Range discrimination in an RFID electronic article surveillance system

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.

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.

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.