Patent Publication Number: US-11661279-B2

Title: Autonomous transports for storage and retrieval systems

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/009,399, filed on Sep. 1, 2020, (now U.S. Pat. No. 11,396,427), which is a continuation of U.S. patent application Ser. No. 16/275,973, filed on Feb. 14, 2019, (now U.S. Pat. No. 10,759,600) which is a continuation of U.S. patent application Ser. No. 15/137,889, filed on Apr. 25, 2016, (now U.S. Pat. No. 10,207,870) which is a continuation of U.S. patent application Ser. No. 13/860,802, filed on Apr. 11, 2013 (now U.S. Pat. No. 9,321,591) which is a continuation of U.S. patent application Ser. No. 12/757,312, filed on Apr. 9, 2010 (now U.S. Pat. No. 8,425,173) and claims the benefit of U.S. Provisional Patent Application No. 61/168,349, filed on Apr. 10, 2009, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     1. Field 
     The exemplary embodiments generally relate to material handling systems and, more particularly, to transports for automated storage and retrieval systems. 
     2. Brief Description of Related Developments 
     Warehouses for storing case units may generally comprise a series of storage racks that are accessible by transport devices such as, for example, fork lifts, carts and elevators that are movable within aisles between or along the storage racks or by other lifting and transporting devices. These transport devices may be automated or manually driven. Generally the items transported to/from and stored on the storage racks are contained in carriers, for example storage containers such as trays, totes or shipping cases, or on pallets. Generally, incoming pallets to the warehouse (such as from manufacturers) contain shipping containers (e.g. cases) of the same type of goods. Outgoing pallets leaving the warehouse, for example, to retailers have increasingly been made of what may be referred to as mixed pallets. As may be realized, such mixed pallets are made of shipping containers (e.g. totes or cases such as cartons, etc.) containing different types of goods. For example, one case on the mixed pallet may hold grocery products (soup can, soda cans, etc.) and another case on the same pallet may hold cosmetic or household cleaning or electronic products. Indeed some cases may hold different types of products within a single case. Conventional warehousing systems, including conventional automated warehousing systems do not lend themselves to efficient generation of mixed goods pallets. In addition, storing case units in, for example carriers or on pallets generally does not allow for the retrieval of individual case units within those carriers or pallets without transporting the carriers or pallets to a workstation for manual or automated removal of the individual items. 
     It would be advantageous to have a storage and retrieval system for efficiently storing and retrieving individual case units without containing those case units in a carrier or on a pallet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of the disclosed embodiments are explained in the following description, taken in connection with the accompanying drawings, wherein: 
         FIG.  1    schematically illustrates an exemplary storage and retrieval system in accordance with an exemplary embodiment; 
         FIGS.  2  and  3 A- 3 C  illustrate a transport robot in accordance with an exemplary embodiment; 
         FIGS.  4 A and  4 B  illustrate partial schematic views of the transport robot of  FIGS.  2 ,  3 A and  3 B  in accordance with an exemplary embodiment; 
         FIG.  4 C  illustrates a schematic view of a transport robot in accordance with an exemplary embodiment; 
         FIGS.  5 A- 5 C and  6 A- 6 D  illustrate a portion of a transfer arm of the transport robot of  FIGS.  12 ,  13 A and  13 B  in accordance with an exemplary embodiment; 
         FIG.  7    schematically illustrates a control system of the transport robot of  FIGS.  2 ,  3 A and  3 B  in accordance with an exemplary embodiment; 
         FIGS.  8 ,  9 A and  9 B  schematically illustrate exemplary operational paths of a transport robot in accordance with the exemplary embodiments; 
         FIG.  10    schematically illustrates a portion of the control system of  FIG.  17    in accordance with an exemplary embodiment; 
         FIGS.  11 A- 11 E,  12 A,  12 B,  13 A and  13 B  schematically illustrate exemplary operational paths of a transport robot in accordance with the exemplary embodiments; 
         FIG.  14 A  illustrates a conventional organization of item storage in a storage bay; 
         FIG.  14 B  illustrates an organization of items in a storage bay in accordance with an exemplary embodiment; 
         FIG.  14 C  illustrates a comparison of unused storage space between the item storage of  FIG.  14 A  and the item storage of  FIG.  14 B ; 
         FIG.  15    schematically illustrates a conveyor system in accordance with an exemplary embodiment; 
         FIGS.  16 A,  16 B,  16 C, and  16 D  illustrate schematic views of a conveyor system in accordance with an exemplary embodiment; 
         FIGS.  17 A- 17 D  schematically illustrate a transfer station in accordance with an exemplary embodiment; 
         FIGS.  18 A and  18 B  illustrate schematic views of a conveyor system in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT(S) 
       FIG.  1    generally schematically illustrates a storage and retrieval system  100  in accordance with an exemplary embodiment. Although the embodiments disclosed will be described with reference to the embodiments shown in the drawings, it should be understood that the embodiments disclosed can be embodied in many alternate forms. In addition, any suitable size, shape or type of elements or materials could be used. 
     In accordance with one exemplary embodiment the storage and retrieval system  100  may operate in a retail distribution center or warehouse to, for example, fulfill orders received from retail stores for case units (where case units as used herein means items not stored in trays, on totes or on pallets, e.g. uncontained). It is noted that the case units may include cases of items (e.g. case of soup cans, boxes of cereal, etc.) or individual items that are adapted to be taken off of or placed on a pallet. In accordance with the exemplary embodiments, shipping cases or case units (e.g. cartons, barrels, boxes, crates, jugs, or any other suitable device for holding items) may have variable sizes and may be used to hold items in shipping and may be configured so they are capable of being palletized for shipping. It is noted that when, for example, pallets of items arrive at the storage and retrieval system the content of each pallet may be uniform (e.g. each pallet holds a predetermined number of the same item—one pallet holds soup and another pallet holds cereal) and as pallets leave the storage and retrieval system the pallets may contain any suitable number and combination of different items (e.g. each pallet may hold different types of items—a pallet holds a combination of soup and cereal). In alternate embodiments the storage and retrieval system described herein may be applied to any environment in which items are stored and retrieved. 
     The storage and retrieval system  100  may be configured for installation in, for example, existing warehouse structures or adapted to new warehouse structures. In one exemplary embodiment, the storage and retrieval system  100  may include in-feed and out-feed transfer stations  170 ,  160 , multilevel vertical conveyors  150 A,  150 B, a storage structure  130 , and a number of autonomous vehicular transport robots  110  (referred to herein as “bots”). In alternate embodiments the storage and retrieval system may also include robot or bot transfer stations (as described in, for example, U.S. patent application Ser. No. 12/757,220, entitled “STORAGE AND RETRIEVAL SYSTEM,” previously incorporated by reference herein) that may provide an indirect interface between the bots and the multilevel vertical conveyor  150 A,  150 B. The in-feed transfer stations  170  and out-feed transfer stations  160  may operate together with their respective multilevel vertical conveyors  150 A,  150 B for transferring items to and from one or more levels of the storage structure  130 . The multilevel vertical conveyors may be substantially similar to those described in U.S. patent application Ser. No. 12/757,354, entitled “LIFT INTERFACE FOR STORAGE AND RETRIEVAL SYSTEMS,” previously incorporated by reference herein in its entirety. It is noted that while the multilevel vertical conveyors are described herein as being dedicated inbound conveyors  150 A and outbound conveyors  150 B, in alternate embodiments each of the conveyors  150 A,  150 B may be used for both inbound and outbound transfer of case units/items from the storage and retrieval system. The bots  110  may be configured to place items, such as the above described retail merchandise, into picking stock in the one or more levels of the storage structure  130  and then selectively retrieve ordered items for shipping the ordered items to, for example, a store or other suitable location. In one exemplary embodiment, the bots  110  may interface directly with the multilevel vertical conveyors  150 A,  150 B through, for example, access provided by transfer areas  295  ( FIGS.  8  and  11 A- 11 E ) while in other exemplary embodiments, the bots  110  may interface indirectly with the respective multilevel vertical conveyors  150 A,  150 B in any suitable manner such as through bot transfer stations. 
     The storage structure  130  may be substantially similar to the storage structure described in U.S. patent application Ser. No. 12/757,381, entitled “STORAGE AND RETRIEVAL SYSTEM,” and U.S. patent application Ser. No. 12/757,220, entitled “STORAGE AND RETRIEVAL SYSTEM,” previously incorporated herein by reference in their entirety. For example, the storage structure  130  may include multiple levels of storage rack modules, where each level includes picking aisles  130 A ( FIGS.  8 - 9 D ) that provide access to the storage racks, transfer decks  130 B ( FIGS.  8 - 9 D ) that provide access to the picking aisles, and charging stations (not shown) that are configured to replenish, for example, a battery pack of the bots  110 . The bots  110  and other suitable features of the storage and retrieval system  100  may be controlled by, for example, one or more central system control computers (e.g. control server)  120  through, for example, any suitable network  180 . In one example, the central control computer and network may be substantially similar to those described in U.S. patent application Ser. No. 12/757,354, entitled “LIFT INTERFACE FOR STORAGE AND RETRIEVAL SYSTEMS,” U.S. patent application Ser. No. 12/757,381, entitled “STORAGE AND RETRIEVAL SYSTEM,” U.S. patent application Ser. No. 12/757,220, entitled “STORAGE AND RETRIEVAL SYSTEM,” and U.S. patent application Ser. No. 12/757,337, entitled “CONTROL SYSTEM FOR STORAGE AND RETRIEVAL SYSTEMS,” previously incorporated by reference herein in their entirety. The network  180  may be a wired network, a wireless network or a combination of a wireless and wired network using any suitable type and/or number of communication protocols. It is noted that, in one exemplary embodiment, the system control server  120  may be configured to manage and coordinate the overall operation of the storage and retrieval system  100  and interface with, for example, a warehouse management system, which in turn manages the warehouse facility as a whole. 
     As an exemplary operation of an order fulfillment process of the storage and retrieval system  100 , case units for replenishing the picking stock are input at, for example, depalletizing workstations so that case units bundled together on pallets (or other suitable container-like transport supports) are separated and individually carried on, for example, conveyors or other suitable transfer mechanisms (e.g. manned or automated carts, etc.) to the in-feed transfer stations  170 . The in-feed transfer stations  170  assembles the case units into pickfaces (e.g. one or more case units that may form a bot load) and loads the pickfaces onto respective multilevel vertical conveyors  150 A, which carry the pickfaces to a predetermined level of the storage structure  130 . Bots  110  located on the predetermined level of the storage structure  130  interface with the multilevel vertical conveyor  150 A at, for example, the transfer areas  295  for removing the pickfaces from the multilevel vertical conveyor  150 A. The bots  110  transfer the pickfaces from the multilevel vertical conveyors  150 A to a predetermined storage module of the storage structure  130 . When an order for individual case units is made the bots  110  retrieve the corresponding pickfaces from a designated storage module of the storage structure  130  and transfer the ordered case units to transfer areas  295  located on a level of the storage structure  130  from which the ordered case units were picked. The bots  110  interfaces with multilevel vertical conveyor  150 B for transferring the pickfaces to the multilevel vertical conveyor  150 B. The multilevel vertical conveyor  150 B transports the ordered case unit(s) of the pickface to the out-feed transfer stations  160  where the individual case units are transported to palletizing workstations by conveyors  230  where the individual case units are placed on outbound pallets (or other suitable container-like transport supports) for shipping to a customer. 
     As may be realized, the storage and retrieval system  100  may include multiple in-feed and out-feed multilevel vertical conveyors  150 A,  150 B that are accessible by, for example, bots  110  on each level of the storage and retrieval system  100  so that one or more case unit(s), uncontained or without containment (e.g. case unit (s) are not sealed in trays), can be transferred from a multilevel vertical conveyor  150 A,  150 B to each storage space on a respective level and from each storage space to any one of the multilevel vertical conveyors  150 A,  150 B on a respective level. The bots  110  may be configured to transfer the uncontained case units between the storage spaces and the multilevel vertical conveyors with one pick (e.g. substantially directly between the storage spaces and the multilevel vertical conveyors). By way of further example, the designated bot  110  picks the uncontained case unit (s) from a shelf  730  of a multilevel vertical conveyor, transports the uncontained case unit(s) to a predetermined storage area of the storage structure  130  and places the uncontained case unit (s) in the predetermined storage area (and vice versa). In one exemplary embodiment, the storage and retrieval system  100  may include a bot positioning system for positioning the bot adjacent the shelves  730  of the multilevel vertical conveyor  150 A,  150 B for picking/placing a desired pickface from a predetermined one of the shelves  730  (e.g. the bot  110  is positioned so as to be aligned with the pickface on the shelf or a position on the shelf designated to receive the pickface). The bot positioning system may also be configured to correlate the extension of the bot transfer arm  1235  with the movement (e.g. speed and location) of the shelves  730  so that the transfer arm  1235  is extended and retracted to remove (or place) pickfaces from predetermined shelves  730  of the multilevel vertical conveyors  150 A,  150 B. It is noted that at least a portion of the bot positioning system may reside within the control system  1220  ( FIG.  7   ) of the bot  110 . 
     Referring now to  FIGS.  2 - 6 D , the bots  110  that transfer loads (e.g. pickfaces formed of at least one case unit) between, for example, the multilevel vertical conveyors  150 A,  150 B and the storage shelves of a respective level of storage structure  130  will be described. It is noted that in one exemplary embodiment the bots  110  may transfer loads directly to and/or from the multilevel vertical conveyors  150 A,  150 B as will be described below, while in alternate embodiments the bots  110  may interface with the multilevel vertical conveyors indirectly such as through the bot transfer stations. In one example, the bots  110  may be configured for substantially continuous operation. For exemplary purposes only, the bots  110  may have a duty cycle of about ninety-five (95) percent. In alternate embodiments the bots may have any suitable duty cycle and operational periods. 
     As can be seen in  FIG.  2   , the bots  110  generally include a frame  1200 , a drive system  1210 , a control system  1220 , and a payload area  1230 . The drive system  1210  and control system  1220  may be mounted to the frame in any suitable manner. The frame may form the payload area  1230  and be configured for movably mounting a transfer arm or effector  1235  to the bot  110 . 
     In one exemplary embodiment, the drive system  1210  may include two drive wheels  1211 ,  1212  disposed at a drive end  1298  of the bot  110  and two idler wheels  1213 ,  1214  disposed at a driven end  1299  of the bot  110 . The wheels  1211 - 1214  may be mounted to the frame  1200  in any suitable manner and be constructed of any suitable material, such as for example, low-rolling-resistance polyurethane. In alternate embodiments the bot  110  may have any suitable number of drive and idler wheels. In one exemplary embodiment, the wheels  1211 - 1214  may be substantially fixed relative to the a longitudinal axis  1470  ( FIG.  4 B ) of the bot  110  (e.g. the rotational plane of the wheels is fixed in a substantially parallel orientation relative to the longitudinal axis  1470  of the bot) to allow the bot  110  to move in substantially straight lines such as when, for example, the bot is travelling on a transfer deck  130 B (e.g.  FIGS.  8 - 9 B ) or within a picking isle  130 A (e.g.  FIGS.  8 - 9 B ). In alternate embodiments, the rotational plane of one or more of the drive wheels and idler wheels may be pivotal (e.g. steerable) relative to the longitudinal axis  1470  of the bot for providing steering capabilities to the bot  110  by turning the rotational planes of one or more of the idler or drive wheels relative to the longitudinal axis  1470 . The wheels  1211 - 1214  may be substantially rigidly mounted to the frame  1200  such that the axis of rotation of each wheel is substantially stationary relative to the frame  1200 . In alternate embodiments the wheels  1211 - 1214  may be movably mounted to the frame by, for example, any suitable suspension device, such that the axis of rotation of the wheels  1211 - 1214  is movable relative to the frame  1200 . Movably mounting the wheels  1211 - 1214  to the frame  1200  may allow the bot  110  to substantially level itself on uneven surfaces while keeping the wheels  1211 - 1214  in contact with the surface. 
     Each of the drive wheels  1211 ,  1212  may be individually driven by a respective motor  1211 M,  1212 M. The drive motors  1211 M,  1212 M may be any suitable motors such as, for exemplary purposes only, direct current electric motors. The motors  1211 M,  1212 M may be powered by any suitable power source such as by, for example, a capacitor  1400  ( FIG.  4 B ) mounted to the frame  1200 . In alternate embodiments the power source may be any suitable power source such as, for example, a battery or fuel cell. In still other alternate embodiments the motors may be alternating current electric motors or internal combustion motors. In yet another alternate embodiment, the motors may be a single motor with dual independently operable drive trains/transmissions for independently driving each drive wheel. The drive motors  1211 M,  1212 M may be configured for bi-directional operation and may be individually operable under, for example, control of the control system  1220  for effecting steering of the bot  110  as will be described below. The motors  1211 M,  1212 M may be configured for driving the bot  110  at any suitable speed with any suitable acceleration when the bot is in either a forward orientation (e.g. drive end  1298  trailing the direction of travel) or a reverse orientation (e.g. drive end  1298  leading the direction of travel). In this exemplary embodiment, the motors  1211 M,  1212 M are configured for direct driving of their respective drive wheel  1211 ,  1212 . In alternate embodiments, the motors  1211 M,  1212 M may be indirectly coupled to their respective wheels  1211 ,  1212  through any suitable transmission such as, for example, a drive shaft, belts and pulleys and/or a gearbox. The drive system  1210  of the bot  110  may include an electrical braking system such as for example, a regenerative braking system (e.g. to charge, for example, a capacitor  1400  ( FIG.  4 B ) powering the bot  110  under braking). In alternate embodiments, the bot  110  may include any suitable mechanical braking system. The drive motors may be configured to provide any suitable acceleration/deceleration rates and any suitable bot travel speeds. For exemplary purposes only the motors  1211 M,  1212 M may be configured to provide the bot (while the bot is loaded at full capacity) a rate of acceleration/deceleration of about 3.048 m/sec 2 , a transfer deck  130 B cornering speed of about 1.524 m/sec and a transfer deck straightaway speed of about 9.144 m/sec or about 10 m/sec. 
     As noted above drive wheels  1211 ,  1212  and idler wheels  1213 ,  1214  are substantially fixed relative to the frame  1200  for guiding the bot  110  along substantially straight paths while the bot is travelling on, for example, the transfer decks  130 B (e.g.  FIGS.  8 - 9 B ). Corrections in the straight line paths may be made through differential rotation of the drive wheels  1211 ,  1212  as described herein. In alternate embodiments, guide rollers  1250 ,  1251  may be mounted to the frame to aid in guiding the bot  110  on the transfer deck  130 B such as through contact with a wall  1801 ,  2100  ( FIG.  8   ) of the transfer deck  130 B. However, in this exemplary embodiment the fixed drive and idler wheels  1211 - 1214  may not provide agile steering of the bot  110  such as when, for example, the bot  110  is transitioning between the picking aisles  130 A, transfer decks  130 B or transfer areas  295  ( FIGS.  8  and  11 A- 11 E ). In one exemplary embodiment, the bot  110  may be provided with one or more retractable casters  1260 ,  1261  for allowing the bot  110  to make, for example, substantially right angle turns when transitioning between the picking aisles  130 A, transfer decks  130 B and bot transfer stations  140 A,  140 B. It is noted that while two casters  1260 ,  1261  are shown and described, in alternate embodiments the bot  110  may have more or less than two retractable casters. The retractable casters  1260 ,  1261  may be mounted to the frame  1200  in any suitable manner such that when the casters  1260 ,  1261  are in a retracted position both the idler wheels  1213 ,  1214  and drive wheels  1211 ,  1212  are in contact with a flooring surface such as surface  1300 S of the rails  1300  or a transfer deck  130 B of the storage structure  130 , whereas when the casters  1260 ,  1261  are lowered the idler wheels  1213 ,  1214  are lifted off the flooring surface. As the casters  1260 ,  1261  are extended or lowered the idler wheels  1213 ,  1214  are lifted off of the flooring surface so that the driven end  1299  of the bot  110  can be pivoted about a point P ( FIG.  14 B ) of the bot through, for example, differential rotation of the drive wheels  1211 ,  1212 . For example, the motors  1211 M,  1212 M may be individually and differentially operated for causing the bot  110  to pivot about point P which is located, for example, midway between the wheels  1211 ,  1212  while the driven end  1299  of the bot swings about point P accordingly via the casters  1260 ,  1261 . 
     In other exemplary embodiments, the idler wheels  1213 ,  1214  may be replaced by non-retractable casters  1260 ′,  1261 ′ ( FIG.  4 C ) where the straight line motion of the bot  110  is controlled by differing rotational speeds of each of the drive wheels  1211 ,  1212  as described herein. The non-retractable casters  1260 ′,  1261 ′ may be releasably lockable casters such that the casters  1260 ′,  1261 ′ may be selectively locked in predetermined rotational orientations to, for example, assist in guiding the bot  110  along a travel path. For example, during straight line motion of the bot  110  on the transfer deck  130 B and/or within the picking aisles  130 A the non-retractable casters  1260 ′,  1261 ′ may be locked in an orientation such that the wheels of the casters  1260 ′,  1261 ′ are substantially in-line with a respective one of the drive wheels  1213 ,  1214  (e.g. the rotational plane of the wheels of the casters is fixed in a substantially parallel orientation relative to the longitudinal axis  1470  of the bot). The rotational plane of the wheels of non-retractable casters  1260 ′,  1261 ′ may be locked and released relative to the longitudinal axis  1470  of the bot  110  in any suitable manner. For example, a controller  1701  ( FIG.  7   ) of the bot  110  may be configured to effect the locking and releasing of the casters  1260 ′,  1261 ′ by for example controlling any suitable actuator and/or locking mechanism. In alternate embodiments any other suitable controller disposed on or remotely from the bot  110  may be configured to effect the locking and releasing of the casters  1260 ′,  1261 ′. 
     The bot  110  may also be provided with guide wheels  1250 - 1253 . As can be best seen in  FIGS.  3 B and  3 C , while the bot  110  is travelling in, for example, the picking aisles  130 A and/or transfer areas  295  ( FIGS.  8  and  11 A- 11 E ) the movement of the bot  110  may be guided by a tracked or rail guidance system. It is noted that the transfer areas  295  may allow the bots  110  to access transport shelves  730  of the multilevel vertical conveyors  150 A,  150 B. The rail guidance system may include rails  1300  disposed on either side of the bot  110 . The rails  1300  and guide wheels  1250 - 1253  may allow for high-speed travel of the bot  110  without complex steering and navigation control subsystems. The rails  1300  may be configured with a recessed portion  1300 R shaped to receive the guide wheels  1250 - 1253  of the bot  110 . In alternate embodiments the rails may have any suitable configuration such as, for example, without recessed portion  1300 R. The rails  1300  may be integrally formed with or otherwise fixed to, for example, one or more of the horizontal and vertical supports  398 ,  399  of the storage rack structure  130 . As can be seen in  FIG.  3 C  the picking aisles may be substantially floor-less such that bot wheel supports  1300 S of the guide rails  1300  extend away from the storage areas a predetermined distance to allow a sufficient surface area for the wheels  1211 - 1214  (or in the case of lockable casters, wheels  1260 ′,  1261 ′) of the bot  110  to ride along the rails  1300 . In alternate embodiments the picking aisles may have any suitable floor that extends between adjacent storage areas on either side of the picking aisle. In one exemplary embodiment, the rails  1300  may include a friction member  1300 F for providing traction to the drive wheels  1211 ,  1212  of the bot  110 . The friction member  1300 F may be any suitable member such as for example, a coating, an adhesive backed strip or any other suitable member that substantially creates a friction surface for interacting with the wheels of the bot  110 . 
     While four guide wheels  1250 - 1253  are shown and described it should be understood that in alternate embodiments the bot  110  may have any suitable number of guide wheels. The guide wheels  1250 - 1253  may be mounted to, for example, the frame  1200  of the bot in any suitable manner. In one exemplary embodiment, the guide wheels  1250 - 1253  may be mounted to the frame  1200 , through for example, spring and damper devices so as to provide relative movement between the guide wheels  1250 - 1253  and the frame  1200 . The relative movement between the guide wheels  1250 - 1253  and the frame may be a dampening movement configured to, for example, cushion the bot  110  and its payload against any change in direction or irregularities (e.g. misaligned joints between track segments, etc.) in the track  1300 . In alternate embodiments, the guide wheels  1250 - 1253  may be rigidly mounted to the frame  1200 . The fitment between the guide wheels  1250 - 1253  and the recessed portion  1300 R of the track  1300  may be configured to provide stability (e.g. anti-tipping) to the bot during, for example, cornering and/or extension of the transfer arm  1235  (e.g. to counteract any tipping moments created by a cantilevered load on the transfer arm). In alternate embodiments the bot may be stabilized in any suitable manner during cornering and/or extension of the transfer arm  1235 . For example, the bot  110  may include a suitable counterweight system for counteracting any moment that is created on the bot through the extension of the transfer arm  1235 . 
     The transfer arm  1235  may be movably mounted to the frame  1200  within, for example, the payload area  1230 . It is noted that the payload area  1230  and transfer arm  1235  may be suitably sized for transporting cases in the storage and retrieval system  100 . For example, the width W of the payload area  1230  and transfer arm  1235  may be substantially the same as or larger than a depth D ( FIG.  6 B ) of the storage shelves  600 . In another example, the length L of the payload area  1230  and transfer arm  1235  may be substantially the same as or larger than the largest item length transferred through the system  100  with the item length being oriented along the longitudinal axis  1470  ( FIG.  4 B ) of the bot  110 . 
     Referring also to  FIGS.  4 A and  4 B , in this exemplary embodiment the transfer arm  1235  may include an array of fingers  1235 A, one or more pusher bars  1235 B and a fence  1235 F. In alternate embodiments the transfer arm may have any suitable configuration and/or components. The transfer arm  1235  may be configured to extend and retract from the payload area  1230  for transferring loads to and from the bot  110 . In one exemplary embodiment, the transfer arm  1235  may be configured to operate or extend in a unilateral manner relative to the longitudinal axis  1470  of the bot (e.g. extend from one side of the bot in direction  1471 ) for increasing, for example, reliability of the bot while decreasing the bots complexity and cost. It is noted that where the transfer arm  1235  is operable only to one side of the bot  110 , the bot may be configured to orient itself for entering the picking aisles  130 A and/or transfer areas  295  with either the drive end  1298  or the driven end  1299  facing the direction of travel so that the operable side of the bot is facing the desired location for depositing or picking a load. In alternate embodiments the bot  110  may be configured such that the transfer arm  1235  is operable or extendable in a bilateral manner relative to the longitudinal axis  1470  of the bot (e.g. extendable from both sides of the bot in directions  1471  and  1472 ). 
     In one exemplary embodiment, the fingers  1235 A of the transfer arm  1235  may be configured such that the fingers  1235 A are extendable and retractable individually or in one or more groups. For example, each finger may include a locking mechanism  1410  that selectively engages each finger  1235 A to, for example, the frame  1200  of the bot  110  or a movable member of the transfer arm  1235  such as the pusher bar  1235 B. The pusher bar  1235 B (and any fingers coupled to the pusher bar), for example, may be driven by any suitable drive such as extension motor  1495 . The extension motor  1495  may be connected to, for example, the pusher bar, through any suitable transmission such as, for exemplary purposes only, a belt and pulley system  1495 B ( FIG.  4 A ). 
     In one exemplary embodiment, the locking mechanism for coupling the fingers  1235 A to, for example, the pusher bar  1235 B may be, for example, a cam shaft driven by motor  1490  that is configured to cause engagement/disengagement of each finger with either the pusher bar or frame. In alternate embodiments, the locking mechanism may include individual devices, such as solenoid latches associated with corresponding ones of the fingers  1235 A. It is noted that the pusher bar may include a drive for moving the pusher bar in the direction of arrows  1471 ,  1472  for effecting, for example, a change in orientation (e.g. alignment) of a load being carried by the bot  110 , gripping a load being carried by the bot  110  or for any other suitable purpose. In one exemplary embodiment, when one or more locking mechanisms  1410  are engaged with, for example, the pusher bar  1235 B the respective fingers  1235 A extend and retract in the direction of arrows  1471 ,  1472  substantially in unison with movement of the pusher bar  1235 B while the fingers  1235 A whose locking mechanisms  1410  are engaged with, for example, the frame  1200  remain substantially stationary relative to the frame  1200 . 
     In another exemplary embodiment, the transfer arm  1235  may include a drive bar  1235 D or other suitable drive member. The drive bar  1235 D may be configured so that it does not directly contact a load carried on the bot  110 . The drive bar  1235 D may be driven by a suitable drive so that the drive bar  1235 D travels in the direction of arrows  1471 ,  1472  in a manner substantially similar to that described above with respect to the pusher bar  1235 B. In this exemplary embodiment, the locking mechanisms  1410  may be configured to latch on to the drive bar  1235 D so that the respective fingers  1235 A may be extended and retracted independent of the pusher bar and vice versa. In alternate embodiments the pusher bar  1235 B may include a locking mechanism substantially similar to locking mechanism  1410  for selectively locking the pusher bar to either the drive bar  1235 D or the frame  1200  where the drive bar is configured to cause movement of the pusher bar  1235 B when the pusher bar  1235 B is engaged with the drive bar  1235 D. 
     In one exemplary embodiment, the pusher bar  1235 B may be a one-piece bar that spans across all of the fingers  1235 A. In other exemplary embodiments, the pusher bar  1235 B may be a segmented bar having any suitable number of segments  1235 B 1 ,  1235 B 2 . Each segment  1235 B 1 ,  1235 B 2  may correspond to the groups of one or more fingers  1235 A such that only the portion of the pusher bar  1235 B corresponding to the finger(s)  1235 A that are to be extended/retracted is moved in the direction of arrows  1471 ,  1472  while the remaining segments of the pusher bar  1235 B remain stationary so as to avoid movement of a load located on the stationary fingers  1235 A. 
     The fingers  1235 A of the transfer arm  1235  may be spaced apart from each other by a predetermined distance so that the fingers  1235 A are configured to pass through or between corresponding support legs  620 L 1 ,  620 L 2  of the storage shelves  600  ( FIG.  5 A ) and corresponding support fingers  910  of the shelves  730  on the multilevel vertical conveyors  150 A,  150 B. In alternate embodiments the fingers  1235 A may be configured to pass through corresponding support fingers of bot transfer stations for passing the bot load to multilevel vertical conveyor through the bot transfer station. The spacing between the fingers  1235 A and a length of the fingers of the transfer arm  1235  allows an entire length and width of the loads being transferred to and from the bot  110  to be supported by the transfer arm  1235 . 
     The transfer arm  1235  may include any suitable lifting device(s)  1235 L configured to move the transfer arm  1235  in a direction  1350  ( FIG.  13 B ) substantially perpendicular to a plane of extension/retraction of the transfer arm  1235 . 
     Referring also to  FIGS.  5 A- 5 C , in one example, a load (substantially similar to loads  750 - 753 ) is acquired from, for example, a storage shelf  600  by extending the fingers  1235 A of the transfer arm  1235  into the spaces  620 S between support legs  620 L 1 ,  620 L 2  of the storage shelf  600  and under one or more target items  1500  located on the shelf  600 . The transfer arm lift device  1235 L is suitably configured to lift the transfer arm  1235  for lifting the one or more target items  1500  off of the shelf  600 . The fingers  1235 A are retracted so that the one or more target items are disposed over the payload area  1230  of the bot  110 . The lift device  1235 L lowers the transfer arm  1235  so the one or more target items are lowered into the payload area  1230  of the bot  110 . In alternate embodiments, the storage shelves  600  may be configured with a lift motor for raising and lowering the target items where the transfer arm  1235  of the bot  110  does not include a lift device  1235 L.  FIG.  5 B  illustrates an extension of three of the fingers  1235 A for transferring a load  1501 .  FIG.  5 C  shows a shelf  1550  having two items or loads  1502 ,  1503  located side by side. In  FIG.  5 C , three fingers  1235 A of the transfer arm  1235  are extended for acquiring only load  1502  from the shelf  1550 . As can be seen in  FIG.  5 C , it is noted that the loads carried by the bots  110  may include cases of individual items (e.g. load  1502  includes two separate boxes and load  1503  includes three separate boxes). It is also noted that in one exemplary embodiment the extension of the transfer arm  1235  may be controlled for retrieving a predetermined number of items from an array of items. For example, the fingers  1235 A in  FIG.  5 C  may be extended so that only item  1502 A is retrieved while item  1502 B remains on the shelf  1550 . In another example, the fingers  1235 A may be extended only part way into a shelf  600  (e.g. an amount less than the depth D of the shelf  600 ) so that a first item located at, for example, the front of the shelf (e.g. adjacent the picking aisle) is picked while a second item located at the back of the shelf, behind the first item, remains on the shelf. 
     As noted above the bot  110  may include a retractable fence  1235 F. Referring to  FIGS.  6 A- 6 D , the fence  1235 F may be movably mounted to the frame  1200  of the bot  110  in any suitable manner so that the loads, such as load  1600 , pass over the retracted fence  1235 F as the loads are transferred to and from the bot payload area  1230  as can be seen in  FIG.  6 A . Once the load  1600  is located in the payload area  1230 , the fence  1235 F may be raised or extended by any suitable drive motor  1610  so that the fence  1235 F extends above the fingers  1235 A of the bot  110  for substantially preventing the load  1600  from moving out of the payload area  1230  as can be seen in  FIG.  6 B . The bot  110  may be configured to grip the load  1600  to, for example, secure the load during transport. For example, the pusher bar  1235 B may move in the direction of arrow  1620  towards the fence  1235 F such that the load  1600  is sandwiched or gripped between the pusher bar  1235 B and the fence  1235 F as can be seen in  FIGS.  6 C and  6 D . As may be realized, the bot  110  may include suitable sensors for detecting a pressure exerted on the load  1600  by the pusher bar  1235 B and/or fence  1235 F so as to prevent damaging the load  1600 . In alternate embodiments, the load  1600  may be gripped by the bot  110  in any suitable manner. 
     Referring again to  FIGS.  4 B and  4 C , the bot  110  may include a roller bed  1235 RB disposed in the payload area  1230 . The roller bed  1235 RB may include one or more rollers  1235 R disposed transversely to the longitudinal axis  1470  of the bot  110 . The rollers  1235 R may be disposed within the payload area  1230  such that the rollers  1235 R and the fingers  1235 A are alternately located so that the fingers  1235 A may pass between the rollers  1235 R for transferring items to and from the payload area  1230  as described above. One or more pushers  1235 P may be disposed in the payload area  1230  such that a contact member of the one or more pushers  1235 P extends and retracts in a direction substantially perpendicular to the axis of rotation of the rollers  1235 R. The one or more pushers  1235 P may be configured to push the load  1600  back and forth within the payload area  1230  in the direction of arrow  1266  (e.g. substantially parallel to the longitudinal axis  1470  of the bot  110 ) along the rollers  1235 R for adjusting a position of the load  1600  longitudinally within the payload area  1230 . In alternate embodiments, the rollers  1235 R may be driven rollers such that a controller of, for example, the bot drives the rollers for moving the load  1600  such that the load is positioned at a predetermined location within the payload area  1230 . In still other alternate embodiments the load may be moved to the predetermined location within the payload area in any suitable manner. The longitudinal adjustment of the load  1600  within the payload area  1230  may allow for positioning of the loads  1600  for transferring the loads from the payload area to, for example, a storage location or other suitable location such as the multilevel vertical conveyors  150 A,  150 B or bot transfer stations  140 A,  140 B. 
     Referring now to  FIG.  7   , the control system  1220  of the bot will be described. The control system  1220  may be configured to provide communications, supervisory control, bot localization, bot navigation and motion control, case sensing, case transfer and bot power management. In alternate embodiments the control system  1220  may be configured to provide any suitable services to the bot  110 . The control system  1220  may include any suitable programs or firmware configured for performing the bot operations described herein. The control system  1220  may be configured to allow for remote (e.g. over a network) debugging of the bot. In one example, the firmware of the bot may support a firmware version number that can be communicated over, for example, the network  180  so the firmware may be suitably updated. The control system  1220  may allow for assigning a unique bot identification number to a respective bot  110  where the identification number is communicated over the network  180  ( FIG.  1   ) to, for example, track a status, position or any other suitable information pertaining to the bot  110 . In one example, the bot identification number may be stored in a location of the control system  1220  such that the bot identification number is persistent across a power failure but is also changeable. 
     In one exemplary embodiment, the control system  1220  may be divided into a front end  1220 F ( FIG.  2   ) and back end  1220 B ( FIG.  2   ) having any suitable subsystems  1702 ,  1705 . The control system  1220  may include an on-board computer  1701  having, for example, a processor, volatile and non-volatile memory, communication ports and hardware interface ports for communicating with the on-board control subsystems  1702 ,  1705 . The subsystems may include a motion control subsystem  1705  and an input/output subsystem  1702 . In alternate embodiments, the bot control system  1220  may include any suitable number of portions/subsystems. 
     The front end  1220 F may be configured for any suitable communications (e.g. synchronous or asynchronous communications regarding bot commands, status reports, etc.) with the control server  120 . The communications between the bot  110  and the control server  120  may, in one exemplary embodiment, provide for a substantially automatic bootstrap from, for example, initial installation of the bot  110 , operational failure of the bot  110  and/or bot replacement. For example, when a bot  110  is initialized, the bot may obtain an identification number and subscribe to a bot proxy  2680  ( FIG.  26 A ) via communication with the front end  1220 F. This allows the bot to become available for receiving tasks. The front end  1220 F may receive and decompose tasks assigned to the bot  110  and reduce the task into primitives (e.g. individual commands) that the back end  1220 B can understand. In one example, the front end  1220 F may consult any suitable resources such as, for example, a map of the storage structure  130  ( FIG.  1   ) to decompose a task into the primitives and to determine the various movement related parameters (e.g. velocity, acceleration, deceleration, etc.) for each portion of the task. The front end  1220 F may pass the primitives and movement related parameters to the back end  1220 B for execution by the bot  110 . The bot front end  1220 F may be configured as a pair of state machines where a first one of the state machines handles communication between the front end  1220 F and the control server  120  and a second one of the state machines handles communication between the front end  1220 F and the back end  1220 B. In alternate embodiments the front end  1220 F may have any suitable configuration. The first and second state machines may interact with each other by generating events for each other. The state machines may include a timer for handling timeouts such as during, transfer deck  130 B access. In one example, when a bot  110  is entering a transfer deck  130 B (e.g.  FIG.  8   ), a bot proxy of the central system control computers may inform the front end  1220 F of a predetermined entrance time that the bot is to enter the transfer deck  130 B. The front end  1220 F may start the timer of the state machines according to a time the bot is to wait (based on the predetermined entrance time) before entering the deck. It is noted that the timers (e.g. clocks) of the state machines and the bot proxy  2680  may be synchronized clocks so as to substantially avoid collisions between bots travelling on the transfer deck  130 B and bots entering the transfer deck  130 B. 
     The back end  1220 B may be configured to effect the functions of the bot described above (e.g. lowering the casters, extending the fingers, driving the motors, etc.) based on, for example, the primitives received from the front end  1220 F. In one example, the back end  122 B may monitor and update bot parameters including, but not limited to, bot position and velocity and send those parameters to the bot front end  1220 F. The front end  1220 F may use the parameters (and/or any other suitable information) to track the bot&#39;s  110  movements and determine the progress of the bot task(s). The front end  1220 F may send updates to, for example, the bot proxy  2680  so that the control server  120  can track the bot movements and task progress and/or any other suitable bot activities. 
     The motion control subsystem  1705  may be part of the back end  1220 B and configured to effect operation of, for example, the drive motors  1211 M,  1212 M,  1235 L,  1495 ,  1490 ,  1610  of the bot  110  as described herein. The motion control subsystem  1705  may be operatively connected to the computer  1701  for receiving control instructions for the operation of, for example, servo drives (or any other suitable motor controller) resident in the motion control subsystem  1705  and subsequently their respective drive motors  1211 M,  1212 M,  1235 L,  1495 ,  1490 ,  1610 . The motion control subsystem  1705  may also include suitable feedback devices, such as for example, encoders, for gathering information pertaining to the drive motor operation for monitoring, for example, movement the transfer arm  1235  and its components (e.g. when the fingers  1235 A are latched to the pusher bar, a location of the pusher bar, extension of the fence, etc.) or the bot  110  itself. For example, an encoder for the drive motors  1211 M,  1212 M may provide wheel odometry information, and encoders for lift motor  1235 L and extension motor  1495  may provide information pertaining to a height of the transfer arm  1235  and a distance of extension of the fingers  1235 A. The motion control subsystem  1705  may be configured to communicate the drive motor information to the computer  1701  for any suitable purpose including but not limited to adjusting a power level provided to a motor. 
     The input/output subsystem  1702  may also be part of the back end  1220 B and configured to provide an interface between the computer  1701  and one or more sensors  1710 - 1716  of the bot  110 . The sensors may be configured to provide the bot with, for example, awareness of its environment and external objects, as well as the monitor and control of internal subsystems. For example, the sensors may provide guidance information, payload information or any other suitable information for use in operation of the bot  110 . For exemplary purposes only, the sensors may include a bar code scanner  1710 , slat sensors  1711 , line sensors  1712 , case overhang sensors  1713 , arm proximity sensors  1714 , laser sensors  1715  and ultrasonic sensors  1716 . 
     The bar code scanner(s)  1710  may be mounted on the bot  110  in any suitable location. The bar code scanners(s)  1710  may be configured to provide an absolute location of the bot  110  within the storage structure  130 . The bar code scanner(s)  1710  may be configured to verify aisle references and locations on the transfer decks by, for example, reading bar codes located on, for example the transfer decks, picking aisles and transfer station floors to verify a location of the bot  110 . The bar code scanner(s)  1710  may also be configured to read bar codes located on items stored in the shelves  600 . 
     The slat sensors  1711  may be mounted to the bot  110  at any suitable location. The slat sensors  1711  may be configured to count the slats or legs  620 L 1 ,  620 L 2  of the storage shelves  600  (e.g.  FIG.  5 A ) for determining a location of the bot  110  with respect to the shelving of, for example, the picking aisles  130 A. The slat information may be used by the computer  1701  to, for example, correct the bot&#39;s odometry and allow the bot  110  to stop with its fingers  1235 A positioned for insertion into the spaces between the legs  620 L 1 ,  620 L 2 . In one exemplary embodiment, the bot may include slat sensors  1711  on the drive end  1298  and the driven end  1299  of the bot to allow for slat counting regardless of which end of the bot is facing the direction the bot is travelling. The slat sensors  1711  may be any suitable sensors such as, for example, close range triangulation or “background suppression” sensors. The slat sensors  1711  may be oriented on the bot  110  so that the sensors see down into the slats and ignore, for example, the thin edges of the legs  620 L 1 ,  620 L 2 . For exemplary purposes only, in one exemplary embodiment the slat sensors  1711  may be mounted at about a 15 degree angle from perpendicular (relative to the longitudinal axis  1470  ( FIG.  4 B ) of the bot  110 ). In alternate embodiments the slat sensors  1711  may be mounted on the bot in any suitable manner. 
     The line sensors  1712  may be any suitable sensors mounted to the bot in any suitable location, such as for exemplary purposes only, on bumpers  1273  ( FIG.  2   ) disposed on the drive and driven ends of the bot  110 . For exemplary purposes only, the line sensors may be diffuse infrared sensors. The line sensors  1712  may be configured to detect guidance lines provided on, for example, the floor of the transfer decks  130 B (e.g.  FIG.  8   ). The bot  110  may be configured to follow the guidance lines when travelling on the transfer decks  130 B and defining ends of turns when the bot is transitioning on or off the transfer decks  130 B. The line sensors  1712  may also allow the bot  110  to detect index references for determining absolute localization where the index references are generated by crossed guidance lines. In this exemplary embodiment the bot  110  may have about six line sensors  1712  but in alternate embodiments the bot  110  may have any suitable number of line sensors. 
     The case overhang sensors  1713  may be any suitable sensors that are positioned on the bot to span the payload area  1230  adjacent the top surface of the fingers  1235 A. The case overhang sensors  1713  may be disposed at the edge of the payload area  1230  to detect any loads that are at least partially extending outside of the payload area  1230 . In one exemplary embodiment, the case overhang sensors  1713  may provide a signal to the computer  1701  (when there is no load or other items obstructing the sensor) indicating that the fence  1235 F may be raised for securing the load(s) within the payload area  1230 . In other exemplary embodiments, the case overhang sensors  1713  may also confirm a retraction of the fence  1235 F before, for example, the fingers  1235 A are extended and/or a height of the transfer arm  1235  is changed. 
     The arm proximity sensors  1714  may be mounted to the bot  110  in any suitable location, such as for example, on the transfer arm  1235 . The arm proximity sensors  1714  may be configured to sense objects around the transfer arm  1235  and/or fingers  1235 A of the transfer arm  1235  as the transfer arm  1235  is raised/lowered and/or as the fingers  1235 A are extended/retracted. Sensing objects around the transfer arm  1235  may, for exemplary purposes only, substantially prevent collisions between the transfer arm  1235  and objects located on, for example, shelves  600  (e.g.  FIG.  5 A ) or the horizontal and/or vertical supports of the storage structure  130 . 
     The laser sensors  1715  and ultrasonic sensors  1716  (collectively referred to as case sensors) may be configured to allow the bot  110  to locate itself relative to each case unit forming the load carried by the bot  110  before the case units are picked from, for example, the storage shelves  600  and/or multilevel vertical conveyor (or any other location suitable for retrieving payload). The case sensors may also allow the bot to locate itself relative to empty storage locations for placing case units in those empty storage locations. This location of the bot relative to the case units to be picked and/or empty storage locations for placing the case units may be referred to as bot localization, which will be described in greater detail below. The case sensors may also allow the bot  110  to confirm that a storage slot (or other load depositing location) is empty before the payload carried by the bot is deposited in, for example, the storage slot. In one example, the laser sensor  1715  may be mounted to the bot at a suitable location for detecting edges of items to be transferred to (or from) the bot  110 . The laser sensor  1715  may work in conjunction with, for example, retro-reflective tape (or other suitable reflective surface, coating or material) located at, for example, the back of the shelves  600  to enable the sensor to “see” all the way to the back of the storage shelves  600 . The reflective tape located at the back of the storage shelves allows the laser sensor  1715  to be substantially unaffected by the color, reflectiveness, roundness or other suitable characteristics of the items located on the shelves  600 . The ultrasonic sensor  1716  may be configured to measure a distance from the bot  110  to the first item in a predetermined storage area of the shelves  600  to allow the bot  110  to determine the picking depth (e.g. the distance the fingers  1235 A travel into the shelves  600  for picking the item(s) off of the shelves  600 ). One or more of the case sensors may allow for detection of case orientation (e.g. skewing of cases within the storage shelves  600 ) by, for example, measuring the distance between the bot  110  and a front surface of the case units to be picked as the bot  110  comes to a stop adjacent the case units to be picked. The case sensors may allow verification of placement of a case unit on, for example, a storage shelf  600  by, for example, scanning the case unit after it is placed on the shelf. 
     It is noted that the computer  1701  and its subsystems  1702 ,  1705  may be connected to a power bus for obtaining power from, for example, the capacitor  1400  through any suitable power supply controller  1706 . It is noted that the computer  1701  may be configured to monitor the voltage of the capacitor  1400  to determine its state of charge (e.g. its energy content). In one exemplary embodiment, the capacitor may be charged through charging stations located at, for example, one or more transfer areas  295  ( FIGS.  8  and  11 A- 11 E ) or at any other suitable location of the storage structure  130  so that the bot is recharged when transferring payloads and remains in substantially continuous use. The charging stations may be configured to charge the capacitor  1400  within the time it takes to transfer the payload of the bot  110 . For exemplary purposes only, charging of the capacitor  1400  may take about 15 seconds. In alternate embodiments, charging the capacitor may take more or less than about 15 seconds. During charging of the capacitor  1400  the voltage measurement may be used by the computer  1701  to determine when the capacitor is full and to terminate the charging process. The computer  1701  may be configured to monitor a temperature of the capacitor  1400  for detecting fault conditions of the capacitor  1400 . 
     The computer  1701  may also be connected to a safety module  1707  which includes, for example, an emergency stop device  1311  ( FIG.  3 A ) which when activated effects a disconnection of power to, for example, the motion control subsystem  1705  (or any other suitable subsystem(s) of the bot) for immobilizing or otherwise disabling the bot  110 . It is noted that the computer  1701  may remain powered during and after activation of the emergency stop device  1311 . The safety module  1707  may also be configured to monitor the servo drives of the motion control subsystem  1705  such that when a loss of communication between the computer and one or more of the servo drives is detected, the safety module  1707  causes the bot to be immobilized in any suitable manner. For example, upon detection of a loss of communication between the computer  1701  and one or more servo drives the safety module  1707  may set the velocity of the drive motors  1211 M,  1212 M to zero for stopping movement of the bot  110 . 
     The communication ports of the control system  1220  may be configured for any suitable communications devices such as, for example, a wireless radio frequency communication device  1703  (including one or more antennae  1310 ) and any suitable optical communication device  1704  such as, for example, an infrared communication device. The wireless radio frequency communication device  1703  may be configured to allow communication between the bot  110  and, for example, the control server  120  and/or other different bots  110  over any suitable wireless protocol. For exemplary purposes only, the wireless protocol for communicating with the control server  120  may be the wireless 802.11 network protocol (or any other suitable wireless protocol). Communications within the bot control system  1220  may be through any suitable communication bus such as, for example, a control network area bus. It is noted that the control server  120  and the bot control system  1220  may be configured to anticipate momentary network communication disruptions. For example, the bot may be configured to maintain operation as long as, for example, the bot  110  can communicate with the control server  120  when the bot  110  transits a predetermined track segment and/or other suitable way point. The optical communication device  1704  may be configured to communicate with, for example, the bot transfer stations for allowing initiation and termination of charging the capacitor  1400 . The bot  110  may be configured to communicate with other bots  110  in the storage and retrieval system  100  to form a peer-to-peer collision avoidance system so that bots can travel throughout the storage and retrieval system  100  at predetermined distances from each other in a manner substantially similar to that described in U.S. patent application Ser. No. 12/757,337, entitled “CONTROL SYSTEM FOR STORAGE AND RETRIEVAL SYSTEMS,” previously incorporated by reference herein in its entirety. 
     Referring to  FIGS.  2 ,  4 B and  8 - 13 B , bot navigation and motion control will be described. Generally, in accordance with the exemplary embodiments, the bot  110  has, for example, three modes of travel. In alternate embodiments the bot  110  may have more than three modes of travel. For exemplary purposes only, in the picking aisles  130 A the bot travels on wheels  1211 - 1214  (or lockable casters  1260 ′,  1261 ′ in lieu of idler wheels  1213 ,  1214 ) and is guided by guide wheels  1250 - 1253  against the sides of track  1300  ( FIG.  3 B ). On the transfer deck  130 B, the bot  110  uses casters  1261 ,  1262  (or releases lockable casters  1260 ′,  1261 ′) while making substantially right angle turns when transitioning from/to the picking aisles  130 A or transfer stations  140 A,  140 B. For traveling long distances on, for example, the transfer deck  130 B the bot  110  travels on wheels  1211 - 1214  (or lockable casters  1260 ′,  1261 ′ in lieu of idler wheels  1213 ,  1214  where the casters  1260 ′,  1261 ′ are rotationally locked as described above) using a “skid steering” algorithm (e.g. slowing down or stopping rotation of one drive wheel relative to the other drive wheel to induce a turning motion on the bot) to follow guidance lines  1813 - 1817  on the transfer deck  130 B. 
     When traveling in the picking aisles  130 A, the bot  110  travels in substantially straight lines. These substantially straight line moves within the picking aisles  130 A can be in either direction  1860 ,  1861  and with either bot orientation (e.g. a forward orientation with the drive end  1298  trailing the direction of travel and a reverse orientation with the drive end  1298  leading the direction of travel). During straight line motion on the transfer deck  130 B the bot  110  travels in, for exemplary purposes only, a counterclockwise direction  1863 , with a forward bot orientation. In alternate embodiments the bot may travel in any suitable direction with any suitable bot orientation. In still other alternate embodiments, there may be multiple travel lanes allowing bots to travel in multiple directions (e.g. one travel lane has a clockwise direction of travel and another travel lane has a counter-clockwise direction of travel). In one example, the turns to and from the picking aisles  130 A and/or transfer areas  295  are about 90 degrees where the center point of rotation P of the bot is located substantially midway between the drive wheels  1211 ,  1212  such that the bot can rotate clockwise or counterclockwise. In alternate embodiments the bot turns may be more or less than about 90 degrees. In another example, the bot may make a substantially 180 degree turn (i.e. two substantially 90 degree turns made in sequence without a stop) as will be described below. 
     As described above, the transfer deck  130 B may include guidance lines  1810 - 1817  for guiding the bot  110 . The guidance lines  1810 - 1817  may be any suitable lines adhered to, formed in or otherwise affixed to the transfer deck  130 B. For exemplary purposes only, in one example the guidance lines may be a tape affixed to the surface of the transfer deck  130 B. In this exemplary embodiment the transfer deck  130 B includes a track  1800  having a first side  1800 A and a second side  1800 B separated by a wall  1801 . The first and second sides  1800 A,  1800 B of the track  1800  are joined by end track sections  1800 E (only one of which is shown in  FIG.  8   ). In alternate embodiments the track  1800  may have any suitable configuration. Each of the first and second sides  1800 A,  1800 B includes two travel lanes defined by, for example, guidance lines  1813 ,  1814  and  1816 ,  1817  respectively. The end track portions  1800 E include, for example, one travel lane defined by, for example, guidance line  1815 . In alternate embodiments the sections/sides of the track  1800  may have any suitable number of travel lanes defined in any suitable manner. In accordance with the exemplary embodiments each picking lane  130 A and/or transfer area  295 , includes a lead in/out guidance line  1810 - 1812 . The lead in/out guidance lines  1810 - 1812  and the single guidance line  1815  of the end track portions  1800 E may be detected by the bot  110  as index marks for bot localization during long line-following moves. The lead in/out guidance lines  1810 - 1812  and guidance line  1815  may also be detected by the bot  110  as reference marks for making turns. 
     When the bot  110  moves in substantially straight lines, such as in the picking aisles  130 A and/or transfer areas  295 , the drives for motors  1211 M,  1212 M may be configured as torque controllers. For example, the computer  1701  may be configured to close a velocity loop as shown in  FIG.  10    using the average of the velocity feedback from both wheels  1211 ,  1212  as the “bot velocity”. To improve performance and avoid velocity loop instabilities, the velocity loop may be augmented with torque-feedforward and operated at a low gain. The computer  1701  may also be configured to close a position loop as also shown in  FIG.  10    for final position at a stop location of the bot  110 . The computer  1701  may also be configured to sum in a differential torque offset to implement line following. It is noted that the drive wheels  1211 ,  1212  may loose traction with the transfer deck  130 A or floor of a picking aisle  130 A or transfer area  295  when the flooring surfaces and/or the wheels are contaminated with liquids, dirt or other particulates. The velocity control loop may be configured to mitigate the loss of traction by backing off the torque to both wheels  1211 ,  1212  whenever feedback provided by, for example, an encoder for one or both wheels  1211 ,  1212  indicates a velocity higher than a predetermined velocity of the bot  110 . 
     When travelling long distances on, for example, the transfer deck, the bot  110  travels on drive wheels  1211 ,  1212  and idler wheels  1213 ,  1214  (or locked casters  1260 ′,  1261 ′) so that the bot is deterred from veering off of the straight line trajectory through the fixed nature of the drive wheels  1211 ,  1212  and idler wheels  1213 ,  1214  (or locked casters  1260 ′,  1261 ′). The computer  1701  may be configured with any suitable line following algorithm to substantially ensure that the bot  110  maintains travel in a straight line. The line following algorithm may also allow for correction of initial line following errors due to, for example, misalignment from turns. In one exemplary embodiment the bot  110  uses line sensors  1712  to estimate its heading and offset from a guidance line  1810 - 1817 . The bot  110  may be configured to use, for example, any suitable algorithm such as a fuzzy logic algorithm to generate corrections in the travel path of the bot  110 . The correction may be applied as a differential torque to the wheels as the bot is travelling (e.g. skid steering—rotating one drive wheel slower than the other drive wheel to produce increased drag on one side of the bot for inducing a turning moment on the bot). 
     For turns, such as for example, substantially right angle turns, the drives for motors  1211 M,  1212 M may be configured as position controllers. For example the drives may be commanded by the computer  1701  to rotate their respective wheels in opposite directions for a predetermined distance to generate a pivot turn of slightly more than about 90 degrees. When for example, line sensors  1712  detect a stopping guidance line, the turning move is terminated. In alternate embodiments the drives for the motors  1211 M,  1212 M may be operated in any suitable manner for driving the bot in substantially straight lines or during turns. 
       FIGS.  9 A and  9 B  illustrate an exemplary turn sequence for a substantially 90 degree turn made by the bot  110  while transitioning onto the transfer deck  130 B from a picking aisle  130 A. In this example, the bot is traveling in a forward orientation in the direction of arrow  1910 . As the bot  110  exits the picking aisle  130 A, the bot  110  lowers the casters  1260 ,  1261  ( FIG.  4 A ) so that the idler wheels  1213 ,  1214  are lifted off of the transfer deck  130 B (or unlocks casters  1260 ′,  1261 ′). Using line sensors  1712  located at for example the driven end  1299  of the bot  110 , the bot  110  detects the inner travel lane guidance line  1814  and then using corrected wheel odometry, stops with its pivot point P at or close to the outer travel lane guidance line  1813 . The bot  110  rotates about 90 degrees in the direction of arrow  1920  using a differential torque in the drive motors  1211 M,  1212 M to turn the drive wheels  1211 ,  1212  in opposite directions such that the bot  110  rotates about point P. The bot  110  detects the guidance line  1813  with the line sensors  1712  and terminates the turn. The bot  110  raises the casters  1260 ,  1260  so that the idler wheels  1213 ,  1214  contact the transfer deck  130 B (or locks casters  1260 ′,  1261 ′) and proceeds to follow guidance line  1813  using, for example, line following. It is noted that turning of the bot to enter, for example, picking aisle  130 A may occur in substantially the same manner as that described above for exiting the picking aisle  130 A. 
       FIGS.  11 A- 13 B  illustrate exemplary travel paths of a bot  110  including straight line travel and turn sequences. It is noted that while the specific examples of bot travel are shown and described, the bot  110  may be configured to perform any suitable number of turns and transition between any suitable number of travel lanes in any suitable manner for travelling throughout a respective level of the storage structure  130 .  FIG.  11 A  illustrates a travel path of the bot  110  where the bot  110  transitions from one, for example, picking aisle (or transfer station), across the transfer deck  130 B and into a transfer area  295  (or other picking aisle) with the bot  110  transitioning from a reverse orientation (e.g. drive end  1298  leading) to a forward orientation (e.g. drive end  1298  trailing). In this example, the bot  110  exits the picking aisle  130 A in a reverse orientation such that a line sensor(s)  1712  disposed substantially at pivot point P detects the inner travel lane guidance line  1814 . The bot  110  pivots about point P in a counterclockwise direction in a manner substantially similar to that described above with respect to  FIGS.  9 A and  9 B . The bot follows guidance line  1814  in a forward orientation until, for example, line sensors  1712  (located proximately to or at one or more of the drive end  1298  and driven end  1299  of the bot  110 ) detect the crossing of guidance lines  1814  and  1815  ( FIG.  8   ) at which point the bot  110  pivots counterclockwise about point P in a manner similar to that described above so that the bot follows line  1815  substantially to a point where guidance line  1815  crosses the inner travel lane guidance line  1816 . At the crossing of guidance lines  1815 ,  1816  the bot  110  pivots counterclockwise to follow guidance line  1816 . The bot follows guidance line  1816  until the line sensors  1712  detect a crossing of guidance lines  1816  and  1812  at which point the bot pivots clockwise to enter transfer area  295  in a forward orientation. 
       FIG.  11 B  illustrates an exemplary travel path of the bot  110  where the bot exits the picking aisle  130 A in a reverse orientation and enters transfer area  295  in a reverse orientation. In this example, the motion of the bot is substantially similar to that described above with respect to  FIG.  11 A , however, after travelling around wall  1801  the bot transitions to the outer travel lane guidance line  1817  so that the bot  110  is in a reverse orientation. The reverse orientation of the bot  110  allows the bot to pivot counterclockwise into an open area of the transfer deck  130 B for entering the transfer area  295  in a forward orientation without colliding with the outside wall  2100  of the transfer deck  130 B. 
       FIG.  11 C  illustrates the bot  110  exiting the picking aisle  130 A in a forward orientation and entering transfer area  295  in a forward orientation. The motion of the bot is substantially similar to that described above however, in this example, the bot  110  transitions from outer travel lane guidance line  1813  to inner travel lane guidance line  1816  so that the bot can enter the transfer area  295  in a forward orientation. 
       FIGS.  11 D and  11 E  illustrate the bot  110  exiting the picking aisle  130 A in a forward orientation and entering the transfer area  295  in a reverse orientation using the outer travel lane guidance lines  1813  and  1817 . The motion of the bot  110  may be substantially similar to that described above, however as the bot  110  travels along guidance line  1815  the bot  110  makes three turns  2110 - 2112  (e.g. where turns  2110 ,  2111  turn the bot substantially 180 degrees along guidance line  1815 ) to orient the bot  110  for making the final turn  2113  into the transfer area  295 . As can be seen in  FIG.  11 E , without the additional turn the bot  110  would not be able to transition from outer travel lane guidance line  1813  onto outer travel lane guidance line  1817  without colliding with the outer wall  2100  (as indicated by the shaded area) using line following as described herein. 
       FIGS.  12 A and  12 B  illustrate an exemplary travel path of bot  110  from picking aisle  130 A 1  to picking aisle  130 A 2  where travel of the bot  110  within the picking aisle  130 A 1  is in a forward orientation and travel within picking aisle  130 A 2  is in a reverse orientation. In this example, the bot uses the outer travel lane guidance line  1813  for transitioning between picking aisles  130 A 1 ,  130 A 2 . As can be seen in  FIG.  12 A  when travelling along the outer travel lane guidance line  1813  and entering picking aisle  130 A 2  the bot pivots in a direction so the driven end  1299  of the bot swings towards the inner travel lane of the transfer deck  130 B so as to avoid colliding with the outer wall  2100  as shown in  FIG.  12 B . 
       FIGS.  13 A and  13 B  illustrate an exemplary travel path of bot  110  from picking aisle  130 A 1  to picking aisle  130 A 2  where travel of the bot  110  within the picking aisles  130 A 1 ,  130 A 2  is in a reverse orientation. In this example, the bot uses the inner travel lane guidance line  1814  for transitioning between picking aisles  130 A 1 ,  130 A 2 . As can be seen in  FIG.  13 A  when travelling along the inner travel lane guidance line  1814  the bot pivots in a direction so the driven end  1299  of the bot swings towards the outer travel lane of the transfer deck  130 B so as to avoid colliding with the inner wall  1801  as shown in  FIG.  13 B . 
     It is noted that the bot may be configured to transition between the tracked travel lanes of the picking aisles  130 A and the open transfer deck  130 B in any suitable manner. In one exemplary embodiment, the guidance lines  1810 - 1812  may guide the bot into the tracks  1300  of the picking aisles. In alternate embodiments, one or more of the sensors  1710 - 1716  may allow the bot  110  to detect, for example, edges or other suitable features of the tracks  1300  ( FIG.  3 B ) and position itself so the bot  110  passes between opposing tracks  1300  with the guide wheels contacting the recesses  1300 R in the tracks  1300 . 
     In accordance with one exemplary embodiment, in the above described examples of the bot travel paths, when the bot is turning the bot is supported by the drive wheels  1211 ,  1212  and the casters  1260 ,  1261  (or casters  1260 ′,  1261 ′). The straight line moves of the bot may be made with the bot supported on the drive wheels  1211 ,  1212  and the idler wheels  1213 ,  1214  (or casters  1260 ′,  1261 ′). As noted above, any corrections to the bot travel path while the bot is traveling in a straight line may be made using skid steering. In alternate embodiments, the bot may travel in a straight line path with the casters  1260 ,  1261  deployed. In still other alternate embodiments, correction to the bots straight line travel paths may be made through steerable wheels. 
     Referring again to  FIG.  7   , the bot  110  can determine its position within the storage and retrieval system  100  for transitioning through the storage structure  130  as described above through, for example, bot localization. In one exemplary embodiment bot localization may be derived through one or more of bot odometrey, slat counting, index counting and bar code reading. As described above, the bot odometry may be provided from encoders associated with, for example, wheels  1211 - 1214  ( FIG.  2   ). It is noted that encoder information from each wheel  1211 - 1214  may be averaged and scaled in any suitable manner to provide a distance traveled by the bot. In alternate embodiments, the distance traveled by the bot may be obtained from the wheel encoder information in any suitable manner. The slat counting may be provided by, for example, the slat sensors  1711  as the bot travels through the picking aisles. The slat counting may supplement the odometry information when the bot is within the picking aisles. The index counting may be provided, by for example, the line sensors  1712  as the bot passes over crossed sections of the guidance lines  1810 - 1817  ( FIG.  8   ). The index counting may supplement the bot odometry when the bot is traveling on the transfer deck  130 B. Bar code reading may be provided by bar code scanner  1710 . The bar code reading may be configured to allow the bot  110  to determine an initial position of the bot such as when the bot is powered up from an off or dormant state. The bar codes may be located at transfer stations  140 A,  140 B ( FIG.  1   ) or any other suitable location within the storage structure  130  for initializing the bot  110 . Bar codes may also be located within the picking aisles and on the transfer deck to, for example, confirm bot location and correct for missed slats or indexes. The on-board computer  1701  of the bot  110  may be configured to use any suitable combination of bot odometrey, slat counting, index counting and bar code reading to determine the bot&#39;s  110  position within the storage structure  130 . In alternate embodiments, the computer  1701  may be configured to determine the location of the bot using only one or any suitable combination of bot odometrey, slat counting, index counting and bar code reading. In still other alternate embodiments, the location of the bot may be determined in any suitable manner such as through, for example, an indoor spatial positioning system. The indoor spatial positioning system may be substantially similar to a global positioning system and use any suitable technique for determining the position of an object such as, for example, acoustic, optical, or radio frequency signaling. 
     In one exemplary embodiment, one or more of the sensors  1710 - 1716  described above may allow for dynamic positioning of bots  110  within the picking aisles  130 A in any suitable manner. The position at which the bot  110  is stopped for dynamically allocating case units may be determined by, for example, the control server  120  ( FIG.  1   ), the control system  1220  of the bot or by a combination thereof. For example, dynamic allocation of the storage space may be determined by, for example, the control server  120  in any suitable manner such that any open storage spaces in the storage structure  130  are filled with items having a size capable of fitting within those open storage spaces. The control server may communicate to the appropriate components of the storage and retrieval system  100 , for example, a location of a predetermined storage location and an appropriately sized case unit or case units for placement in the predetermined storage location. That case unit may be transferred into the storage and retrieval system  100  where a bot  110  delivers the case unit to the predetermined storage location. As a non-limiting example, the sensors  1710 - 1716  of the bots  110  may count slats  620 L 1 ,  620 L 2  ( FIG.  5 A ) and/or detect edges of case units on the storage shelves for dynamically positioning the bots for placing the case unit in the predetermined storage location. Dynamically positioning the bots  110  and/or the dynamic allocation of shelf storage space may allow for the positioning of case units having varying lengths in each storage bay  5000 ,  5001  ( FIGS.  14 A and  14 B ) such that the use of the storage space is maximized. For example,  FIG.  14 A  illustrates a storage bay  5000  divided into storage slots S 1 -S 4  as is done in conventional storage systems. The size of the storage slots S 1 -S 4  may be a fixed size dependent on a size of the largest case unit (e.g. case unit  5011 ) to be stored on the shelf  600  of the storage bay  5000 . As can be seen in  FIG.  14 A , when case units  5010 ,  5012 ,  5013  of varying dimensions, which are smaller than case unit  5011 , are placed in a respective storage slot S 1 , S 2 , S 4  a significant portion of the storage bay capacity, as indicated by the shaded boxes, remains unused. In accordance with an exemplary embodiment,  FIG.  14 B  illustrates a storage bay  5001  having dimensions substantially similar to storage bay  5000 . In  FIG.  14 B  the case units  5010 - 5016  are placed on the shelf  600  using dynamic allocation. As can be seen in  FIG.  14 B , dynamically allocating the storage space allows placement of case units  5014 - 5016  on shelf  600  in addition to case units  5010 - 5013  (which are the same case units placed in storage bay  5000  described above) such that the unused storage space, as indicated by the hatched boxes, is less than the unused storage space using the fixed slots of  FIG.  14 A .  FIG.  14 C  illustrates a side by side comparison of the unused storage space for the fixed slots and dynamic allocation storage described above. It is noted that the unused storage space of bay  5001  using dynamic allocation may be decreased even further by decreasing the amount of space between the case units  5010 - 5016  which may allow for placement of additional case units on the shelf  600 . As may be realized, as items are placed within the storage structure the open storage spaces may be analyzed, by for example the control server  120 , after each case unit placement and dynamically re-allocated according to a changed size of the open storage space so that additional case units having a size corresponding to (or less than) a size of the re-allocated storage space may be placed in the re-allocated storage space. 
     Referring now to  FIG.  15   , the multilevel vertical conveyors, such as conveyor  150 A are supplied with uncontained case units  1000  from in-feed transfer stations  170  ( FIG.  1   ). As described above, the in-feed transfer stations  170  may include one or more of depalletizing workstations, conveyors  240 , conveyor interfaces/bot load accumulators  1010 A,  1010 B and conveyor mechanisms  1030 . As can be seen in  FIG.  15   , uncontained case units  1000  are moved from, for example depalletizing workstations by conveyors  240 . In this example, each of the positions A-D is supplied by a respective in-feed transfer station. As may be realized, while the transfer of case units is being described with respect to shelves  730 ′ it should be understood that transfer of case units to shelves  730  occurs in substantially the same manner. For example, position A may be supplied by in-feed transfer station  170 A and position C may be supplied by in-feed transfer station  170 B. Referring also to  FIG.  16 A  the in-feed transfer stations  170 A,  170 B, for supplying similar sides of the shelf  730  (in this example positions A and C, which are disposed side by side, form a first side  1050  of the shelf  730  and positions B and D, which are disposed side by side, form a second side  1051  of the shelf  730 ), may be located one above the other in a horizontally staggered stacked arrangement (an exemplary stacked arrangement is shown in  FIG.  16 A ). In other exemplary embodiments, the stacked arrangement may be configured so that the in-feed transfer stations are disposed vertically in-line one above the other and extend into the multilevel vertical conveyors by different amounts for supplying, for example, positions A and B or positions C and D where positions A and B (and positions C and D) are disposed one in front of the other, rather than side by side. In alternate embodiments, the in-feed transfer stations may have any suitable configuration and positional arrangement. As can be seen in  FIG.  15   , the first side  1050  and second side  1051  of the shelf  730  are loaded (and unloaded) in opposing directions such that each multilevel vertical conveyor  150 A is located between respective transfer areas  295 A,  295 B where the first side  1050  interfaces with a transfer area  295 B and the second side  1051  interfaces with transfer area  295 A. 
     In this exemplary embodiment, the accumulators  1010 A,  1010 B are configured to form the uncontained case units  1000  into the individual pick faces  750 - 753  prior to loading a respective position A-D on the multilevel vertical conveyor  730 . In one exemplary embodiment, the computer workstation  700  and/or control server  120  may provide instructions or suitably control the accumulators  1010 A,  1010 B (and/or other components of the in-feed transfer stations  170 ) for accumulating a predetermined number of items to form the pickfaces  750 - 753 . The accumulators  1010 A,  1010 B may align the case units in any suitable manner (e.g. making one or more sides of the items flush, etc.) and, for example, abut the items together. The accumulators  1010 A,  1010 B may be configured to transfer the pickfaces  750 - 753  to respective conveyor mechanisms  1030  for transferring the pickfaces  750 - 753  to a respective shelf position A-D. In one exemplary embodiment the conveyor mechanisms  1030  may include belts or other suitable feed devices for moving the pickfaces  750 - 753  onto transfer platforms  1060 . The transfer platforms  1060  may include spaced apart fingers for supporting the pickfaces  750 - 753  where the fingers  910  of the shelves  730  are configured to pass between the fingers of the transfer platforms  1060  for lifting (or placing) the pickfaces  750 - 753  from the transfer platforms  1060 . In another exemplary embodiment, the fingers of the transfer platforms  1060  may be movable and serve to insert the pickfaces  750 - 753  into the path of the shelves  730  in a manner similar to that described below with respect to the bot transfer stations  140 . In alternate embodiments the in-feed transfer stations  170  (and out-feed transfer stations  160 ) may be configured in any suitable manner for transferring case units (e.g. the pickfaces formed by the case units) onto or from respective multilevel vertical conveyors  150 A,  150 B. 
     It is noted that while the interface between the bot transfer stations  140  and the multilevel vertical conveyors  150 A,  150 B are described it should be understood that interfacing between the bots  110  and the multilevel vertical conveyors  150 A,  150 B occurs in a substantially similar manner (e.g. as described in U.S. patent application Ser. No. 12/757,312, previously incorporated by reference herein in its entirety). For exemplary purposes only, referring now to  FIGS.  16 B and  17 A- 17 D , the multilevel vertical conveyors  150 A transfer pickfaces  750 ,  752  from, for example, the in-feed transfer stations  170  (or any other suitable device or loading system) to, for example, the bot transfer stations  140  associated with each of the levels in the storage structure  130 . In other examples, the pickfaces  750 ,  752  may be transferred directly from the multilevel vertical conveyors  150 A to the bots  110  as described below. As may be realized, the bot transfer stations  140  are disposed on respective levels of the storage structure adjacent the path of travel of the shelves  730  of a respective multilevel vertical conveyor  150 A. In one exemplary embodiment, there may be a bot transfer station  140  corresponding to each of the positions A and C on the shelves  730  (and positions A-D with respect to shelf  730 ′). For example, a first bot transfer station  140  may remove load  750  from position A on shelf  730  while another bot transfer station  140  may remove pickface  752  from position C on shelf  730  and so on. In other exemplary embodiments, one bot transfer station  140  may serve to remove or place case units in more than one position A, C on the shelves  730 . For example, one bot transfer station  140  may be configured for removing pickfaces  750 ,  752  from one or more of positions A, C of shelf  730 . In alternate embodiments, referring also to  FIG.  15   , one bot transfer station  140  may be configured for removing pickfaces  750 ,  752  from one or more of positions A, C on a first side  1050  of the shelf  730 ′ while another bot transfer station  140  may be configured to remove pickfaces  751 ,  753  from one or more positions B, D on a second side  1051  of the shelf  730 ′. In alternate embodiments the bot transfer stations  140  may have any suitable configuration for accessing any suitable number of positions A-D of the shelves  730 ,  730 ′. 
     Each bot transfer station  140  may include a frame  1100 , one or more drive motors  1110  and a carriage system  1130 . The frame  1100  may have any suitable configuration for coupling the bot transfer station  140  to, for example, any suitable supporting feature of the storage structure  130 , such as a horizontal or vertical support. The carriage system  1130  may be movably mounted to the frame  1100  through, for example, rails  1120  that are configured to allow the carriage system  1130  to move between retracted and extended positions as shown in  FIGS.  17 A and  17 B . The carriage system  1130  may include a carriage base  1132  and fingers  1135 . The fingers  1135  may be mounted to the carriage base  1132  in a spaced apart arrangement so that the fingers  1135  extend from the carriage base  1132  in a cantilevered fashion. It is noted that each finger  1135  may be removably mounted to the carriage base  1132  for facilitating replacement or repair of individual fingers  1135 . In alternate embodiments the fingers and carriage base may be of unitary one-piece construction. The fingers  1135  of the bot transfer stations  140  may be configured to pass between the fingers  910  ( FIG.  16 B ) of the shelves  730  of the multilevel vertical conveyors  150 A ( FIG.  1   ) for removing pickfaces such as pickfaces  1150  (which may be substantially similar to pickfaces  750 - 753 ) from the shelves  730 . The bot transfer station  140  may also include a load positioning device  1140  that retractably extends between, for example, the spaced apart fingers  1135  in the direction of arrow  1181  for effecting positioning of the pickfaces  1150  in a predetermined orientation relative to the bot transfer station  140 . In still other alternate embodiments the carriage system  1130  may have any suitable configuration and/or components. The one or more drive motors  1110  may be any suitable motors mounted to the frame  1100  for causing the extension/retraction of the carriage system  1130  and the extension/retraction of the positioning device  1140  in any suitable manner such as by, for exemplary purposes only, drive belts or chains. In alternate embodiments, the carriage system and positioning device may be extended and retracted in any suitable manner. 
     In operation, referring also to  FIGS.  16 C,  16 D,  18 A and  18 B , inbound pickfaces (e.g. pickfaces, which include one or more case units, that are being transferred into the storage and retrieval system) such as pickface  1150  are loaded on and will circulate around the multilevel vertical conveyor  150 A and be removed from a respective conveyor by, for example, one or more bots  110  for placement in a storage area of the storage structure. As will be described further below, in the exemplary embodiments the input loading sequencing of case units onto the multilevel vertical conveyors  150 A,  150 B (e.g. such as at corresponding feeder input sides of transfer stations  170  and bot transfer locations on respective storage levels) may be substantially independent from the output or unloading sequence of the multilevel vertical conveyors  150 A,  150 B (e.g. such as at corresponding output sides of transfer stations  160  and bot transfer locations on respective storage levels) and vice versa. In one example, the pickface  1150  may be loaded onto the shelves  730  during an upward travel of the multilevel vertical conveyor  150 A and off loaded from the shelves  730  during downward travel of the multilevel vertical conveyor  150 A. By way of example, multilevel vertical conveyor shelves  730   i  and  730   ii  ( FIG.  16 D ) may be loaded sequentially, but when unloaded, shelf  730   ii  may be unloaded before shelf  730   i . It is noted that the shelves  730  may be loaded through one or more cycles of the multilevel vertical conveyor. In alternate embodiments the pickfaces may be loaded or off loaded from the shelves  730  in any suitable manner. As may be realized, the position of the case units on the multilevel vertical conveyor shelf  730  defines the pickface position that the bot  110  picks from. The bot may be configured to pick any suitable load or pickface from the shelf  730  regardless of the pickface position on the shelf  730  or the size of the pickface. In one exemplary embodiment, the storage and retrieval system  100  may include a bot positioning system for positioning the bot adjacent the shelves  730  for picking a desired pickface from a predetermined one of the shelves  730  (e.g. the bot  110  is positioned so as to be aligned with the pickface). The bot positioning system may also be configured to correlate the extension of a bot transfer arm with the movement (e.g. speed and location) of the shelves  730  so that the transfer arm is extended and retracted to remove (or place) pickfaces from predetermined shelves  730  of the multilevel vertical conveyors  150 A,  150 B. For exemplary purposes only, the bot  110  may be instructed by, for example, the computer workstation  700  or control server  120  ( FIG.  16 A ) to extend the transfer arm into the path of travel of the pickface  1150 . As the pickface  1150  is carried by the multilevel vertical conveyor  150 A in the direction of arrow  860  fingers of the bot the transfer arm (which may be substantially similar to fingers  1135  of the bot transfer station  140 ) pass through the fingers  910  of the shelf  730  for transferring the pickface  1150  from the shelf  730  to the carriage system  1135  (e.g. the pickface  1150  is lifted from the fingers  910  via relative movement of the shelf  730  and the bot transfer arm). As may be realized, the pitch P between shelves may be any suitable distance for allowing the transfer of pickfaces between the multilevel vertical conveyor and the bots  110  while the shelves  730  are circulating around the multilevel vertical conveyor at a substantially continuous rate. The bot transfer arm may be retracted (in a manner substantially similar to that shown in  FIGS.  17 C,  17 D  with respect to the bot transfer station  140 ) so that the pickface  1150  is no longer located in the path of travel of the shelves  730  of the multilevel vertical conveyor  150 A. It is noted that in alternate embodiments, where the bot transfer stations  140  are used, the positioning device  1140  may be extended through the fingers  1135  and the carriage system  1130  ( FIGS.  17 A- 17 D ) may be moved in the direction of arrow  1180  for abutting the pickface  1150  against the positioning device  1140  effecting positioning of the pickface  1150  in a predetermined orientation relative to, for example, the bot transfer station  140 . The carriage system  1130  may be fully retracted as shown in  FIG.  17 D  for transfer of the pickface  1150  to a bot  110 . 
     Referring to  FIGS.  16 D and  18 B , for transferring loads in the outbound direction (e.g. moving pickfaces from or out of the storage and retrieval system) the bots  110  pick one or more pickface, such as pickface  1150 , from a respective predetermined storage area of the storage structure. The pickfaces may be extended into the path of the shelves  730  of the multilevel vertical conveyor  150 B (which is substantially similar to conveyor  150 A) by the transfer arm of bot  110  through an extension of the bot transfer arm relative to a frame of the bot  110 . It is noted that the pickfaces, such as pickface  1150 , may be placed on the multilevel vertical conveyor  150  in a first predetermined order sequence. The first predetermined order may be any suitable order. The substantially continuous rate of movement of the shelves  730  in the direction of arrow  870  cause the fingers  910  of the shelf  730  to pass through the fingers of the bot transfer arm such that the movement of the shelf  730  effects lifting the pickface  1150  from the fingers of the bot transfer arm. The pickface  1150  travels around the multilevel vertical conveyor  150 B to an out-feed transfer station  160  (which is substantially similar to in-feed transfer station  170 ) where is it removed from the shelf  730  by a conveyor mechanism  1030  in a manner substantially similar to that described above. The pickfaces may be removed from the multilevel vertical conveyor  150 B by, for example the out-feed transfer stations  160  in a second predetermined order sequence that may be different and independent from the first predetermined order sequence. The second predetermined order sequence may depend on any suitable factors such as, for example, the store plan rules described below. 
     It is noted that the respective transfer of pickfaces between the multilevel vertical conveyors  150 A,  150 B and the in-feed and out-feed transfer stations  170 ,  160  may occur in a manner substantially similar to that described above with respect to the bots  110  and bot transfer stations  140 . In alternate embodiments transfer of pickfaces between the multilevel vertical conveyors  150 A,  150 B and the in-feed and out-feed transfer stations  170 ,  160  may occur in any suitable manner. 
     It should be understood that the exemplary embodiments described herein may be used individually or in any suitable combination thereof. It should also be understood that the foregoing description is only illustrative of the embodiments. Various alternatives and modifications can be devised by those skilled in the art without departing from the embodiments. Accordingly, the present embodiments are intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.