RFID detection system

• US 6,577,238 B1
• Filed: 03/01/2001
• Issued: 06/10/2003
• Est. Priority Date: 09/28/1998
• Status: Expired due to Fees

• Current Assignee: Tagtec Ltd.
• Original Assignee: Tagtec Ltd.
• Inventors: Whitesmith, Howard William, Palmer, Timothy John, Ball, Alan Edward
• Primary Examiner(s): Wu, Daniel J.
• Assistant Examiner(s): Nguyen, Tai T.
• Application Number:
  US09/786,425
• Time in Patent Office: 831 Days
• Field of Search: 340/572.1, 340/573.1, 340/693.9, 340/572.8, 340/568.1, 340/825.49, 340/10.1, 342/42, 342/47, 342/95, 342/106, 342/107, 342/109, 342/113, 342/114, 342/139, 342/146, 342/118
• US Class Current: 340/572.1
• 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: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer
• CPC Class Codes: G06K 7/10039 interrogator driven, i.e. synchronous
• CPC Class Codes: G08B 13/2462 Asset location systems combined with EAS inventory, tracking, logistic G06Q10/00; entrance control systems G07C9/00

Abstract

A system 100 is disclosed for monitoring the position of one or more RFID tags 201. The system has a detector 301 incorporating circuitry 304 for detecting changes in the range of an RFID tag 201 from the detector and for triggering an alarm 401 if a detected change in range of an RFID tag 201 exceeds a predetermined threshold or if the RFID radio tag cannot be detected by the detector 301. Range may be detected, for example, by measuring the time of a returned radio signal from a tag 201, by measuring the strength of a returned radio signal from a tag, or by detecting changes in a periodic interval at which energy is transmitted by a tag.

Claims

1. A system for monitoring the position of at least one RFID tag, said system comprising:

at least one RFID tag;

at least one detector incorporating means for receiving signals from said RFID tag for detecting changes in the range of said RFID tag from said detector; and

control means for comparing signals sequentially received from said RFID to detect a change in range of said RFID tag, and for triggering an alarm if said detected change in range of said RFID tag exceeds a predetermined threshold.

2. A system according to claim 1, wherein said detector is arranged to measure the time of a returned radio signal from said RFID tag to determine the range of said RFID tag.

3. A system according to claim 1, wherein said detector is arranged to determine the strength or waveform shape of a returned radio signal from said RFID tag to determine the range of said RFID tag.

4. A system for monitoring the position of at least one RFID tag, said system comprising:

at least one RFID tag;

at least one detector incorporating means for receiving signals from said RFID tag for detecting changes in the range of said RFID tag from said detector;

control means comparing the signals received from said RFID tag at different times to detect a change in range of said RFID tag, and triggering an alarm if a detected change in range of said RFID tag exceeds a predetermined threshold; and

said RFID tag including circuitry having means for emitting short bursts of RF energy at periodic intervals.

5. A system according to claim 4, further including an RF transmitter arranged to transmit RF energy to said RFID tag, wherein the said RFID tag includes circuitry arranged to store energy received from said transmitter and to provide energy for transmitting said short bursts of RF energy.

6. A system according to claim 5, wherein said detector includes circuitry for detecting changes in the periodic interval at which said energy is transmitted by said RFID tag.

7. A system according to claim 4, wherein said RFID tag includes a receiving antenna and a transmitting antenna.

8. A system according to claim 4, wherein said control means is additionally arranged to trigger said alarm if said RFID tag cannot be detected by said detector.

9. A system according to claim 4, wherein said detector includes circuitry for detecting changes in the periodic interval at which said energy is transmitted by said RFID tag.

10. A system according to claim 9, wherein said detector includes circuitry for predicting the time of receipt of a burst of energy from said tag and for triggering an alarm if the time of actual receipt varies from the predicted time of receipt by more than a predetermined interval.

11. A system according to claim 9, wherein said detector includes circuitry for analyzing changes in the rate of receipt of bursts of energy from said RFID tag and for triggering an alarm if the rate of change is more than a predetermined level.

12. A system according to claim 9, wherein said detector includes circuitry for triggering an alarm if the rate of change in the periodic interval at which energy is transmitted by said RFID tag is outside a predetermined range.

13. A system according to claim 9, wherein said transmitter includes circuitry arranged to vary the average power transmitted and hence received by said RFID tag to alter the periodic interval at which said RFID tag emits bursts of energy.

14. A system for monitoring the position of an RFID tag, comprising:

a transmitter for transmitting RF energy;

an RFID tag including circuitry having means for receiving said RF energy and transmitting short bursts of RF energy at periodic intervals based on the relative position and orientation of said RFID tag relative to said transmitter;

a detector for receiving said bursts transmitted from said RFID tag; and

a controller interconnected to said detector for determining a change in range of said RFID tag and triggering an alarm if said change in range of said RFID tag exceeds a predetermined threshold.

15. A method for monitoring the position of an RFID tag, comprising:

transmitting RF energy to said RFID tag;

receiving said RF energy in said RFID tag;

using said RF energy for transmitting short bursts of RF energy from said RFID tag at periodic intervals;

detecting said short bursts and determining a change in range of said RFID tag; and

triggering an alarm if said change in range exceeds a predetermined threshold.