Abstract:
An efficient package sorting system in which a stationary matrix of multi-directional conveyor cells sorts packages to a plurality of destination locations. In a preferred embodiment, the matrix delivers sorted packages to a plurality of lift assemblies that further transfer the packages to receiving conveyors or chutes on different levels. According to another preferred aspect of the system, a selectively elevating stop bar is provided to control side transfer between conveyor cells. A controller is operative to plan a path for each of the objects from the input cell to a destination location, to monitor availability of successive conveyor cells along the path, and to cause an object to be moved to the next conveyor cell along its path only when the next conveyor cell is available. The system may include a plurality of sensors positioned to sense the passage of objects from one conveyor cell to another, the sensors being connected to the controller. The sensor input is used to guide the packages through the matrix, and to optimize the speed at which a plurality of packages can be sorted.

Description:
RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 08/595,672 filed Feb. 2, 1996, now U.S. Pat. No. 6,005,211, issued Dec. 21, 1999. 
     Appendix A hereto is a copy of U.S. application Ser. No. 08/421,675, filed Apr. 12, 1995, now abandoned entitled “Method and Apparatus for Lifting Packages.” Appendix B hereto is a copy of an application entitled “Conveyor Control System” issued Jan. 27, 1998 as U.S. Pat. No. 5,711,410, which was originally filed with the above-reference patent application Ser. No. 08/595,672. Both such applications are commonly owned by the assignee of the present application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to material handling systems utilizing conveyors, and more particularly relates to a package or parcel transfer and sorting system useful in a hub facility of a package delivery system. 
     BACKGROUND ART 
     In a large package delivery system, millions of packages picked up from thousands of locations over a large geographical area must be transported, primarily by truck and airplane, to a correspondingly large number of destinations that are also scattered over a large area. Such delivery services are offered within guaranteed times as short as one day. To meet a rigorous schedule and provide accurate deliveries, a package delivery company must use automated transfer systems to match incoming packages with proper transport that is heading to their destinations. 
     Belt and roller conveyor systems have often been used in package sorting systems to move packages from incoming loading docks to outgoing transport. Typically, conveyors carry packages unloaded from a truck to a worker who manually sorts them by reading address information on shipping labels attached to the packages. The worker then places the packages onto receiving conveyors or chutes which carry the packages either to a loading dock for loading onto outgoing trucks, or to another sorting station for a narrower breakdown of destinations. A distribution hub in a package delivery system may have as many as 20 to 60 sorting stations operating simultaneously. By providing vertically stacked rows of receiving conveyors, the sorting operation could be accommodated in a relatively small amount of floor space. 
     To automate handling of articles in conveyor systems, conveyor diverter assemblies have been developed. Examples of conveyor diverters are shown in U.S. Pat. No. 4,798,275 to Leemkuil et. al., and U.S. Pat. No. 4,174,774 to Bourgeois, both of which are incorporated herein by reference. However, such diverters are used primarily to divert articles from a main linear conveyor. Thus, such systems occupy a relatively large amount of space. This problem is overcome by the rotary sorter system shown in U.S. Pat. No. 5,284,252 to Bonnet, assigned to the assignee of the present application. In this system, destination codes on shipping label of packages are machine read, and the packages are transferred onto powered conveyor modules mounted on a rotating distribution assembly. The individual module is then rotated and elevated or lowered into alignment with one of a plurality of destination conveyors that are spaced apart both horizontally and vertically. After such alignment, the modules rollers are operated to discharge the package onto the destination conveyor. In the Bonnet system, packages can be rapidly sorted without human intervention by an apparatus that occupies a small amount of floor space. 
     For some circumstances, it would be advantageous to have a compact package sorting system that did not require moving a conveyor module holding a package from a loading point to a discharge point. 
     SUMMARY OF THE INVENTION 
     The present invention provides an efficient package sorting system in which a stationary matrix of multi-directional conveyor cells sorts packages to a plurality of destination locations. In a preferred embodiment, the matrix delivers sorted packages to a plurality of lift assemblies that further transfer the packages to receiving conveyors or chutes on different levels. According to another preferred aspect of the system, a selectively elevating stop bar is provided to control side transfer between conveyor cells. 
     Generally described, according to one of its aspects, the present invention provides a system for sorting a stream of objects emanating one-by-one from an object source, comprising: a matrix of stationary conveyor cells, including a plurality of the cells positioned to form at least two transversely extending rows of adjacent conveyor cells and at least two longitudinally extending rows of adjacent conveyor cells, an input cell of the conveyor cells in a first transversely extending row being positioned to receive objects from the object source; a controller connected to operate a plurality of the conveyor cells in the matrix individually to discharge an object thereon in one of a plurality of directions; a plurality of destination locations positioned adjacent to the conveyor cells in at least one of the transverse rows other than the first transverse row; a reader positioned adjacent to the stream of objects to read destination information borne by the objects; the controller being operative responsive to the reader to guide each object received by the input cell from cell to cell through the matrix to a destination location corresponding to the destination information. 
     In a preferred embodiment, the controller is operative to plan a path for each of the objects from the input cell to a destination location, to monitor availability of successive conveyor cells along the path, and to cause an object to be moved to the next conveyor cell along its path only when the next conveyor cell is available. The system may include a plurality of sensors positioned to sense the passage of objects from one conveyor cell to another, the sensors being connected to the controller. The sensor input is used to guide the packages through the matrix, and to optimize the speed at which a plurality of packages can be sorted. 
     According to another aspect, the present invention provides a method of sorting objects emanating one-by-one from an object source, comprising the steps of: transferring each of the objects from the object source to a matrix of stationary conveyor cells, the matrix including a plurality of the cells positioned to form at least two transversely extending rows of adjacent conveyor cells and at least two longitudinally extending rows of adjacent conveyor cells, and transferring each of the objects through the matrix from one of the conveyor cells to another and to one of a plurality of destination locations. In a preferred embodiment, the method may further comprise the steps of: reading destination information from each of the objects as the objects enter the matrix; and guiding the objects to one of the destination locations corresponding to the destination information. The method may also comprise monitoring availability of a next conveyor cell in the matrix to which an object is to be moved; and moving the object to the next cell only when the cell is available. 
     According to yet another aspect, the present invention provides, in a conveyor unit including a plurality of spaced apart rollers extending to a unit edge extending perpendicular to a longitudinal axis of the rollers, a side stop plate assembly, comprising: an elongate stop plate extending parallel to the unit edge and spaced inwardly from the unit edge above the rollers, and defining a pair of arms extending downwardly from the stop plate between the rollers to be pivotally attached to a support member of the conveyor unit; the stop plate being pivotal from an elevated position above the rollers to a horizontal position along the unit edge; means for moving the stop plate between the elevated position and the horizontal position. Preferably, the means for moving the stop plate comprises a foldable linkage attached to a linear actuator, and wherein the linkage locks against pressure on the stop plate when the stop plate is in the elevated position. 
     Thus, it is an object of the present invention to provide an improved package sorting system and method. 
     It is another object of the present invention to provide a package sorting system and method in which sorting on one level does not require movement of a conveyor unit from one place to another. 
     It is another object of the present invention to provide a package sorting system and method with improved processing speed. 
     It is another object of the present invention to provide a package sorting system and method applicable to output packages to rows of vertically spaced receiving conveyors or chutes. 
     Other objects, features and advantages of the present invention will become apparent upon review of the following detailed description of a preferred embodiment and the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of a package sorting system embodying the present invention. 
     FIG. 2 is a diagrammatic top view of the main level of the sorting system of FIG.  1 . 
     FIG. 3 is a side cross sectional view of the sorting system of FIG. 1, taken along line  3 — 3  of FIG.  2 . 
     FIG. 4 is a pictorial view of a transfer conveyor with an elevated side transfer stop bar. 
     FIG. 5 is a pictorial view of a transfer conveyor with a lowered side transfer stop bar. 
     FIG. 6 is a side view of the elevated side transfer stop bar. 
     FIG. 7 is a side view of the lowered side transfer stop bar. 
     FIG. 8 is an exploded view of the side transfer stop bar assembly. 
     FIG. 9 is an exploded view of portions of a lift assembly. 
     FIG. 10 is a side cross sectional view of a lift assembly. 
     FIG. 11 is a block diagram showing the device input signals to the controller and the control signal outputs therefrom. 
     FIG. 12 is a state diagram of the sorting system, noting the logic routines utilized in changing between states as packages are sorted. 
     FIG. 13 is a flow diagram of the logic applied in transferring a package from the input conveyor cell. 
     FIG. 14 is a flow diagram of the logic applied in transferring a package from the conveyor cell in the second row adjacent to the input cell. 
     FIG. 15 is a flow diagram of the logic applied in transferring a package from the cell to the right of the cell of FIG.  14 . 
     FIG. 16 is a flow diagram of the logic applied in transferring a package from the cell to the left of the cell of FIG.  14 . 
     FIG. 17 is a flow diagram of the logic applied in transferring a package from the cell to the left of the cell of FIG.  16 . 
     FIG. 18 is a flow diagram of the logic applied in transferring a package from the cell to the right of the cell of FIG.  15 . 
     FIG. 19 is a flow diagram of the logic applied in forward transfer of a package to a succeeding row. 
     FIG. 20 is a flow diagram of the logic applied in side transfer of a package without a separating stop bar. 
     FIG. 21 is a flow diagram of the logic applied in side transfer of a package with side stop bar operation. 
     FIG. 22 is a flow diagram of the logic applied in transfer of a package from a conveyor cell to a lift conveyor. 
     FIG. 23 is a flow diagram of the logic applied in shifting the level of a lift conveyor. 
    
    
     DETAILED DESCRIPTION 
     Referring now in more detail to the drawings, in which like numerals refer to like parts throughout the several views, FIG. 1 shows a package sorting system  100  embodying the present invention. The sorting system  100  is composed of a feed assembly  200 , which feeds packages to a matrix sorting assembly  300 . From the sorting assembly  300 , the packages are transferred to a vertical lift and discharge assembly  400 , from which they are discharged into an chute array  500  of destination-specific output chutes. These assemblies will be described in detail below. The vertical lift assembly  400  contains destination locations from the matrix sorting assembly  300 , and the chute array  500  contains ultimate package destinations in the sorting system  100 . 
     The sorting and transfer process is controlled by a digital controller  180 , the function of which is described below with reference to FIGS. 11-23. The controller may be a programmed general purpose personal computer. The controller receives input from detectors and sensors associated with various conveyors and chutes, all described below, and provides control signals instructing conveyors to operate to transfer packages in a selected direction. 
     The feed assembly  200  includes a feed conveyor  20  which is a horizontally mounted belt conveyor. An optical scanner  22  is mounted above the feed conveyor  20 , so as to be able to scan a package  25  moving under the scanner  22  on the conveyor  20 . The packages  25  bear labels on which are printed optically encoded symbols such as bar codes or two-dimensional dense codes, such as the MaxiCode symbology. The scanner may be either a conventional laser scanner or an over-the-belt video scanner having a charged coupled device (CCD) sensor. An example of the latter system is described in U.S. Pat. No. 5,308,960, which is incorporated herein by reference. A feed input sensor assembly  27  is located at the discharge end of the feed conveyor  20 , as shown in FIG.  2 . The sensor assembly  27  includes a conventional photocell transmitter and receiver. In the drawing, the light path between the transmitter and receiver is indicated by a dotted line, to which the reference numeral lead line is drawn. All package position sensors associated with the sorting system  100  are of this type and are shown in this manner. Spaced back from the discharge end of the feed conveyor a package deceleration sensor  26  is positioned. As explained below, if the next downstream location is occupied by another package, the drive motor (not shown) of the feed conveyor  20  is stopped when the leading edge of the package on the feed conveyor passes the deceleration sensor  26 . This results in the package coming to a stop closely adjacent to the discharge end of the conveyor  20 . A system for accomplishing this is described in Appendix B. 
     The matrix sorting assembly  300  includes an array or matrix of stationary multi-directional conveyor units  30 - 36 , arranged in two closely adjacent rows of closely adjacent conveyor units. The conveyor units  30 - 36  are referred to herein as cells of the matrix, shown in FIGS. 1 and 2. An input cell  30  is positioned adjacent to the discharge end of the feed conveyor  20 . Forming a first transverse row of cells with the cell  30  is a cell  31  positioned to the left side of the cell  30  when viewed from the feed conveyor. A second row of cells is formed by a cell  32 , positioned longitudinally forward of the cell  30 , a cell  33 , positioned forward of the cell  31  and to the left of the cell  32 , a cell  34 , positioned to the right of the cell  32 , a cell  35 , positioned to the right of the cell  34 , and a cell  36 , positioned to the left of the cell  33 . 
     The feed assembly  200  and the matrix sorter  300  are supported at an elevated level as shown in FIG. 1, referred to herein as the feed level. The feed level is located midway up the array of output chutes  500 , to minimize the distance to any particular output chute, as best shown in FIG.  3 . 
     An example of a multi-directional conveyor unit is shown in FIG.  4 . Each individual multi-directional conveyor unit  30 - 36  preferably is a ERMANCO model UBTXR40 line-shaft conveyor with right angle urethane belt transfer unit, modified to increase the size of pneumatic supply fittings, hoses and solenoid valve to one-half inch. The units include side frame members  38  connected by a cross member  39 . A plurality of powered rollers  40  extend between the frame members. In several of the spaces between the rollers  40 , urethane side transfer belts  42  are positioned on pulleys  43 . The side transfer units  42 , 43  can be elevated up from between the rollers  40  to a level above the rollers, in a known manner, and operated to move the belts  42  in either direction perpendicular to the feed direction of the powered rollers  40 . The units  30 - 36  are oriented so that the feed direction of the powered rollers is longitudinal, and the feed direction of the diverter belts  42  is transverse. 
     Alternate multi directional diverter units that could be used as conveyor matrix cells are shown in U.S. Pat. Nos. 4,798,275 and 4,174,774, incorporated by reference above. 
     Conveyor units  30 ,  32 , and  33  also are equipped with modified ERMANCO forward stop plates  48  that can be elevated as shown in FIGS. 1 and 4 to prevent discharge of a package by the powered rollers  40 . The standard forward “pop-up” stop plates are modified to increase their vertical stroke to four inches to raise the elevated height of the stop plate to prevent the packages from tilting or jumping through the stop plate during a high speed impact stop. Also, the pneumatic pipe&#39;s diameter is increased to one-half inch. 
     The conveyor unit  34  includes a collapsible side transfer stop plate assembly  50 , shown in FIGS. 4-8. The purpose of the assembly  50  is to selectively raise a stop plate  52  into a position inside the edges of the powered rollers  40 , as shown in FIG.  4 . In this position, the U-shaped plate  52  holds packages  25  against the action of the side transfer belts  42 , so that when the side transfer belts  42  are lowered, the package will drop onto the powered rollers  40  for forward transfer without danger of the package running off the side edge of the next conveyor unit. The side transfer stop plate assembly  50  also is useful if the downstream conveyor is narrower than the conveyor unit on which the assembly is installed. It will be appreciated that such a side stop plate cannot simply be raised vertically like the forward stop bar  48 , because of the presence of the rollers  40 . 
     The side of the stop plate  52  facing the center of the conveyor unit  34  is lined with a sheet  53  of low friction plastic or Teflon. The arms of the “U-shaped” plate  52  extend between two of the rollers  40  near the ends of the unit  34 . Each of the arms carries a plate support member  56  which terminates in a pivot joint  57  at the bottom of the arm of the plate  52 . A pair of plate mounting blocks  58  are secured on the cross member  39  of the frame  38  of the conveyor unit  34 , one block  58  below each support member  56 . The blocks  58  provide a complementary portion of the pivot joints  57 , and thereby support the stop plate  52 . The stop plate  52  thus is able to pivot from an elevated position, shown in FIGS. 4 and 6, to a retracted position in which the plate lies flat across the frame members  38  of adjacent conveyor units  34  and  35 , as shown in FIGS. 5 and 7. In the retracted position, the stop plate  52  bridges the gap between the rollers  40  of the adjacent conveyor cells, facilitating smooth side transfer of packages by the side transfer belts  42 . 
     To move the stop plate  52  between its elevated and retracted positions, the stop plate assembly  50  includes two multi-link mechanisms  59 , which are operated by a pair of air cylinders  60 . The cylinders  60  are mounted beneath the cross member  39  and are actuated by a common solenoid valve (not shown) to assure simultaneous action. Actuation occurs when an appropriate control signal is received from the controller. Each cylinder  60  includes an adjustable length shaft  62  extending upwardly through a pillow block  63  and terminating in a shaft end  65 . A linear bearing mount  67  is pivotally connected to the shaft end  65  by a pivot pin  68 , and defines a central bore in which a linear bearing  70  is fitted. A link shaft  71  is slidably received within the bearing  70 , and is pivotally connected at a lower shaft end  72  by a pivot pin  77  to a yoke  76 . The yoke  76  is mounted on the cross member  39  on the opposite side of the cylinder shaft  62  from the plate mounting block  58 . 
     An upper shaft end  74  of the link shaft  71  is pivotally connected, on the upper side of the bearing  70 , to a lower end of each of a pair of link arms  80 , by a pivot pin  81 . The upper ends of the link arms  80  are pivotally connected by a pivot pin  82  to the plate support member  56  spaced upwardly from the pivot joint  57  along the arm of the stop plate  52 . As noted, identical multi-link mechanisms  59  are installed at both ends of the stop plate  52 . 
     It may be seen that each multi-link mechanism  59  includes five pivot joints  57 ,  68 ,  77 ,  81  and  82 , and a slider shaft  71 . Upon upward movement of the cylinder shaft  62 , the link mechanism assumes the position shown in FIG. 6, with the link shaft  71  and link arms  80  aligned essentially colinearly. In this position, the link assembly  59  is “locked” such that the force induced by a package on the stop plate  52  will be directed along the link arms  80  and link shaft  71 , and will not tend to collapse the link assembly. This configuration creates a rigid connection between the stop plate  52  and the frame  38 , which is enforced and maintained by the fully extended cylinder shaft  62 . When the cylinder shaft  62  is retraced, it folds the mechanism and pivots the stop plate into its horizontal position shown in FIGS. 5 and 7. It should be understood that this collapsible side transfer stop plate assembly can have other mechanical linkage arrangements, such as providing pivoting action without the slider shaft, or can have other types of actuators. Also, it can be utilized as an erectable barrier in systems other than a diverter conveyor unit of the type shown. 
     Transverse motion of packages in the matrix of conveyor cells is also controlled by fixed side stop bars  85 ,  86 , and  87  attached along the outer sides of conveyor cells  31 ,  35 , and  36 , respectively, best shown in FIG.  2 . Fixed transfer plates  89 ,  90 , and  91  are positioned horizontally to bridge the gaps between rollers of conveyor cell pairs  30 / 31 ,  33 / 36 , and  32 / 34 , respectively, as shown in FIG.  1 . 
     As shown in FIG. 2, many position sensor assemblies similar to the feed input sensor assembly  27  are utilized to enable the controller to track packages through the matrix sorter  300  and the lift and discharge assembly  400  to the output chutes  500 . All of the sensor assemblies are capable of providing signals to the controller when the leading edge of a package breaks the photocell beam, and when the trailing edge leaves the beam. The manner in which these signals are used by the controller is discussed below in connection with logic flow diagrams. Two forward transfer sensor assemblies  94  and  95  are positioned at the entrance to conveyor cells  32  and  33 , respectively, to monitor transfer of packages from the first row of cells to the second row. A side transfer sensor assembly  97  is positioned along the left side of the input cell  30  to monitor transfer of packages to the cell  31 . A side transfer sensor assembly  98  is positioned along the right side of the input cell  30  to monitor transfer of packages to a return conveyor or chute  99  used primarily to remove from the sorter packages whose labels cannot be read. A side transfer sensor assembly  96  is positioned along the left side of the conveyor cell  33  to monitor transfer of packages to the cell  36 . A side transfer sensor assembly  101  is positioned along the right side of the cell  34  to monitor transfer of packages to the cell  35 . A side transfer sensor assembly  102  is positioned along the right side of the cell  32  to monitor transfer of packages to the cell  34 . Forward transfer sensor assemblies  104 ,  105 ,  106 ,  107 , and  108  are positioned along the forward or exit edge of the cells  36 ,  33 ,  32 ,  34 , and  35 , respectively, to monitor transfer of packages to the lift and discharge assembly  400 . 
     The lift and discharge assembly  400  may be seen in FIGS. 1-3,  9  and  10 . The assembly  400  includes five vertical lift units,  110 - 114 , positioned to receive packages from the conveyor cells  36 ,  33 ,  32 ,  34 , and  35 , respectively, of the second row of the matrix sorter  300 . The lift units are described in commonly owned U.S. application Ser. No. 08/421,675, filed Apr. 12, 1995, entitled “Method and Apparatus for Lifting Packages,” which is incorporated herein by reference. As shown in FIG. 10 of the present application, each of the vertical lift units includes a reversible, height-adjustable, belt conveyor  121  which is configured to receive the parcels  25  when the conveyor  121  is at the feed level. The power conveyor  121  is mounted for movement up and down a support structure  120  and is reversible so that the packages  25  may be alternatively discharged at various heights on opposite sides of the support structure. 
     FIG. 9 sets forth the construction of the support structure  120 . As can be seen in the drawing, the support structure includes two “A” shaped support frames  122  distanced apart by a base plate  126 . A pair of linear actuators  128  extend up the inside of the two “A” shaped support frames  122 . The linear actuators  128  may be driven in any manner known in the art, but preferably include a timing belt drive. Alternately, a screw-type linear actuator may be utilized. The linear actuators  28  are supported by vertical plates  130  to prevent warping and for reinforcement. Each of the linear actuators  128  includes a linear actuator carriage  132  configured to travel along the linear actuator&#39;s length. An encoder (not shown) monitors the vertical position of the carriage  132  of the linear actuator, and provides a signal to the controller  180 . An actuator speed reducer  134  is attached by two drive shafts  136  to simultaneously drive the linear actuators  128 , and is driven by a motor  138 . The controller  20  sets the speed and direction of the motor  38 . 
     As best shown in FIG. 10, the power conveyor  121  is attached to the linear actuator carriages  132  by two conveyor support plates  140 , such that the power conveyor  121  can move up and down the support structure  120  along the linear actuators  128 . The power conveyor  121  includes a continuous belt  141  which is driven by a motor  142  and a speed reducer  144 , via timing belts  146 ,  148 , in a manner known in the industry. The controller  20  sets the speed and direction of the motor  42 . 
     Each lift conveyor  121  includes a deceleration sensor  150  positioned to sense when the leading edge of a package traveling onto the belt conveyor  121  reaches a predetermined location intermediate the ends of the conveyor  121 . The deceleration sensors  150  are best seen in FIG. 2, and are similar to the deceleration sensor  26  of the feed conveyor  20 . As described below, if the package cannot immediately be discharged from the conveyor  121 , the drive motor  142  connected to the conveyor  121  is stopped as soon as the package reaches the position of the deceleration sensor  150 , so that the package come to rest fully on the lift conveyor  121  for any vertical travel that may be necessary. 
     The set of output chutes  500  is a plurality of chutes  160  whose entrance ends are positioned to receive packages from one of the lift conveyors  121 . In FIG. 1, the chutes  160  are numbered  1 - 18 . A group of the output chutes  160  (labeled  1 - 15  and  18 ) form a two-dimensional array in a vertical plane adjacent to the vertical path of travel of the lift conveyors  121  of the vertical lifts  110 - 114 . Five of the chutes (labeled  2 ,  5 ,  8 ,  11 , and  14 ) are positioned at the feed level so that packages destined for these output chutes are moved from conveyor cells of the sorter matrix directly into the chutes without vertical movement of the lift conveyors  121 . The chutes labeled  16  and  17  are positioned on the opposite side of the vertical lift  114  from the chutes labeled  13  and  15 . Discharge into the chutes  16  and  17  requires reverse operation of the lift conveyor  121  of the vertical lift  114 . It will be understood that additional output chutes  160  could be provided above and under the matrix sorter  300 . 
     Each output chute includes a chute input sensor assembly  165  positioned at its entrance, for detecting the discharge of a package from an adjacent lift conveyor  121 . Alternately, sensor assemblies may be placed at both ends of the lift conveyor  121 , rather than in the chutes, to reduce the total number of sensors utilized. 
     An optional consolidation belt conveyor  170  is shown in FIG. 1, held by legs above the conveyor cells  32  and  33 . The conveyor  170  may receive packages from vertical lifts  110  or  111 , and transfer such packages to output chute  16 . This may be useful if a very large number of packages are known to go to the destination of chute  16 . By consolidating their output, two vertical lifts and associated sorting cells can be used for one destination, increasing the throughput of the sorting system  100 . An input sensor assembly and a discharge sensor assembly (not shown) may be associated with the conveyor  170 . 
     Operation 
     Referring now to FIG. 11, the controller  180  coordinates the routing of packages through the sorting system  100 . The controller receives information about packages and their location within the system from the scanner  22  and the various position sensor assemblies  26 ,  27 ,  94 - 99 ,  101 - 108 ,  150 , and  165 . The controller sends control signals to operate the feed conveyor  20 , the powered rollers  40  of the various conveyor cells  30 - 36 , the lift conveyors  121 , the linear actuators of the vertical lifts  110 - 114 , the stop bars  48  and  52 , and the deceleration mode of the feed conveyor and the lift conveyors. FIG. 12 is a state diagram showing the states of a package passing through the system, and noting the logic routines utilized in changing between states as packages are sorted. The shorthand notations used in FIGS. 11 and 12 are as follows: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 F-T 
                 forward transfer 
                 (FIG. 19) 
               
               
                 S-T 
                 side transfer 
                 (FIG. 20) 
               
               
                 S-T/SB 
                 side transfer with side stop bar operation 
                 (FIG. 21) 
               
               
                 LB-T 
                 transfer to lift conveyor belt 
                 (FIG. 22) 
               
               
                 L-S-D 
                 level shift and discharge 
                 (FIG. 23) 
               
               
                 D 
                 discharge 
                 (FIG. 22) 
               
               
                   
               
             
          
         
       
     
     Generally described, a package  25  to be sorted is placed onto the feed conveyor  20  either automatically from an upstream conveyor or chute system, or manually. A stream of single packages may be fed through the sorting system  100 . Conventional apparatus for resolving packages on a conveyor into a single file may be utilized upstream from the feed conveyor  20 . When the package is in state  210  of FIG. 12, the controller  180  operates the feed conveyor  20  to carry the package  25  beneath the scanner  22 , which reads a label on the package and transmits information from the label, such as zip code information encoded in a bar code or MaxiCode symbol, to the controller. The controller assigns a destination bin  160  to the package based on the zip code or other address information read by the scanner, and maps a path through the system from cell to cell, then onto a lift conveyor and to an output chute at the appropriate level. The controller also monitors whether the transfer conveyor cells  30 - 36  are occupied or empty. If the input cell  30  is occupied, the controller decelerates and stops the feed conveyor  20  when the leading edge of the package reaches the deceleration sensor  26 . 
     When the input cell  30  is empty, the controller operates the feed conveyor to transfer the package in the forward direction onto the input cell  30  (state  212 ). Preferably, the powered rollers  40  of all the cells  30 - 36  are operating continuously at the same speed, so that any package being transferred onto a cell is immediately drawn onto the cell by its powered rollers. Also, the forward stop plates  48  and side stop plate  52  are normally in a raised position. Unless the package is to immediately move in another forward transfer to the cell  32 , the forward stop bar  48  stops the motion of the package. The controller monitors the signal from the sensor  27  to note the entrance of the package leading edge onto the cell  30  as well as when the trailing edge of the package clears the sensor  27 . At this time, the controller memory marks the cell  30  as occupied by the package and its destination. 
     The controller  180  continually monitors the status of each of the conveyor cells  30 - 36 , and applies logic to dispose of any package that arrives at a cell. First, the logic routines applied to transfer any arriving package to another cell will be described (FIGS.  13 - 18 ), and then the subroutines that are called by such logic routines for particular transfer operations (FIGS.  19 - 22 ). 
     The logic used by the controller for moving a package from the cell  30  is shown in FIG.  13 . At step  240 , the controller determines if cell  30  is occupied. If so, the controller determines if the path for the package includes cell  31  at step  242 . If so, a side transfer subroutine is executed at step  243 , moving the package to cell  31 , state  226  in FIG.  12 . The side transfer subroutine is described below in connection with FIG.  20 . If cell  31  is not in the path, the controller determines if the path includes cell  32  at step  244 . If so, at step  245  a forward transfer subroutine is executed, moving the package to state  214 . The forward transfer subroutine is described below in connection with FIG.  19 . If cell  32  is not in the path, a side transfer subroutine is executed at step  246  to transfer the package to the return chute  99 , which is state  227 . 
     The logic used by the controller for moving a package from the cell  32  is shown in FIG.  14 . At step  248 , the controller determines if the trailing edge of the package has cleared the entry sensor  94 . When this is true, at step  250  the controller determines if the path includes cell  34 . If so, at step  251  the controller executes a side transfer as shown in FIG. 20 to move the package to cell  34  (state  216 ). If not, at step  252  the controller determines if the lift conveyor belt  121  of the vertical lift  110  is present at feed level. If so, at step  253  the controller determines if the belt  121  is full and not discharging a package. If true, the controller waits until the belt  121  is empty or in the process of discharging a package into the chute  5 . Then, at step  254  the controller executes a transfer to lift belt subroutine to move the package to the lift belt  121  (state  224 ). The transfer to lift belt subroutine is described below in connection with FIG.  22 . 
     The logic used by the controller for moving a package from the cell  34  is shown in FIG.  15 . At step  258 , the controller determines if cell  34  is occupied. Movement of a package into cell  34  always involves a side transfer from cell  32  by the side transfer belts  42 . The package is held against the movable stop plate  52  by the continuing action of the belts  42 . If this condition exists, then at step  260  the controller determines if the path includes cell  35 . If so, at step  261  the controller executes a side transfer with side stop plate operation to move the package to cell  35  (state  220 ). The side transfer with side stop plate operation subroutine is described below in connection with FIG.  21 . If the path does not include cell  35 , at step  262  the controller determines if the lift conveyor belt  121  of the vertical lift  112  is present at feed level. If so, at step  263  the controller determines if the belt  121  is full and not discharging a package. If true, the controller waits until the belt  121  is empty or in the process of discharging a package into the chute  2 . Then, at step  264  the controller stops operation of the side transfer belts  42  without dropping the side stop plate  52  executes the transfer to lift belt subroutine to move the package to the lift belt  121  (state  218 ). 
     The logic used by the controller for moving a package from the cell  31  consists only of an immediate forward transfer subroutine, moving the package to the cell  33  (state  228 ). 
     The logic used by the controller for moving a package from the cell  33  is shown in FIG.  16 . The steps are identical to those of FIG. 14, except that at step  270  the first possible path is to the cell  36  (state  232 ), and the vertical lift referred to in steps  272 - 274  is lift  111  (state  230 ). 
     The logic used by the controller for moving a package from the cell  36  is shown in FIG.  17 . At step  276 , the controller determines if cell  36  is occupied. As in the case of cell  34 , movement of a package into cell  36  always involves a side transfer, here from cell  33 , by the side transfer belts  42 . The package is held against the movable fixed side plate  87  by the continuing action of the belts  42 . If this condition exists, then at step  277  the controller determines if the lift conveyor belt  121  of the vertical lift  114  is present at feed level. If so, at step  278  the controller determines if the belt  121  is full and not discharging a package. If true, the controller waits until the belt  121  is empty or in the process of discharging a package into the chute  14 . Then, at step  279  the controller stops operation of the side transfer belts  42  and executes the transfer to lift belt subroutine to move the package to the lift belt  121  (state  234 ). 
     The logic used by the controller for moving a package from the cell  35  is shown in FIG.  18 . The steps are identical to those of FIG. 17, except that the vertical lift referred to in steps  287 - 290  is lift  113  (state  222 ). 
     The routines of FIGS. 19-23 will be explained in connection with example package paths. As a first example, assume that the package is destined for the vertical lift  112  and one of the output chutes  1 ,  2 , or  3 . A forward transfer logic subroutine, as shown in FIG. 19, is run by the controller. Referring to FIG. 19, the routine is shown generically for forward transfer from a cell i to a cell j. At decision step  310 , the controller determines if the current cell i (in this case cell  30 ) is empty. If so, the process waits until a package occupies the current cell. When a package is present and is to be moved forward, at decision step  312 , the controller determines if the next cell j in the forward direction (in this case cell  32 ) is full and not in the process of exiting a package. If so, the controller waits until cell j is either empty or is exiting another package. Then, at decision step  314 , the controller checks to see if the exiting is a side transfer, which would not be consistent with a forward transfer onto cell j. If so, the controller waits until cell j is not executing a side transfer. Then, at decision step  316 , the controller determines if cell i has a forward stop plate (which is the case for cell  30 ). If so, the stop plate is lowered at step  320 , which immediately allows the running rollers  40  to advance the package onto the running rollers of cell j. At steps  322  and  324 , the exit sensor of cell i (here sensor  97 ) confirms to the controller that the package has cleared cell i. When this is confirmed and i&#39;s stop bar exists (step  326 ), the stop plate  48  is again raised at step  328 . At this point, the package has been transferred to cell  32  (state  214  in FIG. 12) and rests against the forward stop plate of cell  32 . 
     The next step in the path of the example package to vertical lift  113  is a side transfer to cell  34 , according to the logic subroutine of FIG.  20 . Again, the routine is shown generically for side transfer from cell i to cell j. It applies only when there is no side stop plate between cells i and j. At decision step  340 , the controller determines if the current cell i (in this case cell  32 ) is empty. If so, the process waits until a package occupies the current cell. When a package is present and is to be moved to the side, at decision step  342 , the controller determines if the next cell j in the side direction (in this case cell  34 ) is empty. If, not, the controller waits until cell j is empty. Then, at step  344 , side transfer is initiated by raising the side transfer belts  42  of both cells i and j, and operating them to carry the package in the direction of cell j. At step  346 , the entry sensor for cell j (here sensor  102 ) sends a signal to the controller when the trailing edge of the package clears the sensor, and thus is clear of the cell i. Then at step  348 , the side transfer action is stopped, and the side transfer belts  42  are lowered. 
     It should be noted that all cells of the sorting system  300  that are capable of receiving a side transfer have either fixed or movable side stop plates to hold the package on the cell until it either continues sideways movement (possible in the case of movement from cell  34  to cell  35 ), or is lowered onto the powered rollers  40  for forward transfer. In the case of the present example, the package is stopped by the elevated stop bar  52 . When the side transfer belts  42  of the cell  34  are lowered, the powered rollers of the cell  34  immediately begin to transfer the package in the forward direction, and the controller executes steps  416  through  426  of a transfer to lift belt logic subroutine shown in FIG.  22 . At step  416 , the controller activates the belt drive motor  142  of the lift conveyor belt  121  of vertical lift  112  to operate the belt in the forward direction. When the leading edge of the package triggers the exit sensor  107  of the cell  34 , as monitored by decision step  418 , the controller determines at step  420  if the package must stop on the belt  121  for a level shift. If so, when the leading edge triggers the deceleration sensor  150  at step  422 , the controller stops the motor  142  at step  424 . Also, the controller prevents any further action until the trailing edge of the package clears the sensor  107 , monitored at step  426 . At this point, the package is in state  218  as shown in FIG.  12 . 
     If, at step  420 , it is determined that the package is to be directly discharged into the chute  2 , the process proceeds directly to step  426  without stopping the belt  121 . The controller is informed when package has cleared the exit sensor  107  of cell  34 , and the package moves immediately into the chute  2 . 
     If, on the other hand, the package has stopped in state  218  for a level shift, a level shift and discharge logic subroutine shown in FIG. 23 is executed. At step  430 , the controller determines if the package is on the belt  121 , and the belt is stopped. When this is true, at step  432 , the controller operates the lift motor  138  to drive the belt  121  to the desired output chute level, either up to chute  1  or down to chute  3 . When the encoder signal associated with the carriage  132  of the linear actuator indicates the belt  121  is coming close to the destination level, as monitored at step  434 , the controller at step  436  begins to drive the belt  121 , in this case in the forward direction, so that the package is moving when the chute level is reached, and the package immediately exits the belt  121  into the chute. At step  438 , the controller determines when the trailing edge of the package clears the chute sensor  165 , and then drives the linear actuator at step  439  to return the belt  121  to the feed level. When the encoder output indicates the belt  121  is approaching the feed level, at step  440 , the controller decelerates the belt  121  to a stop at the feed level, and waits for another package to be loaded. 
     This completes the sorting of the first example package. Considering a different example, assume the package was destined for the vertical lift  113 . In this case, the package must continue transversely from cell  34  to cell  35 , according to a logic subroutine shown in FIG.  21 . At the end of the side transfer subroutine of FIG. 20, operation of the side transfer belts  42  of the cell  34  is not terminated if the path mapped for the package continues to cell  35 . At step  350 , the controller determines if a package is present on a current cell i (here cell  34 ) that is equipped with a movable side stop plate assembly  50 . If so, the controller determines if the next cell j (here cell  35 ) is empty and the lift conveyor  121  of the vertical lift  113  is empty at the feed level. When this is true, the stop plate  52  is dropped and side transfer by the belts  42  of both cells is operated at step  354  until the trailing edge of the package passes the side transfer sensor  101  as monitored at step  356 . The package comes to rest against the fixed side stop  86 . The package is now in state  220  in FIG.  12 . Then, at step  358 , operation of the side transfer belts is stopped and the belt  121  drive is started. The package drops onto the powered rollers of the cell  35  and is carried onto the belt  121  of the vertical lift  113 , state  222  in FIG.  12 . The belt  121  is controlled according to steps  416  to  426  of FIG. 22, as described above. Movement of the lift to a proper output level, if necessary, is executed in accordance with FIG. 23, as described above. 
     Considering still another example, assume the package is destined for vertical lift  110  and one of the output chutes  4 ,  5 ,  6 , or  18 . Progress of the package to state  214  is as described above. Then a transfer subroutine in accordance with FIG. 22 is executed beginning with step  410 , at which the controller determines if the current cell  32  is empty. When the controller determines that the cell  32  is occupied, then at step  412  the controller determines if the lift conveyor  121  of the vertical lift  110  is full and not discharging a package into the chute  5 . If so, the controller waits until the lift conveyor  121  is ready. Then, at step  414 , the forward stop plate  48  of the cell  32  is dropped, allowing the powered rollers  40  to begin to move the package forward. At the same time, the lift conveyor belt  121  is operated in the forward direction at step  416 . From this point, the routine of FIG. 22 continues as described above, moving the package to state  224 . 
     It will be understood that transfers within the matrix not specifically described above, such as involving cells  31 ,  33 , and  36 , are carried out in a manner similar to the examples described above. If the destination output chute is chute  16  or  17 , when the lift conveyor reaches the chute level, it is operated by the controller in reverse direction at step  436  of the level shift and discharge subroutine of FIG.  23 . 
     From the foregoing, it will be understood that the sorter matrix  300  functions as a package buffer and sorter before the package is fed forward onto the lift conveyors. Multiple packages can be found at different locations within the matrix buffer at the same time. The buffer regulates the package horizontal transfer based upon its destination. A package is moved within the matrix and onto the lift conveyors only if its next position is clear. Position sensors are used to provide status information about the current conveyor on which the package is located, and about other conveyors farther along the planned path of the package, so that conveyor operation can be optimized to move packages through the matrix rapidly. No movement of the cells of the matrix from place to place occurs; only the lift conveyors  121  move, and they only move vertically. 
     While this invention has been described in detail with particular reference to a preferred embodiment thereof, it will be understood that modifications and variations may be made without departing from the spirit and scope of the invention as defined in the appended claims.