Abstract:
An inventory system includes a plurality of mobile drive units with a processor control and with a positioning system that enables the mobile drive units to navigate a factory floor. The mobile drive units interface with a material handling system to receive order requests and deliver inventory items to pack stations located on the factory floor. The inventory items are stored in trays stacked into movable inventory pods, which may be transported by the mobile drive units throughout the factory floor. The mobile drive units dock and undock with the movable inventory pods using a docking mechanism. The movable inventory pods are stored in a virtual storage grid when they are not being transported by the mobile drive units. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Description:
RELATED APPLICATIONS 
     This is a continuation-in-part (CIP) application of application Ser. No. 10/196,772 filed Jul. 15, 2002, entitled, “Material Handling System and Method Using Mobile Autonomous Inventory Trays and Peer-to-Peer Communications.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of material handling; more particularly, to systems and methods of material handling using autonomous mobile drive units and movable inventory trays. 
     BACKGROUND 
     The order fulfillment step in the distribution system process is often one of the largest cost components in moving inventory from production to end consumer. This is due to the fact that final order assembly is typically labor intensive and time consuming as operators move among inventory locations and manually handle items. The order fulfillment step involves selecting multiple individual inventory items from among a large assortment of possible items. In contrast, the steps prior to the order fulfillment step in the distribution system process are generally more efficient since they handle inventory in bulk operations such as moving a truckload at a time, a full pallet of one product, or even whole cases. 
     Due to its large labor costs, order fulfillment operations have long been the focus of innovations designed to reduce labor. These developments have taken the form of pick-to-light technology, wireless barcode readers, conveyor systems that move orders to operators and even automated storage and retrieval systems (“ASRS”) that bring the inventory to the worker. Common ASRS solutions are sometimes called carousels or stockers. A typical carousel may have several thousand storage bins installed in a rotating structure that operates similar to the spinning clothes rack at a dry cleaning facility. Another type of solution known as a sorter is used in conjunction with inventory storage equipment and comprises a revolving tray mechanism that helps sort items coming from inventory into their target order bins. Yet another solution is to provide fixed racking aisles served by a gantry robot that moves in and out of the aisles to bring inventory to the front of the storage system. 
     These solutions have been embraced by the distribution industry for their ability to streamline operations and cut operating costs. Yet even with these often expensive systems, fulfillment costs remain high and distribution system managers continue to search for ways to reduce operating costs. 
     Another major shortcoming of the current set of order fulfillment solutions is complexity. These automated systems often involve complex control software, lengthy installation, integration and bring-up time, and fail to perform robustly over long periods. Current solutions must be monitored, tuned, and managed by experts with sophisticated knowledge of the system&#39;s workings. In addition, these systems are often inflexible to new processes that may be required as an organization&#39;s needs change. 
     What is needed is an order fulfillment system that is simple to install, operate, and maintain, and that would further reduce operating costs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description that follows and from the accompanying drawings, which however, should not be taken to limit the invention to the specific embodiments shown, but are for explanation and understanding only. 
     FIG. 1 is a top perspective view of a mobile inventory tray according to one embodiment of the present invention. 
     FIG. 2A is a bottom perspective view of a mobile inventory tray according to one embodiment of the present invention. 
     FIG. 2B is a front side view of the mobile inventory tray of FIG.  2 A. 
     FIG. 3 is a high-level system block diagram of tray subsystems according to one embodiment of the present invention. 
     FIG. 4A is a block diagram of a system interface to a warehouse management system according to one embodiment of the present invention. 
     FIG. 4B is a flow chart showing the steps of an order fulfillment process using mobile inventory trays. 
     FIG. 5 is a top view of mobile inventory trays located on a factory floor according to one embodiment of the present invention. 
     FIG. 6 is a perspective view of mobile inventory trays located on a factory floor according to one embodiment of the present invention. 
     FIG. 7 is a perspective view of mobile inventory trays populating multiple vertical floor levels within a factory space according to one embodiment of the present invention. 
     FIG. 8 is a perspective view of mobile inventory trays on a factory floor showing openings in the floor enclosure according to one embodiment of the present invention. 
     FIGS. 9A-9C are a side view and first and second perspective views of movable inventory trays and detachable mobile drive units according to one embodiment of the present invention. 
     FIG. 10 is a top view showing movable inventory trays arranged in a cellular grid with support lanes and queue locations on a factory floor according to one embodiment of the present invention. 
     FIG. 11 is a three-part movable inventory tray including a base tray, a stack tray, and a mobile drive unit according to one embodiment of the present invention. 
     FIG. 12 is a mobile drive unit docked to a base tray according to one embodiment of the present invention. 
     FIG. 13 is a flow chart illustrating a mobile drive unit selection process to fill an order request according to one embodiment of the present invention. 
     FIG. 14 is a flow chart illustrating an order fill process using autonomous mobile drive units and movable inventory trays according to one embodiment of the present invention. 
     FIG. 15 is a flow chart illustrating an inventory replenishment process using autonomous mobile drive units and movable inventory trays according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A material handling system and method using mobile autonomous inventory trays and peer-to-peer communications is disclosed. In the following description numerous specific details are set forth, such as the particular configuration of mobile inventory trays, the use of mobile inventory trays on a factory floor, and details regarding communication technologies, etc., in order to provide a thorough understanding of the present invention. However, persons having ordinary skill in the material handling arts will appreciate that these specific details may not be needed to practice the present invention. 
     According to an embodiment of the present invention, autonomous mobile inventory trays, which are robotic devices, are used to extend the concept of bringing a storage location to an operator (e.g., person, robot, etc.) in a novel way. Inventory is stored in mobile trays that can move in any direction under their own power within an established storage area of an organization (e.g., a factory floor). There are no predetermined storage locations for the mobile inventory trays other than that they exist somewhere within a designated space (e.g., an enclosed factory floor). The mobile inventory trays are free to move in any direction necessary including up and down ramps to other inventory floor levels. In this manner, the mobile inventory trays can respond to pick requests and move to pack station locations as part of the pick-and-pack order filling process. The mobile inventory trays may communicate with each other via radio frequency (“RF”) technology (e.g., the Bluetooth wireless protocol link) or other types of peer-to-peer communication. The mobile inventory trays may use a pseudolite indoor global positioning system (“GPS”) to provide themselves with an accurate position of their location within the predefined inventory storage area. Alternate positioning technologies may also be used such as 802.11-based localization technology or Ultra Wide Band (UWB) signaling based location technology. The mobile inventory trays may then use this information to calculate routes to a pack station, and their peer-to-peer communications ability to coordinate clear paths on the factory floor, or to queue with other trays at control nodes. 
     The mobile inventory trays of the present invention are thus automatic unguided vehicles (an “AUV”) rather than automatic guided vehicle (an “AGV”). They are able to navigate the factory floor autonomously using information obtained from the on-board GPS and RF communication systems without any guidance assistance from a remote central computer. This system of mobile inventory trays is self-tuning and self-optimizing. Frequently requested trays migrate closer to the pack stations, while trays containing slower moving inventory items drift back and to the sides and may even move to upper levels. In this sense, the material handling system and method of the present invention is a complex adaptive system and demonstrates emergent system behavior. 
     As with all material handling systems, the autonomous storage and retrieval system and method of the present invention may integrate with existing warehouse management software (“WMS”) systems. For example, order requests may be made from a WMS to the material handling system (“MHS”) and relayed to the appropriate pack station computers which then direct the order fulfillment from inventory brought to the pack stations utilizing the mobile inventory trays. Orders may be processed in parallel, i.e., multiple orders may be filled simultaneously at a given pack station and multiple pack stations can operate concurrently. Parallel processing of orders allows for real-time fulfillment of orders, in that multiple orders may be filled in minutes rather than in hours. Operators pick the inventory items from the arriving trays, place the items in the order container and, when the order is complete, the pack station computer relays this information to the MHS which in turn notifies the WMS. 
     Referring now to FIG. 1 there is shown a perspective view of a mobile inventory tray  101  according to one embodiment of the present invention. Mobile inventory tray  101  is designed so that it may move autonomously on a surface, such as a factory floor (not shown in this view). Although mobile inventory tray  101  may be specifically discussed in reference to its movement on a factory floor, it should be noted that mobile inventory tray  101  may be used in a variety of capacities including those typified by pick-and-pack operations, order fulfillment operations, or assembly line operations where a few items are drawn from a large population of possible items. An example of such an operation is where a single item is drawn from a large population of books, movies, food supplies, subsystem parts, etc. 
     Mobile inventory tray  101  comprises an enclosure  102  to contain various inventory items (not show in this view). In the embodiment illustrated by FIG. 1, the enclosure is a circular, one-piece assembly container having a base or bottom wall  103  and a side wall  104  extending upwardly from the bottom wall  103  to create a compartment  105  for the inventory items. It should be noted that the mobile inventory tray does not necessarily need to be circular, as is shown in FIG.  1 . The design of the mobile inventory tray  101  may vary in size and shape based on the type of inventory items the factory stores. Mobile inventory tray  101  also contains a housing  106  for its drive system and control electronics which will be described in more detail later. 
     Referring now to FIG. 2A there is shown a bottom perspective view of a mobile inventory tray  101 . Two driving wheels  111  and  112  and three small freely-rotating casters  113 - 115  are shown mounted to the base  103  of the mobile inventory tray  101 . The driving wheels  111  and  112  are operated by motors (not shown in this view) located in the housing  106  of mobile inventory tray  101 . The drive wheels  111  and  112  always remain in contact with the factory floor. Casters  113 - 115  function to support the load and maintain mobile inventory tray  101  in rolling contact with the floor despite imbalances in the items contained in enclosure  102 . The motors may be attached to the driving wheels  111  and  112  in a conventional manner. 
     FIG. 2B is a front side view of the mobile inventory tray of FIG.  2 A. Casters  113 - 115  roll freely and balance the mobile inventory tray  101  as it moves along a surface (not shown in this view) by using the driving wheels  111  and  112 . It should be noted that the mobile inventory tray  101  may use other locomotion means as well, including motor driven tracks, propellers, ball-wheels or a combination of locomotion devices. 
     FIG. 3 is a high-level block diagram of the subsystems of a mobile inventory tray according to one embodiment of the present invention. The mobile inventory tray subsystem may be implemented as a computer-based (i.e., microprocessor-based) device. For instance, all of the elements shown in FIG. 3 may be contained within housing  106  (see FIG. 1) secured to the mobile inventory tray. 
     A motor controller  122  controls the movement of the mobile inventory tray in response to drive movement commands received from microprocessor  121 . Motor controller  122  is coupled to provide pulse signals to a left motor  123  and a right motor  124 . The motors  123  and  124  are coupled to the drives wheels (see FIG. 1) which propel the mobile inventory tray forward and backward in response to the signals provided by controller  122 . A control battery  125  and a drive battery  126  provide the electrical power for operating the electrical systems  122  and drive motors  123  and  124 . The mobile inventory tray may move to and couple with charging stations (not shown) as needed to replenish the battery power. 
     Microprocessor  121  of the mobile inventory tray subsystem  119  provides the intelligence for the mobile inventory tray. A random-access (“RAM”)  129  memory may be included to provide memory storage and as a source of data. A global positioning system (“GPS) receiver  127 , radio frequency (“RF”) communication transceiver  128 , and sensors  120  provide signals to microprocessor  121 . For example, GPS receiver  127  outputs position coordinates (x, y, z), while transceiver  128  provides command and other messages, and sensors  120  provide signals to microprocessor  121 . Sensors may include infrared, optical, acoustic, contact, laser, sonar, magnetic, etc. common to mobile robotic vehicles for the purpose of identifying obstacles, avoiding collisions, finding edge limits etc. Microprocessor  121  may also send information (e.g., location, status, diagnostics, etc.) to a remote receiver utilizing transceiver  128 . 
     As the mobile inventory tray moves about the factory floor it may provide itself with an accurate position of its location at all times using the GPS receiver  127 . The GPS receiver  127  or equivalent system receives signals for determination of its position coordinates. This position information may include geographic longitude and latitude, as well as the height above normal zero or Cartesian coordinates in a manner that is commonly known. Those skilled in the art will appreciate that other guidance methods and systems including radar-based inertial navigation using gyroscopes, laser triangulation, cell-based locator logic (e.g., such as the emergency 911 positioning technology), and visual referencing may also be used by the mobile inventory tray to determine its position coordinates. The mobile inventory tray utilizes the position coordinates obtained from the GPS receiver  127  to calculate routes on the factory floor. It may also utilize position information when navigating to clear paths or queue with other mobile inventory trays, as will be described in detail shortly. 
     The mobile inventory tray may communicate its position and other data (e.g., the content of its inventory, its destination pack station, etc.) in a peer-to-peer fashion to other mobile inventory trays using RF communication as provided through receiver  128 . In the embodiment illustrated by FIG. 3, a short-range communications medium such as a Bluetooth wireless protocol link or an ordinary infrared communication link may be used to provide a direct wireless link between mobile inventory trays. It should be understood that various wireless and terrestrial communications technologies may be employed. For example, the mobile inventory tray may be equipped with a device for communicating using the Global System for Mobile Communications (“GSM”) protocol, the General Packet Radio Service (“GPRS”) protocol, the 802.11b Wi-Fi networking protocol, and/or any other communication protocol/standard capable of communicating data. In a two-way mode of operation, transceiver  128  is equipped with an interface for both receiving and transmitting data over the direct wireless link. The wireless link may also communicate with the material handling system (“MHS”) (not shown in this view) which interfaces with the individual mobile inventory trays. In this manner, the mobile inventory trays may be directed to various check-in stations and/or pack stations to process orders requested by the MHS. The mobile inventory tray may use the RF communication system provided by transceiver  128  and the GPS receiver  127  to navigate to appropriate check-in stations and/or pack stations. 
     Referring now to FIG. 4A there is shown a block diagram of a system interface to a WMS  130  according to one embodiment of the present invention. The WMS  130  comprises a host computer that communicates data such as a production order (i.e., a request for an item(s) of inventory) to a Materials Handling System (“MHS”)  131 . The WMS  130  may be implemented as any one of a number of well known systems used to manage inventory in a factory or warehouse. WMS  130  transmits orders for shipments, tracks receipts, monitors factory inventory, etc. The WMS  130  transmits the request for the item(s) of inventory to the MHS  131  through a network connection, such as an intranet network  132 . It should be noted that a variety of wireless and/or terrestrial communications technologies may also be used to transmit this request, including a wide area network (“WAN”), a local area network (“LAN”), or any other system of interconnections enabling two or more computers to exchange information. The MHS  131  then transmits the data using the above network connection methods to one or more pack station controllers  133 ,  134 , etc. In turn, the pack station controller  133 ,  134 , etc., wirelessly transmits the data request for the item(s) of inventory to one or more of the mobile inventory trays  135 ,  136  via a communication device in the pack station controller  133 ,  134 , etc., using an RF link  137 . 
     There may be multiple mobile inventory trays  135 ,  136 , etc., moving about on a factory floor, with each mobile inventory tray  135 ,  136 , etc., carrying a particular item(s) of inventory. Note, that in certain implementations, it is also possible for a single mobile inventory tray to carry multiple different types of inventory items in order to reduce the overall number of trays needed in the system. When the request for an item(s) of inventory is received by one or more mobile inventory trays  135 ,  136 , etc., the mobile inventory trays transmit the request to peer mobile inventory trays  135 ,  136 , etc. using the RF link  137 . In a matter of seconds (or in a smaller increment of time), every mobile inventory tray  135 ,  136 , etc., has received the request. Mobile inventory trays  135 ,  136 , etc., containing the requested items(s) of inventory are instructed by their microprocessor  121  (see FIG. 3) to move to the pack station controller  133 ,  134 , etc., all the while locating themselves on the factory floor with their GPS (not shown in this view). During movement, the mobile inventory trays  135 ,  136 , etc. may also communicate with other control nodes  138  such as charging stations, obstacle markers, ramp markers, etc. using the RF link  137 . When mobile inventory trays  135 ,  136 , etc., arrive at the pack station controller  133 ,  134 , etc., an operator (e.g., a human, a robot, etc.) removes the requested inventory item(s) from the mobile inventory trays  135 ,  136 , etc. Pack station controller  133 ,  134 , etc., tracks inventory item(s) requests as they are satisfied. This tracking function may be performed by scanning a barcode affixed to the inventory item(s). Pack station controller  133 ,  134 , etc., communicates with the microprocessor  121  on mobile inventory trays  135 ,  136 , etc., so that once an order is satisfied (e.g., requested item(s) is removed from the mobile inventory trays  135 ,  136 , etc., and scanned by the barcode scanner) the mobile inventory trays  135 ,  136 , etc., are released so that they may again move about the factory floor to fill other orders. The pack station controller  133 ,  134 , etc., may also communication with the MHS  131  via the intranet network  132  or via some other wireless and/or terrestrial link, which in turn communicates with the WMS so that it may also track when order requests have been satisfied. 
     It should be noted that each mobile inventory tray  135 ,  136 , etc., receives a supply of a particular item(s) of inventory at one or more check-in station(s)  139 ,  140 , etc., where pallets may arrive from vendors on a regular basis. An operator at the check-in station  139 ,  140  etc. removes items of inventory from the pallets and places the items in the mobile inventory tray  135 ,  136 , etc. For example, mobile inventory tray  135  may carry tubes of toothpaste while mobile inventory tray  136  may carry cartons of milk. Mobile inventory trays  135 ,  136 , etc. know to move themselves to a check-in station  139 ,  140 , etc. to replenish their inventory item(s) as they are depleted. When depleted, the empty mobile inventory tray may take on any new inventory item as determined by the operator at the check-in station. Mobile inventory trays  135 ,  136 , etc., may also receive requests from the MHS  131  to move to check-in station  139 ,  140 , etc. as more pallets arrive. 
     Another embodiment of the present invention provides for giving inventory certain intelligence. According to this embodiment, as depicted by FIG. 4B, not only can the pack station controller  143  communicate with the inventory, the inventory can also essentially communicate with other inventory via mobile inventory trays. FIG. 4B is a flow chart showing the steps of an order fulfillment process using mobile inventory trays interfacing with each other and with the material handling system of FIG.  4 A. In one embodiment, an order (e.g., for bread and milk) is transmitted from the WMS  141  to the MHS. The MHS  142  then relays this order to a pack station controller. The pack station controller  143  transmits the order to mobile inventory trays using an RF link. The mobile inventory trays then communicate among themselves to locate the trays that contain the requested inventory items  144 . When a tray does not contain a requested item it relays the request to peer trays. (e.g., “I do not have bread, but does anyone else have bread?”). The system relays the request all the way across the factory floor in this fashion. In a matter of seconds, every mobile inventory tray that contains requested items begins moving toward the pack station controller  145 . As mobile inventory trays containing requested items move toward the pack station, other mobile inventory trays which are not part of this order coordinate to move aside. If two mobile inventory trays attempting to fill the same item request come within a short range of each other (e.g., 30 feet), they may communicate to determine who should fill the order  146 . One mobile inventory tray may state that it has two loaves of bread, and another mobile inventory tray may state that it has five loaves. Then according to embedded tray selection algorithms, one tray moves aside and the other tray continues to move toward the pack station, because it is the optimum mobile inventory tray to fill the order. In this manner, the system is not only self-regulating but also self-optimizing in that item(s) of inventory that are requested more often drift closer to the pack station for more rapid response on subsequent order requests. As mobile inventory trays arrive at pack station, they communicate with each other to form an orderly queue  147  so that an operator can remove the requested items. 
     Referring now to FIG. 5 there is shown a top view of multiple mobile inventory trays located on a factory floor according to one embodiment of the present invention. According to the embodiment illustrated by FIG. 5, check-in stations  150 ,  151 ,  152 , etc., and pack stations  161 ,  162 ,  163 , etc., are located on opposite sides of a factory floor  170 . It should be noted that the configuration of the factory floor  170  and the location of the check-in stations  150 ,  151 ,  152 , etc., and the pack stations  161 ,  162 ,  163 , etc., in relation to the factory floor  170  may change depending on a variety of considerations (e.g., size and quantity of the inventory item(s) processed, types of inventory item(s), size of the factory floor, etc.). Mobile inventory trays  171 ,  172 ,  173 , etc., are free to move about the factory floor  170  in any direction using the propulsion means disclosed above (see FIGS.  1  and  2 ). The mobile inventory trays  171 ,  172 ,  173 , etc., may be directed to various check-in stations  150 ,  151 ,  152 , etc., and/or pack stations  161 ,  162 ,  163 , etc., to fill order requests by the MHS (not shown in this view). The mobile inventory trays  171 ,  172 ,  173 , etc., form orderly queues as they enter the input areas  181 ,  182  of the check-in stations  150 ,  151 ,  152 , etc., and/or pack stations  161 ,  162 ,  163 , etc. Operators (not shown in this view) move inventory item(s) (not shown in this view) into and out of the mobile inventory trays  171 ,  172 ,  173 , etc., as the mobile inventory trays move through the check-in  150 ,  151 ,  152 , etc. and pack stations  161 ,  162 ,  163 , etc. 
     Referring now to FIG. 6 there is shown a perspective view of multiple mobile inventory trays located on a factory floor according to one embodiment of the present invention. The mobile inventory trays  190 ,  191 ,  192 , etc., may be of varying sizes and shapes. As shown in FIG. 6, the mobile inventory trays  190 ,  191 ,  192 , etc., are circular and vary in size and shape. Mobile inventory trays  190 ,  191 ,  192 , etc., may also be customized to transport specialty items (e.g., items that require special care). There are no predetermined storage locations for the mobile inventory trays  190 ,  191 ,  192 , etc., other than that they exist somewhere within the designated inventory storage area on a factory floor  195 . This is due to the fact that the mobile inventory trays  190 ,  191 ,  192 , etc., are “smart” trays. They direct themselves wherever they need to be on the factory floor  195 . As described herein, the location of the mobile inventory trays  190 ,  191 ,  192 , etc., is not tracked, assigned, or controlled, until they are directed to a pack station or a check-in station (not shown in this view). In this sense, the material handling system and method of the present invention provides for a location-less inventory storage and retrieval system. 
     Referring now to FIG. 7 there is shown is a perspective view of mobile inventory trays populating multiple vertical floor levels within a factory space according to one embodiment of the present invention. Mobile inventory trays  201 ,  202 ,  203 , etc., are located and free to move about on all vertical floor levels  210 ,  211 ,  212 , etc., within the factory space of a multi-floor inventory storage area  220 . Floor enclosure openings  215  and ramp access  216 ,  217 ,  218 , etc., is provided on every vertical floor level  210 ,  211 ,  212 , so that the mobile inventory trays  201 ,  202 ,  203 , etc. may move freely from floor to floor. Check-in stations and pack stations (not shown in this view) may be located on one floor level  210  or every floor level  211 ,  212 , etc., depending on the configuration of the facility. 
     Referring now to FIG. 8 there is shown a perspective view of mobile inventory trays on a factory floor showing openings in the floor enclosure according to one embodiment of the present invention. In the embodiment illustrated by FIG. 8, mobile inventory trays  221 ,  222 , etc., move through floor enclosure openings  230 ,  231 ,  232 , etc. to gain access to pack stations, check-in stations etc. Ramps may be provided (see FIG. 7) for the mobile inventory trays  221 ,  222 , etc., to move in any direction necessary including up and down the ramps to other inventory floor levels. In this way, mobile inventory trays  221 ,  222 , etc., can respond to pick requests and move to pack station locations (not shown in this view) to fill orders. The mobile inventory trays may also move to other inventory floor levels using other types of mechanisms as well (e.g., elevators). 
     There are several different embodiments by which mobile autonomous inventory trays can be used to facilitate inventory management. Two such embodiments are described as fixed drive units and detachable drive units. In the detachable drive unit embodiment of the present invention, the drive mechanism is a detachable unit described as a mobile drive unit that can dock and undock with movable inventory trays as needed to carry out inventory movement requests. The movable inventory trays remain motionless until such time as a mobile drive unit docks with a movable inventory tray and moves it through the inventory picking process and releases it into a storage location. The mobile drive units may place the movable inventory trays down in an organized virtual layout such that other mobile drive units can locate the trays. Movable inventory trays may be stacked to form inventory pods that are moved by the mobile drive units. Another characteristic of the detachable drive unit embodiment is that mobile drive units may send and receive instruction messages in a broadcast fashion over a typical wireless network and may still communicate in a peer-to-peer fashion for such things as collision avoidance. 
     In the embodiment of the invention described in FIGS. 1 through 8 herein, the description refers generally to the fixed drive unit embodiment. FIGS. 9 through 15 refer generally to the detachable drive unit embodiment. 
     Referring now to FIGS. 9A-9C there is shown a side view and first and second perspective views of movable inventory trays and detachable mobile drive units according to one embodiment of the present invention. The movable inventory trays or “pods”  301 ,  302 , and  303  are designed so that they may be moved about on a factory floor or other type of surface by detachable, mobile drive units  304 ,  305 , and  306 . The mobile drive units  304 ,  305 , and  306  may use a docking mechanism to dock to base trays  301 ,  307 , and  308  of the movable inventory pods  301 ,  302 , and  303 , using a docking mechanism (not shown in this view) located on the bottom of the base trays  301 ,  307 , and  308 . Although the embodiments illustrated by FIGS. 9A-9C show the mobile drive units  304  and  306  docked to the base  301 ,  307 , and  308 , a whole variety of docking mechanisms may be employed. 
     The control units for the mobile drive units  304 ,  305 , and  306  may be contained within a housing  320  secured to the mobile drive units  304 ,  305 , and  306  by conventional means. The control unit may be implemented as a microcontroller-based device, or other electromechanical system and may contain the components described in FIG. 3 herein. As is described in more detail in FIG. 3, the control unit for each mobile drive unit  304 ,  305 , and  306  includes a communication link coupled to a processor and a power unit coupled to a drive mechanism configured to move the mobile drive units about the warehouse in response to control signals of the processor. The mobile drive units  304 ,  305 , and  306 , receive command signals via this communication link. For example, the command signals may be transmitted by one or more pack stations (not shown in the view) responsive to an order request issued by the MHS. 
     The movable inventory pods  301 ,  302 , and  303 , are simply stationary units on casters  310 ,  311 ,  312 , etc., that do not have the drive capability and control electronics necessary to move about on the factory floor without the assistance of the mobile drive units  304 ,  305 , and  306 . As in the previous embodiments disclosed herein, casters  310 ,  311 ,  312 , etc., roll freely and balance the movable inventory pods  301 ,  302 , and  303  as they are moved along a floor surface by the driving wheels  313 ,  314 , etc., of the mobile drive units  304 ,  305 , and  306 . In other embodiments, casters  310 ,  311 ,  312 , etc., may be replaced by fixed wheels, roller ball-type bearings or other conventional means for rolling the movable inventory pods across a floor. 
     The mobile drive units  304 ,  305 , and  306  navigate the factory floor autonomously using information obtained from their on-board GPS and RF communication systems in an exemplary embodiment. The autonomous mobile drive units  304 ,  305 , and  306 , integrate with a MHS in the same way that the mobile inventory trays communicate with the MHS, except that the mobile drive units  304 ,  305 , and  306  may contain additional RAM or other storage and computational circuits to perform more extensive calculations and docking procedures. 
     The movable inventory pods  301 ,  302 , and  303  may be configured as vertically stackable trays, with each stackable tray  321 ,  322 , etc., joined in an interlocked stack to create the pod. The stack height of the stackable trays is limited by the material and structural characteristics of the base trays  301 ,  307 , and  308 , operator picking ergonomics and the ability of the mobile drive units  304 ,  305 , and  306  to effectively transport the movable inventory pods  301 ,  302 , and  303  on the factory floor. Each stackable tray  321 ,  322 , etc., may be further sub-divided into bins  323 ,  324 ,  325 , etc., for the storage of small parts and/or slow-moving products that require only a few units of inventory. The partitions  326 ,  327 , etc., creating the bins  323 ,  324 ,  325 , etc., may be removable and reconfigurable. In addition, the movable inventory pods  301 ,  302 , and  303  may have molded parts made of plastic or some other suitable material conventionally designed to facilitate positive locating, docking of the mobile drive units  304 ,  305 , and  306 , and transfer of force during movement on the factory floor. 
     Referring now to FIG. 10 there is shown a top view showing movable inventory pods arranged in a virtual cellular grid with support lanes and queue locations on a factory floor according to one embodiment of the present invention. The movable inventory pods  351  may be organized in a virtual grid  360  on the factory floor  390  so that they may be easily located by the mobile drive units  361 . The position of the movable inventory pods  351  on the factory floor, however, are not typically predetermined or fixed by a system computer/controller. Mobile drive units will typically store pods in the closest available storage grid from their current location without regard to any overall inventory layout plan. At other times, the system controller may direct a mobile drive unit to move a pod to a location farther from the pack stations since it does not get called upon frequently for example. The term “virtual” is used to indicate that the grid is only a mathematical construct and that no markings, guides, or fixed hardware is used on the active factory floor area. There may be many more movable inventory pods  351  than mobile drive units  361  located on a given factory floor. For example, there may be 10,000 movable inventory pods on a factory floor and only 2,000 mobile drive units. This configuration achieves a high inventory storage density while using few mobile drive units. Of course, the ratio of movable inventory pods to mobile drive units may vary depending on the configuration and requirements of the operation. 
     Movable inventory pods  351  dwell in storage grids  360  on the active factory floor  390 . The mobile drive units  361  are located in random locations on the factory floor  390  and may communicate with each other and with the MHS  400  wirelessly about the movable inventory pods  351  they are carrying and about the moves they are making. The mobile drive units  361  may also communicate position coordinates and/or changes in the storage locations of the movable inventory pods  351  on the factory floor  390  to peer mobile drive units, such that all of the mobile drive units  361  on the factory floor  390  may receive and store the changing coordinates of the movable inventory pods  351 . In this way, the mobile drive units  361  keep a mental mapping of the current position of all inventory on the active floor area. In the alternative, the mobile drive units  361  may communicate changes and/or position coordinates of movable inventory pods  351  directly to the MHS  400  via the wireless network  410 . The MHS  400  may then, in turn, inform all of the mobile drive units  361  about these changes and/or position coordinates via the wireless network  410  and may store this location information in a central MHS database. 
     If a movable inventory pod contains an item(s) needed by one of the pack stations  395  a pack station computer  450  chooses an appropriate mobile drive unit to transport the movable inventory pod to the pack station so that an operator may remove the requested item(s) from the movable inventory pod. As multiple orders are processed, mobile drive units docked to movable inventory pods  380  form orderly queues  401  at the pack stations  395  by communicating with each other globally via the wireless network  410  and locally via built-in infrared (IR) type communications. In this manner, movable inventory pods  380  may reorganize such that earlier order requests may move ahead in the queues. The mobile drive units docked to movable inventory pods  380  may rotate to orient the movable inventory pods accordingly to assist operators at the pack stations  395  in locating the correct item(s) of inventory. 
     Once an item(s) of inventory has been removed from a movable inventory pod and scanned across a barcode scanner attached to the pack station computer  450 , the pack station computer notifies that mobile drive unit docked to a movable inventory pod  380  through a wireless communication that it is free to return the movable inventory pod to the closest possible available storage location among the storage grids  360  located on the factory floor  390 . In this manner, the item(s) of inventory that are most commonly requested migrate to the front areas of the factory floor  390 , maximizing the efficiency of the material handling system. 
     Mobile drive units docked to empty movable inventory pods  380 , or pods containing at least one empty bin, and/or docked to movable inventory pods that may need particular item(s) of inventory to be replenished form orderly queues  415  and  416  along the sides of the factory floor area  390  to await the assignment and loading of new item(s) of inventory at check-in stations  460 . Empty movable inventory pods  380  may accept any new item(s) of inventory that the operators at the check-in stations  460  see fit. Empty movable inventory pods  380  are not dedicated to particular item(s) of inventory. An operator at the check-in stations  460  may use a barcode scanner or some other device attached to computers  465  at the check-in stations  460  to scan and identify an item(s) of inventory and to scan the movable inventory pod/bin to make the association as the item is being placed in a movable inventory pod. An operator at the check-in stations  460  may indicate the quantity of the item(s) being placed in the movable inventory pod. The computers  465  at the check-in stations  460  may communicate wirelessly with mobile drive units transporting the movable inventory pods as to when to move into and out of the check-in stations  460  to replenish empty movable inventory pods  380 . In addition, once the check-in process is completed, the computers  465  at the check-in stations transmit the type and quantity of units of inventory loaded on to the movable inventory pods  380  and other types of information as needed to the MHS  400  and/or to all of the mobile drive units on the factory floor  390  using the wireless network  410 . 
     Each movable inventory pod  351  may contain an embedded wireless RFID chip or some other form of identification that may be used to uniquely identify that movable inventory pod and the item(s) of inventory contained by the movable inventory pod among the thousands of movable inventory pods  351  on the factory floor  390 . When the check-in operation is complete, the check-in station computer  465  may wirelessly notify the mobile drive unit that it is free to move the movable inventory pod back to the storage grids  360  located on the factory floor  390 . At this point, the mobile drive units are free to transport the movable inventory pods  380  back to the closest available storage location in the storage grids  360 . 
     The mobile drive units  361  may move movable inventory pods  351  to the check-in stations  460  via a response to a wireless request by the computers  465  at the check-in stations. Alternatively, after the mobile drive units docked to the movable inventory pods  380  at the pack stations  395  have dropped off the item(s) of inventory, they may move immediately to check-in stations  460  if the mobile drive units determine that bins in the movable inventory pods are empty and/or have depleted to a certain predetermined level. 
     In yet another embodiment, a centralized computer, such as the MHS  400 , may track the position coordinates of the movable inventory pods  380  and the mobile drive units  361  using the wireless network. The MHS  400  may also store information about the particular items of inventory contained in each movable inventory pod in memory, or a central database. For example, the MHS  400  may receive an order request for an item of inventory. Instead of transmitting this request to a pack station computer  450 , the MHS  400  may itself use algorithms and software processes to select a particular mobile drive unit  361  to fulfill the order request at a particular pack station  395 . The algorithm may instruct the MHS  400  to choose the mobile drive unit  361  to fulfill the order request based on the proximity of a mobile drive unit  361  to a particular movable inventory pod  380  that contains the item of inventory in the order request. Other types of algorithms may be used to determine the mobile drive unit  361  selection processes. It should also be appreciated that the MHS  400  may also select the particular pack station  395  based on an algorithm. The pack station  395  may be selected either before or after the MHS  400  selects the mobile drive unit  361  to fulfill the order. 
     MHS  400  may transmit the request to the mobile drive unit  361  using the wireless network  410 . The mobile drive unit  361  then transports the movable inventory pod  380  to the pack station  395  to fulfill the order request. The mobile drive unit  361  may move under its own direction using its control electronics as previously described herein. Mobile drive unit  361  may still communicate with peer mobile drive units for various purposes. For example, peer-to-peer communication between mobile drive units  361  may be utilized to avoid congestion on the factory floor  390  and/or to effectively navigate into queuing positions  401  at pack stations  395  and/or check-in stations  460 . 
     Once an item of inventory has been removed from the movable inventory pod  380  at the pack station  395 , mobile drive unit  361  may be notified, either by a computer  450  at pack station  395  or by MHS  400 , that it is free to return movable inventory pod  380  to a storage location in virtual grid  360  of factory floor  390 . MHS  400  may determine an optimum location for movable inventory pod  380  on factory floor  390 . For example, MHS  400  may plan and optimize storage by directing the movement of less utilized movable inventory pods  380  to locations farther away from pack stations  395  and check-in stations  460 . 
     MHS  400  may also be used to control and direct requests for the replenishment of items of inventory. These requests may be transmitted wirelessly to MHS  400  from check-in station computers  465  or may come from the WMS (not shown). MHS  400  may determine an appropriate mobile drive unit  361  and movable inventory pod  380  to fulfill the order according to an algorithm or by some other method. Once MHS  400  selects the mobile drive unit  361  to fulfill the order, the mobile drive unit  361  moves an empty movable inventory pod  380  to the appropriate check-in station  460 . After the item of inventory has been replenished, mobile drive unit  361  may be instructed by the MHS  400  to return the movable inventory pod  380  back to virtual grid  360  in the manner described herein. 
     MHS  400  may continually poll the entire population of mobile drive units  361  for position coordinate information using the wireless network. In addition, each mobile drive unit may transmit its position coordinates in regular intervals, for example every minute, to MHS  400  using the wireless network. In this manner, MHS  400  can maintain current location information about mobile drive units  361  and movable inventory pods  380  and may determine the most efficient mobile drive units  361  to fulfill each order request. 
     Referring now to FIG. 11 there is shown a three-part movable inventory pod including a base tray, a stack tray, and a mobile drive unit according to one embodiment of the present invention. A base tray  510  is used in the first (lowest) position. The base tray  510  includes casters  511 - 513 , and interfaces with the mobile drive unit  520 . Base tray  510  may also include an RFID chip  540  embedded in base tray  510 . Mobile drive unit  520  docks to base tray  510  to move the movable inventory pod through specific inventory tasks and to release the movable inventory pod back into storage locations. Mobile drive unit  520  interrogates the RFID chip  540  during docking to ensure pick up of the correct movable inventory pod. Moreover, when mobile drive unit  520  is docked to the movable inventory pod, mobile drive unit  520  identifies the orientation of all of the “pickable” openings  531 ,  532 , etc., in the movable inventory pod such that it can align the correct opening  531 ,  532 , etc., toward the operator in the pack stations and/or check-in stations. A stack tray unit  530  is designed to sit on top of base tray  510  and/or other stack trays, thus creating movable inventory pods with multiple stackable trays. The stack trays may also include RFID chips for unique identification by the mobile drive units. 
     Referring now to FIG. 12 there is shown a mobile drive unit docked to a base tray according to one embodiment of the present invention. In this particular embodiment, as the movable inventory pod  550  moves along a factory floor  555  it occupies a path three feet wide. The inner diameter of the storage compartment  551  may be slightly smaller than the three-foot outer diameter based on the wall  552  thickness. The storage compartment height of the base tray may be 15 inches. This particular geometry yields a payload capacity of approximately 14,800 cubic inches in this particular embodiment. 
     The height of the base tray  550  above the factory floor  555  determines the location of the pick openings and thus the reaching and bending of the operators at the pack stations and/or check-in stations. With a floor clearance between 15 and 26 inches above the surface of the factory floor  555 , and a pod consisting of 2 or 3 trays, a top pick opening may typically be between 49 and 60 inches. The openings may be 15 inches wide by 11 inches tall. This particular movable inventory pod  550  may accept up to 200 pounds of inventory. Items of inventory may range from very small objects, such as a pack of gum or nail polish, to larger items such as an eight-pack of paper towels or a VCR. Other embodiments may utilize trays in very different configurations, designed for vastly different payloads. 
     Referring now to FIG. 13 there is shown a flow chart illustrating a mobile drive unit selection process to fill an order request according to one embodiment of the present invention. An order is transmitted from a WMS to the MHS (block  601 ). The MHS then relays the order to a pack station computer (block  602 ) according to specified load leveling algorithms. The pack station computer transmits the order request for items of inventory to the entire population of mobile drive units located on the factory floor via the wireless network (block  603 ). The mobile drive units respond to the order request with bids that represent the amount of time each mobile drive unit calculates it would take to deliver the requested item (block  604 ). If the packs station computer receives bids from one or more mobile drive units (block  605 ), the pack station computer will fill the request by selecting the mobile drive unit that bids the shortest amount of time (block  606 ). A winning bidder message is broadcast back to the entire mobile drive unit population by the pack station controller (block  607 ) such that the losing mobile drive units may delete the open bid and respond to other requests. This process may be handled in an alternative fashion whereby the MHS maintains the status of all mobile drive units and movable inventory pods and centrally calculates and assigns mobile drive unit activities to include which drive is to transport which pod from which location and later, into which location to replace the pod. 
     If the pack station computer does not receive a bid from one or more mobile drive units (block  608 ), this means there are no idle mobile drive units available. The pack station computer will then queue the request internally until it hears from an idle mobile drive unit (block  609 ). When a mobile drive unit becomes idle, it will broadcast that message to the pack station computers in the event that there is an open item request requiring a mobile drive unit (block  610 ). All pack station computers that have work bid for the use of the mobile drive unit (block  611 ). The mobile drive unit selects the winning pack station based on the age of the request (block  612 ) or by some other selection formula. 
     Referring now to FIG. 14 there is shown a flow chart illustrating an order fill process using autonomous mobile drive units and movable inventory pods according to one embodiment of the present invention. Once a mobile drive unit has been notified that it has been selected by a pack station computer to fill an order request, the mobile drive unit reserves that movable inventory pod with a wireless message to peer drive units and moves to the location of the movable inventory pod containing the requested item(s) of inventory (block  620 ) and docks with the movable inventory pod ( 621 ). The peer mobile drive units update there maps in RAM with this information (block  622 ) at this point, or earlier when the reservation was made against the movable inventory pod. The mobile drive unit moves to the pack station with the movable inventory pod (block  623 ). An operator at the pack station removes the item(s) of inventory from the movable inventory pod. 
     Once the item(s) have been scanned by the operator, the mobile drive unit docked to the movable inventory pod is released from the pack station (block  624 ). Using its internal map of open storage locations in the storage grid, the mobile drive unit reserves an open storage location with a wireless message to peer mobile drive units, then proceeds to that location and undocks from the movable inventory pod (block  625 ) and upon completion notifies the peer mobile drive units of the new storage location of the movable inventory pod (block  626 ). 
     Referring now to FIG. 15 there is shown a flow chart illustrating an inventory replenishment process using autonomous mobile drive units and movable inventory pods according to one embodiment of the present invention. A request for a movable inventory pod containing at least one empty bin to be transported to a check-in station is transmitted by a check-in station computer to all of the mobile drive units currently docked to pods containing an empty bin and located in the check-in queue lanes on either side of the factory floor (block  630 ). The oldest mobile drive unit in the queue lanes responds to the request by moving to the check-in station (block  631 ). An operator at the check-in station supplies an empty bin in the movable inventory pod with item(s) of inventory (block  632 ) and the check-in station computer then notifies the mobile drive unit that it is free to move the replenished movable inventory pod back to the factory floor (block  633 ). The mobile drive unit transports the replenished movable inventory pod to the closest available place in the storage grid on the factory floor (block  634 ) and wirelessly transmits to the MHS and peer drive units the new location of the movable inventory pod (block  635 ). 
     In the foregoing, a material handling system and method using autonomous mobile drive units and movable inventory pods has been disclosed. Although the present invention has been described with reference to specific exemplary embodiments, it should be understood that numerous changes in the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit and scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalent.