Patent Publication Number: US-2022212869-A1

Title: Bot position sensing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/917, 555, filed Jun. 30, 2020, (Now U.S. Pat. No. 11,279,557, issued Mar. 22, 2022), which is a continuation of U.S. patent application Ser. No. 16/292,611, filed Mar. 5, 2019 (Now U.S. Pat. No. 10,696,479, issued Jun. 30,2020), which is a continuation of U.S. patent application Ser. No. 16/107,068, filed Aug. 21, 2018 (Now U.S. Pat. No. 10,221,014, issued Mar. 5, 2019), which is a continuation of U.S. patent application Ser. No. 15/094,214, filed on Apr. 8, 2016 (Now U.S. Pat. No. 10,053,286, issued Aug. 21,2018), which is a continuation of U.S. patent application Ser. No. 14/684,715, filed on Apr. 13, 2015 (Now U.S. Pat. No. 9,309,050, issued Apr. 12, 2016), which is a continuation of U.S. patent application Ser. No. 13/327,035, filed on Dec. 15, 2011 (Now U.S. Pat. No. 9,008,884, issued Apr. 14, 2015), which is a non-provisional of and claims the benefit of U.S. Provisional Patent Application No. 61/423,206 filed on Dec. 15, 2010, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     1. Field 
     The embodiments generally relate to storage and retrieval systems and, more particularly, to autonomous transports of the 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. 
     When transporting the cases to and from the storage racks with automated transports it would be advantageous to be able to locate the automated transports relative to a case holding area for accurately picking and placing cases to and from the case holding area. 
    
    
     
       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 the embodiments; 
         FIG. 2  illustrates a schematic plan view of an exemplary storage and retrieval system in accordance with the embodiments; 
         FIG. 3  illustrates a structural portion of a storage and retrieval system in accordance with the embodiments; 
         FIGS. 4A and 4B  illustrate storage shelves and an exemplary autonomous transport vehicle in accordance with the embodiments; 
         FIG. 4C  is a schematic illustration of an assembly jig in accordance with the embodiments; 
         FIG. 5  is a schematic illustration of an autonomous transport vehicle and a portion of a storage shelf in accordance with the embodiments; 
         FIG. 6  is a schematic illustration of sensor output signals in accordance with the embodiments; 
         FIG. 7  is a schematic illustration of a portion of a storage shelf and sensor beam in accordance with the embodiments; 
         FIG. 8  is a flow diagram in accordance with the embodiments; 
         FIG. 9  is a schematic illustration of an autonomous transport vehicle and a conveyor shelf in accordance with the embodiments; 
         FIG. 10  is a flow diagram in accordance with the embodiments; 
         FIG. 11  is a schematic illustration of an autonomous transport vehicle in accordance with the embodiments; 
         FIG. 12  is a schematic illustration of a portion of a storage shelf in accordance with the embodiments; 
         FIG. 13  is a schematic illustration of a portion of the transport vehicle of  FIG. 11  in accordance with the embodiments; 
         FIG. 14  is a schematic illustration of a portion of the transport vehicle of  FIG. 11  in accordance with the embodiments; 
         FIG. 14A  is a schematic illustration of a portion of a positioning system in accordance with the embodiments; 
         FIG. 15  is a flow diagram in accordance with the embodiments; 
         FIG. 16  is a schematic illustration of a portion of a picking aisle and a transport vehicle in accordance with the embodiments; and 
         FIG. 17  is a schematic illustration of a portion of the storage and retrieval system in accordance with the embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT(s) 
       FIG. 1  schematically illustrates an exemplary storage and retrieval system in accordance with the embodiments. Although the disclosed embodiments will be described with reference to the embodiments shown in the drawings, it should be understood that the disclosed embodiments 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 the embodiments 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 or items stored in trays, totes or on pallets). It is noted that the case units may include cases of items (e.g. cases 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 embodiments, shipping cases or case units (e.g. cartons, barrels, boxes, crates, jugs, totes, pallets or any other suitable device for holding case units) 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, bundles or pallets of case units 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). It should be understood that the embodiments of the storage and retrieval system described herein may be applied to any environment in which case units 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 the embodiments, the storage and retrieval system 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 transport vehicles or robots  110  (referred to herein as “bots”). 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” and filed on Apr. 9, 2010, the disclosure of which is incorporated by reference herein in its entirety) that may provide an indirect interface between the bots  110  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 bi-directionally transferring case units to and from one or more levels of the storage structure  130 . It is noted that while the multilevel vertical conveyors are described herein as being dedicated inbound or in-feed conveyors  150 A and outbound or out-feed conveyors  150 B, 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 multilevel vertical conveyors may be any suitable lifting devices for transporting case units between levels of the storage and retrieval system. It is noted that while multilevel vertical conveyors are described herein in other aspects the conveyors may be any suitable conveyors or transfer/picking devices having any suitable transport path orientation. Some non-limiting suitable examples of multilevel vertical conveyors can be found in, for example, U.S. patent application Ser. No. 13/327,088, entitled “MULTILEVEL VERTICAL CONVEYOR PLATFORM GUIDES” filed on Dec. 15, 2011, and U.S. patent application Ser. No. 12/757,354, entitled “LIFT INTERFACE FOR STORAGE AND RETRIEVAL SYSTEMS” and filed on Apr. 9, 2010 (the disclosures of which are incorporated by reference herein in their entireties) and U.S. patent application Ser. No. 12/757,220, entitled “STORAGE AND RETRIEVAL SYSTEM,” (previously incorporated by reference). For example, the multilevel vertical conveyors may have any suitable number of support shelves for transporting the case units to a predetermined level of the storage and retrieval system. The support shelves may have slatted supports configured to allow fingers of the bots  110  or in-feed/out-feed transfer stations  170 ,  160  to pass between the slats for transferring case units to and from the conveyor. It is noted that in the embodiments transfer of case units between the bots and the multilevel vertical conveyors may occur in any suitable manner. 
     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) 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 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 case unit(s) from a shelf of a multilevel vertical conveyor, transports the case unit(s) to a predetermined storage area of the storage structure  130  and places the case unit(s) in the predetermined storage area (and vice versa). 
     The bots  110  may be configured to place case units, 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 the embodiments, the bots  110  may interface in any suitable manner with the multilevel vertical conveyors  150 A,  150 B such as through, for example, extension of a transfer arm or effector  110 A ( FIG. 9 ) of the bot (which may have fingers  110 F ( FIGS. 4A and 9 )) for interfacing with slatted support shelves of the multi-level vertical conveyors) relative to a frame of the bot. Suitable examples of bots are described in U.S. patent application Ser. No. 12/757,312, entitled “AUTONOMOUS TRANSPORTS FOR STORAGE AND RETRIEVAL SYSTEMS” and filed on Apr. 9, 2010, U.S. Provisional Patent Application entitled “BOT PAYLOAD ALIGNMENT AND SENSING” with Attorney Docket Number 1127P014263-US (-#1) (Ser. No. 61/423,220) and filed on December 15, 2010 (now U.S. patent application Ser. No. 13/327,040 with Attorney Docket Number 1127P014263-US (PAR) filed on Dec. 15, 2011), U.S. Provisional Patent Application entitled “AUTOMATED BOT WITH TRANSFER ARM” with Attorney Docket Number 1127P014264-US (-#1) (Ser. No. 61/423,365) and filed on Dec. 15, 2010 (now U.S. patent application Ser. No. 13/326,952 with Attorney Docket Number 1127P014264-US (PAR) filed on Dec. 15, 2011), and U.S. Provisional Patent Application entitled “AUTOMATED BOT TRANSFER ARM DRIVE SYSTEM” with Attorney Docket Number 1127P014265-US (-#1) (Ser. No. 61/423,388) and filed on December 15, 2010 (now U.S. patent applilcation Ser. No. 13/326,993 with Attorney Docket Number 1127P014265-US (PAR) filed on Dec. 15, 2011), the disclosures of which are incorporated by reference herein in their entireties. 
     The storage structure  130  may include multiple levels of storage rack modules where each level includes an array of storage spaces (arrayed on the multiple levels and in multiple rows on each level), picking aisles  130 A formed between the rows of storage spaces, and transfer decks  130 B. It is noted that the bots  110  may be configured to traverse the transfer decks  130 B while being mechanically unconstrained and may be configured to traverse the picking aisles  130 A while being mechanically constrained by, for example, rails or other guiding features located in the picking aisles  130 A. Any bot  110  traveling on a level of the storage structure may enter any one of the picking aisles  130 A located on that level which may allow for a variance between a frame of the bot  110  and targets or positioning determining features  1201 - 1203  ( FIG. 12 ) to exist. It is also noted that each level may also include respective bot transfer stations that provide an indirect interface between the bots and the multilevel vertical conveyors. In the embodiments, the picking aisles  130 A and transfer decks  130 B may be arranged for allowing the bots  110  to traverse respective levels of the storage structure  130  for placing case units into picking stock and to retrieve the ordered case units. As may be realized, the storage and retrieval system may be configured to allow random accessibility to the storage spaces. For example, all storage spaces in the storage structure  130  may be treated substantially equally when determining which storage spaces are to be used when picking and placing case units from/to the storage structure  130  such that any storage space of sufficient size can be used to store items. The storage structure  130  of the embodiments may also be arranged such that there is no vertical or horizontal array partitioning of the storage structure. For example, each multilevel vertical conveyor  150 A,  150 B is common to all storage spaces (e.g. the array of storage spaces) in the storage structure  130  such that any bot  110  can access each storage space and any multilevel vertical conveyor  150 A,  150 B can receive case units from any storage space on any level so that the multiple levels in the array of storage spaces substantially act as a single level (e.g. no vertical partitioning). The multilevel vertical conveyors  150 A,  150 B can also receive case units from any storage space on any level of the storage structure  130  (e.g. no horizontal partitioning). It is noted that the storage and retrieval system may be configured so that each multilevel vertical conveyor serves a predetermined area of the array of storage spaces. 
     The storage structure  130  may also include charging stations  130 C for replenishing, for example, a battery pack of the bots  110 . In the embodiments, the charging stations  130 C may be located at, for example, transfer areas  295  ( FIG. 2 ) of the transfer deck  130 B so that the bots  110  can substantially simultaneously transfer items, for example, to and from a multilevel vertical conveyor  150 A,  150 B while being charged. 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 . 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 the embodiments, 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  125 , which in turn manages the warehouse facility as a whole. The control server  120  may be substantially similar to that described in, for example, U.S. patent application Ser. No. 12/757,337, entitled “CONTROL SYSTEM FOR STORAGE AND RETRIEVAL SYSTEMS” and filed on Apr. 9, 2010 (the disclosure of which is incorporated by reference herein in its entirety). 
     Referring also to  FIG. 2 , an exemplary configuration of the storage and retrieval system  100  is shown. Other suitable exemplary configurations of storage and retrieval systems can be found in, for example, U.S. patent application Ser. No. 12/757,381, entitled “STORAGE AND RETRIEVAL SYSTEM” and filed on Apr. 9, 2010, and United States Provisional Patent Application entitled “Warehousing Scalable Storage Structure” with Attorney Docket Number 1127P014551-US (-#1) (Ser. No. 61/423,340) and filed on Dec. 15, 2010 (now U.S. patent application Ser. No. 13/326,674 with 1127P014551-US (PAR) filed on Dec. 15, 2011), the disclosures of which are incorporated by reference herein in their entireties. It should be understood that the storage and retrieval system may have any suitable configuration. As can be seen in  FIG. 2 , the storage and retrieval system  200  is configured, for exemplary purposes only, as a single-ended picking structure in which only one side of the system  200  has a transfer section or deck  130 B. The single-ended picking structure may be used in, for example, a building or other structure having loading docks disposed only on one side of the building. In this example, the storage and retrieval system  200  includes transfer deck(s)  130 B and picking aisles  130 A that allow bots  110  to traverse an entirety of a level of the storage structure  130  on which that bot  110  is located for transporting items between any suitable storage locations/picking aisles  130 A and any suitable multilevel vertical conveyors  150 A,  150 B. The multilevel vertical conveyors  150 A,  150 B provide transport of case units into the storage and retrieval system  200  through input workstations  210  and provide output of case units from the storage and retrieval system  200  through output workstations  220 . In the embodiments, the storage and retrieval system  200  includes a first and second storage section  230 A,  230 B located side by side so that the picking aisles of each section are substantially parallel with each other and facing the same direction (e.g. towards transfer deck  130 B). It should be understood that in the embodiments the storage and retrieval system may have any suitable number of storage sections arranged relative to each other in any suitable configuration. 
     Referring to  FIGS. 1, 3, 4A and 4B , each of the storage bays  510 ,  511  of the storage structure  130  may hold the picking stock on storage shelves  600  that are separated by aisle spaces  130 A. In the embodiments the storage bays  510 ,  511  and storage shelves  600  may be substantially similar to those described in, for example, U.S. patent application Ser. No. 12/757,220, entitled “STORAGE AND RETRIEVAL SYSTEM,” and U.S. patent application Ser. No. 12/757,381, entitled “STORAGE AND RETRIEVAL SYSTEM” (both of which being previously incorporated by reference). For example, one or more support legs  620 L 1 ,  620 L 2  may be provided on the storage shelves  600  so that the support legs extend from, for example, the horizontal supports  610 ,  611 ,  613  (which are supported by vertical supports  612 ) ( FIG. 8 , Block  900 ). The support legs  620 L 1 ,  620 L 2  may be integrally formed with the storage rack structure in any suitable manner. For example, the support legs  620 L 1 ,  620 L 2  may have any suitable configuration and may be part of, for example, a substantially U-shaped channel  620  such that the support legs  620 L 1 ,  620 L 2  are connected to each other through channel portion  620 B. The channel portion  620 B may provide an attachment point between the channel  620  and one or more horizontal supports  610 ,  611 ,  613 . It should be understood that each support leg  620 L 1 ,  620 L 2  may also be configured to individually mount to the horizontal supports  610 ,  611 ,  613 . 
     As may be realized, Referring also to  FIG. 4C , the support legs  620 L 1 ,  620 L 2  may be installed on the horizontal supports  610 ,  611 ,  613  with an installation jig  802 . The installation jig  802  may include a body  800  having a first set of grooves  801  configured so that the support legs  620 L 1 ,  620 L 2 , generally referred to as slats  620 L, fit into the grooves for accurately locating the slats  620 L on the horizontal supports  610 ,  611 ,  613  within a predetermined tolerance. The installation jig  802  may include a second groove  803 , substantially orthogonal to the first set of grooves  801 . The second groove may be configured to accept a vertical support  612  for locating the installation jig  802  relative to the storage rack structure. It should be understood that the installation jig  802  may be located relative to the storage rack structure in any suitable manner for installing the slats  620 L. The slats  620 L may be affixed to the storage structure in any suitable manner such as by, for example, snaps, fasteners, welds, chemical bonding agents and the like. It should be understood that the slats  620 L may be installed on the storage rack structure in any suitable manner using any suitable alignment tools. 
     In the embodiments, each support leg  620 L 1 ,  620 L 2  includes a bent portion  620 H 1 ,  620 H 2  having a suitable surface area configured to support case units stored on the shelves  600 . The bent portions  620 H 1 ,  620 H 2  may be configured to substantially prevent deformation of the case units stored on the shelves. It should be understood that the leg portions  620 H 1 ,  620 H 2  may have a suitable thickness or have any other suitable shape and/or configuration for supporting case units stored on the shelves. As can be seen in  FIGS. 4A and 4B , the slats  620 L or channels  620  may form a slatted or corrugated shelf structure where spaces  620 S between, for example, the support legs  620 L 1 ,  620 L 2  allow for arms or fingers  110 F of the bots  110  to reach into the shelving for transferring case units to and from the shelves as well as allowing the bot  110  to track its position within the storage rack structure. The slats  620 L may be mounted to the storage shelf  600  such that the distance  620 S (e.g. space between slats) places the slats  620 L at known increments  130 A for bot position location during picking and placing case units to the storage shelves  600 . In one example, the spacing  620 S between the slats  620 L can be arranged to provide an incremental bot positioning system (e.g. the spacing  620 S is substantially the same between all of the slats  620 L where the bot location is tracked from a base or reference point such as an end of the picking aisle  130 A). In another example, the spacing  620 S between the support legs  620 L 1 ,  620 L 2  can be arranged to provide an absolute bot positioning system (e.g. the spacing  620 S follows a predetermined pattern so that each space when detected by the bot provides a unique identifiable location of the bot within the picking aisle) while still allowing the fingers  110 F of the bot  110  to be inserted between the slats  620 L for picking and placing case units from the storage shelves  600 . In the embodiments, substantially the same absolute encoder slat pattern may be used in each of the picking aisles while in other alternate embodiments each of the picking aisles may have a unique absolute encoder slat pattern so as to identify the aisle as well as the bot location within the aisle. It should be understood that in the embodiments, the spacing between the slats  620 L on the shelves  600  may be any suitable spacing to provide any suitable measurement scale for determining the location of the bot such as, for example, a combination of incremental and absolute positioning scales. The position of the bot may also be determined using a “map” or “fingerprint” of the cases on the storage shelves as will be described in greater detail below. It is also noted that transfer of case units to and from the multilevel vertical conveyors  150 A,  150 B (whether the transfer is made directly or indirectly by the bot  110 ) may occur in a substantially similar manner to that described above with respect to storage shelves  600 . 
     Referring now to  FIGS. 4A and 5 , any suitable number of sensors for detecting or sensing the slats  620 L may be provided on the bot  110  ( FIG. 8 , Block  910 ). In the embodiments the bot  110  includes two sensors  700 ,  701  for exemplary purposes only. In the embodiments the sensors  700 ,  701  are described as beam sensors including an emitter and a receiver. The emitter and receiver of each sensor  700 ,  701  may be housed in a unitary sensor casing or separate sensor casings of the respective sensor  700 ,  701 . It should be understood that the sensors  700 ,  701  may be any suitable types of sensors including, but not limited to, beam sensors and proximity sensors such as magnetic sensors, capacitance sensors, inductance sensors and the like. The sensor  700  may be located towards the front of the bot  110  and the sensor  701  may be located towards the rear of the bot  110 . It should be realized that the terms “front” and “rear” are relative terms and used herein for exemplary purposes only as the bot  110  may be configured to travel down the picking aisle  130 A in any direction such that the front and rear of the bot, relative to the direction of bot travel, may be reversed. It should be understood that one or more sensors may be located at any suitable positions on the bot such as for example, along any suitable length of any suitable side of the bot  110 . The sensors  700 ,  701  may be mounted to the bot  110  in any suitable manner such as to the chassis or any other portion of the bot  110  structure. 
     The sensors  700 ,  701  may be mounted to the bot  110  for detecting or otherwise sensing the slats  620 L to provide, for example, an incremental (or absolute) and discrete position encoder ( FIG. 8 , Block  920 ) for determining a location of the bot within, for example, a picking aisle  130 A or any other suitable location within the storage and retrieval system  100 . The sensors  700 ,  701  may be mounted at any suitable angle θ (shown exaggerated in  FIGS. 5, 7 and 9 ) relative to, for example, the bot chassis and/or the face  620 LF of the slats  620 L for generating a signal when a respective slat  620 L is sensed. It is noted that the angle θ may allow, for example, a beam emitted from the sensor to be reflected off of, for example, the slats  620 L and be received by a receiver of the sensor as will be described below. As may be realized, the emitter of the beam sensor may be configured such that the emitter is angled relative to the sensor housing so that the housing can be mounted to the bot substantially parallel and/or perpendicular to one or more structural features of the bot. As may also be realized where the sensors used are proximity sensors, the sensors may not be angled as the slats are detected through, for exemplary purposes only, changes in capacitance, inductance or magnetic fields as will be described in greater detail below. It is noted that the sensors may have any suitable arrangement/configuration relative to the slats for detecting the slats and determining a position of the bot. As a non-limiting example only, the back surface of the shelf may have an anti-reflective property that allows the sensors to be placed so that the sensor beam of a reflective type sensor is substantially parallel to a longitudinal axis of the slats (e.g. not at an angle to the slats). 
     Referring also to  FIG. 7 , as the bot moves through the picking aisle  130 A in, for example, the direction of arrow  799  the beam  700 B emitted from the emitter of sensor  700  strikes the side  620 LS of the slat  620  and is reflected away from the sensor (e.g. the beam is not returned to the receiver of the sensor  700 ). As the bot continues to move in the direction of arrow  799  the beam  700 B strikes a face  620 LF of the slat  620 L such that the beam  700 B is reflected back to the receiver of sensor  700  so that the sensor produces an output signal indicating the presence of the slat  620 L. During the continual movement of the bot  110  in the direction of, for example, arrow  799  the beam  700 B sweeps the face  620 LF of the slat  620 L such that the beam  700 B continues to be reflected back to the receiver of sensor  700 . As the receiver of sensor  700  receives the beam  700 B the sensor  700  provides a substantially constant output signal to, for example, any suitable controller  1220  of the bot  110  (or storage and retrieval system  100  such as control server  120 ). As the bot continues to move in the direction of, for example, arrow  799  the beam  700 B moves off of the slat face  620 LF and is no longer reflected back to the receiver of the sensor  700  such that the sensor discontinues to output the substantially constant output signal to indicate no slat is present. As may be realized, as the bot moves past successive slats  620 L the output signals (e.g. slat present, no slat present, slat present, etc.) generated by the sensor  700  may form of an “on/off” signal S 5700  as shown in  FIG. 6  where the on/off output signals correspond to a pitch P (or spacing) of the slats ( FIG. 8 , Block  930 ). In this example, the signal S 700  is illustrated as a square wave but may have any suitable waveform/shape. Sensor  701  may operate in the same manner as that described above with respect to sensor  700  such that the beam  701 B from sensor  701  is reflected off the slat faces  620 LF to produce another “on/off” signal S 701 . As may be realized, the “on/off” signal may be generated in a similar manner using proximity sensors where the signal is “on” when the slat is in proximity to the sensor (e.g. slat presence is detected) and “off” when there is no slat presence detected. 
     The two signals S 700 , S 701  generated by the respective sensors  700 ,  701  form, for example, incremental encoder patterns (e.g. substantially equal pitch between slats) that may be interpreted by the controller  1220  for determining a position of the bot within, for example, the picking aisle  130 A. It is noted that the pitch between slats may vary in a unique manner (while still allowing enough room for fingers  110 F of the bot  110  to be inserted between the slats for picking and placing case units from the storage shelves  600 ) to provide an absolute encoder pattern that can be interpreted by the controller  1220  for determining the location of the bot independent of previously detected slats of the picking aisle  130 A. 
     It is noted that the accuracy or resolution of the sensors  700 ,  701  may be increased by, for example, placing the sensors  700 ,  701  on the bot  110  such that the distance between sensors or the angle of the different sensors results in at least one of the sensors being offset from the slat pitch P by a predetermined fractional amount to effectively increase a number of slats detected by the bot for creating a finer resolution. For example, the distance L between sensors can be as follows: 
     
       
         
           
             
               L 
               = 
               
                 
                   m 
                   ⁢ 
                   P 
                 
                 + 
                 w 
               
             
             , 
           
         
       
     
     where m is an integer and w is a predetermined fraction of the pitch P (e.g. P/2, P/4, . . . P/x). It is noted that the location of the slats  620 L within the storage shelves  600  may be located in a predetermined configuration relative to, for example, the vertical supports  612  of the storage structure. In one example, the vertical supports  612  may not be slatted and the higher position resolution may assist in confirming the bot location so that, for example, fingers  110 F ( FIG. 4A ) of the bot  110  do not contact the vertical supports  612  or support slats  612 L while picking/placing case units from the storage shelves  600 . In another example, the vertical supports  612  may have false slats disposed thereon in a manner substantially similar to that described below with respect to the transfer areas  295  of the storage and retrieval system. In still other examples, the bot position can be determined using RFID tags or barcode labels mounted throughout the storage and retrieval structure. In this example the bot  110  may include any suitable RFID or barcode reader so that the RFID tags and/or barcodes can be read as the bot  110  travels throughout the storage and retrieval system. In still other examples the location of the bot can be determined based on odometry information and feedback from the bot drive motors and their interaction with the surface the bot rides on or against as will be described below. It should be understood that any suitable combination of the above features can be used to determine the location of the bot. 
     The controller  1220  of the bot  110  may have access to a storage and retrieval system structure file. The structure file may include the location of each structural feature of the storage and retrieval system including the positions for each slat  620 L within their respective picking aisles  130 A. The structure file may be located in any suitable memory accessible by the controller  1220 . In one example, the structure file may be resident in a memory  1221  of the bot  110 . In other examples, the structure file may be resident in a memory of, for example, the control server  120  and accessed by the bot  110  or uploaded to a bot memory when the location of the bot  110  is being determined. The slat locations specified by the structure file may assist in qualifying the location of the slats for determining the position of the bot  110  within, for example, a picking aisle  130 A. For example, when the bot qualifies a slat such as slat  620 L 1  of the storage shelves  600  with one of the sensors  700 ,  701  the controller  1220  of the bot compares an estimated location of the bot  110  using bot odometry (obtained from e.g. wheel encoders  720  as described below, which accounts for changes in diameter of the wheels due to, e.g. wear) at the instant in time when the slat  620 L 1  is detected with the location of the slat  620 L 1  as specified by the information in the structure file ( FIG. 8 , Blocks  940  and  950 ). If the comparison between the estimated bot location and the location of the slat from the structure file coincide within a predetermined tolerance the location of the bot (and the sensor sensing the slat) is qualified with the slat such that the bot  110  knows its substantially exact location within the picking aisle  130 A. It is noted that the sensors  700 ,  701  may be located at a predetermined distance relative to, for example, a location of an effector or arm  110 A ( FIG. 9 ) of the bot  110  so that the arm  110 A can be positioned, based on the sensor&#39;s determined location relative to the storage slats  620 L, for inserting fingers  110 F of the arm  110 A between the slats for transferring containers between the bot  110  and the storage shelves  600 . It is also noted that the controller  1220  may be configured to determine a state (acceleration, speed, direction, etc.) of the bot  110  as well as account for wheel slippage when determining the position of the bot within the storage and retrieval system as described in, for example, U.S. Provisional Patent Application entitled “BOT HAVING HIGH SPEED STABILITY” with Attorney Docket Number 1127P014266-US (-#1) (Ser. No. 61/423,359) and filed on Dec. 15, 2010 (now U.S. patent application Ser. No. 13/326,447 with 1127P014266-US (PAR) filed on Dec. 15, 2011), the disclosures of which are incorporated by reference herein in their entireties. 
     In the area between slats  620 L 1 ,  620 L 2  the bot  110  may be configured to obtain odometry information from wheel encoders  720  of the bot  110  to substantially continuously update an estimated position of the bot  110  (e.g. by adding the distance traveled by the bot as determined from the rotation of one or more of the bot&#39;s wheels to the bots last qualified position or any other suitable previously determined position of the bot). The estimated position of the bot  110  may be based off of, for example, the position of the last slat  620 L 1  detected and qualified (e.g. the location is verified through comparison with the structure file) by the bot  110  ( FIG. 8 , Block  960 ). For example, when the bot  100  encounters a subsequent slat  620 L 2  in the direction of travel  799  through the picking aisle  130 A the bot  110  calculates its estimated position using the verified position of the previously detected slat  620 L 1  and the information from the wheel encoders  720 . The bot  110  compares this estimated position against the slat position information contained in the structure file for slat  620 L 2  and if the two locations (i.e. the bots estimated position and the position of the slat  620 L 2  obtained from the structure file) coincide within the predetermined tolerance then the bot  110  knows substantially exactly where it is located within the picking aisle  130 A and the bot&#39;s position within the picking aisle  130 A is updated by, for example, the bot controller  1220 . If the estimated location of the bot  110  (when the sensor senses the subsequent slat  620 L 2 ) is confirmed using the information in the structure file then the slat/bot location is qualified. If there is no match or confirmation then the signal output from one or more of the sensors  700 ,  701  is ignored and the substantially exact position of the bot is not updated, rather the controller  1220  of the bot continues to use the estimated position obtained from the wheel encoders  720  until the location of a subsequently sensed slat is confirmed/qualified. It is noted that in the embodiments, the bot odometry may be reset each time a slat position is qualified. The resetting of the bot odometry may substantially eliminate any built up tolerance or other cumulative tracking errors generated by, for example, the wheel encoders  720 . Alternatively, the bot odometry may not be reset when each slat is qualified such that the bot controller or any other suitable controller of the storage and retrieval system may be configured to account for any tolerance or cumulative tracking errors in the wheel encoders  720  when qualifying the locations of the slats and determining a position of the bot. 
     Referring to  FIGS. 2 and 9  a similar bot location system, such as that described above with respect to the location of the bot in the picking aisle  130 A may be used for determining the location of the bot  110  relative to holding locations A, B on shelves of the multilevel vertical conveyors  150 A,  150 B. As can be seen in  FIG. 9  each shelf  1000  of the multilevel vertical conveyors  150 A,  150 B may be configured to hold multiple case units. In this example, two case units  1001 ,  1002  are held on the conveyor shelf  1000  in holding areas A, B having a side by side arrangement. The conveyor shelf  1000  is connected to a drive system so as to rotate around a predetermined path so that the shelf  1000  passes by the different levels of the storage and retrieval system for delivering case units to the different levels as described in, for example, U.S. patent application Ser. No. 12/757,354, entitled “LIFT INTERFACE FOR STORAGE AND RETRIEVAL SYSTEMS,” and U.S. patent application Ser. No. 12/757,220, entitled “STORAGE AND RETRIEVAL SYSTEM” (both previously incorporated herein by reference). 
     The storage and retrieval system is configured so that the bot can travel into a transfer area  295  for transferring case units between the bot  110  and a holding area A, B of the conveyor shelf  1000 . The transfer area  295  may have a wall  1100  or other suitable structure or surface configured to support, for example, any suitable number of false slats  1620  ( FIG. 10 , Block  1500 ). The wall  1100  may be located between the bot  110  and the conveyor shelf  1000  as the bot travels in the transfer area  295 . In the embodiments, the false slats  1620  may be substantially similar to slats  620 L but are merely mounted to the wall  1100  (rather than extend the depth of a storage shelf) and are not configured to hold or otherwise support case units. The false slats  1620  may be of any sufficient length (e.g. extend from the surface of the wall) to allow one or more sensors  700 ,  701  of the bot  110  ( FIG. 10 , Block  1510 ) to detect the false slats  1620 . It should be understood that the false slats may have any suitable configuration for, in the case of beam sensors  700 ,  701 , reflecting the sensor beams  700 B,  710 B back to the sensors for locating the bot  110  relative to the holding locations A, B of the conveyor shelf  1000  in a manner substantially similar to that described above. Where, for example, proximity sensors are used the false slats may have any suitable configuration for interacting with the proximity sensors. It should also be understood that the false slats  1620  may have any suitable configuration for interacting with any suitable sensors of the bot  110  for causing the sensors to output the “on/off” signal(s) described above. 
     While the false slats are illustrated in  FIG. 9  as protruding from the wall  100 , the false slats  1620  may be substantially flat surfaces configured to interact with the sensors  700 ,  701  in the manner described herein. For example the wall or structure  1100  may have an anti-reflective surface on which reflective objects are mounted. The reflective objects may be used in a manner substantially similar to that of the false slats  1620  for interacting with the sensors  700 ,  701  and generating the on/off sensor signals  5700 ,  5701 . 
     In operation, the bot  110  may receive instructions from, for example, the control server  120  to transfer a case unit, such as case unit  1001 ,  1002 , to or from the conveyor shelf  1000 . The instructions may indicate which holding area A, B of the conveyor shelf  1000  the case unit is located. The bot  110  may travel into a transfer area  295  corresponding to the conveyor shelf  1000  from/to which the bot  110  is to transfer a case unit. During travel in the transfer area  295 , one or more sensors  700 ,  701  of the bot  110  may sense or otherwise detect the false slats  1620  in the manner described above with respect to slats  620 L ( FIG. 10 , Block  1520 ). As each slat is detected an “on/off” signal, similar to signals  5700 ,  5701  described above may be generated through sensor output ( FIG. 10 , Block  1530 ). The bot  110  may compare the location of the bot at the times the false slats  1620  are detected with, for example, predetermined false slat locations within the storage and retrieval system structure file ( FIG. 10 , Block  1540 ). The position of each of the false slats  1620  may be correlated to a respective holding position A, B of the conveyor shelf  1000  such that if the false slat position detected by the bot and the predetermined position match within a predetermined tolerance the bot knows substantially exactly where it is located within the transfer area  295  relative to the holding areas A, B of the conveyor shelf  1000  ( FIG. 10 , Block  1550 ). It is noted that the locations of the false slats  1620  correspond to the location of the fingers  1000 F of the conveyor shelf  1000  so that the fingers  110 F of the bot arm  110 A can be aligned between the false slots  1620  for extending between the fingers  1000 F without contact for picking/placing case units to the conveyor shelf  1000 . 
     In a manner substantially similar to that described above, if the false slat  1620  position detected by the bot  110  and the predetermined position of the false slats, as specified in the structure file, do not match within the predetermined tolerance the sensor signal corresponding to the detected false slat may be ignored. Where the number of false slats or the length of the transfer area  295  does not allow for the bot  110  to travel to another false slat for determining its position within the transfer area  295 , the bot may change its travel direction so that the false slats  1620  can be re-detected by the bot  110 . There may be a “starting false slat” that provides the bot  110  with an absolute position location within the storage structure. The starting false slat may be located at a predetermined position within the transfer area  295  such as at a beginning or entrance of the transfer area  295 . If the bot&#39;s  110  position cannot be determined within the transfer area via the false slat detection, the bot may travel to the location of the “starting false slat” and re-detect the false slats  1620  in the manner described herein. The bot  110  may also obtain information from the wheel encoder(s)  720  for continually updating an estimate of its position in a manner similar to that described above ( FIG. 10 , Block  1560 ) when, for example, the bot sensors are located between the false slats or if the position of the bot  110  cannot otherwise be determined from the false slats  1620 . 
     In a manner similar to that described above, the false slats  1620  may be arranged to form an incremental or absolute encoding system for determining the location of the bot  110  relative to the holding areas A, B of the conveyor shelf so that the fingers  110 F of the bot  110  can be aligned with a case unit, which in this example is case unit  1002 , on the conveyor shelf  1000 . It is noted that, in one example, the false slats  1620  may extend the length of the transfer area  295  while in other examples the false slats  1620  may be located only at the multilevel vertical conveyor access location (e.g. where the bot  110  stops to access the conveyor shelf  1000 ) of the transfer area  295 . It is noted that lines may be affixed or otherwise disposed on decks or other suitable locations, such as the walls, of the transfer area  295  and/or multilevel vertical conveyor access location. These lines may be disposed transverse to the direction of bot travel at predetermined locations so that sensors on the bot  110  can detect the lines as the bot travels through the transfer area  295  and/or multilevel vertical conveyor access location for determining a position of the bot within the storage and retrieval system. It should be realized that the line or lines may alternatively be placed on the bottom or sides of the bot and sensors may be located on the deck or walls of the storage and retrieval system so that the sensor can detect the lines on the bot as the bot passes by the sensor for determining a location of the bot. 
     Referring again to  FIG. 4  in the embodiments the bot  110  may also include one or more suitable case sensors  703 ,  704  configured for sensing case units  101  stored on the shelves  600 . Some non-limiting examples, of case unit sensors can be found in, for example, U.S. patent application Ser. No. 12/757,312, previously incorporated by reference herein. In one example, the case sensors  703 ,  704  may include one or more of a laser sensor and ultrasonic sensor. In another example, the case sensors  703 ,  704  may be substantially similar to sensors  700 ,  701  described above. The case sensors  703 ,  704  may be configured to allow the bot  110  to sense each case unit  101  as the bot travels along a picking aisle. The case sensors  703 ,  704  may be connected to any suitable controller such as, for example, control server  120  and/or bot controller  1220  such that patterns or sequences of case units  101  may be recognized for assisting in a location determination of the bot  110 . For example, the control server  120  may include a “map” or “fingerprint” of case units (including their respective sizes, positions, spacing between the case units, etc.) for each picking aisle. As the bot  110  travels through the picking aisle the controller, such as control server  120  (or bot controller  1220 ) may receive and interpret signals from the case sensors  703 ,  704  indicating, for example, the sizes and relative positions of the case units  101  the bot is passing. The control server  120 , for example, may compare these signals with the case unit map/fingerprint for determining, for example, which aisle the bot is in and which portion of the aisle the bot is in (e.g. the location of the bot within the aisle). In one example, as the bot  110  turns down a picking aisle the case units  101  may be sensed and the control server  120  may determine if the bot  110  is in the correct aisle based on the sensed case units. It is noted that the fingerprint of cases may be dynamic as cases are added and removed from the shelves  600 . 
     Referring to  FIGS. 11-15 , a bot location system using proximity sensors for determining the location of the bot in the picking aisle  130 A and/or relative to holding locations A, B on shelves of the multilevel vertical conveyors  150 A,  150 B is illustrated. In this aspect the bot  110  includes at least one proximity sensor module  1101  mounted to the frame of the bot ( FIG. 15 , Block  2500 ). The proximity sensor module  1101  may be mounted to the frame at any suitable location and for exemplary purposes is shown as being mounted to the frame below the payload holding area of the bot. Here the sensors are located on the bot as a position for sensing targets or position determining features  1201 - 1203  disposed on the rails  1300  on which the bot travels through the picking aisles  130 A (and/or on walls of the transfer area  295  and/or multilevel vertical conveyor access location—not shown—in a manner substantially similar to that described above). In one aspect the proximity sensor module  1101  includes a sensor mount  1101 M that is movably mounted to the frame of the bot  110  in any suitable manner. In one example, the sensor mount  1101 M may be spring loaded or otherwise compliant such that the sensor mount is slidably movable in the direction of arrow  1400  and biased outwards towards/against the rail  1300  (and/or walls of the transfer area  295  and/or multilevel vertical conveyor access location) as the bot  110  moves through the picking aisles (or transfer areas/multilevel vertical conveyor access locations). In one aspect, the sensor mount  1101 M may have an integrally formed guide member  1101 G that rides along in substantial contact with the rail  1300  (e.g. the guide member is held against the rail  1300  by the biasing force BF of the spring loaded mount) so that a substantially constant distance SX is maintained between the targets  1201 - 1203  and the proximity sensor  1101 S regardless of position variance between the bot  110  frame and the targets. In other aspects the guide member  1101 G may be affixed or otherwise mounted to the sensor mount  1101 M in any suitable manner. The distance SX may be any suitable distance that allows the proximity sensor to sense the targets  1201 - 1203 . In one example, the distance SX may be about 2 mm while in other examples the distance SX may be more or less than about 2 mm. The proximity sensor  1101 S may be mounted or otherwise affixed to the sensor mount  1101 M in any suitable manner and may be any suitable proximity sensor (e.g. magnetic sensors, capacitance sensors, inductance sensors and the like). For exemplary purposes only the proximity sensor may be a Hall effect sensor. It is also noted that while only one sensor module  1101  is shown on the bot  110  in other aspects there may be more than one sensor module  1101  disposed at any suitable locations on the bot for sensing the targets  1201 - 1203 . 
     As noted above, and referring to  FIG. 12 , the targets  1201 - 1203  may be provided on the rails  1300  of the picking aisles and/or walls of the transfer area  295  and/or multilevel vertical conveyor access location ( FIG. 15 , Block  2510 ). In one aspect, the targets  1201 - 1203  may be provided on the rails on both sides of the picking aisle  130 A so that the proximity sensor  1101  of the bot may determine its position within the picking aisle by sensing the targets regardless of which travel orientation the bot enters the picking aisle to allow the bot to pick from both sides of the aisle. In other aspects the targets may be provided on but one side of the picking aisle and at least one proximity sensor module  1101  may be disposed on both lateral sides  110 S 1 ,  110 S 2  of the bot so that the targets on but one side of the aisle can be sensed by the proximity sensors of the bot regardless of the travel orientation of the bot for allowing the bot to pick from both sides of the aisle. As may be realized the targets  1201 - 1203  may be located on rails in a reference frame of the storage shelf or storage shelf area. For example, the targets  1201 - 1203  may have a predetermined relationship with the slats  620 L 1 ,  620 L 2  or other any other suitable feature of the storage shelf (such as when the storage shelf is configured without slats or otherwise). The targets  1201 - 1203  may be integrally formed with the rails  1300  or otherwise mounted to or affixed to the rails  1300  in any suitable manner. In one aspect the targets  1201 - 1203  may be formed in the rails  1300  during manufacture of the rails  1300 . The targets  1201 - 1203  may have any suitable configuration that allows the targets to be sensed or otherwise detected by the proximity sensor  1101  of the bot  110 . For exemplary purposes only, in one aspect the targets  1201 - 1203  may be apertures, such as e.g. slots or holes, or recesses provided in a side wall  1300 B of the rails  1300 . Also for exemplary purposes only, the slots may be about 6 mm wide by about 12 mm tall slots or slots having any other suitable dimensions. In other aspects the targets  1201 - 1203  may be any suitable target for influencing the proximity sensor  1101  to produce an on/off signal as will be described below. The targets  1201 - 1203  may be provided in the rails  1300  at predetermined spaced intervals (e.g. the distances between the targets and the location of each target is known) so that the targets  1201 - 1203 , along with the proximity sensor  1101 , form an incremental (or absolute) and discrete position encoder for determining a location of the bot within, for example, a picking aisle  130 A or any other suitable location within the storage and retrieval system  100 . In one aspect the targets  1201 - 1203  may be spaced about 0.3048 m (about 1 ft) from each other. In other aspects the targets  1201 - 1203  may be spaced by a distance that is more or less than about 0.3048 m. In still other aspects the targets  1201 - 1203  may have a varied spacing between the targets that provides for an absolute position determination within, for example, a picking aisle or any other suitable location of the storage structure. 
     As noted above the targets  1201 - 1203  may also be disposed at walls of the transfer area  295  and/or multilevel vertical conveyor access location. In a manner substantially similar to that described above, the targets  1201 - 1203  may be integrally formed in the walls of the transfer area  295  and/or multilevel vertical conveyor access location or otherwise affixed in any suitable manner to the walls. The targets  1201 - 1203  at the transfer area  295  and/or multilevel vertical conveyor access location may be located on the walls in a reference frame of a respective one of the transfer area  295  and/or multilevel vertical conveyor so that the targets  1201 - 1203  have a predetermined relationship with, for example, a shelf of the multilevel vertical conveyor or any other reference point at the transfer station or of the multilevel vertical conveyor in a manner substantially similar to that described above with respect to the picking aisles. 
     In a manner similar to that described above, the controller  1220  of the bot  110  may have access to a storage and retrieval system structure file. The structure file may include the location of each structural feature of the storage and retrieval system including the positions for each target  1201 - 1203  within their respective picking aisles  130 A. The target  1201 - 1203  locations specified by the structure file may assist in qualifying the location of the targets for determining the position of the bot  110  within, for example, a picking aisle  130 A. For example, as the bot travels along, for example, a picking aisle the bot  110  senses the targets  1201 - 1203  ( FIG. 15 , Block  2520 ) with the proximity sensor module  1101  such that the proximity sensor module  1101  produces an on/off signal ( FIG. 15 , Block  2530 ) in a manner substantially similar to that described above. The bot  110  qualifies the target(s)  1201 - 1203  of the rail  1300  with the proximity sensors  1101  where the controller  1220  of the bot compares an estimated location of the bot  110  using bot odometry (obtained from e.g. wheel encoders  720  in a manner substantially similar to that described above) at the instant in time when the target  1201 - 1203  is sensed with the location of the target  1201 - 1203  as specified by the information in the structure file ( FIG. 15 , Block  2540 ). If the comparison between the estimated bot location and the location of the target  1201 - 1203  from the structure file coincide within a predetermined tolerance the location of the bot (and the sensor sensing the slat) is qualified ( FIG. 15 , Block  2550 ) with the target  1201 - 1203  such that the bot  110  knows its substantially exact location within the picking aisle  130 A. 
     In a manner substantially similar to that described above, in the area between targets  1201 - 1203  the bot  110  may be configured to obtain odometry information from wheel encoders  720  of the bot  110  to substantially continuously update an estimated position of the bot  110  (e.g. by adding the distance traveled by the bot as determined from the rotation of one or more of the bot&#39;s wheels to the bots last qualified position or any other suitable previously determined position of the bot) for updating a position of the bot  110  with the wheel encoders ( FIG. 15 , Block  2560 ). For example, the estimated position of the bot  110  in the area between targets  1201 - 1203  may be based off of, for example, the position of the last target  1201 - 1203  detected and qualified (e.g. the location is verified through comparison with the structure file) in a manner substantially similar to that described above. The bot odometry may be used to align the fingers  110 F of the arm  110 A with the slats for transferring containers between the bot  110  and the storage shelf  600 . 
     It is noted that, the positioning of the bot within, for example, the picking aisles  130 A may be decoupled from the structure of the storage shelves  600 . For example, if the slats  620 L 1 ,  620 L 2  become deformed or bent this deformation will have substantially no impact on the location determination of the bot within the picking aisles as the targets  1201 - 1203  being sensed by the proximity sensor  1101 S are disposed on the rails  1300 . This allows for the modification and/or replacement of the slats  620 L 1 ,  620 L 2  without substantially impacting the ability of the bot to determine its location within the storage and retrieval system. As may be realized, in one aspect, there may be some correlation between the targets  1201 - 1203  and the slats  620 L 1 ,  620 L 2  to allow for inserting fingers  110 F of the arm  110 A between the slats for transferring containers between the bot  110  and the storage shelves  600 . In other aspects the bot  110  may include any suitable sensors, such as those described above, for detecting the positions of the slats to allow for inserting fingers  110 F of the arm  110 A between the slats for transferring containers between the bot  110  and the storage shelves  600 . 
     Referring now to  FIG. 16 , in one aspect of the disclosed embodiment, the beam sensors  700 ,  701  described above with respect to  FIGS. 4A-10  may be positioned on the frame of the bot below the payload carrying area in a manner substantially similar to the proximity sensor  1101 . The sensors  700 ,  701  may be positioned to sense the targets  1201 - 1203  on the rails  1300  so that as each target  1201 - 1203  is sensed by a respective sensor  700 ,  701  that sensor produces an on/off signal in a manner substantially similar to that described above with respect to the slat detection for determining a position of the bot in a manner substantially similar to that described above. As may be realized, the bot may have sensors  700 ,  701  on both lateral sides of the bot  110 S 1 ,  110 S 2  so that the sensors  700 ,  701  may detect the targets  1201 - 1203  regardless of the travel orientation of the bot where the targets  1201 - 1203  are located on but one rail  1300  in the picking aisle  130 A. 
     In other aspects the bot  110  may include both the beam sensors  700 ,  701  and one or more proximity sensors  1101  that are used in conjunction with each other for determining a position of the bot within the storage structure. In one aspect the proximity sensors  1101  may be used to determine a location of the bot within the picking aisle  130 A while the beam sensors  700 ,  701  may be used to determine a location of the bot in an area between the targets  1201 - 1203  for aligning the arm  110 A of the bot with the slats on the storage shelf  600  for transferring containers between the bot  110  and the shelf  600 . In other aspects the beam sensors  700 ,  701  and proximity sensors  1101  may be used in any suitable manner for determining a location of the bot within the storage structure and for transferring containers between the bot  100  and the storage shelves  600 . 
     Referring now to  FIG. 17  the storage and retrieval system may also include a bot location system for locating the bot upon, for example, initialization of the bot and during, for example, travel of the bot along the transfer deck  130 B. In one aspect, the bot location system may use radio waves for determining a location of the bot and include any suitable number of transmitters and receivers. In other aspects the bot location system may use any suitable devices capable of allowing for a position determination of the bot such as, for example, optical transmitters and receivers and acoustic transmitter and receivers. In one aspect radio device  2600 , such as transponders, transceivers, transmitters, etc., may be placed at any suitable locations within the storage structure on, for example, the vertical  612  or horizontal  610 ,  611  ( FIG. 3 ) supports of the storage structure. For exemplary purposes, the radio devices  2600  may be placed at the intersection between each picking aisle  130 A and the transfer deck  130 B and at each storage bay  510 ,  511  of the picking aisles  130 A. In one aspect the radio devices  2600  may be passive radio devices such as radio frequency identification (RFID) tags while in other aspects the radio devices may be active devices. Where the radio devices  2600  are passive the bot  110  may include a transceiver and antenna  110 AN that is configured to communicate with and energize the transponders  2600  for receiving information stored in the transponders  2600 . The information stored in the transponders may include a storage aisle identification, a storage bay identification, a multilevel vertical conveyor location, transfer deck location and/or any other location information pertaining to a location within the storage structure. The bot  110  may be configured to interrogate the radio devices  2600  at any suitable times during the operation of the bot such as when travelling through the storage structure or upon initialization (e.g. turning on) of the bot  110 . In one aspect when a bot  110  is initialized within the storage structure the bot  110  may interrogate one or more nearby radio devices  2600  and receive position information from the devices  2600  as to where the bot is located. In the example, shown in  FIG. 17  the bot  110  may receive information from radio devices  2600 A,  2600 B that is processed by, for example, controller  1220  indicating the bot is located in aisle  130 A 1  between bays  510  and  511 . This position information may provide an initial location of the bot  110  that may be supplemented and refined by position information received from one or more of the sensors  700 ,  701 ,  1101 . In other aspects, the radio devices  2600  may be interrogated by the bot  110  while the bot  110  is moving at substantially high speeds along the transfer deck and picking aisles such that when the bot  110  receives position information from the radio devices  2600  that the bot  100  is located at a predetermined location the bot may slow down and obtain position information from one or more of the sensors  700 ,  701 ,  1101 . As may be realized, the radio devices  2600  and the transceiver and antenna  110 AN may also be used to obtain a position of the bot  110  with any desired accuracy such as through any suitable analysis of the signals received from the radio devices  2600  that may or may not be supplemented by position information obtained from sensors  700 ,  701 ,  1101 . 
     In a first aspect of the disclosed embodiment a storage and retrieval system is provided. The storage and retrieval system includes a storage structure having storage shelves, each storage shelf having slats for supporting stored items where the slats are spaced apart from each other by a predetermined distance. An autonomous transport vehicle is also provided where the autonomous transport vehicle includes at least one sensor configured to sense each of the slats and output a signal indicating when a slat is sensed. A controller is provided for verifying a location of the autonomous transport vehicle within the storage structure based on at least the output signal. 
     In accordance with a first sub-aspect of the first aspect of the disclosed embodiment the controller is configured to compare a location of the autonomous transport vehicle at a time the slat is sensed with a predetermined location of the slat and updating a verified location of the autonomous transport vehicle if the locations substantially coincide. 
     In accordance with the first sub-aspect of the first aspect of the disclosed embodiment, the controller is configured to ignore the output signal of the at least one sensor where the locations do not substantially coincide. 
     In accordance with a second sub-aspect of the first aspect of the disclosed embodiment, the controller is configured to continuously update an estimated location of the autonomous transport vehicle based on a last known verified location of the autonomous transport vehicle. 
     In accordance with the second sub-aspect of the first aspect of the disclosed embodiment, the autonomous transport vehicle includes at least one wheel encoder and the controller is configured to obtain wheel encoder information for updating the estimated location of the autonomous transport vehicle. 
     In accordance with a third sub-aspect of the first aspect of the disclosed embodiment the autonomous transport vehicle is configured to align transfer arm fingers of the autonomous transport vehicle with spaces located between the slats of a respective storage shelf based on the determined location of the autonomous transport vehicle for extending the transfer arm fingers into the spaces without contacting the slats. 
     In accordance with the first aspect of the disclosed embodiment the autonomous transport vehicle includes a case unit detection sensor configured for detecting case units located on the storage shelf and the controller is configured to determine a position of the autonomous transport vehicle based on the sensed case units. 
     In accordance with a second aspect of the disclosed embodiment, a storage and retrieval system is provided. The storage and retrieval system includes at least one multilevel vertical conveyor having at least one shelf having support finger. At least one wall is also provided adjacent the multilevel vertical conveyor, the wall including protrusions substantially aligned with the support fingers. An autonomous transport vehicle is provided where the autonomous transport vehicle includes at least one sensor configured to sense each of the protrusions and output a signal indicating when a protrusion is sensed. A controller is provided and is configured to determine a location of the autonomous transport vehicle relative to the support fingers based on the output signal from the at least one sensor. 
     In accordance with the second aspect of the disclosed embodiment the autonomous transport vehicle includes a transfer arm having transfer fingers, the autonomous transport vehicle being configured to align the transfer arm fingers with spaces located between the support fingers of the at least one shelf based on the determined location of the autonomous transport vehicle for extending the transfer arm fingers into a path of the shelf without substantial contact with the supporting fingers. 
     In accordance with the second aspect of the disclosed embodiment, the at least one shelf includes at least two item holding locations and the autonomous transport vehicle includes a transfer arm, the autonomous transport vehicle being configured to align the transfer arm with one of the at least two item holding locations based on the output signal from the at least one sensor. 
     In accordance with a third aspect of the disclosed embodiment an encoder for determining a position of an autonomous transport vehicle is provided. The encoder includes at least one slat mounted adjacent a travel lane of the autonomous transport vehicle, at least one sensor mounted on the at least one autonomous transport vehicle where the at least one sensor is configured to sense the at least one slat and output a presence signal when each of the at least one slat is sensed, and a controller configured to receive the presence signal and determine a location of the autonomous transport vehicle along the travel path based on the presence signal. 
     In accordance with the third aspect of the disclosed embodiment, the at least one slat comprises item supports of a storage shelf. 
     In accordance with the third aspect of the disclosed embodiment, the at least one slat comprises a protrusion mounted on a wall adjacent the travel lane. 
     In accordance with the third aspect of the disclosed embodiment, the at least one sensor comprises at least one of a beam sensor and a proximity sensor. 
     In accordance with the third aspect of the disclosed embodiment, wherein each of the at least one slats are spaced from each other by a predetermined pitch and a distance between each of the at least one sensors are spaced apart from each other by a fractional portion of the pitch. 
     In accordance with the third aspect of the disclosed embodiment, the at least one sensor is angled relative to a face of the at least one slat. 
     In accordance with the third aspect of the disclosed embodiment, a spacing between each of the at least one slats effects an incremental determination of the location of the autonomous transport vehicle. 
     In accordance with the third aspect of the disclosed embodiment, a spacing between each of the at least one slats effects an absolute determination of the location of the autonomous transport vehicle. 
     In accordance with a first sub-aspect of the third aspect of the disclosed embodiment, the controller is configured to compare a location of the autonomous transport vehicle at the time a slat is sensed with a predetermined location of the sensed slat for verifying the location of the autonomous transport vehicle. 
     In accordance with the first sub-aspect of the third aspect of the disclosed embodiment, the controller is configured to update a location of the autonomous transport vehicle when the location of the autonomous transport vehicle at the time a slat is sensed and the predetermined location of the sensed slat coincide. 
     In accordance with the first sub-aspect of the third aspect of the disclosed embodiment, the controller is configured to ignore the presence signal generated when the location of the autonomous transport vehicle at the time a slat is sensed and the predetermined location of the sensed slat do not coincide. 
     In accordance with the third aspect of the disclosed embodiment, the autonomous transport vehicle includes at least one wheel encoder, the controller being configured to obtain information from the wheel encoder and determine an estimated location of the autonomous transport vehicle from the wheel encoder information and based on a previously determined location of the autonomous transport vehicle. 
     In accordance with a fourth aspect of the disclosed embodiment, a storage and retrieval system is provided. The storage and retrieval system includes a storage shelf structure having stationary positioning determining features with respect to a reference feature of the storage shelf structure where the positioning determining features are spaced apart from each other by a predetermined distance, an autonomous transport vehicle including at least one sensor configured to sense each of the positioning determining features and output a signal when a target is sensed as the autonomous transport vehicle moves past the positioning determining features, where the bot is configured for both mechanically constrained travel and mechanically unconstrained travel, and a controller configured to verify a location of the autonomous transport vehicle relative to the storage shelf structure based on at least the output signal. 
     In accordance with a fourth aspect of the disclosed embodiment, the targets include slats forming part of the storage shelf structure and configured to support stored items on the storage shelf structure. 
     In accordance with a first sub-aspect of the fourth aspect of the disclosed embodiment, the storage and retrieval system further includes rails disposed in picking aisles and configured to mechanically constrain travel of the autonomous transport vehicle and to provide access to the storage shelves wherein the positioning determining features include apertures formed in the rails. 
     In accordance with the first sub-aspect of the fourth aspect of the disclosed embodiment, the positioning determining features are of unitary construction with the storage shelf structure that defines the positioning determining features. 
     In accordance with the fourth aspect of the disclosed embodiment, the at least one sensor includes an optical sensor. 
     In accordance with the fourth aspect of the disclosed embodiment, the at least one sensor includes a proximity sensor. 
     In accordance with the fourth aspect of the disclosed embodiment, the at least one sensor includes a Hall effect sensor. 
     In accordance with a second sub-aspect of the fourth aspect of the disclosed embodiment, the positioning determining features include radio devices disposed at predetermined locations on supports of the storage structure and the at least one sensor includes at least an antenna for interrogating the radio devices and obtaining information regarding a predetermined location of an interrogated radio device. 
     In accordance with the second sub-aspect of the fourth aspect of the disclosed embodiment, the positioning determining features and at least the antenna are configured to provide a position of the autonomous transport vehicle upon an initialization of the autonomous transport vehicle. 
     In accordance with the fourth aspect of the disclosed embodiment, the sensor is movably mounted to the autonomous transport vehicle and biased towards the stationary positioning determining features. 
     It should be understood that the exemplary embodiments disclosed herein can 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.