Locating RFID tags

• US 8,289,129 B2
• Filed: 12/16/2008
• Issued: 10/16/2012
• Est. Priority Date: 12/20/2007
• Status: Active Grant

• Current Assignee: Kyndryl Incorporated (Kyndryl Holdings, Inc.)
• Original Assignee: International Business Machines Corporation
• Inventors: Bauchot, Frederic, Clement, Jean-Yves, Marmigere, Gerard, Picon, Joaquin
• Primary Examiner(s): Mehmood, Jennifer
• Assistant Examiner(s): SMALL, NAOMI J
• Application Number:
  US12/335,601
• Publication Number:
  US 20090160603A1
• Time in Patent Office: 1,400 Days
• Field of Search: 340/3.51, 340/539.13, 342/47, 342/118
• US Class Current: 340/3.51
• CPC Class Codes: G01S 11/02 using radio waves G01S19/00 takes precedence
• 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

A method and RFID tag for locating RFID tags. A passive RFID tag A receives a polling command transmitted from a RFID reader and addressed to another passive RFID tag B. The RFID tag A determines that the polling command is not addressed to the RFID tag A and consequent1y, at time t1, switches the RFID tag A to a catching mode for catching echos from other tags. The RFID tag A receives at time t3>t1 an echo of a message sent by the RFID tag B to the RFID reader. The RFID tag A determines a distance (D.t2t) between the RFID tag A and the RFID tag B based on t3 minus t1. The distance D.t2t does not exceed a specified radius limit and the identification of the RFID tag B and the distance D.t2t are stored in a database within the RFID tag A.

Claims

1. A method for locating Radio Frequency Identification (RFID) tags, said method comprising:

a passive RFID tag A receiving a polling command previously transmitted from a RFID reader and addressed to another passive RFID tag B;

said RFID tag A determining that the polling command is not addressed to the RFID tag A;

in response to the RFID tag A determining that the polling command is not addressed to the RFID tag A, at a time t1 said RFID tag A setting the RFID tag A to a catching mode configured to catch echos from other tags;

said RFID tag A receiving at a time t3 an echo of a message sent by the RFID tag B to the RFID reader in response to the polling command received by the RFID tag B at a time t2, wherein the echo comprises an identification of the RFID tag B, and wherein t3>

t2>

t1;

said RFID tag A determining a distance (D.t2t) between the RFID tag A and the RFID tag B based on a total timing (TT) equal to t3 minus t1;

said RFID tag A ascertaining that the distance D.t2t does not exceed a specified radius limit; and

responsive to said ascertaining, said RFID tag A storing the identification of the RFID tag B and the distance D.t2t in a database within the RFID tag A.

2. The method of claim 1, wherein the method further comprises:

at a time later than the time t3, said RFID tag A sending an identification message to the RFID reader, wherein the identification message includes an identifier of the RFID tag A, a distance between the RFID tag A and the RFID reader, the radius limit, the identification of the RFID tag B, and the distance D.t2t.

3. The method of claim 1, wherein said determining the distance D.t2t comprises determining D.t2t according to:

D.t2t=(TT−

Ttrig−

TE.r2t.tagB−

TE.r2t.tagA)*C

wherein Ttrig is a specified delay that is not less than a sum of a tag access time for reading the polling command and a granted time for processing the polling command, wherein TE.r2t.tagA is a time for the polling command to be transmitted from the RFID reader to the RFID tag A, wherein TE.r2t.tagB is a time for the polling command to be transmitted from the RFID reader to the RFID tag B, wherein the polling command comprises TE.r2t.tag B, and wherein C is the speed of light.

4. The method of claim 3, wherein Ttrig and TE.r2t.tagA were stored in the RFID tag A during an initialization of the RFID tag A which occurred prior to the time t1, wherein the polling command comprises a distance (D.r2t.tagB) between the RFID tag B and the RFID reader, and wherein TE.r2t.tagB=D.r2t.tagB/C.

5. The method of claim 1, wherein the echo of the message sent by the RFID tag B to the RFID reader is a reflection of the polling command received from the RFID reader at the RFID tag B.

6. The method of claim 1, wherein the RFID tag A comprises an antenna and an impedance variation system, and wherein the method further comprises:

responsive to RFID tag A determining that the polling command is not addressed to the RFID tag A, said impedance variation system adjusting the impedance of the antenna so that the RFID tag A totally absorbs the echo of the message received by the RFID tag A.

7. The method of claim 6, wherein the impedance variation system comprises a three state driver circuit, and wherein said adjusting comprises said three state driver circuit adjusting the impedance of the antenna by adapting a sufficient impedance termination at a base of the antenna to cause the RFID tag A to totally absorb the echo of the message received by the RFID tag A.

8. A computer program product comprising a computer usable storage medium having a computer readable program code embodied in the medium, said program code configured to be executed on a processor of a computer system to perform a method for locating Radio Frequency Identification (RFID) tags, said method comprising:

a passive RFID tag A receiving a polling command previously transmitted from a RFID reader and addressed to another passive RFID tag B;

said RFID tag A determining that the polling command is not addressed to the RFID tag A;

in response to the RFID tag A determining that the polling command is not addressed to the RFID tag A, at a time t1 said RFID tag A setting the RFID tag A to a catching mode configured to catch echos from other tags;

said RFID tag A receiving at a time t3 an echo of a message sent by the RFID tag B to the RFID reader in response to the polling command received by the RFID tag B at a time t2, wherein the echo comprises an identification of the RFID tag B, and wherein t3>

t2>

t1;

said RFID tag A determining a distance (D.t2t) between the RFID tag A and the RFID tag B based on a total timing (TT) equal to t3 minus t1;

said RFID tag A ascertaining that the distance D.t2t does not exceed a specified radius limit; and

responsive to said ascertaining, said RFID tag A storing the identification of the RFID tag B and the distance D.t2t in a database within the RFID tag A.

9. The computer program product of claim 8, wherein the method further comprises:

at a time later than the time t3, said RFID tag A sending an identification message to the RFID reader, wherein the identification message includes an identifier of the RFID tag A, a distance between the RFID tag A and the RFID reader, the radius limit, the identification of the RFID tag B, and the distance D.t2t.

10. The computer program product of claim 8, wherein said determining the distance D.t2t comprises determining D.t2t according to:

D.t2t=(TT−

Ttrig−

TE.r2t.tagB−

TE.r2t.tagA)*C

wherein Ttrig is a specified delay that is not less than a sum of a tag access time for reading the polling command and a granted time for processing the polling command, wherein TE.r2t.tagA is a time for the polling command to be transmitted from the RFID reader to the RFID tag A, wherein TE.r2t.tagB is a time for the polling command to be transmitted from the RFID reader to the RFID tag B, wherein the polling command comprises TE.r2t.tagB, and wherein C is the speed of light.

11. The computer program product of claim 10, wherein Ttrig and TE.r2t.tagA were stored in the RFID tag A during an initialization of the RFID tag A which occurred prior to the time t1, wherein the polling command comprises a distance (D.r2t.tagB) between the RFID tag B and the RFID reader, and wherein TE.r2t.tagB=D.r2t.tagB/C.

12. The computer program product of claim 8, wherein the echo of the message sent by the RFID tag B to the RFID reader is a reflection of the polling command received from the RFID reader at the RFID tag B.

13. The computer program product of claim 8, wherein the RFID tag A comprises an antenna and an impedance variation system, and wherein the method further comprises:

responsive to RFID tag A determining that the polling command is not addressed to the RFID tag A, said impedance variation system adjusting the impedance of the antenna so that the RFID tag A totally absorbs the echo of the message received by the RFID tag A, wherein the impedance variation system comprises a three state driver circuit, and wherein said adjusting comprises said three state driver circuit adjusting the impedance of the antenna by adapting a sufficient impedance termination at a base of the antenna to cause the RFID tag A to totally absorb the echo of the message received by the RFID tag A.

14. A passive Radio Frequency Identification (RFID) tag, said RFID tag denoted as a RFID tag A, said RFID tag A comprising:

means for receiving a polling command previously transmitted from a RFID reader and addressed to another passive RFID tag denoted as a RFID tag B;

means for determining that the polling command is not addressed to the RFID tag A;

means for setting the RFID tag A at a time t1 to a catching mode configured to catch echos from other tags in response to the RFID tag A determining that the polling command is not addressed to the RFID tag A;

means for receiving at a time t3 an echo of a message sent by the RFID tag B to the RFID reader in response to the polling command received by the RFID tag B at a time t2, wherein the echo comprises an identification of the RFID tag B, and wherein t3>

t2>

t1;

means for determining a distance (D.t2t) between the RFID tag A and the RFID tag B based on a total timing (TT) equal to t3 minus t1;

means for ascertaining that the distance D.t2t does not exceed a specified radius limit; and

means for storing the identification of the RFID tag B and the distance D.t2t in a database within the RFID tag A in response to said ascertaining.

15. The RFID tag of claim 14, further comprising:

means for sending an identification message to the RFID reader at a time later than the time t3, wherein the identification message includes an identifier of the RFID tag A, a distance between the RFID tag A and the RFID reader, the radius limit, the identification of the RFID tag B, and the distance D.t2t.

16. The RFID tag of claim 14, wherein said means for determining comprises means for determining the distance D.t2t according to:

D.t2t=(TT−

Ttrig−

TE.r2t.tagB−

TE.r2t.tagA)*C

wherein Ttrig is a specified delay that is not less than a sum of a tag access time for reading the polling command and a granted time for processing the polling command, wherein TE.r2t.tagA is a time for the polling command to be transmitted from the RFID reader to the RFID tag A, wherein TE.r2t.tagB is a time for the polling command to be transmitted from the RFID reader to the RFID tag B, wherein the polling command comprises TE.r2t.tagB, and wherein C is the speed of light.

17. The RFID tag of claim 16, means for storing Ttrig and TE.r2t.tagA in the RFID tag A during an initialization of the RFID tag A which occurred prior to the time t1, wherein the polling command comprises a distance (D.r2t.tagB) between the RFID tag B and the RFID reader, and wherein TE.r2t.tagB=D.r2t.tagB/C.

18. The RFID tag of claim 14, wherein the echo of the message sent by the RFID tag B to the RFID reader is a reflection of the polling command received from the RFID reader at the RFID tag B.

19. The RFID tag of claim 14, wherein the RFID tag A comprises:

an antenna; and

an impedance variation system configured to adjust an impedance of the antenna so that the RFID tag A totally absorbs the echo of the message received by the RFID tag A in response to the RFID tag A determining that the polling command is not addressed to the RFID tag A.

20. The RFID tag of claim 19, wherein the impedance variation system comprises a three state driver circuit configured to adjust the impedance of the antenna by adapting a sufficient impedance termination at a base of the antenna to cause the RFID tag A to totally absorb the echo of the message received by the RFID tag A.