Bot having high speed stability

An autonomous transport vehicle for transporting items in a storage and retrieval system is provided. The autonomous transport vehicle includes at least two drive wheels and a controller, where each drive wheel is independently driven and a drive wheel encoder is disposed adjacent each drive wheel. The controller, in communication with the drive wheel encoders, is configured to determine a kinematic state of the autonomous transport vehicle within the storage and retrieval system based on incremental data from the drive wheel encoders only and independent of drive wheel slippage.

BACKGROUND

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 transport the cases at high speeds using autonomous transport vehicles.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT(S)

FIG. 1schematically 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 system100may 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. 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 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). In the embodiments 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 system100may 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 stations170,160, multilevel vertical conveyors150A,150B, a storage structure130, and a number of autonomous transport vehicles or robots110(referred to herein as “bots”). In the 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,” 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 bots110and the multilevel vertical conveyor150A,150B. The in-feed transfer stations170and out-feed transfer stations160may operate together with their respective multilevel vertical conveyors150A,150B for bi-directionally transferring case units to and from one or more levels of the storage structure130. It is noted that while the multilevel vertical conveyors150are described herein as being dedicated inbound or in-feed conveyors150A and outbound or out-feed conveyors150B, each of the conveyors150A,150B may be used for both inbound and outbound transfer of case units/items from the storage and retrieval system. The multilevel vertical conveyors150may 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. Provisional Patent Application No. 61/423,298, entitled “MULTILEVEL VERTICAL CONVEYOR PLATFORM GUIDES” and filed on Dec. 15, 2010, 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. In the embodiments transfer of case units between the bots110and the multilevel vertical conveyors may occur in any suitable manner through any suitable interface between the bots110and the conveyors.

As may be realized, the storage and retrieval system100may include multiple in-feed and out-feed multilevel vertical conveyors150A,150B that are accessible by, for example, bots110on each level of the storage and retrieval system100so that one or more case unit(s) can be transferred from a multilevel vertical conveyor150A,150B to each storage space on a respective level and from each storage space to any one of the multilevel vertical conveyors150A,150B on a respective level. The bots110may 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 bot110picks 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 structure130and places the case unit(s) in the predetermined storage area (and vice versa).

The bots110may 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 structure130and then selectively retrieve ordered items for shipping the ordered items to, for example, a store or other suitable location. In the embodiments, the bots110may interface in any suitable manner with the multilevel vertical conveyors150A,150B such as through, for example, extension of a transfer arm or effector110A (FIG. 3A) of the bot 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” (Ser. No. 61/423,220) and filed on Dec. 15, 2010 with U.S. Ser. No. 13/327,040 filed on Dec. 15, 2011), U.S. Provisional Patent Application entitled “AUTOMATED BOT WITH TRANSFER ARM” with (Ser. No. 61/423,365) and filed on Dec. 15, 2010 with U.S. Ser. No. 13/326,952 filed on Dec. 15, 2011), and U.S. Provisional Patent Application entitled “AUTOMATED BOT TRANSFER ARM DRIVE SYSTEM” with (Ser. No. 61/423,388) and filed on Dec. 15, 2010 with U.S. Ser. No. 13/326,993 filed on Dec. 15, 2011), the disclosures of which are incorporated by reference herein in their entireties.

The storage structure130may 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 aisles130A formed between the rows of storage spaces, and transfer decks130B. In the embodiments, each level may also include respective bot transfer stations for providing an indirect interface between the bots and the multilevel vertical conveyors. In this exemplary embodiment, the picking aisles130A and transfer decks130B may be arranged for allowing the bots110to traverse respective levels of the storage structure130for 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 structure130may be treated substantially equally when determining which storage spaces are to be used when picking and placing case units from/to the storage structure130such that any storage space of sufficient size can be used to store items. The storage structure130of 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 conveyor150A,150B is common to all storage spaces (e.g. the array of storage spaces) in the storage structure130such that any bot110can access each storage space and any multilevel vertical conveyor150A,150B 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 conveyors150A,150B can also receive case units from any storage space on any level of the storage structure130(e.g. no horizontal partitioning). In the embodiments the storage and retrieval system may also be configured so that each multilevel vertical conveyor serves a predetermined area of the array of storage spaces.

The storage structure130may also include charging stations130C for replenishing, for example, a battery pack of the bots110. The charging stations130C may be located at, for example, transfer areas295(FIG. 2) of the transfer deck130B so that the bots110can substantially simultaneously transfer items, for example, to and from a multilevel vertical conveyor150A,150B while being charged. The bots110and other suitable features of the storage and retrieval system100may be controlled by, for example, one or more central system control computers (e.g. control server)120through, for example, any suitable network180. The network180may 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 server120may be configured to manage and coordinate the overall operation of the storage and retrieval system100and interface with, for example, a warehouse management system125, which in turn manages the warehouse facility as a whole. The control server120may 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 toFIG. 2, an exemplary configuration of the storage and retrieval system100is 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 U.S. Provisional Patent Application entitled “MULTILEVEL VERTICAL CONVEYOR PLATFORM GUIDES” with (Ser. No. 61/423,340) and filed on Dec. 15, 2010 with U.S. Ser. No. 13/326,674 filed on Dec. 15, 2011), the disclosures of which are incorporated by reference herein in their entireties. It should be understood that in the embodiments the storage and retrieval system may have any suitable configuration. As can be seen inFIG. 2, the storage and retrieval system200is configured, for exemplary purposes only, as a single-ended picking structure in which only one side of the system200has a transfer section or deck130B. 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 system200includes transfer deck(s)130B and picking aisles130A that allow bots110to traverse an entirety of a level of the storage structure130on which that bot110is located for transporting items between any suitable storage locations/picking aisles130A and any suitable multilevel vertical conveyors150A,150B. The multilevel vertical conveyors150A,150B provide transport of case units into the storage and retrieval system200through input workstations210and provide output of case units from the storage and retrieval system200through output workstations220. The storage and retrieval system200includes a first and second storage section230A,230B 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 deck130B). 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 toFIGS. 3A-3C, as described above, the bots110are configured to traverse the transfer deck(s)130B and picking aisles130A. The speed at which the bot110travels along the transfer deck(s)130B may be higher than the speed at which the bot travels in the picking aisles. In the embodiments the bot110may travel at any suitable speeds within the travel decks and picking aisles. On the transfer decks130B the bots110may be expected to travel at speeds up to about 10 m/s and higher speeds may be desired. The bot may travel along the transfer deck130B in an unconstrained or unrestricted manner (e.g. substantially without mechanical guidance such as tracks, rails, etc.) while the bot110may be guided by, mechanical guide constraints such as tracks or rails during travel within the picking aisles130A such as, for example, described in U.S. patent application Ser. No. 12/757,312, previously incorporated by reference. The bot may include any suitable sensors, such as one or more line following sensors380A,380B and one or more wheel encoders381,382, for providing feedback to, for example, bot controller1220for guiding the bot along one or more of the transfer deck(s)130B and picking aisles130A. In the embodiments the bot controller1220may be located on board the bot110and/or be located remotely from the bot110but in, for example, bi-directional communication with the bot110. In one example the control server120may act at least in part as the remotely located bot controller.

The bot110may include at least two independently driven drive wheels1211,1212and at least one swivelable (not steered, i.e., not provided with an independent steering input) wheel or caster1262,1261. In another aspect, the drive wheels1211,1212may be driven by a common motor and a transmission that is capable of providing or generating differential torque in any suitable manner to the commonly driven wheels for a desired yaw input. It is noted that the casters1261,1262and the drive wheel1211,1212are disposed at substantially opposite longitudinal (e.g. front to back) ends of the bot110where a wheel is located substantially at each of the four corners of the bot. This caster wheel/drive wheel configuration may provide improved high speed stability for the bot and ease of control. Each drive wheel1211,1212may have its own respective motor383,384that is controlled by the bot controller1220in the manner described herein. As will be described in greater detail below, the casters1262,1261may be selectively locked to allow stable travel of the bot110during, for example, substantially high speed bot travel along the transfer deck130B. It is noted that during low speed travel the casters1262,1261may be unlocked so that the bot110can enter, for example, picking aisles and/or multilevel vertical conveyor interface stations with either a front or a back of the bot110leading a direction of bot travel as described in, for example, U.S. Provisional Patent Application entitled “AUTONOMOUS TRANSPORT VEHICLE” (Ser. No. 61/423,409) and filed on Dec.15, 2010 with U.S. Ser. No. 13/326,423 filed on Dec. 15, 2011), the disclosures of which are incorporated by reference herein in their entireties. Steering of the bot110along the transfer deck may be effected by applying a differential torque to the drive wheels1211,1212through the respective independently controller motors383,384. The differential torque T may result in each of the drive wheels rotating at different speeds (e.g. rotations). The different rotational speeds of each wheel may cause the bot110to yaw or turn.

As an example of bot travel along the transfer deck and referring toFIGS. 3C and 5, during straight line travel of the bot along, for example, guide line395(e.g. in 1 degree of freedom) both drive wheels1211,1212rotate at substantially the same speed/angular velocity. If the travel direction of the bot has to be corrected for any suitable reason, one of the drive wheels1211,1212may be slowed down to cause yawing of the bot. For example, if the bot110is to turn or yaw in the direction of arrow396the bot controller1220may cause drive wheel1211to rotate slower than drive wheel1212for turning the bot110in the direction of arrow396. Likewise, if the bot110is to turn or yaw in the direction of arrow397the bot controller1220may cause drive wheel1212to rotate slower than drive wheel1211for turning the bot110in the direction of arrow397. It is noted that the bot110is capable of bidirectional travel along guide line395.

In the embodiments, as described above, the bot110may include one or more line following sensors380A,380B configured to detect one or more guidance lines391-395(which are inclusive of guide lines391A which may be disposed on the transfer deck130B at predetermined intervals in a direction transverse to the bot travel along the transfer deck may be provided as shown inFIG. 5) such as line395disposed on a surface of, for example, the transfer deck130B for providing steering feedback to the bot controller1220. It is noted that the one or more line following sensors380A,380B may be located at the front and rear of the bot110or the line following sensors may be located in any suitable location on the bot. The bot controller1220may be configured to receive signals generated by the one or more line following sensors380A,380B and or wheel encoders381,382from which the controller1220determines suitable torque commands for driving the respective motors383,384of drive wheels1211,1212.

The casters1261,1262may be un-steered as noted before, but may have a directional or swivel lock configured to aid in steering and control of the bot110. It is noted that during substantially straight line travel of the bot on the transfer deck130B the casters1261,1262may be locked (e.g. the plane of rotation of the caster wheel400is substantially aligned with a 1 degree of freedom axis110X of the bot where the 1 degree of freedom axis110X may coincide with the bot's straight line direction of travel) unless the degree at which the bot110is to turn (e.g. to account for corrections to the direction of travel) exceeds a predetermined amount (e.g. a predetermined turn angle for correcting tracking of the substantially straight line travel of the bot) or if the bot travel speed is below a predetermined amount. The casters may also be unlocked to provide the bot with steering that is more responsive than when the casters are locked so that quicker corrections to the bot's direction of travel can be made. It is noted that, as shown inFIG. 5, the positioning of the casters1261,1262at an end of the bot100that is opposite the drive wheels1211,1212operates to amplify the slip angle or amount of lateral movement at, e.g. a front of the bot where the casters are located. The slip angle or amount of lateral movement may be generated from the differential rotation at the drive wheels1211,1212. The slip angles will generally be small, and thus can be generated by correspondingly smaller differential rotation at the drive wheels which minimizes slipping of the drive wheels (which is undesirable). The slipping of the caster wheels may occur when the swivel lock of the casters is locked and a differential torque is applied to the drive wheels such that the caster wheels are “slid” in a direction substantially transverse to a plane of rotation of the caster wheel as it travels along, e.g., the transfer deck130B. During the slipping of the caster wheel it is noted that the caster wheel may slow its rotation or substantially stop rotating.

Still referring toFIGS. 3A-3Cand5as a non-limiting example only, the bot may travel on the transfer deck130B along line395where the bot is to turn down picking aisle130A1in a manner substantially similar to that described in U.S. patent application Ser. No. 12/757,312 (previously incorporated by reference herein). The casters1261,1262may be locked to allow for stable high speed bot travel until the bot makes or is about to make the turn onto the picking aisle130A1. For example, as the bot slows below a predetermined travel speed, the casters1261,1262may unlock allowing the caster wheels to swivel (e.g. giving the bot 2-degrees of freedom of movement). The bot controller1220may also determine that the bot is to make a turn (such as the substantially 90-degree turn into picking aisle130A1) where the angle of the turn exceeds a predetermined turn angle. The casters1261,1262may unlock upon the determination that the turn angle exceeds the predetermined turn angle for correcting tracking of the substantially straight line travel of the bot. The bot controller1220may also account for turn angle and bot speed when determining when to unlock the casters1261,1262. The controller1220may also be configured to cause the unlocking of the casters1262,1261at substantially the same time that a differential torque is applied to the drive wheels1211,1212.

Referring now toFIGS. 4A-4Ethe swiveling caster wheels1261,1262having the selectively lockable swivel lock will be described. It is noted that while the casters are described with respect to caster1262, caster1261is substantially similar. In accordance with the embodiments, the caster1262includes a frame412, a wheel yoke410, a wheel400, a first locking member440, a second locking member450, an actuator420and a spring430. It is noted that the caster may have any suitable components arranged in any suitable manner. In this example, the wheel400is rotatably mounted to the yoke410in any suitable manner. The second locking member450may be fixedly mounted to the yoke450so that the second locking member450and yoke410form an integral unit. In the embodiments the second locking member may be integrally formed with the yoke410in a unitary construction. The yoke410(and the second locking member450mounted thereto) may be pivotally mounted to the frame412so that the yoke assembly (e.g. including the second locking member450, the yoke410and the wheel400) can be freely rotated 360-degrees in the direction of arrow499(see alsoFIG. 3B). It is noted that the axis of rotation of the yoke assembly is substantially perpendicular to a surface on which the bot110travels, such as, for example, the surface of the transfer deck130B. The first locking member440may be pivotally mounted to the frame412by, for example, any suitable pivot441having a pivot axis that is substantially parallel with a swivel axis of the yoke450(and wheel400). The pivot441may be for example, a shoulder bolt or any other suitable axle and/or retaining device. A spring430or any other suitable resilient member may be mounted between the frame430and a portion of the first locking member440so as to bias the first locking member440about the pivot441as will be described in greater detail below. The actuator420may also be mounted to the frame412for engaging a portion of the first locking member440in an opposing relation to the spring430. The actuator420may be any suitable actuator, such as a solenoid, suitably connected to, for example, the bot controller1220where the controller is configured to cause actuation of the actuator420. In one aspect the actuator420may be horizontally oriented (e.g. arranged to extend and retract horizontally) but in other aspects the actuator may have any suitable orientation. The swivel lock of the casters1262,1261may be configured for rapid lock and release through, for example, a pulse actuation of the actuator420(e.g. electrical current is sent to the actuator for a predetermined amount of time so that the actuator is activated only for a limited time so that a piston420P of the actuator is retracted for releasing the swivel lock of the caster and extended for locking the swivel lock of the caster in succession). In one example, the pulse actuation of the actuator420may activate the actuator for approximately one or two seconds for unlocking the swivel lock of the casters. In other examples the pulse actuation of the actuator may activate the actuator for less than about one second. In still other examples, the pulse actuation of the actuator may activate the actuator for more than about two seconds or any other suitable amount of time. The actuator may also be configured so that the caster is released when the piston420P is extended and locked when the piston420P is retracted. As may be realized, in one example, any suitable sensors may be provided and be in communication with the controller1220for sensing when each of the swivel locks for the casters is locked and/or unlocked. In other examples, sensors may not be provided for sensing when the casters are locked and/or unlocked.

In this exemplary embodiment the first locking member or lock bar/link440may have a substantially dog-leg or hooked shape. In alternate embodiments the first locking member440may have any suitable shape. The first locking member440may have a pivot hole440H configured to accept the pivot441for allowing the first locking member to pivot about the pivot pin441. A first portion440B of the first locking member440may extend in a first direction from the pivot hole440H while a second portion440C of the first locking member440extends in a second direction from the pivot hole440H. The first direction and second direction may be angled relative to one another by any suitable angle (e.g. from 0 to 360 degrees). In this example, the first portion440B includes a protrusion or other locking feature440L configured to engage a reciprocally shaped engagement feature450S1,450S2of the second locking member450. The second portion440C may include an attachment feature440S for attaching the spring430to the first locking member440. The second portion440C may also include an actuator engagement surface440A. In this example, the ratio between the lengths of the first and second portions440B,440C of the first locking member may be such that forces applied to the second portion440C cause the precise movement of the first portion440B for releasing locking feature440L from the second locking member450. For example, the lever action of the first locking member440converts the stroke of the actuator420to a short engagement stroke of the protrusion440L into and out of the one or more engagement features450S1,450S2that may result in substantial on/off control of the swivel lock to reliably engage and disengage the swivel lock while the bot is travelling at speed (e.g. about 10 m/s or more) and/or while the bot is travelling at any suitable speed.

In the embodiments the second locking member450is in the form of a disc but the second locking member may have any suitable configuration. The second locking member may include one or more engagement features or slots450S1,450S2or other retaining feature configured to accept and reciprocally engage the protrusion440L of the first locking member440or the second locking member may include a protrusion for engaging a reciprocally engagement feature or recess in the first locking member. In the embodiments, the second locking member may include two slots arranged so that the caster wheel400can be locked at about 0-degrees and about 180-degrees where the about 0 and 180-degree positions are arranged so that when locked the rotational plane of the wheel400is substantially aligned with a 1-degree of freedom axis110X of the bot (which may coincide with the direction of straight line travel of the bot110). In the embodiments the second locking member may have any suitable number of locking features arranged in any suitable manner for locking the wheel at any angle(s).

In operation, to lock swivel lock of the caster1262the actuator may be retracted (e.g. a piston420P or other suitable engagement feature of the actuator may be retracted) so that it is not in substantial contact with the actuator engagement surface440A of the first locking member440. The spring430may be configured to pull or otherwise bias the first locking member to rotate about pivot441in the direction of arrow495so that the locking feature440L is substantially forced against a side surface450S of the second locking member450. As the yoke assembly rotates in the direction of arrow499and the biased locking feature440L rides along the side surface450S the force exerted by the spring430causes the locking feature440L to be inserted into one of the slots450S1,450S2for substantially preventing further rotation of the yoke assembly such that the yoke assembly of the caster is in a locked configuration (e.g. is locked from rotation). As noted above, the slots450S1,450S2may be positioned so that when the locking feature440L is inserted into one of the slots450S1,450S2the rotational plane of the wheel499is substantially aligned with the 1 degree of freedom axis110X of the bot. As such, one of the slots450S1,450S2will be aligned with the locking feature440L by virtue of the bot travelling along a substantially straight line path to facilitate the locking of the caster1262.

To unlock the swivel lock of the caster, the actuator420is actuated (e.g. the piston420P or other engagement feature is extended in the direction of arrow491) to engage the actuator engagement surface440A of the first locking member440. As described above, the actuation of the actuator420may be a pulse actuation (e.g. electrical current is sent to the actuator for a predetermined amount of time so that the actuator is activated only for a limited time so that a piston420P of the actuator is retracted for releasing the swivel lock of the caster and extended for locking the swivel lock of the caster in succession). The force exerted by the actuator420on the first locking member440opposingly overcomes the force of the spring430causing the first locking member440to rotate about pivot441in the direction of arrow496such that the locking feature440L disengages the slot450S1,450S2for unlocking the swivel lock of the caster1262. As a differential torque T is applied to the drive wheels the caster wheels1262,1261want to rotate according to the way the bot110is turning. The disengagement or release of the locking member440L from the slot450S1,450S2allows the wheel400(e.g. through a rotation of the components of the yoke assembly) to freely rotate or swivel in the direction of arrow499by virtue of the differential torque applied to the drive wheels1211,1212. It is noted that the actuator is activated for a time period that is long enough for the slot450S1,450S2to become misaligned with the locking feature440L after which time the spring430causes the locking feature440L to ride along the side450S as described above to allow for unlocking and locking of the caster1262,1261as described above.

As can be seen in e.g.FIGS. 4A-4E, the configuration of the first locking member440is such that locking engagement between the first and second locking members440,450occurs substantially laterally relative to the longitudinal axis or 1-degree of freedom axis of the bot110for increased rigidity (e.g. creating a greater moment arm for resisting rotation of the yoke assembly). However, the first locking member (and second locking member) may have any suitable configuration and spatial relationship with each other and the 1-degree of freedom axis of the bot for rigidly preventing rotation of the caster1262,1261. In the embodiments, a caster drive motor may be coupled to, for example, the yoke assembly, for providing steerable rotational movement of the caster1262,1261. It is noted that the coupling between the caster drive motor and the yoke assembly may be any suitable coupling including substantially rigid links, belts/pulleys, gear trains and any other suitable drive couplings.

Referring again toFIG. 3Cand as described above, the bot110may include one or more wheel encoders381,382configured to sense rotational movement of each of the drive wheels1211,1212. The wheel encoders281,282may be any suitable encoders such as, for example, incremental encoders. In the embodiments the encoders may also be absolute encoders. In this example, the encoders381,382provide data to, for example, the bot controller1220for determining a location of the bot110within the storage and retrieval system100. An example of determining the location of the bot110within the storage and retrieval system100using encoders can be found in U.S. Provisional Patent Application entitled “BOT POSITION SENSING” (Ser. No. 61/423.206) and filed on Dec. 15, 2010 with U.S. Ser. No. 13/327,035 filed on Dec. 15, 2011), the disclosures of which are incorporated by reference herein in their entireties. The encoders381,382on each of the drive wheels may also provide information for determining a state (e.g. acceleration, speed, direction, etc.) of the bot110as will be described below. However, the drive wheels1211,1212may be subject to wheel slipping that adversely affects the confidence of the data received from the encoders381,382. Wheel slipping may be caused by, for example, a loss of traction. This loss of traction may be caused by, for example, a reduction in friction between each of the drive wheels1211,1212and the drive surface of, for example, the transfer deck130B or picking aisle130A or one of the drive wheels1211,1212lifting off of the drive surface of the transfer deck130B or picking aisle130A. Where the bot drive wheels1211,1212lose traction the wheel position measured by the respective encoders381,382will no longer reflect the actual bot110position. For example, if the bot110is accelerating then the wheel will tend to rotate faster than the bot is travelling. Conversely, if the bot is decelerating then the slipping wheel will tend to rotate slower than the bot is travelling.

In accordance with the embodiments, the bot controller1220, or any other suitable controller of the storage and retrieval system (e.g. control server120) may be configured to account for wheel slip by, for example, using the data from the encoder381,382from the drive wheel1211,1212that is not slipping. In one example, the controller1220may be configured to determine which drive wheel1211,1212is slipping based on the torque being applied to the drive wheels1211,1212by their respective drive motors383,384. As a non-limiting example, if a positive torque is being applied then a slipping wheel will accelerate in a positive manner so the encoder from the wheel with a lower velocity and/or rotational speed will be used to estimate the speed and position of the bot110within the storage and retrieval system100. If a negative torque is being applied to the drive wheels1211,1212then a slipping wheel will accelerate in a negative manner (e.g. decelerate) so the encoder from the drive wheel1211,1212with a higher rotational velocity and/or rotation speed will be used to estimate the speed and position of the bot110within the storage and retrieval system100. In the embodiments, one or more encoders may be placed on idler wheels (e.g. wheels that are not driven) such as the casters1261,1262such that the controller can estimate the position and speed of the bot110based on the rotation of the non-driven wheels.

In the embodiments, the controller1220may be configured with predetermined drive wheel1211,1212velocities or rotational speeds for any predetermined time of bot operation. The controller1220when determining the position and speed of the bot110when, for example, one wheel is slipping may compare the actual velocities and/or rotational speeds of the drive wheels1211,1212with the expected or predetermined velocities and/or rotational speeds for each drive wheel1211,1212. In one example, the controller1220may be configured to perform inertia modeling to determine how much velocity/rotation speed change to expect due to, for example, acceleration of the bot110(and its drive wheels). If the velocities/rotational speeds of one or more of the drive wheels1211,1212does not substantially match the expected or predetermined velocities/rotational speeds the encoder data from the one or more of the drive wheels1211,1212whose data does not substantially match may be ignored and replaced with the expected or predetermined data when determining the bot's100position and speed.

It is noted that the determination of the bot's speed and position may be performed with, for example, models that both utilize and do not utilize suitable filters. In the embodiments the controller1220may be configured to filter the spurious data from the drive wheel encoders381,382when determining bot110location/speed and state estimations. As an example, the controller1220may include, for example, a Kalman (or other suitable) filter to substantially eliminate the effects of wheel slip error in bot state estimation and location calculations. In one aspect the controller1220may include an extended Kalman filter1220K that may be employed using substantially real time encoder updates (for example, at 2 kHz, every 0.5 milliseconds, every 50 milliseconds, or any other suitable frequency or time increments), along with any suitable time data regarding each sensor transition event (e.g. such as when a sensor detects a suitable guide line, seeFIG. 5on the transfer deck130B). Referring toFIG. 6a bot position determination method employing an extended Kalman filter is depicted. In general, calculating a bot110position may be performed using an extended Kalman filter by applying data received from any suitable encoder (FIG. 6, Block16000), such as one or more of encoders381,382, to determine the bot110position and/or predict sensor380A,380B transitions. However, as a variation to the general method depicted inFIG. 6, the bot position finding Kalman model may be updated periodically. More specifically, sensor data may be received at each sensor380A,380B transition over a suitable guide line, such as guide lines391-394(as may be realized, additional guide lines391A that are disposed on the transfer deck130B at predetermined intervals in a direction transverse to the bot travel along the transfer deck may be provided as shown inFIG. 5), that includes a time of the transition and as appropriate, an identity and/or location of the guide line (FIG. 6, Block16001). Based upon the data from sensors380A,380B, an error may be calculated between an expected transition time for the location and the measured transition time (FIG. 6, Block16002). This error may then be employed to update the extended Kalman filter for more accurate subsequent estimations of bot positioning as determined by the wheel encoders381,382(FIG. 6, Block16003). In general, one or more of the wheel encoder and line sensor data is employed to provide bot positioning data for control of the bot110, while actual detected guide line transitions may be employed to update the pot positioning model, for example, the equations of an extended Kalman filter.

By way of example, for bot positioned at a particular position (Xe, Ye) on, for example the transfer deck130B, and travelling at an estimated velocity and acceleration V, a, the model might predict a guide line being sensed or transitioned by one of the line sensors380A,380B at time te, and the system may identify the actual transition of the guide line at time ts. The encoder381,382time ts(or optionally at the time stamp) may generate an error expressed as:

Then, extended Kalman filter equations may be used as described for example, in Applied Optimal Estimation by Arthur Gelb (MIT Press 1794). An adaptation of the formulation described in Gelb may be briefly stated as a system model:
{dot over (x)}(t)=f(x(t),t)+w(t);w(t)≈N(0,Q(t))

with state estimate propagation:
{circumflex over (x)}(t)=f({circumflex over (x)}(t),t)

and error covariance propagation:
{dot over (P)}(t)=F({circumflex over (x)}(T),T)P(t)FT({circumflex over (x)}(t),t)+Q(t)

As a significant advantage, this generalized technique permits use of individual sensor events incrementally, rather than requiring some number of sensor event to identify the location of the bot110. It should be understood that, while a particular order of steps is implied inFIG. 6, that the depicted operations are repetitively performed during operation of the bot110, and that no particular order or timing of steps should be inferred. Nonetheless, it will be generally true in some implementations that encoder data from wheel encoders381,382may be provided substantially continuously in real time, while line sensor380A,380B transitions of guide lines391-394(and391A) may occur intermittently as the bot traverses, for example the transfer deck130B. It should also be understood that, while an extended Kalman filter is one useful technique for converting encoder data into bot position information, other filters or linear modeling techniques may similarly be applied.

The data from the encoders381,382may be further weighted based on signals from the one or more line following sensors380A,380B that provide signals to the controller1220corresponding to deviation of the bot from a guide line GL (FIG.5) on, for example, the transfer deck130B. For example, when one of the drive wheels1211,1212slips a differential torque results between the drive wheels1211,1212which causes the bot110to turn and deviate from the guide line GL.

The bot controller1220may also be configured to determine a position of the bot using, for example, the wheel encoders381,382in combination with other position tracking features of the storage and retrieval system. In the embodiments, when in the picking aisles130A the wheel encoders381,382may be used in combination with slat counting (e.g. tracking the position of the bot relative to slats on the storage shelves with one or more slat detection sensors387) as described in U.S. Provisional Patent Application No. 61/423,206 entitled “BOT POSITION SENSING”, previously incorporated by reference herein. In the embodiments, when travelling on the transfer deck130B the controller1220may be configured to use the wheel encoders381,382in combination with a tracking of the guide lines391-395on the floor (e.g. driving surface) of the transfer deck130B. Referring toFIGS. 3C and 5when travelling on the transfer deck130B the one or more line following sensors380A,380B of the bot detect guide lines391-395(FIG. 7, Block17000). The controller1220receives signals from the one or more line following sensors380A,380B (FIG. 7, Block17001) and drives the individual drive motors383,384accordingly (e.g. by adjusting the torque applied to each respective wheel for steering the bot as described above) for keeping the bot travelling along a desired one of the guide lines391-395(FIG. 7, Block17002). As can be seen inFIG. 5, each guide line GL running around the transfer deck130B is crossed by, for example, transverse guide lines, such as guide line392located at turns on the transfer deck as well as guide lines391,393,394located at the picking aisle130A locations. The crossing locations R, P of each of these guide lines391-395may be at known predetermined locations which are stored in any suitable memory accessible by the bot110such as for example, a memory of the bot110or the control server120. Where the crossing locations R, P are stored remotely from the bot110, such as in a memory of the control server, the bot110may be configured to access the crossing location information in any suitable manner such that the information is downloaded to the bot110or remotely read by the bot110.

In one exemplary operation of bot travel, it is noted that verification or position qualification of the bot110on, for example, the transfer deck130B may be determined from the crossing locations/position datum lines R, P on the transfer deck130B (FIG. 7, Block17003) in a manner substantially similar to that described in U.S. patent application Ser. No. 12/757,312 (previously incorporated by reference herein). For example, as the bot110travels along, for example, guide line395the bot passes the crossing location P and may verify its location using the predetermined location of the crossing location P. At this point the data from the wheel encoders may be reset (FIG. 7, Block17004) so that the encoders incrementally track, for example, a distance traveled by the bot110starting from crossing location P (which was previously verified) so that, for example, the controller1220can estimate the bot position relative to a last known verified position of the bot (FIG. 7, L.17005). It is noted that resetting the wheel encoder data may substantially eliminate any tolerance and/or error stack ups generated by the wheel encoders. As the bot110continues to travel along the guide line395the bot110detects a second crossing location R. At substantially the time the second crossing location R is detected by the bot110the controller1220uses an estimated location of the bot110as determined from the wheel encoders381,382and the last verified location of the bot (which in this example is at crossing location P) and compares the estimated location of the bot with the predetermined location of the crossing location R (FIG. 7, Block17006). If the estimated location of the bot substantially matches the predetermined location of the crossing location R then the controller verifies or otherwise qualifies the position of the bot110and updates the bots110location accordingly (FIG. 7, Block17007). The controller1220may also be configured to verify or otherwise qualify the bot's location in any suitable manner such as, for example, through the use of an extended Kalman filter as described above. As may be realized during the determination of the bot's position/location using the wheel encoders381,382and line following the controller1220may be configured to account for wheel slip in the manner described above. If the estimated position does not match the predetermined location of the crossing location R the signal generated by the one or more line following sensors380A,380B corresponding to guide line394(at location R) may be ignored such that the location of the bot is estimated using the wheel encoders381,382(FIG. 7, Block17008) until a next crossing location can be verified or otherwise qualified with the bot's estimated location (FIG. 7, Blocks17006,17007). In the embodiments, if the estimated location of the bot110and the predetermined crossing location do not match the bot110may also be configured to verify its position in any suitable manner such as, for example, returning to a last known location and resetting the encoder information.

Referring toFIGS. 3C and 5, the bot controller1220(or other suitable controller configured to control the bot, such as a remote controller as described above) may be configured with a state estimator module1220E for estimating, for example, the kinematic state of the bot (e.g. position X, Y; position rates or speed {dot over (x)}, {dot over (y)}; and/or yaw angle α all with respect to time t) to determine command logic for controlling the bot rather than controlling the bot directly from sensor input. For example, using the state estimator module1220E the bot110can determine, based on one or more sensor inputs, how fast it is travelling, what direction it is travelling, etc. and then determine what bot parameters need to be changed in order to arrive at a new desired speed, direction, etc. rather than, using bot speed as an example, reduce the torque on the drive wheels and use the wheel encoders as a gauge for stopping the reduction in motor torque when the encoders send signals corresponding to the desired new speed.

In one example, the command logic may allow, for example, the controller1220to know the speed, acceleration, and direction of the bot and calculate corrective control commands to, for example, the drive motors383,384for maintaining a predetermined directional course and speed. For convention purposes, the Y (e.g. longitudinal) position of the bot is in a direction substantially parallel with a predetermined datum line or line of travel (in this example e.g. line395) and may be an estimation of bot position between, for example, line crossing locations R, P or any other suitable lateral datum lines. The X (lateral) position of the bot is in a direction substantially transverse to the predetermined datum line391-395and may be an estimation of the amount of offset between, for example, the 1 degree of freedom axis110X of the bot and the predetermined datum line391-395. The yaw angle α (e.g. the divergence/convergence angle with the predetermined datum line) may be an estimation of the angle between the 1 degree of freedom axis110X of the bot and the predetermined datum line391-395. The state estimator module1220E of the controller1220may also be configured to determine or otherwise estimate dynamic states and commands such as, for example, forces Fx, Fy exerted on the bot, the differential torque T applied by the drive motors383,384, a position rate or speed of the bot (e.g. linear or angular/yaw speeds) and/or changes in the position rate (e.g. acceleration) of the bot110.

In addition to the wheel encoders381,382and line following sensors380A,380B described above, the bot110may or may not also include any suitable inertial sensor389, such as a 1 or 2 dimensional accelerometer. The controller may be configured to use signals from the inertial sensor389in combination with, for example, wheel odometry information from the wheel encoders381,382and the line following sensors380A,380B for estimating the state of the bot110.

Input may be provided to the controller1220for use with the state estimator module1220E from, for example, one or more of the wheel encoders381,382, the line following sensors380A,380B and the inertial sensor389(FIG. 8, Block18000). Input may also be provided to the controller for use with the state estimator from slat sensors (e.g. sensors that sense the storage shelf slats). An example, of storage slat sensors can be found in U.S. Provisional Patent Application No. 61/423,206 entitled “BOT POSITION SENSING”, previously incorporated by reference herein.

In the embodiments the bot may have, for example, a steady state of operation (e.g. no acceleration/deceleration) and a dynamic state of operation (e.g. during acceleration/deceleration). During steady state operation with the differential motor torque T substantially equal to zero the state estimator may obtain data for longitudinal position Y and longitudinal speed {dot over (y)} from, for example, wheel odometry as described above (FIG. 8, Block18001). It is noted that the longitudinal speed {dot over (y)} may be obtained from the wheel encoders using, for example, a Kalman or other suitable filter accounting for wheel slip as described above. Lateral position X and lateral speed {dot over (x)} may be obtained from, for example, the line following sensors380A,380B when the line following sensors380A,380B at, for example, the front and rear of the bot110cross the lines393,394,391. The lateral position X of the bot and the yaw angle α can be calculated from the front and rear positions of the bot110as determined by the sensors380A,380B when the sensors cross the lines393,394,391(FIG. 8, Block18002). In one example, between line393,394,391crossings the differential movement of the wheels (as determined from e.g. wheel encoders or other suitable sensors) indicate rotation or yaw of the bot. In another example, between line393,394,391crossings the rotation or yaw angle α of the bot110may be determined from a dynamic model in, for example, an extended Kalman filter. The lateral speed {dot over (x)} can be determined by the state estimator, for example, as follows:
{dot over (x)}={dot over (y)}tan α
where {dot over (y)} may be verified using, for example, crossing locations R, P or any other suitable transverse datum lines similar to guide lines393,394,391.

Referring also toFIG. 4A-4E, in one exemplary embodiment the controller1220may command actuation of the actuators420for unlocking the casters1262based on the state of the bot110so that quick steering response can be obtained for correcting the yaw angle α of the bot. The yaw angle α of the bot may also be corrected with the casters locked, which may provide a slower steering response with more bot stability. The controller1220may know (via the state estimator module1220E) an estimate of the differential torque T to apply to the drive wheels1211,1212so that a maximum differential torque T can be applied to the drive wheels1211,1212for correcting the yaw angle α of the bot110. It is noted that the estimate for the transverse speed {dot over (x)} (or any other suitable kinematics of the bot) can be updated (FIG. 8, Block18003) at any suitable time such as when, for example, there is no line sense signal from speed {dot over (y)} and yaw angle α. The yaw angle α and the rate of change of the yaw angle {dot over (α)} can be estimated using, for example, the inertial sensor389.

Once the differential torque T is applied based on the estimated value of the yaw angle α, the controller1220waits to receive a guide line sense signal from one or more of the line following sensors380A,380B. When the line sense signal is received, the controller1220may command a reduction in differential torque T and continue to update the kinematic information of the bot110until the yaw angle α is at a predetermined value such that the bot is substantially travelling along the desired guide line391-395. It is noted that the differential torque T may be reduced proportionately to the approach rate (e.g. {dot over (x)}, {dot over (α)}) to, for example, the center of the guide line391-395. When the differential torque T is reduced to substantially zero the bot is travelling in a substantially straight line enabling the casters to lock into their locked configuration as described above (e.g. the locking feature440L may ride along the side surface450S of the second locking member450until it is substantially aligned with and engages one of the slots450S1,450S2. It is noted that when the differential torque T is applied, the transverse position X and speed {dot over (x)} may be determined by the controller1220(via the state estimator module1220E) from, for example, a mean of the wheel encoder signals where the differential torque T is known and the wheel encoder signals (from each drive wheel1211,1212) are related to one another (or otherwise weighted) based on the differential torque T such that, for example, a difference of inner to outer wheel encoder signals (depending on which way the bot is turning where the inner encoder is on the inside of the turn) is applied to the encoder signal based on the magnitude of the differential torque T.

In the dynamic state of bot operation, e.g. where the bot110is undergoing positive acceleration or negative acceleration (i.e. deceleration) and the differential torque T is substantially zero or greater than zero the kinematic state of the bot may be determined in a manner substantially similar to that described above with respect to the steady state of bot operation. It is noted that where travel of the bot110is guided by a contact linear guide system (e.g. tracks or rails) as in, for example, the picking aisles130A (see for example, U.S. patent application Ser. No. 12/757,312, entitled “AUTONOMOUS TRANSPORTS FOR STORAGE AND RETRIEVAL SYSTEMS,” previously incorporated herein) there is no data sent to the controller1220by, for example, the line following sensors380A,380B. As such, the state estimation of the bot110within the contact linearly guided travel areas is only in the longitudinal direction. The position of the bot110within the picking aisles may be determined by the state estimator (without direct wheel encoder readings) in combination with, for example, the storage shelf slat location validation as described in U.S. Provisional Patent Application No. 61/423,206 entitled “BOT POSITION SENSING”, previously incorporated by reference herein.

In a first aspect of the embodiments an autonomous transport vehicle for transporting items in a storage and retrieval system is provided. The autonomous transport vehicle includes at least two drive wheels and a controller, where each drive wheel is independently driven and a drive wheel encoder is disposed adjacent each drive wheel. The controller, in communication with the encoders, is configured to determine a kinematic state of the autonomous transport vehicle within the storage and retrieval system based on incremental data from the drive wheel encoders only and independent of drive wheel slippage.

In accordance with a first aspect of the first aspect of the embodiments, the controller is configured to determine command logic for operating the autonomous transport vehicle based on the kinematic state.

In accordance with the first aspect of the first aspect of the embodiments, the autonomous transport vehicle includes one or more of a wheel encoder for each drive wheel, an inertial sensor, at least one storage slat sensor and at least one line following sensor, wherein the controller is configured to receive data from the one or more of the wheel encoder for each drive wheel, the inertial sensor, the at least one storage slat sensor and the at least one line following sensor for determining the kinematic state.

In accordance with a second aspect of the first aspect of the embodiments, the controller is further configured to determine a position of the autonomous transport vehicle within the storage and retrieval system independent of drive wheel slippage.

In accordance with the second aspect of the first aspect of the embodiments, the controller is configured to determine the position of the autonomous transport vehicle based on a drive wheel having a lowest velocity of the at least two drive wheels when a positive torque is applied by the at least two drive wheels.

In accordance with the second aspect of the first aspect of the embodiments, the controller is configured to determine the position of the autonomous transport vehicle based on a drive wheel having a highest velocity of the at least two drive wheels when a negative torque is applied by the at least two drive wheels.

In accordance with the second aspect of the first aspect of the embodiments, the controller is configured with an extended Kalman filter for filtering spurious data from each of the encoders.

In accordance the second aspect of the first aspect of the embodiments, the autonomous transport vehicle includes at least one line following sensor configured to sense guide lines on a surface of the storage and retrieval system, the controller being further configured to weight data from each of the encoders based on a guide line deviation signals provided by the at least one line following sensor.

In accordance the second aspect of the first aspect of the embodiments, wherein the controller is configured to verify a position of the autonomous transport vehicle by detecting one or more datum lines on a surface of the storage and retrieval system.

In accordance with the first aspect of the embodiments, the autonomous transport vehicle includes at least one releasably lockable caster wheel.

In accordance with a second aspect of the embodiments, an autonomous transport vehicle for transporting items in a storage and retrieval system is provided. The autonomous transport vehicle includes a frame, a controller, at least two independently driven drive wheels mounted to the frame and at least one caster wheel mounted to the frame and having a releasably lockable swivel lock. The controller being configured to lock and unlock the releasably lockable swivel lock during a transport of items through the storage and retrieval system based on a predetermined kinematic state of the autonomous transport vehicle.

In accordance with the second aspect of the embodiments, the controller is configured to individually drive the independently driven drive wheels based on command logic generated from a determination of the predetermined kinematic state of the autonomous transport vehicle.

In accordance with the second aspect of the embodiments, the controller is configured to apply a differential torque to the independently driven drive wheels for effecting curvilinear travel of the autonomous transport vehicle.

In accordance with the second aspect of the embodiments, the controller is configured to maintain the releasably lockable swivel lock in a locked state when a lateral position and yaw angle of the autonomous transport vehicle are below a predetermined deviation value.

In accordance with the second aspect of the embodiments, the controller is configured to unlock the releasably lockable swivel lock substantially during an application of differential torque to the independently driven drive wheels.

In accordance with a first aspect of the second aspect of the embodiments, the at least one caster wheel includes an actuator, a first locking member and a second locking member connected to a wheel of the at least one releasably lockable caster wheel, the actuator being configured for a pulse release of the first locking member from the second locking member for unlocking the releasably lockable swivel lock.

In accordance with the first aspect of the second aspect of the embodiments, the at least one caster wheel is configured such that the first locking member and second locking member are substantially aligned during substantially straight line travel of the autonomous transport vehicle and the first locking member is biased to automatically engage the second locking member for locking the releasably lockable swivel lock.

In accordance with the first aspect of the second aspect of the embodiments, wherein the autonomous transport vehicle has a longitudinal axis substantially aligned with the straight line travel of the autonomous transport vehicle and a lateral axis that is transverse to the longitudinal axis, the first locking member being configured to engage the second locking member in a lateral direction.

In accordance with a third aspect of the embodiments, an autonomous transport vehicle is provided. The autonomous transport vehicle includes at least two independently driven drive wheels, at least one releasably lockable caster wheel and a controller. The controller includes a state estimator configured to estimate a kinematic state of the autonomous transport vehicle wherein the controller issues control commands to the at least two independently driven drive wheels and the at least one releasably lockable casters based on the estimated kinematic state of the autonomous transport vehicle.

In accordance with the third aspect of the embodiments, the controller is configured to estimate a state of the autonomous transport vehicle based on data obtained from one or more sensors of the autonomous transport vehicle.

In accordance with a fourth aspect of the embodiments an autonomous transport vehicle for transporting items in a storage and retrieval system is provided. The autonomous transport vehicle includes a controller, at least one wheel encoder in communication with the controller and at least one line following sensor in communication with the controller and configured to detect guide lines disposed on a deck of the storage and retrieval system. The controller being configured to estimate a position of the autonomous transport vehicle within the storage and retrieval system using an extended Kalman filter and sensor data from the at least one wheel encoder. The controller is further configured to update the extended Kalman filter using sensor data from the at least one line following sensor so that an accuracy of the estimated position of the autonomous transport vehicle increases over previous estimated position determinations.

In accordance with a fifth aspect of the embodiments an autonomous transport vehicle for transporting items in a storage and retrieval system is provided. The autonomous transport vehicle including a controller, a frame, at least two driven wheels mounted to the frame and a wheel encoder for each of the driven wheels in communication with the controller and configured to detect rotation of a respective driven wheel. The controller being configured to determine a position of the autonomous transport vehicle within the storage and retrieval system from sensor data from an encoder for a driven wheel having the best wheel odometry of the at least two driven wheels.

It should be understood that the 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.