Abstract:
A method for rapidly stacking and dropping sheets is provided. The method comprises receiving a plurality of sheets from a sheet delivery device onto a landing platform of a stacking and dropping apparatus. The method also comprises lowering the landing platform from an original upper position. Lowering comprises maintaining a drop height between the sheet delivery device and a stack of delivered sheets. The method also comprises sending a delay signal that comprises a command to delay the receiving of the plurality of sheets from the sheet delivery device. The delay signal is sent in response to a detection of a predetermined stack height of the stack of delivered sheets. The method also comprises sending a movement signal, immediately following the sending of the delay signal. The movement signal comprises a command to move the landing platform, such that the plurality of sheets are received by a catch tray unit. The method also comprises dropping, utilizing the catch tray unit, the plurality of sheets onto a sheet receiving device. The method also comprises returning, automatically, the landing platform to the original upper position. The method also comprises sending a resume signal, that includes a command to the sheet delivery device to resume sheet delivery. The delay signal, movement signal and resume signal are sent by a central processor component of the stacking and dropping apparatus.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a Divisional Application of U.S. patent application Ser. No. 14/841,885, filed Sep. 1, 2015, which is based on and claims the benefit of U.S. Provisional Patent Application Ser. No. 62/047,886, filed Sep. 9, 2014, entitled Dual-Stage Sheet Stacking and Dropping Apparatus, the contents of which is hereby incorporated in its entirety. 
     
    
     BACKGROUND 
       [0002]    A stacking apparatus is often placed adjacent to, or attached to, a discharge end of a sheet-feeding device, conveyor system, or other sheet-delivery mechanism. The stacking apparatus generally functions to allow discharged, for example, envelopes, labels, stock, cards, sheets, and the like to stack neatly until an operator or an automated system removes the discharged sheets from the stacking apparatus. Preferably, the stacking apparatus can be adjusted in two dimensions to accommodate items of varying dimensions. 
         [0003]    A dropper-style stacking apparatus generally includes a mechanism for dropping a predetermined number of stacked sheets into an appropriate receiving device below, such as a catch bin, conveyor system or secondary sheet-feeding apparatus. When a dropper-style stacking apparatus is designed to accommodate a relatively large stack of sheets, the sheet-feeding device must generally feed sheets slowly into such a large-capacity stacking apparatus. The slower sheet-feeding speed is required because the sheets initially being fed into a larger stacking apparatus must fall a greater distance. Sheets dropping a greater distance at high speeds often tend to become disorganized on their descent into the larger stacking device, creating undesirable jams. 
       SUMMARY 
       [0004]    A method for rapidly stacking and dropping sheets is provided. The method comprises receiving a plurality of sheets from a sheet delivery device onto a landing platform of a stacking and dropping apparatus. The method also comprises lowering the landing platform from an original upper position. Lowering comprises maintaining a drop height between the sheet delivery device and a stack of delivered sheets. The method also comprises sending a delay signal that comprises a command to delay the receiving of the plurality of sheets from the sheet delivery device. The delay signal is sent in response to a detection of a predetermined stack height of the stack of delivered sheets. The method also comprises sending a movement signal, immediately following the sending of the delay signal. The movement signal comprises a command to move the landing platform, such that the plurality of sheets are received by a catch tray unit. The method also comprises dropping, utilizing the catch tray unit, the plurality of sheets onto a sheet receiving device. The method also comprises returning, automatically, the landing platform to the original upper position. The method also comprises sending a resume signal, that includes a command to the sheet delivery device to resume sheet delivery. The delay signal, movement signal and resume signal are sent by a central processor component of the stacking and dropping apparatus. 
         [0005]    These and other various features and advantages that characterize the claimed embodiments will become apparent upon reading the following detailed description and upon reviewing the associated drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1 a    illustrates an exemplary process flow diagram depicting a process flow in which embodiments described herein are useful. 
           [0007]      FIG. 1 b    is a block diagram of an exemplary dual-stage sheet stacking system in accordance with an embodiment of the present invention. 
           [0008]      FIG. 2 a - c    are perspective views of a dual-stage dropper with which embodiments described herein may be useful. 
           [0009]      FIG. 3 a - c    illustrate isolated perspective views of several embodiments of the second-stage module of the dual-stage dropper. 
           [0010]      FIG. 3 d - e    illustrate isolated perspective views of one embodiment of the first-stage module of the dual-stage dropper. 
           [0011]      FIG. 4  illustrates an exemplary method of stacking and dropping in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    To provide an understanding of some of the basic principles of some embodiments of the present invention, reference is made to the embodiments shown in the drawings, and specific terms will be employed to describe the same. It should be understood, however, that no limitation of the scope of the invention is thereby intended. Instead, the invention includes any and all such alterations and improvements of the illustrated device that would normally occur to one skilled in the art to which the invention relates. 
         [0013]    These and other features of some embodiments of the present invention will be more readily and fully understood by reference to the accompanying drawings and the detailed description that follows. 
         [0014]    Many existing accumulators/droppers are configured to accumulate and drop material in stacks under 2 inches in height. Most droppers focus on a smaller count of material that creates a stack under 1 inch in height. The dropper platforms are typically within 1 to 1.5 inches below the discharge point of the feeder/dispenser to which they are attached. With this relatively shallow position, the material or sheet can leave the feeder at an angle under 20 degrees relative to horizontal, which may fall within an angle range for maximizing the speed at which the material can be fed into the dropper. The steeper the downward angle the material is when leaving the feeder, the more the material needs to bend or deflect back to horizontal in order to stack properly. This makes it more difficult to control the material after leaving the feeder, which leads to increased jams as speeds increase. Often, existing droppers slow the feeder to reduce jams. While jams may be reduced, a job time is increased, which is inefficient. 
         [0015]    It is desired for an accumulator/dropper to be configured such that it can accumulate taller stacks, for example with heights in excess of 2 inches. However, as the dropper is lowered relative to the discharge point from the feeder in order to accumulate taller stacks, the material has to dispense at increasingly steeper angles, which increases the risk of jams. 
         [0016]      FIG. 1 a    illustrates an exemplary process flow diagram that may be useful in one embodiment of the present invention. The dual-stage dropper  10 , in one embodiment, is configured for placement adjacent to a discharge end of a sheet-feeding device, conveyor system, or other appropriate sheet-delivery mechanism  12 . The dual stage dropper  10 , in one embodiment, is configured to receive and handle envelopes, cards, labels, sheets, and the like (hereinafter sheets) discharged by the sheet-delivery mechanism  12 . The dual stage dropper, in one embodiment, is configured to drop the sheets uniformly onto a sheet receiving device  14 . 
         [0017]      FIG. 1 b    illustrates a block diagram of an exemplary dual-stage sheet stacking system in accordance with an embodiment of the present invention. The dual-stage dropper  10 , in one embodiment, includes a central processing unit (CPU)  18 , a first stage module  100  and a second stage module  200 . In one embodiment, the first stage module  100  includes; a first actuator  155 , a second actuator  165 , a first catch tray unit  150  and a second catch tray unit  160 . The second-stage module, in one embodiment, includes a first-stage-stack sensor  20 , a second-stage-stack sensor  22 , a vertical actuator unit  240 , a horizontal actuator unit  250  and a landing platform  253 . 
         [0018]    Adding a second stage module  200  to the dropper  10  assists the material in dispensing at the optimum angle for maximized speed, while accommodating taller stacks and maintaining an efficient speed. The second stage module  200  provides a platform at a position relative to the feeder discharge such that the material angle is in a range allowing for efficient completion of a stacking job. The second stage platform  253  then descends as material is accumulated so the dispensed material is constantly fed on to the stack at a constant discharge angle. When the stack is complete, the second stage platform  253  is retracted to place the stack into the first stage module  100 . The second stage platform  253  can then ascend to the upper most position ready for the next stack to start dispensing. 
         [0019]    The second stage platform  253  can be controlled through sensors and/or software (described in further detail below) to adjust the uppermost position so it is in the optimum position for the specific type and size of material being dispensed. The second stage module  200  can also be controlled to retract prior to completion and return to the upper most position while the feeder is completing the dispensing of the stack into the first stage module  100 . In applications requiring stack heights shorter than the first stage module  100  can hold, the second stage module  200  can retract and place the first stack in the first stage module  100 , then move into the upper most position and the next stack can start and potentially finish prior to the first stage module  100  dropping the earlier stack onto the main system line. This can substantially increase the overall cycle rate of the system. With the second stage platform  200  integrated to the first stage module  100 , the limiting factor to performance becomes a speed of the feeder/dispenser. 
         [0020]    Referring to  FIGS. 3 b  and 3 c   , the second-stage module  200  includes a pair of attachment units  210  designed to connect the second-stage module  200  to the first-stage module  100 . Each of the pair of attachment units  210 , in one embodiment, comprises a sliding block  214  affixed to one of two outer ends of a top side  221  of a second-stage upper platform  220 . A central bore  215 , passing through a length of the sliding block  214 , is sized to slidably receive a disengagement rail  218 , in one embodiment. A disengagement rail first end  217  extends through the central bore  215  toward the sheet-delivery mechanism  12  and is firmly attached to a fixed block  212 , in one embodiment. 
         [0021]    The dual-stage dropper  10 , in one embodiment, can be adjusted by an operator to accommodate sheets of varying dimensions. Further, mechanical interaction between a second-stage module  200  and a first-stage module  100  of the dual-stage dropper  10  may allow, in one embodiment, the stacking of relatively large stacks of sheets being fed by a sheet-delivery mechanism  12  at relatively high speeds without creating frequent paper jams. In the unlikely event of a paper jam, the dual-stage dropper  10  is configured for easy separation of the second-stage module  200  from the first-stage module  100 , so that an operator can efficiently clear a jammed sheet from the dual-stage dropper  10 . 
         [0022]    In one embodiment, one of the pair of fixed blocks  212  is affixed proximate to a first end  121  of on an upper side  123  of a first-stage upper platform  120 . In one embodiment, the other one of the pair of fixed blocks  212  is affixed proximate to a second end  122  of the upper side  123  of the first-stage upper platform  120 . Each one of the pair of sliding blocks  214  is held adjacent to one of the pair of fixed blocks  212  by a holding mechanism  216 , such as a release clip. In one embodiment the release clip is a manually operated release clip. A hinged end  216 A of the holding mechanism  216 , in one embodiment, is hingedly attached to the sliding block  214 . A nose end  216 B of the release clip  216  is snapped into a securing recess  213  in the fixed block  212 . This allows an operator to draw the second-stage module  200  away from the first-stage module  100  by lifting each of the pair of nose ends  216 B out of each of the securing recesses  213 . However, in another embodiment, a different holding mechanism  216  can be used that allows for separation of the first-stage module  100  and second-stage module  200 . The operator may then be able to slide both of the pair of sliding blocks  214  along both of the pair of disengagement rails  218  toward a disengagement rail second end  219  opposite the disengagement rail first end  217 . 
         [0023]    A plurality of sheet-width-adjustment slots  213  pass through the second-stage upper platform  220 , in one embodiment. One or more fastening mechanisms  215 , such as set screws, pass through the plurality of sheet-width-adjustment slots  213 , in one embodiment. In one embodiment, the fastening mechanism  215  is a set screw. The one or more fastening mechanisms  215 , may be, a threaded attachment with one or more back plates  230  descending vertically from a bottom side  222  of the second-stage upper platform  220 . This may, in one embodiment, allow an operator to loosen the one or more fastening mechanisms  215  to reposition the one or more back plates  230  to accommodate sheets of different width being delivered to the dual-stage dropper  10  from the sheet-delivery mechanism  12 . In another embodiment, a lower edge  233  of a centrally positioned back plate  231  descends to a position just above an upper-most vertical position of a retractable landing platform  253  (described below). In one embodiment, at least one material control band (not shown) is mounted to the centrally positioned back plate  231  or the one or more back plates  230 . The material control band is made of flexible material, for example spring steel, designed to absorb the force of the sheets being fed into the dual-stage dropper  10  by the sheet-delivery mechanism  12 . However, in another embodiment, another flexible material is used to absorb the force. 
         [0024]    In one embodiment, a vertical actuator unit  240  is affixed to the second-stage upper platform  220 . Upon receiving an electronic signal, for example from the CPU  18  in one embodiment, at least one vertical piston  241  is configured to descend or ascend vertically from the vertical actuator unit  240  through the second-stage upper platform  220 , with a distal end  243  of the at least one vertical piston  241  attached to a horizontal actuator unit  250 . Upon receiving an electronic command, in one embodiment, at least one horizontal piston  251  may also be configured to extend or retract horizontally from the horizontal actuator unit  250 . In one embodiment, a distal end of the at least one horizontal piston  252  is attached to a retractable landing platform  253 . 
         [0025]    In one embodiment, a second-stage-stack sensor  22  is mounted on the centrally positioned back plate  231  in a position indicated by position  260 . In one embodiment, the second-stage-stack sensor  22  is a photoelectric sensor. In another embodiment, the second-stage-stack sensor  22  is a different sensor type from the first-stage-stack sensor  20 . In another embodiment, the first-stage-stack sensor  20  and second-stage-stack sensor  22  are the same sensor type. The second-stage-stack sensor  22  may be configured, in one embodiment, to be directed downward toward the retractable landing platform  253 . 
         [0026]    In one embodiment, the second-stage-stack sensor  22  is configured to sense a distance to a top of a growing stack of sheets accumulating on the retractable landing platform  253 . For example, when the top of the growing stack of sheets reaches a predetermined level, the second-stage-stack sensor  22  sends a signal to the CPU  18  to direct the vertical actuator unit  240  to lower the retractable landing platform  253 , for example, utilizing at least one vertical piston  241 . The retractable landing platform  253  continues to descend so that the top of the growing stack of sheets stays at a uniform distance, for example no more than 2 inches below a discharge level of the sheet-delivery mechanism  12 . 
         [0027]      FIG. 3 d - e    illustrate isolated perspective views, of one embodiment of the first-stage module of the dual-stage dropper. In one embodiment, a first side member  130  is perpendicularly affixed to a lower side  124  of the first end  121  of the first-stage upper platform  120 . In one embodiment, a second side member  140  is perpendicularly affixed to the lower side  124  of the second end  122  of the first-stage upper platform  120 . A plurality of first-stage suspension rails  170  are mounted in parallel between the first side member  130  and the second side member  140 . 
         [0028]    In one embodiment, a first sliding catch tray unit  150  may be configured such that it is slidably mounted on the plurality of suspension rails  170  in proximity to the first side member  130 . A first catch tray sheet-length-adjustment slot  113  passes through the first-stage upper platform  120 . One or more first catch tray fastening mechanisms  114 , such as set screws, pass through the first catch tray sheet-length-adjustment slot  113 . The one or more first catch tray fastening mechanisms  114  are in threaded attachment with the first sliding catch tray unit  150 . The operator may be able to, in one embodiment, loosen the one or more first catch tray fastening mechanisms  114  to reposition the first sliding catch tray unit  150  to accommodate sheets of different length being delivered to the dual-stage dropper  10  from the sheet-delivery mechanism  12 . The first sliding catch tray unit  150  may comprise, in one embodiment, a first side panel  151  that projects perpendicularly away from the first-stage upper platform  120 . A first stacking ledge  153 , in one embodiment, is perpendicularly affixed to a first bottom edge  152  of the first side panel  151 . The first stacking ledge  153 , in one embodiment, projects away from the first side member  130  and toward the second side member  140 . 
         [0029]    A second sliding catch tray unit  160 , in one embodiment, is slidably mounted on the plurality of suspension rails  170  such that is in proximity to the second side member  140 . A second catch tray sheet-length-adjustment slot  118  passes through the first-stage upper platform  120 . One or more second catch tray fastening members  119  pass through the second catch tray sheet-length-adjustment slot  118 . In one embodiment, the fastening mechanisms  119  could be set screws. In another embodiment, the fastening mechanisms  119  could be another appropriate fastener. 
         [0030]    In one embodiment, the one or more second catch tray fastening members  119  are in threaded attachment with the second sliding catch tray unit  160 . The operator may be able to, in one embodiment, loosen the one or more second catch tray fastening members  119  to reposition the second sliding catch tray unit  160  to accommodate sheets of different length being delivered to the dual-stage dropper  10  from the sheet-delivery mechanism  12 . The second sliding catch tray unit  160  may also comprise, in one embodiment, a second side panel  161  that projects perpendicularly away from the first-stage upper platform  120 . A second stacking ledge  163 , in one embodiment, is perpendicularly affixed to a second bottom edge  162  of the second side panel  161 , the second stacking ledge  163  projects away from the second side member  140  and toward the first side member  130 . 
         [0031]    The first sliding catch tray unit  150 , in one embodiment, further comprises a first catch tray actuator  155  in mechanical communication with the first side panel  151 . Upon receipt of an electronic signal from the CPU  18 , the first catch tray actuator  155 , in one embodiment, is configured to cause the first side panel  151  (and the attached first stacking ledge  153 ) to slide along the plurality of suspension rails  170  toward the first side member  130 . Upon receipt of another electronic signal from the CPU  18 , in one embodiment, the first catch tray actuator  155  is configured to cause the first side panel  151  (and the attached first stacking ledge  153 ) to return to an original position on the plurality of suspension rails  170 . 
         [0032]    The second sliding catch tray unit  160 , in one embodiment, further comprises a second catch tray actuator  165  that may be configured such that is in mechanical communication with the second side panel  161 . Upon receipt of an electronic signal from the CPU  18 , in one embodiment, the second catch tray actuator  165  may be configured to cause the second side panel  161  (and the attached second stacking ledge  163 ) to slide along the plurality of suspension rails  170  toward the second side member  140 . In one embodiment, upon receipt of another electronic signal from the CPU  18 , the second catch tray actuator  165  may be configured to cause the second side panel  161  (and the attached second stacking ledge  163 ) to return to an original position on the plurality of suspension rails  170 . In one embodiment, the first catch tray actuator  155  and the second catch tray actuator  165  may be operated simultaneously. 
         [0033]    During operation of the dual stage dropper  10 , in one embodiment, the sheet-delivery mechanism  12  may initially deliver a plurality of sheets onto the retractable landing platform  253  of the second-stage module  200 . As described above, the retractable landing platform  253  may be gradually lowered by the vertical actuator unit  240  so that the top of the growing stack of sheets stays at a distance of approximately 1.5 inches to 2 inches below the discharge level of the sheet-delivery mechanism  12 . Maintenance of a relatively consistent and minimal drop distance allows sheets to be discharged at a rapid rate by the sheet-delivery mechanism  12 . 
         [0034]    In one embodiment, when a first predetermined number of sheets is delivered onto the retractable landing platform  253 , a signal is sent, by the CPU  18 , to the sheet-delivery mechanism  12  to interrupt delivery of sheets to the dual stage dropper  10 . A signal may, in one embodiment, be sent from the CPU  18  to the horizontal actuator unit  250  causing the at least one horizontal piston  251  to retract the retractable landing platform  253 . In one embodiment, the first predetermined number of sheets is then dropped onto the first stacking ledge  153  and second stacking ledge  163  of the first-stage module  100  positioned below. The first predetermined number of sheets is, in one embodiment, cradled between the first stacking ledge  153  and the second stacking ledge  163  until a signal is sent by the CPU  18  to the first catch tray actuator  155  and the second catch tray actuator  165  to cause the first stacking ledge  153  and the second stacking ledge  163  to separate and drop the first predetermined number of sheets onto a sheet receiving device  14  configured to receive the first-predetermined number of sheets. In one embodiment, the sheet receiving device  14  comprises a catch bin, conveyor or a secondary sheet-delivery system  14 . 
         [0035]    After the first predetermined number of sheets is dropped from the retractable landing platform  253 , the CPU  18  may, in one embodiment, be configured to send a signal to the horizontal actuator unit  250  to return the retractable landing platform  253  to an extended position. The CPU  18  may be configured, in one embodiment, to also send a signal to the vertical actuator unit  240  to return the horizontal actuator unit  250  to an upper position. The CPU  18  may then be configured to send a signal to the sheet-delivery mechanism  12  to begin delivery of a second predetermined number of sheets to the dual stage dropper  10 . As described above, in one embodiment, the retractable landing platform  253  may gradually lowered by the vertical actuator unit  240  so that the top of the growing stack of sheets stays at a distance of approximately 1.5 inches to 2 inches below the discharge level of the sheet-delivery mechanism  12 . 
         [0036]    A first-stage-stack sensor  20 , in one embodiment, maybe mounted on the horizontal actuator unit  250  in a position indicated by  270 . The first-stage-stack sensor  20  may be directed toward a gap between the first stacking ledge  153  and the second stacking ledge  163  of the first-stage module  100 . The first-stage-stack sensor  20  is lowered along with the horizontal actuator unit  250  as sheets are fed onto the retractable landing platform  253 . If the first-stage-stack sensor  20  is lowered to a point where it senses a presence of sheets cradled in the gap between the first stacking ledge  153  and the second stacking ledge  163  of the first-stage module  100 , the first-stage-stack sensor  20  is, in one embodiment, configured to send a signal to the CPU  18 , which may automatically direct the sheet-delivery mechanism  12  to interrupt the flow of sheets being delivered to the dual stage dropper  10  until the sheets are dropped from the from the first-stage module  100 . In another embodiment, the sensor may be configured to remain stationary and detect a distance between the retractable landing platform  253  and a stack of sheets. After the sheets are cleared from the first-stage module  100 , in one embodiment, the CPU  18  may be configured to automatically direct the sheet-delivery mechanism  12  to resume delivery of the second predetermined number of sheets to the dual stage dropper  10 . Delivery of additional pre-set numbers of sheets can be cycled through the dual-stage dropper  10  as described above. 
         [0037]      FIG. 4  illustrates an exemplary method of stacking and dropping in accordance with an embodiment of the present invention. In block  402 , the sheet-delivery mechanism  12  initially delivers a plurality of sheets onto the retractable landing platform  253  of the second-stage module  200 , in one embodiment. In block  404 , the retractable landing platform  253  may, in one embodiment, be gradually lowered by a vertical actuator unit  240 . In one embodiment, a top of a growing stack of sheets stays at a distance of approximately 1.5 to 2 inches below a discharge level of the sheet-delivery mechanism  12 . Maintenance of a consistent and minimal drop distance may, in one embodiment, allow sheets to be discharged at a rapid rate by the sheet-delivery mechanism  12  without the creation of jams. 
         [0038]    When a first, predetermined, number of sheets is delivered onto the retractable landing platform  253 , in block  412 , a signal is sent to the sheet-delivery mechanism  12  by the CPU  18  to interrupt delivery of sheets to the dual-stage dropper  10 . Substantially, in one embodiment, in block  406 , a signal is sent from the CPU  18  to the horizontal actuator unit  250  to retract the retractable landing platform  253 . The first predetermined number of sheets may be dropped below, and cradled between, a first sliding catch tray unit  150  and a second sliding catch tray unit  160  of the first-stage module  100 , in one embodiment. Upon receiving a command from the CPU  18 , for example in block  408 , one or more actuators may separate the first sliding catch tray unit  150  from the second sliding catch tray unit  160 , dropping the first predetermined number of sheets into a sheet receiving device  14 , in one embodiment. The sheet receiving device  14  may, in one embodiment, be a secondary sheet-feeding device located below the retractable landing platform  253 . 
         [0039]    After the first predetermined number of sheets is dropped from the retractable landing platform  253 , the CPU  18  may, in one embodiment, send a signal to the horizontal actuator unit  250  to return the retractable landing platform  253  to an extended position, in block  410 . The CPU  18  may also in block  410 , send a signal to the vertical actuator unit  240  to return the horizontal actuator unit  250  to an upper position. The CPU  18  may then, in one embodiment, send a signal to the sheet-delivery mechanism  12 , in block  414 , to begin delivery of a second predetermined number of sheets to the dual stage dropper  10 . 
         [0040]    The retractable landing platform  253 , in one embodiment, after receiving the signal for the second delivery, may be gradually lowered by the vertical actuator unit  240  so that the top of the growing stack of sheets stays at a distance of at most 1.5 inches or at most 2 inches below the discharge level of the sheet-delivery mechanism  12 . In one embodiment, a first-stage-stack sensor  20  is mounted on the horizontal actuator unit  250  and directed toward a gap between the first sliding catch tray unit  150  and the second sliding catch tray unit  160  of the first-stage module  100 . In one embodiment, the first-stage-stack sensor  20  is lowered along with the horizontal actuator unit  250  as sheets are fed onto the retractable landing platform  253 . When the first-stage-stack sensor  20  is lowered to a point where it senses a presence of sheets cradled in the gap between the first sliding catch tray unit  150  and the second sliding catch tray unit  160 , in one embodiment, the first-stage-stack sensor  20  automatically sends a signal to the CPU  18 , which directs the sheet-delivery mechanism  12  to interrupt the flow of sheets being delivered to the dual stage dropper  10  until the sheets are dropped from the from the first-stage module  100 . In one embodiment, the sensing of existing sheets and signaling a stop of the dual stage dropper  10  lowers the risk of jam. 
         [0041]    In one embodiment, after the sheets are cleared from the first-stage module  100 , the CPU  18  directs the sheet-delivery mechanism  12  to resume delivery of the second predetermined number of sheets to the dual stage dropper  10 . Delivery of additional pre-set numbers of sheets can be cycled through the dual-stage dropper  10  as described above. 
         [0042]    The addition of a second-stage module  200  allows for an increased stack height that can be accommodated by the dual stage dropper  10 . For example, the dropper  10  may be able to accommodate stacks in excess of 2 inches, or even in excess of 3 inches, or even in excess of 4 inches, or even in excess of 5 inches, or even in excess of 6 inches. 
         [0043]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.