls, using the driving motor  90 , the operation of the main belt  126  so as to control the supply speed or feed amount of the mail items P with the supply mechanism  2 . If mail item absence information is output from at least one of the first letter sensor  122  and the second letter sensor  127 , the control unit  200  executes a feed operation of the supply mechanism  2  to feed the mail items P to the feeding position  87 . Specifically, if it is detected that there is no mail item P at the feeding position  87 , the control unit  200  determines that there is no or few mail items P and therefore the feeding of the mail item P may become intermittent, and the control unit  200  performs a supply operation. Thereafter, when mail item presence information is signaled from the first letter sensor  122  or the second letter sensor  127 , the control unit  200  stops the mail item P supply operation of the supply mechanism  2 . 
     The control unit  200  also monitors, during the mail item P feeding operation, the relationship between signal information of the first letter sensor  122  and signal information of the second letter sensor  127 . If a signal relationship that is not normal as compared to that of normal feeding occurs, the control unit  200  ignores the signal information of the optical sensor that has been determined as having an anomaly, and continues the supply operation of the supply mechanism  2  based only on the signal information of the other optical sensor. For example, as shown in  FIG. 9 , the control unit  200  calculates an integral value Δt of mail item absence time or mail item presence time for each predetermined period ΔT, for the mail item presence information or the mail item absence information of the first letter sensor  122  and the mail item presence information or the mail item absence information of the second letter sensor  127 , and compares the calculated integral values. If the difference between the integral values is greater than or equal to a predetermined value, for example, 0.5 or greater, then the control unit  200  determines that the signal information of one of the sensors is not normal as compared to that of normal feeding. 
     Usually, in the case where a mail item P is present in the vicinity of the feeding position  87 , there is not much difference in the time integral values of the mail item presence signals between the first and second letter sensors  122  and  127 . However, for example, if black mail items having a low reflectivity are continuously conveyed, the second letter sensor  127  cannot detect the reflected light so much as it usually does, and thus a difference is likely to occur in the time integral values of the mail item presence signals between the two sensors. For this reason, when the difference between the integral values reaches a predetermined value (for example, 500 msec), the control unit  200  determines that a state that is different from the normal state has occurred, such as, for example, that a black mail item is present at the feeding position  87 , and the control unit  200  ignores the signal information from the second letter sensor  127 . Specifically, the control unit  200  retracts the signal sent from the second letter sensor  127  and enters an anomaly mode. In the anomaly mode, the supply operation is continued based only on the signal information from the first letter sensor  122  while the second letter sensor  127  is being retracted. In other words, even when the second letter sensor is dark and outputs a mail item absence signal, the supply/pushing operation by the mail item absence signal is ignored. 
     The control unit  200  continuously calculates the difference between the integral values, and when the difference returns to a predetermined value, for example, 0.3 (300 msec), the control unit  200  releases the retraction of the second letter sensor  127 , or in other words, the control unit  200  ends the state in which the second letter sensor is ignored and goes back to the normal mode. 
     According to the configuration described above, for example, even if sheets having a low light reflectivity such as black mail items are continuously conveyed and a detection anomaly occurs in the optical sensors, by controlling the supply mechanism  2 , it is possible to prevent supply operation anomalies such as jamming of mail items in the vicinity of the feeding position. Consequently, the frontmost mail item P can be fed in a stable manner without causing skewing or overlapped feeding. 
     With the mail item handling apparatus including the loading unit configured as described above, it is possible to suppress anomalies in the sheet supply operation and perform an appropriate sheet supply operation. Consequently, sheets can be taken out in a stable manner, and a sheet handling apparatus with an improved handling speed can be obtained. 
     The present disclosure is not limited to the embodiments given above, and can be embodied by modifying the constituent elements without departing the spirit of the disclosure when the disclosure is carried out. The present disclosure can be implemented in various forms using appropriate combinations of the constituent elements disclosed in the embodiments given above. For example, some constituent elements may be removed from the constituent elements disclosed in the embodiments. Furthermore, the constituent elements of different embodiments may be combined as appropriate. 
     The number of sensors that detect the presence of a mail item at the feeding position  87  is not limited to two, and may be three or more. The first and second letter sensors  122  and  127  are not limited to the transmissive optical sensor and the reflective optical sensor, and may be configured using other sensors. Furthermore, the configuration of the second letter sensor  127  is not limited to the configuration in which detection is performed in the direction that intersects with the mail item surface, and the second letter sensor may be disposed, for example, on the bottom wall  51   a  of the loading unit  51 , and configured to detect mail items from under the mail items along the mail item surface direction. The sheets used in the present invention are not limited to mail items, and the present invention is applicable to any other types of sheets.