Patent Publication Number: US-2023158552-A1

Title: Material handling apparatus for delivering or retrieving items

Description:
PRIORITY CLAIMS 
     The present application is a continuation of co-pending of U.S. patent application Ser. No. 17/949,008 filed on Sep. 20, 2022, which is a continuation of U.S. patent application Ser. No. 17/870,706 filed on Jul. 21, 2022, which is a continuation of U.S. patent application Ser. No. 17/743,403 filed on May 12, 2022, which is a continuation of co-pending U.S. patent application Ser. No. 17/515,122 filed on Oct. 29, 2021, which is a continuation of U.S. patent application Ser. No. 16/750,596, filed Jan. 23, 2020 issued on Dec. 7, 2021, which is a continuation of U.S. patent application Ser. No. 16/039,713, filed Jul. 19, 2018 issued as U.S. Pat. No. 10,576,505, which is a continuation of U.S. patent application Ser. No. 15/618,744, filed Jun. 9, 2017 issued as U.S. Pat. No. 10,052,661, which is a continuation of U.S. patent application Ser. No. 14/690,541, filed Apr. 20, 2015 issued as U.S. Pat. No. 9,687,883, which is a continuation of U.S. patent application Ser. No. 14/149,282, filed Jan. 7, 2014 issued as U.S. Pat. No. 9,010,517, which is a continuation of U.S. patent application Ser. No. 13/631,817, filed Sep. 28, 2012 issued as U.S. Pat. No. 8,622,194, which is a continuation of U.S. patent application Ser. No. 13/361,490 filed Jan. 30, 2012 issued as U.S. Pat. No. 8,276,740, which is a continuation of U.S. patent application Ser. No. 12/983,726 filed Jan. 3, 2011 issued as U.S. Pat. No. 8,104,601, which is a continuation of U.S. patent application Ser. No. 12/014,011 filed Jan. 14, 2008 issued as U.S. Pat. No. 7,861,844, which claims priority to U.S. Provisional Patent Application No. 60/884,766 filed on Jan. 12, 2007. The present application claims priority to each of the foregoing applications and the entire disclosure of each of the foregoing applications is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a material handling system for sorting or retrieving items. More specifically, the present invention relates to a material handling system incorporating a plurality of destination areas and a plurality of vehicles for carrying items to and/or from the destination areas. 
     BACKGROUND OF THE INVENTION 
     Sorting documents and mail pieces manually is laborious and time consuming. For example, thousands of large organizations employ numerous people full-time to manually sort and deliver incoming and interoffice mail and documents. For instance, a large company may receive 5,000 mail pieces that need to be sorted and delivered each day to different departments and/or individuals. Such volumes require a significant number of employees dedicated to sorting and delivering the mail. Nonetheless, such volume is not typically sufficient to justify the expense of traditional automated sorting equipment, which is quite expensive. Additionally, the mail for such organizations is typically quite diverse, which makes it more difficult, and therefore more expensive, to automate the sorting procedures. 
     Various systems for sorting have been developed to address the needs of mail rooms for large organizations. However, the known systems suffer from several problems; the most significant are cost and size. Accordingly, there is a need for a compact and affordable automated sorting system that is able to meet the needs of mid-to large-sized organization that handle several thousand mail pieces each day. 
     Similarly, many large organizations have extensive storage areas in which numerous items are stored. Sorting and retrieving items from the hundreds or thousands of storage areas requires significant labor to perform manually, and the known systems of automatically handling the materials are either very expensive or have limitations that hamper their effectiveness. Accordingly, there is a need in a variety of material handling applications for automatically storing and/or retrieving items. 
     SUMMARY OF THE INVENTION 
     In light of the foregoing, a system provides a method and apparatus for delivering items to storage locations. The system includes a plurality of storage locations, such as bins, and a plurality of delivery vehicles for delivering items to the storage locations or retrieving items from the storage locations. 
     In one embodiment, the system may includes a plurality of delivery vehicles that deliver items to destination areas. The destination areas may include a first set of destination areas and a second set of destination areas spaced apart from the first set of destination areas by a gap that forms part of an aisle. The delivery vehicles are operable to deliver items by following a path that forms a vertical loop within the aisle. The vehicles may be operated such that the orientation of the vehicle stays constant relative to the horizon as the vehicles are displaced along the vertical loop. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary and the following detailed description of the preferred embodiments of the present invention will be best understood when read in conjunction with the appended drawings, in which: 
         FIG.  1    is a perspective view of a sorting apparatus; 
         FIG.  2    is a plan view of the sorting apparatus illustrated in  FIG.  1   ; 
         FIG.  3    is a fragmentary perspective view of the sorting apparatus illustrated in  FIG.  1   , shown without an input station; 
         FIG.  4    is a right side view of the sorting apparatus illustrated in  FIG.  3   ; 
         FIG.  5    is a front elevational view of the sorting apparatus illustrated in  FIG.  3   , shown without discharge bins; 
         FIG.  6    is a fragmentary sectional view of a loading station of the sorting apparatus illustrated in  FIG.  1   ; 
         FIG.  7    is an enlarged fragmentary perspective view of a portion of the loading station of the apparatus illustrated in  FIG.  3   ; 
         FIG.  8    is an enlarged fragmentary view of a portion of track of the apparatus illustrated in  FIG.  1   , showing details of a gate in an open position; 
         FIG.  9    is an enlarged fragmentary view of a portion of track of the apparatus illustrated in  FIG.  1   , showing details of a gate in a closed position; 
         FIG.  10    is an enlarged fragmentary perspective view of a portion of the track illustrated in  FIG.  1   , showing details of a gate; 
         FIG.  11    is an enlarged fragmentary perspective view of a portion of the track illustrated in  FIG.  1   , showing details of a gate, with the gate shown in an open position in phantom; 
         FIG.  12    is a top perspective view of a delivery vehicle of the apparatus illustrated in  FIG.  1   ; 
         FIG.  13    is a plan view of the delivery vehicle illustrated in  FIG.  12   ; 
         FIG.  14    is a right side view of the delivery vehicle illustrated in  FIG.  12   ; 
         FIG.  15    is a front elevational view of the delivery vehicle illustrated in  FIG.  12   ; 
         FIG.  16    is a bottom perspective view of the delivery vehicle illustrated in  FIG.  12   ; 
         FIG.  17    is a bottom view of the delivery vehicle illustrated in  FIG.  12   ; 
       and 
         FIG.  18    is an enlarged view of a wheel of the delivery vehicle illustrated in  FIG.  12   , shown in relation to the track of the sorting apparatus illustrated in  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS.  1 - 18   , an apparatus for sorting items such as documents or mail pieces is designated generally  10 . The apparatus  10  includes a plurality of delivery vehicles or cars  200  to deliver items to a plurality of sort locations, such as output bins  190 . At a loading station  310 , each car  200  receives an item from an input station  50  and delivers it to the appropriate bin. 
     The cars  200  travel along a track  110  to the sort locations. The track has a horizontal upper rail  135  and a horizontal lower rail  140 , which operates as a return leg. A number of parallel vertical track legs  130  extend between the upper rail and the lower return leg. In the present instance, the bins  190  are arranged in columns between the vertical track legs  130 . 
     After a piece is loaded onto a car, the car travels upwardly along two pairs of vertical tracks legs and then horizontally along two upper tracks  135 . The car  200  travels along the upper rail until it reaches the appropriate column containing the bin for the piece that the car is carrying. The track  110  includes gates  180  that fire to direct the car  200  down the vertical legs and the car stops at the appropriate bin. The car  200  then discharges the piece into the bin. 
     After discharging the piece, the car  200  continues down the vertical legs  130  of the column until it reaches the lower rail  140 . Gates fire to direct the car along the lower rail, and the car follows the lower rail to return to the loading station  310  to receive another piece. 
     The cars  200  are semi-autonomous vehicles that each have an onboard power source and an onboard motor to drive the cars along the track  110 . The cars also include a loading/unloading mechanism  210 , such as a conveyor, for loading pieces onto the cars and discharging the pieces from the cars. 
     Since the system  10  includes a number of cars  200 , the positioning of the cars is controlled to ensure that the different cars do not crash into each other. In one embodiment, the system  10  uses a central controller  350  that tracks the position of each car  200  and provides control signals to each car to control the progress of the cars along the track. The central controller  350  may also control operation of the various elements along the track, such as the gates  180 . 
     Input Station 
     At the input station  50 , the mail pieces are separated from one another so that the pieces can be conveyed serially to the loading station  310  to be loaded onto the cars  200 . Additionally, at the input station information is determined for each piece so that the piece can be sorted to the appropriate bin. 
     A variety of configurations may be used for the input station, including manual or automatic configurations or a combination of manual and automated features. In a manual system, the operator enters information for each piece and the system sorts the mail piece accordingly. In an automatic system, the input system includes elements that scan each mail piece and detect information regarding each piece. The system then sorts the mail piece according to the scanned information. 
     In an exemplary manual configuration, the input system includes a work station having a conveyor, an input device, such as a keyboard, and a monitor. The operator reads information from a mail piece and then drops in onto a conveyor that conveys the piece to the loading station  310 . Sensors positioned along the conveyor track the piece as the conveyor transports the mail piece toward the loading station. An example of a work station having a conveyor for receiving dropped pieces and tracking the pieces is provided in pending U.S. application Ser. No. 10/862,021, filed Jun. 4, 2004, which was published Jan. 27, 2005 under Publication No. US 2005-0018214 A1 and which is incorporated herein by reference. The conveyor receives mail pieces dropped by an operator and tracks the mail pieces as they are transported along the conveyor. 
     In an exemplary automatic configuration, the system includes an imaging station, having an imaging device such as a high speed line scanning camera. The imaging station scans each mail piece to detect information regarding the destination for each piece. The system analyzes the image data to determine the destination information and then electronically tags the mail piece with the destination and sorts the piece accordingly. An example of a system having an automated imaging station for scanning pieces as they are conveyed is described in U.S. patent application Ser. No. 09/904,471, filed Jul. 13, 2001, which was published Jan. 16, 2003 under Publication No. US 2003-0014376 A1, and which is incorporated herein by reference. 
       FIGS.  1  and  2    illustrate such an automated system. The input station includes an input bin  55  for receiving a stack of mail. A feeder  60  in the input bin serially feeds mail pieces from the input bin to a conveyor  65 . An imaging station  70  positioned along the conveyor scans the mails pieces as the pieces are conveyed to the loading station  310 . The system  10  analyzes the image data to read information for the mail piece, such as the recipient&#39;s address. 
     The conveyor  65  conveys the mail piece to the loading station  310 . At the loading station the conveyor  65  conveys the mail piece onto a car  200 . As discussed further below, after the mail piece is loaded onto the car, the car moves away from the loading station and another car moves into position at the loading station to receive the next piece of mail. 
     In certain instances, the system may not be able to automatically identify the relevant information for a mail piece. To process such pieces, the system may include an operator to input the relevant information so that the mail piece can be sorted. For instance, the system may include an operator station having an input device and a display, such as a monitor. If the system cannot automatically determine the address within a pre-determined time period, the system displays the scanned images for the mail piece to the monitor so that the operator at the work station can view the images and manually enter the information using the input device. 
     In addition to the automated and manual systems described above, the system may be configured in a hybrid or semi-automated configuration having some operations performed manually and others automated. For instance, the system may include a manual input station that also has an imaging station. Since the system can handle a wide variety of items, it may be desirable to have an operator input the pieces manually so that the pieces are properly oriented and separated. The imaging station then scans the items and processes the imaging data to determine the address information for the pieces. Additionally, the operator station may include an input device and a display for inputting information if the address for a piece cannot be automatically determined, as discussed above. The operator can input the information as soon as the system indicates to the operator that it cannot determine the information for a piece. Alternatively, as discussed below, the car may be directed to a buffer if the information for a piece cannot be determined. In such an instance, the cars having such pieces will remain in the buffer while the system continues to process pieces for which the system can determine the relevant information. The operator can continue to manually drop pieces and wait until a number of pieces need manual keying of information. The operator can then switch from the operation of dropping pieces to the operation of manually keying the pieces, sometimes referred to as local video encoding (LVE). The operator can continue keying until some or all of the pieces in the buffer have been successfully coded, and then the operator can go back to the operation of manually dropping pieces. As yet another alternative, it may be desirable to incorporate a separate operator station having the input device and display so that one operator can input the mail at the input station and a separate operator can input the information for pieces having addresses that cannot be automatically determined. 
     As can be seen from the foregoing, the input station  50  may be configured in a wide range of options. The options are not limited to those configurations described above, and may include additional features, such as an automated scale for weighing each piece, a labeler for selectively applying labels to the mail pieces and a printer for printing information on the mail pieces or on the labels. 
     Additionally, in the foregoing description, the system is described as having a single input station  50 . However, it may be desirable to incorporate a plurality of input stations positioned along the system  10 . By using a plurality of input stations, the feed rate of pieces may be increased. In addition, the input stations may be configured to process different types of items. In this way, each input station could be configured to efficiently process a particular category of items. For instance, if the system is configured to process documents, such as mail, one input station may be configured to process standard envelopes, while another input station may be configured to process larger mails, such as flats. Similarly, one input station may be configured to automatically process mail by scanning it and automatically determining the recipient. The second input station may be configured to process rejects, such as by manually keying in information regarding the recipient. 
     Sorting Station 
     Referring to  FIGS.  1 - 6   , the system includes a sorting station  100 , such as an array of bins  190  for receiving the pieces. In the present instance, the sorting station includes a number of bins arranged in columns. Additionally, the sorting station  100  includes a track  110  for guiding the cars  200  to the bins  190 . 
     The track  110  includes a horizontal upper rail  135  and a horizontal lower rail  140 . A plurality of vertical legs  130  extend between the upper horizontal leg and the lower horizontal leg  140 . During transport, the cars travel up a pair of vertical legs from the loading station  310  to the upper rail  135  (as described below, the cars actually travel up two pairs of rails because the track includes a forward track and a parallel opposing track). The car then travels along the upper rail until reaching the column having the appropriate bin. The car then travels downwardly along two front vertical posts and two parallel rear posts until reaching the appropriate bin, and then discharges the mail piece into the bin. The car then continues down the vertical legs until reaching the lower horizontal leg  140 . The car then follows the lower rail back toward the loading station. 
     As can be seen in  FIG.  2   , the track  110  includes a front track  115  and a rear track  120 . The front and rear tracks  115 ,  120  are parallel tracks that cooperate to guide the cars around the track. As shown in  FIG.  13   , each of the cars includes four wheels  220 : two forward wheel and two rearward wheels. The forward wheels  220  ride in the front track, while the rearward wheel ride in the rear track. It should be understood that in the discussion of the track the front and rear tracks  115 ,  120  are similarly configured opposing tracks that support the forward and rearward wheels  220  of the cars. Accordingly, a description of a portion of either the front or rear track also applies to the opposing front or rear track. 
     Referring to  FIG.  18    the details of the track will be described in greater detail. The track  110  includes an outer wall  152  and an inner wall  154  that is spaced apart from the outer wall and parallel to the outer wall. The track also has a back wall  160  extending between the inner and outer walls. As can be seen in  FIG.  18   , the outer and inner walls  152 ,  154  and the back wall form a channel. The wheels  220  of the car ride in this channel. 
     Referring to  FIG.  11   , the track includes both a drive surface  156  and a guide surface  158 . The drive surface positively engages the cars to enable the car to travel along the track. The guide surface  158  guides the car, maintaining the car in operative engagement with the drive surface  156 . In the present instance, the drive surface is formed of a series of teeth, forming a rack that engages the wheels of the cars as described further below. The guide surface  158  is a generally flat surface adjacent the rack  156 . The rack  156  extends approximately halfway across the track and the guide surface  158  extends across the other half of the track. As shown in  FIGS.  11  and  18   , the rack  156  is formed on the inner wall  154  of the track. The opposing outer wall  152  is a generally flat surface parallel to the guide surface  158  of the inner wall. 
     As described above, the track includes a plurality of vertical legs extending between the horizontal upper and lower rails  135 ,  140 . An intersection  170  is formed at each section of the track at which one of the vertical legs intersects one of the horizontal legs. Each intersection includes an inner branch  172  that is curved and an outer branch  176  that is generally straight.  FIG.  10    illustrates both a right-hand intersection  170   c  and a left-hand intersection  170 , which are mirrors of one another. In  FIG.  10   , the intersections  170   c ,  170   d  illustrate the portion of the track in which two vertical legs  130  intersect the upper horizontal leg  135 . The intersections of the vertical legs with the lower rail incorporate similar intersections, except the intersections are reversed. Specifically, the point at which vertical leg  130   c  intersects the lower rail incorporates an intersection configured similar to intersection  170   d , and the point at which vertical leg  130   d  intersects the lower rail incorporates an intersection configured similar to intersection  170   c.    
     Each intersection  170  includes a pivotable gate  180  that has a smooth curved inner race and a flat outer race that has teeth that correspond to the teeth of the drive surface  156  for the track. The gate  180  pivots between a first position and a second position. In the first position, the gate  180  is closed so that the straight outer race  184  of the gate is aligned with the straight outer branch  176  of the intersection. In the second position, the gate is open so that the curved inner race  182  of the gate is aligned with the curved branch  172  of the intersection. 
     Accordingly, in the closed position, the gate is pivoted downwardly so that the outer race  184  of the gate aligns with the drive surface  156 . In this position, the gate blocks the car from turning down the curved portion, so that the car continues straight through the intersection. In contrast, as illustrated n  FIG.  10   , when the gate is pivoted into the open position, the gate blocks the car from going straight through the intersection. Instead, the curved inner race  182  of the gate aligns with the curved surface of the inner branch  172  and the car turns through the intersection. In other words, when the gate is closed, a car goes straight through the intersection along either the upper rail  130  or the lower rail, depending on the location of the intersection. When the gate is opened, the gate directs the car from either a vertical rail to a horizontal rail or from a horizontal rail to a vertical rail, depending on the location of the intersection. 
     As can be seen in  FIG.  11   , the end of the gate remote from the pivot point of the gate flares outwardly so that the curved inner race matches the curved profile of the inner branch when the gate is open. As a result, the gate has a generally L-shaped configuration. To accommodate the flared end of the gate  180 , the drive surface  156  of the inner branch has a notch or recessed portion. When the gate is closed, the notch provides clearance so that the outer race  184  of the gate lies flat, parallel with the drive surface of the outer branch  176 . Further, in the example shown in  FIG.  11   , the gate is positioned along the upper rail  135  of the track  110 . When the gate is closed, the recess in the inner branch of the intersection  170  allows the gate to lie flat so that it is aligned with the drive surface of the upper rail. 
     In the foregoing description, the gates allow one of the cars to either continue in the same direction (e.g. horizontally) or turn in one direction (e.g. vertically). However, in some applications, the system may include more than two horizontal rails that intersect the vertical columns. In such a configuration, it may be desirable to include a different rail that allows the cars to turn in more than one direction. For instance, if a car is traveling down a column, the gate may allow the car to turn either left or right down a horizontal rail, or travel straight through along the vertical column. Additionally, in some applications it may be desirable to allow the cars to travel upwardly, whereas in the system described above, the cars only travel downwardly through the sorting station. If the cars also travel upwardly in the sorting station, then the gates should be configured to accommodate and guide the cars when the cars travel upwardly through an intersection. 
     The gates  180  are controlled by signals received from the central controller  350 . Specifically, each gate is connected with an actuator  186  that displaces the gate from the opened position to the closed position and back. There may be any of a variety of controllable elements operable to displace the gate. In the present instance, the actuator  186  is a solenoid having a linearly displaceable piston. 
     In the foregoing description, the sorting station  100  is described as a plurality of output bins  190 . However, it should be understood that the system may include a variety of types of destinations, not simply output bins. For instance, in certain applications it may be desirable to sort items to a storage area, such as an area on a storage shelf. Alternatively, the destination may be an output device that conveys items to other locations. According to one example of an output device, the system may include one or more output conveyors that convey pieces away from the sorting system toward a different material handling or processing system. For instance, an output conveyor designated A may convey pieces to a processing center designated A. Therefore, if a piece is to be delivered to processing center A, the car will travel along the track to output conveyor A. Once the car reaches output conveyor A, the car will stop and transfer the piece onto output conveyor A. Output conveyor A will then convey the piece to processing center A. Further, it should be understood that the system may be configured to include a plurality of output devices, such as output conveyors. 
     In some embodiments, the system may include a plurality of output conveyors in addition to the output bins. In other embodiments, the system may only include a plurality of output devices, such as conveyors, and the system is configured to sort the pieces to the various output devices. Further still, the system may be configured to retrieve pieces from storage locations. In such embodiments, the cars may sort pieces to a storage location, such as a bin. Subsequently, one of the cars may travel to the storage location and retrieve the item from the storage location and transport it to one of the output devices. 
     One manner that the cars may retrieve items from the storage locations is by including a conveyor at the storage locations. In this way, an item at a storage location can be conveyed by the conveyor toward the track. When a car arrives at the storage location, the conveyor at the storage location conveys the item onto the car, similar to the manner in which a piece is loaded onto the car at the loading column. Accordingly, the system can sort pieces to a plurality of output devices, in addition to sorting pieces to a plurality of storage locations before subsequently retrieving the pieces and conveying the pieces to the output devices. 
     As discussed above, the system is operable to sort a variety of items to a plurality of destinations. One type of destination is a bin; a second type is a shelf or other location on which the item is to be stored; and a third type of destination is an output device that may be used to convey the item to a different location. The system may include one or more of each of these types or other types of destinations. 
     Delivery Vehicles 
     Referring now to  FIGS.  12 - 17   , the details of the delivery vehicles  200  will be described in greater detail. Each delivery vehicle is a semi-autonomous car having an onboard drive system, including an onboard power supply. Each car includes a mechanism for loading and unloading items for delivery. 
     The car  200  may incorporate any of a variety of mechanisms for loading an item onto the car and discharging the item from the car into one of the bins. Additionally, the loading/unloading mechanism  210  may be specifically tailored for a particular application. However, in the present instance, the loading/unloading mechanism  210  is a conveyor belt. Specifically, referring to  FIGS.  12  and  15   , the loading/unloading mechanism includes a plurality of narrow belts  212  that extend along the top surface of the car between two opposing walls  202 ,  204 . The conveyor belts are reversible. Driving the belts in a first direction displaces the item toward the rearward end of the car; driving the belt in a second direction displaces the item toward the forward end of the car. 
     A conveyor motor  255  mounted on the underside of the car drives the conveyor belts  212 . Specifically, the conveyor belts  212  are entrained around a forward roller  213  at the forward edge of the car, and a rearward roller at the rearward edge of the car. The conveyor motor  255  is connected with the forward roller  213  to drive the forward roller, thereby operating the conveyor belts. 
     The car includes four wheels  220  that are used to transport the car along the track  110 . The wheels  220  are mounted onto two parallel spaced apart axles  215 , so that two or the wheels are disposed along the forward edge of the car and two of the wheels are disposed along the rearward edge of the car. 
     Referring to  FIG.  18   , each wheel comprises an inner idler roller  224  and an outer gear  222  that cooperates with the drive surface  156  of the track. The idler roller  224  rotates freely relative to the axles, while the outer gear is fixed relative to the axle onto which it is mounted. In this way, rotating the axle operates to rotate the gear  222 . Additionally, the idler roller is sized to have a diameter slightly smaller than the distance between the upper wall  152  and the lower wall  154  of the track. In this way, the idler roller may rotate freely within the track, while ensuring that the gear  222  of each wheel remains in operative engagement with the drive surface (i.e. the teeth)  156  of the track. Accordingly, when the vehicle is moving horizontally, the rollers carry the weight of the cart, while the gears  222  cooperate with the drive surface  156  of the track to drive the vehicle along the track. 
     The car includes an onboard motor  250  for driving the wheels  220 . More specifically, the drive motor  250  is operatively connected with the axles to rotate the axles  215 , which in turn rotates the gears  222  of the wheels. As shown in  FIG.  16   , the drive motor  250  is interconnected to the axles  215  via a pair of drive belts  254  that are driven by the drive motor. 
     The drive system for the car may be configured to synchronously drive the car along the track. In the present instance, the drive system is configured so that each gear is driven in a synchronous manner. Specifically, each gear  222  is connected to an end of one of the axles in a manner that substantially impedes rotation of the gear relative to the axle. In this way each axle drives the attached two gears in a synchronous manner. Additionally, in the present instance, both axles are driven in a synchronous manner so that all four gears are driven in a synchronous manner. There are various mechanisms that can be used to synchronously drive the axles. For instance, a pair of drive motors can be used to drive the axles, and the drive motors can be synchronized. However, in the present instance, a single drive motor  250  is used to drive both axles. Each axle includes a timing pulley  226  that is rigidly connected to the axle to prevent rotation of the pulley relative to the axle. Similarly, a timing pulley  228  is connected to the motor shaft. The drive belt  254  connecting the timing pulley  226  on the axle with the motor is a timing belt so that the rotation of the drive motor is precisely linked to the rotation of the axle. Although a single timing belt can be used to drive both axles synchronously, in the present instance, a pair of timing pulleys is connected to the motor shaft, and each timing pulley is connected to a corresponding timing pulley on one of the axles, as shown in  FIG.  16   . 
     The drive motor  250  includes a sensor that is operable to detect the rotation of the motor to thereby determine the distance the car has traveled. Since the gears  222  are rigidly connected with the axles, which are in turn synchronously connected with the drive motor, the forward distance that the car moves corresponds can be exactly controlled to correlate to the distance that the drive motor is displaced. Accordingly, the distance that a car has traveled along the determined path depends on the distance through which the car motor is rotated. 
     To detect the rotation of the drive motor  250 , the motor includes a sensor  252  for detecting the amount of rotation of the drive motor. In the present instance the sensor  252  is a hall sensor. A portion of rotation of the motor corresponds to what is referred to as a tick. The sensor detects the number of ticks and sends a signal to the central processor  350 , which determines how far along the designate path the car has traveled based on the known information regarding the path and the number of ticks that the sensor detects for the motor. 
     As the car travels along the track, an item on top of the car may tend to fall off the car, especially as the car accelerates and decelerates. Therefore, in the present instance, the car includes a retainer  230  to retain the element on the car during delivery. As illustrated in  FIGS.  12 - 17   , the retainer  230  is a hold down that clamps the item against the top surface of the car. 
     The retainer includes an elongated pivotable arm  232 . A biasing element, such as a spring, biases the arm downwardly against the top surface of the retainer  230 . The retainer  230  further includes an operator  234  in the form of a tab. Pushing downwardly on the tab raises the clamp from the top surface of the conveyor to allow a piece to be loaded onto the car or discharged from the car. 
     The car  200  may be powered by an external power supply, such as a contact along the rail that provides the electric power needed to drive the car. However, in the present instance, the car includes an onboard power source  240  that provides the requisite power for both the drive motor  250  and the conveyor motor  255 . Additionally, in the present instance, the power supply is rechargeable. Although the power supply may include a known power source, such as a rechargeable battery, in the present instance, the power supply  240  is made up of one or more ultracapacitors. Ultracapacitors are extremely high energy density capacitors. Capacitors store electrical energy by physically separating positive and negative charges, in contrast to the chemical means a battery uses. Ultracapacitors rely on an electrostatic effect, which is physical rather than chemical, and highly reversible. The ultracapacitors can accept very high amperage to recharge the ultracapacitors. By using a high current, the ultracapacitors can be recharged in a very short time, such as a few seconds or less. 
     The car includes one or more contacts for recharging the power source  240 . In the present instance, the car includes a plurality of brushes  245 , such as copper brushes that are spring-loaded so that the brushes are biased outwardly. The brushes  245  cooperate with a charging rail in the loading station to recharge the power source, as described further below. 
     Each car includes at least one and preferably two load sensors for detecting the items as it is loaded onto the car. The sensor(s) ensure that the mail piece is properly positioned on the car. In the present instance, the car includes a forward loading sensor  260  and a rearward loading sensor  262 . The forward loading sensor detects the leading edge of the item as it is loaded onto the car. The forward loading sensor  260  also detects the trailing edge of the item to ensure that the entire length of the item is loaded onto the car. Similarly, the rearward sensor  262  detects the leading edge and in certain instances, may detect the trailing edge of the mail piece. The loading sensors  260 ,  262  may be simple I/R sensors that detect the presence or absence of a document or mail piece. 
     Although the car operates in response to signals received from the central controller  350 , which tracks the location of each car, the car may also include a reader  265  for reading indicia along the track to confirm the position of the car. For instance, each bin may be assigned a unique bar code, and the forward reader may scan the track or other area around the bin  190  at which an item is to be delivered. The data that the central processor has regarding the path that the car is to follow and the data regarding the distance the car has traveled based on the data regarding the rotation of the drive motor  250  should be sufficient to determine whether the car  200  is positioned at the appropriate bin. Nonetheless, it may be desirable to double check the location of the car before the item is discharged into the appropriate bin. Therefore, the scanner may operate to scan and read information regarding the bin at which the car is stopped. If the scanned data indicates that the bin is the appropriate bin, then the car discharges its item into the bin. Similarly, the car may have a second reader  266  for reading indicia adjacent the rearward edge of the car. The second reader  266  may be used in applications in which the system is set up to utilize a first series of bins  190  along the forward side and a second series of bins along the rearward side of the track  110 . 
     In foregoing description, the cars have drive gears that interact with teeth in the track to guide the cars around the track. Additionally, as described further below in the operation section, the location of the car may be controlled based on information regarding how far the car has traveled. In such applications it is desirable to synchronize the drive wheels of the car. However, in some applications alternative control systems may be used. For instance, the location of the cars can be controlled based on signals from sensors positioned along the track or indicators positioned along the track. In such instances, the cars may be configured to use a drive mechanism that is not synchronous as described above. 
     As discussed further below, the car further includes a processor for controlling the operation of the car in response to signals received from the central processor. Additionally, the car includes a wireless transceiver so that the car can continuously communicate with the central processor as it travels along the track. Alternatively, in some applications, it may be desirable to incorporate a plurality of sensors or indicators positioned along the track. The car may include a reader for sensing the sensor signals and/or the indicators, as well as a central processor for controlling the operation of the vehicle in response to the sensors or indicators. 
     Loading Column 
     Referring now to  FIGS.  6 - 7    the details of the loading column  300  will be described in greater detail. The loading column  300  is formed adjacent the output end of the input station  50 . The loading column  300  is formed of a front pair of vertical rails  305   a ,  305   b  and a corresponding rearward set of vertical rails. The loading station  310  is positioned along the loading column. The loading station  310  is the position along the track in which the car  200  is aligned with the discharge end of the conveyor of the input station  50 . In this way, a mail piece from the input station may be loaded onto the car as it is conveyed toward the car from the input station. 
     Although the central processor  350  tracks the position of the car, a home sensor  312  is positioned adjacent the loading station  310 . When the home sensor detects the car, the position for the car is known relative to a fixed point along the track, and the central processor resets the position of the car to the home or zero position. 
     Referring to  FIG.  7   , a pair of charging rails  314  are disposed along the vertical rails  305   a ,  305   b . The charging rails are conductive strips connected with an electrical supply. The charging contacts  245  of the car  200  engage the conductive strips to recharge the ultracapacitors  240 . Specifically, the biasing element of the brushes  245  biases the brushes outwardly toward the charging contacts. The electricity flowing through the charging contact  245  is a high amperage, low voltage source that allows the ultracapacitors to recharge in a few seconds or less. In addition, since the power supply provided by the ultracapacitors last for only a few minutes, the car recharges each time it travels through the loading column. 
     Additionally, it may be desirable to incorporate a startup charging rail similar to the charging rails described above, but disposed along either the return rail or the rails in the column adjacent to the loading column, depending on where the cars are stored when the cars are shut down. Since the cars use ultracapacitors, it is possible that the ultracapacitors will discharge while the system is shut down. Therefore, upon startup the cars will not have any charge and will not be able to move to the loading column to charge the ultracapacitors. Accordingly, the system may include a startup charging rail disposed along a rail that the cars contact when the cars are stored during shutdown. If the cars are stored in the loading column and the adjacent column during shutdown, then the startup rail is disposed in the column adjacent the loading column. Alternatively, if the cars are stored on the return rail and the loading column during shutdown, then the startup rail is disposed along the return rail. In this way, when the system is started, a charging current is supplied to the cars through the startup charging rail and the charging rail in the loading column. 
     As discussed previously, each car  200  includes a retainer  230  to hold down items on the car during transport. The retainer should be opened at the loading station to allow an item to be loaded onto the car. Accordingly, as shown in  FIG.  6   , an actuator  316  is positioned along the column. The actuator  316  projects inwardly toward the cars as the cars are conveyed up the loading column. As a car is conveyed upwardly in the loading column  300 , the hold down actuator  316  contacts the hold down operator or tab  236 . The interaction between the actuator  316  and the tab  236  causes the retainer to open, so that items can be loaded onto the car. As the car moves upwardly past the actuator  316 , the tab  236  on the car disengages the actuator, thereby releasing the retainer, thereby holding down or clamping the mail piece against the top surface of the vehicle. 
     In the foregoing description, the loading station has been described as a column in which an item is loaded onto the car and the car then travels upwardly to the horizontal upper rail  135 . However, in some applications in may be desirable to configure the loading station so that the items are loaded onto the cars at or near the top of the vertical column. In such an application, the load on the cars would be reduced since the car will not have to lift the item loaded on the car. In order to load the items on the cars at the top of the conveyor, a vertical conveyor may be added to the system. For instance, a conveyor angled upwardly may convey the items upwardly to the top of the column to load the items onto the cars. Alternatively, one or more of a variety of conveyor configurations can be used to transport to items toward the top of the loading column to load the items onto the cars. 
     Operation 
     The system  10  operates as follows. An item is processed at the input station  50  to identify a characteristic of the piece that is indicative of where the piece should be sorted. For instance, the item may be a mail piece that is to be sorted according to department, box number or recipient. If the mail pieces are sorted by department, the piece may be processed to identify either an indicator of the department (such as box number) or the piece may be processed to identify the recipient. The central controller maintains a database that correlates various data to identify the destination bin. For instance, the database may correlate the recipient names with the appropriate department if the mail is being sorted according to department. In other embodiments, the piece may be a part that has a product code and the database may correlate the product code with the sort location. 
     As discussed previously, the input station may process the items automatically or manually. In a manual mode, the operator manually enters information regarding a piece and then drops the piece on a conveyor. The system electronically tags the piece with the sort information and the conveyor conveys the piece toward the loading station. Alternatively, if the input system is an automated system, the piece is automatically scanned to identify the relevant sort characteristic. For instance, the input station may use a scanner, such as a bar code scanner to read the postnet code on a piece, or the input station may include an imaging device, such as a high speed line scan camera in combination with an OCR engine to read information on the piece. 
     To prepare to receive an item, a car  200  moves along the track toward the loading station  310  in the loading column  300 . As the car approaches the loading station, the operator  236  for the hold down  230  engages the actuator  316 , which pivots the hold down upwardly to prepare the car to receive an item, as illustrated in  FIG.  6   . When the car  200  moves into position at the loading station  310  the home sensor detects the presence of the car and sends a signal to the central processor  350  indicating that the car is positioned at the loading station. In the following description, the item being sorted is described as being a mail piece. It should be understood that such an item is an exemplary application of the system. As described above, the system can be configured to sort a variety of items in a variety of material handling applications. 
     Once the car is positioned at the loading station, the input station conveys a mail piece onto the car. As the mail piece is being conveyed onto the car  200 , the loading mechanism  210  on the car loads the mail piece onto the car. Specifically, the input station conveys the mail piece into contact with the conveyor belts  212  on the car. The conveyor belts  212  rotate toward the rearward side of the car, thereby driving the mail piece rearwardly on the car. 
     The operation of the conveyor belts is controlled by the loading sensors  260 ,  262 . The forward loading sensor detects the leading edge of the mail piece as the mail piece is loaded onto the car. Once the forward loading sensor  260  detects the trailing edge of the mail piece, a controller onboard the car determines that the mail piece is loaded on the car and stops the conveyor motor. Additionally, the onboard controller may control the operation of the conveyor in response to signals received from the rearward sensor  262 . Specifically, if the rearward sensor  262  detects the leading edge of the mail piece, then the leading edge of the mail piece is adjacent the rearward edge of the car. To ensure that the mail piece does not overhang from the rearward edge of the car, the controller may stop the conveyor once the rearward sensor detects the leading edge of the mail piece. However, if the rearward sensor detects the leading edge of the mail piece before the forward sensor detects the trailing edge of the mail piece, the controller may determine that there is a problem with the mail piece (i.e. it is too long or two overlapping mail pieces were fed onto the car. In such an instance, the car may communicate an error message with the central controller, which may declare an error and provide an indicator to the operator that the car at the loading station requires attention. Alternatively, a reject bin  325  may be positioned behind the loading station so that mail pieces on the car at the loading station can be ejected into the reject bin  325 . In this way, if there is an error loading a mail piece onto a car, the mail piece can simply be ejected into the reject bin, and a subsequent mail piece can be loaded onto the car. 
     After a mail piece is loaded onto the car, the car moves away from the loading station. Specifically, once the onboard controller detects that a mail piece is properly loaded onto the car, the onboard controller sends a signal to start the drive motor  250 . The drive motor  250  rotates the axles, which in turn rotates the gears  222  on the wheels  220 . The gears  222  mesh with the drive surface  156  of the vertical rails  305  in the loading column to drive the car upwardly. Specifically, the gears and the drive surfaces mesh and operate as a rack and pinion mechanism, translating the rotational motion of the wheels into linear motion along the track  110 . 
     Since the cars move up the loading column from the loading station, the destination for the car does not need to be determined until after the car reaches the first gate along the upper rail  135 . For instance, if an automated system is used at the input station to scan and determine the characteristic used to sort the mail pieces, it may take some processing time to determine the relevant characteristic. The time that it takes to convey the mail piece onto the car and then convey the car up the loading column will typically be sufficient time to determine the relevant characteristic for the mail piece. However, if the characteristic is not determined by the time the car reaches the upper rail, the car may be directed down the second column, which is the column next to the loading column. The car travels down the second column to the lower rail  140 , and then back to the loading column. The car may stop in the second column to provide additional time to determine the characteristic. However, after waiting for a pre-determined period the system may declare that the address cannot be determined and the car may be advanced from the second column and the piece may be discharged to a reject bin. Alternatively, rather than declare an error the car may continue to travel around the loop from the loading column to the second column until the characteristic is determined or until a predetermined time at which the central controller declares an error. Additionally, rather than using the reject bin when the system is unable to determine the characteristic for a mail piece, one of the bins in the second column can also be used as a reject bin. In this way, the cars are ready to receive a mail piece as soon as the car reaches the loading station, without having to eject the problem mail piece into the reject bin  325  at the loading station. 
     As described above, the system includes a loop that can be utilized as a buffer track to provide additional processing time to analyze the characteristic for the mail piece if necessary. Although the first and second columns can be used as the buffer loop, other columns can be used as a buffer loop if desired. 
     The foregoing discussion described the process for buffering a car if the system is unable to determine the characteristic for the mail piece by the time the car reaches the top rail. However, for most mail pieces, the system should be able to identify the characteristic without having to buffer the car. The following discussion describes the operation of the system assuming that the characteristic for the mail piece is determined before the car reaches the upper rail  135 . 
     Once the characteristic for the mail piece is determined, the central controller  350  determines the appropriate bin  190  for the mail piece. Based on the location of the bin for the mail piece, the route for the car is determined. Specifically, the central controller determines the route for the car and communicates information to the car regarding the bin into which the mail piece is to be delivered. The central controller then controls the gates along the track to direct the car to the appropriate column. Once the car reaches the appropriate column the car moves down the column to the appropriate bin. The car stops at the appropriate bin  190  and the onboard controller sends an appropriate signal to the conveyor motor  255  to drive the conveyor belts  212 , which drives the mail piece forwardly to discharge the mail piece into the bin. Specifically, the top of the car aligns with the gap between the appropriate bin  190  and the bottom edge of the bin that is immediately above the appropriate bin. 
     As discussed above, the central controller  350  controls the operation of the gates  180  in response to the location of the car  200  and the route that the car is to follow to deliver the mail piece. Additionally, as discussed below, the central controller controls the gates in response to the position of other cars on the track. 
     As the car  200  travels along the upper rail  135  and approaches a column, the gates for the vertical rails  130  are controlled as follows. If the car is to pass over the column on the way to the next column, the gates are displaced into the closed position, as shown in  FIG.  9   . Specifically, both gates at the top of the column are closed so that the outer race  184  of the gate aligns with the straight track, with the outer race aligning with the drive surface  156  of the track  110 . In this way, the gates provide a straight drive surface that cooperates with the drive surface  156  to allow the car to travel over the column. 
     When the car comes to a column that it is to turn down, the gates are controlled as follows. Referring to  FIG.  5   , the columns can be seen without the bins attached. The view in  FIG.  5    is from the front of the apparatus  10 , so the car will be traveling along the upper rail from the right to the left in the perspective of  FIG.  5   . In the following discussion, the car is to be conveyed to a bin in the column designated C in  FIG.  5   . Column C includes two pairs of vertical legs. The first pair is front and back vertical legs  130   c  on the left side of column C; the second pair is front and back vertical legs  130   d  on the right side of column C. 
     In order for the car to travel down column C, the wheels on the left side of the car travel down legs  130   c  and the right side wheels travel down legs  130   d . Therefore, as the car approaches column C, the gates at the top of  130   d  are displaced to the closed position so that the left side wheels remain on the upper rail and pass over the right side legs  130   d . After the left side wheels of the car pass over the right legs  130   c , the gates  180  at the top of the right legs  130   d  are displaced into the open position so that the right side wheels can turn down legs  130   d . Specifically, after the left side wheels pass right legs  130   d , the central controller operates the solenoids  186  of the gates  180  at the top of legs  130  to displace the gates into the open position, as shown in  FIG.  8    (note that the view in  FIG.  8    is taken from the rear side of the apparatus so that the perspective of the gates is reversed relative to the front side). The gates  180  block the straight path through the intersection  170  and the curved inner race  182  of the gates direct the right side wheels down vertical legs  130   d . Similarly, the gates  180  at the top of the left side legs  130   c  are displaced into the open position to direct the left side wheels down vertical legs  130   c.    
     As the car approaches the intersections at the bottom of legs  130   c  and  130   d , the gates are operated similarly to the above description, but in reverse. Specifically, as the car approaches the intersections  170  at the bottom of legs  130   c  and  130   d , the gates  180  in the intersections are displaced into the opened position so that the gates direct the forward and leading wheels to turn down the lower rail. From the perspective of  FIG.  5   , the car travels from left to right after the car reaches the lower rail. After the car passes though the intersections at the bottom of the rails  130   c ,  130   d , the gates at the bottom of right side legs  130   d  are displaced into the closed position before the left side wheels of the car reach the intersection at the bottom of the right side legs  130   d . In this way, the left side wheels of the car pass straight through the intersection at the bottom of legs  130   d  along the bottom rail  140 . 
     As discussed above, the central controller  350  controls the operation of the gates in response to the position of the car and more specifically in response to the position of the left hand and right hand wheels of the car. The gates are fired sequentially to ensure that the different pairs of wheels are directed down the proper vertical legs. Alternatively, the operation of the gates may be controlled by signals received from the cars. Specifically, the cars may include a transmitter that transmits a signal to the central controller indicating that it is in proximity to a gate that is to be fired. Further still, the car may include an indicator that may be scanned as the car approaches the gate. Based on the indicator and the know destination for the car, the gate may fire. Still further, the car may include an mechanical actuator that selectively triggers or actuates a gate to appropriately direct the car. 
     One of the advantages of the system as described above is that the orientation of the cars does not substantially change as the cars move from travelling horizontally (along the upper or lower rails) to vertically (down one of the columns). Specifically, when a car is travelling horizontally, the two front geared wheels  220  cooperate with the upper or lower horizontal rail  135  or  140  of the front track  115 , and the two rear geared wheels  220  cooperate with the corresponding upper or lower rail  135  or  140  of the rear track  120 . As the car passes through a gate and then into a column, the two front geared wheels engage a pair of vertical legs  130  in the front track  115 , and the two rear geared wheels engage the corresponding vertical legs in the rear track  120 . 
     As the car travels from the horizontal rails to the vertical columns or from vertical to horizontal, the tracks allow all four geared wheels to be positioned at the same height. In this way, as the car travels along the track it does not skew or tilt as it changes between moving horizontally and vertically. Additionally, it may be desirable to configure the cars with a single axle. In such a configuration, the car could be oriented generally vertically as opposed to the generally horizontal orientation of the cars described above. In the single axle configuration, the weight of the cars could maintain the orientation of the cars. However, when using a vertical single axle car, the orientation of the sort locations would be re-configured to accommodate the vertical orientation of the cars. Similarly, the loading station would also be re-configured to load the pieces onto the cars in the vertical orientation. 
     Traffic Control 
     Since the system includes a number of cars  200 , the system controls the operation of the different cars to ensure the cars do not collide into one another. In the following discussion, this is referred to as traffic control. 
     A variety of methodologies can be used for traffic control. For instance, the traffic control can be a distributed system in which each car monitors its position relative to adjacent cars and the onboard controller controls the car accordingly. One example of such as system utilizes proximity sensors on each car. If the proximity sensor for a car detects a car within a predefined distance ahead of the car, the onboard controller for the trailing car may control the car by slowing down or stopping the trailing car. Similarly, if a car detects a car within a predefined distance behind the car, the lead car may speed up unless the lead car detects a car ahead of it within the predefined distance. In this way, the cars may control the speed of the cars independently based on the feedback from the proximity sensors. 
     Although the system may use a distributed system for traffic control, in the present instance, the system uses a centralized system for traffic control. Specifically, the central controller  350  tracks the position of each car  200  and provides traffic control signals to each car based on the position of each car relative to adjacent cars and based on the route for each car. 
     In the present instance, the central controller  350  operates as the traffic controller, continuously communicating with the cars as the cars travel along the track  110 . For each car, the central controller determines the distance that each car can travel, and communicates this information with the cars. For instance, if car B is following car A along the track, and car A is at point A, car B can safely travel to a point just before point A without crashing into car A. As car A advances to a subsequent point B along the track, car B can travel safely to a point just before point B without crashing into car A. 
     The cars continuously communicate with the central controller to provide information indicative of their positions, so that the central controller can continuously update the safe distances for each car as the cars advance around the track. 
     Although the foregoing discussion is limited to determining safe zones based on the positions of the various cars on the track, the determination of safe zones is based on other factors that affect the traffic. For instance, when calculating the safe distance for a car, the central controller considers the distance between the car and the next gate, as well as the distance to the destination bin for the car. 
     As can be seen from the foregoing, increasing the frequency of communication between the cars and the central controller increases the efficiency of the traffic flow along the track. Accordingly, in the present instance, the traffic control is designed to communicate with a car once for every inch the car travels along the track. Therefore, if a car travels at 25 inches per second, the central controller communicates with the car every 40 msec. Further, it is desirable to have the cars travel at up to 50 inch/sec. Therefore, it is desirable to configure the communications to allow the cars to communicate with the central controller every 20 msec. 
     In addition, to the foregoing variables used to calculate safe distances, information regarding the track profile ahead of each car is used to calculate safe distances. For instance, the central controller determines whether the path ahead of a car is sideways movement, uphill movement (i.e. movement vertically upwardly) or downhill movement (i.e. movement vertically downwardly). 
     One of the issues in traffic control relates to merging at intersections  170 . The problem arises when a car needs to merge onto the return rail  140 . If two cars will arrive at the intersection close enough to collide, one of the cars needs to have priority and the other car needs to wait or slow down to allow the first car to go through. 
     A first method for controlling merging traffic is based on determining the next gap large enough for a car to have time to pass through an intersection without colliding with another car. In other words, if a first car approaches an intersection and it is determined that the gap between the first car and a second car is not sufficient for the first car to pass through, the first car waits at the intersection until there is a gap large enough to allow the first car to pass through. 
     A second method for controlling merging traffic is based on determining which car is closest to the homing sensor at the loading station  310 . The car with the shortest distance to the homing sensor gets priority at the intersection. 
     Another factor that the traffic controller considers when calculating safe distances relates to the position of cars in adjacent columns. In the present instance, most of the adjacent columns share a common vertical rail. For instance, in  FIG.  5   , the leftmost column uses vertical rails  130   a  and  130   b . The column next to the leftmost column uses vertical rails  130   b  and  130   c.    
     However, in the present instance, some of the columns may have two vertical rails  130  that are independent from the adjacent columns. For instance, the loading column  300  has two independent rails that are not shared with the adjacent column. Therefore, cars can travel up the loading column without regard to the position of cars in the column next to the loading column. Furthermore, as shown in  FIG.  5   , it may be desirable to configure the column next to the loading column so that it also has two independent vertical rails. In this way, cars can more freely travel up the loading column and down the adjacent column to provide a buffer loop as described previously. 
     Accordingly, when calculating safe distances, the traffic controller evaluates the position of cars in adjacent columns if the cars share a common vertical rail to ensure that the two cars do not collide as the car travel down the adjacent columns. 
     In the foregoing discussion, the sorting of items was described in relation to an array of bins disposed on the front of the sorting station  100 . However, as illustrated in  FIGS.  2  &amp;  4   , the number of bins in the system can be doubled by attaching a rear array of bins on the back side of the sorting station. In this way, the cars can deliver items to bins on the front side of the sorting station by traveling to the bin and then rotating the conveyor on the car forwardly to eject the piece into the front bin. Alternatively, the cars can deliver items to bins on the rear side of the sorting station by traveling to the bin and then rotating the conveyor on the car rearwardly to eject the piece into the rear bin. 
     Additionally, the sorting station  100  is modular and can be readily expanded as necessary simply by attaching an additional section to the left end of the sorting station. Further, although the foregoing describes the array of bins as being essentially a two dimensional array in which the cars simply travel in X and Y directions, the sorting station can be expanded to add additional “runs” of track. Specifically, a separate sorting station parallel to or perpendicular to the sorting station illustrated in  FIG.  2    may be connected to the sorting station. In this way, the car would travel in a third dimension relative to the X and Y directions of the sorting station illustrated in  FIG.  2   . For instance, additional sections of track may be connected to the sorting station illustrated in  FIG.  2    perpendicular to the illustrated sorting station, so that the additional track forms an L-shape intersecting the loading column. In such a configuration, gates selectively direct the cars either down the upper rail  135  or rearwardly toward the additional track. Similarly, a plurality of parallel rows of sorting stations can be interconnected so that the cars selectively travel along a crossover rail until the car reaches the appropriate row. The car then travels down the row until it reaches the appropriate column as described above. 
     It will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. For instance, in the foregoing description, the operation of the sorting station is described as being centralized with the central controller. However, it may be desirable to have the cars control the operation of the gates. According to one alternative, the cars incorporate one or more mechanical actuators that cooperate with an operator on the gate. The actuators on the cars are operable between first and second positions. In a first position, the actuator engages the gate operator to displace the gate into the closed position. In a second position, the actuator engages the gate to displace the gate into the open position. Alternatively, the gate may be biased toward the opened position, so that when the car actuator is in the second position it does not engage the gate operator. In another alternative, each car includes a mechanism for communicating with each gate. If the gate needs to be pivoted to direct an approaching car along a particular path, the car sends a signal to the gate indicating whether the gate should be opened or closed. In response to the signal from the car, the gate pivots to the appropriate position. 
     Further, in the above description, the system uses a wireless communication between the cars and the central controller. In an alternative embodiment, a communication line may be installed on the track and the cars may communicate with the central controller over a hard wired communication link. Still further, the system has been described as being useful in sorting incoming mail. However, the system may also be utilized to sort and prepare outgoing mail. For instance, after determining a characteristic for a mail piece, the system may print a marking onto the mail piece. For instance, after determining the recipient&#39;s address for a mail piece, the system determines which bin the mail piece is to be sorted to. As the mail piece is conveyed to the bin, a printer prints the appropriate postnet bar code on the piece before sorting the piece. To provide the printing functionality, the system may include a printer disposed along the track. When the car approaches the printer the car stops and at least partially discharges the mail piece to extend the mail piece toward the printer. The printer then prints the appropriate postnet code. The car then reverses the conveyors to load the piece back onto the car all the way, and then travels to the appropriate bin. Similarly, the system may include a device for selectively applying labels to the pieces. Similar to the above example of printing markings onto the pieces, the labeler may be positioned along the track. The cars selectively stop at the labeler on route to the appropriate bin and at least partially discharge the mail piece toward the labeler. The labeler then applies a label onto the mail piece and the conveyor on the car then reverses to load the piece back onto the car. 
     In addition to outgoing mail applications, it may be desirable to incorporate a printer and/or a labeler in systems configured to process incoming mail. For instance, when sorting incoming mail pieces, it may be desirable to print certain information, such as sort codes, a time stamp or audit trail information onto some or all of the pieces being processed. In some instances such information may be printed directly onto the mail pieces. In other instances, a label may be applied to the mail pieces and the information may be printed on the label. 
     In addition to a printer and a labeler, the system may include a scale for weighing the mail pieces. The scale may be positioned along the track  110 , such as along the loading column. To weigh a piece, the car stops adjacent the scale, and ejects the piece from the car onto the scale by driving the conveyor belts  212 . Preferably, the scale includes a conveyor or transfer mechanism for discharging the piece from the scale and back onto the car or onto a subsequent car. When the piece is loaded onto the car from the scale, the car drives the conveyors to load the piece as discussed above in connection with the loading station. 
     It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the claims.