Abstract:
A sorter conveyor system according to the invention includes at least one endless conveyor loop including a rail. One or more conveyor segments are mounted on the rail. Each segment is a series of cart units each having wheel structures mounted for rolling movement along the rail, a tray for carrying one or more items thereon, a selectively actuable mechanism for tilting the tray laterally in at least one direction to unload an item from the tray to an unloading station adjacent the conveyor loop, and a coupling mechanism for joining each cart unit in each series in a head to tail relationship. One or more drive elements are connected to one or more of the cart units and configured to permit the conveyor segment to be driven by a linear drive unit. A drive system is provided which includes a plurality of linear drive units, preferably linear induction motors (LIM&#39;s) disposed at spaced positions along the conveyor loop for driving each of the drive elements of the cart units in each segment, such that each conveyor segment can each be driven independently of each other conveyor segment by selective actuation of the linear induction drive units.

Description:
FIELD OF THE INVENTION 
     The invention relates to sorting using a tilt tray conveyor, particularly to an apparatus and method for sorting items using multiple carts traveling around a closed loop. 
     BACKGROUND OF THE INVENTION 
     The postal system and high volume package shipping industry use tilt tray conveyor systems to sort bundles of letters and packages according to their respective destinations. Specialized sorters sort a bundle or package by destination zip code. During operation, an input stream of parcels is placed on a tilt tray conveyor and sorted into multiple output streams. The conveyor sorts the packages by tilting and ejecting them to either another appropriate conveyor or to an intermediate destination such as an unloading station. 
     Prior art tilt tray conveyor systems comprise a series of tilt tray carts linked together in a continuous loop. According to one known tilt tray conveyor system, the trays are secured to an endless drive chain, which pulls the trays around the loop. See Muller U.S. Pat. No. 3,662,874, issued May 16, 1972. According to another known tilt tray conveyor system known as the Mantissa Scorpion, linear induction motors (LIM&#39;s) are disposed at intervals around the loop for acting on a horizontally or vertically disposed plate (drive element) on each cart. The frame of each cart is T-shaped with a single axle, so that each cart depends on an adjoining cart for support. 
     FIG. 1 illustrates a conventional loop  5  of LM-driven tilt tray carts  10  connected head to tail and mounted on an endless, generally oval-shaped rail  12 . The continuous loop of carts creates significant inefficiencies in the conveyor system. First, the system&#39;s strength depends literally on its weakest link. For example, if one cart  10  or its tray fails, the entire system must be stopped until the cart is repaired or replaced. Second, inefficient loading frequently occurs. The system may skip carts to maintain conveyor speed. This creates a situation in which empty carts are pulled around the loop, thereby resulting in wasted energy and system capacity. Additionally, some applications require large distances between input and output streams. Increased costs associated with longer cart chains may prohibit using a continuous chain conveyor system in a large loop. 
     Referring now to FIG. 2, transferring parcels between multiple loops  5 A and  5 B requires unloading the parcel from loop  5 A and transferring it to the other loop  5 B by a gravity slide  22  which feeds parcels to conventional conveyor belt  24 . Belt  24  delivers the parcels to a powered induction station  26  which loads it onto a tray of a cart  10  in loop  5 B. The potential for parcel damage occurs with each transfer to and from the carts  10 . This manner of transfer between loop  5 A and loop  5 B introduces many opportunities for the item to be damaged because moving an item to or from trays involves subjecting the item to forceful impacts. 
     Inefficiencies caused by the method of locomotion also exist. According to another known conveyor design called the NovaSort, a product of Siemens ElectroCom, L.P., a train or segment of tilt tray carts connected end to end is drawn by a leading cart having an engine in the manner of a monorail. The lead cart draws power from a sliding electrical contact on the rail. This design suffers the customary drawbacks of systems that rely on sliding electrical contacts. In addition, the carts of each segment contain a solenoid that actuates the tilting mechanism on each cart, thus adding to the weight and complexity of the system. 
     Accordingly, a low-maintenance cart system is needed that reduces the potential for parcel damage created by cart transfers between loops. 
     SUMMARY OF THE INVENTION 
     A sorter conveyor system according to the invention includes at least one endless conveyor loop including a rail. One or more conveyor segments are mounted on the rail. Each segment is a series of cart units each having wheel structures mounted for rolling movement along the rail, a tray for carrying one or more items thereon, a selectively actuable mechanism for tilting the tray laterally in at least one direction to unload an item from the tray to an unloading station adjacent the conveyor loop, and a pivotable coupling mechanism for joining each cart unit in each series in a head to tail relationship. One or more drive elements are connected to one or more of the cart units and configured to permit the conveyor segment to be driven by a linear drive unit. A drive system is provided which includes a plurality of linear drive units, preferably linear induction motors (LIM&#39;s) disposed at spaced positions along the conveyor loop for driving each of the drive elements of the cart units in each segment, such that each conveyor segment can each be driven independently of each other conveyor segment by selective actuation of the linear induction drive units. The first and last carts in each segments are connected to only one adjoining cart, that is, are not connected or adjacent to each other in a manner effective to form a continuous cart loop as in the prior art. Where the system has two or more cart segments, for example, selective control of the LIM&#39;s can be used to move one segment independently of other segments on the same rail, but without need for an “engine”, i.e., a front or rear cart that pulls or pushes the series of carts in a manner analogous to a railroad train engine. 
     A linear drive unit as referred to herein means any form of conveyor drive, including both mechanical and linear induction, that exerts a force on a cart as it passes by, propelling the cart linearly (in the direction of the rail the cart is traveling on). The force may be exerted intermittently, as when a fin or plate on the cart passes by the linear drive unit, or continuously, as where the fin or plate spans multiple carts in the segment. In the alternative, spaced drive elements may be deployed on some carts and not others, such as on every other cart in the segment, as long as there are enough drive elements to keep the entire segment moving as required by the system design. 
     The invention further provides a method for sorting and conveying using a sorter conveyor system as described above. The method comprises the steps of moving the conveyor segment past a loading station, loading items onto the trays of one or more of the carts as the carts pass the loading station, actuating the linear drive system to move the segment of carts past a row of unloading stations, and unloading items from the cart trays to the unloading stations in accordance with a sorting scheme. Since the cart segment does not occupy the entire rail, the linear drive units may if desired be actuated only as a drive element of a cart is passing by. Similarly, two or more cart segments may be independently controlled on the same rail, for example, as where one is passing the loading station as the other is passing the unloading stations, after which the two cart segments exchange roles. These and other aspects of the invention are discussed in the detailed description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements, and: 
     FIG. 1 is a schematic diagram of a conventional linear induction drive (LID) tilt tray sorter tilt tray sorter) having carts connected head to tail; 
     FIG. 2 is a schematic diagram of a conventional method for transferring parcels between loops of tilt tray sorter systems; 
     FIG. 3 is a perspective view of conventional LID tilt tray sorter components usable in the present invention; 
     FIG. 4 is a partial perspective view of a LID with a drive element for the sorter of FIG. 3; 
     FIG. 5 is a schematic diagram of a segmented LID tilt tray sorter according to the present invention showing two segments; 
     FIG. 6 is a schematic diagram of a cart segment according to the invention. 
     FIG. 7 is a schematic diagram of a multi-loop segmented LID tilt tray sorter according to the invention showing a transfer system between loops; 
     FIG. 8 is an alternative form of the sorter of FIG. 7; 
     FIG. 9 is a schematic diagram of a segmented LID tilt tray sorter according to the invention having a set of sidetracks for isolating broken or out of service conveyor segments; and 
     FIG. 10 is a schematic diagram of a segmented LID tilt tray sorter control system. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 3 and 4, carts  10  for use in the present invention may be substantially the same as systems presently in use, but with certain key differences as described hereafter. Carts  10  have rollers  11  that allow carts  10  to follow and move freely around the track  12 . The undersides of carts  10  also have centrally mounted vertical linear induction drive elements  13 . Electric linear induction motors (LIMs)  14  spaced around track  12  at regular intervals act upon linear induction drive elements  13  and propel carts  10 . Each cart  10  is fitted with a tilt tray mechanism including a tiltable tray  15 . A variety of items, for example packages, bundles of mail, or parcels, are loaded onto the trays  15  from a loading station  2  and conveyed around the track  12  until the item reaches a row of unloading stations  4 . The items may be off-loaded into one or more output streams that correspond to a parcel&#39;s destination by selectively tilting trays  15  by actuating tilting mechanisms  16  to specific unloading stations  4  according to a sort scheme in a manner known in the art. The Mantissa Scorpion tilt tray conveyor system, made by Mantissa Corporation of Charlotte, N.C. is a preferred tilt tray mechanism for use in the present invention, but other commercially available tilt tray mechanisms could be used. 
     Referring now to FIG. 5, a LID tilt tray sorter according to the invention includes two independent segments  6 A,  6 B of carts on a single closed loop track  12 . Because each cart has a linear induction drive element  13 , LIMs  14  may drive each cart segment  6 A,  6 B independently around track  12 . This feature eliminates the necessity of linking all carts  10  in a closed loop. Carts  10  used to make separately movable trains or cart segments  6 A,  6 B are most preferably Mantissa Scorpion LID carts as described above. However, as shown in FIG. 6, the Scorpion carts are essentially T-shaped and rely on each other for support as illustrated. Each cart  10  is joined by a suitable pivoting coupling, such as a ball and socket joint  17 , tail to head with the cart behind it. Accordingly, the last cart  10 A in the segment is preferably modified to have an additional set of rollers  11 A and has a double axle  23  rather than a single one. Rollers  11 A may if necessary be provided with casters to permit cart  10 A to travel around curves. The equivalent arrangement in reverse, wherein each cart frame is an inverted T-shape and the front cart  10  has the double axle, is also within the scope of the present invention. Thus, in the present invention it is most advantageous to have one double axle cart  10 A per segment at an end position, while the remainder of the carts are single  23  axle carts relying at one end for support on an adjacent cart  10  or  10 A. 
     Multiple segments  6 A,  6 B allow greater flexibility in system design. Segments  6 A,  6 B may be operated with only the number of carts  10  necessary for a desired process. This eliminates the expense of extra carts that are only required to complete the chain around the loop. The length of each segment  6 A,  6 B may be adjusted to match the volume of packages conveyed to a particular location. Independently operating segments  6 A,  6 B on a single track allows for a more efficient sorting process. A LID tilt tray sorter of the invention having several segments  6 A,  6 B of varying lengths can accommodate many different sorting processes. According to a preferred aspect of the invention, the spacing of LIMs  14  may vary from conventional spacing based on a predetermined minimum size for segments  6 . The preferred minimum distance between adjacent LIMs in the main loops  5 A,  5 B is the length of the shortest segment  6 , such that a segment  6  is always over at least one LIM  14 . 
     FIG. 7 illustrates a LID tilt tray sorter according to the invention having multiple loops or sub-loops  5 A,  5 B connected by a pair of parallel crossover tracks  28 A,  28 B and switches  30 A- 30 D at opposite ends of each segment  28 A,  28 B. Each switch  30  has a movable track section  31 A or  31 B that operates in either a transfer position or a loop position in the manner of a raiload switch. In the transfer position, switch  30 A directs a segment of carts  6 C to follow interconnecting track  28 A to transfer the segment from loop  5 A to loop  5 B as shown. In the loop position, switch  30 B sends segment  6 B around loop  5 A. 
     Tracks  28 A,  28 B may be provided with spaced LIM&#39;s in the same manner as loops  5 A,  5 B. If tracks  28  are short in comparison to the cart segments, it may not prove necessary to provide spaced linear induction motors along tracks  28 , since the LIM&#39;s of the respective loops and the momentum of the cart segments may be sufficient to make the transfer. On the other hand, if tracks  28  are long and transfers between loops  5 A,  5 B are rare, it may be more economical to find an alternative means for moving the segments along, such changing the elevation of the loops to rely on gravity to make the transfer, moving the cart segments manually, or providing a suitable propulsion system other than a linear induction drive which acts on the segment during transfer. 
     A multi-loop system according to FIG. 7 may be operated so that certain cart segments  6 A and  6 B, act as “local” carriers and remain on loop  5 A and/or  5 B at all times, whereas others (such as  6 C) are regularly transferred at switches  30 A- 30 D so that these segments circulate about the larger oval defined by both of loops  5 A,  5 B and tracks  28 A,  28 B. In the alternative, the sort scheme logic may be designed to cause crossover to occur any time a segment has been loaded with an item (or items) destined for unloading stations in each of rows  4 A,  4 B. 
     FIG. 8 illustrates a simplified version of the system of FIG. 7, wherein switches  30 C,  30 D are elininated, resulting in a first small oval shaped loop  5 A and a second, larger loop  5 B created as an extension of loop  5 A. Segments  6 A- 6 D are loaded with items from a common loading station  2  on loop  5 A. Segments  6 A,  6 B deliver only to a first row of local unloading stations  4 A representing more common destinations, whereas segments  6 C and  6 D also unload at remote unloading stations  4 B located on loop  5 B representing less common destinations. This embodiment of the invention permits four segments  6 A- 6 D to pass the more common unloading stations  4   a , whereas only two segments pass and sort to the less common unloading stations  4 B. At the single loading station  2 , computer controlled sort scheme logic may, for example, ensure that items destined for one of unloading stations  4 B are loaded onto one of segments  6 C or  6 D only. 
     FIG. 9 shows a LID tilt tray sorter according to the invention having several additional loops  5 C to  5 F which can serve as a holding area for carts with tray contents that require delayed delivery. One loop  5 C can optionally be used as a “bone yard” or maintenance/storage area for an unused or broken cart segment  6 E. In this embodiment, switches  30 B and  30 D may be three position switches as shown. Segments  6  may be transferred from the sorting loops  5 A,  5 B to one of the loops  5 C- 5 F by associated switches  30 D- 30 G. Loop  5 C can provide an area separate from the active sorting process to perform preventative maintenance or repair work on the carts  10 , and may adjoin a storage rack  19  for carts that have been removed from the system. A loaded or unloaded cart segment that has been diverted to one of loops  5 D- 5 F can be reactivated when ready and moved through return switches  30 I- 30 K along a common return track  7  and back into loop  5 A through switch  30 B. 
     FIG. 10 shows one example of a LID tilt tray sorter control system for operating a tilt tray sorting system of the invention as shown in FIG. 7. A personal computer  40  actuates a series of solenoids  42 A- 42 D that control the switches  30 A- 30 D. Computer  40  controls the LIMs  14 , which drive carts  10 , and the tilting mechanisms  16  which tilt the trays  15  for unloading at stations  4 . Programming computer  40  allows a user to automatically control the path and movements of segments  6 A- 6 D in accordance with a predetermined sorting scheme. Each segment  6 A- 6 D can be directed to sort items around the loops  5 A,  5 B (or enter one of the loops  5 C- 5 F, in the embodiment of FIG.  9 ). Computer  40  also controls loading items onto the cart  10  at loading stations  2 A,  2 B as well as actuation of the tray tilt mechanisms  16  at specific unloading stations  4 A or  4 B. 
     For control purposes, it may prove useful to provide readable panels, such as reflective panels or light-scanable bar codes on each cart as so that unloading only occurs when the correct identification is detected at the unloading station, as for example, by scanning a bar code affixed to the cart frame. However, it may also prove possible using computer  40  to operate the system without uniquely identifying each individual cart for unloading purposes. By tracking the location of the lead cart in a segment  6  and storing data identifying the number of carts  10  in that segment and the respective contents relative to a corresponding row of unloading stations  4 A or  4 B, sorting logic may then be used to match each specific cart  10  with its respective unloading station  4 . It may prove necessary in some cases to provide sensors throughout the loops  5 A- 5 C, not merely proximate the unloading stations  4 A,  4 B, so that computer  40  knows the exact or approximate position of each segment  6 A- 6 E at all times so that switching errors and the like can be avoided. 
     In a typical operation using the embodiment of FIG. 5, computer  40  receives information from a sensor  46  such as a bar code scanner concerning the destination of each of series of packages. The stream of packages is loaded from loading station  2  onto successive carts of a segment  6 A. Computer  40  stores in memory a table of the item destination for each successive cart  10 . Computer  40  also has in memory a table of the successive unloading stations  4  and the destination corresponding to each. As segment  6 A passes the row of unloading stations  4 , computer  40  activates the tilt mechanism of each cart  10  to be actuated when that cart  10  is in registration with the matching unloading station  4 . As noted above, where the number of carts is known relative to the number of unloading stations and the carts are configured with the same spacing as the row of unloading stations, then the position of the lead cart sufficiently identifies the position of all carts in the segment for unloading purposes. However, to ensure accuracy, each cart may be detected as it enters each unloading station in a manner known in the art. 
     As segment  6 A is unloading, the other segment  6 B is loading at loading station  2 , and the computer  40  operates LIM&#39;s  14  as needed to keep segments  6 A,  6 B in opposing positions on loop  5 . Segments  6 A,  6 B then reverse roles again as segment  6 B approaches unloading stations  4 . Under conditions where less than all carts in a full loop are filled with items, this embodiment avoids wasted energy associated with driving empty carts continuously around the circle. 
     In the embodiment of FIG. 8, computer  40  additionally maintains in memory a table of common unloading stations  4 A and rare unloading stations  4 B. If a cart  10  is loaded with an item that must be unloaded at a rare destination  4 B, then computer  40  operates switch  30 A upon the approach of that cart segment  6  and sends it to pass by unloading stations  4 B. In the alternative, the system may be controlled so that segments  6 C,  6 D always travel on loop  5 B and pass by stations  4 B, and segments  6 A,  6 B remain on loop  5 A. Items destined for stations  4 B are diverted and set aside to be loaded only onto one of carts  6 C or  6 D. Details of the specific control scheme will vary depending on the purpose for which the system is designed, and may be simple or complex as conditions dictate. 
     It will be understood that the foregoing description is of preferred exemplary embodiments of the invention, and that the invention is not limited to the specific forms shown, but is limited only by the scope of the invention as expressed in the appended claims.