Object location system and method

• US 6,882,315 B2
• Filed: 10/18/2001
• Issued: 04/19/2005
• 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
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): MULL, FRED H
• Application Number:
  US09/978,750
• Publication Number:
  US 20040108954A1
• Time in Patent Office: 1,279 Days
• Field of Search: 342/465, 342/463, 342/387
• US Class Current: 342/465
• 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 reference signal,

at least two monitoring stations positioned at known locations, each of said monitoring stations being coupled to a common clock source of relatively low frequency and including a respective time counting device being indexed at a relatively higher frequency and cyclically frequency-locked with said common clock source to maintained a fixed offset relative to time counting devices of other monitoring stations, each of said monitoring stations further including a receiver that receives the reference signal and the tag signal, and

a processor to obtain times of arrival of said tag signals at said monitoring stations, to determine offsets of time counting devices of the monitoring stations, and to determine the position of the object based on said offsets and known locations.

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 reference signal from a known location, plural monitoring stations also positioned at known locations and including respective timers having cycles that are relatively phase-locked with a common clock reference to measure times of arrival of the reference signal and the tag signal, and processor in communication with the monitoring stations to obtain said times of arrival to compute object location based on said known locations and offsets in respective timers relative to each other.

3. The object location system of claim 2, wherein said timer comprises a counter that is indexed at a frequency multiple of said common clock reference and further includes a phase locked loop circuit to relatively lock cycling of said counter with occurrences of clock pulse of said common clock reference.

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

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

6. The object location system of claim 3, wherein said object comprises at least one of an asset, inventory, and personnel.

7. The object location system as recited in claim 3, wherein said RF rag and reference transmitter includes one of an ultra wideband and a short pulse transmitter.

8. The object location system as recited in claim 3, wherein said location processor uses nonlinear optimization techniques to measure differential times.

9. The object location system as recited claim 3, wherein said location processor includes a routine employing nonlinear optimization techniques to determine object location.

10. The object location system as recited in claim 3, wherein the tag signal further includes at least one of an identification signal, a packet sequence number, and data associated with object.

11. The object location system as recited in claim 3, wherein the tag signal comprises one of a periodic, aperiodic, random, and an externally prompted signal.

12. The object location system as recited in claim 2, wherein respective timers of said monitoring stations include non-resettable counters to measure times of arrival of said tag and reference signals and said timers have an index frequency higher than said common clock reference to provide incremental count values indicative of said times of arrival.

13. The object location system as recited in claim 2, wherein the respective monitoring stations further include a memory that stores a digital representation of times-of-arrival and offset information, and said processor accesses said memory to obtain time of arrival information from said 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 signal includes a sequence number, said location processor uses sequence number of said tag signal to correlate data from respective monitoring stations.

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

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

transmitting a reference signal from a known location,

providing a common clock signal to respective timers of plural monitoring stations, said timers having an index frequency being a frequency multiple of said common clock signal to provide incremental count values indicative of times of arrival of said tag signal and said reference signal,

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

determining relative offsets among timers of the monitoring stations using times of arrival of said reference signal measured in said measuring step,

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

determining the position of the object according to the relative offsets and 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 using nonlinear optimization techniques to measure differential times of arrival.

18. The method of claim 17, wherein each said transmitting step includes transmitting an ultra wideband signal.

19. The method of claim 18, wherein in said measuring step, utilizing a relatively phase-locked, non-resettable counter that is indexed at an integer multiple of the frequency of said common clock signal to provide incremental count values that measure times of arrival.

20. The method of claim 18, wherein the providing step include 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, and according to an external event.

23. The method of claim 18, further including storing digital representations of measured times of arrival in a memory located at the monitoring stations, and sending said digital representations 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 a position of an object in a monitored region, said method comprising:

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

transmitting a reference signal from a fixed location,

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

providing a common clock signal to plural monitoring stations,

at said fixed monitoring stations, generating incremental count values using a counter having a cycle that is relatively phase-locked with, and index at a frequency relatively higher than a frequency of said common clock source to measure, times of arrival of the reference signal and the tag signal, and

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

26. The method of claim 25, further including transmitting said common clock source to said monitoring stations via an network data line.

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 counter being indexed at an integer multiple frequency of said common clock source to measure times of arrival.

31. The method of claim 26, further including daisy chaining the monitoring stations via said network data line.

32. The method of claim 26, further including transmitting with the tag signal at least one of an 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 time of arrival data in memory located within the monitoring stations and sending said data via a local area network to a processor that determines object location.

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

35. The method as recited in claim 33, further including said processor utilizing sequence numbers of multiple tag signals to correlate data obtained from the respective monitoring stations in order to determine object location.

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

a daisy-chained clock source applied to each monitoring station, each said monitoring station having a high-frequency timer indexed at a frequency multiple of said clock source and a phase locking circuit to lock relative phase relation between cycles of said timer and said clock source, said high-frequency timer being operative to measure times of arrival of the reference signal and the tag signal, and

said location processor being operative to determine object position using nonlinear optimization techniques to measure differential times of arrival of said tag signals at said monitoring stations.

37. In a system for determining a position of an object, said system comprising an RF tag that transmits a tag signal, a reference transmitter that transmits a reference signal from a reference station positioned at known location, and plural linked monitoring stations positioned at known locations and that include a receiver that detects relative times of arrival of the 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 reference signal, said time of arrival processor including a counter being indexed at a frequency multiple of and relatively phase-locked in cycles with a common clock, and

a location processor to determine relative timing offsets among monitoring stations based on known distances from the reference station and that determines object position according to said offsets and times of arrival of said tag signal.

38. The improvement of claim 37, 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 37, wherein the time-of-arrival processor comprises a counter to measure times of arrival.

40. The improvement of claim 39, wherein digital representations of offset and times of arrival are stored in a memory located within each 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 the tag signal to correlate data from respective monitoring stations.

42. A system to determine a location of an object that emits a UWB signal, said system comprising:

a clock source that generates periodic clock pulses at a fixed-frequency,

at least two monitoring stations positioned at known locations each including an incremental time counter having an index frequency higher than said fixed frequency to measure arrival times of UWB signals in discrete increments of time and a phase-locking circuit to lock cycles of said counter with clock source,

a reference transmitter also positioned at a known location to transmit a reference signal that effects latching of incremental time values indicative of times of arrival of the reference signal at the monitoring stations, and

a processor that utilizes the incremental time values to determine relative offsets between respective timers of the monitoring station based on known locations thereof and to determine the location of the object based on said relative offsets and times of arrival of the UWB signal at the monitoring stations.

43. The system of claim 42, further comprising an network data line to convey the clock pulses to the monitoring stations.

44. The system of claim 43, wherein said data line conveying said clock pulses is daisy-chained to multiple monitoring stations.

45. The system of claim 44, further comprising a LAN connection between the processor and the monitoring stations to covey a digital representation of said time values.

46. The system of claim 42, wherein each incremental time counter comprises a non-resettable cyclic counter having a count index frequency being a multiple of said fixed-frequency of the clock source, whereby said count index frequency defines position resolution of object location.

47. The system of claim 46, further comprising a PLL multiplier responsive to said clock source to index said counter at a multiple of said fixed frequency.

48. The system of claim 46, wherein respective counters of the monitoring stations latch respective time values to detect arrival of the UWB signal, and said processor obtains and utilizes respective time values of the monitoring stations to locate the object.

49. The system of claim 42, wherein each said incremental time counter further includes a delay line having multiple taps to further increase resolution accuracy of object location.

50. The system of claim 42, wherein said reference signal includes a identification component to enable identification thereof prior to latching a time value indicative of receipt of a signal, and said processor utilizes said identification component to match respective pairs of offsets obtained from the monitoring stations.

51. The system of claim 50, wherein said reference signal comprises a data packet comprising a series of UWB pulses that provide framing bits to enable extraction and identification of a unique bit sequence from the reference signal.

52. The system of claim 42, wherein the incremental time counters said monitoring stations are phase-locked relative to each other by frequency locking cycling of the time counters of the monitoring station with the clock source.

53. The system of claim 42, further comprising an identification code associated with the object UWB signal to identify the object.

54. The system of claim 53, further comprising digital data embedded within the UWB signal to provide information associated with said object.

55. The system of claim 53, further comprising a timing circuit associated with transmission of said object UWB signal to adjust an interval between successive packet transmissions of the object thereby to reduce the chance of data collisions when multiple objects transmit packets.

56. The system of claim 53, further including a memory located within each monitoring station and said processor polls said monitoring station to obtain timing and identification information about the reference tag and the object.

57. The system of claim 56, further including a network interface at said monitoring stations, and said processor communicates with said network interface via a local area network to obtain said information and to command the monitoring station to alter an operational characteristic thereof.

58. The system of claim 57, wherein said operational characteristic includes at least one of gain and detection threshold applied to received signals.

59. A method of locating an object comprising the steps of:

positioning multiple monitoring stations at known locations,

transmitting a UWB tag signal from the object,

generating a periodic clock signal of a relatively low fixed-frequency,

at the monitoring stations, receiving the periodic clock signal and phase-locking respective incremental timers of the monitoring stations relative to each other using said periodic clock signal, said timers operating at a relatively higher frequency than said periodic clock signal to provide discrete count values indicative of times of arrival of received signals,

transmitting a reference signal from a known location to effect latching of respective count values at the monitoring stations indicative of times of arrival of the reference signal,

determining relative offsets in the incremental timers of the monitoring stations based on the known locations and count values associated with receipt of the reference signal at respective monitoring stations, and

locating the object based on the relative offsets and times of arrival of the UWB tag signal at the respective monitoring stations.

60. A method of locating an object comprising:

positioning multiple monitoring stations at known locations,

transmitting a UWB tag signal from the object,

generating a fixed-frequency clock signal,

receiving and utilizing the fixed-frequency clock signal at the monitoring stations to relatively phase-lock cycling of respective counters of the monitoring stations relative to each other utilizing the fixed-frequency clock signal to frequency-lock cycling of the respective counters, said counters being indexed at a frequency multiple of said fixed frequency clock signal to generate and latch count values indicative of times of arrival of received signals,

transmitting a reference signal from a known location to effect latching of count values indicative of times of arrival of the reference signal at the monitoring stations,

determining relative offsets in the counters of the monitoring stations based on the known locations and count values associated with receipt of the reference signal at respective monitoring stations, and

locating the object based on the relative offsets and times of arrival of the UWB tag signal at the monitoring stations.

61. A system to locate an object that transmits a tag signal, said system comprising:

a primary clock to generate periodic clock pulses of a relatively low fixed frequency,

at least two monitoring stations positioned at known locations, each of said monitoring stations including a secondary clock of relatively high frequency to produce count values indicative of times of arrival and a phase-locking circuit to lock cycle in intervals of the secondary clock with occurrences of said periodic clock pulses of the primary clock whereby to maintain a constant phase offset between cycles of respective secondary clocks of the monitoring stations,

a reference transmitter also positioned at a known location to transmit a reference signal that effects latching of clock count values indicative of times of arrival of the reference signal at the monitoring stations, and

a processor that obtains digital representations of the count values from the monitoring stations to determine offsets between the secondary clocks of the monitoring stations based on the known locations and to locate the object based on the offsets and times of arrival of the tag signal at the monitoring stations.

62. The system of claim 61, wherein said secondary clock of each monitoring station comprises a non-resettable cyclic counter having an index frequency being a multiple of said relatively low fixed frequency of the primary clock.

63. The system of claim 62, wherein said secondary clock further includes a multi-tapped delay line to measure further increments of time.

64. The system of claim 61, further including an network cable to convey said periodic clock pulses of the primary clock to said monitoring stations.

65. The system of claim 64, wherein the frequency of said primary clock is ten MHz and the index frequency of said secondary clock is a multiple thereof.