Range discrimination in an RFID electronic article surveillance system
View Patent ↗RFID-based electronic surveillance article systems are provided with first and second receiving antennas. An RF signal is transmitted to an RFID device, which transmits a return signal that is received by the receiving antennas. A position of the RFID device may be determined based on a difference between the strength of the return signal when received by the first antenna and the strength of the return signal when received by the second antenna. If RF signals are transmitted by the receiving antennas, the position of the RFID device may be determined by changing the strengths of the RF signals transmitted by each antenna and comparing the strength of the RF signal transmitted by the first antenna when the RFID device is at a threshold for receiving the signal to the strength of the RF signal transmitted by the second antenna when the RFID device is at the threshold.
1. A method of determining a position of an RFID device in an electronic article surveillance system having first and second read zones, comprising:
transmitting an RF signal to an RFID device;
receiving a return signal from the RFID device at a first location and at a second location;
calculating the difference between a first strength of the return signal at the first location and a second strength of the return signal at the second location;
determining whether the RFID device is positioned in the first read zone based at least in part on the difference between the first and second strengths,
wherein said determining whether the RFID device is positioned in the first read zone is based on the difference between the first and second strengths is based on signal loss occurring between the first and second locations and is independent of signal loss occurring between the RFID device and whichever of the first and second locations is closer to the RFID device;
determining an approximate two-dimensional position of the RFID device based at least in part on the difference between the first and second strengths,
sequentially transmitting a plurality of RF signals to the RFID device via a transmitting antenna,
receiving sequentially transmitted return signal from the RFID device via the first and second receiving antennas;
calculating the approximate two-dimensional position of the RFID device for each RF signal; and
determining a direction of movement of the RFID device based at least in part on the two-dimensional position of the RFID device for each RF signal.
2. The method of claim 1 , wherein
the second location is positioned farther from the first read zone than the first location, and
it is determined that the RFID device is positioned in the first read zone when the difference between the first and second strengths is positive and less than a positive threshold value.
3. The method of claim 1 , wherein
the second location is positioned farther from the first read zone than the first location, and
it is determined that the RFID device is positioned in the second read zone when the difference between the first and second strengths is greater than a negative threshold value.
4. The method of claim 1 , wherein the RF signal is transmitted from a location different from the first and second locations.
5. The method of claim 1 , wherein said transmitting an RF signal to the RFID device includes transmitting a first RF signal from the first location and a second RF signal from the second location.
6. An electronic article surveillance system for determining a position of an RFID device configured to transmit a return signal upon receiving an RF signal, the electronic surveillance system comprising:
a first read zone;
a second read zone;
a transmitting antenna configured to sequentially transmit a plurality of RF signals to the RFID device;
a first receiving antenna configured to receive the return signal at a first strength;
a second receiving antenna configured to receive the return signal at a second strength;
wherein the first and second receiving antennas are configured to receive sequentially transmitted return signal from the RFID device; and
a controller configured to:
determine whether the RFID device is positioned in the first read zone based at least in part on a difference between the first and second strengths, wherein said determining whether the RFID device is positioned in the first read zone based on the difference between the first and second strengths is based on the difference between the first and second strengths is based on signal loss occurring between the first and second locations and is independent of signal loss occurring between the RFID device and whichever of the first and second locations is closer to the RFID device,
determine an approximate two-dimensional position of the RFID device based at least in part on the difference between the first and second strengths, and
determine the approximate two-dimensional position of the RFID device for each return signal, and to determine a direction of movement of the RFID device based at least in part on the two-dimensional position of the RFID device for each return signal.
7. The electronic article surveillance system of claim 6 , wherein
the second location is positioned farther from the first read zone than the first location, and
the controller is configured to determine that the RFID device is positioned in the first read zone when the difference between the first and second strengths is positive and less than a positive threshold value.
8. The electronic article surveillance system of claim 6 , wherein
the second location is positioned farther from the first read zone than the first location, and
the controller is configured to determine that the RFID device is positioned in the second read zone when the difference between the first and second strengths is greater than a negative threshold value.
9. The electronic article surveillance system of claim 6 , further comprising a transmitting antenna configured to transmit said RF signal.
10. The electronic article surveillance system of claim 6 , wherein
the first receiving antenna is configured to transmit a first RF signal, and
the second receiving antenna is configured to transmit a second RF signal.
11. The electronic article surveillance system of claim 6 , wherein at least one of the first and second receiving antennas comprises a dipole antenna.
12. The electronic article surveillance system of claim 6 , wherein at least one of the first and second receiving antennas comprises a directional antenna.
13. The electronic article surveillance system of claim 6 , wherein at least one of the first and second receiving antennas comprises a transmission line antenna.
14. A method of determining a position of an RFID device in an electronic article surveillance system having first and second read zones, comprising:
transmitting a first RF signal to an RFID device from a first location and changing a power of the first RF signal to a first power corresponding to a threshold at which a first return signal from the RFID device is received at the first location;
transmitting a second RF signal to the RFID device from a second location and changing a power of the second RF signal to a second power corresponding to a threshold at which a second return signal from the RFID device is received at the second location;
calculating the difference between the first strength and the second strength; and
determining whether the RFID device is positioned in the first read zone based at least in part on the difference between the first and second strengths, wherein said determining whether the RFID device is positioned in the first read zone based on the difference between the first and second strengths is based on signal loss occurring between the first and second locations and is independent of signal loss occurring between the RFID device and whichever of the first and second locations is closer to the RFID device,
determining an approximate two-dimensional position of the RFID device based at least in part on the difference between the first and second strengths;
sequentially transmitting a plurality of RF signals to the RFID device via a transmitting antenna;
receiving sequentially transmitted return signal from the RFID device via the first and second receiving antennas;
calculating the approximate two-dimensional position of the RFID device for each RF signal; and
determining a direction of movement of the RFID device based at least in part on the two-dimensional position of the RFID device for each RF signal.
15. An electronic article surveillance system for determining a position of an RFID device configured to transmit return signals upon receiving RF signals, the electronic surveillance system comprising:
a first read zone;
a second read zone;
a transmitting antenna configured to sequentially transmit a plurality of RF signals to the RFID device;
a first receiving antenna configured to transmit a first RF signal to the RFID device and to change a power of the first RF signal to a first power corresponding to a threshold at which a first return signal from the RFID device is received by the first receiving antenna;
a second receiving antenna configured to transmit a second RF signal to the RFID device and to change a power of the second RF signal to a second power corresponding to a threshold at which a second return signal from the RFID device is received by the second receiving antenna,
wherein the first and second receiving antennas are configured to receive sequentially transmitted return signal from the RFID device; and
a controller configured to:
determine whether the RFID device is positioned in the first read zone based at least in part on a difference between the first and second strengths, wherein said determining whether the RFID device is positioned in the first read zone based on the difference between the first and second strengths is based on signal loss occurring between the first and second locations and is independent of signal loss occurring between the RFID device and whichever of the first and second locations is closer to the RFID device
determine an approximate two-dimensional position of the RFID device based at least in part on the difference between the first and second strengths, and
determine the approximate two-dimensional position of the RFID device for each return signal, and to determine a direction of movement of the RFID device based at least in part on the two-dimensional position of the RFID device for each return signal.