Patent Publication Number: US-10760947-B1

Title: Modular item stowage system

Description:
PRIORITY 
     This application is a continuation of, and claims priority to, U.S. patent application Ser. No. 14/745,045, filed on Jun. 19, 2015, entitled “INSTRUMENTED ITEM STOWAGE SYSTEM WITH MODULAR ELEMENTS,” which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Retailers, wholesalers, and other product distributors typically maintain an inventory of various items that may be ordered, purchased, leased, borrowed, rented, viewed, and so forth, by clients or customers. For example, an e-commerce website may maintain inventory in a fulfillment center. When a customer orders an item, the item is picked from inventory, routed to a packing station, packed, and shipped to the customer. Likewise, physical stores maintain inventory in customer accessible areas, such as in a shopping area, and customers can pick items from inventory and take them to a cashier for purchase, rental, and so forth. 
     Many physical stores also maintain inventory in a storage area, fulfillment center, or other facility that can be used to replenish inventory located in the shopping areas or to satisfy orders for items that are placed through other channels (e.g., e-commerce). Other examples of entities that maintain facilities holding inventory include libraries, museums, rental centers, and so forth. In each instance, for an item to be moved from one location to another, it is picked from its current location and transitioned to a new location. It is often desirable to monitor quantity of inventory within the facility. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The detailed description is set forth with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical items or features. 
         FIG. 1  is a block diagram illustrating a materials handling facility (facility) using a modular item stowage system using modular elements coupled to platforms, according to some implementations. 
         FIG. 2  is a block diagram illustrating additional details of the facility, according to some implementations. 
         FIG. 3  is a block diagram of a server configured to support operation of the facility, according to some implementations. 
         FIG. 4  illustrates a side view of an inventory location comprising the platforms, according to some implementations. 
         FIG. 5  illustrates top views of a platform before and after mounting of modular elements including instrumented auto-facing units, dividers, and spacers, according to some implementations. 
         FIG. 6  illustrates a side view of the platform along line A-A depicting various features of the platform including a cable recess, according to some implementations. 
         FIG. 7  illustrates a side view of the platform along line B-B depicting various features of the platform including weight sensors, according to some implementations. 
         FIG. 8  illustrates top views of an instrumented auto-facing unit, according to some implementations. 
         FIG. 9  illustrates side views of the instrumented auto-facing unit, according to some implementations. 
         FIG. 10  illustrates a side view of a portion of the instrumented auto-facing unit, according to some implementations. 
     
    
    
     While implementations are described herein by way of example, those skilled in the art will recognize that the implementations are not limited to the examples or figures described. It should be understood that the figures and detailed description thereto are not intended to limit implementations to the particular form disclosed but, on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including, but not limited to. 
     DETAILED DESCRIPTION 
     This disclosure describes a modular item stowage system and associated modular elements that provide inventory locations. These inventory locations facilitate stowage of items at a materials handling facility (facility) or other setting. The facility may include, or have access to, an inventory management system. The inventory management system may be configured to maintain information about items, users, condition of the facility, and so forth. For example, the inventory management system may maintain data indicative of a number of items at a particular inventory location, what items a particular user is ordered to pick, how many items have been picked or placed at the inventory location, requests for assistance, environmental status of the facility, and so forth. Operation of the facility may be facilitated by using one or more sensors to acquire information about interactions in the facility. Interactions may comprise the user picking an item from an inventory location, placing an item at an inventory location, touching an item, bringing an object such as a hand or face close to an item, and so forth. For example, the inventory management system may use interaction data that indicates what item a user picked from a particular inventory location to adjust the count of inventory stowed at the particular inventory location. 
     Described in this disclosure is a modular item stowage system that includes devices for stowing and facilitating stowage of items. A platform is described that may be mounted to a support structure such as a counter, upright rack, and so forth. The platform provides physical support and functional support for operation of one or more modular elements. The platform may include various sensors to provide sensor data to the inventory management system. For example, the platform may include a plurality of weight sensors to generate weight data about a load on the platform. For example, the load may comprise one or more items. The platform may also include electronics to receive data from the modular elements. The modular elements may include electronics to provide sensor data, such as instrumented auto-facing units (AFUs). 
     Instrumented and uninstrumented AFUs may hold one or more items. The AFU may include a sled that is under tension from a spring to push items towards a front of the AFU, where the items are readily accessible to a user. In the instrumented AFU, a linear position sensor may report sensor data indicative of the position of the sled, such as how close the sled is to the front of the instrumented AFU. Using the sensor data indicative of the position, assuming items in the same instrumented AFU have the same depth, and given information about the depth of a single product, a quantity of items in the instrumented AFU may be determined. Based on a change in quantity from a first time to a second time, a quantity of items that have been picked from or placed to the instrumented AFU may be determined. This information may be used by the inventory management system to operate the facility. 
     Other modular elements may contain no electronics but support stowage of the items. The modular elements may include dividers, spacers, hangers, bins, and so forth. The dividers may comprise a vertical member or wall that maintains separation between different types of items that may be adjacent to one another on the platform. The spacers may comprise surfaces that provide horizontal distance between other modular elements such as dividers and AFUs. For example, a wide item may extend from a first modular element over a spacer. The hangers may provide pegs, hooks, or other structures from which items may hang. The bins may contain compartments or recesses within which items may be stowed. 
     By using the devices and techniques described herein, operation of the facility may be improved. The modular item stowage system may be easily reconfigured to hold items in a desired configuration. For example, an operator of the facility may easily reconfigure the AFUs, spacers, dividers, and so forth, on one or more platforms to arrange items to conform to a desired planogram that specifies how items are to be arranged in the inventory locations of the facility. Sensors on the platform, sensors on the modular elements such as the instrumented AFUs, or other sensors in the facility provide sensor data that may be used by the inventory management system to determine quantity on hand at a particular inventory location, quantity picked or placed by the user, and so forth. 
     Illustrative System 
     An implementation of a materials handling system  100  configured to store and manage inventory items is illustrated in  FIG. 1 . A materials handling facility  102  (facility) comprises one or more physical structures or areas within which one or more items  104 ( 1 ),  104 ( 2 ), . . . ,  104 (Q) may be held. As used in this disclosure, letters in parenthesis such as “(Q)” indicate an integer value greater than or equal to zero. The items  104  comprise physical goods, such as books, pharmaceuticals, repair parts, electronic gear, and so forth. 
     The facility  102  may include one or more areas designated for different functions with regard to inventory handling. In this illustration, the facility  102  includes a receiving area  106 , a storage area  108 , and a transition area  110 . 
     The receiving area  106  may be configured to accept items  104 , such as from suppliers, for intake into the facility  102 . For example, the receiving area  106  may include a loading dock at which trucks or other freight conveyances unload the items  104 . 
     The storage area  108  is configured to store the items  104 . The storage area  108  may be arranged in various physical configurations. In one implementation, the storage area  108  may include one or more aisles  112 . The aisle  112  may be configured with, or defined by, inventory locations  114  on one or both sides of the aisle  112 . The inventory locations  114  may include one or more of shelves, racks, cases, cabinets, bins, floor locations, or other suitable storage mechanisms for holding, supporting, or storing the items  104 . The inventory locations  114  may be affixed to the floor or another portion of the facility&#39;s  102  structure. The inventory locations  114  may also be movable such that the arrangements of aisles  112  may be reconfigurable. In some implementations, the inventory locations  114  may be configured to move independently of an outside operator. For example, the inventory locations  114  may comprise a rack with a power source and a motor, operable by a computing device to allow the rack to move from one location within the facility  102  to another. 
     One or more users  116  and totes  118  or other material handling apparatuses may move within the facility  102 . For example, the user  116  may move about within the facility  102  to pick or place the items  104  in various inventory locations  114 , placing them on the tote  118  for ease of transport. The tote  118  is configured to carry or otherwise transport one or more items  104 . For example, the tote  118  may include a basket, cart, bag, bin, and so forth. In other implementations, other material handling apparatuses such as robots, forklifts, cranes, aerial drones, and so forth, may move about the facility  102  picking, placing, or otherwise moving the items  104 . For example, a robot may pick an item  104  from a first inventory location  114 ( 1 ) and move the item  104  to a second inventory location  114 ( 2 ). 
     One or more sensors  120  may be configured to acquire information in the facility  102 . The sensors  120  may include, but are not limited to, optical sensors, cameras, three-dimensional (3D) sensors, weight sensors, radio frequency (RF) receivers, temperature sensors, humidity sensors, vibration sensors, and so forth. The sensors  120  may be stationary or mobile, relative to the facility  102 . For example, the inventory locations  114  may contain cameras configured to acquire images of picking or placement of items  104  on shelves, of users  116  in the facility  102 , and so forth. In another example, the floor of the facility  102  may include weight sensors configured to determine a weight of objects thereupon. The sensors  120  are discussed in more detail below with regard to  FIG. 2 . 
     While the storage area  108  is depicted as having one or more aisles  112 , inventory locations  114  storing the items  104 , sensors  120 , and so forth, it is understood that the receiving area  106 , the transition area  110 , or other areas of the facility  102  may be similarly equipped. Furthermore, the arrangement of the various areas within the facility  102  is depicted functionally rather than schematically. For example, in some implementations, multiple different receiving areas  106 , storage areas  108 , and transition areas  110  may be interspersed rather than segregated in the facility  102 . 
     The facility  102  may include, or be coupled to, an inventory management system  122 . The inventory management system  122  is configured to interact with users  116  or devices such as sensors  120 , robots, material handling equipment, computing devices, and so forth, in one or more of the receiving area  106 , the storage area  108 , or the transition area  110 . 
     During operation of the facility  102 , the sensors  120  may be configured to provide information suitable for tracking the location of objects within the facility  102 , their movement, and so forth. For example, a series of images acquired by the camera may indicate removal of an item  104  from a particular inventory location  114  by the user  116  and placement of the item  104  on or at least partially within the tote  118 . Objects may include, but are not limited to, items  104 , users  116 , totes  118 , and so forth. In another example, sensor data from an instrumented auto-facing unit may be used to determine a quantity on hand at a particular inventory location  114 , change in quantity of items  104  resulting from a pick or place, and so forth. 
     The facility  102  may be configured to receive different kinds of items  104  from various suppliers and to store them until a customer orders or retrieves one or more of the items  104 . A general flow of items  104  through the facility  102  is indicated by the arrows of  FIG. 1 . Specifically, as illustrated in this example, items  104  may be received from one or more suppliers, such as manufacturers, distributors, wholesalers, and so forth, at the receiving area  106 . In various implementations, the items  104  may include merchandise, commodities, perishables, or any suitable type of item  104 , depending on the nature of the enterprise that operates the facility  102 . 
     Upon being received from a supplier at the receiving area  106 , the items  104  may be prepared for storage in the storage area  108 . For example, in some implementations, items  104  may be unpacked or otherwise rearranged. The inventory management system  122  may include one or more software applications executing on a computer system to provide inventory management functions. These inventory management functions may include maintaining information indicative of the type, quantity, condition, cost, location, weight, or any other suitable parameters with respect to the items  104 . The items  104  may be stocked, managed, or dispensed in terms of countable units, individual units, or multiple units, such as packages, cartons, crates, pallets, or other suitable aggregations. Alternatively, some items  104 , such as bulk products, commodities, and so forth, may be stored in continuous or arbitrarily divisible amounts that may not be inherently organized into countable units. Such items  104  may be managed in terms of measurable quantity such as units of length, area, volume, weight, time, duration, or other dimensional properties characterized by units of measurement. Generally speaking, a quantity of an item  104  may refer to either a countable number of individual or aggregate units of an item  104  or a measurable amount of an item  104 , as appropriate. 
     After arriving through the receiving area  106 , items  104  may be stored within the storage area  108 . In some implementations, like items  104  may be stored or displayed together in the inventory locations  114  such as in instrumented AFUs, bins, on shelves, hanging from pegboards, and so forth. In this implementation, all items  104  of a given kind are stored in one inventory location  114 . In other implementations, like items  104  may be stored in different inventory locations  114 . For example, to optimize retrieval of certain items  104  having frequent turnover within a large physical facility  102 , those items  104  may be stored in several different inventory locations  114  to reduce congestion that might occur at a single inventory location  114 . 
     When a customer order specifying one or more items  104  is received, or as a user  116  progresses through the facility  102 , the corresponding items  104  may be selected or “picked” from the inventory locations  114  containing those items  104 . In various implementations, item picking may range from manual to completely automated picking. For example, in one implementation, a user  116  may have a list of items  104  they desire and may progress through the facility  102  picking items  104  from inventory locations  114  within the storage area  108 , and placing those items  104  into a tote  118 . In other implementations, employees of the facility  102  may pick items  104  using written or electronic pick lists derived from customer orders. These picked items  104  may be placed into the tote  118  as the employee progresses through the facility  102 . 
     After items  104  have been picked, they may be processed at a transition area  110 . The transition area  110  may be any designated area within the facility  102  where items  104  are transitioned from one location to another or from one entity to another. For example, the transition area  110  may be a packing station within the facility  102 . When the item  104  arrives at the transition area  110 , the items  104  may be transitioned from the storage area  108  to the packing station. Information about the transition may be maintained by the inventory management system  122 . 
     In another example, if the items  104  are departing the facility  102 , a list of the items  104  may be obtained and used by the inventory management system  122  to transition responsibility for, or custody of, the items  104  from the facility  102  to another entity. For example, a carrier may accept the items  104  for transport with that carrier accepting responsibility for the items  104  indicated in the list. In another example, a user  116  may purchase or rent the items  104  and remove the items  104  from the facility  102 . During use of the facility  102 , the user  116  may move about the facility  102  to perform various tasks, such as picking or placing the items  104  in the inventory locations  114 . 
     The inventory management system  122  may be configured to access physical layout data  124 , item data  126 , or other information during operation. The physical layout data  124  comprises information such as the arrangement of inventory locations  114  and modular elements of the modular item stowage system, such as described below in more detail. The item data  126  may comprise information about one or more of the items  104 . The item data  126  may include, but is not limited to, weight of a single item  104  (or package of items), physical dimensions of packaging, images of a single item  104  from different points of view, and so forth. The physical dimensions of the packaging may include height, width, depth, and so forth, of the single item  104 , a package of items, or other unit. The item data  126  may also include information indicative of a particular inventory location  114  at which the item  104  may be stowed, and so forth. 
     In some implementations, items  104  may be processed, such as at the receiving area  106 , to generate at least a portion of the item data  126 . For example, an item  104  not previously stored by the inventory management system  122  may be scanned or measured to determine the physical dimensions as part of a process to receive the item  104  into the facility  102 . In another example, the receiving process at the facility  102  may include receiving or accessing previously generated information about the item  104 . Continuing the example, an electronic manifest record may include an item identifier, weight, physical dimensions, and so forth. 
     The one or more sensors  120  may produce sensor data  128 . For example, the cameras may produce image data, weight sensors may produce weight data, instrumented AFUs may provide position data indicative of a position of the sled or amount of displacement of the sled, and so forth. 
     The modular item stowage system may include one or more platforms  130 . The platform  130  may be freestanding, mounted to a support structure, suspended from an overhead structure, and so forth. Each platform  130  provides a structure to which one or more modular elements may be coupled. An overall shape of the platform  130  may be a rectangle. In other implementations, the platform  130  may have an overall shape that is a quadrilateral, triangle, hexagon, circle, or other polyhedron. The modular elements may include, but are not limited to, instrumented auto-facing units (AFUs)  132 , dividers  134 , spacers  136 , and so forth. The platform  130  may provide one or more bays for holding processing components such as sensor controllers, computing devices, and so forth. These bays may provide environment protection for the devices therein. The platform  130  may include one or more sensors  120 . For example, the platform  130  may include a plurality of weight sensors to generate weight data of a load supported by the platform  130 . A controller may determine one or more first weight values from the weight measured by the plurality of weight sensors. The controller may determine a change in weight of the load. The change in weight may be associated with mounting of an empty modular element to the platform  130 . For example, the association may be based on a lookup table that compares the change in weight of the load with predetermined weights of different modular elements. When the change in weight matches one of the predetermined weights within a threshold tolerance, the weight change may be associated with the addition or removal of a particular type of modular element. The controller may then determine one or more tare weight values of the load on the platform, such as including the empty modular element which has been added or removed. Features of the platform  130  are discussed in more detail below with regard to  FIGS. 4-7 . 
     A mechanical coupling may be maintained between the modular element and the platform  130 . For example, the platform  130  may provide physical support to the modular element. The mechanical coupling may include the use of one or more of mechanical engagement features, magnets, gravity, and so forth. For example, the modular element may rest atop the platform  130  that supports it. In another example, the modular element may comprise an AFU  132  (instrumented or uninstrumented) having tabs to engage corresponding slots on the platform  130  and a magnet on the AFU  132  to apply a magnetic force to hold the platform  130  and the AFU  132  together. 
     The coupling between the modular element and the platform  130  may include providing data communication, electrical power, and so forth, between the modular element and the platform  130 . For example, the instrumented AFU  132  may electrically couple to the platform  130  to receive power for onboard electronics and provide sensor data  128 . In another example, data may be transferred between the modular element and the platform  130  optically, such as using optical waveguides, infrared transmission, and so forth. The instrumented AFU  132  is discussed in more detail below with regard to  FIGS. 8-10 . 
     The platforms  130  may be positioned throughout the facility  102  and reconfigured at will by an operator of the facility  102 . For example, the platform  130  may be configured to use platform base supports to mount to a support member, such as a rack. Upon a particular platform  130 , various modular elements may be arranged in various permutations. For example, the platform  130  may be configured with AFUs  132  alternating with dividers  134 . The AFUs  132  are used to stow the items  104  therein, while the dividers  134  maintain tidiness of the facility  102  by constraining items  104  to a particular lane defined by the inventory location  114 . The dividers  134  may comprise vertical walls. These vertical walls may divide the items  104  held by a first modular element from a second modular element. The walls may comprise one or more of solid sheets, wire, slats, and so forth. The solid sheets may comprise aluminum, steel, plastic, and so forth. The one or more wires may be straight or bent to form the vertical wall. 
     In some situations, the item  104  to be stowed may exceed the width of the AFU  132 . In this situation, spacers  136  may be placed adjacent to the AFU  132  to provide additional width for the item  104  to extend into or over. As the needs of the facility  102  change, the modular elements may be added, removed, or rearranged to suit changing configurations of items  104  while the platform  130  is emplaced. For example, the modular elements on a platform  130  that is mounted to a support member may be rearranged without removal of the platform  130  itself. The platforms  130  may be added, removed, repositioned, and so forth, within the facility  102 . For example, a platform  130  may be removed or added to the support member without affecting neighboring platforms  130 . 
     The modular elements may be configured to have a common sizing to facilitate modular operation. A common depth (front-to-back length) of the modular elements may be used. The width of the modular elements may vary as an integer multiple of a minimum size increment. Different modular elements may have different widths. For example, the minimum size increment may be ⅓ inch, with modular elements such as the AFUs  132 , the dividers  134 , and the spacers  136  available in widths such 1⅓ inch, 2 inches, 2⅔ inches, and so forth. 
     By utilizing the modular item stowage system described, the facility  102  may be easily configured to support different items  104 , different arrangements of items  104 , and so forth. Furthermore, the sensors  120  in the platform  130  and other modular elements provide sensor data  128  to the inventory management system  122 . This sensor data  128  may be used to maintain information such as quantity of an item  104  picked, placed, currently on hand, and so forth. 
       FIG. 2  is a block diagram  200  illustrating additional details of the facility  102 , according to some implementations. The facility  102  may be connected to one or more networks  202 , which in turn connect to one or more servers  204 . The network  202  may include private networks such as an institutional or personal intranet, public networks such as the Internet, or a combination thereof. The network  202  may utilize wired technologies (e.g., wires, fiber optic cables, and so forth), wireless technologies (e.g., radio frequency, infrared, acoustic, optical, and so forth), or other connection technologies. The network  202  is representative of any type of communication network, including one or more of data networks or voice networks. The network  202  may be implemented using a wired infrastructure (e.g., copper cable, fiber optic cable, and so forth), a wireless infrastructure (e.g., cellular, microwave, satellite, and so forth), or other connection technologies. 
     The servers  204  may be configured to execute one or more modules or software applications associated with the inventory management system  122 . While the servers  204  are illustrated as being in a location outside of the facility  102 , in other implementations, at least a portion of the servers  204  may be located at the facility  102 . The servers  204  are discussed in more detail below with regard to  FIG. 3 . 
     The users  116 , the totes  118 , or other objects in the facility  102  may be equipped with one or more tags  206 . The tags  206  may be configured to emit a signal  208 . In one implementation, the tag  206  may be a radio frequency identification (RFID) tag configured to emit an RF signal  208  upon activation by an external signal. For example, the external signal may comprise a radio frequency signal or a magnetic field configured to energize or activate the RFID tag  206 . In another implementation, the tag  206  may comprise a transmitter and a power source configured to power the transmitter. For example, the tag  206  may comprise a Bluetooth Low Energy (BLE) transmitter and battery. In other implementations, the tag  206  may use other techniques to indicate presence of the tag  206 . For example, an acoustic tag  206  may be configured to generate an ultrasonic signal  208 , which is detected by corresponding acoustic receivers. In yet another implementation, the tag  206  may be configured to emit an optical signal  208 . 
     The inventory management system  122  may be configured to use the tags  206  for one or more of identification of the object, determining a location of the object, and so forth. For example, the users  116  may wear tags  206 , the totes  118  may have tags  206  affixed, and so forth, which may be read and, based at least in part on signal strength, used to determine identity and location. 
     Generally, the inventory management system  122  or other systems associated with the facility  102  may include any number and combination of input components, output components, and servers  204 . 
     The one or more sensors  120  may be arranged at one or more locations within the facility  102 . For example, the sensors  120  may be mounted on or within a floor, wall, at a ceiling, at an inventory location  114 , on a tote  118 , may be carried or worn by a user  116 , and so forth. 
     The sensors  120  may include one or more cameras  120 ( 1 ). The one or more cameras  120 ( 1 ) may include imaging sensors configured to acquire images of a scene. The imaging sensors are configured to detect light in one or more wavelengths including, but not limited to, terahertz, infrared, visible, ultraviolet, and so forth. The imaging sensors may comprise charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) devices, microbolometers, and so forth. The inventory management system  122  may use image data acquired by the cameras  120 ( 1 ) during operation of the facility  102 . For example, the inventory management system  122  may identify items  104 , users  116 , totes  118 , and so forth, based at least in part on their appearance within the image data acquired by the cameras  120 ( 1 ). The cameras  120 ( 1 ) may be mounted in various locations within the facility  102 . For example, cameras  120 ( 1 ) may be mounted overhead, on inventory locations  114 , and so forth. 
     One or more 3D sensors  120 ( 2 ) may also be included in the sensors  120 . The 3D sensors  120 ( 2 ) are configured to acquire spatial or 3D data, such as depth information, about objects within a field of view of a sensor  120 . The 3D sensors  120 ( 2 ) include range cameras, lidar systems, sonar systems, radar systems, structured light systems, stereo vision systems, optical interferometry systems, and so forth. The inventory management system  122  may use the 3D data acquired by the 3D sensors  120 ( 2 ) to identify objects, determine a location of an object in 3D real space, and so forth. 
     One or more buttons  120 ( 3 ) may be configured to accept input from the user  116 . The buttons  120 ( 3 ) may comprise mechanical, capacitive, optical, or other mechanisms. For example, the buttons  120 ( 3 ) may comprise mechanical switches configured to accept an applied force from a touch of the user  116  to generate an input signal. The inventory management system  122  may use data from the buttons  120 ( 3 ) to receive information from the user  116 . For example, the tote  118  may be configured with a button  120 ( 3 ) to accept input from the user  116  and send information indicative of the input to the inventory management system  122 . 
     The sensors  120  may include one or more touch sensors  120 ( 4 ). The touch sensors  120 ( 4 ) may use resistive, capacitive, surface capacitance, projected capacitance, mutual capacitance, optical, Interpolating Force-Sensitive Resistance (IFSR), or other mechanisms to determine the position of a touch or near-touch. For example, the IFSR may comprise a material configured to change electrical resistance responsive to an applied force. The location within the material of that change in electrical resistance may indicate the position of the touch. The inventory management system  122  may use data from the touch sensors  120 ( 4 ) to receive information from the user  116 . For example, the touch sensor  120 ( 4 ) may be integrated with the tote  118  to provide a touchscreen with which the user  116  may select from a menu of one or more particular items  104  for picking, enter a manual count of items  104  at an inventory location  114 , and so forth. 
     One or more microphones  120 ( 5 ) may be configured to acquire information indicative of sound present in the environment. In some implementations, arrays of microphones  120 ( 5 ) may be used. These arrays may implement beamforming techniques to provide for directionality of gain. The inventory management system  122  may use the one or more microphones  120 ( 5 ) to acquire information from acoustic tags  206 , accept voice input from the users  116 , determine ambient noise level, and so forth. 
     One or more weight sensors  120 ( 6 ) are configured to measure the weight of a load. For example, the platform  130  may include weight sensors  120 ( 6 ) to measure objects supported thereby, such as modular elements, items  104 , and so forth. The weight sensors  120 ( 6 ) may be configured to measure the weight of the load at the tote  118 , on the floor of the facility  102 , and so forth. The weight sensors  120 ( 6 ) may include one or more sensing mechanisms to determine the weight of a load. These sensing mechanisms may include piezoresistive devices, piezoelectric devices, capacitive devices, electromagnetic devices, optical devices, potentiometric devices, microelectromechanical devices, and so forth. The sensing mechanisms of weight sensors  120 ( 6 ) may operate as transducers that generate one or more signals based on an applied force, such as that of the load due to gravity. For example, the weight sensor  120 ( 6 ) may comprise a load cell having a strain gauge and a structural member that deforms slightly when weight is applied. By measuring a change in the electrical characteristic of the strain gauge, such as capacitance or resistance, the weight may be determined. The inventory management system  122  may use the data acquired by the weight sensors  120 ( 6 ), such as on the platform  130 , to identify an object, determine a change in the quantity of objects, determine a location of an object, maintain shipping records, and so forth. 
     The sensors  120  may include one or more optical sensors  120 ( 7 ). The optical sensors  120 ( 7 ) may be configured to provide data indicative of one or more of color or intensity of light impinging thereupon. For example, the optical sensor  120 ( 7 ) may comprise a photodiode and associated circuitry configured to generate a signal or data indicative of an incident flux of photons. For example, the optical sensor  120 ( 7 ) may comprise an ambient light sensor such as the ISL76683 as provided by Intersil Corporation of Milpitas, Calif., USA, or the MAX44009 as provided by Maxim Integrated Products Inc. of San Jose, Calif., USA. In other implementations, other optical sensors  120 ( 7 ) may be used. The optical sensors  120 ( 7 ) may be sensitive to one or more of infrared light, visible light, or ultraviolet light. For example, the optical sensors  120 ( 7 ) may be sensitive to infrared light. 
     The optical sensors  120 ( 7 ) may include photodiodes, photoresistors, photovoltaic cells, quantum dot photoconductors, bolometers, pyroelectric infrared detectors, and so forth. For example, the optical sensor  120 ( 7 ) may use germanium photodiodes to detect infrared light. In some implementations, the optical sensors  120 ( 7 ) may be arranged in a two dimensional array (such as rows and columns) and mounted beneath, above, or to the side of an inventory location  114 . For example, the array may be below the AFU  132 , incorporated into a spacer  136 , and so forth. The sensor data  128  from the array may be used to detect shadows cast by the items  104 , users  116 , and so forth. This information may be used by the inventory management system  122  to track objects, determine interactions with items  104 , and so forth. 
     One or more radio frequency identification (RFID) readers  120 ( 8 ), near field communication (NFC) systems, and so forth, may be included as sensors  120 . For example, the RFID readers  120 ( 8 ) may be configured to read the RF tags  206 . Information acquired by the RFID reader  120 ( 8 ) may be used by the inventory management system  122  to identify an object associated with the RF tag  206  such as the item  104 , the user  116 , the tote  118 , and so forth. For example, based on information from the RFID readers  120 ( 8 ) detecting the RF tag  206  at different times and RFID readers  120 ( 8 ) having different locations in the facility  102 , a velocity of the RF tag  206  may be determined. 
     One or more RF receivers  120 ( 9 ) may also be included as sensors  120 . In some implementations, the RF receivers  120 ( 9 ) may be part of transceiver assemblies. The RF receivers  120 ( 9 ) may be configured to acquire RF signals  208  associated with Wi-Fi, Bluetooth, ZigBee, 3G, 4G, LTE, or other wireless data transmission technologies. The RF receivers  120 ( 9 ) may provide information associated with data transmitted via radio frequencies, signal strength of RF signals  208 , and so forth. For example, information from the RF receivers  120 ( 9 ) may be used by the inventory management system  122  to determine a location of an RF source, such as a communication interface onboard the tote  118 . 
     The sensors  120  may include one or more accelerometers  120 ( 10 ), which may be worn or carried by the user  116 , mounted to the tote  118 , and so forth. The accelerometers  120 ( 10 ) may provide information such as the direction and magnitude of an imposed acceleration. Data such as rate of acceleration, determination of changes in direction, speed, and so forth, may be determined using the accelerometers  120 ( 10 ). 
     A gyroscope  120 ( 11 ) may provide information indicative of rotation of an object affixed thereto. For example, the tote  118  or other objects may be equipped with a gyroscope  120 ( 11 ) to provide data indicative of a change in orientation of the object. 
     A magnetometer  120 ( 12 ) may be used to determine an orientation by measuring ambient magnetic fields, such as the terrestrial magnetic field. The magnetometer  120 ( 12 ) may be worn or carried by the user  116 , mounted to the tote  118 , and so forth. For example, the magnetometer  120 ( 12 ) mounted to the tote  118  may act as a compass and provide information indicative of which direction the tote  118  is oriented. 
     A position sensor  120 ( 13 ) provides information indicative of a position of an object. In one implementation, the position sensor  120 ( 13 ) may be incorporated into the instrumented AFU  132  to provide information about a position of one or more of items  104 , a sled of the instrumented AFU  132 , a position target on the sled, and so forth. The position sensor  120 ( 13 ) may use optical, magnetic, capacitive, inductive, resonant inductive, resistive, ultrasonic, or other techniques to determine presence of an object. For example, the position sensor  120 ( 13 ) may comprise an ultrasonic transducer to determine a distance to a portion of the sled. In another example, the position sensor  120 ( 13 ) may comprise a linear potentiometer or string potentiometer that measures position or displacement of an object based on a change in electrical resistance. The position sensor  120 ( 13 ) may report a value as one or more of an analog signal or a digital signal. For example, a value of the amplitude of an analog signal such as the electrical resistance of the linear potentiometer may be indicative of the position. A digital signal may be indicative of the position. For example, the position may be expressed as an 8 bit value. In some implementations, the position sensor  120 ( 13 ) may include a controller to generate sensor data  128  indicative of the position or displacement of the object. For example, the controller may determine a linear measurement in inches or meters based on the amplitude of the analog signal, the 8 bit value, and so forth. The position sensor  120 ( 13 ) is discussed in more detail below with regard to  FIG. 9 . 
     The sensors  120  may include other sensors  120 (S) as well. For example, the other sensors  120 (S) may include proximity sensors, ultrasonic rangefinders, thermometers, barometric sensors, hygrometers, and so forth. For example, the inventory management system  122  may use information acquired from thermometers and hygrometers in the facility  102  to direct the user  116  to check on delicate items  104  stored in a particular inventory location  114 , which is overheating, too dry, too damp, and so forth. 
     In some implementations, the camera  120 ( 1 ) or other sensors  120  may include hardware processors, memory, and other elements configured to perform various functions. For example, the cameras  120 ( 1 ) may be configured to generate image data, send the image data to another device such as the server  204 , and so forth. 
     The facility  102  may include one or more access points  210  configured to establish one or more wireless networks. The access points  210  may use Wi-Fi, NFC, Bluetooth, or other technologies to establish wireless communications between a device and the network  202 . The wireless networks allow the devices to communicate with one or more of the sensors  120 , the inventory management system  122 , the tag  206 , a communication device of the tote  118 , or other devices. 
     Output devices  212  may also be provided in the facility  102 . The output devices  212  are configured to generate signals which may be perceived by the user  116  or detected by the sensors  120 . In some implementations, the output devices  212  may be used to provide illumination of an optical sensor array. 
     Haptic output devices  212 ( 1 ) are configured to provide a signal that results in a tactile sensation to the user  116 . The haptic output devices  212 ( 1 ) may use one or more mechanisms such as electrical stimulation or mechanical displacement to provide the signal. For example, the haptic output devices  212 ( 1 ) may be configured to generate a modulated electrical signal, which produces an apparent tactile sensation in one or more fingers of the user  116 . In another example, the haptic output devices  212 ( 1 ) may comprise piezoelectric or rotary motor devices configured to provide a vibration, which may be felt by the user  116 . 
     One or more audio output devices  212 ( 2 ) may be configured to provide acoustic output. The acoustic output includes one or more of infrasonic sound, audible sound, or ultrasonic sound. The audio output devices  212 ( 2 ) may use one or more mechanisms to generate the acoustic output. These mechanisms may include, but are not limited to, the following: voice coils, piezoelectric elements, magnetostrictive elements, electrostatic elements, and so forth. For example, a piezoelectric buzzer or a speaker may be used to provide acoustic output. 
     The display devices  212 ( 3 ) may be configured to provide output, which may be seen by the user  116  or detected by a light-sensitive sensor such as a camera  120 ( 1 ) or an optical sensor  120 ( 7 ). In some implementations, the display devices  212 ( 3 ) may be configured to produce output in one or more of infrared, visible, or ultraviolet light. The output may be monochrome or color. 
     The display devices  212 ( 3 ) may be emissive, reflective, or both. An emissive display device  212 ( 3 ), such as using light emitting diodes (LEDs), is configured to emit light during operation. In comparison, a reflective display device  212 ( 3 ), such as using an electrophoretic element, relies on ambient light to present an image. Backlights or front lights may be used to illuminate non-emissive display devices  212 ( 3 ) to provide visibility of the output in conditions where the ambient light levels are low. 
     The display devices  212 ( 3 ) may include, but are not limited to, microelectromechanical systems (MEMS), spatial light modulators, electroluminescent displays, quantum dot displays, liquid crystal on silicon (LCOS) displays, cholesteric displays, interferometric displays, liquid crystal displays (LCDs), electrophoretic displays, and so forth. For example, the display device  212 ( 3 ) may use a light source and an array of MEMS-controlled mirrors to selectively direct light from the light source to produce an image. These display mechanisms may be configured to emit light, modulate incident light emitted from another source, or both. The display devices  212 ( 3 ) may operate as panels, projectors, and so forth. 
     The display devices  212 ( 3 ) may be configured to present images. For example, the display device  212 ( 3 ) may comprise an addressable display  212 ( 3 )( 1 ). The addressable display  212 ( 3 )( 1 ) may comprise elements that may be independently addressable to produce output, such as pixels. For example, the addressable display  212 ( 3 )( 1 ) may produce an image using a two-dimensional array of pixels. 
     In some implementations, the display devices  212 ( 3 ) may be configured to provide non-image data, such as text characters, colors, and so forth. For example, an addressable display  212 ( 3 )( 1 ) may comprise a segmented electrophoretic display device  212 ( 3 ), segmented LED, and so forth, and may be used to present information such as a stock keeping unit (SKU) number, quantity on hand, and so forth. The display devices  212 ( 3 ) may also be configurable to vary the color of the segment, such as using multicolor/multi-wavelength LED segments. 
     The display devices  212 ( 3 ) may include image projectors  212 ( 3 )( 2 ). For example, the image projector  212 ( 3 )( 2 ) may be configured to project an image onto objects, illuminate at least a portion of an optical sensor array, and so forth. The image may be generated using MEMS, LCOS, and so forth. 
     The display devices  212 ( 3 ) may include a light array  212 ( 3 )( 3 ). The light array  212 ( 3 )( 3 ) may comprise a plurality of discrete emissive elements configurable to emit light. The discrete emissive elements (or assemblies thereof) may be separated from one another by a distance such that, when image data of the light array  212 ( 3 )( 3 ) is acquired, one emissive element may be distinguished from another. For example, the light array  212 ( 3 )( 3 ) may comprise a plurality of infrared LEDs separated by at least 0.5 centimeters. 
     Other display devices  212 ( 3 )(D) may also be used in the facility  102 . The display devices  212 ( 3 ) may be located at various points within the facility  102 . For example, the addressable displays  212 ( 3 )( 1 ) or the light arrays  212 ( 3 )( 3 ) may be located on inventory locations  114 , totes  118 , in or on the floor of the facility  102 , and so forth. 
     Other output devices  212 (P) may also be present. For example, the other output devices  212 (P) may include scent/odor dispensers, document printers, 3D printers or fabrication equipment, and so forth. 
       FIG. 3  illustrates a block diagram  300  of a server  204  configured to support operation of the facility  102 , according to some implementations. The server  204  may be physically present at the facility  102 , may be accessible by the network  202 , or a combination of both. The server  204  does not require end-user knowledge of the physical location and configuration of the system that delivers the services. Common expressions associated with the server  204  may include “on-demand computing”, “software as a service (SaaS)”, “platform computing”, “network-accessible platform”, “cloud services”, “data centers”, and so forth. Services provided by the server  204  may be distributed across one or more physical or virtual devices. 
     One or more power supplies  302  may be configured to provide electrical power suitable for operating the components in the server  204 . The one or more power supplies  302  may comprise batteries, capacitors, fuel cells, photovoltaic cells, wireless power receivers, conductive couplings suitable for attachment to an external power source such as provided by an electric utility, and so forth. The server  204  may include one or more hardware processors  304  (processors) configured to execute one or more stored instructions. The processors  304  may comprise one or more cores. One or more clocks  306  may provide information indicative of date, time, ticks, and so forth. For example, the processor  304  may use data from the clock  306  to associate a particular interaction with a particular point in time. 
     The server  204  may include one or more communication interfaces  308  such as input/output (I/O) interfaces  310 , network interfaces  312 , and so forth. The communication interfaces  308  enable the server  204 , or components thereof, to communicate with other devices or components. The communication interfaces  308  may include one or more I/O interfaces  310 . The I/O interfaces  310  may comprise Inter-Integrated Circuit (I2C), Serial Peripheral Interface bus (SPI), Universal Serial Bus (USB) as promulgated by the USB Implementers Forum, RS-232, and so forth. 
     The I/O interface(s)  310  may couple to one or more I/O devices  314 . The I/O devices  314  may include input devices such as one or more of a sensor  120 , keyboard, mouse, scanner, and so forth. The I/O devices  314  may also include output devices  212  such as one or more of a display device  212 ( 3 ), printer, audio speakers, and so forth. In some embodiments, the I/O devices  314  may be physically incorporated with the server  204  or may be externally placed. 
     The network interfaces  312  may be configured to provide communications between the server  204  and other devices, such as the totes  118 , routers, access points  210 , and so forth. The network interfaces  312  may include devices configured to couple to personal area networks (PANs), local area networks (LANs), wide area networks (WANs), and so forth. For example, the network interfaces  312  may include devices compatible with Ethernet, Wi-Fi, Bluetooth, ZigBee, and so forth. 
     The server  204  may also include one or more busses or other internal communications hardware or software that allow for the transfer of data between the various modules and components of the server  204 . 
     As shown in  FIG. 3 , the server  204  includes one or more memories  316 . The memory  316  may comprise one or more non-transitory computer-readable storage media (CRSM). The CRSM may be any one or more of an electronic storage medium, a magnetic storage medium, an optical storage medium, a quantum storage medium, a mechanical computer storage medium, and so forth. The memory  316  provides storage of computer-readable instructions, data structures, program modules, and other data for the operation of the server  204 . A few example functional modules are shown stored in the memory  316 , although the same functionality may alternatively be implemented in hardware, firmware, or as a system on a chip (SoC). 
     The memory  316  may include at least one operating system (OS) module  318 . The OS module  318  is configured to manage hardware resource devices such as the I/O interfaces  310 , the I/O devices  314 , the communication interfaces  308 , and provide various services to applications or modules executing on the processors  304 . The OS module  318  may implement a variant of the FreeBSD operating system as promulgated by the FreeBSD Project; other UNIX or UNIX-like variants; a variation of the Linux operating system as promulgated by Linus Torvalds; the Windows operating system from Microsoft Corporation of Redmond, Wash., USA; and so forth. 
     Also stored in the memory  316  may be a data store  320  and one or more of the following modules. These modules may be executed as foreground applications, background tasks, daemons, and so forth. The data store  320  may use a flat file, database, linked list, tree, executable code, script, or other data structure to store information. In some implementations, the data store  320  or a portion of the data store  320  may be distributed across one or more other devices including the servers  204 , network attached storage devices, and so forth. 
     A communication module  322  may be configured to establish communications with one or more of the totes  118 , sensors  120 , display devices  212 ( 3 ), other servers  204 , or other devices. The communications may be authenticated, encrypted, and so forth. 
     The memory  316  may store an inventory management module  324 . The inventory management module  324  is configured to provide the inventory functions as described herein with regard to the inventory management system  122 . For example, the inventory management module  324  may track items  104  between different inventory locations  114 , to and from the totes  118 , and so forth. 
     The inventory management module  324  may include one or more of a data acquisition module  326  or a processing module  328 . The data acquisition module  326  may be configured to acquire and access information associated with operation of the facility  102 . For example, the data acquisition module  326  may be configured to acquire sensor data  128  from one or more sensors  120 . This information may be stored in the data store  320 . 
     The processing module  328  may be configured to process the sensor data  128  to generate information such as a quantity of items  104  at an inventory location  114 , change in quantity over time, and so forth. The processing module  328  may utilize one or more of the physical layout data  124 , item data  126 , sensor data  128 , or threshold data  330  during operation to generate processed data  332 . The threshold data  330  may specify one or more thresholds, such as permissible tolerances or variances. For example, the thresholds may specify a percentage variance between an estimated change in position based on item data  126  and change in quantity and a measured change in position measured by the position sensor  120 ( 13 ). 
     The processing module  328  may be configured to process the sensor data  128  from the weight sensors  120 ( 6 ). For example, the sensor data  128  from the weight sensors  120 ( 6 ) may be used to determine a change in quantity, determine where from upon the platform  130  an item  104  was removed, and so forth. In another example, the processing module  328  may access the sensor data  128  acquired by the instrumented AFU  132 ( 27 ). The sensor data  128  from the position sensor  120 ( 13 ) may indicate a linear change in position of a sled of the instrumented AFU  132 ( 27 ) of 3.2 inches. The processing module  328  may access the physical layout data  124  to determine that instrumented AFU  132 ( 27 ) is used to store item  104 ( 114 ) described as “canned dog food”. The physical characteristics for the item  104 ( 114 ) may be retrieved from the item data  126 , indicating a per-item depth of 3 inches for each can of dog food. The processing module  328  may divide the linear change in position by the per-item depth to generate a measured count of quantity change. Continuing the example, 3.2/3=1.07. The quotient may be rounded to produce a result that a quantity of 1 of item  104 ( 114 ) was removed from the instrumented AFU  132 ( 27 ). As a result, the inventory management module  324  may decrease the quantity of the item  104 ( 114 ) stored at instrumented AFU  132 ( 27 ) by 1. 
     In some implementations, the processing module  328  may use sensor data  128 , such as image data obtained from the cameras  120 ( 1 ), to determine proximity of the user  116  to the inventory location  114  that includes the instrumented AFU  132 . As a result, a quantity associated with the user  116  may be changed based on the sensor data  128  obtained from the instrumented AFU  132 . Continuing the example, a quantity of 1 of the item  104 ( 114 ) may be added to a manifest or order pick list associated with the user  116 . 
     Processing of the sensor data  128  or other data may be performed by the processing module  328  or other modules implementing at least in part using the OpenCV library as developed by Intel Corporation of Santa Clara, Calif., USA; Willow Garage of Menlo Park, Calif., USA; and Itseez of Nizhny Novgorod, Russia, with information available at www.opencv.org. In another implementation, functions available in the OKAO machine vision library as promulgated by Omron Corporation of Kyoto, Japan, may be used to process the sensor data  128 . 
     Techniques such as artificial neural networks (ANN), active appearance models (AAM), active shape models (ASM), principal component analysis (PCA), cascade classifiers, and so forth, may also be used to process the sensor data  128  or other data. For example, the ANN may be a trained using a supervised learning algorithm such that object identifiers are associated with images of particular objects within training images provided to the ANN. Once trained, the ANN may be provided with the sensor data  128  such as the image data from a camera  120 ( 1 ), and may provide, as output, the object identifier. 
     Other modules  334  may also be present in the memory  316  as well as other data  336  in the data store  320 . For example, the other modules  334  may include an accounting module while the other data  336  may include billing data. The accounting module may be configured to assess charges to accounts associated with particular users  116  or other entities, while the billing data may include information such as payment account numbers. 
     Modular Item Stowage Hardware 
     Features in the following figures are depicted for purposes of illustration and not necessarily as limitations. The figures are not to scale. 
       FIG. 4  illustrates a side view  400  of inventory locations  114  comprising the platforms  130 , according to some implementations. As specified by an operator of the facility  102 , the inventory location  114  may include a group of platforms  130  (such as an entire rack), an individual platform  130 , or the discrete locations for storage of items  104  therein, such as the AFUs  132 , bins, hangers, and so forth. Two AFUs  132  are depicted mounted to two of the platforms  130  in this illustration. 
     A support member  402  provides a structure to which one or more platform base supports  404  may be affixed. For example, the support member  402  may comprise an upright member with slots or other mechanical engagement features. The slots or other mechanical engagement features may be regularly spaced. The platform base supports  404  may be able to mechanically engage the support member  402  and the platform  130 . In some implementations, the platform base supports  404  may be integral or otherwise affixed to the platform  130 . For example, hooks or other mechanical engagement features may extend from a back of the platform  130 . In this illustration, the support member  402  is depicted as positioned to the back of the platform  130 . In other implementations, the support member  402  may be in different positions relative to the platforms  130 . For example, the support member  402  may support the platform  130  from a front of the platform  130 , where the front is proximate to the location of a user  116  during typical use of picking or placing items  104 . 
     The support member  402  may be vertical, at an angle to vertical, or horizontal. For example, the support member  402  may comprise a horizontal rail supported by legs. In other implementations, the support member  402  may suspend one or more platforms  130  from an overhead structure. 
     A plurality of platforms  130  may be supported from one or more of the support members  402 . The support member  402  may provide for one or more of horizontal or vertical separation between platforms  130 . For example, a first platform  130 ( 1 ) may be above a second platform  130 ( 2 ), side-by-side, and so forth. 
     A cable management system  406  may be provided to guide cabling connecting the platforms  130  to other devices. For example, the cable management system  406  may route cabling between the platform  130  and Ethernet switches, power supplies, and so forth. In some implementations, integrated busses, cabling, electrical conductors, optical waveguides, and so forth, may be integrated into the support member(s)  402 . 
     In other implementations, a system may be provided for thermal management of the platforms  130  and the devices therein. For example, ductwork or piping may be provided to move a working fluid such as air or water through the platform  130  to remove heat dissipated during the operation of the electronics therein. Continuing the example, the cable management system  406  may incorporate ducts to deliver cool air to the platform  130  and remove warm air from the platform  130 . 
     The cabling integrated into the support member(s)  402  may also provide the physical media for communication of the network  202 , or a portion thereof. For example, the platform  130  may send sensor data  128  to the inventory management module  324  executing on one or more of the servers  204 . The inventory management module  324  or another module may also communicate with one or more of the devices onboard the platform  130 . For example, the inventory management module  324  may provide configuration information to a computing device onboard the platform  130 , the instrumented AFU  132 , and so forth. In other implementations, wireless power transfer, wireless data transfer, and so forth, may be used instead of, or in addition to, the cabling. For example, a wireless access point, NFC transceiver, or other devices may be incorporated into the support member  402 . 
     One or more cameras  120 ( 1 ) are positioned within the facility  102 . Each camera  120 ( 1 ) has a field of view (FOV)  408 . Cameras  120 ( 1 ) may be arranged within the facility  102  to have a FOV  408  that includes at least a portion of one or more of the inventory locations  114 . For example, a camera  120 ( 1 ) may be mounted overhead in the facility  102 , such as from the ceiling. In another example, the camera  120 ( 1 ) may be mounted above a platform  130  with the FOV  408  oriented to where the items  104  may be stowed during use. 
       FIG. 5  illustrates two views  500  of a platform  130 . A first view  502  depicts the platform  130  in an unpopulated configuration, such as before modular elements have been affixed. A second view  504  depicts the platform  130  in a populated configuration, in which a plurality of modular elements such as AFUs  132 , dividers  134 , spacers  136 , and so forth, have been coupled to the platform  130 . 
     The platform  130  includes a shelf base  506 . The shelf base  506  may comprise an upper portion of the platform  130 . For example, the shelf base  506  may comprise stainless steel that has been powder coated. In some implementations, the shelf base  506  may have a predominantly planar configuration, describing a flat surface upon which one or more of the modular elements may rest, or be positioned immediately above. The shelf base  506  may have a first elevation at a first height. Within at least a portion of the shelf base  506 , a cable recess  508  may be provided. 
     Located beneath the shelf base  506  may be a plurality of weight sensors  120 ( 6 ). For example, the location of the weight sensors  120 ( 6 )( 1 ) and  120 ( 6 )( 2 ) under a left side of the shelf base  506  are indicated by a dotted outline. A cutaway view shows the weight sensors  120 ( 6 )( 3 ) and  120 ( 6 )( 4 ) under a right side of the shelf base  506 . The plurality of weight sensors  120 ( 6 ) may provide sensor data  128  such as weight of a load on the shelf base  506 , weight distribution of the load, and so forth. The configuration of the weight sensors  120 ( 6 ) in the platform  130  are described in more detail below with regard to  FIG. 7 . 
     The weight distribution provides information indicative of weight applied to weight sensors  120 ( 6 ) at different points under the load. Sensor data  128  from a plurality of weight sensors  120 ( 6 ) may be combined. For example, the weights from the weight sensors  120 ( 6 )( 1 ) and  120 ( 6 )( 2 ) may be summed to provide a weight measured at a left side of the platform, and weights from the weight sensors  120 ( 6 )( 3 ) and  120 ( 6 )( 4 ) may be summed to provide a weight measured at a right side of the platform  130 . The weight distribution may be expressed as a measured weight at a particular weight sensor  120 ( 6 ), a ratio or percentage of weight on a weight sensor  120 ( 6 ), and so forth. For example, the weight distribution data may be expressed as “3213 g left, 2214 g right”, as a dimensionless ratio such as “0.59 left, 0.41 right”, and so forth. 
     The cable recess  508  has a second elevation that is lower than the first elevation. In this illustration the second elevation may be distinguished from the first elevation by different crosshatch patterns. In some implementations, the shelf base  506  and the cable recess  508  may be a single piece of material or may be several pieces of material joined together, such as by welding, riveting, adhesives, fasteners, and so forth. 
     The cable recess  508  provides a volume for routing cabling that connects electronics within the platform  130  to one or more of the modular elements, such as the instrumented AFU  132 . The cable recess  508  may be arranged around one or more of the weight sensors  120 ( 6 ). For example, in this illustration, the cable recess  508  includes two incursions or peninsulas to allow for placement of the weight sensors  120 ( 6 )( 2 ) and  120 ( 6 )( 4 ) beneath the shelf base  506 . In this illustration, the cable recess  508  describes a contiguous area. However, in other implementations, the cable recess  508  may comprise separate compartments or sections. For example, the shelf base  506  may include a plurality of cable recesses  508 . The cable recess  508  may be arranged generally towards the front of the shelf base  506 . For example, the cable recess  508  may be arranged in a front third of the distance from the front to the back of the shelf base  506  and extending across the width of the shelf base  506 . 
     Within the cable recess  508  may be one or more drains  510 . The drains  510  provide a path for spilled liquids to exit the cable recess  508 . For example, the instrumented AFU  132  above the cable recess  508  may hold containers of bottled water. Should one of the containers leak, the drain  510  prevents accumulation of the water. 
     Also arranged within the cable recess  508  are one or more connectors  512 . The connectors  512  are configured to couple to components in one or more of the modular elements. In one implementation, the connectors  512  may comprise connectors utilizing the RJ-45 form factor. The connectors  512  may be rated to the Ingress Protection (IP) standards IP67 or IP68 as described by the International Electrotechnical Commission (IEC) in specification IEC 60529. 
     A front lip  514  is arranged along a front of the platform  130 . In some implementations, the front lip  514  may comprise an extension or portion of the shelf base  506  and may share a common elevation with the shelf base  506 . In other implementations, the front lip  514  may be at a different elevation, such as above or below the elevation of the shelf base  506 . 
     The front lip  514  may include one or more front engagement features  516 . A plurality of front engagement features  516  may be arranged at predetermined spacing intervals with respect to one another along the front lip  514 . The front engagement features  516  may include, but are not limited to, one or more of the following: slots, recesses, tabs, hooks, latches, rails, lips, ferrous material, hook and loop fasteners, and so forth. For example, the front lip  514  may include a steel member. The front engagement features  516  may be able to accept and mechanically engage corresponding features on the front of the modular element. 
     A slot may comprise an opening in another material. For example, the slot may comprise a hole that has a rectangular, square, elliptical, or other shape. A hook may comprise a member having an asymmetrical shape. For example, a hook may have a shaft with a barb or tip that protrudes perpendicular to a long axis of the hook. The barb or tip may mechanically engage a corresponding engagement feature on another member. A tab may comprise a member that protrudes from another structure. In some implementations the tab may have a ridge or other engagement feature that may mechanically engage a corresponding feature on another member. A bar may comprise a member that has a cross-section that is generally a parallelogram. A rod may comprise a member that is arcuate in cross-section, such as circular or elliptical. A tube may comprise a member that has a cross-section that is generally a parallelogram, arcuate in cross-section, and so forth. For example, the tube may have a circular cross-section. 
     The engagement features may use magnetic forces to engage two or more members. For example, a magnet may be attracted to another magnet, attracted to a ferromagnetic material, repelled from another magnet, and so forth. The ferromagnetic material may include, but is not limited to, iron, nickel, cobalt, neodymium, and so forth. 
     In one implementation, the front engagement features  516  may comprise indexing slots. The indexing slots may be oriented vertically such that the corresponding indexing features from a modular element may be inserted from above. For example, the AFU  132  (instrumented or uninstrumented) may include one or more indexing features such as tabs or pegs that slide into the front engagement feature  516  upon installation. For example, an indexing tab may extend from the AFU  132  to engage the front engagement feature  516 . The front engagement features  516  may be configured to provide lateral stability, preventing the modular element from shifting left to right along the width of the platform  130 . In some implementations, the front engagement features  516  may not provide for vertical engagement with the indexing features. For example, an indexing feature of the AFU  132  may not “catch” or otherwise be restrained from vertical motion by the front engagement feature  516 . 
     In some implementations, the front lip  514  may incorporate one or more ferrous materials that are attractive to a magnet. In some implementations, the front lip  514  may incorporate one or more magnets. As described below, the magnets may be used to secure the AFU  132  or other modular elements after installation on the platform  130 . The magnets may be used to maintain vertical engagement, maintaining a pull between a front of the modular element and the front lip  514 . In other implementations, nonmagnetic techniques may be used to retain the front of the modular element to the front lip  514  during operation while maintaining the ability to remove the modular element. For example, a hook and loop fastener, low-tack pressure sensitive adhesive, suction cups, vacuum clamps, and so forth, may be used instead of or in addition to magnets. 
     Arranged from left to right along the back of the shelf base  506  may be one or more back engagement features  518 . The back engagement features  518  may be part of a back wall, extending vertically from a back portion of the shelf base  506 . A plurality of back engagement features  518  may be arranged at regularly spaced intervals with respect to one another along the back of the shelf base  506 . The back engagement features  518  may include, but are not limited to, one or more of the following: slots, recesses, tabs, hooks, latches, rails, lips, hook and loop fasteners, and so forth. The back engagement features  518  may be horizontally accessible, that is a corresponding feature from a modular element may be inserted in a generally horizontal motion if the shelf base  506  is substantially flat with respect to the Earth. For example, the back engagement features  518  may comprise slots in a back wall extending vertically upward from the shelf base  506 , the slots having a longest axis that is perpendicular to a plane described by the shelf base  506 . An angle between the back wall and the shelf base  506  may be, within a threshold tolerance, a right angle. The back engagement features  518  may be able to accept and mechanically engage corresponding features on the back of the modular element. For example, the AFU  132  may have tabs that engage the slots. 
     In some implementations, the back engagement feature  518  may comprise one or more bars, rods, or other members. The modular element may be configured to engage at least a portion of this member. For example, the modular element may be suspended in a cantilever fashion from the back engagement feature  518 . 
     The platform  130  may include one or more internal electronics bays  520 . The internal electronics bay  520  may be arranged under the shelf base  506 . A perimeter of the internal electronics bay  520  is presented in this illustration as a dotted line. The internal electronics bay  520  may comprise one or more devices. For example, the internal electronics bay  520  may contain one or more of the controllers or other electronics associated with the weight sensors  120 ( 6 ), one or more computing devices, and so forth. One or more cable harnesses provide communication between the connectors  512  and one or more of the devices within the internal electronics bay  520 . The internal electronics bay  520  is described in more detail below with regard to  FIG. 6 . 
     One or more platform base supports  404  may also be provided. As described above, platform base supports  404  may be integral with or otherwise part of the platform  130 . In some implementations, platform base supports  404  may include a platform base upon which the platform  130  may rest. For example, the shelf base  506 , modular elements affixed thereto, and so forth, may be supported from the platform base by the weight sensors  120 ( 6 ). In some implementations, the internal electronics bay  520  may rest on the platform base or may be mounted to the shelf base  506 . 
     Also depicted in this illustration are cross-sectional lines. A cross section along line A-A is described below with regard to  FIG. 6 . A cross section along line B-B is described below with regard to  FIG. 7 . 
     In a populated second view  504  depicted in this illustration, a number of different modular elements have been affixed to the platform  130 . For example, several AFUs  132  have been installed. This installation may include the connection of a data cable (not shown) from the instrumented AFU  132  to one or more of the connectors  512 . Adjacent to the AFUs  132  may be dividers  134 . The dividers  134  may comprise vertical members extending upwards such as walls that act as partitions between adjacent modular elements. In some implementations, modular elements may incorporate built-in walls or partitions. In another implementation, the divider  134  may comprise wire, tubing, or other features to direct the items  104 . For example, the divider  134  may comprise one or more bent wires. To accommodate items  104  that have a width exceeding that of the AFU  132 , or to hold items  104  without the use of an AFU  132 , a spacer  136  may be employed. To accommodate larger items  104 , two or more AFUs  132  may be placed adjacent to one another or may be separated by one or more spacers  136 . In some implementations, the two or more AFUs  132  may be instrumented, or one may be instrumented and the other non-instrumented. 
     The surface of the modular elements may utilize one or more of textures, coatings, or features to facilitate movement of items  104 . For example, linear ridge features extending from the front to the back of the modular element may be used to reduce friction between the items  104  and the surface of the modular element to facilitate sliding of the items  104 . 
     The modular elements may be configured to have a common sizing to facilitate the modular operation. A common depth (front-to-back length) of the modular elements may be used. For example, each of the modular elements may have the same depth dimension. 
     The width of the modular elements may vary. In some implementations, modular elements may vary in width as an integer multiple of a minimum size increment. For example, the minimum size increment may be ⅓ inch with modular elements such as the AFUs  132 , the dividers  134 , and the spacers  136  available in widths such 1⅓ inch, 2 inches, 2⅔ inches, and so forth. 
     In other implementations, other modular elements may be used. For example, the other modular elements may include bins, dispensers, hangers, hooks, and so forth. 
     While the platform  130  has been described in terms of a structure upon which the load such as the modular elements sits, in other implementations, the modular elements may be suspended from the platform  130 . For example, the platform  130  may support a plurality of hanger rods or pegs, from which one or more items  104  depend. 
       FIG. 6  illustrates a side view  600  of the platform  130  along line A-A, according to some implementations. 
     The platform  130  may include one or more spill diverters  602 . The spill diverter  602  may comprise one or more physical features such as a lip, ridge, trough, and so forth, that divert spills of liquids or solids away from one or more of the connectors  512 . For example, as illustrated in  FIG. 6 , the spill diverter  602  comprises an overhang extending away from a vertical face to which the connector  512  is mounted, partially into the cable recess  508  above the connector  512 . The extent of the overhang by the spill diverter  602  into the cable recess  508  may be less than, equal to, or greater than the portion of the connector  512  within the cable recess  508 . In some implementations, the spill diverter  602  may be contiguous across the width of the shelf base  506  proximate to the cable recess  508 . In other implementations, the spill diverter  602  may comprise a plurality of separate features, such as discrete ridges with each ridge associated with a particular connector  512 . The spill diverter  602  may be arranged such that a spilled liquid is directed towards one or more of the drains  510 . 
     The internal electronics bay  520  provides a volume within which electronics may be protected from the environment around the platform  130 . For example, the internal electronics bay  520  may provide protection from moisture, fluids, dust, and so forth. In some implementations, the internal electronics bay  520  may provide temperature control as well. For example, where the platform  130  is deployed into a refrigerated area, the internal electronics bay  520  may include a heater to maintain a minimum acceptable temperature of the devices within the internal electronics bay  520 . 
     Devices within the internal electronics bay  520  may be accessed by one or more removable access panels  604 . In the implementation depicted here, the access panel  604  is arranged along an underside of the platform  130 . The access panel  604  may be secured to the platform  130  using one or more mechanical fasteners, latches, magnets, interference fit, and so forth. A seal  606  may be arranged around the interface or junction between a plurality of bay walls  608  and the access panel  604 . For example, the seal  606  may comprise silicone rubber, room temperature vulcanizing rubber, and so forth. 
     The bay walls  608 , in conjunction with the access panel  604  (when in place), encompass a sealed volume of the internal electronics bay  520 . This sealed volume within the internal electronics bay  520  prevents contaminants such as dust, moisture, and so forth, from affecting the contents of the internal electronics bay  520 . Points of entry for cable harnesses or other cabling into the internal electronics bay  520  may be sealed as well. 
     In some implementations, the bay walls  608  may comprise a portion of the platform  130  structure itself. For example, the shelf base  506  and one or more of the bay walls  608  may be the same piece of material. 
     As described above, the platform  130  may have elements at different elevations. Depicted here is the shelf base  506  at a first elevation  610 . Also depicted is a second elevation  612  of the cable recess  508 . As shown, the second elevation  612  of the cable recess  508  is lower than the first elevation  610  of the shelf base  506 . While the cable recess  508  is depicted with a flat bottom, in other implementations, other cross sectional shapes may be used. For example, the cable recess  508  may slant downwards from underneath the connector  512  towards the lowest point of the second elevation  612 . 
     A front stop  614  is also shown as part of the platform  130 . The front stop  614  may be positioned in front of the front lip  514 , such as on a side of the front lip  514  opposite the cable recess  508 . The front stop  614  may comprise a removable piece that is retained in place during normal use by a front stop retainer  616 . The front stop retainer  616  may be secured to the platform  130  using one or more mechanical fasteners, latches, magnets, interference fit, and so forth. For example, threaded bolts or screws may be used to hold the front stop retainer  616  to the platform  130 . The front stop  614  may comprise a plate or planar piece of rigid or semi-rigid material. The front stop  614  may help retain items  104  within the modular elements. For example, the front stop  614  may prevent the items  104  held by the AFU  132  from being pushed off of the AFU  132  and onto the floor below. In some implementations, the front stop  614  or another structure may be configured to support one or more labels, tags, or other displays for presenting information to the users  116  of the facility  102 . 
     One or more lights  620  may be arranged on an underside of the platform  130 . These lights  620  may include LEDs, incandescent lights, fluorescent lights, electroluminescent lights, quantum dots, lasers, and so forth. The lights  620  may provide illumination for objects below the platform  130 . For example, the lights  620  may illuminate items  104  on the modular elements below. The lights  620  may be positioned proximate to the front of the platform  130 . The lights  620  may be configured to direct emitted light, such as down and towards the back of the inventory location  114  below, such as another platform  130 . The lights  620  may include optical elements such as reflectors, lenses, light pipes, and so forth, to provide desired illumination. 
       FIG. 7  illustrates a side view of the platform  130  along line B-B, according to some implementations. Along the line B-B are the weight sensors  120 ( 6 )( 1 ) and  120 ( 6 )( 2 ) arranged underneath the shelf base  506 . In the implementation depicted here, an internal support structure  702  is depicted that provides support between the shelf base  506  and the weight sensors  120 ( 6 ). For example, the internal support structure  702  may comprise one or more “C” channel rails that support the shelf base  506  and transfer a mechanical load from the shelf base  506  to the weight sensors  120 ( 6 ). In some implementations the internal support structure  702  may be optional. For example, the weight sensors  120 ( 6 ) may directly support the shelf base  506 . 
     The platform  130  may rest upon a platform base  704 . For example, the platform base  704  may be affixed to the platform base supports  404 . Continuing the example, the platform base  704  and a platform base supports  404  may comprise a shelf upon which the platform  130  rests. The weight sensors  120 ( 6 ) act as an interface between the platform  130  and the platform base  704 , transferring the weight of the platform  130  and a load thereupon to the platform base  704 . 
     The weight sensors  120 ( 6 ) may include a load cell  706 . The load cell  706  may include a load cell body  708 . The load cell body  708  may comprise a structure or material that, under the influence of an applied force such as the weight of a load thereupon, will deflect or bend. In some implementations, such as shown here, cutouts or voids may be provided within the load cell body  708 . The load cell body  708  may include, or have affixed thereto, a transducer such as a strain gauge to measure the extent of the deflection. In other implementations, the load cell body  708  itself may comprise a transducer material such that distortion or deflection due to an imposed load generates a detectable signal that may be used to determine an applied weight. 
     One or more mounting features  710  may be used to retain the load cell  706  to the platform  130  and the platform base  704 . For example, the mounting feature  710  may include one or more pins, bolts, threaded sockets, and so forth. Continuing the example, a mounting feature  710  on the underside of the load cell  706  at the interface between the load cell  706  and the platform base  704  may comprise a foot to rest upon the platform base  704  or a bolt to pass through a corresponding hole in the platform base  704  and be retained with a nut. 
     A spacer  712  having a thickness  714  is placed between the load cell  706  and the corresponding mating surface of the platform  130 . The mounting feature  710  such as a bolt or pin through the spacer  712  may maintain the position of the spacer  712  and also affix the load cell  706  to the platform  130 . 
     The spacer  712  provides a gap or standoff between the load cell  706  and the rest of the platform  130 . For example, the spacer  712  may be used to maintain a gap of thickness  714  between a top of the load cell  706  and the bottom of the internal support structure  702  when the platform  130  is in a base load or unpopulated state. The spacer  712  may comprise a separate piece, such as a discrete piece of metal, plastic, ceramic, and so forth. In other implementations, the load cell body  708  may be formed, machined, or otherwise designed such that the spacer  712  is integrated thereto. For example, a first end of the load cell body  708  may be stepped or thicker than a second end. During use, as modular elements and items  104  are added to the platform  130 , the weight on the platform  130  and on the respective load cells  706  increases. 
     The thickness  714  of the spacer  712  may be configured such that overloading the platform  130  with too much weight will result in the load cell  706  “bottoming out” before there is irreparable damage to the load cell body  708  or the other components of the load cell  706 . For example, the load cell  706  may be described as having two load support areas  716 . A first load support area  716 ( 1 ) of each load cell  706  is positioned medially, that is on an end of the load cell  706  that is closest to the neighboring load cell  706 . The first load support area  716 ( 1 ) may be the point at which a mounting feature  710  and the spacer  712  transfer the weight of a load from the platform  130  to the load cell  706 . A second load support area  716 ( 2 ) is distal from the first load support area  716 ( 1 ) and the neighboring load cell  706 . The second load support area  716 ( 2 ) transfers the weight of the load from the load cell  706  to the platform base  704 . 
     During normal operation, the weight of the load on the load cell  706  such as the weight of the platform  130  and any modular elements thereupon is applied to the first load support area  716 ( 1 ). The load cell body  708  may undergo a slight deformation or change in shape that is detectable and may be used to determine a numeric value of the weight. The force of the weight then passes through the load cell body  708  to the second load support area  716 ( 2 ) and onto the platform base  704 . During an overload condition, the upper surface of the load cell body  708  at the second load support area  716 ( 2 ) may come in contact with the internal support structure  702 , shelf base  506 , or other structure to which the upper mounting feature  710  of the load cell  706  is affixed. The thickness  714  of the spacer  712  allows sufficient travel for the distal end of the load cell body  708  to be displaced and provide for measurement of a desired range of weights, while preventing excessive travel that may result in irreparable damage to the load cell  706 . The spacer  712  that provides a safety mechanism to avoid damaging load cell  706  should a user  116  inadvertently overload the platform  130 . 
     In other implementations, other configurations of load cell  706  may be used. For example, the load cell  706  may be configured as a single-ended beam, double-ended beam, S-beam, and so forth. 
     The weight sensors  120 ( 6 ) may be readily accessible and field replaceable. For example, weight sensors  120 ( 6 ) incorporating load cells  706  having different weight ranges or capacities may be readily changed out to accommodate different types of loads. Continuing the example, where the platform  130  will be supporting modular elements holding items  104  that are heavy such as canned foods, load cells  706  and spacers  712  designed for the estimated total load of the platform  130  may be installed in the platform  130 . In comparison, where the platform  130  will be supporting items  104  that are lighter, such as potato chips, different load cells  706  and spacers  712  may be installed in the platform  130 . The weight sensors  120 ( 6 ) may be selected to provide a particular operating range such as a minimum weight, maximum weight, desired weight resolution, and so forth, as desired by operators of the facility  102 . 
       FIG. 8  illustrates top views  800  of an instrumented AFU  132 , according to some implementations. The AFU  132  may be instrumented to provide sensor data  128  that may be used to determine information such as quantity of items  104  held by the instrumented AFU  132 . 
     The AFU  132  may include an AFU base  802 . One or more items  104  that may be stowed by the AFU  132  may sit upon the AFU base  802 . One or more rails  804  may be integral with, or affixed to, the AFU base  802 . For example, two rails  804  may be used as illustrated here, with each rail  804  on an opposite side of the AFU base  802 . 
     A front clip  806  is arranged at a front of the AFU base  802  proximate to a front end of the rails  804 . As described below in more detail, the front clip  806  may constrain the travel of the sled towards the front of the AFU base  802 . The front clip  806  may have one or more mechanical engagement features that retain the front clip  806  to the AFU base  802 . For example, the front clip  806  may have one or more ridges or tabs that engage corresponding slots within the AFU base  802 . 
     A back clip  808  is arranged at a back of the AFU base  802  proximate to a backend of the rails  804 . As described below in more detail, the back clip  808  may constrain the travel of the sled towards the back of the AFU base  802 . The back clip  808  may have one or more mechanical engagement features that retain the back clip  808  to the AFU base  802 . For example, the back clip  808  may have one or more ridges or tabs that engage corresponding slots within the AFU base  802 . 
     Extending from a back of the AFU base  802  or other portion of the AFU  132  may be one or more AFU back engagement features  810 . The AFU back engagement features  810  are configured to engage one or more of the back engagement features  518  of the platform  130 . In some implementations, the AFU back engagement features  810  may extend from the back of the AFU  132 . 
     A sled  812  comprises an assembly that is movable relative to the AFU base  802 . The sled  812  may include one or more features that are engaged by the rail  804 . The sled  812  may travel linearly from front to back of the AFU base  802  as constrained by the front clip  806  and the back clip  808 . The one or more rails  804  retain the sled  812  with respect to the AFU base  802 . 
     The sled  812  may include a pushplate  814 , an actuator  816 , and the position target  818 . The pushplate  814  comprises a member that comes in contact with one or more items  104  that may be stowed by the AFU  132 . The pushplate  814  may comprise a member having a substantially flat, curvilinear, or other shape with a long axis generally perpendicular to the AFU base  802 . 
     The actuator  816  may comprise one or more of a spring or motor. In this illustration, the actuator  816  is depicted as being located on the sled  812 . In other implementations, the actuator  816  may be positioned within AFU base  802 , the front clip  806 , or another portion of the AFU  132 . The actuator  816  is configured to apply a force  820  to the sled  812  such that the sled  812  is biased to move towards the front clip  806 . The actuator  816  may comprise a linear motor, rotary motor, and so forth. For example, a linear or rotary motor may be used to move the sled  812  towards or away from the front of the AFU base  802 . 
     The force  820  provided by the actuator  816  may be sufficient to push one or more of the items  104  that are on the AFU base  802  towards the front stop  614 . For example, the actuator  816  may comprise a variable force spring with one end attached to the sled  812  and the other attached to one or more of the front of the AFU base  802  or the front clip  806 . In one implementation, a portion of the variable force spring may be bent, with the bent portion engaging a portion of the front clip  806 , such as an edge of a slot. In another implementation, the variable force spring may be riveted, screwed, glued, laminated, or otherwise affixed to the front clip  806 . 
     Under the influence of the actuator  816  that produces the force  820 , as items  104  are added to or removed from the AFU base  802 , the sled  812  moves towards or away from the front clip  806 . Position data may be generated by a position sensor  120 ( 13 ) within the instrumented AFU  132  that is indicative of a position  822  of the sled  812 . The position  822  may be relative to a particular origin or a reference point on the instrumented AFU  132 , such as the rearmost portion of the back clip  808  as illustrated here. In other implementations, other origins or reference points may be used, such as a front of the AFU base  802 , relative to the position sensor  120 ( 13 ) itself, and so forth. The position  822  and the corresponding position data may be represented in terms of a binary value, absolute measurement, and so forth. For example, the position  822  may be represented as a four bit value, distance in inches, and so forth. 
     In this illustration, at time=1, three items  104  are shown stored by the instrumented AFU  132 . For example, the position  822 ( 1 ) is indicative of first position data such as “1 inch”. At time=2, one item  104  has been removed, leaving two items  104  still stowed by the instrumented AFU  132 . As a result of the force  820  applied by the actuator  816 , the sled  812  has moved to a new position  822 ( 2 ). Continuing the example, the position  822 ( 2 ) is indicative of second position data such as “4 inches”. The controller device onboard the instrumented AFU  132 , or another device such as the server  204 , may process sensor data  128  indicative of one or more of the first position  822 ( 1 ), the second position  822 ( 2 ), or a difference between the two (such as “+3 inches”). Based on the sensor data  128  providing information about one or more of the change in position  822 , the magnitude of the change in position  822 , the sign of the change in position  822 , speed of change in position  822 , and so forth, information indicative of an interaction with the items  104  stowed by the instrumented AFU  132  may be determined. For example, based on the change in position of 3 inches, the item data  126  indicative of the depth of the type of item  104  stowed by the instrumented AFU  132  being 3 inches each, and a positive sign of the difference, the processing module  328  may determine that a quantity of one of the items  104  stowed by the AFU  132  has been picked. 
     The position target  818  is detectable by a position sensor located within or underneath the AFU base  802 . The position target  818  may comprise an active or passive component. An active component may use a battery or electrical energy received via wires to generate a signal. For example, an infrared emitter is an active component. In comparison, a passive component may interact with a signal produced by a detector. For example, a passive component may comprise an inductive target. The position target  818  and its interaction with the position sensor  120 ( 13 ) are discussed in more detail below with regard to  FIG. 9 . 
     While the AFU  132  is described in terms of discrete components, in some implementations, a single structure may be used instead of discrete components. For example, the front clip  806 , the rails  804 , the AFU base  802 , and the back clip  808  may comprise a single piece or structure that is formed, milled, or otherwise fabricated. Likewise, the sled  812  may comprise a single piece or structure. 
       FIG. 9  illustrates side views  900  of different versions of the instrumented AFUs  132 , according to some implementations. At  902 , an active force mechanism version of the instrumented AFU  132  is depicted. At  904 , a gravity feed version of the instrumented AFU  132  is depicted. As  FIG. 9  depicts different versions of the instrumented AFUs  132 , portions of the platform  130 , such as the front stop  614 , are not presented. 
     With regard to the active force mechanism version at  902 , several features are designed to assist in the mounting of the AFU  132  to the platform  130 . One or more engagement features such as magnets  906  may be placed proximate to a front end of the AFU base  802 . The one or more magnets  906  may be located on an underside of the AFU base  802 , on an upper side of the AFU base  802 , or within the AFU base  802 . When installed at the platform  130 , the magnet  906  is attracted to a ferrous material or another magnet positioned within the front lip  514 . The attraction of the magnet  906  to the front lip  514  or other portion of the shelf base  506  provides a force to retain the AFU base  802  in position, with respect to the shelf base  506 , during use. For example, as a user  116  pushes to insert or remove items  104  from the AFU  132 , the magnet  906  keeps the front of the AFU base  802  from tipping upwards. 
     In other implementations, other engagement features including, but not limited to, hook and loop fasteners, low-tack pressure sensitive adhesives, suction cups, vacuum clamps, a portion of a spring from the actuator  816 , and so forth, may be used instead of or in addition to magnets  906 . For example, a portion of the spring from the actuator  816  may have one or more bends or features therein that are configured to engage into a corresponding engagement feature in the front lip  514 . Continuing the example, the front end of the spring from the actuator  816  may be bent to form a ridge that may then engage a slot within the front lip  514 . The magnet  906  or other mechanism provides a force that retains the front of the AFU  132  in contact with the front lip  514 . 
     One or more front indexing features  908  may be proximate to the front of the AFU  132 . In some implementations, one or more of the front indexing features  908  may extend from or be integral with the front clip  806 . The front indexing feature  908 , when engaged with the front engagement features  516  of the front lip  514 , may be configured to provide lateral stability to prevent the AFU  132  from shifting left to right along the width of the platform  130 . The front indexing feature  908  may comprise a member extending downwards perpendicularly from the front of the AFU base  802 , the front clip  806 , or another portion of the AFU  132 . In some implementations, the front indexing feature  908  may not provide for vertical engagement with the indexing features. For example, the front indexing feature  908  may comprise a smooth sided tab of a constant cross section. When engaged, the front indexing feature  908  prevents lateral motion of the front end of the AFU  132 . 
     One or more position sensors  120 ( 13 ) may be incorporated into the instrumented AFU  132 . The position sensor  120 ( 13 ) provides information indicative of a position of one or more of items  104 , the sled  812 , the position target  818  held by the sled  812 , and so forth. The position sensor  120 ( 13 ) may use optical, magnetic, capacitive, inductive, or other techniques to determine presence of an object. For example, the position sensor  120 ( 13 ) may comprise a resonant inductive position sensor such as the 205 mm Type 6.8 Linear Sensor provided by Cambridge Integrated Circuits Ltd. of Cambridge, United Kingdom. This position sensor  120 ( 13 ) uses a plurality of sensor coils mounted to a printed circuit board that detect the presence of an inductive resonator that acts as the position target  818 . For example, the position target  818  may comprise an inductive resonator element, such as a coil, having a resonant frequency. The position sensor  120 ( 13 ) may include an exciter coil to generate a magnetic field at the resonant frequency. The position sensor  120 ( 13 ) may also include a plurality of sensor coils. During operation, the exciter coil emits a magnetic field that induces an electromotive force (EMF) in the inductive resonator element. This EMF then produces a signal that may be detected by sensor coils. A processor or controller may then determine proximity of the position target based on strength of the signal at the plurality of sensor coils, and given the known location of and distance between the sensor coils. The processor may then generate sensor data  128  indicative of the relative position of the sled  812 . 
     In one implementation, the position sensor  120 ( 13 ) may comprise a linear array of inductive sensors. The inductive sensors may be configured to emit an electromagnetic signal that interacts with the position target  818 . When a particular inductive sensor of the linear array detects a signal corresponding to position target  818 , the position  822  may be determined as corresponding to that of the particular inductive sensor. In another implementation, the position sensor  120 ( 13 ) may comprise a linear array of Hall effect sensors or other sensors able to detect a magnetic field. The position target  818  may comprise a magnet, with the magnetic field produced by that magnet detectable by the sensors in the linear array. In yet another implementation, the position sensor  120 ( 13 ) may comprise optical emitters, optical transmitters, and so forth. For example, an infrared LED may provide illumination while infrared photodiodes are used to detect a particular light pattern reflected by the position target  818 , underside of the sled  812 , and so forth. 
     In other implementations, an active portion of the position sensor  120 ( 13 ) may be located on the sled  812 . For example, optically detectable targets such as a barcode may be printed on or otherwise arranged on an upper surface of the AFU base  802 . An optical transceiver comprising an infrared LED and an infrared photodiode may be configured to detect the barcode and determine position information based on information encoded therein. In another implementation, an optoelectronic sensor comprising a low-resolution two-dimensional array of monochromatic detectors and a light emitter may be used to detect motion and distance traveled. It is recognized that a wide variety of other mechanisms may be used to measure the relative displacement of the sled  812  with respect to the AFU base  802 . 
     A data cable  910  that is terminated in a plug  912  may be used to couple the electronics of the position sensor  120 ( 13 ) to one or more of the connectors  512  within the cable recess  508  of the platform  130 . The data cable  910  may be used to transfer the sensor data  128  to an external device, such as a computing device in the platform  130 . The data cable  910  may exit the AFU base  802  in approximately a front half or front third of the AFU base  802 . For example, the exit point for the data cable  910  from the AFU base  802  may be arranged to correspond with the cable recess  508  when the instrumented AFU  132  is installed onto the platform  132 . 
     In some implementations, the data cable  910  may be omitted, and a connector  512  may be provided within the front lip  514 . In such an implementation, the cable recess  508  may be omitted from the platform  130 . In other implementations, the data cable  910  or similar physical interconnects may be omitted. For example, the position sensor  120 ( 13 ) may provide the resulting sensor data  128  wirelessly. The components of the position sensor  120 ( 13 ) that use electrical power may receive power wirelessly. In other implementations, power may be provided from one or more conductive rails or features arranged at one or more of the front lip  514 , such as within the front engagement features  516 , the back engagement feature  518 , and so forth. In yet another implementation, the position sensor  120 ( 13 ) or other components of the instrumented AFU  132  may be powered by batteries, capacitors, photovoltaic cells, thermocouples, kinetic energy harvesting devices, and so forth. For example, movement of the sled  812  may be used to generate electrical energy that powers the position sensor  120 ( 13 ) or other components. 
     While the modular element may couple to the connectors  512  in the shelf base  506 , in some implementations the modular elements may be daisy-chained. For example, a first data cable  910  of a first modular element may connect to a connector on a second modular element. The second modular element may then connect to one of the connectors  512  using a second data cable  910 . The first modular element may perform one or more data processing functions, or may pass through any signals or data between the devices in the shelf base  506  and the second modular element. 
     In some implementations, one or more of the components within the instrumented AFU  132  may be sealed or held in place using a potting compound. For example, epoxy may be placed within the AFU base  802  to seal and retain components such as the position sensor  120 ( 13 ), the magnet  906 , a portion of the data cable  910 , and so forth. Portions of the AFU base  802  or other components may include integrated features such as ridges that are produced during manufacture. These features may act as dams to retain the potting compound while in its liquid state during manufacture. 
     With regard to the gravity feed version  904 , the actuator  816  may be omitted. Instead, a tilt support  914  may maintain the AFU base  802  at an angle. For example, the AFU base  802  may have a first elevation at the front and a second elevation at the back, with the second elevation being higher than the first elevation. The AFU base  802  may include one or more rollers  916  to facilitate movement of the items  104  towards a front of the AFU base  802 . The weight of the items  104  and the weight of the sled  812  may provide the force  820  in this version. In some implementations of this version, the sled  812  may include additional mass to increase the force  820 . 
       FIG. 10  illustrates a side view  1000  of the sled  812  of the instrumented AFU  132 , according to some implementations. As described above, the sled  812  may include a pushplate  814 , an actuator  816 , and a position target  818 . These components may be mounted or otherwise secured to a sled base  1002 . The sled base  1002  may include one or more rail engagement features  1004 . The rail engagement features  1004  assist in maintaining the sled  812  proximate to the AFU base  802  during operation. The rail engagement features  1004  may comprise ridges or protrusions that mechanically engage one or more sides of the rails  804 . For example, the ridges may mechanically engage an upper face and a lower face of the rails  804 . The rail engagement features  1004  in combination with the rails  804  may constrain motion of the sled  812  along a single axis. 
     In this illustration, the actuator  816  comprises spring  1006 . For example, the spring  1006  may comprise a stainless steel spring having one end affixed to a portion of the sled  812  and the other end affixed to the front of the AFU base  802  or the front clip  806 . In some implementations, the spring  1006  may engage a front of the AFU base  802  and the front clip  806  may hold the spring  1006  in place to maintain that engagement. In some implementations, the spring  1006  may be dismountable from the front clip  806 . By being readily dismountable, such as without tools or with simple tools, changing of springs  1006  that have become worn or broken is performed more easily. Spring  1006  may also be changed to provide a different level of force  820  at the AFU  132 , and so forth. For example, an AFU  132  holding heavy items  104  may need a more forceful spring  1006  than another AFU  132  holding lightweight items  104 . 
     In other implementations, the actuator  816  may be arranged in other positions. For example, the actuator  816  may be located within the AFU base  802 , within or affixed to the platform  130 , and so forth. 
     One or more pushplate supports  1008  extend from the sled base  1002  to the pushplate  814 . The sled base  1002 , the pushplate support  1008 , and the pushplate  814  may comprise one or more different mechanical elements. For example, a single structure may be formed that incorporates all three elements. 
     The pushplate support  1008  may be configured to maintain a front edge of the pushplate  1012  at a point which is forward of a front edge of the sled  1010 . During operation of the AFU  132 , if the front stop  614  is removed, the front edge of the pushplate  1012  may extend past the front clip  806 . Said another way, if the front clip  806  is removed, the spring  1006  and the arrangement of the front edge of the pushplate  1012  may be such that all items  104  that are stowed by the AFU  132  would be pushed clear of the AFU base  802  and onto the floor. 
     In some implementations, the pushplate  814  may be removable or may have additional elements affixed thereto. For example, the pushplate  814  may be removable from the pushplate support  1008  to enable use of a pushplate  814  that has a larger area, smaller area, particular contour, particular height, particular width, and so forth. For example, a large pushplate  814  may be installed to the pushplate support  1008  to accommodate bulky products. 
     The other modular elements may include one or more of the features described above with regard to the AFU  132 . For example, the dividers  134 , spacers  136 , or other modular elements may include engagement features, particular surface textures or treatments, and so forth. 
     Other implementations may be used in the facility  102 . For example, the magnet  906  may be proximate to the back of the modular element rather than the front, and the various engagement features may be flipped to correspond. Continuing the example, the indexing feature may be positioned at the back of the AFU  132 , while engagement features are located at the front of the AFU  132 . 
     In some implementations, the position sensors  120 ( 13 ) may be within the platform  130 . For example, a plurality of position sensors  120 ( 13 ) may be arranged in lanes. A modular unit may then be attached to the platform  130 . In one variation of this implementation, the platform  130  may include a shelf base  506  configured with engagement features to hold other elements, such as the sled  812 . For example, the shelf base  506  may include a plurality of rails  804  or similar features onto which the sled  812  may be engaged at a desired location. The position sensors  120 ( 13 ) in the platform  130  may determine the position of a sled  812  or position target  818  therein. 
     The processes discussed herein may be implemented in hardware, software, or a combination thereof. In the context of software, the described operations represent computer-executable instructions stored on one or more non-transitory computer-readable storage media that, when executed by one or more processors, perform the recited operations. Generally, computer-executable instructions include routines, programs, objects, components, data structures, and the like that perform particular functions or implement particular abstract data types. Those having ordinary skill in the art will readily recognize that certain steps or operations illustrated in the figures above may be eliminated, combined, or performed in an alternate order. Any steps or operations may be performed serially or in parallel. Furthermore, the order in which the operations are described is not intended to be construed as a limitation. 
     Embodiments may be provided as a software program or computer program product including a non-transitory computer-readable storage medium having stored thereon instructions (in compressed or uncompressed form) that may be used to program a computer (or other electronic device) to perform processes or methods described herein. The computer-readable storage medium may be one or more of an electronic storage medium, a magnetic storage medium, an optical storage medium, a quantum storage medium, and so forth. For example, the computer-readable storage media may include, but is not limited to, hard drives, floppy diskettes, optical disks, read-only memories (ROMs), random access memories (RAMs), erasable programmable ROMs (EPROMs), electrically erasable programmable ROMs (EEPROMs), flash memory, magnetic or optical cards, solid-state memory devices, or other types of physical media suitable for storing electronic instructions. Further, embodiments may also be provided as a computer program product including a transitory machine-readable signal (in compressed or uncompressed form). Examples of transitory machine-readable signals, whether modulated using a carrier or unmodulated, include, but are not limited to, signals that a computer system or machine hosting or running a computer program can be configured to access, including signals transferred by one or more networks. For example, the transitory machine-readable signal may comprise transmission of software by the Internet. 
     Separate instances of these programs can be executed on or distributed across any number of separate computer systems. Thus, although certain steps have been described as being performed by certain devices, software programs, processes, or entities, this need not be the case, and a variety of alternative implementations will be understood by those having ordinary skill in the art. Additionally, those having ordinary skill in the art readily recognize that the techniques described above can be utilized in a variety of devices, environments, and situations. 
     Although the subject matter has been described in language specific to structural features or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as illustrative forms of implementing the claims.