Range identification in RFID electronic article surveillance system

Abstract

To provide a method for determining the position of an RFID device in an electronic article surveillance system having first and second read zones.SOLUTION: An RFID-based electronic article surveillance system includes first and second receiving antennas. An RF signal is transmitted to an RFID device which transmits a return signal received by a receiving antenna. The location of the RFID device may be determined on the basis of a difference between the strength of the return signal as received by the first antenna and the strength of the return signal as received by the second antenna. When the RF signal is transmitted by the receiving antenna, by modifying the strength of the RF signal 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 value for receiving the signal and the strength of the RF signal transmitted by the second antenna when the RFID device is at the threshold value, the location of the RFID device may be determined.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of U.S. Provisional Patent Application No. 62 / 981,206, filed February 25, 2020, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to radio frequency identification ("RFID") devices. More particularly, the present invention relates to determining the location of RFID devices in electronic article surveillance ("EAS") systems. [Background technology]

[0003] In retail stores, an accurate count of products on display and / or in storage is important. Additionally, it is important to have an effective anti-theft system in place. RFID tags and labels (collectively referred to herein as "RFID devices") are used to perform all of these functions.

[0004] An EAS system using RFID technology has two primary read zones 10 and 12, each containing an associated RFID reader, as shown in FIG. 1. The first read zone 10 is the area in the store where products are presented to consumers (referred to herein as the "inventory zone"), and the second read zone 12 is the exit area of ​​the store (referred to herein as the "detection zone") where an improperly deactivated RFID device may be detected to trigger some sort of alarm indicating an attempt to steal merchandise is occurring. If the customer correctly purchases the item, the cashier removes or deactivates the RFID device associated with that item. If the RFID device is not removed or deactivated, the RFID reader will read the device and trigger an alarm or other alert in the detection zone 12.

[0005] Although the above-mentioned systems are widespread, they do have some drawbacks. When using RFID devices / systems for EAS systems, one common problem is whether the read range of the RFID device in a particular situation is large enough that an RFID device in the inventory zone 10 will be read in the detection zone 12, or vice versa. To reduce such risks, a transition zone 14 is frequently provided between the inventory zone 10 and the detection zone 12, physically separating the two read zones. However, due to different RFID devices having higher sensitivity at the operating frequency and / or different items that affect the performance of the associated RFID device differently, the transition zone 14 needs to be relatively large. A larger transition zone results in a smaller inventory zone, and therefore fewer items that a retailer can present to a customer for purchase. It would therefore be advantageous to provide an RFID device configured to reduce the size of the transition zone 14.

[0006] Many RFID-based EAS systems attempt to identify the range between an RFID device and an EAS reader system by measuring factors such as when an RFID device begins to respond (such as when the reader system transmits at a certain power) and the response level, commonly referred to as Received Signal Strength Indicator (RSSI). However, the results of such an approach may be unreliable due to the attenuation factor K and the sensitivity T of the RFID device. K and T may be affected by environmental conditions such as reflective and absorbing materials between the RFID device and the reader system, and losses may be incurred in stealing items where a thief may place the RFID device close to the human body to attenuate the signal (called the human body model or human body effect). With high T and low K, RFID devices far away (e.g., devices in an inventory zone) may respond at a similar level to tags in an EAS zone, causing false alarms. An RFID reader transmitting at full power is ideal for detection when K is high due to deliberate attempts to disable the EAS system, but this also increases the chance of false alarms. Therefore, an identification method independent of K is advantageous. Summary of the Invention

[0007] There are various aspects of the present subject matter that may be implemented individually or together in the devices, systems and methods described and claimed below. These aspects may be used alone or in combination with other aspects of the subject matter described herein, and describing these aspects together does not exclude using these aspects separately or claiming them individually or in different combinations as set forth in the claims appended hereto.

[0008] Described herein is a method for determining a location of an RFID device in an electronic article surveillance system having first and second read zones. The method includes transmitting an RF signal to the RFID device and receiving a return signal from the RFID device at the first location and the second location. A 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 is calculated, and a determination is made as to whether the RFID device is located in the first read zone based at least in part on the difference between the first strength and the second strength.

[0009] Also described herein is an electronic article surveillance system for determining a location of an RFID device configured to transmit a return signal upon receiving an RF signal. In some embodiments, the electronic surveillance system includes first and second read zones, first and second receive antennas, and a controller. In some embodiments, the first receive antenna is configured to receive the return signal at a first strength, while the second receive antenna is configured to receive the return signal at a second strength. The controller is configured to determine whether the RFID device is located in the first read zone based at least in part on a difference between the first strength and the second strength.

[0010] Described herein is a method for determining a location of an RFID device in an electronic article surveillance system having first and second read zones. In some embodiments, the method includes transmitting a first RF signal from a first location to the RFID device and varying a power of the first RF signal to a first power corresponding to a threshold at which a first reply signal from the RFID device is received at the first location. A second RF signal is transmitted to the RFID device from a second location and the power of the second RF signal is varied to a second power corresponding to a threshold at which a second reply signal from the RFID device is received at the second location. A difference between the first intensity and the second intensity is determined and then it is determined whether the RFID device is located in the first read zone based at least in part on the difference between the first intensity and the second intensity.

[0011] Also described herein is an electronic article surveillance system for determining a location of an RFID device configured to transmit a reply signal upon receiving an RF signal. In some embodiments, the electronic surveillance system includes first and second read zones, first and second receive antennas, and a controller. The first receive antenna is configured to transmit a first RF signal to the RFID device and change a power of the first RF signal to a first power corresponding to a threshold at which a first reply signal from the RFID device is received by the first receive antenna. The second receive antenna is configured to transmit a second RF signal to the RFID device and change a power of the second RF signal to a second power corresponding to a threshold at which a second reply signal from the RFID device is received by the second receive antenna. The controller is configured to determine whether the RFID device is located in the first read zone based at least in part on a difference between the first and second intensities. [Brief description of the drawings]

[0012] [Figure 1] 1 is a schematic diagram of a conventional EAS system that uses RFID devices. [Diagram 2] 1 is a schematic diagram of an exemplary embodiment of an RFID-based EAS system according to the present disclosure. [Diagram 3]1 is a schematic diagram of another exemplary embodiment of an RFID-based EAS system in accordance with the present disclosure. [Figure 4] 1 is a schematic diagram of an exemplary arrangement of antennas of a gate of an EAS system, according to an embodiment of the present disclosure. [Diagram 5] 1 is a schematic diagram of an EAS system of the present disclosure used to determine the two-dimensional location of an RFID device. [Figure 6A] 1 illustrates an approach for determining the movement of an RFID device using an EAS system according to the present disclosure. [Figure 6B] 1 illustrates an approach for determining the movement of an RFID device using an EAS system according to the present disclosure. [Figure 6C] 1 illustrates an approach for determining the movement of an RFID device using an EAS system according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Where necessary, detailed embodiments of the present invention are disclosed herein; however, it should be understood that the disclosed embodiments are merely exemplary of the present invention, which may be embodied in various forms. Thus, the specific details disclosed herein should not be construed as limiting, but merely as a basis for the claims and as a representative basis for teaching one of ordinary skill in the art to use the present invention in its various forms in substantially any suitable manner.

[0014] FIG. 2 illustrates an exemplary embodiment of an RFID-based EAS system 16 according to the present disclosure. In the embodiment of FIG. 2, the EAS system 16 includes a transmitting antenna 18 and two receiving antennas 20 and 22. The antennas of an EAS system according to the present disclosure may be configured in a variety of ways without departing from the scope of the present disclosure, but it is desirable to use antenna designs that have equal gain in the zone of interest or have a means for compensating the measured RSSI / power values. By way of example, the receiving antennas 20 and 22 of FIG. 2 may be configured as dipole antennas, directional antennas, transmission line antennas, or combinations thereof. Different antenna configurations have different performance characteristics and therefore different advantages. For example, a dipole antenna provides an appropriate angular range for an EAS system configured to detect the two-dimensional location of an RFID device. On the other hand, a directional antenna may be configured to keep the focus forward of the detection zone relative to the gate of the EAS system. Thus, it should be understood that the present disclosure is not limited to EAS systems having particularly configured antennas, and the aspects described herein may be implemented with a variety of differently configured antennas.

[0015] 2, a transmitting antenna 18 transmits an RF signal "S" to an RFID device 24 (e.g., an RFID tag or label attached to an item) located at a location in the EAS system 16. The RFID device 24 receives the RF signal S from the transmitting antenna 18 and transmits back a return signal that is received by a first receiving antenna 20 and a second receiving antenna 22.

[0016] Environmental conditions increase the distance the return signal travels, thereby decreasing the strength of the return signal. In the orientation of Figure 2, the first receive antenna 20 is located closer to the RFID device 24 than the second receive antenna 22, and the return signal strength or RSSI is greater at the first receive antenna 20 than at the second receive antenna 22. In Figure 2, the distance between the RFID device 24 and the first receive antenna 20 is represented by "r", the distance between the first receive antenna 20 and the second receive antenna 22 is represented by "θ", and the distance between the RFID device 24 and the second receive antenna 22 is r+θ.

[0017] As mentioned above, using a single antenna to measure the strength or RSSI of a return signal from an RFID device may not be particularly beneficial or useful. However, by using two antennas 20 and 22 located a known distance apart from each other and comparing the strength or RSSI of the return signal, the EAS system 16 can more reliably determine the approximate location of the RFID device 24. As mentioned above, the strength or RSSI of the return signal is a function of the distance traveled by the return signal. The common distance (represented by "r" in FIG. 2) that the return signals travel in reaching the first and second receive antennas 20 and 22 may be offset by comparing the strength or RSSI of the return signal received by the first and second receive antennas 20 and 22. Thus, the difference between the strength or RSSI of the return signal received by the first and second receive antennas 20 and 22 will indicate a loss of strength as the return signal travels the known distance θ between the first and second receive antennas 20 and 22.

[0018] Because the change in strength or RSSI of an RF signal follows a square law, the difference in strength or RSSI of the return signal received by the first receive antenna 20 and the second receive antenna 22 indicates the distance r between the RFID device and the first receive antenna 20. Generally speaking, the difference between the strength or RSSI of the return signal received by the first receive antenna 20 and the second receive antenna 22 will be relatively large in magnitude when the distance r is relatively small, and the difference will be relatively small in magnitude when the distance r is relatively large. While the exact magnitude of the difference depends on a number of factors, the magnitude of the difference in an exemplary embodiment is on the order of about 6 dB when r=1, about 1.6 dB when r=5, and about 0.83 dB when r=10.

[0019] Regardless of the particular difference between the strength or RSSI of the return signals at the two receive antennas 20 and 22, it can be seen that any RFID device 24 that does not exhibit a significant change in strength or RSSI can be considered to be a significant distance away from the receive antennas 20 and 22. Although the embodiment is not very accurate at long distances (e.g., at r=5 and r=10, the difference in signal strength or RSSI is small), the EAS system 16 can be configured so that high accuracy is not required at relatively long ranges. For example, in one embodiment, the two receive antennas 20 and 22 (or a single antenna with one or more reference planes if the receive antennas are incorporated into a line antenna) are positioned between the inventory zone 10 and the detection zone 12, with the first receive antenna 20 located closer to the inventory zone 10 than the second receive antenna 22. If the RFID device 24 is located closer to the inventory zone 10 than to the detection zone 12 (i.e., closer to the first receive antenna 20 than to the second receive antenna 22), the difference between the strength or RSSI of the return signal received by the first receive antenna 20 and the second receive antenna 22 should be positive (i.e., the return signal should be stronger at the first receive antenna 20 than at the second receive antenna 22).

[0020] Thus, in this exemplary configuration, a difference in the strength or RSSI of the return signals received by the first receive antenna 20 and the second receive antenna 22 that is positive and relatively small (i.e., in the exemplary embodiment, less than a positive threshold somewhere between 1.6 dB and 6 dB) is sufficient to indicate that the RFID device 24 is located somewhere in the inventory zone 10. The exact location of the RFID device 24 within the inventory zone (e.g., whether r=5 or r=10) cannot be precisely determined, but it is sufficient to know that the RFID device 24 is in the inventory zone 10 and not in the detection zone 12 or the transition zone 14. The exact positive threshold for determining whether the RFID device 24 is sufficiently far away from the receive antennas 20 and 22 depends on numerous factors (e.g., the location of the receive antennas 20, 22 within the EAS system 16 and the size of the transition zone 14), and therefore the present disclosure is not limited to a particular positive threshold.

[0021] Similarly, to determine when an RFID device 24 is located anywhere in the inventory zone 10, an EAS system 16 according to the present disclosure can also determine when an RFID device 24 is located anywhere in the detection zone 12. If the first receive antenna 20 is located closer to the inventory zone 10 than the second receive antenna 22, and if the RFID device 24 is located closer to the detection zone 12 than the inventory zone 10, the difference between the strength or RSSI of the return signals received by the first receive antenna 20 and the second receive antenna 22 should be negative. Thus, in this exemplary configuration, a difference in the strength or RSSI of the return signals received by the first receive antenna 20 and the second receive antenna 22 that is negative and relatively small (i.e., greater than a negative threshold or close to zero) is sufficient to indicate that an RFID device 24 is located anywhere in the detection zone 12. The exact negative threshold for determining whether the RFID device 24 is sufficiently far away from the receiving antennas 20 and 22 depends on many factors (e.g., the location of the receiving antennas 20, 22 within the EAS system 16 and the size of the transition zone 14), so this disclosure is not limited to a particular negative threshold.

[0022] It should be understood that the configuration of Figure 2 is merely exemplary, and that EAS systems according to the present disclosure may be configured differently. For example, Figure 3 illustrates an EAS system 26 in which RF signals are transmitted by two receive antennas 28 and 30, rather than a third transmit antenna (as in Figure 2). The EAS system 26 of Figure 3 may be referred to as operating in a "monostatic" mode, whereas the EAS system 16 of Figure 2 may be referred to as operating in a "bistatic" mode.

[0023] In the embodiment of FIG. 3, each receive antenna 28, 30 transmits an RF signal to RFID device 24 and receives a return signal. As in the embodiment of FIG. 2, the difference in strength or RSSI of the return signals received by the first receive antenna 28 and the second receive antenna 30 can be used to determine the general location of RFID device 24 (i.e., whether RFID device 24 is located somewhere in inventory zone 10 or somewhere in detection zone 12). However, because two different RF signals are sent to RFID device 24 in the EAS system of FIG. 3, care must be taken to ensure that the return signal from RFID device 24 is transmitted at the same power. Upon receiving the RF signals from the first receive antenna 28 and the second receive antenna 30, the power of the return signal transmitted by RFID device 24 is similar to when RFID device 24 receives enough power to transmit a return signal (referred to herein as a "threshold"), which represents a constant power input and a constant power output.

[0024] In one embodiment, each receive antenna 28, 30 initially transmits a low power RF signal and then increases the strength of the RF signal until it first receives a return signal from RFID device 24, which is the RF signal strength of that receive antenna at the threshold of RFID device 24. Alternatively, rather than starting at low power, the threshold may be reached by initially transmitting a high power RF signal that is strong enough for the receive antennas 28 and 30 to reach RFID device 24, and then the power is lowered until no return signal is transmitted. It should be understood that the threshold may be reached using any of a number of suitable approaches, which may include, in practice, a linear sweep or a binary search.

[0025] Just as the difference in strength between the two return signals can be used to determine the general location of the RFID device 24, the difference in strength between the RF signal emitted by the first receive antenna 28 at the threshold of the RFID device 24 and the RF signal emitted by the second receive antenna 30 at the threshold of the RFID device 24 can indicate the general location of the RFID device 24. The RF signals emitted by the two receive antennas 28 and 30 have the same (or at least substantially the same) strength or RSSI in reaching the RFID device 24. Since the two RF signals traverse the same distance r in reaching the RFID device 24, the additional strength required to reach the RFID device 24 to the threshold by the more distant receive antenna (which is the second receive antenna 30 in the orientation of FIG. 3, but which may be the first receive antenna 28 depending on the location of the RFID device 24) is entirely due to losses associated with the distance θ between the receive antennas 28 and 30. This information can be used (by using the principles described herein) to determine whether RFID device 24 is a significant distance away from receiving antennas 28 and 30, with a positive or negative difference indicating the side of receiving antennas 28 and 30 on which RFID device 24 is located.

[0026] For optimal performance, it is preferred that RFID device 24 have no or only relatively small changes in its position during the measurements described above. Bistatic systems may be advantageous in this regard since the power of the RF signal transmitted by transmitting antenna 18 is sufficient to elicit a response from RFID device 24, whereas monostatic systems must adjust the transmitted power to keep a particular RFID device 24 at the threshold slower. However, monostatic systems allow for a second approach to determining the general location of RFID device 24 that may be preferred in certain situations.

[0027] Although an EAS system according to the present disclosure may be configured to be less accurate at longer ranges, it may be advantageous for the EAS system to more accurately monitor the movement of the RFID device from the transition zone 14 to the detection zone 12 to prevent false alarms. The determination of the movement of the RFID device may be based on a comparison of the approximate location of the RFID device at a first time to the approximate location of the RFID device at a subsequent second time. In an exemplary embodiment, an EAS system of the type described above may be used to determine the general location of the RFID device 24 at a first time based on the difference in strength or RSSI between the return signals received by the two receiving antennas (as in the embodiment of Figures 2 and 3), or the difference in the power of the RF signals emitted by the two receiving antennas that causes the RFID device 24 to reach a threshold (as in the embodiment of Figure 3). The same approach may be used to determine the general location of the same RFID device 24 at a second time, where the difference in the general location between the first and second times indicates the direction in which the RFID device 24 is moving.

[0028] While the EAS systems 16 and 26 of Figures 2 and 3 can be used to determine the general location and movement of the RFID device 24, an EAS system having more receiving antennas can more precisely determine the location, and therefore movement, of the RFID device 24. Figure 4 illustrates an exemplary gate 32 of an EAS system having four receiving antennas 34a-34d, while Figure 5 illustrates an exemplary approach for determining the location of the RFID device 24 using a system of the type illustrated in Figure 4. It should be understood that an EAS system may have more than three receiving antennas, but such antennas may be variously located (including at different heights, such as one or more antennas associated with a ceiling and other antennas located at ground level) without departing from the scope of this disclosure.

[0029] Regardless of the exact number and location of the receiving antennas in the EAS system, each receiving antenna has a known location within the EAS system and relative to the other receiving antennas. The approximate distances r1-r4 between the RFID device 24 and each receiving antenna 34 (FIG. 5) can be determined based on the strength or RSSI of the return signal received by each receiving antenna 34, or (for receiving antennas configured to transmit RF signals to the RFID device 24) the strength of the RF signal transmitted by each receiving antenna 34 that causes the RFID device 24 to reach its threshold. By simultaneously solving for the differentials, the absolute and relative positions of the receiving antennas 34a-34d, as well as the distances r1-r4 between the RFID device 24 and the receiving antennas 34a-34d can be used to determine (i.e., by triangulation) the two-dimensional location of the RFID device.

[0030] After the two-dimensional location of the RFID device 24 is determined at a first time, the process may be repeated at a second subsequent time to determine the two-dimensional location of the RFID device 24 at the second time. The locations of the RFID device 24 at the two times may be compared to determine the direction of movement of the RFID device through the EAS system. As mentioned above, this may be particularly relevant to determine when the RFID device 24 is moving through the transition zone 14 towards the detection zone 12, which may indicate an attempt to steal merchandise associated with the RFID device 24. The two-dimensional location of the RFID device 24 may be determined multiple times to more precisely track the path of the RFID device 24 through the EAS system. In some cases, the receiving antenna 34 may be located adjacent to the detection zone 12 to more precisely track the movement of the RFID device through the transition zone 14 towards the detection zone 12, where the receiving antenna 34 may be advantageous to more precisely determine the location of the RFID device 24 at a short distance, in which case it may be advantageous to have the receiving antenna 34 located adjacent to the detection zone 12 to track the movement of the RFID device through the transition zone 14 towards the detection zone 12.

[0031] Figures 6A-6C show the movement of RFID device 24 through gate 32 of Figure 4 from a first position on one side of gate 32 (Figure 6A) to a second position on the gate 32 (Figure 6B) to a third position on the other side of gate 32 (Figure 6C). In Figures 6A-6C, RFID device 24 is monitored by two receiving antennas 34a and 34b of gate 32. The two receiving antennas 34a and 34b are separated by a distance θ (as in Figures 2 and 3). When the RFID device 24 is at a distance of more than four times θ (r>4×θ) from the gate 32 (as in FIG. 6A ) and the RFID device 24 is moving towards the gate 32, the vector distance can be considered to be mainly the separation θ of the two receive antennas 34a and 34b, since the difference in angle between the line between the first receive antenna 34a and the RFID device 24 (represented as r1 in FIG. 6A ) and the line between the second receive antenna 34b and the RFID device 24 (represented as r2 in FIG. 6A ) is small. This is similar to the arrangement shown in FIG. 2 and FIG. 3 , where the two antennas are treated or considered to be aligned with the RFID device 24.

[0032] As the range decreases (i.e., as the RFID device 24 moves closer to the gate 32), the difference between the vector distances between the first and second receive antennas 34a, 34b decreases, so the range estimate (as well as the combined RSSI or strength of the return signals received by the receive antennas 34a and 34b) begins to increase. When the RFID device 24 is exactly the same distance from the first and second receive antennas 34a, 34b (as in FIG. 6B), the estimated range is essentially infinite. As the RFID device 24 passes through the gate 32, the range estimate (as well as the combined RSSI or strength of the return signals received by the receive antennas 34a and 34b) begins to decrease again, but then points in the opposite direction (as in FIG. 6C), until the RFID device 24 is again more than four times θ away from the gate 32 (r>4×θ), at which point the range has increased and the range can be measured more accurately. This passing morphology is characteristic of passing through gate 32 and can be analyzed by looking at the difference in the calculated change in range over time and the tendency of the combined RSSI to peak in the middle of gate 32 (Figure 6B).

[0033] 6A-6C, only the pair of receive antennas 34a and 34b of gate 32 of FIG. 4 is shown as being used to track the movement of RFID device 24. When multiple gates or receive antenna pairs are provided (as in FIG. 4), a system controller may select the most appropriate receive antenna pair for monitoring the movement of RFID device 24. The most accurate estimate of range is the minimum of the receive antenna pair (e.g., receive antennas 34a and 34b or receive antennas 34c and 34d of FIG. 4) indicating that RFID device 24 is most closely aligned with that receive antenna pair.

[0034] It will be understood that the above-described embodiments are illustrative of some of the applications of the principles of the present invention. Numerous modifications may be made by those skilled in the art without departing from the spirit and scope of the present invention as claimed, including any combination of features that are individually disclosed or claimed herein. For this reason, the scope of the present invention is not limited to the above description, but is understood to be directed to any feature of the present invention as set forth in the following claims, including any combination of features that are individually disclosed or claimed herein.

Claims

1. 1. A method for determining a location of an RFID device in an electronic article surveillance system having a first read zone and a second read zone, comprising: transmitting an RF signal to an RFID device; receiving return signals from the RFID devices at the first location and at the second location; calculating a difference between a first magnitude of the return signal at the first location and a second magnitude of the return signal at the second location; determining whether the RFID device is located in the first read zone based at least in part on a difference between the first intensity and the second intensity, the difference being based on signal loss occurring between the first location and the second location and independent of signal loss occurring between the RFID device and either of the first location and the second location that is closer to the RFID device; determining an approximate two-dimensional location of the RFID device based at least in part on a difference between the first intensity and the second intensity; sequentially transmitting a plurality of RF signals to the RFID device via a transmitting antenna; receiving return signals sequentially transmitted from the RFID device via a first receiving antenna and a second receiving antenna; calculating an approximate two-dimensional location of the RFID device for each RF signal; determining a direction of movement of the RFID device based at least in part on a two-dimensional position of the RFID device relative to each RF signal; A method comprising:

2. the second location is located farther from the first read zone than the first location; The method of claim 1 , wherein if a difference between the first intensity and the second intensity is positive and less than a positive threshold, the RFID device is determined to be located in the first read zone.

3. the second location is located farther from the first read zone than the first location; The method of claim 1 , wherein the RFID device is determined to be located in the second read zone if a difference between the first intensity and the second intensity exceeds a negative threshold.

4. The method of claim 1 , wherein the RF signal is transmitted from a location different from the first location and the second location.

5. 4. The method of claim 1, wherein the step of transmitting an RF signal to the RFID device includes the steps of transmitting a first RF signal from the first location and transmitting a second RF signal from the second location.

6. 1. An electronic article surveillance system for determining a location of an RFID device configured to transmit a reply signal upon receiving an RF signal, 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 receive antenna configured to receive the return signal at a first strength; a second receive antenna configured to receive the return signal at a second strength; A controller; Equipped with the first receiving antenna and the second receiving antenna are configured to receive return signals transmitted sequentially from the RFID device; The controller: determining whether the RFID device is located in the first read zone based at least in part on a difference between the first intensity and the second intensity; determining an approximate two-dimensional location of the RFID device based at least in part on a difference between the first intensity and the second intensity; determining an approximate two-dimensional location of the RFID device for each return signal; determining a direction of movement of the RFID device based at least in part on a two-dimensional position of the RFID device relative to each return signal; determining whether the RFID device is located in the first read zone based at least in part on a difference between the first intensity and the second intensity is based on a signal loss occurring between a first location and a second location and is independent of a signal loss occurring between the RFID device and any of the first and second locations closer to the RFID device; Electronic article surveillance system.

7. the second location is located farther from the first read zone than the first location; 7. The electronic article surveillance system of claim 6, wherein the controller is configured to determine that the RFID device is located in a first read zone if a difference between the first intensity and the second intensity is positive and less than a positive threshold.

8. the second location is located farther from the first read zone than the first location; 7. The electronic article surveillance system of claim 6, wherein the controller is configured to determine that the RFID device is located in a second read zone if a difference between the first intensity and the second intensity exceeds a negative threshold.

9. 9. An electronic article surveillance system according to any one of claims 6 to 8, further comprising a transmitting antenna configured to transmit said RF signal.

10. the first receive antenna configured to transmit a first RF signal; 9. An electronic article surveillance system according to any one of claims 6 to 8, wherein the second receiving antenna is configured to transmit a second RF signal.

11. 11. An electronic article surveillance system according to any one of claims 6 to 10, wherein at least one of the first receiving antenna and the second receiving antenna comprises a dipole antenna.

12. 11. An electronic article surveillance system according to any one of claims 6 to 10, wherein at least one of the first receive antenna and the second receive antenna comprises a directional antenna.

13. 11. An electronic article surveillance system according to any one of claims 6 to 10, wherein at least one of the first receive antenna and the second receive antenna comprises a line antenna.

14. 1. A method for determining a location of an RFID device in an electronic article surveillance system having a first read zone and a second read zone, comprising: transmitting a first RF signal from a first location to an RFID device and varying 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 from a second location to the RFID device and varying 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 a difference between the first intensity and the second intensity; determining whether the RFID device is located in the first read zone based at least in part on a difference between the first intensity and the second intensity; determining an approximate two-dimensional location of the RFID device based at least in part on a difference between the first intensity and the second intensity; sequentially transmitting a plurality of RF signals to the RFID device via a transmitting antenna; receiving, via a first receiving antenna and a second receiving antenna, return signals transmitted sequentially from the RFID devices; calculating an approximate two-dimensional location of the RFID device for each RF signal; determining a direction of movement of the RFID device based at least in part on a two-dimensional position of the RFID device relative to each RF signal; Including, determining whether the RFID device is located in the first read zone based on a difference between the first and second intensities is based on a signal loss occurring between the first and second locations and is independent of a signal loss occurring between the RFID device and any of the first and second locations closer to the RFID device; method.

15. 1. An electronic article surveillance system for determining a location of an RFID device configured to transmit a reply signal upon receiving an RF signal, 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 receive antenna configured to transmit a first RF signal to the RFID device and to change the 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 receive antenna; a second receive antenna configured to transmit a second RF signal to the RFID device and change the 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 receive antenna; A controller; Equipped with the first receiving antenna and the second receiving antenna are configured to receive return signals transmitted sequentially from the RFID device; The controller: determining whether the RFID device is located in a first read zone based at least in part on a difference between the first intensity and the second intensity; determining an approximate two-dimensional location of the RFID device based at least in part on a difference between the first intensity and the second intensity; determining an approximate two-dimensional location of the RFID device for each return signal; determining a direction of movement of the RFID device based at least in part on a two-dimensional position of the RFID device relative to each return signal; determining whether the RFID device is located in the first read zone based at least in part on a difference between the first intensity and the second intensity is based on a signal loss occurring between a first location and a second location and is independent of a signal loss occurring between the RFID device and any of the first and second locations closer to the RFID device; Electronic article surveillance system.

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