Patent Publication Number: US-10321275-B1

Title: Multi-frequency user tracking system

Description:
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 or movement of users, inventory, or other objects within the facility. 
     Other types of facilities may also benefit from tracking of users or other objects. For example, hospitals may wish to track patients, airports may wish to track passengers, and so forth. 
    
    
     
       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. The figures are not necessarily drawn to scale, and in some figures, the proportions or other aspects may be exaggerated to facilitate comprehension of particular aspects. 
         FIG. 1  illustrates a system using signals emitted by smart floor tiles to generate tracking data about movement of users within a facility, according to some implementations. 
         FIG. 2  illustrates an arrangement of smart floors tiles and their respective segments, according to some implementations. 
         FIG. 3  illustrates the arrangement of components included in a smart floor tile, according to some implementations. 
         FIG. 4  illustrates the mixing of signals transmitted simultaneously by the smart floor tiles, according to some implementations. 
         FIG. 5  illustrates a graph of combined received signal characteristics for a first user, such as received by a receiver in a fixture or a smart floor tile, according to some implementations. 
         FIG. 6  illustrates a graph of combined received signal characteristics for a second user, such as received by a receiver in a fixture or a smart floor tile, according to some implementations. 
         FIG. 7  illustrates tracking of a user as they move across the smart floor tiles, according to some implementations. 
         FIG. 8  illustrates the use of a portable receiver to detect the signals transmitted by the smart floor tiles, according to some implementations. 
         FIG. 9  illustrates the use of a signal transmitted by the smart floor tiles to determine a particular user is interacting with a particular portion of a fixture, according to some implementations. 
         FIG. 10  illustrates an enlarged view of the use of an electromagnetic signal to generate gesture data and other information indicative of which item a user interacted with at the fixture, according to some implementations. 
         FIG. 11  depicts a block diagram of a fixture such as a shelf that is configured to generate gesture data, characteristic data, and so forth, according to some implementations. 
         FIG. 12  depicts a scenario showing the signal strengths as received using different antennas at the fixture, according to some implementations. 
         FIG. 13  is a block diagram illustrating a materials handling facility (facility) using the system, according to some implementations. 
         FIG. 14  is a block diagram illustrating additional details of the facility, according to some implementations. 
         FIG. 15  is a block diagram of a server to support operation of the facility, according to some implementations. 
         FIG. 16  depicts a flow diagram of a process of using smart floor tiles to generate tracking data, 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 
     Described in this disclosure are systems and techniques for generating data in a materials handling facility (facility). 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 fixture, what items a particular user is ordered to pick, how many items have been picked or placed at the fixture, 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. The inventory management system may process the sensor data from the one or more sensors to determine tracking data, interaction data, and so forth. The tracking data provides information about the location of a user within the facility, their path through the facility, and so forth. The interaction data is indicative of an action such as picking or placing an item at a particular location on the fixture, touching an item at a particular location on the fixture, presence of the user at the fixture without touching the item, and so forth. For example, the inventory management system may use the sensor data to generate tracking data and interaction data that determines a type of item a user picked from a particular fixture. 
     A fixture may include one or more item stowage areas such as shelves, hangers, and so forth, that hold or otherwise support a type of item. The fixture may be arranged into sections, such as lanes on a shelf. For example, a shelf may have three lanes, with each lane holding a different type of item. Items may be added to (placed) or removed (picked) from the fixture, moved from one fixture to another, and so forth. 
     The floor of the facility may comprise a plurality of smart floor tiles. The smart floor tiles may include transmitters that generate low frequency radio signals and a receiver that detects the low frequency radio signals. For example, the carrier of these signals may be less than or equal to 30 MHz. The smart floor tiles may also include sensors such as touch or pressure sensors that provide object data indicative of an object such as a foot or wheel that is in contact with the smart floor tile. 
     The floor of the facility is composed of clusters of smart floor files. Each cluster includes a plurality of smart floor tiles. Each smart floor tile, in turn, has a plurality of segments. During operation, the transmitters of the smart floor tiles may be configured to transmit several signals. Each smart floor tile transmits a tile signal at a particular tile frequency. Within the cluster, each smart floor tile transmits on a different frequency. The same tile frequencies may be reused by other smart floor tiles in another cluster. 
     Each segment of the smart floor tile transmits a segment signal at a particular segment frequency. Each segment signal within a given smart floor tile is transmitted at a different frequency. The same segment frequencies may be reused by other smart floor tiles in the cluster. Each segment includes at least one antenna that radiates the segment signal assigned to that segment. The tile signal may be transmitted using one or more of the antennas at the different segments. 
     An object may electromagnetically couple to a proximate antenna in the smart floor tile. For example, when a user is standing with their left foot on a first segment in a first smart floor tile, their left foot electromagnetically couples to the antenna in that segment. As a result of this coupling, a first set of the signals transmitted by the first segment are transferred along the body of the user by way of this electromagnetic coupling. Continuing the example, the tile signal and the segment signal are propagated along the body of the user standing on that segment to the other extremities such as the right foot and both hands. 
     As the user walks, their right foot comes to rest on a second smart floor tile. The body of the user now acts as a bridge, providing a signal path along which signals may travel between the first and second smart floor tiles. A receiver in the second smart floor tile detects the first set of signals that originated by the first smart floor tile under the left foot. Meanwhile, the reverse happens with the first smart floor tile detecting a second set of signals that originate from the second smart floor tile and are passed from the right foot through the user&#39;s body to the left foot. 
     The smart floor tiles may generate tile output data that includes received characteristic data. The received characteristic data provides information about the signals received and may include the received signal strength of those signals. The tile output data from the first and second smart floor tiles may be used to determine that the user is in contact with both tiles. For example, a server may receive the tile output data and determine that these two smart floor tiles and their respective segments are reporting received characteristic data indicative of the other smart floor tile. Given this correspondence, the two locations of the received characteristic data may be associated with the feet of a single user, and a location of the user may be determined. The server may also analyze the received characteristic data obtained from several segments and estimate a shape of a user&#39;s foot. 
     By determining a successive series of locations of the user over time, tracking data may be generated. The tracking data comprises information indicative of the user&#39;s path through the facility. 
     The signals provided by the transmitters may be used to determine the relative position of the user&#39;s hand(s) with respect to a fixture, to determine an item interacting with a location, and so forth. For example, a smart floor tile may transmit the signals that are then conducted through the user and detected using antennas arranged along a shelf that are connected to one or more receivers. By using the relative signal strength at the different antennas and the known position of the antennas, a position of the user&#39;s hand may be determined with respect to the shelf. When the user touches an item stored on the shelf, the signals transfer from the user to the item and from there transfer to the shelf. For example, the amplitude of the electromagnetic signal received at an antenna that is located beneath the item that is being touched may increase significantly relative to the level obtained when there is no contact. As a result of this increase, the user may be deemed to have had contact with the item stored at that location on the shelf. 
     Information about which user is interacting with the fixture, touching an item, and so forth, may be determined by analyzing the particular combination of signals that are received. For example, the receiver of the shelf may generate characteristic data for the signal received at the shelf. This characteristic data may be compared with the tile output data described above to determine which user is in contact with the particular smart floor tiles and segments. This contact produces a characteristic pattern of signals that corresponds the received characteristic data for the signal received at the shelf. In some implementations, the characteristic data may be obtained using signals received from a subset of the antennas at the shelf. For example, the antennas corresponding to peak received signal strength values that are used to determine the relative position may be used to produce the characteristic data. The spatial diversity between different antennas on the shelf may be used to separate out different hands, and the different characteristics may be used to distinguish one user from another. 
     By using the techniques described herein, operation of the facility may be improved. Details about interactions between users and items in the facility may be quickly and accurately determined. For example, as items are picked, placed, and so forth, information such as inventory levels based on changes in the count of items at the fixtures may be readily and more accurately determined. As a result, the inventory management system may be able to quickly track what item a user has interacted with, maintain up-to-date inventory information, and so forth. Tracking of users may be facilitated, allowing for enhanced services to the users of the facility, such as making the facility respond to the presence of a user. For example, as an authorized user approaches a fixture holding items that is locked, the fixture may unlock to provide access. 
     The smart floor tiles provide various technical advantages including, but not limited to, reductions in bandwidth compared to other sensor methodologies, improved tracking of individual users in congested environments, detection of potential hazards, detection of user incapacity, and so forth. The smart floor tiles are mechanically robust and provide high resolution tracking data for users as well as providing the ability to identify who is interacting with a particular fixture, item, and so forth. The system described herein allows for reduced capital expenditures, as well as reduced operating expenditures relative to other sensor methodologies. For example, compared to vision tracking systems, installation of smart floor tiles is less expensive, and during operation requires fewer computational resources, is less prone to failure or environmental interference, and so forth. The smart floor tiles and the information obtained thereby may be used in conjunction with other systems, such as vision tracking systems, tag tracking systems, and so forth. 
     The system described herein may be used in other types of facilities, both commercial and non-commercial. For example, the smart floor tiles may be installed within a home or care facility and provide information such as user tracking, if the user is standing, lying on the floor, and so forth. The system may be used to improve user safety by determining the whereabouts of the user, determining if the user has fallen, and so forth. The system may also provide enhanced functionality, such as operating in conjunction with building operation. For example, by tracking the user in the facility, lighting, environmental controls, and so forth, may be controlled based on the location of the user. 
     Illustrative System 
       FIG. 1  illustrates a system  100  using a variety of sensors to generate tracking data and other information within a facility, according to some implementations. The facility includes a floor  102 . The floor  102  may comprise a plurality of smart floor tiles (SFTs)  104 . A group of the SFTs  104  is a cluster. The floor  102  may include a plurality of clusters. 
     Each of the SFTs  104  may include various components such as antennas, transmitters, receivers, hardware processors, sensors, and so forth. The SFT  104  may itself be subdivided into segments. For example, each segment may comprise a different antenna. The SFT  104  may be configured to transmit and receive electromagnetic signals (EMS)  106 . Each segment may transmit at a particular frequency that is different from the other segments in the SFT  104 . Each segment may also transmit at a frequency that is different from the other segment frequencies and is different from the other SFTs  104  in the cluster. As a result, within the cluster, a particular SFT  104  may be distinguished by the EMS  106  that it transmits at a particular frequency. The EMS  106  may be transmitted at a low power. For example, the EMS  106  may have a power level of less than 500 microwatts. 
     These EMS  106  may be propagated by the body of a user. For example, the EMS  106  may be propagated along the skin or clothing of the user, travelling from one SFT  104  to another, or from one SFT  104  to another device such as the fixtures  108 . Each SFT  104  may transmit several signals, each at different frequencies. The transmissions may be continuous or may be made at particular times. The different types of signals that may be transmitted are discussed in more detail below with regard to  FIG. 2 . The SFT  104  is discussed in more detail below with regard to  FIG. 3 . 
     Within the facility may be one or more fixtures  108 . The fixture  108  may include shelves, hangers, and so forth, that hold or otherwise support a type of item. The fixture  108  may be arranged into sections, such as lanes on a shelf. For example, a shelf may have three lanes, with each lane holding a different type of item. Items may be added to (placed) or removed (picked) from the fixture  108 , moved from one fixture  108  to another, and so forth. In some implementations, the SFTs  104  may be installed, and the fixtures  108  and other objects may then be installed on the SFTs  104 . In other implementations, the fixtures  108  may be installed and then the SFTs  104  may be installed around the fixtures  108 . Some portions of the floor  102  may omit SFTs  104 . For example, SFTs  104  may be omitted from around the perimeter of a room, immediately adjacent to a wall, underneath a fixture  108 , and so forth. 
     An entry  110  provides access for a user  112  to the facility. For example, the entry  110  may comprise a foyer, door, gated entry area, and so forth. In some implementations, an identity of the user  112  may be asserted at the entry. For example, the user  112  may provide identification credentials such as swiping a card, carrying a device that transmits or displays authentication credentials, and so forth. The user  112  may move throughout the facility, with movement depicted in this illustration as a user path  114  across the floor  102 . The user  112  may use various tools while in the facility, such as a tote  116 , pallet jack, and so forth. The tote  116  may include a basket, cart, bin, bag, and so forth. During operation of the facility, users  112  thus move around, picking, placing, or otherwise interacting with items at the fixtures  108 . 
     The SFTs  104  may obtain electrical power from a power supply  118 . For example, the power supply  118  may provide 24 volts direct current (VDC) to one or more of the SFTs  104 . The power supply  118  may be configured to obtain power from building mains and then provide conditioned power for use. The SFTs  104  are connected to a network  120 . The network  120  allows for communication between SFTs  104  and other devices, such as described below. 
     A clock  122  may provide a clock signal  124  or other clock data that is transmitted to the SFTs  104  using the network  120 . In some implementations, the clock signal  124  may be distributed via another mechanism, such as by the power supply  118  by way of a power distribution network. For example, the clock signal  124  may be overlaid as an alternating current signal along one or more of the electrical conductors used to supply direct current power to the SFTs  104 . In some implementations, the clock signal  124  may be omitted, with each SFT  104  operating with independent clocks  122  or “free running”. 
     One or more processors of the SFTs  104  may generate tile output data  126 . The tile output data  126  may include characteristic data  128 . The characteristic data  128  is indicative of a plurality of signals, each at different frequencies, and the received signal strength of the signals at each of the different frequencies. The characteristic data  128  is indicative of a particular SFT  104  and one or more segments of the SFT  104 . The tile output data  126  may include information about the SFT  104  itself and the segments thereon that received the signals that are represented by the characteristic data  128 . For example, the tile output data  126  may comprise characteristic data  128  for the EMS  106  received at each segment. 
     During operation a first foot of the user  112  is in contact with a first SFT  104 ( 1 ). The particular mix of EMS  106  transmitted by the first SFT  104 ( 1 ) is electromagnetically coupled to the body of the user  112  and transferred along a signal path that includes the body of the user  112  from the first foot to the second foot of the user  112 . Meanwhile, a first receiver in the first SFT  104 ( 1 ) is listening for EMS  106 . As the second foot comes into contact with a second SFT  104 ( 2 ), a bidirectional exchange of EMS  106  takes place. The first SFT  104 ( 1 ) transmits a first set of EMS  106 ( 1 ) (at a first tile frequency and one or more segment frequencies), which is received by a receiver of the second SFT  104 ( 2 ). Meanwhile, the second SFT  104 ( 2 ) transmits a second set of EMS  106 ( 2 ) (at a second tile frequency and one or more segment frequencies), which is received by a receiver of the first SFT  104 ( 1 ). 
     As the user  112  walks across the floor  102 , they act as a bridge between successive SFTs  104 , resulting in a trail of pairs of SFTs  104  (or the segments therein) that have been trod upon. Tile output data  126  may be generated that is indicative of the identity of the receiving SFT  104  and the characteristic data  128  indicative of the EMS  106  that were received. The tile output data  126  may be transferred from the SFT  104  in the floor  102  to an inventory management system  130  via the network  120 . Other information, such as the fixture data  132 , may also be provided to the inventory management system  130 . 
     The inventory management system  130  may include a tracking module  134 . The tracking module  134  may use one or more of the tile output data  126  or the fixture data  132  to generate tracking data  136 . The tracking data  136  may include one or more of information indicative of the user path  114  within the facility, current location, location at a particular time, and so forth. In some implementations, the tracking module  134  may be executed as a tracking system, such as provided by one or more computing devices. In some implementations, the tracking module  134  may use the characteristic data  128  to further distinguish between users  112  or other objects. For example, the user  112 , tote  116 , or other object may include a transmitter that emits a discrete EMS  106  or a receiver that receives the EMS  106  and provides characteristic data  128 . In some implementations, the distribution of received EMS  106  signal amplitude with respect to feet (such as greater signal strength at the toe than the heel) may be used to determine an approximate shape of the foot that is indicative of a particular user  112  or other object to be tracked. This data may be used instead of, or in conjunction with, the characteristic data  128  to generate the tracking data  136 . 
     An analysis module  138  may use the tracking data  136  to generate group data  140 . The group data  140  may comprise information that associates a plurality of users  112  as belonging to a common group or having a common affiliation. For example, members of a family within the facility may be deemed to be a group, members of the same picking crew may be members of a group, and so forth. In some implementations, the tile output data  126  may be processed to determine the group data  140 . For example, several users  112  may be holding hands or otherwise in physical contact with one another. As a result of this contact, the EMS  106  from a first SFT  104 ( 1 ) may be transferred through those users  112  to the receivers of the SFTs  104  beneath each of the other members of the group. By determining the presence of a plurality of users  112 , such as by multiple footprints detected by the sensors within the SFTs  104  that share a common EMS  106  encoding of the same characteristic data  128 , group data  140  may be determined. 
     The analysis module  138  may also generate interaction data  142 . The interaction data  142  is indicative of an action such as picking or placing an item at a particular fixture  108 , approaching but not touching an item stowed at the fixture  108 , presence of the user  112  at the fixture  108 , and so forth. For example, the analysis module  138  may use tracking data  136  to determine that a particular user  112  was in front of a particular fixture  108  at a time when that fixture  108  experienced a change in quantity of items stowed therein. Based on this correspondence, a particular user  112  may be associated with that change in quantity, and interaction data  142  indicative of this may be generated. 
     The analysis module  138  may also use the fixture data  132  or other data obtained from one or more sensors or other devices located at or near the fixture  108  to generate the interaction data  142 . In one implementation, the fixture  108  may include one or more receivers that are able to receive the EMS  106 . As the user  112  comes into contact with the item stowed at the fixture  108 , their body and the item itself provide a pathway for the EMS  106  to be transferred to an antenna located at the fixture  108 . As a result, use of the SFT  104  and the EMS  106  provides the additional benefit of unambiguously identifying an item that the particular user  112  interacted with. The analysis module  138  is configured to generate the interaction data  142  based on inputs including, but not limited to, the tile output data  126 , the fixture  108 , and so forth. 
     While  FIG. 1  depicts the floor  102  as being completely covered with SFTs  104 , in some implementations, only a portion of the floor  102  may include SFTs  104 . For example, SFTs  104  may be placed down an aisle and not underneath the fixtures  108 . In another example, the SFTs  104  may be deployed in front of the fixtures  108 . 
     The inventory management system  130  may access data from other sensors within the facility. For example, image data may be obtained from a plurality of cameras located within the facility. Various image processing techniques may be used, such as object recognition, blob tracking, and so forth, to generate information from this image data. In some implementations, the image data may be processed by human operators. For example, a human operator may be presented with images as well as tracking data  136  to resolve an ambiguity or loss of tracking. 
       FIG. 2  illustrates an arrangement  200  of SFTs  104  and their respective segments, according to some implementations. 
     A portion of the floor  102  is depicted which is made up of several clusters  202 . A cluster  202  is a grouping of SFTs  104 . For example, the portion of the floor  102  depicted here includes 25 clusters  202 , each cluster  202  including 16 SFTs  104 . Each SFT  104  in turn may include one or more segments  204 . Continuing the example depicted here, each SFT  104  includes nine segments. In other implementations, the cluster  202  may include different numbers of SFTs  104 , each SFT  104  may include different numbers of segments  204 , and so forth. 
     In some implementations, segments  204  may comprise portions of a SFT  104  or may be discrete devices that are joined together to form a SFT  104 . For example, the segments  204  may be connected to one another, a backplane, wiring harness, and so forth, to form a SFT  104 . 
     The physical size of a cluster  202  may be determined in some implementations based on a maximum expected stride length of a user  112 . For example, a user  112  may be expected to have a stride length that is less than 3 feet while walking. If the SFTs  104  are 1 foot on each side, then the cluster  202  depicted here is 4 feet by 4 feet. Likewise, each segment  204  is 4 inches by 4 inches. In other implementations, other sizes of segments  204 , SFTs  104 , and clusters  202  may be used. Also, other shapes of segments  204 , SFTs  104 , and clusters  202  may be used. For example, the segments  204  may be triangular shaped, SFTs  104  may be rectangular, and so forth. 
     The SFT  104  transmits a tile signal  206  and one or more segment signals  208 . Together, these signals comprise the EMS  106  emitted by the SFT  104 . The tile signal  206  is transmitted at a first frequency that is representative of that particular SFT  104  within a particular cluster  202 . The segment signal  208  is transmitted at a second frequency that is different from the first and is representative of the particular segment  204  within a particular SFT  104 . 
     In this illustration, the particular frequency of a particular tile signal  206  is represented by a letter, such as “A”, “B”, “C”, and so forth, while the frequency of the segment signal  208  is represented by a number “1”, “2”, “3”, and so forth. For example, the tile frequencies  206  may begin at 40 kHz with 1 kHz spacing, resulting in “A” representing 40 kHz, “B” representing 41 kHz, “C” representing 42 kHz, and so forth. Continuing the example, the segment signals  208  may begin at 50 kHz with 1 kHz spacing, resulting in “1” representing 50 kHz, “2” representing 51 kHz, and so forth. This notation and these frequencies are provided by way of illustration and not necessarily as limitations. 
     The EMS  106  as emitted may exhibit sinusoidal waveforms. In other implementations, other waveforms such as square, triangle, sawtooth, and so forth, may be used. Use of sinusoidal waveforms may allow for reduced channel spacing and minimize adjacent channel interference. The EMS  106  may be transmitted at fixed carrier frequencies of between 20 kilohertz and 15 megahertz. In other implementations, other frequencies may be used. 
     In some implementations, each SFT  104  may utilize the same spatial arrangement of segments  204 . For example, the SFTs  104  in the floor  102  may have the same arrangement of segments  204 , such as beginning at the top left of the SFT  104  with segment  1  and increasing from left to right and into subsequent rows, such as in SFT  104 (A). 
     In other implementations, such as depicted here, the SFTs  104  may be arranged such that adjacent segments  204  of adjacent SFTs  104  use the same frequencies. For example, the SFTs  104  may be arranged such that a first physical arrangement of segments  204  and their respective segment signal  208  frequencies for a first SFT  104 (A) are mirrored in a second SFT  104 (B) that is adjacent to the first SFT  104 (A). In this configuration, immediately adjacent segments  204  utilize the same segment signal  208  frequencies. Continuing the example, SFT  104 (A)&#39;s upper rightmost segment  204  is using segment signal  208  frequency “3”, while SFT  104 (B)&#39;s upper leftmost segment  204  that is immediately adjacent is also using segment signal  208  frequency “3”. 
     Use of this mirrored arrangement may improve performance of the system by producing an increase in the total amplitude of the segment signals  208  in situations where a user  112  or other object has a foot in contact with two different SFTs  104 . This arrangement may also provide additional benefits with regard to computing the location of an object, such as the position of the feet of the users  112 . 
     In this illustration, three users  112  are depicted. The left foot of user  112 ( 1 ) is above the following SFTs  104  and their respective segments:  104 (E)( 2 ),  104 (E)( 3 ),  104 (I)( 2 ),  104 (I)( 3 ),  104 (I)( 6 ),  104 (J)( 3 ), and  104 (J)( 6 ). The right foot of user  112 ( 1 ) is above the following SFTs  104  and their respective segments:  104 (B)( 5 ),  104 (B)( 8 ),  104 (F)( 8 ), and  104 (F)( 9 ). Also shown are the feet of users  112 ( 2 ) and  112 ( 3 ) at other locations within the cluster  202 . A representation of the characteristic data  128  associated with the first user  112 ( 1 ) is depicted below with regard to  FIG. 5 , while the characteristic data  128  associated with the second user  112 ( 2 ) is depicted below with regard to  FIG. 6 . 
     The SFTs  104  may be configurable, such that they may be installed and then configured to provide a particular segment signal  208  at a particular frequency after physical installation of the SFT  104 . For example, SFT  104 (A) may be electronically switched to provide segment signals  208  in the pattern shown with SFT  104 (P). 
       FIG. 3  illustrates the arrangement  300  of components included in a SFT  104 , according to some implementations. A side view of a portion of the SFT  104  depicts a top layer comprising a protective material, such as flooring material  302 . The flooring material  302  is electrically non-conductive under ordinary conditions. For example, the flooring material  302  may include plastic, ceramic, wood, textile, or other material. Beneath a layer of flooring material  302  may be one or more antennas  304  and one or more sensors  306 . The antennas  304  may comprise structures designed to accept or emit radio frequency energy. In some implementations, the antennas  304  may also serve as the flooring material  302 . For example, the antennas  304  may comprise aluminum or steel sheets upon which the users  112  walk. The active portion of the antenna  304  comprises that portion of the antenna  304  that is used to radiate or receive an EMS  106 . 
     The SFT  104  may include a plurality of antennas  304 . For example, the antennas  304  may be arranged to form an array. In some implementations, the active portion of the antennas  304  may have a surface area that occupies at least 1 square inch. Each segment  204  includes at least one segment antenna  304 . The segment antenna  304  of the segment  204  may be the same size as the segment  204  or may be smaller. For example, the segment  204  may be 4 inches by 4 inches square, but the segment antenna  304  in that segment  204  may only be 2 inches by 2 inches square. In another example, the segment  204  may be 4 inches by 4 inches square and the segment antenna  304  in that segment  204  may be 4 inches by 4 inches square. In one implementation, antennas  304  may be shared, with a single antenna being used to both transmit and receive simultaneously or at different times. In another implementation, separate antennas  304  may be used to transmit and receive. 
     The SFT  104  may also include a plurality of sensors  306 , and the sensors  306  may also be arranged to form an array. For example, the sensors  306  may include weight sensors that measure the weight applied to a particular segment  204 . The sensors  306  provide sensor output data. The arrangement of these two arrays may differ from one another. In some implementations, the sensors  306  may include a magnetometer that provides information about local magnetic fields. 
     As illustrated here, the antennas  304  may be located within a common plane. In other implementations, the antennas  304  may be arranged within a layer that is above the sensors  306 , below the sensors  306 , and so forth. A load bearing support structure  308  may be beneath the sensors  306  and the antennas  304  and provides mechanical and physical separation between the underlying subfloor  310  upon which the SFT  104  rests and the flooring material  302 . The support structure  308  may comprise a series of pillars, posts, ribs, or other vertical elements. The support structure  308  may comprise a composite material, plastic, ceramic, metal, or other material. In some implementations, the support structure  308  may be omitted, and electronics  312  or structures associated with the electronics  312  may be used to support a load on the flooring material  302 . For example, the electronics  312  may comprise a glass fiber circuit board that provides mechanical support while also providing a surface for mounting the electronics  312 . The subfloor  310  may comprise concrete, plywood, or existing flooring materials over which the SFT  104  is installed. In some implementations, the SFT  104  may be affixed to the subfloor  310 , or may be unaffixed or “floating”. For example, the SFT  104  may be adhered to the subfloor  310  using a pressure sensitive adhesive. 
     The SFT  104  includes the electronics  312 . The electronics  312  may include the elements described elsewhere in more detail. In the implementation depicted here, electronics  312  are arranged within the support structure  308 . In some implementations, one or more of the antennas  304  or the sensors  306  may be located within the support structure  308 . The support structure  308  may operate as a heat sink to dissipate heat generated by operation of the electronics  312 . 
     The SFT  104  may incorporate a wiring recess  314  on an underside of the SFT  104 . For example, the support structure  308  and the electronics  312  may be formed or arranged to provide a pathway for a wiring harness  316  to pass beneath at least a portion of the SFT  104 . The wiring recess  314  may extend from one edge of the SFT  104  to another, may extend in different directions, and so forth. For example, the wiring recess  314  may be arranged in a “+” or cross shape, allowing for wiring harnesses  316  to pass along the X or Y axes as depicted here. 
     The wiring harness  316  may provide a coupling to one or more of the power supply  118 , the network  120 , and so forth. For example, the wiring harness  316  may include conductors that allow for the SFT  104  to receive electrical power from an electrical distribution network, allow for connection to a Controller Area Network (CAN) bus network that services a cluster  202  of SFTs  104 , and so forth. The wiring harness  316  may include electrical conductors, electromagnetic waveguides, fiber optics, and so forth. In some implementations, a plurality of wiring harnesses  316  may be used. For example, a first wiring harness  316 ( 1 ) may provide electrical power while a second wiring harness  316 ( 2 ) provides network connectivity. In some implementations, the wiring harness  316  may be used to provide information used to determine a relative arrangement of SFTs  104 . 
     The electronics  312  of the SFT  104  may include a power supply  318 . The power supply  318  may include an electric power interface that allows for coupling to the power supply  118 . For example, the electrical power interface may comprise connectors, voltage converters, frequency converters, and so forth. The power supply  318  may include circuitry that is configured to provide monitoring or other information with regard to the consumption of electrical power by the other electrical power components of the SFT  104 . For example, the power supply  318  may include power conditioning circuitry, DC to DC converters, current limiting devices, current measurement devices, voltage measurement devices, and so forth. In some implementations, the SFT  104  may be configured to connect to redundant power buses. For example, a first electrical distribution network such as an “A” bus and a second electrical distribution network such as a “B” bus may be provided, each of which can provide sufficient electrical power for operation. In some implementations, the SFT  104  may incorporate redundant power supplies  318 . 
     The SFT  104  may include one or more hardware processors  320 . Hardware processors  320  may include microprocessors, microcontrollers, systems on a chip (SoC), field programmable gate arrays (FPGAs), and so forth. The SFT  104  may also include one or more memories  322 . The memory  322  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  322  provides storage of computer-readable instructions, data structures, program modules, and other data for the operation of the SFT  104 . 
     The SFT  104  may include sensor electronics  312 . The sensor electronics  312  may be configured to acquire information from sensors  306 . In one implementation, the sensors  306  may comprise electrodes or other electrically conductive elements that are used as part of a capacitive sensor array. In one implementation, the electrodes may be arranged in an array. Each electrode may be rectangular with a first side and a second side, with the length of the first side and the second side being between 10 millimeters and 50 millimeters. In other implementations, other shapes and sizes may be used. 
     The sensor electronics  312  may include capacitive measurement circuitry that generates capacitance data. The capacitance measurement circuitry may use various techniques to determine capacitance. For example, the capacitance measurement circuitry may include a source that provides a predetermined voltage, a timer, and circuitry to measure voltage of the conductive element relative to the ground. By determining an amount of time that it takes to charge the conductive element to a particular voltage, the capacitance may be calculated. The capacitance measurement circuitry may use one or more of analog or digital circuits to determine capacitance. During operation, the capacitive sensor uses a conductive element located beneath the flooring material  302  to produce capacitance data indicating capacitance values at particular times. Based on the capacitance data, information such as a presence of an object, shape of an object, and so forth, may be generated to produce sensor output data  324 . The sensor electronics  312  may be configured to scan the sensors  306  and generate sensor output data  324  at least 30 times per second. The sensor output data  324  may include information about proximity of an object with respect to a particular electrode. The sensor output data  324  may be further processed to generate the other data  330 . 
     In other implementations, the sensors  306  may comprise optical touch sensors comprising one or more illuminators and one or more photodetector elements, resistive touch sensors comprising electrically resistive material, acoustic touch sensors comprising one or more transducers, and so forth. The sensors  306  may include other sensors, such as weight sensors, moisture detectors, microphones, and so forth. 
     The SFT  104  may include a receiver  326 . The receiver  326  is configured to detect the EMS  106 . The receiver  326  may be implemented as discrete circuitry, as a software defined radio (SDR), and so forth. The receiver  326  is coupled to one or more of the antennas  304 . In some implementations, a single receiver  326  may be coupled to a single antenna  304 . In other implementations, a single receiver  326  may be coupled to a plurality of antennas  304  by way of switching circuitry, matching network, and so forth. The switching circuitry may allow the selective connection of a particular antenna  304  to the receiver  326 . The receiver  326  may be configured to detect the EMS  106  at a particular frequency and generate information indicative of a received signal strength. 
     In some implementations, elements of the sensors  306  may be combined or used in conjunction with the antennas  304 . For example, electrically conductive elements may be used for both capacitive sensing by the sensor  306  and as antennas  304 . This dual use may occur at the same time or may be multiplexed over time. For example, switching circuitry may, at a first time, selectively connect the sensor electronics  312  to the electrically conductive element for use as a capacitive sensor pad. The switching circuitry may then selectively connect, at a second time, the receiver  326  to the same electrically conductive element for use as an antenna  304 . 
     The EMS  106  is acquired by the antenna  304  and then provided to the receiver  326 . For example, the receiver  326  may comprise a superheterodyne receiver, with an incoming radio signal being converted to an intermediate frequency by a mixer. At the intermediate frequency stage, the downconverted signal is amplified and filtered before being fed to a demodulator. One or more antennas  304  may be dedicated for use by the receiver  326 , while one or more other antennas  304  may be dedicated for use by the transmitter(s)  328 . The use of separate antennas to transmit and receive may improve isolation between the receiver  326  and the transmitter  328 . The receiver  326  or the hardware processor  320  processes the EMS  106  to determine the characteristic data  128 , such as a received frequency and the signal strength received at that frequency. In another implementation, the receiver  326  may comprise a SDR. 
     In some implementations, the EMS  106  may encode data. The receiver  326  or the hardware processor  320  decode, decrypt, or otherwise demodulate and process the demodulated signal to determine the characteristic data  128 . For example, the receiver  326  may provide as output the digital representation of a signal that incorporates binary phase shift keying (BPSK). The hardware processor  320  may process this digital representation to recover a serial data stream that includes framing, error control data, payload, and other information. The payload may then be processed to produce output. The error control data may include error detection data such as parity check data, parity bits, hash values, and so forth. For example, a hash function may be applied to the characteristic data  128  to generate hash output. A comparison of the hash output may be made to determine if an error is present. 
     The SFT  104  includes one or more transmitters  328 . For example, the transmitter  328  may comprise a voltage controlled oscillator that generates an output signal that is fed directly to a power amplifier. The transmitter  328  couples to an antenna  304 , which then radiates the EMS  106 . The transmitter  328  may be implemented as discrete circuitry, SDR, or combination thereof. 
     The transmitter  328  may accept multiple signals to generate the EMS  106  that is emitted from an antenna  304  connected to the transmitters  328  output. For example, a mixer may be used to combine a tile signal  206  with a segment signal  208  to produce the EMS  106  that is transmitted from a particular segment  204  of the SFT  104 . In some implementations, each segment  204  may utilize a single transmitter  328  that produces an EMS  106  that includes at least the segment signal  208 . In other implementations, a single transmitter  328  may be used to generate all of the EMS  106  from a given SFT  104 . For example, the transmitter  328  may generate the tile signal  206  and all the respective segment signals  208  for that SFT  104 . Filters may be used on the output such that the antenna  304  at a particular segment  204  emits only the desired tile signal  206  and the segment signal  208  for that particular segment  204 . 
     The transmitter  328  may be configured to produce a phase modulated output signal. The transmitters  328  for the SFTs  104  in a given floor  102  may operate on a single frequency, or may be frequency agile and operate on a plurality of different frequencies. For example, a single transmitter  328  may generate, at different times, the tile signal  206  and the segment signals  208 , transitioning in rapid succession between producing signals at these different frequencies. In some implementations, the receiver  326  and the transmitter  328  may be combined or share one or more components. For example, the receiver  326  and the transmitter  328  may share a common oscillator or frequency synthesizer. 
     In some implementations, a single antenna  304  may be used to both transmit and receive. For example, the receiver  326  may include notch filters to attenuate the frequencies of the transmitted tile signal  206  and the transmitted segment signal  208  for that segment  204 . A single antenna  304  may also be used to transmit different signals. For example, a single antenna  304  may be used to transmit the tile signal  206  and a segment signal  208 . In some implementations, a diplexer may be used that accepts input from two or more transmitters  328  and provides output of the EMS  106  to an antenna  304  or group of antennas  304 . In other implementations, the diplexer or other filtering may be omitted and one or more transmitters  328  may be coupled to a single antenna  304  or group of antennas  304 . 
     The hardware processor  320  may acquire data from one or more of the sensors  306 , the receiver  326 , transmitter  328 , and so forth, to generate other data  330 . The other data  330  comprises information about an object that is resting on or proximate to the flooring material  302 . The information may be indicative of a shape of the object. In some implementations, the other data  330  may comprise information that is representative of the contours of an object. For example, the other data  330  may comprise a bitmap representative of the output from a plurality of sensors  306  and indicative of their relative arrangement. In another example, the other data  330  may comprise a vector value that is indicative of polygons used to represent an outline of an object. In some implementations, the other data  330  may be indicative of an area of the object. For example, the other data  330  may indicate that the total area of an object is 48 square centimeters. The other data  330  may include other information such as information about amplitude of a received EMS  106  with respect to different portions of the object. For example, other data  330  may be generated that indicates the shape of the object with information about amplitude, frequency, or other details about the EMS  106  at particular points or areas within that shape. 
     In some implementations, one or more of the receiver  326  or the transmitter  328  may be used to generate the sensor output data  324 . For example, sensors  306  may communicate with the power supply  318  to determine the amount of electrical current that is being drawn at a particular time by the transmitter  328 . As the electrical coupling between an object above the SFT  104  and one or more of the antennas  304  changes, one or more operating characteristics of the devices in the SFT  104  may change. For example, the impedance of the antenna  304  may experience change. Changes in the impedance may result in a change in the power output of the transmitter  328  during operation. For example, the transmitter  328  may exhibit an impedance mismatch with the antenna  304  in the presence of an object, such as a foot. This impedance mismatch may result in reduced power consumption by the radio frequency amplifier of the transmitter  328 . Information about changes in the operational characteristics, such as a change in current draw by the transmitter  328 , may be processed to determine the presence or absence of an object with respect to the antennas  304 . The operating characteristics may include, but are not limited to: received signal strength at the receiver  326 , power consumption of the transmitter  328 , radio frequency power output of the transmitter  328 , impedance presented at an antenna  304 , standing wave ratio (SWR), and so forth. For example, the impedance of the antenna  304  may be measured at a radio frequency input to the receiver  326 , a radio frequency output of the transmitter  328 , and so forth. In another example, the SWR presented by one or more of the antennas  304  may be similarly measured. In other implementations, other operating characteristics may be used. For example, a change in the noise detected by the receiver  326  may be used to determine presence or absence of an object. In yet another implementation, the transmitter  328  of the SFT  104  may generate a signal that is then received by the receiver  326  of the same SFT  104 . A change in the received signal at a particular antenna  304  may be used to determine the presence of an object. In still another implementation, the EMS  106  received from the other SFT  104  may be measured, and the received signal strength at particular segments  204  may be used to generate information indicative of the presence of an object. 
     By combining information from a plurality of antennas  304 , other data  330  may be generated. In other implementations, other characteristics of the receiver  326  or the transmitter  328  may be assessed to generate the other data  330  or other information indicative of proximity of an object to the antenna  304 . For example, the change in impedance may be measured, a change in background noise level may be measured, and so forth. In some implementations, radio ranging may be utilized in which the transmitter  328  emits a pulse and the receiver  326  listens for a return or echo of that pulse. Data indicative of proximity from several antennas  304  may then be processed to generate the other data  330 . In another implementation, distance between the object and the antenna  304  may be determined using the amplitude of the received EMS  106 . For example, a lookup table may be used that associates a particular received signal strength with a particular distance from the antenna  304 . 
     The communication interface  332  connects the SFT  104  to the network  120 . For example, the communication interface  332  may be able to connect to one or more of a Controller Area Network (CAN bus), Inter-Integrated Circuit (I2C), Serial Peripheral Interface bus (SPI), 1-Wire bus, Universal Serial Bus (USB) as promulgated by the USB Implementers Forum, RS-232, Ethernet, Wi-Fi, Bluetooth, and so forth. The communication may be facilitated by data connectors, such as optical connectors, electrical connectors, and so forth. The data connectors provide a pathway for signals to be exchanged between the communication interface  332  and the network  120 . 
     The SFT  104  may include non-transitory computer readable media that is used to store instructions, data, and so forth. Tile identifier data  334  comprises information indicative of a particular SFT  104 . The tile identifier data  334  may be unique within the particular network  120 , the facility, unique across the production of all SFTs  104  manufactured, and so forth. In some implementations, a media access control (MAC) address, network address, bus address, and so forth, that is associated with the communication interface  332  may be used as tile identifier data  334 . 
     During operation, the hardware processor  320  may generate tile output data  126 . As described above, the tile output data  126  may include the characteristic data  128 . In some implementations, the tile output data  126  may indicate the characteristic data  128  that was received by the SFT  104 , the particular antennas  304  or segments  204  associated with that reception, information about the frequencies of EMS  106  that are being transmitted, and so forth. The tile output data  126  may also include the tile identifier data  334 , timestamp data, and so forth. For example, the timestamp data included in the tile output data  126  may indicate when the characteristic data  128  was received by the receiver  326 . 
     The SFT  104  may include multiple hardware processors  320  with different capabilities. For example, individual segments of the sensors  306  may utilize dedicated state machines to perform simple processing functions. These dedicated state machines may then send output data to a microcontroller that provides additional processing to generate sensor output data  324 . In one implementation, the dedicated state machine may comprise a complex programmable logic device (CPLD). Continuing the example, a dedicated state machine may provide a 4 bit value indicative of the capacitance measured by a capacitive sensor  306  at a particular segment. The microcontroller may have information that describes a relative arrangement of the sensors  306 , and may use this information in conjunction with the dedicated state machine output to generate a bitmap that may be included in the other data  330 . 
     Various techniques may be used to increase the overall uptime of an individual SFT  104 , and functionality of the floor  102  as a whole. In one implementation, the SFT  104  may include additional components to provide for failover redundancy. For example, the SFT  104  may include at least two hardware processors  320 , each of which is able to generate other data  330 , generate tile output data  126 , and so forth. In another example, the SFT  104  may include two power supplies  318 , each connected to a different bus or power supply  118 . 
     To provide additional redundancy, adjacent SFTs  104  may be connected to different networks  120 . For example, an SFT  104  may be connected to a first network  120 ( 1 ) while the SFT  104  immediately to the right may be connected to a second network  120 ( 2 ). 
     The SFT  104  may be configured to perform diagnostics of onboard components, adjacent SFTs  104 , and so forth. For example, the SFT  104  may be configured to test the receiver  326  and the transmitter  328  by transmitting a signal from the first antenna  304 ( 1 ) and listening with the receiver  326  with a second antenna  304 ( 2 ) that is adjacent to the first antenna  304 ( 1 ). In some implementations, the SFT  104  may be configured to send diagnostic data using the network  120 . For example, diagnostic data may be sent to the inventory management system  130  indicating that a particular SFT  104  has a fault and requires repair or replacement. The SFT  104  may be designed in a modular fashion to allow for repair or replacement without affecting adjacent SFTs  104 . 
     In some implementations, operation of the SFT  104  or the segments  204  therein may be responsive to presence or absence of an object. For example, segments  204  that are proximate to or underneath the object forming a shape may be deemed active segments. Antennas  304  associated with these active segments may be used transmit or receive the EMS  106 . Inactive segments comprise segments  204  that are not underneath or proximate to the shape. The determination of whether a segment  204  is active or not may be based at least in part on output from the sensor elements, antennas  304 , or other sensors. For example, a segment  204  may be deemed to be an active segment when the associated sensor element exhibits a capacitance value that exceeds a threshold level. 
     During operation, the determination of which segments  204  are active may be used to determine which antennas  304  are used to one or more of transmit or receive the EMS  106 . For example, the antennas  304  beneath inactive segments may be disconnected from receivers  326 , or the receivers  326  associated with those antennas  304  may be placed in a low power mode or turned off. As an object is detected by the sensor element as driven using the sensor electronics  312 , a particular segment  204  may be designated as an active segment. In this illustration, the active segments are represented with a crosshatch pattern. The antenna  304  and associated radio frequency elements such as the receiver  326  and the transmitter  328  associated with that antenna  304  may be transitioned to an operational mode. Continuing the example, the receiver  326  may begin listening for an EMS  106 . 
     As described above, the SFT  104 , or portions thereof such as segments  204 , may transition from a receive mode to a transmit mode or vice versa. This transition may be responsive to the detection of an object by the sensor  306 . For example, the presence of an object followed by the absence of the object may result in the SFT  104  transitioning from the transmit mode to the receive mode. 
     By selectively transmitting the EMS  106  using antennas  304  that are within a threshold distance of the shape as determined by the sensors  306 , performance of the system may be improved. For example, power consumption may be reduced by transmitting using only those antennas  304  that are proximate to the object producing the shape. In other implementations, the transmitters  328  may be activated on a particular schedule, such as transmitting for 50 milliseconds duration with a gap waiting time of 100 ms before the next transmission. This reduction in duty cycle decreases power consumption. 
     In some implementations, segments  204  may be in transmit mode while the receiver  326  is still active. For example, the transmitters  328  may transmit while the receiver  326  is listening. 
     The sensors  306  in the SFT  104  may be used to determine the presence of hazardous conditions at the SFT  104 . For example, the sensors  306  may be able to detect a liquid that is present on the flooring material  302  that may comprise a slipping hazard. Continuing the example, a puddle of water on the flooring material  302  may be detected. Information indicative of the puddle may be provided to the inventory management system  130  for mitigation, such as clean up. In another example, the sensors  306  may be able to detect a user  112  lying on the flooring material  302 . Upon such detection, an attendant of the facility may be alerted to provide assistance to the user  112 . With this example, the floor  102  provides information to the operators the facility that may be used to improve the safety of the facility for the users  112 . 
       FIG. 4  illustrates at  400  the mixing of EMS  106  transmitted simultaneously by the SFTs  104 , according to some implementations. The body of the user  112 , or another object proximate to the antenna  304 , may electromagnetically couple to the antenna  304 . This electromagnetic coupling may include, but is not limited to, capacitive coupling, electrostatic coupling, inductive coupling, and so forth. In other implementations, other types of coupling may take place. Once coupled, a signal path  402  is provided that incorporates the body of the user  112 , their clothing, other users  112  they are in contact with, and so forth. 
     As described above and illustrated here, each SFT  104  and the respective segments  204  thereof are transmitting signals at particular frequencies. Each segment  204  emits from its respective one or more antennas  304  both the tile signal  206  and the respective segment signal  208 . In this simplified example, each SFT  104  includes two segments  204 . The user  112  is standing with a left foot on SFT  104 (K) at segment  204 ( 6 ), and thus the body of the user acts as the signal path  402  of the EMS  106  to a right foot on the SFT  104 (L) at segment  204 ( 6 ), and vice versa. Each of the SFTs  104  produces tile output data  126  that is indicative of the tile identifier data  334  of the receiving SFT  104 . 
     The SFT  104 (K) produces tile output data  126 ( 21 ) that is indicative of the various signals received by the SFT  104 (K) at the different frequencies and their received signal strength, while the tile output data  126 ( 22 ) is indicative of the various signals received by the SFT  104 (L). With the characteristic data  128 , the combination of the various frequencies presented and their received signal strength provides information as to the placement of the foot with respect to a SFT  104 . As the foot of the user  112  rests across different segments  204 , and possibly different SFTs  104 , it electromagnetically couples to the antennas  304  therein. 
     The tile output data  126  thus provides information about the location of a foot. Given the exchange of EMS  106  from one SFT  104  to another, a pair of SFTs  104  (or locations therein) may be determined. Given the reciprocity of the exchanges of EMS  106  and the resulting characteristic data  128 , the two feet may be associated with a single user  112 . In some implementations, a location of the user  112  may be determined to be between the feet locations. For example, the tracking module  134  may generate tracking data  136  that indicates the location of the user  112 ( 1 ) is at a midpoint between their left and right footprints. 
     The tracking module  134  may utilize certain assumptions or rules in the determination of a location of the user  112 . For example, the user  112  may be assumed to have two feet, the feet may be assumed to have a minimum length of 4 inches but less than 20 inches, and so forth. The tracking module  134  may also utilize data about the physical layout of the facility. For example, the physical arrangement of the SFTs  104  with respect to one another, the arrangement of the segments  204  therein, and so forth, may be used. 
       FIG. 5  illustrates a graph  500  of combined received signal characteristic data  128  for the first user  112 ( 1 ) shown in  FIG. 2 . The characteristic data  128  may be generated using data obtained by a receiver  326  in an SFT  104  or a fixture  108 , according to some implementations. 
     In this illustration, the EMS  106  that are combined and propagated through the signal path  402  of the user  112 ( 1 ) are shown. For example, the graph  500  may result from the combination of characteristic data  128  obtained from the SFTs  104  indicated. 
     Along a horizontal axis are bins indicative of the different frequencies  502  in use in a cluster  202  while a vertical axis indicates the received signal strength  504  of the signals in the respective bins. For example, the frequency  502  depicts 25 bins including sixteen tile frequencies  506  A-P for tile signals  206  and nine segment frequencies  508  for segment signals  208 . The received signal strength  504  may be indicative of a maximum value for all signals received for that frequency  502  bin, a cumulative signal strength that comprises a sum of the received signal strengths received for that frequency  502  bin, an average signal strength of the values of received signal strengths for that frequency  502  bin, and so forth. For example, the cumulative signal strength may comprise a sum of signal strength values. Continuing the example here, the left foot of the user  112 ( 1 ) may be resting on three different segments. The SFTs  104  and their received signal strength values may be as follows:  104 (E)( 3 ) received signal strength  7 ,  104 (I)( 3 ) received signal strength  10 ,  104 (J)( 3 ) received signal strength  5 . The cumulative signal strength may be 22. 
     The characteristic data  128  is depicted here in graphical format, but it is understood that the characteristic data  128  may be represented using various data structures including, but not limited to tables, linked lists, delimiter separated values, serialized data, and so forth. 
       FIG. 6  illustrates a graph  600  of combined received signal characteristic data  128  for the second user  112 ( 2 ) shown in  FIG. 2 . The characteristic data  128  may be generated using data obtained by a receiver  326  in an SFT  104  or a fixture  108 , according to some implementations. As above with respect to  FIG. 5 , a horizontal axis is indicative of frequency  502  bins while the vertical axis is indicative of the received signal strength  504 . The tile frequencies  506  and the segment frequencies  508  are also shown. 
     Note the differences that are apparent in the characteristic data  128 ( 1 ) of the first user  112 ( 1 ) shown in  FIG. 5  and the characteristic data  128 ( 2 ) of the second user  112 ( 2 ) shown in  FIG. 6 . For example, the characteristic data  128 ( 1 ) shows a tile signal  206 (B) while the characteristic data  128 ( 2 ) shows no such signal. The configuration of the received signal strengths  504  for the tile frequencies  506  are thus different. Also, because the users  112  are standing on different portions of their respective SFTs  104 , each user  112  exhibits different arrangements of segment frequencies  508 , as well as tile frequencies  506 . 
     By processing the characteristic data  128 , a location for each foot may be obtained. For example, based on the characteristic data  128 ( 2 ), the user  112 ( 2 ) is determined to be standing on SFTs  104 (C), (D), and (H). Given the distribution of the segment frequencies  508  and the known arrangement of the SFTs  104  and segments  204  with respect to one another, the relative positions of the left and right feet of a user  112  may be reconstructed. 
     In one implementation, another frequency allocation scheme may be used. For example, each segment  204  in the cluster  202  may emit a segment signal  208  that is distinct within the cluster  202 , and the tile signal  206  may be omitted. Continuing the example, given the sixteen SFTs  104  with their respective nine segments, 144 different frequencies may be used in a cluster  202 . 
     In another implementation, segments  204  may be omitted and each SFT  104  within a cluster  202  may emit a tile signal  206  that is distinct within the cluster  202 . For example, given the sixteen SFTs  104  in a cluster  202 , sixteen different frequencies may be used. 
       FIG. 7  illustrates tracking  700  of a user  112  as they move across the SFTs  104 , according to some implementations. At  702 , the user  112  is shown at a first time=0. As described above, based on the tile output data  126 ( 11 ), a first location of the user  112  is determined as being between SFT  104 (M) and SFT  104 (J) based on the location of the left and right feet of the user  112 . At  704 , the user  112  is shown at a second time t=1. A second location of the user  112  is determined as being between SFT  104 (J) and SFT  104 (E). A time series of these user locations may be used to describe the user path  114 . As described above, if the entry  110  involves identification, authentication, or other functions, this identity may be asserted with the user  112  as they move throughout the facility  102  along the user path  114 . 
       FIG. 8  illustrates the use  800  of a portable receiver  802  to detect the signals transmitted by the SFTs  104 , according to some implementations. The user  112 , a tote  116 , or other object may be equipped with the portable receiver  802 . The portable receiver  802  may be electromagnetically coupled to the user  112  and is configured to receives the EMS  106  and generate data such as the characteristic data  128 , an identifier of the portable receiver  802 , a timestamp, and so forth. For example, the portable receiver  802  may include a communication interface such as a Wi-Fi or Bluetooth compliant network interface that allows for wireless exchange of data with another computing device. The portable receiver  802  may be acquire the characteristic data  128  and send this characteristic data  128  to a server or other computing device. The portable receiver  802  may be associated with a particular user account, such as that of an associate or affiliate of the facility. The portable receiver  802  may obtain the characteristic data  128  such as shown in  FIG. 5 . The characteristic data  128  may be sent via the communication interface to a server that determines the user  112 ( 1 ) is located at a position centered on SFT  104 (F), segment ( 6 ). 
     In another implementation, the user  112  or other object may utilize a portable transmitter  804 . The portable transmitter  804  transmits an EMS  106  at one or more particular frequencies that will result in a receiver generating characteristic data  128  associated with that particular object. Similar to that described above, the system may use the characteristic data  128  to specifically identify one or more of a particular category or specific identity of a particular user  112 , tote  116 , or other object. For example, all totes  116  may be issued a portable transmitter  804  that emits a signal at 76 kHz. In another example, each tote  116  may have a different assigned frequency, such that tote  116 ( 1 ) has a portable transmitter  804  that emits at 78 kHz while another tote  116 ( 2 ) transmits at 81 kHz. As a result, the receiver(s)  326  of the SFTs  104  proximate to the wheels of the tote  116  detect the signal and produce tile output data  126  with characteristic data  128  showing the signal(s) emitted by the portable transmitter  804 . 
     The portable transmitter  804  may be provided in a variety of different form factors. For example, the portable transmitter  804  may comprise a device that may be mounted on the belt, worn as a wristband, a necklace, or a headband, attached to safety equipment worn by the user  112 , and so forth. In some implementations, the portable transmitter  804  may be incorporated into another device, such as a smartphone, point-of-sale terminal, and so forth. 
     Other information may be gathered with this configuration, or in the earlier configurations, without the portable transmitter  804 . For example, it may be determined which user  112  is in contact with a particular tote  116  based on the characteristic data  128  reported by their portable receiver  802 . In this example, the characteristic data  128  may include the EMS  106  emitted from the SFTs  104  under the tote  116  that are propagated via the tote  116  into the user  112 , the EMS  106  emitted by the portable transmitter  804 , and so forth. 
     In some implementations, the portable receiver  802 , portable transmitter  804 , and so forth, may be in communication with the inventory management system  130 . For example, these devices may communicate using Wi-Fi with an access point. In another example, data may be transferred using the SFTs  104 . Continuing this example, a signal may be transferred that encodes data which is then received by the receiver  326  in the floor  102 . Likewise, the transmitter  328  in the SFT  104  may send data to a receiver onboard the tote  116  or other device. 
     In some implementations, the functions of the portable receiver  802  and the portable transmitter  804  may be combined in a single device. For example, a portable transceiver may be configured to transmit EMS  106  and receive EMS  106 . 
     Fixed installations may also use these devices. For example, the components and functions of the portable receiver  802  may be incorporated into or associated with a fixed device, such as a door handle. When the user  112  touches the door handle, the EMS  106  propagated through their body from the SFT  104  to the door handle may provide characteristic data  128  that may be used to identify that user  112 . In another example, the components and functions of the portable transmitter  804  may be incorporated into or associated with a fixed device, such as a handrail. The handrail may emit the EMS  106  and a receiver, such as the portable receiver  802  or a receiver in the SFT  104 , may be used to provide characteristic data  128  that may be used to identify that user  112 . 
       FIG. 9  is an illustration  900  of the use of EMS  106  to determine a particular user  112  is interacting with a particular portion of fixture  108 , according to some implementations. 
     As described above, the fixtures  108  may be used to store one or more items  902 . As illustrated here, the fixture  108  includes items  902  stowed on four shelves  904 ( 1 ),  904 ( 2 ),  904 ( 3 ), and  904 ( 4 ). In other implementations, the fixture  108  may comprise racks, bins, hangers, and so forth. 
     As depicted here, a first SFT  104 ( 1 ) transmits a first combination of EMS  106 ( 1 ) along a signal path  402  of the body of the first user  112 ( 1 ), while a second SFT  104 ( 2 ) transmits a second combination of EMS  106 ( 2 ) along a signal path  402  of the body of the second user  112 ( 2 ). The first EMS  106 ( 1 ) conveys first characteristic data  128 ( 1 ), while the second EMS  106 ( 2 ) conveys second characteristic data  128 ( 2 ). As respective users  112  pick or place items  902  on one or more of the shelves  904 , their respective combinations of EMSs  106  are propagated along their respective bodies. The shelves  904  are equipped with one or more antennas  304  and one or more receivers  326  (not shown). In some implementation, shields or other arrangements of antennas  304  may be present to provide directionality to the patterns of the antennas  304 . The electronics of the shelves  904  generate the fixture data  132 . The fixture data  132  may comprise the characteristic data  128  of the EMS  106  that has been received by the shelf  904 . In some implementations, the fixture data  132  may include fixture identifier data indicative of a particular fixture  108  or portion thereof, a timestamp, and so forth. 
     The shelves  904  may include an array of antennas  304 , allowing for a determination of gesture data indicative of where the hand of the user  112  is relative to the fixture  108 , motion of the hand, and so forth. For example, each shelf  904  may include two antennas  304 , one on the left side and one on the right side. By analyzing the relative signal strength of the EMS  106  as conveyed by a signal path  402  from the foot of the user  112  to their hand as it is near or in contact with the shelf  904 , a position of the hand at a particular time may be determined. 
     By utilizing data from the antennas  304  and receivers  326  on different shelves  904 , information about the position of the hand in three-dimensional space may be determined. For example, antennas  304  on shelf  904 ( 1 ) and  904 ( 2 ) may be used to determine the position of the hand of the user  112  relative to those shelves  904 . 
     In some implementations, an antenna  304  may be located beneath the item  902 . As a result of the user  112  coming into contact with the item  902 , an increase in the amplitude of the EMS  106  as measured by the receiver  326  connected to the antenna  304  may be determined. Given predetermined information specifying that a particular type of item  902  is stowed on the shelf  904  proximate to the antenna  304 ( 1 ), based on the fixture data  132 , the inventory management system  130  is able to generate interaction data  142 . For example, an item  902  of the type “pet food” is assigned for stowage on shelf  904 ( 1 ) in a lane that is above antenna  304 ( 16 ). The fixture data  132  may indicate that the signal strength of one or more frequencies of the second EMS  106 ( 2 ) that conveyed the second characteristic data  128 ( 2 ) exceeded a threshold value. The amplitude of the signals as indicated in the second characteristic data  128 ( 2 ) is thus indicative of the user  112  coming into contact with the item  902 . Based on the particular characteristic data  128 , a particular user  112  may thus be associated with a particular user account, and the particular user  112  may be assessed a charge for the pick of the can of pet food. 
     Other sensors  306 , such as weight sensors, capacitive sensors, and so forth, may also be used. Data from these other sensors  306  may then be used in conjunction with the characteristic data  128  and information obtained from the receivers  326  about the EMS  106  to generate the interaction data  142 . The characteristic data  128  transferred by way of the EMS  106  to the antenna  304  in the shelf  904  may be used to determine who is picking what item  902 . A change in weight of the shelf  904  as measured by one or more weight sensors  306  may be used to determine the quantity of the items  902  that are either picked or placed. For example, the change in weight may be divided by a known weight of a sample of the item  902 . By using these techniques, the inventory management system  130  is able to quickly and inexpensively determine which user  112  interacted with a particular item  902 , the fixtures  108 , or portion thereof. 
     In some implementations, information about how the EMS  106  is propagated may be used to distinguish between one type of item  902  and another type of item  902  that the user  112  may be interacting with. For example, the same antenna  304  may service two lanes on the shelf  904 . In a first lane are stowed boxes of dried pasta, while the second lane stows metal cans of tomato sauce. The metal can provides a better signal pathway for the EMS  106  compared to the box of dried pasta. By analyzing the received signal strength of the EMS  106 , the user  112  coming into contact with the metal can may be distinguished from the user  112  coming into contact with the box of dried pasta. For example, if the received signal strength of the EMS  106  exceeds a threshold value, the contact may be determined to be with the metal can in the second lane. Similarly, if the received signal strength of the EMS  106  is below a threshold value, the contact may be determined to be with the box of dried pasta in the first lane. 
     As described above with regard to  FIG. 8 , in some implementations, the EMS  106  may be transmitted by portable transmitter  804 . In other implementations, the shelf  904  may emit one or more EMS  106  that are detected using the receiver  326  in the SFT  104  or the portable receiver  802 . 
     In other implementations, the same techniques may be used to determine if the user  112  is touching other objects in the environment. For example, the placement of a user&#39;s  112  hand with respect to a table or countertop may be determined. In another example, the techniques may be used to determine that a user  112  is touching a door handle, sitting in a chair, sitting on a bench, and so forth. 
       FIG. 10  illustrates an enlarged side view  1000  of the use of an EMS  106  to generate gesture data and other information indicative of which item  902  a user  112  interacted with at the fixture  108 , according to some implementations. 
     As described above, the shelves  904  or other fixtures  108  may incorporate one or more antennas  304  that are coupled to one or more receivers  326 . As a hand  1002  of the user  112  approaches the fixture  108 , antennas  304  may receive the EMS  106  as transmitted by a SFT  104 , portable transmitter  804 , and so forth. 
     Electronics  1004  associated with the shelf  904  recover the characteristic data  128  conveyed by the EMS  106 . The electronics  1004  may be similar to the electronics  312  described above with regard to the SFT  104 . For example, the electronics  1004  may include a power supply  318 , a receiver  326 , the hardware processor  320 , a communication interface  332 , one or more antennas  304 , and so forth. In some implementations, the electronics  1004  may include one or more transmitters  328 . 
     The hardware processor  320  may be configured to generate the fixture data  132 . The fixture data  132  may include one or more of characteristic data  128 , fixture identifier data  1006 , gesture data  1008 , and so forth. As described above, the characteristic data  128  comprises information that is conveyed by an EMS  106 . Fixture identifier data  1006  is used to identify a particular fixture  108  or portion thereof, such as a shelf  904 , lane upon the shelf  904 , and so forth. The gesture data  1008  may comprise information indicative of a location of the hand  1002  of the user  112  with respect to the fixture  108  or portion thereof, duration of contact by the hand  1002 , direction of movement of the hand  1002 , and so forth. The gesture data  1008  may be generated based on information about the EMS  106  obtained by one or more antennas  304 . For example, based on the changes over time of an amplitude or received signal strength of the EMS  106  at a given antenna  304 , a position of the hand  1002  or portion thereof may be determined. 
     The gesture data  1008  may include information such as a time series of a position of the hand  1002 . In some implementations, the gesture data  1008  may be used to generate trajectory data indicative of a trajectory of the hand  1002 . This trajectory may then be used to help determine which lane the user  112  is interacting with, disambiguate the user  112  from among several users  112  if the characteristic data  128  is unavailable, and so forth. 
     The gesture data  1008  may include information indicative of contact duration between the user  112  and the item  902 . For example, a contact threshold time may indicate a minimum amount of time that the user  112  has to be in contact with the item  902  before a contact is deemed to occur. The comparison of the contact duration and the contact threshold time may be used to reduce false positives, minimize the impact of noise, and so forth. In some implementations, the contact may also be determined at least in part by the received signal strength of the EMS  106  during contact. For example, contact may be determined when the received signal strength is above a threshold strength value. Contact may be determined when the contact duration exceeds the contact threshold time and the received signal strength is above the threshold strength value. 
     The gesture data  1008  may comprise a time series of coordinates, each set of coordinates indicating a position of the hand  1002  at successive times. The gesture data  1008  may provide coordinates in one, two, or three-dimensional spaces. For example, coordinates in a one-dimensional space for the gesture data  1008  may indicate where along the shelf  904  from left to right the hand  1002  is determined to be. In another example, coordinates in three-dimensional space for the gesture data  1008  may indicate where the hand  1002  is in terms of left to right, front to back and height above the shelf  904 . 
     To generate gesture data  1008 , the hand of the user  1002  does not necessarily need to be in contact with the portion of the fixture  108 . For example, proximity of the hand  1002  may be sufficient to allow for coupling between the hand  1002  and the antenna  304  that is sufficient to transfer the EMS  106 . 
     As described below in more detail with regard to  FIG. 11 , the fixture  108  may incorporate other sensors as well. 
     While  FIGS. 9 and 10  depict the EMS  106  as originating in the SFT  104 , in other implementations, the signal pathway may be reversed. For example, a transmitter  328  may be located at the shelf  904  that generates an EMS  106  associated with a particular type of item  902 . As the user  112  approaches and then grasps the item  902 , a signal path  402  may be provided that conveys the EMS  106  from the shelf  904  to a receiver in the SFT  104 . In other implementations, the EMS  106  may be produced by the portable transmitter  804 . 
       FIG. 11  depicts a block diagram  1100  of a fixture  108  such as a shelf  904  that is configured to generate gesture data  1008 , characteristic data  128 , and so forth, according to some implementations. A top view  1102  of a shelf  904  and a side view  1104  of an enlarged portion of the shelf  904  are depicted. 
     As shown in the top view  1102 , a plurality of conductive elements  1106  are distributed in rows and columns across the shelf  904  to form an array. The conductive elements  1106  may be planar and formed into shapes such as rectangles (as shown here). Arranged proximate to each of the four corners of the shelf  904  are weight sensors  1108 . The conductive elements  1106  may be configured for dual use as antennas  304  and as elements of a capacitive sensor array. In other implementations, other shapes and arrangements of the conductive elements  1106  may be used. 
     The conductive elements  1106  may be connected by wire or other electrical conductor. The wire transfers a capacitive signal  1110  between the conductive element  1106  and other circuitry, such as a switch module  1112 . The switch module  1112  may in turn connect to a capacitance measurement/receiver module  1114 . For example, the capacitive signal  1110  may be used to supply a charge to the conductive element  1106 . The capacitance measurement/receiver module  1114  determines a change in this charge over time and generates capacitance data  1116 . The capacitance measurement/receiver module  1114  may also generate the characteristic data  128  in some implementations. 
     The switch module  1112  may comprise switching circuitry that allows for the capacitance measurement/receiver module  1114  to be selectively connected to a particular conductive element  1106 . In some implementations, a plurality of switch modules  1112  may be used to allow for different switching configurations. For example, a first switch module  1112 ( 1 ) may have 4 outputs, each connecting to additional switch modules  1112 ( 2 ),  1112 ( 3 ),  1112  ( 4 ),  1112  ( 5 ). Each of those switch modules  1112 ( 2 )-( 5 ) may have 4 outputs in which each output is connected to additional switch modules  1112 , and so forth. The switching circuitry may comprise microelectromechanical switches, relays, transistors, diodes, and so forth. Other configurations or networks of switch modules  1112  may be implemented as well. 
     The capacitance measurement/receiver module  1114  may be used to generate the capacitance data  1116 . The capacitance data  1116  may include information such as a capacitance value, information indicative of a particular conductive element  1106 , timestamp, and so forth. In some implementations, circuitry or functionality of the switch module  1112  and the capacitance measurement/receiver module  1114  may be combined. The capacitance measurement/receiver module  1114  may also include a receiver  326  to allow for the reception of the EMS  106 . 
     A bottom plate  1118  may provide mechanical support for one or more of the conductive elements  1106 . In some implementations, the bottom plate  1118  may comprise an electrical conductor that acts as a shield for an electric field present at the conductive element  1106 . 
     A shelf top  1120  may be arranged atop one or more of the conductive elements  1106  and the bottom plate  1118 . One or more items  902  may rest on or above the shelf top  1120 . For example, the shelf top  1120  may comprise a non-conductive material such as a plastic or ceramic. 
     The conductive element  1106  may comprise one or more electrically conductive materials. The electrically conductive elements  1106  may be formed as one or more of a coating, thin-film, paint, deposited material, foil, mesh, and so forth. For example, the conductive element  1106  may comprise an electrically conductive paint, silver paste, aluminum film, a copper sheet, and so forth. The conductive element  1106  may be deposited upon, embedded within, laminated to, or otherwise supported by the bottom plate  1118 , the shelf top  1120 , and so forth. These conductive elements  1106  may then be connected to the capacitance measurement circuitry in the capacitance measurement/receiver module  1114 . 
     One or more shields  1122  may be provided. A shield  1122  may be adjacent to one or more of the conductive elements  1106 . The shield  1122  comprises an electrically conductive material and is separated by an electrical insulator, such as air, plastic, ceramic, and so forth, from the conductive element  1106 . A single shield  1122  may be used to provide shielding for one or more conductive elements  1106 . During operation, the shield  1122  may be driven at the same voltage potential of the input of the capacitive signal  1110 . In this configuration, there is no difference in electrical potential between the shield  1122  and the conductive element  1106 . External interference may then couple to the shield  1122  producing little interaction with the conductive element  1106 . The shield  1122  may also be used to direct the electric field produced by the conductive element  1106  during operation. For example, the electric field is directed generally away from the shield  1122 . Using this technique, the capacitive sensor may detect objects on the side opposite that of the shield  1122 , with the shield  1122  preventing the capacitive sensor from “seeing” or being affected by an object behind the shield  1122 . 
     The shelf  904  may include other layers or structures. For example, an electrical insulator  1124  such as polyethylene terephthalate may be arranged between the bottom plate  1118  and the shield  1122  (if present) or the conductive element  1106 . Wires, circuit traces, or other electrically conductive pathways may conduct the capacitive signal  1110  between the capacitance measurement/receiver module  1114  and the conductive element  1106 . 
     The bottom plate  1118  may be supported by one or more of the weight sensors  1108 . In some implementations, the bottom plate  1118  may comprise an electrically conductive material and act as a ground plane, such as if connected to an earth ground. The weight sensor  1108  may in turn be supported by a shelf support  1126 . 
     The one or more of the weight sensors  1108  may be connected to the weight sensor module  1128 . The weight sensor module  1128  may comprise circuitry that is used to generate the weight data  1130 . The weight data  1130  may include information such as a weight value, information indicative of a particular weight sensor  1108 , timestamp, and so forth. In some implementations, circuitry or functionality of the weight sensor module  1128  and the weight sensor  1108  may be combined. 
     One or more image sensors (not shown) may be used to acquire image data at or near the shelf  904  or other fixture  108 . The image data may comprise one or more still images, video, or combination thereof. The image sensor may have a field of view (FOV) that includes at least a portion of the shelf  904  or other type of fixture  108 . For example, a camera may be mounted within the shelf  904  to acquire image data of one or more lanes of items  902  on the shelf  904 . 
       FIG. 12  depicts a scenario  1200  showing the signal strengths as received using different antennas  304  at the fixture  108 , according to some implementations. In this scenario, a graph is depicted along with a corresponding schematic of the antennas  304  laid out on a shelf  904 . With regard to the graph, along a horizontal axis are bins indicative of shelf position  1202 . Along the vertical axis of the graph is the received signal strength  504  received at the particular shelf position  1202 . 
     Below the graph are the array of antennas  304  that may be positioned along the shelf  904 . In this scenario, the shelf  904  includes sixteen antennas  304  arranged side by side. In one implementation, each lane of the shelf  904  may be associated with a particular antenna  304 . A right hand  1002  of the first user  112 ( 1 ) is shown reaching towards a first antenna. A left hand  1002  of the second user  112 ( 2 ) is shown reaching towards a second antenna. As depicted by the graph above, the received signal strength  504  for the respective hands  1002  exhibit spikes at a first location  1204  and a second location  1206 . As described above, the received signal strength  504  may be for a particular frequency, group of frequencies, and so forth. 
     To distinguish between the hands  1002  detected at the first location  1204  and the second location  1206 , the characteristic data  128  may be assessed. For example, given where the respective users  112  are standing, they will exhibit particular set of characteristics or spectra that is the combination of the tile signals  206 , segment signals  208 , and so forth. 
       FIG. 13  is a block diagram  1300  illustrating a materials handling facility (facility)  1302  using the system  100 , according to some implementations. A facility  1302  comprises one or more physical structures or areas within which one or more items  902 ( 1 ),  902 ( 2 ), . . . , may be held. The items  902  may comprise physical goods, such as books, pharmaceuticals, repair parts, electronic gear, and so forth. 
     The facility  1302  may include one or more areas designated for different functions with regard to inventory handling. In this illustration, the facility  1302  includes a receiving area  1304 , a storage area  1306 , and a transition area  1308 . Throughout the facility  1302 , the plurality of SFTs  104  may be deployed as described above. 
     The receiving area  1304  may be configured to accept items  902 , such as from suppliers, for intake into the facility  1302 . For example, the receiving area  1304  may include a loading dock at which trucks or other freight conveyances unload the items  902 . In some implementations, the items  902  may be processed, such as at the receiving area  1304 , to generate at least a portion of item data as described below. For example, an item  902  may be tested at the receiving area  1304  to determine the attenuation of an EMS  106  passing through it, and this information stored as item data. 
     The storage area  1306  is configured to store the items  902 . The storage area  1306  may be arranged in various physical configurations. In one implementation, the storage area  1306  may include one or more aisles  1310 . The aisle  1310  may be configured with, or defined by, the fixtures  108  on one or both sides of the aisle  1310 . The fixtures  108  may include one or more of a shelf  904 , a rack, a case, a cabinet, a bin, a floor location, or other suitable storage mechanisms for holding, supporting, or storing the items  902 . For example, the fixtures  108  may comprise shelves  904  with lanes designated therein. The fixtures  108  may be affixed to the floor  102  or another portion of the structure of the facility  1302 . The fixtures  108  may also be movable such that the arrangements of aisles  1310  may be reconfigurable. In some implementations, the fixtures  108  may be configured to move independently of an outside operator. For example, the fixtures  108  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  1302  to another. 
     One or more users  112 ( 1 ),  112 ( 2 ), . . . ,  112 (U) and totes  116 ( 1 ),  116 ( 2 ), . . . ,  116 (T) or other material handling apparatus may move within the facility  1302 . For example, the user  112  may move about within the facility  1302  to pick or place the items  902  in various fixtures  108 , placing them on the tote  116  for ease of transport. The tote  116  is configured to carry or otherwise transport one or more items  902 . For example, the tote  116  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  1302  picking, placing, or otherwise moving the items  902 . For example, a robot may pick an item  902  from a first fixture  108 ( 1 ) and move the item  902  to a second fixture  108 ( 2 ). 
     One or more sensors  1312  may be configured to acquire information in the facility  1302 . The sensors  1312  may include, but are not limited to, weight sensors  1312 ( 1 ), capacitive sensors  1312 ( 2 ), image sensors  1312 ( 3 ), depth sensors  1312 ( 4 ), and so forth. The weight sensors  1312 ( 1 ) may comprise the same or different hardware as the weight sensors  1108  described above. The sensors  1312  may be stationary or mobile, relative to the facility  1302 . For example, the fixtures  108  may contain weight sensors  1312 ( 1 ) to acquire weight sensor data of items  902  stowed therein, image sensors  1312 ( 3 ) to acquire images of picking or placement of items  902  on shelves  904 , optical sensor arrays  1312 ( 14 ) to detect shadows of the user&#39;s  112  hands  1002  at the fixtures  108 , and so forth. In another example, the facility  1302  may include image sensors  1312 ( 3 ) to obtain images of the user  112  or other objects in the facility  1302 . The sensors  1312  are discussed in more detail below with regard to  FIG. 14 . 
     While the storage area  1306  is depicted as having one or more aisles  1310 , fixtures  108  storing the items  902 , sensors  1312 , and so forth, it is understood that the receiving area  1304 , the transition area  1308 , or other areas of the facility  1302  may be similarly equipped. Furthermore, the arrangement of the various areas within the facility  1302  is depicted functionally rather than schematically. For example, in some implementations, multiple different receiving areas  1304 , storage areas  1306 , and transition areas  1308  may be interspersed rather than segregated in the facility  1302 . 
     The facility  1302  may include, or be coupled to, the inventory management system  130 . The inventory management system  130  is configured to interact with one or more of the users  112  or devices such as sensors  1312 , robots, material handling equipment, computing devices, and so forth, in one or more of the receiving area  1304 , the storage area  1306 , or the transition area  1308 . 
     During operation of the facility  1302 , the sensors  1312  may be configured to provide sensor data, or information based on the sensor data, to the inventory management system  130 . The sensor data may include the weight data  1130 , the capacitance data  1116 , the image data, and so forth. The sensors  1312  are described in more detail below with regard to  FIG. 14 . 
     The inventory management system  130  or other systems may use the sensor data to track the location of objects within the facility  1302 , movement of the objects, or provide other functionality. Objects may include, but are not limited to, items  902 , users  112 , totes  116 , and so forth. For example, a series of images acquired by the image sensor  1312 ( 3 ) may indicate removal by the user  112  of an item  902  from a particular location on the fixture  108  and placement of the item  902  on or at least partially within the tote  116 . 
     The facility  1302  may be configured to receive different kinds of items  902  from various suppliers and to store them until a customer orders or retrieves one or more of the items  902 . A general flow of items  902  through the facility  1302  is indicated by the arrows of  FIG. 13 . Specifically, as illustrated in this example, items  902  may be received from one or more suppliers, such as manufacturers, distributors, wholesalers, and so forth, at the receiving area  1304 . In various implementations, the items  902  may include merchandise, commodities, perishables, or any suitable type of item  902 , depending on the nature of the enterprise that operates the facility  1302 . 
     Upon being received from a supplier at the receiving area  1304 , the items  902  may be prepared for storage in the storage area  1306 . For example, in some implementations, items  902  may be unpacked or otherwise rearranged. The inventory management system  130  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  902 . The items  902  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  902 , 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  902  may be managed in terms of a 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  902  may refer to either a countable number of individual or aggregate units of an item  902  or a measurable amount of an item  902 , as appropriate. 
     After arriving through the receiving area  1304 , items  902  may be stored within the storage area  1306 . In some implementations, like items  902  may be stored or displayed together in the fixtures  108  such as in bins, on shelves  904 , hanging from pegboards, and so forth. For example, all items  902  of a given kind are stored in one fixture  108 . In other implementations, like items  902  may be stored in different fixtures  108 . For example, to optimize retrieval of certain items  902  having frequent turnover within a large physical facility  1302 , those items  902  may be stored in several different fixtures  108  to reduce congestion that might occur at a single fixture  108 . 
     When a customer order specifying one or more items  902  is received, or as a user  112  progresses through the facility  1302 , the corresponding items  902  may be selected or “picked” from the fixtures  108  containing those items  902 . In various implementations, item picking may range from manual to completely automated picking. For example, in one implementation, a user  112  may have a list of items  902  they desire and may progress through the facility  1302  picking items  902  from the fixtures  108  within the storage area  1306  and placing those items  902  into a tote  116 . In other implementations, employees of the facility  1302  may pick items  902  using written or electronic pick lists derived from customer orders. These picked items  902  may be placed into the tote  116  as the employee progresses through the facility  1302 . 
     After items  902  have been picked, the items  902  may be processed at a transition area  1308 . The transition area  1308  may be any designated area within the facility  1302  where items  902  are transitioned from one location to another or from one entity to another. For example, the transition area  1308  may be a packing station within the facility  1302 . When the item  902  arrives at the transition area  1308 , the items  902  may be transitioned from the storage area  1306  to the packing station. Information about the transition may be maintained by the inventory management system  130 . 
     In another example, if the items  902  are departing the facility  1302 , a list of the items  902  may be obtained and used by the inventory management system  130  to transition responsibility for, or custody of, the items  902  from the facility  1302  to another entity. For example, a carrier may accept the items  902  for transport with that carrier accepting responsibility for the items  902  indicated in the list. In another example, a user  112  may purchase or rent the items  902  and remove the items  902  from the facility  1302 . During use of the facility  1302 , the user  112  may move about the facility  1302  to perform various tasks, such as picking or placing the items  902  in the fixtures  108 . 
     The inventory management system  130  may generate the interaction data  142 . The interaction data  142  may be based at least in part on one or more of the tile output data  126 , the fixture data  132 , and so forth. The interaction data  142  may provide information about an interaction, such as a pick of an item  902  from the fixture  108 , a place of an item  902  to the fixture  108 , a touch made to an item  902  at the fixture  108 , a gesture associated with an item  902  at the fixture  108 , and so forth. The interaction data  142  may include one or more of the type of interaction, duration of interaction, interaction location identifier indicative of where from the fixture  108  the interaction took place, item identifier, quantity change to the item  902 , user identifier, and so forth. The interaction data  142  may then be used to further update the item data. For example, the quantity of items  902  on hand at a particular lane on the shelf  904  may be changed based on an interaction that picks or places one or more items  902 . 
     The inventory management system  130  may combine or otherwise utilize data from different sensors  1312  of different types. For example, weight data  1130  obtained from weight sensors  1312 ( 1 ) at the fixture  108  may be used instead of, or in conjunction with, one or more of the capacitance data  1116  to determine the interaction-data  142 . 
       FIG. 14  is a block diagram  1400  illustrating additional details of the facility  1302 , according to some implementations. The facility  1302  may be connected to one or more networks  1402 , which in turn connect to one or more servers  1404 . The network  1402  may include private networks such as an institutional or personal intranet, public networks such as the Internet, or a combination thereof. The network  1402  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  1402  is representative of any type of communication network, including one or more of data networks or voice networks. The network  1402  may be implemented using 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  1404  may be configured to execute one or more modules or software applications associated with the inventory management system  130  or other systems. While the servers  1404  are illustrated as being in a location outside of the facility  1302 , in other implementations, at least a portion of the servers  1404  may be located at the facility  1302 . The servers  1404  are discussed in more detail below with regard to  FIG. 15 . 
     The users  112 , the totes  116 , or other objects in the facility  1302  may be equipped with one or more tags  1406 . The tags  1406  may be configured to emit a signal  1408 . In one implementation, the tag  1406  may be a RFID tag  1406  configured to emit a RF signal  1408  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  1406 . In another implementation, the tag  1406  may comprise a transmitter and a power source configured to power the transmitter. For example, the tag  1406  may comprise a Bluetooth Low Energy (BLE) transmitter and battery. In other implementations, the tag  1406  may use other techniques to indicate presence of the tag  1406 . For example, an acoustic tag  1406  may be configured to generate an ultrasonic signal  1408 , which is detected by corresponding acoustic receivers. In yet another implementation, the tag  1406  may be configured to emit an optical signal  1408 . 
     The inventory management system  130  may be configured to use the tags  1406  for one or more of identification of the object, determining a location of the object, and so forth. For example, the users  112  may wear tags  1406 , the totes  116  may have tags  1406  affixed, and so forth, which may be read and, based at least in part on signal strength, used to determine identity and location. In other implementations, such as described above, the users  112  may wear portable transmitters  804 , the totes  116  may be equipped with a portable receiver  802 , portable transmitter  804 , and so forth. In some implementations, the two may be combined, such as tags  1406  and the use of a portable transmitter  804 . 
     Generally, the inventory management system  130  or other systems associated with the facility  1302  may include any number and combination of input components, output components, and servers  1404 . 
     The one or more sensors  1312  may be arranged at one or more locations within the facility  1302 . For example, the sensors  1312  may be mounted on or within a floor  102 , wall, at a ceiling, at fixture  108 , on a tote  116 , may be carried or worn by a user  112 , and so forth. 
     The sensors  1312  may include one or more weight sensors  1312 ( 1 ) that are configured to measure the weight of a load, such as the item  902 , the tote  116 , or other objects. The weight sensors  1312 ( 1 ) may be configured to measure the weight of the load at one or more of the fixtures  108 , the tote  116 , on the floor  102  of the facility  1302 , and so forth. For example, the shelf  904  may include a plurality of lanes or platforms, with one or more weight sensors  1312 ( 1 ) beneath each one to provide weight sensor data about an individual lane or platform. The weight sensors  1312 ( 1 ) 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  1312 ( 1 ) 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  1312 ( 1 ) 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. In another example, the weight sensor  1312 ( 1 ) may comprise a force sensing resistor (FSR). The FSR may comprise a resilient material that changes one or more electrical characteristics when compressed. For example, the electrical resistance of a particular portion of the FSR may decrease as the particular portion is compressed. The inventory management system  130  may use the data acquired by the weight sensors  1312 ( 1 ) 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  1312  may include capacitive sensors  1312 ( 2 ). As described above with regard to  FIG. 11 , the capacitive sensor  1312 ( 2 ) may comprise one or more conductive elements  1106  and the capacitance measurement/receiver module. In some implementations, the capacitive sensor  1312 ( 2 ) may include or utilize a switch module  1112 . The capacitive sensor  1312 ( 2 ) may be configured to use a far-field capacitance effect that may comprise measuring the self-capacitance of the conductive elements  1106 , rather than a mutual capacitance. In one implementation, a fixed charge may be provided to the conductive element  1106 , and the resultant voltage may be measured between the conductive element  1106  and the ground. 
     In other implementations, the capacitive sensor  1312 ( 2 ) may be configured to operate in a mutual capacitance mode, surface capacitance mode, and so forth. In mutual capacitance mode, at least two conductive layers are arranged in a stack with a dielectric material between the layers. The dielectric may be a solid, such as a plastic, a gas such as air, a vacuum, and so forth. The mutual capacitance at points between these layers is measured. When another object touches the outermost conductive layer, the mutual capacitance between the two layers changes, allowing for detection. In surface capacitance mode, voltages are applied to different points of a conductive element  1106  to produce an electrostatic field. By measuring the changes in current draw (or another electrical characteristic) from the different points at which voltage is applied, a location of an object may be determined. 
     The sensors  1312  may include one or more image sensors  1312 ( 3 ). The one or more image sensors  1312 ( 3 ) may include imaging sensors configured to acquire images of a scene. The image sensors  1312 ( 3 ) are configured to detect light in one or more wavelengths including, but not limited to, terahertz, infrared, visible, ultraviolet, and so forth. The image sensors  1312 ( 3 ) may comprise charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) devices, microbolometers, and so forth. The inventory management system  130  may use image data acquired by the image sensors  1312 ( 3 ) during operation of the facility  1302 . For example, the inventory management system  130  may identify items  902 , users  112 , totes  116 , and so forth, based at least in part on their appearance within the image data acquired by the image sensors  1312 ( 3 ). The image sensors  1312 ( 3 ) may be mounted in various locations within the facility  1302 . For example, image sensors  1312 ( 3 ) may be mounted overhead, on the fixtures  108 , may be worn or carried by users  112 , may be affixed to totes  116 , and so forth. 
     One or more depth sensors  1312 ( 4 ) may also be included in the sensors  1312 . The depth sensors  1312 ( 4 ) are configured to acquire spatial or three-dimensional (3D) data, such as depth information, about objects within a FOV. The depth sensors  1312 ( 4 ) may 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  130  may use the 3D data acquired by the depth sensors  1312 ( 4 ) to identify objects, determine a location of an object in 3D real space, and so forth. 
     One or more buttons  1312 ( 5 ) may be configured to accept input from the user  112 . The buttons  1312 ( 5 ) may comprise mechanical, capacitive, optical, or other mechanisms. For example, the buttons  1312 ( 5 ) may comprise mechanical switches configured to accept an applied force from a touch of the user  112  to generate an input signal. The inventory management system  130  may use data from the buttons  1312 ( 5 ) to receive information from the user  112 . For example, the tote  116  may be configured with a button  1312 ( 5 ) to accept input from the user  112  and send information indicative of the input to the inventory management system  130 . 
     The sensors  1312  may include one or more touch sensors  1312 ( 6 ). The touch sensors  1312 ( 6 ) 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  130  may use data from the touch sensors  1312 ( 6 ) to receive information from the user  112 . For example, the touch sensor  1312 ( 6 ) may be integrated with the tote  116  to provide a touchscreen with which the user  112  may select from a menu one or more particular items  902  for picking, enter a manual count of items  902  at a fixture  108 , and so forth. 
     One or more microphones  1312 ( 7 ) may be configured to acquire information indicative of sound present in the environment. In some implementations, arrays of microphones  1312 ( 7 ) may be used. These arrays may implement beamforming techniques to provide for directionality of gain. The inventory management system  130  may use the one or more microphones  1312 ( 7 ) to acquire information from acoustic tags  1406 , accept voice input from the users  112 , determine ambient noise level, and so forth. 
     The sensors  1312  may include one or more optical sensors  1312 ( 8 ). The optical sensors  1312 ( 8 ) may be configured to provide data indicative of one or more of color or intensity of light impinging thereupon. For example, the optical sensor  1312 ( 8 ) may comprise a photodiode and associated circuitry configured to generate a signal or data indicative of an incident flux of photons. As described below, the optical sensor array  1312 ( 14 ) may comprise a plurality of the optical sensors  1312 ( 8 ). For example, the optical sensor array  1312 ( 14 ) may comprise an array of ambient light sensors such as the ISL76683 as provided by Intersil Corporation of Milpitas, Calif., USA, or the MAX44009 as provided by Maxim Integrated of San Jose, Calif., USA. In other implementations, other optical sensors  1312 ( 8 ) may be used. The optical sensors  1312 ( 8 ) may be sensitive to one or more of infrared light, visible light, or ultraviolet light. For example, the optical sensors  1312 ( 8 ) may be sensitive to infrared light, and infrared light sources such as light emitting diodes (LEDs) may provide illumination. 
     The optical sensors  1312 ( 8 ) may include photodiodes, photoresistors, photovoltaic cells, quantum dot photoconductors, bolometers, pyroelectric infrared detectors, and so forth. For example, the optical sensor  1312 ( 8 ) may use germanium photodiodes to detect infrared light. 
     One or more radio frequency identification (RFID) readers  1312 ( 9 ), near field communication (NFC) systems, and so forth, may be included as sensors  1312 . For example, the RFID readers  1312 ( 9 ) may be configured to read the RF tags  1406 . Information acquired by the RFID reader  1312 ( 9 ) may be used by the inventory management system  130  to identify an object associated with the RF tag  1406  such as the item  902 , the user  112 , the tote  116 , and so forth. For example, based on information from the RFID readers  1312 ( 9 ) detecting the RF tag  1406  at different times and RFID readers  1312 ( 9 ) having different locations in the facility  1302 , a velocity of the RF tag  1406  may be determined. 
     One or more RF receivers  1312 ( 10 ) may also be included as sensors  1312 . In some implementations, the RF receivers  1312 ( 10 ) may be part of transceiver assemblies. The RF receivers  1312 ( 10 ) may be configured to acquire RF signals  1408  associated with Wi-Fi, Bluetooth, ZigBee, 4G, 3G, LTE, or other wireless data transmission technologies. The RF receivers  1312 ( 10 ) may provide information associated with data transmitted via radio frequencies, signal strength of RF signals  1408 , and so forth. For example, information from the RF receivers  1312 ( 10 ) may be used by the inventory management system  130  to determine a location of an RF source, such as a communication interface onboard the tote  116 . 
     The sensors  1312  may include one or more accelerometers  1312 ( 11 ), which may be worn or carried by the user  112 , mounted to the tote  116 , and so forth. The accelerometers  1312 ( 11 ) 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  1312 ( 11 ). 
     A gyroscope  1312 ( 12 ) may provide information indicative of rotation of an object affixed thereto. For example, the tote  116  or other objects may be equipped with a gyroscope  1312 ( 12 ) to provide data indicative of a change in orientation of the object. 
     A magnetometer  1312 ( 13 ) may be used to determine an orientation by measuring ambient magnetic fields, such as the terrestrial magnetic field. The magnetometer  1312 ( 13 ) may be worn or carried by the user  112 , mounted to the tote  116 , and so forth. For example, the magnetometer  1312 ( 13 ) mounted to the tote  116  may act as a compass and provide information indicative of which direction the tote  116  is oriented. 
     An optical sensor array  1312 ( 14 ) may comprise one or more optical sensors  1312 ( 8 ). The optical sensors  1312 ( 8 ) may be arranged in a regular, repeating, or periodic two-dimensional arrangement such as a grid. The optical sensor array  1312 ( 14 ) may generate image data. For example, the optical sensor array  1312 ( 14 ) may be arranged within or below fixture  108  and obtain information about shadows of items  902 , a hand  1002  of the user  112 , and so forth. 
     The sensors  1312  may include proximity sensors  1312 ( 15 ) used to determine presence of an object, such as the user  112 , the tote  116 , and so forth. The proximity sensors  1312 ( 15 ) may use optical, electrical, ultrasonic, electromagnetic, or other techniques to determine a presence of an object. In some implementations, the proximity sensors  1312 ( 15 ) may use an optical emitter and an optical detector to determine proximity. For example, an optical emitter may emit light, a portion of which may then be reflected by the object back to the optical detector to provide an indication that the object is proximate to the proximity sensor  1312 ( 15 ). In other implementations, the proximity sensors  1312 ( 15 ) may comprise a capacitive proximity sensor  1312 ( 15 ) configured to provide an electrical field and determine a change in electrical capacitance due to presence or absence of an object within the electrical field. 
     The proximity sensors  1312 ( 15 ) may be configured to provide sensor data indicative of one or more of a presence or absence of an object, a distance to the object, or characteristics of the object. An optical proximity sensor  1312 ( 15 ) may use time-of-flight (ToF), structured light, interferometry, or other techniques to generate the distance data. For example, ToF determines a propagation time (or “round-trip” time) of a pulse of emitted light from an optical emitter or illuminator that is reflected or otherwise returned to an optical detector. By dividing the propagation time in half and multiplying the result by the speed of light in air, the distance to an object may be determined. In another implementation, a structured light pattern may be provided by the optical emitter. A portion of the structured light pattern may then be detected on the object using a sensor  1312  such as an image sensor  1312 ( 3 ). Based on an apparent distance between the features of the structured light pattern, the distance to the object may be calculated. Other techniques may also be used to determine distance to the object. In another example, the color of the reflected light may be used to characterize the object, such as skin, clothing, tote  116 , and so forth. 
     The sensors  1312  may also include an instrumented auto-facing unit (IAFU)  1312 ( 16 ). The IAFU  1312 ( 16 ) may comprise a position sensor configured to provide data indicative of displacement of a pusher. As an item  902  is removed from the IAFU  1312 ( 16 ), the pusher moves, such as under the influence of a spring, and pushes the remaining items  902  in the IAFU  1312 ( 16 ) to the front of the fixture  108 . By using data from the position sensor, and given item data such as a depth of an individual item  902 , a count may be determined, based on a change in position data. For example, if each item  902  is 1 inch deep, and the position data indicates a change of 17 inches, the quantity held by the IAFU  1312 ( 16 ) may have changed by 17 items  902 . This count information may be used to confirm or provide a cross check for a count obtained by other means, such as analysis of the weight data  1130 , the capacitance data  1116 , the image data  1532 , and so forth. 
     The sensors  1312  may include other sensors  1312 (S) as well. For example, the other sensors  1312 (S) may include light curtains, ultrasonic rangefinders, thermometers, barometric sensors, air pressure sensors, hygrometers, and so forth. For example, the inventory management system  130  may use information acquired from thermometers and hygrometers in the facility  1302  to direct the user  112  to check on delicate items  902  stored in a particular fixture  108 , which is overheating, too dry, too damp, and so forth. 
     In one implementation, a light curtain may utilize a linear array of light emitters and a corresponding linear array of light detectors. For example, the light emitters may comprise a line of infrared LEDs or vertical cavity surface emitting lasers (VCSELs) that are arranged in front of the fixture  108 , while the light detectors comprise a line of photodiodes sensitive to infrared light arranged below the light emitters. The light emitters produce a “lightplane” or sheet of infrared light that is then detected by the light detectors. An object passing through the lightplane may decrease the amount of light falling upon the light detectors. For example, the user&#39;s  112  hand  1002  would prevent at least some of the light from light emitters from reaching a corresponding light detector. As a result, a position along the linear array of the object may be determined that is indicative of a touchpoint. This position may be expressed as touchpoint data, with the touchpoint being indicative of the intersection between the hand  1002  of the user  112  and the sheet of infrared light. In some implementations, a pair of light curtains may be arranged at right angles relative to one another to provide two-dimensional touchpoint data indicative of a position of touch in a plane. Input from the light curtain, such as indicating occlusion from a hand  1002  of a user  112  may be used to generate interaction data  142 . 
     In some implementations, the image sensor  1312 ( 3 ) or other sensors  1312 (S) may include hardware processors, memory, and other elements configured to perform various functions. For example, the image sensors  1312 ( 3 ) may be configured to generate image data  1532 , send the image data  1532  to another device such as the server  1404 , and so forth. 
     The facility  1302  may include one or more access points  1410  configured to establish one or more wireless networks. The access points  1410  may use Wi-Fi, NFC, Bluetooth, or other technologies to establish wireless communications between a device and the network  1402 . The wireless networks allow devices to communicate with one or more of the sensors  1312 , the inventory management system  130 , the optical sensor arrays  1312 ( 14 ), the tag  1406 , a communication device of the tote  116 , or other devices. 
     Output devices  1412  may also be provided in the facility  1302 . The output devices  1412  are configured to generate signals, which may be perceived by the user  112  or detected by the sensors  1312 . In some implementations, the output devices  1412  may be used to provide illumination of the optical sensor array  1312 ( 14 ). 
     Haptic output devices  1412 ( 1 ) are configured to provide a signal that results in a tactile sensation to the user  112 . The haptic output devices  1412 ( 1 ) may use one or more mechanisms such as electrical stimulation or mechanical displacement to provide the signal. For example, the haptic output devices  1412 ( 1 ) may be configured to generate a modulated electrical signal, which produces an apparent tactile sensation in one or more fingers of the user  112 . In another example, the haptic output devices  1412 ( 1 ) may comprise piezoelectric or rotary motor devices configured to provide a vibration, which may be felt by the user  112 . 
     One or more audio output devices  1412 ( 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  1412 ( 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, magnetorestrictive elements, electrostatic elements, and so forth. For example, a piezoelectric buzzer or a speaker may be used to provide acoustic output. 
     The display devices  1412 ( 3 ) may be configured to provide output, which may be seen by the user  112  or detected by a light-sensitive sensor such as an image sensor  1312 ( 3 ) or an optical sensor  1312 ( 8 ). In some implementations, the display devices  1412 ( 3 ) may be configured to produce output in one or more of infrared, visible, or ultraviolet light. The output may be monochrome or in color. The display devices  1412 ( 3 ) may be one or more of emissive, reflective, microelectromechanical, and so forth. An emissive display device  1412 ( 3 ), such as using LEDs, is configured to emit light during operation. In comparison, a reflective display device  1412 ( 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  1412 ( 3 ) to provide visibility of the output in conditions where the ambient light levels are low. 
     The display devices  1412 ( 3 ) may be located at various points within the facility  1302 . For example, the addressable displays may be located on the fixtures  108 , totes  116 , on the floor of the facility  1302 , and so forth. 
     Other output devices  1412 (P) may also be present. For example, the other output devices  1412 (P) may include scent/odor dispensers, document printers, 3D printers or fabrication equipment, and so forth. 
       FIG. 15  illustrates a block diagram  1500  of a server  1404  configured to support operation of the facility  1302 , according to some implementations. The server  1404  may be physically present at the facility  1302 , may be accessible by the network  1402 , or a combination of both. The server  1404  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  1404  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  1404  may be distributed across one or more physical or virtual devices. 
     One or more power supplies  1502  may be configured to provide electrical power suitable for operating the components in the server  1404 . The one or more power supplies  1502  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  1404  may include one or more hardware processors  1504  (processors) configured to execute one or more stored instructions. The processors  1504  may comprise one or more cores. One or more clocks  1506  may provide information indicative of date, time, ticks, and so forth. For example, the processor  1504  may use data from the clock  1506  to associate a particular interaction with a particular point in time. 
     The server  1404  may include one or more communication interfaces  1508  such as input/output (I/O) interfaces  1510 , network interfaces  1512 , and so forth. The communication interfaces  1508  enable the server  1404 , or components thereof, to communicate with other devices or components. The communication interfaces  1508  may include one or more I/O interfaces  1510 . The I/O interfaces  1510  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)  1510  may couple to one or more I/O devices  1514 . The I/O devices  1514  may include input devices such as one or more of a sensor  1312 , keyboard, mouse, scanner, and so forth. The I/O devices  1514  may also include output devices  1412  such as one or more of a display device  1412 ( 3 ), printer, audio speakers, and so forth. In some embodiments, the I/O devices  1514  may be physically incorporated with the server  1404  or may be externally placed. 
     The network interfaces  1512  may be configured to provide communications between the server  1404  and other devices, such as the SFTs  104 , totes  116 , routers, access points  1410 , and so forth. The network interfaces  1512  may include devices configured to couple to personal area networks (PANs), local area networks (LANs), wireless local area networks (WLANS), wide area networks (WANs), and so forth. For example, the network interfaces  1512  may include devices compatible with Ethernet, Wi-Fi, Bluetooth, ZigBee, and so forth. 
     The server  1404  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  1404 . 
     As shown in  FIG. 15 , the server  1404  includes one or more memories  1516 . The memory  1516  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  1516  provides storage of computer-readable instructions, data structures, program modules, and other data for the operation of the server  1404 . A few example functional modules are shown stored in the memory  1516 , although the same functionality may alternatively be implemented in hardware, firmware, or as a system on a chip (SoC). 
     The memory  1516  may include at least one operating system (OS) module  1518 . The OS module  1518  is configured to manage hardware resource devices such as the I/O interfaces  1510 , the I/O devices  1514 , the communication interfaces  1508 , and provide various services to applications or modules executing on the processors  1504 . The OS module  1518  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  1516  may be a data store  1520  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  1520  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  1520  or a portion of the data store  1520  may be distributed across one or more other devices including the servers  1404 , network attached storage devices, and so forth. 
     A communication module  1522  may be configured to establish communications with one or more of the totes  116 , sensors  1312 , display devices  1412 ( 3 ), other servers  1404 , or other devices. The communications may be authenticated, encrypted, and so forth. 
     The memory  1516  may store an inventory management module  1524 . The inventory management module  1524  is configured to provide the inventory functions as described herein with regard to the inventory management system  130 . For example, the inventory management module  1524  may track items  902  between different fixtures  108 , to and from the totes  116 , and so forth. 
     The inventory management module  1524  may include one or more of a data acquisition module  1526 , the tracking module  134 , the analysis module  138 , an action module  1528 , and so forth. The data acquisition module  1526  may be configured to acquire and access information associated with operation of the facility  1302 . For example, the data acquisition module  1526  may be configured to acquire tile output data  126  from the SFTs  104 , fixture data  132 , sensor data  1530  such as the weight data  1130 , capacitance data  1116 , image data  1532 , other sensor data  1534 , and so forth. The sensor data  1530  may be accessed by the other modules for use. 
     The data store  1520  may also store item data  1536 . The item data  1536  provides information about a particular type of item  902 , including characteristics of that type of item  902  such as physical dimensions, where that type of item  902  is located in the facility  1302 , characteristics about how the item  902  appears, capacitance values associated with the type of item  902 , attenuation characteristics of an EMS  106 , and so forth. For example, the item data  1536  may indicate that the type of item  902  is “Bob&#39;s Low Fat Baked Beans, 10 oz can” with a stock keeping unit number of “24076513”. The item data  1536  may indicate the types and quantities of items  902  that are expected to be stored at that particular fixture  108  such as in a particular lane on a shelf  904 , width and depth of that type of item  902 , weight of the item  902  individually or in aggregate, sample images of the type of item  902 , and so forth. 
     The item data  1536  may include an item identifier. The item identifier may be used to distinguish one type of item  902  from another. For example, the item identifier may include a stock keeping unit (SKU) string, Universal Product Code (UPC) number, radio frequency identification (RFID) tag data, and so forth. The items  902  that are of the same type may be referred to by the same item identifier. For example, cans of beef flavor Brand X dog food may be represented by the item identifier value of “9811901181”. In other implementations, non-fungible items  902  may each be provided with a unique item identifier, allowing each to be distinguished from one another. 
     The item data  1536  may include one or more of geometry data, item weight data, sample image data, sample capacitance data, or other data. The geometry data may include information indicative of size and shape of the item  902  in one-, two-, or three-dimensions. For example, the geometry data may include the overall shape of an item  902 , such as a cuboid, sphere, cylinder, and so forth. The geometry data may also include information such as length, width, depth, and so forth, of the item  902 . Dimensional information in the geometry data may be measured in pixels, centimeters, inches, arbitrary units, and so forth. The geometry data may be for a single item  902 , or a package, kit, or other grouping considered to be a single item  902 . 
     The item weight data comprises information indicative of a weight of a single item  902 , or a package, kit, or other grouping considered to be a single item  902 . The item data  1536  may include other data. For example, the other data may comprise weight distribution of the item  902 , point cloud data for the item  902 , and so forth. 
     The sample capacitance data may comprise data indicative of a previously measured or calculated change in capacitance obtained by a representative capacitive sensor  1312 ( 2 ) based on the presence or absence of a sample of the type of item  902 . For example, during processing or intake of the item  902  at the facility  1302 , a sample of the type of item  902  may be placed on a capacitive sensor  1312 ( 2 ) to generate the sample capacitance data. Similar data may be obtained for the attenuation or propagation of the EMS  106  across the item  902 . 
     The sample image data may comprise one or more images of one or more of that type of item  902 . For example, sample image data may be obtained during processing or intake of the item  902  to be used by the facility  1302 . 
     The item data  1536  may include one or more fixture identifiers (IDs). The fixture ID is indicative of a particular area or volume of fixture  108  such as a shelf  904  that is designated for stowage of the type of item  902 . For example, a single shelf  904  may have several lanes, each with a different fixture ID. Each of the different fixture IDs may be associated with a lane having a particular area on the shelf  904  designated for storage of a particular type of item  902 . A single type of item  902  may be associated with a particular fixture ID, a plurality of fixture IDs may be associated with the single type of item  902 , more than one type of item  902  may be associated with the particular fixture ID, and so forth. 
     The item data  1536  may also include quantity data. The quantity data may comprise a count or value indicative of a number of items  902 . The count may be a measured or an estimated value. The quantity data may be associated with a particular fixture ID, for an entire facility  1302 , and so forth. For example, the same type of item  902  may be stored at different shelves  904  within the facility  1302 . The quantity data may indicate the quantity on hand for each of the different fixtures  108 . 
     The tracking module  134  may access physical layout data  1538  and generate account item data  1540 . The tracking module  134  may be configured to determine a location within the facility  1302  of the user  112 , a user account associated with the user  112 , and so forth. For example, the tracking module  134  may determine that an item  902  has been removed from a lane and placed into the tote  116  based on the fixture data  132  indicative of the user&#39;s  112  characteristic data  128  having been received at the lane. The tracking module  134  may then determine that the tote  116  is associated with the user  112  or the user account that represents the user  112 . Based on this information, the analysis module  138  may generate the interaction data  142 . 
     The analysis module  138  may utilize the tile output data  126 , fixture data  132 , weight data  1130 , capacitance data  1116 , item data  1536 , and other information to generate interaction data  142 . The interaction data  142  is indicative of action such as picking or placing an item  902  for a particular fixture  108 , presence of the user  112  at the fixture  108 , and so forth. 
     In some implementations, the analysis module  138  may generate output data  1544  about the user  112 . The analysis module  138  may determine if the user  112  is standing, moving, lying on the floor  102 , and so forth. For example, the analysis module  138  may determine an area of contact with the floor  102  based on the tile output data  126 . If the area of contact exceeds a threshold value, the user  112  may be determined to be lying on the floor  102 . Based on this determination, other actions may be taken. For example, alarm data may be generated to summon assistance if a user  112  is deemed to be lying on the floor  102 . 
     The analysis module  138 , or other modules, may be configured to determine portions of the SFTs  104  which are to be deactivated or from which information is to be disregarded. In one implementation, during setup of the system, the antennas  304  of a SFT  104  that are located underneath a fixture  108  may be deactivated. In another implementation, the analysis module  138  may determine SFTs  104  or portions thereof that report presence of an object that is unchanging over long periods of time, such as hours or days. These objects, such as a fixture  108  above the SFT  104 , may then be subsequently disregarded and information about these positions may be removed from further processing. If a change is detected, such as when the fixture  108  above the SFT  104  is moved, information about that change may be used to re-enable consideration of data from that SFT  104  or portion thereof. 
     The inventory management module  1524  may utilize the physical layout data  1538 . The physical layout data  1538  may provide information indicative of location of the SFTs  104 , where sensors  1312  and the fixtures  108  are in the facility  1302  with respect to one another, FOV of sensors  1312  relative to the fixture  108 , and so forth. For example, the physical layout data  1538  may comprise information representative of a map or floor plan of the facility  1302  with relative positions of the fixtures  108 , location of individual SFTs  104  therein, arrangements of the segments  204 , planogram data indicative of how items  902  are to be arranged at the fixtures  108 , and so forth. Continuing the example, the physical layout data  1538  may be based on using the relative arrangement of the SFTs  104  in conjunction with their physical dimensions to specify where the SFTs  104  are placed within the facility  1302 . 
     The physical layout data  1538  may associate a particular fixture ID with other information such as physical location data, sensor position data, sensor direction data, sensor identifiers, and so forth. The physical location data provides information about where in the facility  1302  objects are, such as the fixture  108 , the sensors  1312 , and so forth. In some implementations, the physical location data may be relative to another object. For example, the physical location data may indicate that a particular weight sensor  1312 ( 1 ), capacitive sensor  1312 ( 2 ), or image sensor  1312 ( 3 ) is associated with the shelf  904  or portion thereof. 
     The inventory management module  1524  may utilize the physical layout data  1538  and other information during operation. For example, the tracking module  134  may utilize physical layout data  1538  to determine what capacitance data  1116  acquired from particular capacitive sensors  1312 ( 2 ) corresponds to a particular shelf  904 , lane, or other fixture  108 . 
     The tracking module  134  may access information from sensors  1312  within the facility  1302 , such as those at the shelf  904  or other fixture  108 , onboard the tote  116 , carried by or worn by the user  112 , and so forth. For example, the tracking module  134  may receive the fixture data  132  and use the characteristic data  128  to associate a particular user  112  with a pick or place of an item  902  at the associated fixture  108 . 
     The account item data  1540  may also be included in the data store  1520  and comprises information indicative of one or more items  902  that are within the custody of a particular user  112 , within a particular tote  116 , and so forth. For example, the account item data  1540  may comprise a list of the contents of the tote  116 . Continuing the example, the list may be further associated with the user account representative of the user  112 . In another example, the account item data  1540  may comprise a list of items  902  that the user  112  is carrying. The tracking module  134  may use the account item data  1540  to determine subsets of possible items  902  with which the user  112  may have interacted. 
     The inventory management module  1524 , and modules associated therewith, may access sensor data  1530 , threshold data  1542 , and so forth. The threshold data  1542  may comprise one or more thresholds, ranges, percentages, and so forth, that may be used by the various modules in operation. 
     The inventory management module  1524  may generate output data  1544 . For example, the output data  1544  may include the interaction data  142 , inventory levels for individual types of items  902 , overall inventory, and so forth. 
     The action module  1528  may be configured to initiate or coordinate one or more actions responsive to output data  1544 . For example, the action module  1528  may access output data  1544  that indicates a particular fixture  108  is empty and in need of restocking. An action such as a dispatch of a work order or transmitting instructions to a robot may be performed to facilitate restocking of the fixture  108 . 
     Processing sensor data  1530 , such as the image data  1532 , may be performed by a module implementing, at least in part, one or more of the following tools or techniques. In one implementation, processing of the image data  1532  may be performed, at least in part, using one or more tools available in 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  1530 . In still another implementation, functions such as those in the Machine Vision Toolbox for Matlab (MVTB) available using MATLAB as developed by MathWorks, Inc. of Natick, Mass., USA, may be utilized. 
     Techniques such as artificial neural networks (ANNs), active appearance models (AAMs), active shape models (ASMs), principal component analysis (PCA), cascade classifiers, and so forth, may also be used to process the sensor data  1530  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  1530  and the item data  1536  to allow for a determination of similarity between two or more images. 
     The sensor data  1530  obtained from different sensors  1312  may be used to compare or validate output data  1544 . For example, the image data  1532  may indicate the presence of a person based on a coat or jacket that is arranged across the back of a chair. However, the tile output data  126  provides information that no user  112  is currently present at that location in the facility  1302 . This difference may be used to generate an alarm, notify an associate in the facility  1302 , and so forth. 
     Other data  1546  may be stored in the data store  1520  as well as other modules  1548  in the memory  1516 . For example, the other modules  1548  may include a billing module while the other data  1546  may include billing data. 
       FIG. 16  depicts a flow diagram  1600  of a process for using SFTs  104  to generate tracking data  136 , according to some implementations. 
     At  1602 , a SFT  104  transmits a tile signal  206 . As described above, the tile signal  206  is representative of the SFT  104  within the cluster  202 . 
     At  1604 , the SFT  104  transmits segment signals  208  from the segments  204  of the SFT  104 . As described above, the segment signals  208  are representative of particular segments  204  within the SFT  104 . 
     For example, a transmitted plurality of EMS  106  that are transmitted from a plurality of SFTs  104  may include the tile signal  206  at a first frequency and a plurality of segment signals  208  at frequencies different from one another. 
     At  1606 , a plurality of EMS  106  are received using one or more receivers  326  of at least a portion of the plurality of SFTs  104 . 
     At  1608 , received characteristic data  128  is determined using the received plurality of EMS  106 . As described above, the characteristic data  128  is indicative of a frequency and signal strength of the received plurality of signals. 
     At  1610 , tile output data  126  is generated that is indicative of a particular SFT  104  and the received characteristic data  128  obtained by that particular SFT  104 . For example, the tile output data  126  may include tile identifier data  334 . 
     At  1612 , tracking data  136  is generated using the tile output data  126 . For example, the locations of the user  112  at different times may be generated based on the tile output data  126 , and a time series created that represents these different locations at their respective times. The tracking data  136  may be based on the reciprocity of EMS  106  exchanged between the feet of the user  112 . 
     For example, first received characteristic data  128  received at a first SFT  104  may be determined. The first received characteristic data  128  is indicative of the transmitted plurality of EMS  106  from the second SFT  104 . Second received characteristic data  128  received at the second SFT  104  may be determined. The second received characteristic data  128  is indicative of the transmitted plurality of EMS  106  from the first SFT  104 . Thus, there is reciprocity between the two SFTs  104  and the respective characteristic data  128 . The tracking data  136  may then be generated that is indicative of a location of the user  112  at the first SFT  104  and the second SFT  104 , or a location relative thereto. Continuing the example, the tracking data  136  may indicate allocation that is at a midpoint between the left foot and the right foot of the user  112 . 
     As described above, the tracking data  136  may be based on the pairwise presence of two feet at different locations. For example, a first pair of SFTs  104  having respective tile output data  126  indicative of contemporaneous transmission of their respective transmitted plurality of EMS  106  at a first time may be determined. A first location of an object (such as the user  112 ) may be determined at the first time using a location of the SFTs  104  in the first pair. A second pair of SFTs  104  that have respective tile output data  126  indicative of contemporaneous transmission of their respective transmitted plurality of EMS  106  at a second time may be determined. The second pair includes one of the SFTs  104  of the first pair. A second location of an object is determined at the second time using a location of the SFTs in the second pair. Tracking data  136  indicative of a change in location of the object from the first location at the first time to the second location at the second time may then be generated. 
     As described above with regard to  FIG. 12 , the location of a hand  1002  relative to a fixture  108 , or portion thereof such as a shelf  904 , may be determined using the EMS  106 . One or more receivers  326  having a plurality of antennas  304  installed at a fixture  108  may be used to receive a plurality of EMS  106 . Using received signal strength of at least a portion of the plurality of EMS  106  as acquired using the plurality of antennas  304  installed at the fixture  108 , a position of an object with respect to the fixture  108  may be determined. For example, as described above, given a known arrangement of the antennas  304  and a received signal strength  504 , a location may be determined. The object provides a signal path  402  for the plurality of EMS  106  to be transferred between the SFT  104  and the fixture  108 . Shelf received characteristic data  128  may be determined that is indicative of the plurality of EMS  106  and the respective signal strengths of individual signals therein. A user account associated with the shelf received characteristic data  128  may be determined, and associated with the action being taken. For example, the received characteristic data  128  may be compared with information that associates a particular set of characteristic data, that is a particular combination of signals, with the user  112 . Interaction data  142  indicative of the interaction of the user  112  with the fixture  108  at the particular location may then be generated. For example, the interaction data  142  may indicate that user  112  “John Adams” picked an item  902  from a particular place on the shelf  904 . 
     The system described above may be utilized in a variety of different settings including but not limited to commercial, non-commercial, medical, and so forth. For example, the SFTs  104  may be deployed in a home, hospital, care facility, correctional facility, transportation facility, office, and so forth. The tracking module  134  may provide tracking data  136 , such as the location of users  112  within the facility. In some implementations, the tracking module  134  may provide tracking data  136  that is indicative of the identity of a particular user. The analysis module  138  may be used to generate output data  1544  that is indicative of a status of the user  112 , such as whether the user  112  is standing, sitting, lying on the floor, and so forth. 
     The processes discussed in this disclosure 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 computer-readable storage media that, when executed by one or more hardware 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 will 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.