Patent Application
20080157970
1. Field
The present description relates to determining the location of a tagged item and, in particular to combining a mobile fine location determination system with a coarse location determination infrastructure in inventory tracking and maintenance applications.
2. Related Art
Location systems are currently used in inventory tracking and management systems. Existing location systems can determine the location of a person or piece of mobile equipment within a range of about 5 m for systems based on measuring the strength of received radio signals (RSSI, Received Signal Strength Indicator) and within a range of about 3 m for systems that use difference in arrival times between signals sent from different known locations (TDOA, Time Difference of Arrival). This is often sufficient to determine a room, a section, or a corridor, but no better. For many applications, the location must be known to within a range of less than 1 m. Existing systems are not able to provide this level of accuracy.
While high accuracy may theoretically be possible for a location system, limitations exist in many practical applications. Radio-based technologies often suffer from multipath signals. This is particularly true in enclosed or crowded areas, such as the warehouses in which inventory systems are typically used. TDOA, TOA (Time of Arrival), UWB (Ultra Wideband), and CSS (Chirp Spread Spectrum) technologies are all particularly susceptible to multipath errors.
A related difficulty occurs when the item to be located and the location system cannot communicate through LOS (Line of Sight). Variations in the radio landscape also limit the accuracy of location determinations. As people and objects move around and as other radio devices are turned on and off, the accuracy of the measurements required for location determination is reduced.
Many of these difficulties may be reduced using a wider signal band for TDOA, TOA or UWB location. However, in practical applications, the amount of radio spectrum allocated for location systems is limited and regulatory agencies may be unlikely to provide more.
Another way to reduce these limitation is with tags containing transmitters and receivers mounted on the building infrastructure. This is commonly done in some UWB systems, however, it requires a large number of specialized receivers to be able to perform location tasks. This requires a significant installation expense and ongoing maintenance.
The location of an item may be determined by first determining a coarse location and then a fine location. In one example, a coarse position of a tagged item is determined using a coarse positioning system and the item's tag. A mobile unit, carrying a fine positioning system, is moved to the determined coarse position. Then, a fine position of the tagged item is determined by communicating between the fine positioning system of the moved mobile unit and the item's tag.
The invention may be more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which like reference numerals refer to corresponding parts throughout the several views of the drawings, and in which:
Precise locations may be determined using a two-stage, coarse-fine positioning system. The coarse positioning system may be a conventional infrastructure system. Such systems typically use equipment that is fixed in position but this is not necessary to the present invention. The fine positioning system relies on a mobile locating unit, carried, for example, by a person or mounted on a vehicle. The coarse positioning system, which may be any of a variety of different standard locating systems, is used to find the item to be located and to position the mobile unit close to the item. The typical coarse positioning system may be able to place the mobile unit within 3 m, 5 m or 6 m of the item even in poor conditions.
The fine positioning system then uses a locating device carried on the mobile unit to locate the device to within better than 1 m location accuracy. The mobile unit may be any of a variety of different things or persons depending on the particular application for the system. The mobile unit, for example, may be a fork-lift, a security robot, or a member of a warehouse, shipping or hospital staff.
The signal is typically, but not necessarily a radio signal within a predetermined wavelength band and may include identification information. While three receivers are shown, more or fewer may be used. The tag may be an active or passive RFID (Radio Frequency Identification) tag.
Alternatively, the tagged item may be located by a choke point. In some applications, a tag reader is placed at the entrances and exits to rooms, compartments, or containers. Every tag that goes through the entrance or exit is polled. This allows the location of the tagged item to be determined as being within the room, compartment, or container.
The receivers 103 use the received signal to determine the location of the tagged item to within some margin of error. This may be done using triangulation between the three receivers, or in any other way. This margin is shown as a circle 107 with an error radius 107. In typical commercial infrastructure systems, the radius of the circle is typically 5 m or so, however, it may be larger or smaller, depending on the particular application and the location system being used. If the coarse location system relies on a choke point, the circle or error may be an entire room or section. Accordingly, while shown as a circle, the circle or error may not be round.
As mentioned above, the infrastructure location system may use any one or more of a variety of different ways to determine the position of the tagged item. These may include RSSI, TDOA, TOA, AOA, and choke points, among others. Even with radio based location systems, the circle of error may not be round due to distortions and obstacles in the radio path. In the example of
Based on the information from the coarse location infrastructure location system, a mobile unit has gone to the identified location of the tagged item. In
Once the mobile unit has arrived at the approximate location of the tagged item, the mobile unit can attempt to communicate with the tagged item directly. This is shown as a two-way communication link 115 in
The infrastructure location system in
The fine location mechanism may work better if the mobile location device has some intelligence, for example if it is capable of supporting TDOA, TOA or AOA (Angle of Arrival) measurements. Alternatively, some or all of this intelligence may be in a central infrastructure that is in communication with the mobile system. In one example, the measurement system of the mobile location device provides a direction correction to the mobile unit as the unit moves in order to direct the mobile unit toward the item. This might be done either autonomously or with the help of other services within the overall location system infrastructure, either for the coarse system or the fine system.
A fine positioning or location system may be used in a wide variety of different contexts. In a conventional inventory tracking and maintenance system, it may be used, for example in a variety of different warehouse or storeroom contexts. In one example, the coarse position system may be used to locate pallets, shipping containers, or boxes to within the coarse system's accuracy level. As mentioned above, this may typically be on the order of about 5m. However, under different circumstances or with different systems, the accuracy may be significantly less or more. A forklift, forklift operator, or other mobile unit may then be directed to the location determined by the coarse positioning system. A fine location tag system mounted on the forklift and a tag on the pallet then work together to close the position accuracy to less than 1 m. In practical applications, this means that after taking the forklift to the general location of the pallet, the fine positioning system then is able to direct the forklift directly to the particular pallet desired or even a package on the pallet.
A very different possible application for the coarse-fine location system is with a record tracking system as may exist in a hospital, a large office, an agency or an archive. In one example, assume that hospital records for a particular patient have been lost but the coarse location system can place the nurse within the general location (5 m or so) of the records. The nurse or record keeper goes to this approximate area. This may mean to a particular file room or office. Then, a fine location tag system carried by or worn by the nurse and the tag mounted on the documents work together to close the several meter gap to less than 1 m. In other words, the coarse position system directs the nurse to the right office, wing or file room, then the fine location system directs the nurse directly to within an arm's reach of the right file. The same approach may also be used for valuable equipment in the hospital, a workshop, or a service facility.
Such a system may also be used in search and rescue contexts. In one such scenario. Emergency services personnel may enter a building and fire up an emergency infrastructure location system. This could be a portable system brought in by the emergency services personnel or they may use the existing infrastructure that is installed in the building. In a “person down” scenario, a person wearing a tag may be in need of help. The location system may be used to place the rescue team within a close distance of the person. The fine location tag system mounted on a rescue team member and the tag mounted on the person needing help may then work together to close the gap and allow the person to be found.
In better circumstances, it may be easier and just as quick for the rescue team member, once he is within 5 m or 6 m to call out to the person down. However, this may not always be possible or practical. First the emergency infrastructure may not be as accurate as a system that is permanently installed in a warehouse or even a hospital. This would mean that the coarse location may have an error of 12 m or 20 m. Second, the “person down” may not be able to hear or respond to another's voice. Third, the environment may make it difficult to find someone, there may be smoke, rubble, fire or other substances obscuring the view or masking sounds, or the whole scenario may be under snow, under water, or in a dark network of small rooms or caves.
The fine location system may be implemented in a variety of different ways. One approach is to use a TOA (Time of Arrival) based mechanism to give ranging. The tagged item sends or backscatters an RF (Radio Frequency) signal and the mobile unit measures its time of arrival. From this, the travel time of the signal may be determined which will provide the distance between the tagged item and the mobile device. The timing may be based on a shared clock, a response time for a poll from the mobile unit or on some other measure. The TOA mechanism may be used to determine the actual distance to the tagged item or only a relative distance. The mobile device may then move about to determine whether it is coming closer or nearer to the tagged item. By repeatedly moving and measuring the TOA or a value related to the TOA, the mobile unit, by trial and error can move toward the tagged item until the desired position accuracy is reached.
Rather than TOA, the mobile unit may use a RSSI (Received Signal Strength Indication) based mechanism or a similar type of mechanism instead. RSSI and similar measures are more accurate when the sender and receiver are in close proximity to each other. Since the mobile unit is already in or within the circle of error, the amplitude of the received signal is a more useful measure than it is for the coarse location system. RSSI falls off rapidly with distance and the SNR (Signal to Noise Ratio) is high when the two are in close proximity. As a result, close RSSI readings can be turned into distance estimates. If the characteristics of the tagged unit's sender or the mobile unit's receiver are not well known, then the RSSI may instead be used directly as a relative measure without knowing the actual distance. As with the TOA, the mobile unit may move and then measure the RSSI again. It can determine whether it is getting closer or further and with enough attempts and measurements can come to within the desired fine location accuracy.
Either approach may be combined with some way to measure the direction from which the tagged item's signal is being received. If the direction is known, then the mobile unit can move in the measured direction and come to the tagged item much more quickly than by trial and error. Alternatively, the direction and distance may be combined to compute the location of the tagged item without any further movement by the mobile unit.
The TOA or RSSI system may be coupled, for example, with an RF beam forming system to deliver Angle of Arrival (AOA) information. Using two or more antennas, or two or more receivers mounted on the mobile unit, or even two cooperating mobile units allows the system to compare the tagged item's signal as it is received by the two different antennas or receivers. In one example, the time of arrival of the two signals at the two antennas is compared. The time difference together with the distance between the two antennas allows the angle of arrival of the signal to be determined. This angle may also be based on the geometry of the antenna locations and the relative RSSI measurements.
Generally, the greater the distance between the antennas and the larger the number of antennas, the more accurately the location can be determined. A more precise location is also provided by accurately calibrating the timing, distances, and other parameters of the receiver. Depending on the application and the operating environment, the multiple antenna system may be made to deliver an approximation or a very accurate measurement. While accurate calibration and larger arrays allows the tagged item to be located more quickly, it adds to the size, complexity and cost of the receiver. Not only are more antennas required, but the measurements must be made and combined to determine the angle.
In order to increase the accuracy of the fine location system, acoustic signals may be used in addition to or instead of radio signals. While RF signals present advantages at close range in avoiding multipath, and interference, the closer range and desired accuracy make it difficult to obtain high precision using any system that relies on the travel times of an RF signal. The travel time for an RF signal to travel 5 m is only a few nanoseconds. The difference in arrival times between two antennas will be a small fraction of that. Acoustic signals, such as sound waves and other pressure waves, on the other hand travel at much slower speeds. While the speed of light is about 300,000,000 m/s, the speed of sound at normal room conditions is only about 350 m/s. Using acoustic waves instead of electromagnetic waves increases the travel time from a few nanoseconds to a few milliseconds. Inexpensive modern electronics systems are easily able to accurately measure and compare times in the millisecond range.
An acoustic fine location system may be operated in a variety of different ways. In one example, the mobile unit first arrives at the general location of the tagged item. It then sends a command to the tag. This command may be an RF command or an acoustic command. For larger sized commands, an RF transmission may be used. The tag responds by sending a location beacon or signal. This may be an acoustic beacon or an RF location beacon together with an acoustic location beacon. The two beacons may be sent at the same time or the acoustic beacon may follow after a known fixed delay.
If an RF location beacon is sent, then in the example of
If the tagged item sends an RF beacon as well as an acoustic beacon, then the time of transmission is approximately known. The actual travel time of the RF signal is trivial and can be ignored compared to the travel time of the sound wave. Alternatively, the travel time can be determined using some type of RF ranging algorithm.
Using the RF beacon for a time base, the mobile unit can range the distance to the tagged item with a one way transmission from the tagged item. One simple ranging algorithm is to simply compare the arrival time of the acoustic wave to that of the RF beacon and divide by the speed of sound. A round robin technique is not required, and the clock rates are so much faster than the speed of sound transmission that clock errors are not a significant problem. For higher accuracy, the mobile unit can measure environmental conditions such as temperature, pressure, and humidity and adjust the value used for the speed of sound accordingly.
As with the RF approaches described above, the fine location determination may be made more quickly using an angle determination. Angle of arrival can be provided by acoustic beam forming. This may be done in a manner similar to the RF beam forming approach. Multiple spaced apart microphones may be used to receive the acoustic signal and then geometry may be applied to the difference in arrival time in order to calculate the angle between the microphone array and the tagged item. Alternatively, a simple signal strength comparison algorithm may also provide suitable angle information.
The mobile unit arrives in the proximity of the tagged item and sends a signal 203 that is a request for an acoustic beacon. This request may be sent with an RF or an acoustic signal. As a result, the tag with signal 205 sends an RF timestamp and an acoustic location beacon. As mentioned above, these may be sent simultaneously or according to some known interval.
If the mobile unit and the tagged item share a timing reference, then the acoustic location beacon may be sent at a time based on the shared timing reference. Such a timing reference could come from timing signals from the infrastructure receivers, from some external reference or it could be established by either the tagged item or the mobile unit for the benefit of these signals only. In the example described above, the timing reference is the RF portion of the location beacon. Based on the received beacons, the mobile unit is then able to determine a fine location of the nearby tagged item.
Other signals may be added to the example of
As a further alternative, the complex electronics and signal processing systems of the mobile unit may be avoided altogether. The coarse location infrastructure may be used to make a coarse location determination of the tagged item. A person may then be sent to that location with a simple radio activation device. This device, when triggered, sends a command for the tagged item to transmit a visible or audible beacon. Once the person is placed close to the tag, the person can look for or listen for the beacon or both. People are able to perform their own approximate TOA and AOA judgments based on sounds and lights and the person should be able, in some environments, to find the tagged item quickly.
The passive RFID tag 310 works in the proximity of and in conjunction with a tag reader or interrogator that may be a part of, attached to, or carried by the mobile unit 109. As described above, the tag reader includes one or more RF antennas for sending RF energy to and receiving RF energy from the tag and may also include acoustic elements.
In a standardized backscatter system, the interrogator transmits information to the passive tag by modulating an RF signal in, for example, the 860 MHz-960 MHz frequency range. The tag receives both information and operating energy from the RF signal. A passive tags is one that receives all of its operating energy from the interrogator's RF waveform.
The interrogator receives information from the passive tag by transmitting a continuous-wave (CW) RF signal to the tag. The tag responds by modulating the reflection coefficient of its antenna, thereby backscattering an information signal to the interrogator. A conventional passive RFID system is ITF, meaning that the passive tag modulates its antenna reflection coefficient with an information signal only after being directed to do so by an interrogator.
In the example of
In a simple example, the FSM is coupled to an ID (identification) number register 324 that holds the ID number for the tag. When queried through the receive chain, the FSM will retrieve the ID number from the register, modulate it, and transmit it through the transmit chain and the antenna. Additional registers may be used to store additional values and the values may be fixed or rewriteable. Additional registers (not shown) may be used to store values from clocks, counters and environmental sensors. These values may be backscattered together with the ID number or upon a specific request.
The RF signal transmitted by the tag reader or mobile unit and received by the tag's antenna is demodulated. The subsequent bit stream may be designed to control the FSM that controls the transmit modulator. The modulator backscatters data via the antenna. This provides for two-way communication.
In one example, at least some of the tag functions (for example tag singulation) are based on whether the signal received by the antenna matches a predetermined code stored in the tag. The register 324 may, in this instance, be used to compare the incoming data stream to the tag's unique number. The result of the comparison may be used to control the tag back-scatter or be used by the tag to cause it to progress through its state transition diagram.
Singulation allows the tag reader to distinguish the backscattered signal of a tag from all of the tags around it. There are a variety of different mechanisms for singulation including tree walking, in which a tag responds based on its serial number and ALOHA, in which a tag resends its data after a random wait time.
The passive tag also has an energy harvest circuit 326 coupled to the transmit and receive chains. This circuit harvests energy received by the antenna from outside sources of RF energy including the tag reader to power the tag circuitry, including the FSM, the receiver, and the transmitter. The energy harvester may be used to eliminate any requirement for another power supply, such as an external current or a battery. This also eliminates any maintenance of the power supply or a battery allowing the tag to operate indefinitely. The tag of
As described above, the tag 310 when attached to the tagged item 101 may also be able to generate an acoustic signal. This is shown as a speaker 352 attached to the FSM 322 for control purposes. The speaker may, for example, be a piezoelectric transducer that produces a fundamental frequency in the human hearing range or in the ultrasonic to enhance propagation. The particular type of acoustic transducer may be selected based on power, frequency, environmental, and cost requirements. The tag also may have a light 354 to help guide a person to the tagged item. The light may be an LED (Light Emitting Diode) or any other type of illumination depending upon the particular application. In one example, the tag 310 operates as a passive tag until it receives a command to produce an acoustic signal or a visual signal. These devices are then powered by the battery.
The active tag 430 communicates with the passive tag 310 through its own antenna or antenna array 436. One or more antennas may be used. Alternatively, this may be the antenna for the tagged item 101 that communicates with the tag reader. Such a wireless communication interface may also be used to communicate with the coarse location infrastructure system or a central control station. This may be done through WiFi access points, or in any of a variety of other ways. The antenna may also be used to communicate with the infrastructure receivers, as described above, to lead the mobile unit 109 to the approximate location of the tagged item 101. As mentioned above, for angle determinations, an array of antennas may be used. The difference in arrival time or received signal strength between the two antennas may be used to determine the angle to the tagged item.
The active tag has a receive chain 438 coupled to the antenna with a demodulator and a transmit chain 440 with a modulator that are both, in this example, coupled to the CPU 442. While the antenna elements of the array may share a receive chain as shown in
A battery 450 may be used to power the active tag, however, any other type of energy storage or generation cell may be used instead of, or in addition to the battery including a solar cell, an energy harvester 326 or other power supply.
The tag may further include a microphone or array of microphones 452 to receive an acoustic signal from the tagged item. The received signal may then be converted to timing or phase differences in a sensor 454 for interpretation by the CPU 454. As with the antenna array, the time of arrival at one or more of the microphones may be used to determine distance to the tagged item. The difference in amplitude or time of arrival of a single acoustic signal at each microphone may be used to determine angular direction.
Data may be transmitted to the tag 430 from another tag, or from an infrastructure receiver, or from a wireless access point (AP). Received data may be demodulated in the receive chain 438 and presented to the CPU 442. The CPU may be used to control the modulator in the transmit chain 440 to send data back or to generate queries. The received data may be a poll, a query, values to store in the memory 444, or new programming instructions. It may also be parameters to be used in running the programs in the memory.
Depending on the programming, the active tag, acting as an active RFID tag attached to the tagged item 101, may send a periodic ID signal or respond to polling signals according to any of a variety of different protocols or routines. The position of the tag and the best connections for RF communication may be determined in a variety of different ways. In one example, a group of APs measure the RSSI (Received Signal Strength Indicator) of the tag to triangulate the position and determine the best AP for communications.
In the example of
The tag for the mobile unit may also have a user interface (not shown). A user interface would allow a user or operator to move the mobile unit closer to the tagged item, to read the measured location of the tagged item or to provide commands to the unit. The user interface may be a display, a touch screen, or a full display and keyboard or keypad interface. Alternatively, the tag for the mobile unit may have an electronic interface to the mobile unit. Such an interface may allow the tag to send directions, position information, or complete user interface information to the mobile unit for use in operating the mobile unit either autonomously or by a human operator.
At box, 503, this coarse position is received by a fine positioning system. The fine positioning system is at least partially mobile and at block 505 is moved to the coarse position. As shown in
At block 507, the fine positioning system determines the fine position of the tagged item. This may be done in any of a variety of different ways as described above. The fine positioning system is able to take advantage of the close range to the tagged item. This allows a higher accuracy to be obtained using approaches that would be less effective if used from a greater range.
A lesser or more complex passive transceiver structure, active transceiver structure, tag, coarse and fine positioning system, communications protocol, and supporting infrastructure may be used than those shown and described herein. Therefore, the configurations may vary from implementation to implementation depending upon numerous factors, such as price constraints, performance requirements, technological improvements, and other circumstances. Embodiments of the invention may also be applied to other types of inventory tracking and control systems and different RFID systems that use different types of transponders and protocols than those shown and described herein.
In the description above, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form.
The present invention may include various steps. The steps of the present invention may be performed by hardware components, such as those shown in the figures, or may be embodied in machine-executable instructions, which may be used to cause general-purpose or special-purpose processor or logic circuits programmed with the instructions to perform the steps. Alternatively, the steps may be performed by a combination of hardware and software.
The present invention may be provided as a computer program product which may include a machine-readable medium having stored thereon instructions which may be used to program an agent or a computer system to perform a process according to the present invention. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or optical cards, flash memory, or other type of machine-readable media suitable for storing electronic instructions.
Many of the methods and apparatus are described in their most basic form but steps may be added to or deleted from any of the methods and components may be added or removed from any of the described apparatus without departing from the basic scope of the present invention. Many further modifications and adaptations may be made. The particular embodiments are not provided to limit the invention but to illustrate it. In the description above, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. For example, well-known equivalent circuits, components, assemblies and configurations may be substituted in place of those described herein, and similarly, well-known equivalent techniques, processes, and protocols may be substituted in place of the particular techniques described. In other instances, well-known circuits, structures and techniques have not been shown in detail to avoid obscuring the understanding of this description.
While the embodiments of the invention have been described in terms of several examples, those skilled in the art may recognize that the invention is not limited to the embodiments described, but may be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.
The present application is related to provisional application Ser. No. 60/785,703, filed Mar. 23, 2006, entitled Coarse-Fine Location System, the priority of which is hereby claimed.
| Number | Date | Country | |
|---|---|---|---|
| 60785703 | Mar 2006 | US |
