Locating RFID tags
View Patent ↗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 t 1 , switches the RFID tag A to a catching mode for catching echos from other tags. The RFID tag A receives at time t 3 >t 1 an echo of a message sent by the RFID tag B to the RFID reader. The RFID tag A determines a distance (D.t 2 t ) between the RFID tag A and the RFID tag B based on t 3 minus t 1 . The distance D.t 2 t does not exceed a specified radius limit and the identification of the RFID tag B and the distance D.t 2 t are stored in a database within the RFID tag A.
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 t 1 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 t 3 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 t 2 , wherein the echo comprises an identification of the RFID tag B, and wherein t 3 >t 2 >t 1 ;
said RFID tag A determining a distance (D.t 2 t ) between the RFID tag A and the RFID tag B based on a total timing (TT) equal to t 3 minus t 1 ;
said RFID tag A ascertaining that the distance D.t 2 t 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.t 2 t 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 t 3 , 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.t 2 t .
3. The method of claim 1 , wherein said determining the distance D.t 2 t comprises determining D.t 2 t according to:
D.t 2 t =( TT−T trig− TE.r 2 t .tag B−TE.r 2 t .tag A )* 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.r 2 t .tagA is a time for the polling command to be transmitted from the RFID reader to the RFID tag A, wherein TE.r 2 t .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.r 2 t .tag B, and wherein C is the speed of light.
4. The method of claim 3 , wherein Ttrig and TE.r 2 t .tagA were stored in the RFID tag A during an initialization of the RFID tag A which occurred prior to the time t 1 , wherein the polling command comprises a distance (D.r 2 t .tagB) between the RFID tag B and the RFID reader, and wherein TE.r 2 t. tagB=D.r 2 t .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 t 1 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 t 3 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 t 2 , wherein the echo comprises an identification of the RFID tag B, and wherein t 3 >t 2 >t 1 ;
said RFID tag A determining a distance (D.t 2 t ) between the RFID tag A and the RFID tag B based on a total timing (TT) equal to t 3 minus t 1 ;
said RFID tag A ascertaining that the distance D.t 2 t 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.t 2 t 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 t 3 , 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.t 2 t.
10. The computer program product of claim 8 , wherein said determining the distance D.t 2 t comprises determining D.t 2 t according to:
D.t 2 t =( TT−T trig− TE.r 2 t .tag B−TE.r 2 t .tag A )* 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.r 2 t .tagA is a time for the polling command to be transmitted from the RFID reader to the RFID tag A, wherein TE.r 2 t .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.r 2 t .tagB, and wherein C is the speed of light.
11. The computer program product of claim 10 , wherein Ttrig and TE.r 2 t .tagA were stored in the RFID tag A during an initialization of the RFID tag A which occurred prior to the time t 1 , wherein the polling command comprises a distance (D.r 2 t .tagB) between the RFID tag B and the RFID reader, and wherein TE.r 2 t .tagB=D.r 2 t .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 t 1 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 t 3 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 t 2 , wherein the echo comprises an identification of the RFID tag B, and wherein t 3 >t 2 >t 1 ;
means for determining a distance (D.t 2 t ) between the RFID tag A and the RFID tag B based on a total timing (TT) equal to t 3 minus t 1 ;
means for ascertaining that the distance D.t 2 t does not exceed a specified radius limit; and
means for storing the identification of the RFID tag B and the distance D.t 2 t 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 t 3 , 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.t 2 t.
16. The RFID tag of claim 14 , wherein said means for determining comprises means for determining the distance D.t 2 t according to:
D.t 2 t =( TT−T trig− TE.r 2 t .tag B−TE.r 2 t .tag A )* 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.r 2 t .tagA is a time for the polling command to be transmitted from the RFID reader to the RFID tag A, wherein TE.r 2 t .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.r 2 t .tagB, and wherein C is the speed of light.
17. The RFID tag of claim 16 , means for storing Ttrig and TE.r 2 t .tagA in the RFID tag A during an initialization of the RFID tag A which occurred prior to the time t 1 , wherein the polling command comprises a distance (D.r 2 t .tagB) between the RFID tag B and the RFID reader, and wherein TE.r 2 t .tagB=D.r 2 t .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.