RFID TAG LOCATION SYSTEMS

• US 20130201003A1
• Filed: 04/18/2011
• Published: 08/08/2013
• Est. Priority Date: 04/26/2010
• Status: Active Grant

• Current Assignee: Cambridge Enterprise Limited (University of Cambridge)
• Original Assignee: Cambridge Enterprise Limited (University of Cambridge)
• Inventors: Penty, Richard, White, Ian H., Sabesan, Sithamparanathan, Crisp, Michael
• Granted Patent: US 9,367,785 B2
• Time in Patent Office: Days
• US Class Current: 340/10.1
• CPC Class Codes: G01S 13/84 for distance determination by phase measurement
• CPC Class Codes: G01S 13/878 Combination of several spaced transmitters or receivers of known location for determining the position of a transponder or a reflector G01S13/874 takes precedence
• CPC Class Codes: G01S 7/42 Diversity systems specially adapted for radar
• CPC Class Codes: G06K 19/0723 the record carrier comprising an arrangement for non-contact communication, e.g. wireless communication circuits on transponder cards, non-contact smart cards or RFIDs

Abstract

We describe a methods of locating an RFID tag. One method transmits tag location signals at a plurality of different frequencies from a plurality of different antennas spaced apart by more than a near field limit distance. The processing determines a phase difference at the plurality of different frequencies by determining a phase difference between either i) two or more of the transmit signals resulting in a maxima in the returned signal RSSI or ii) a first transmit signal and its corresponding return signal. The range determining uses return signals weighted by signal strength. Further data which may be used for averaging may be generated by using the above techniques along with changes in the polarisation state of the transmit and receive antennas and/or physical reconfiguration of the antennas (e.g. switch the transmit and receive elements).

Claims

1-38. -38. (canceled)

39. A method of locating an RFID tag, the method comprising:

transmitting tag location signals from a plurality of different transmit antennas, wherein said antennas are spaced apart by more than a near field limit distance at a frequency of a said signal;

receiving a corresponding plurality of receiving return signals from said tag; and

processing said tag return signals to determine a range to said tag;

wherein said transmitting comprises transmitting at a plurality of different frequencies;

wherein said processing comprises determining a phase difference at said plurality of different frequencies to determine said range, and wherein said determining of said phase difference determines a phase difference between either i) two of said transmit signals or ii) a first transmit signal and its corresponding return signal; and

wherein said determining of said range to said tag uses said return signals weighted responsive to a respective received signal strength of the return signal.

40. A method as claimed in claim 39, wherein said determining of said phase difference comprises determining a phase difference between a said transmit signal and its corresponding return signal at a first frequency and at a second frequency, and repeating said determining of said phase difference at a plurality of said first and second frequencies, maintaining a substantially constant frequency offset between said first and second frequencies.

41. A method as claimed in claim 40, further comprising exchanging between antennas used to transmit said transmit signal and receive said return signal to use a transmit antenna for a receiver antenna and vice-versa when determining a said phase difference.

42. A method as claimed in claim 39, wherein said determining of said phase difference comprises determining a phase difference between two transmit signals simultaneously transmitted towards said RFID tag which substantially maximises a return signal from said RFID tag.

43. A method as claimed in claim 42, comprising adjusting a relative phase of said signals simultaneously transmitted towards said RFID tag to substantially maximise said return signal.

44. A method as claimed in claim 39, further comprising determining a said phase difference using antennas of a plurality of different beamwidths.

45. A method as claimed in claim 39, wherein weighting of said return signals comprises thresholding said return signals to discard return signals below a threshold received signal strength when determining said range to said tag.

46. A method as claimed in claim 39, further comprising applying a prediction-correction filter to a plurality of said phase differences at said plurality of different frequencies to determine a plurality of corrected estimates of said phase differences, wherein a said corrected estimate of a said phase difference is dependent on a phase variance value responsive to said received signal strength for the return signal from which the said phase difference was derived.

47. A method as claimed in claim 46, wherein said prediction-correction filter comprises a Kalman filter.

48. A method as derives in claim 39, herein said RFID tag is a passive, backscatter modulation tag.

49. A system for locating an RFID tag, the system comprising:

a transmitter to transmit tag location signals from a plurality of different transmit antennas, wherein said antennas are spaced apart by more than a new field limit distance at a frequency of a said signal;

a receiver to receive a corresponding plurality of receiving return signals from said tag; and

a processor to process said tag return signals to determine a range to said tag;

wherein said transmitting comprises transmitting at a plurality of different frequencies;

wherein said processing comprises determining a phase difference at said plurality of different frequencies to determine said range, and wherein said determining of said phase difference determines a phase difference between either i) two of said transmit signals or ii)

a first transmit signal and its corresponding return signal; and

wherein said determining of said range to said tag uses said return signals weighted responsive to a respective received signal strength of the return signal.

50. A data carrier carrying processor control code for the system of claim 49, to, when running, process said tag return signals to determine a range to said tag;

wherein said transmitting comprises transmitting at a plurality of different frequencies;

wherein said processing comprises determining a phase difference at said plurality of different frequencies to determine said range, and wherein said determining of said phase difference determines a phase difference between either i) two of said transmit signals or ii) a first transmit signal and its corresponding return signal; and

wherein said determining of said range to said tag uses said return signals weighted responsive to a respective received signal strength of the return signal.

51. A method of determining tag-related data from a tag employing modulation of a reflected signal, the method comprising:

transmitting data towards said tag from at least one transmit antenna, wherein said transmitted signal comprises a signal having at least two different discrete transmit frequencies simultaneously, and wherein either i) the same tag command data is transmitted on each of said discrete frequencies, or ii) one or more of said discrete frequencies consists of a carrier and one or more others of said discrete frequencies transmit the or the same tag command data;

modulating, at said tag, said at least two different discrete frequencies at the same time to generate a modulated return signal having said at least two different discrete frequencies at the same time;

receiving said modulated return signal at at least one receive antenna; and

determining said tag-related data from signals received from said tag at said different discrete frequencies.

52. A method as claimed in claim 51, comprising transmitting a said transmitted signal simultaneously from at least two said transmit antennas.

53. A method as claimed in claim 51, wherein said modulating comprises backscatter modulating a said transmitted signal to provide said modulated return signal.

56. A method as claimed in claim 51, wherein said tag-related claim comprises data read from said tag, wherein said determining comprises determining said data responsive to a received signal strength at each of said different frequencies.

57. A method as claimed in claim 51, wherein said tag-related claim comprises location data, wherein said determining comprises determining said data responsive to one or more of a received signal strength at each of said different frequencies and relative phase of each of said discrete frequencies.

58. A method as claimed in claim 57, further comprising filtering said relative phase using a Kalman filter having phase variance data dependent on a received signal strength of a signal from which said relative phase is derived to determine said location data.

59. A method as claimed in claim 51, wherein said determining of said tag related data further comprises demodulating each of said discrete frequencies to determine one or more of a received signal strength at each of said different frequencies and at at least one receive antenna and a relative phase at each of said discrete frequencies.

60. A method as claimed in claim 51, wherein said tag is a passive RFID tag.

61. An interrogation transceiver for use in the method of claim 51, the interrogation transceiver comprising a plurality of transceiver circuits for simultaneous operation, each configured to transmit at a different frequency, wherein each of said transceiver circuits is coupled to a common antenna interface.

62. A method as claimed in claim 61, further comprising a transceiver controller to apply one or both of frequency control and phase control to each of said transceiver circuits.

82. A method as claimed in claim 61, further comprising a transceiver controller to apply one or both of frequency control and phase control to each of said transceiver circuits.

54. A method of determining tag-related data from a tag employing backscatter modulation of a reflected signal, the method comprising:

transmitting data towards said tag from a plurality of transmit antennas using a plurality of transmitted signals, wherein the same tag command data is transmitted from each said antenna, and wherein each of said transmitted signals comprises a signal having at least two different discrete transmit frequencies simultaneously;

modulating, at said tag using backscatter modulation, said at least two different discrete frequencies at the same time to generate a plurality of modulated backscatter signals each having said at least two different discrete frequencies at the same time;

receiving said modulated backscatter signals at at least one receive antenna; and

determining said tag-related data from a combination of said modulated backscatter signals at said different discrete frequencies.

55. A method as claimed in claim 54, further comprising transmitting said transmitted signals simultaneously from said plurality of transmit antennas.

77. A method as claimed in claim 54, wherein said tag-related claim comprises data read from said tag, wherein said determining comprises determining said data responsive to a received signal strength at each of said different frequencies.

78. A method as claimed in claim 54, wherein said tag-related claim comprises location data, wherein said determining comprises determining said data responsive to one or more of a received signal strength at each of said different frequencies and relative phase of each of said discrete frequencies.

79. A method as claimed in claim 78, further comprising filtering said relative phase using a Kalman filter having phase variance data dependent on a received signal strength of a signal from which said relative phase is derived to determine said location data.

80. A method as claimed in claim 54, wherein said determining of said tag related data further comprises demodulating each of said discrete frequencies to determine one or more of a received signal strength at each of said different frequencies and at at least one receive antenna and a relative phase at each of said discrete frequencies.

81. An interrogation transceiver for use in the method of claim 54, the interrogation transceiver comprising a plurality of transceiver circuits for simultaneous operation, each configured to transmit at a different frequency, wherein each of said transceiver circuits is coupled to a common antenna interface.

63. A method of locating an RF device using an RF device interrogation system having a plurality of antennas to communicate with said RF device, said interrogation system comprising a transmitter to transmit an interrogation signal to a said RF device simultaneously from said plurality of antennas and a receiver to receive a combined signal simultaneously from said plurality of antennas, wherein said antennas are distributed over a region of space and spaced apart from one another sufficiently for one said antenna not to be within the near field of another said antenna, the method comprising:

calibrating a response of said interrogation system over said region of space by moving a said RF device over a plurality of known locations to determine calibration data for said RF device interrogation system, wherein said calibrating further comprises;

i) changing one or both of;

a relative phase of said interrogation signal transmitted from one of said antennas with respect to said interrogation signal transmitted from another of said antennas, and a frequency of said interrogation signal transmitted from said plurality of antennas; and

ii) determining a combined received signal strength from said plurality of antennas, wherein said combined received signal strength comprises a signal strength of combined RF signals from said plurality of antennas;

determining device-locating data defining an RF signal strength from each of said plurality of antennas and a combined received signal strength from combined RF signals received from said plurality of antennas for a said RF device to be located, wherein said determining comprises changing one or both of;

a relative phase of said interrogation signal transmitted from one of said antennas with respect to said interrogation signal transmitted from another of said antennas, and a frequency of said interrogation signal transmitted from said plurality of antennas; and

determining, using said calibration data, a location for said RF device to be located from said device-locating data including said combined received signal strength.

64. A method as claimed in claim 63, wherein said calibrating comprises changing said relative phase or frequency whilst said RF device is at a said known location.

65. A method as claimed in claim 63, wherein one or both of said calibrating and said determining of said device-locating data further comprises determining said calibration data and/or said device-locating data for one or more sets of partially combined received signal strengths, a said set of partially combined received signal strengths comprising a signal strength for a combination of RF signals from a subset of said plurality of antennas comprising at least two of said antennas together with a signal strength of at least one RF signal from a third of said antenna not forming part of said subset.

67. A method as claimed in claim 63, wherein said determining of a location of said RF device comprises determining a closest match of a said combined received signal strength at a said relative phase or frequency to a said known location of said calibration.

68. A method as claimed in claim 63, wherein said calibration data defines one of more parameters of an analytical expression relating said device-locating data to said location for said RF device.

69. A method as claimed in claim 68, wherein said determining of a location of said RF device further comprises selecting a locus of permitted said locations for said RF device using RF signal strength from one of said antennas, and determining a position on said locus of permitted locations using RF signal strengths from two others of said antennas.

70. A method as claimed in claim 63, wherein said antennas are re-used for data communications system covering said region of space, wherein said plurality of antennas comprises at least three antennas defining a triangle, and wherein said antennas are configured to define a plurality of data communications cells, each said antenna serving a said data communications cell, such that said region of space is covered by at least three said data communications cells.

71. A method as claimed in claim 63, wherein said RF device comprises a passive RFID tag having a protocol for reading multiple said tags, and wherein the method uses said protocol to sequentially locate a plurality of said tags within said region of space.

72. A carrier carrying computer program code to, when running implement the method of any one of claim 63.

76. A method as claimed in claim 63, wherein said RF device is an RFID tag and wherein said RF device interrogation system is an RFID tag reading system.

73. A system for locating an RF device using an RF device interrogation system having a plurality of antennas to communicate with said RF device, said interrogation system comprising a transmitter to transmit an interrogation signal to a said RF device simultaneously from said plurality of antennas and a receiver to receive a combined signal simultaneously from said plurality of antennas, wherein said antennas are distributed over a region of space and spaced apart from one another sufficiently for one said antenna not to be within the near field of another said antenna, the system comprising:

means for calibrating a response of said interrogation system over said region of space by moving a said RF device over a plurality of known locations to determine calibration data for said RF device interrogation system, wherein said means for calibrating further comprises;

i) means for changing one or both of;

a relative phase of said interrogation signal transmitted from one of said antennas with respect to said interrogation signal transmitted from another of said antennas, and a frequency of said interrogation signal transmitted from said plurality of antennas; and

ii) means for determining a combined received signal strength from said plurality of antennas, wherein said combined received signal strength comprises a signal strength of a combined RF signals from said plurality of antennas;

means for determining device-locating data defining an RF signal strength from each of said plurality of antennas and a combined received signal strength from combined RF signals received from said plurality of antennas for a said RF device to be located, wherein said determining comprises changing one or both of;

a relative phase of said interrogation signal transmitted from one of said antennas with respect to said interrogation signal transmitted from another of said antennas, and a frequency of said interrogation signal transmitted from said plurality of antennas; and

means for determining a location for said RF device to be located from said device-locating data including said combined received signal strength, using said calibration data.

74. A system as claimed in claim 73, wherein said RF device is an RFID tag.

75. An RFID tag interrogation system signal processor for locating an RFID tag using an RFID tag interrogation system having a plurality of antennas, to communicate with said tag, said interrogation system comprising a transmitter to transmit an interrogation signal to a said tag simultaneously from said plurality of antennas and a receiver to receive a combined signal simultaneously from said plurality of antennas, wherein said antennas are distributed over a region of space and spaced apart from one another sufficiently for one said antenna not to be within the near field of another said antenna, the signal processor comprising:

a calibration module to calibrate a response of said interrogation system over said region of space by moving a said RFID tag over a plurality of known locations to determine calibration data for said RFID tag interrogation system, said calibration module being configured to;

change one or both of;

a relative phase of said interrogation signal transmitted from one of said antennas with respect to said interrogation signal transmittal from another of said antennas, and a frequency of said interrogation signal transmittal fro said plurality of antennas;

determine combined a combined received signal strength from said plurality of antennas, wherein said combined received signal strength comprises a signal strength of combined RF signals from said plurality of antennas;

determine tag-locating data defining an RF signal strength from each of said plurality of antennas and a combined received signal strength from combined RF signals received from said plurality of antennas for a said RFID tag to be located, wherein said determining comprises changing one or both of;

a relative phase of said interrogation signal transmitted from one of said antennas with respect to said interrogation signal transmitted from another of said antennas, and a frequency of said interrogation signal transmitted from said plurality of antennas; and

determine, using said calibration data, a location for said RFID tag to be located from said tag-locating data including said combined received signal strength.