Methods and Apparatuses For RFID Tag Range Determination

• US 20100109844A1
• Filed: 11/03/2009
• Published: 05/06/2010
• Est. Priority Date: 11/03/2008
• Status: Active Grant

• Current Assignee: Novanta Corporation (Novanta Inc.)
• Original Assignee: ThingMagic, Inc. (Novanta Inc.)
• Inventors: Maguire, Yael G., Carrick, John C.
• Granted Patent: US 8,446,254 B2
• Time in Patent Office: Days
• US Class Current: 340/10.100
• CPC Class Codes: G01S 11/02 using radio waves G01S19/00 takes precedence
• CPC Class Codes: G01S 2013/466 by Trilateration, i.e. two antennas or two sensors determine separately the distance to a target, whereby with the knowledge of the baseline length, i.e. the distance between the antennas or sensors, the position data of the target is determined
• CPC Class Codes: G01S 5/14 Determining absolute distances from a plurality of spaced points of known location
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer

Abstract

Apparatus and methods are described which are useful for determining a location characteristic between an RFID tag and an RFID tag reader or a second RFID tag. In various embodiments, signals backscattered from a singulated tag over a range of frequencies are evaluated for in-phase I and in-quadrature Q signal components. The I-Q data is processed to determine phase delay angles associated with each signal frequency. The phase delay data can be processed by a sum of squared errors method or Fourier transform method to determine a distance to the singulated tag. The methods can also be used to determine any of a location, a radial velocity, a directional velocity of the singulated tag, and proximity of the singulated tag to a second tag.

Claims

1. A method of determining a position of an RFID tag, the method comprising:

broadcasting, by the tag reader, interrogation signals to the tag at a plurality of discrete frequencies;

receiving, by the tag reader, returned signals from the tag in response to the interrogation signals;

determining, by a phase estimator, phase delays associated with the returned signals;

producing, by a processor, a data set from the determined phase delays;

calculating, by the processor, a Fourier transform of the produced data set;

evaluating, by the processor, the Fourier transform; and

determining, by the processor, a distance between the RFID tag and the RFID tag reader based upon the evaluation of the Fourier transform.

2. The method of claim 1, wherein producing a data set further comprises interpolating phase delay data points between the determined phase delays.

3. The method of claim 1, wherein the evaluation of the Fourier transform comprises determining the location of one or more peaks of the Fourier transform.

4. The method of claim 1, wherein the determining of phase delays uses an IIR phase-tracking loop, the phase-tracking loop receiving as input in-phase and in-quadrature components of a returned signal and providing as output an estimated phase delay.

5. The method of claim 4 wherein the determining of phase delays further comprises clipping, by the phase estimator, the output of the IIR phase-tracking loop.

6. The method of claim 4 wherein the determining of phase delays further comprises approximating, by the phase estimator, a residual angle of a rotated sample as the ratio of an in-quadrature component to an in-phase component of the rotated sample, and wherein the sample is representative of a phase delay of a returned signal.

7. The method of claim 1, wherein the determining of phase delays uses a first IIR phase-tracking loop providing an estimated phase delay having a first uncertainty and a second IIR phase-tracking loop providing an estimated phase delay having a second uncertainty, the second uncertainty substantially being larger than the first uncertainty.

8. The method of claim 7 wherein the determining of phase delays further comprises clipping, by the phase estimator, the output of at least one of the IIR phase-tracking loops.

9. The method of claim 1 further comprising:

comparing an EPC value read for the RFID tag at each frequency; and

excluding from the produced data set determined phase delays at each frequency for which the EPC value read is not in agreement with the commonly read EPC value.

10. The method of claim 1 further comprising:

moving, by a user, the tag reader'"'"'s antenna to plural locations;

repeating the steps of the method of claim 1 at each location; and

calculating, by the processor, a location of the RFID tag from the distances determined at each of plural locations.

11. The method of claim 1, wherein

the step of broadcasting comprises broadcasting the interrogation signals from one of plural antennas in communication with the RFID tag reader;

and

the step of receiving comprises receiving the interrogation signals from one of plural antennas in communication with the RFID tag reader.

12. The method of claim 11 further comprising:

repeating, by the tag reader, the steps of the method of claim 11 for each of plural antennas; and

calculating, by the processor, a position of the RFID tag from the distances determined using each of plural antennas.

13. A method of determining a position of an RFID tag, the method comprising:

broadcasting, by the tag reader, interrogation signals to the tag at a plurality of discrete frequencies;

receiving, by the tag reader, returned signals from the tag in response to the interrogation signals;

determining, by a phase estimator, a phase delay associated with each of the returned signals;

monitoring, by the phase estimator, an uncertainty associated with each determined phase delay of each received signal;

assigning, by a processor, a weighting coefficient to each determined phase delay, the weighting coefficient based upon the uncertainty associated with each determined phase delay;

producing, by the processor, a data set comprising determined phase delays, frequencies associated with the phase delays, and weighting coefficients associated with the phase delays;

determining, by the processor, a distance between the RFID tag and the RFID tag reader using at least a portion of the data from the produced data set.

14. A method of determining a relative velocity between an RFID tag and an RFID tag reader, the method comprising:

broadcasting, by the tag reader, interrogation signals to the tag at a single frequency;

receiving, by the tag reader, returned signals from the tag in response to the interrogation signals;

determining, by a phase estimator, a phase delay associated with each of the returned signals;

determining, by a processor, a rate of change of phase delays associated with the returned signals as a function of time; and

calculating, by the processor, a relative velocity between the RFID tag and the RFID tag reader from the rate of change of the phase delay as a function of time.

15. A method of determining a position of an RFID tag, the method comprising:

broadcasting, by the tag reader, interrogation signals to the tag at a plurality of discrete frequencies;

receiving, by the tag reader, returned signals from the tag in response to the interrogation signals;

determining, by a phase-estimator, phase delays associated with the returned signals;

producing, by a processor, a data set from the determined phase delays;

comparing, by the processor, the data set with a second data set of a second RFID tag; and

determining, by the processor, proximity of the RFID tag to the second RFID tag based on the comparison.

16. The method of claim 15, wherein comparing comprises calculating a correlation metric based on the data set and the second data set.

17. The method of claim 16, wherein determining proximity of the RFID tag to the second RFID tag further comprises comparing the calculated correlation metric with a threshold.

18. The method of claim 15, wherein determining proximity of the RFID tag to the second RFID tag further comprises estimating the location of the tag relative to the second tag based on a known location of the second tag.

19. The method of claim 15, further comprising:

comparing, by the processor, the data set with a third data set of a third RFID tag, wherein the locations of the second tag and the third tags are known; and

determining, based on the comparison of data sets, the location of the tag relative to the second tag and the third tag.

20. The method of claim 15, wherein the returned signals from the tag include at least one signal from a reflective path.