Object location system and method

• US 20040108954A1
• Filed: 10/18/2001
• Published: 06/10/2004
• Est. Priority Date: 10/18/2001
• Status: Active Grant

• Current Assignee: Zebra Technologies Corporation
• Original Assignee: Multispectral Solutions Incorporated (Zebra Technologies Corporation)
• Inventors: Perino, Donald V., Richley, Edward A., Fontana, Robert J., Ameti, Aitan
• Granted Patent: US 6,882,315 B2
• Time in Patent Office: Days
• US Class Current: 342/387
• CPC Class Codes: G01S 5/0226 Transmitters
• CPC Class Codes: G01S 5/06 Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements G01S5/12 takes precedence

Abstract

An RF object locating system and method that uses or includes a set of N (N>2) receivers (monitoring stations) located at fixed positions in and/or about a region to be monitored, one or more reference transmitters that transmit a timing reference, a location processor that determines object location based on time-of-arrival measurements, and at least one object having an untethered tag transmitter that transmits RF pulses, which may additionally include object ID or other information. Free-running counters in the monitoring stations, whose phase offsets are determined relative to a reference transmitter, are frequency-locked with a centralized reference clock. Time-of-arrival measurements made at the monitoring stations may be stored and held in a local memory until polled by the location processor. The invention permits rapid acquisition of tag transmissions thereby enabling the monitoring of large numbers of objects, and also provides a unique approach to data correlation in severe multipath environments.

Claims

1. An object location system for determining the position of an object in a monitored region, said system comprising:

an object tag that transmits a tag signal in the monitored region,

a reference transmitter having a known location to transmit a timing reference signal in the monitored region,

at least two monitoring stations positioned at known locations, each of said monitoring stations having a common clock source and a respective time counting device that measures increments of time, a receiver that receives the timing reference signal and the tag signal, and a processor to detect time of arrival of said tag signal and to determine an offset (skew) of the time counting device relative to the timing reference signal, and

a location processor that receives the respective offsets and measured time-of-arrival information from said monitoring stations to determine the position of the object.

2. An object location system that determines the position of an object, said system comprising an RF tag that transmits a tag signal, a reference transmitter that transmits a timing reference signal from a known location, plural linked monitoring stations sharing a common clock reference also positioned at known locations and that include respective receivers each of which having capability to measure times of arrival of the timing reference signal and the tag signal, and a processor in communication with the monitoring stations to compute object location based on a determination of offsets in respective timing circuit relative to each other and measured times of arrival of the tag signal and the timing reference signal at the respective monitoring stations.

3. The object location system of claim 2, wherein said processor is linked with said monitoring stations.

4. The object location system as recited in claim 3, further including daisy changed link between said monitoring stations.

5. The object location system as recited claim 4, further including a daisy changed link between said location processor and monitoring stations.

6. The object location system of claim 2, wherein said object comprises at least one of an asset, inventory, and personnel.

7. The object location system as recited in claim 2, wherein said RF tag and reference transmitter includes one of an ultra wideband and a short pulse transmitter.

8. The object location system as recited in claim 2, wherein respective monitoring stations include a free-running, non-resettable counter to measure times of arrival of said tag and timing reference signals.

9. The object location system as recited in claim 2, wherein said location processor uses nonlinear optimization techniques to measure differential times.

10. The object location system as recited claim 6, wherein said location processor includes a routine employing nonlinear optimization techniques to determine object location.

11. The object location system as recited in claim 2, where the tag signal further include at least one of an identification signal, a packet sequence number, and data associated with object.

12. The object location system as recited in claim 2, wherein the tag signal comprises one of a periodic, aperiodic, random, externally prompted signal.

13. The object location system as recited in claim 2, wherein the respective monitoring stations further include a memory that stores times-of-arrival and offset information, and said processor obtaining information from memory to determine the location of the object.

14. The object location system as recited in claim 13, wherein said location processor polls the memory to obtain offset and time-of-arrival information.

15. The object location system as recited in claim 2, wherein the tag includes a sequence number, said location processor uses sequence number of tag signal to correlate data at respective monitoring stations.

16. A method of determining the position of an object in a monitored region, said method comprising:

transmitting an RF tag signal from an object to be located,

transmitting a timing reference signal from a known location,

providing a common clock signal to plural linked monitoring stations,

measuring, at the monitoring stations, times of arrival of the timing reference signal and the tag signal,

adjusting measured times of arrival according to relative offsets among the monitoring stations, and

determining the position of the object according to the relative offsets and the measured times of arrival of the tag signal at the respective monitoring stations.

17. The method of claim 16, further including:

in said determining step, determining the position of the object by using nonlinear optimization techniques to measure differential times of arrival.

18. The method of claim 18, wherein each said transmitting step includes transmitting an ultra wideband signal.

19. The method of claim 18, wherein in said measuring step, utilizing a free-running, non-resettable counter to measure times of arrival.

20. The method of claim 18, wherein the providing step includes daisy-chaining said monitoring stations.

21. The method of claim 18, wherein in said first transmitting step, said tag signal further includes at least one of a tag ID, a sequence number, and information associated with the object to be located.

22. The method of claim 21, further including transmitting the tag signal by at least one of periodically, aperiodically, randomly, or according to an external event.

23. The method of claim 18, further including storing digital representations of measured times of arrival in a memory associated with the monitoring stations, and sending the stored information to a processor for subsequent determination of object location.

24. The method of claim 18, further including transmitting a sequence number with said tag signal in order to provide a basis for a processor to correlate respective tag signals stored at respective monitoring stations when determining object location in noisy environments.

25. A method of determining the position of an object in a monitored region, said method comprising:

transmitting an RF tag signal from an object to be located,

transmitting a timing reference signal from a fixed location,

receiving the RF tag signal and the timing reference signal at fixed monitoring stations,

providing a common clock signal to plural linked monitoring stations,

measuring, at the monitoring stations, times of arrival of the timing reference signal and the tag signal,

determining the position of the location according to measured times of arrival of the tag signal and the timing reference signal at the respective monitoring stations.

26. The method of claim 25, further including correcting the measured times of arrival of said timing reference signal according to relative offsets.

27. The method of claim 26, further including using nonlinear optimization techniques to measure differential times of arrival.

28. The method of claim 27, wherein said RF tag signal comprises an ultra wideband pulse.

29. The method of claim 27, where the RF tag signal comprises a short wide band pulse.

30. The method of claim 26 further comprising using a free-running counter to measure times of arrival.

31. The method of claim 26, further including daisy chaining the monitoring stations via a common clock source.

32. The method of claim 26, further including transmitting with the tag signal at least one of ID code, sequence number, and information associated with the object.

33. The method as recited in claim 26, further including storing digital representations of offset and TOA data in memory associated with the monitoring stations and sending memory data to a processor that determines object location.

34. The method as recited in claim 33, wherein a processor polls said memory to obtain data pertaining to said offset and TAO data.

35. The method as recited in claim 33, wherein said processor uses sequence numbers of said tag signal to correlate data obtained from the respective monitoring stations in order to determine object location.

36. In an object location system for determining the position of an object, said system comprising an RF tag that transmits a tag signal, a reference transmitter that transmits a timing reference signal from a known location, and a location processor in communication with the monitoring stations to determine object location based on measured times of arrival, the improvement comprising:

plural linked monitoring stations positioned at known locations wherein each of said monitoring stations shares a common daisy-chained clock source and includes a receiver that measures relative times of arrival of the timing reference signal and the tag signal, and

said location processor determines object position using nonlinear optimization techniques to measure differential times of arrival of said timing reference signals at said monitoring stations relative to a timing reference signal.

37. In a system for determining the position of an object, said system comprising an RF tag that transmits a tag signal, a reference transmitter that transmits a timing reference signal from a monitoring station positioned at a known location, and plural linked monitoring stations positioned at known locations and that include a receiver that detects relative times of arrival of the timing reference signal and the tag signal, the improvement comprising:

a time-of arrival processor located at each monitoring station to measure times of arrival of the timing reference signal and to adjust relative timing offsets among the monitoring stations based on known distances from the timing monitoring station, and

a location processor that determines object position according to times of arrival of said tag signal and said timing reference signal sent to the location processor but measured at the monitoring stations.

38. The improvement of claim 38, wherein the tag signal comprises an UWB pulse that includes at least one of an ID code, a sequence number, and data associated with the object.

39. The improvement of claim 38, wherein the monitoring station further includes a free-running counter to is used to measure times of arrival.

40. The improvement of claim 39, wherein digital representations of offset and TOA are stored in a memory associated with the monitoring station, and said memory is accessed by the location processor to determine the location of the object.

41. The improvement of claim 40, wherein the location processor uses a sequence number of tag signal to correlate data at respective monitoring stations.