Patent ID: 12249225

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriate manner.

FIG.2illustrates an exemplary embodiment of an RFID-based EAS system16according to the present disclosure. In the embodiment ofFIG.2, an EAS system16includes a transmitting antenna18and two receiving antennas20and22. The antennas of an EAS system according to the present disclosure may be variously configured without departing from the scope of the present disclosure, though it may be preferable to employ antenna designs that have either equal gain in the zone of interest or a means for compensating for the values of RSSI/power measured. By way of example, the receiving antennas20and22ofFIG.2may be configured as dipole antennas, directional antennas, transmission line antennas, or combinations thereof. Differently configured antennas will have different performance characteristics and, thus, different advantages. Dipole antennas, for example, will give good angular coverage for an EAS system configured to detect the two-dimensional position of an RFID device. Directional antennas, on the other hand, are better configured for keeping the detection zone for a gate of an EAS system focused forward. Accordingly, it should be understood that the present disclosure is not limited to EAS systems having particularly configured antennas, but rather that the aspects described herein may be practiced using a variety of differently configured antennas.

In the EAS system16ofFIG.2, the transmitting antenna18transmits an RF signal “S” to an RFID device24(e.g., an RFID tag or label attached to a piece of merchandise) positioned somewhere in the EAS system16. The RFID device24receives the RF signal S from the transmitting antenna18and returns a return signal, which is received by the first receiving antenna20and the second receiving antenna22.

On account of environmental conditions, the strength of the return signal will decrease as the distance traveled by the return signal increases. In the orientation ofFIG.2, the first receiving antenna20is positioned closer to the RFID device24than the second receiving antenna22, such that the strength or RSSI of the return signal will be greater at the first receiving antenna20than at the second receiving antenna22. InFIG.2, the distance between the RFID device24and the first receiving antenna20is represented by “r” and the distance between the first and second receiving antennas20and22is represented by “θ”, such that the distance between the RFID device24and the second receiving antenna22is r+θ.

As described above, measuring the strength or RSSI of the return signal from an RFID device using a single antenna may not be particularly informative or useful. However, by comparing the strength or RSSI of a return signal using two antennas20and22positioned a known distance away from each other, it is possible to more reliably determine the approximate position of an RFID device24in an EAS system16. As described above, the strength or RSSI of a return signal is a function of the distance traveled by the return signal. The common distance traveled by the return signal in reaching the first and second receiving antennas20and22(which is represented inFIG.2by “r”) may be canceled out when comparing the strength or RSSI of the return signal received by the first and second receiving antennas20and22. Thus, the difference between the strength or RSSI of the return signal received by the first receiving antenna20and the second receiving antenna22will be indicative of the loss of strength as the return signal travels the known distance θ between the first and second receiving antennas20and22.

The change in strength or RSSI of an RF signal obeys a square law, such that the difference in strength or RSSI of the return signal received by the first receiving antenna20and the second receiving antenna22will be indicative of the distance r between the RFID device24and the first receiving antenna20. Generally speaking, the difference between the strength or RSSI of the return signal received by the first receiving antenna20and the second receiving antenna22will be relatively large in magnitude when the distance r is relatively small, while the difference will be relatively small in magnitude when the distance r is relatively large. The exact magnitude of the difference will depend on a number of factors, but in an exemplary embodiment the magnitude of the difference will be on the order of approximately 6 dB when r=1, on the order of approximately 1.6 dB when r=5, and on the order of approximately 0.83 dB when r=10.

Regardless of the particular difference between the return signal strength or RSSI at the two receiving antennas20and22, it will be seen that any RFID device24that does not show a significant change in strength or RSSI can be considered to be a significant distance away from the receiving antennas20and22. While the illustrative example is not highly accurate at long distances (e.g., the difference in signal strength or RSSI is minor at r=5 and r=10), the EAS system16may be configured so as to not need high accuracy at relatively long ranges. For example, in one embodiment, the two receiving antennas20and22(or a single antenna with more than one reference plane, in the case of the receiving antennas being incorporated into a transmission line antenna) are placed between the inventory zone10and the detection zone12, with the first receiving antenna20positioned closer to the inventory zone10than the second receiving antenna22. When an RFID device24is positioned closer to the inventory zone10than to the detection zone12(i.e., closer to the first receiving antenna20than to the second receiving antenna22), the difference between the strength or RSSI of the return signal received by the first receiving antenna20and the second receiving antenna22should be positive (i.e., the return signal should be stronger at the first receiving antenna20than at the second receiving antenna22).

Thus, in this illustrative configuration, a difference in strength or RSSI of the return signal received by the first receiving antenna20and the second receiving antenna22that is positive and relatively small (i.e., less than a positive threshold value, which value is somewhere between 1.6 dB and 6 dB in the exemplary embodiment) will be sufficient to indicate that the RFID device24is positioned somewhere in the inventory zone10. The exact location of the RFID device24within the inventory zone (e.g., whether r=5 or r=10) may not be accurately determinable, but it is sufficient just to know that the RFID device24is in the inventory zone10, rather than in the detection zone12or the transition zone14. The exact positive threshold value for determining whether or not an RFID device24is sufficiently far away from the receiving antennas20and22will depend on a number of factors (e.g., the positions of the receiving antennas20and22within the EAS system16and the size of the transition zone14), so the present disclosure is not limited to any particular positive threshold value.

Similarly, to determining that an RFID device24is somewhere in the inventory zone10, an EAS system16according to the present disclosure may also determine when an RFID device24is somewhere in the detection zone12. When the first receiving antenna20is positioned closer to the inventory zone10than the second receiving antenna22, and when an RFID device24is positioned closer to the detection zone12than to the inventory zone10, the difference between the strength or RSSI of the return signal received by the first receiving antenna20and the second receiving antenna22should be negative. Thus, in this illustrative configuration, a difference in strength or RSSI of the return signal received by the first receiving antenna20and the second receiving antenna22that is negative and relatively small (i.e., greater or closer to zero than a negative threshold value) will be sufficient to indicate that the RFID device24is positioned somewhere in the detection zone12. The exact negative threshold value for determining whether or not an RFID device24is sufficiently far away from the receiving antennas20and22will depend on a number of factors (e.g., the positions of the receiving antennas20and22within the EAS system16and the size of the transition zone14), so the present disclosure is not limited to any particular negative threshold value.

It should be understood that the configuration ofFIG.2is merely exemplary and that EAS systems according to the present disclosure may be differently configured. For example,FIG.3illustrates an EAS system26in which RF signals are transmitted by two receiving antennas28and30, rather than by a third, transmitting antenna (as inFIG.2). The EAS system26ofFIG.3may be referred to as operating in a “mono-static” mode, while the EAS system16ofFIG.2may be referred to as operating in a “bi-static” mode.

In the embodiment ofFIG.3, each receiving antenna28,30transmits an RF signal to an RFID device24and receives a return signal. As in the embodiment ofFIG.2, the difference in the strength or RSSI of the return signal received by the first receiving antenna28and the second receiving antenna30may be used to determine the general position of the RFID device24(i.e., whether the RFID device24is located somewhere in the inventory zone10or somewhere in the detection zone12). However, as two different RF signals are being sent to the RFID device24in the EAS system ofFIG.3, care must be taken to ensure that the return signals from the RFID device24are transmitted at the same power. The power of the return signal transmitted by the RFID device24upon receiving RF signals from the first and second receiving antennas28and30will be the same when the RFID device24receives just enough power to transmit a return signal (which is referred to herein as the “threshold”), which represents a constant power in and a constant power out.

In one embodiment, each receiving antenna28,30will begin by transmitting a low-strength RF signal and then increasing the strength of the RF signal until first receiving a return signal from the RFID device24, which will be the strength of the RF signal of that receiving antenna at the threshold of the RFID device24. Alternatively, rather than starting at a low power, the threshold may be reached by the receiving antennas28and30initially transmitting a higher power RF signal that is sufficiently strong to reach the RFID device24, with the power being lowered until a return signal is no longer transmitted. Indeed, it should be understood that the threshold can reached using any of a number of suitable approaches, which can include a linear sweep or a binary search.

Just as the difference in the strength between two return signals may be used to determine the general location of the RFID device24, the difference in strength between the RF signal emitted by the first receiving antenna28at the threshold of the RFID device24and the RF signal emitted by the second receiving antenna30at the threshold of the RFID device24may be indicative of the general location of the RFID device24. The RF signals emitted by the two receiving antennas28and30will have the same (or at least substantially the same) strength or RSSI when reaching the RFID device24. The two RF signals will traverse the same distance r in reaching the RFID device24, such that the additional strength required to bring the RFID device24to threshold by the farther receiving antenna (which is the second receiving antenna30in the orientation ofFIG.3, but may be the first receiving antenna28, depending on the location of the RFID device24) is entirely due to the losses associated with the distance θ between the receiving antennas28and30. This information may be used (by employing the principles described herein) to determine whether the RFID device24is a substantial distance away from the receiving antennas28and30, with a positive or negative difference indicating the side of the receiving antennas28and30on which the RFID device24is positioned.

For optimum performance, it is preferred for an RFID device24to have no changes or only relatively small changes in its position during the above-described measurements. A bi-static system may be advantageous in this regard, as it is only required that the power of the RF signal transmitted by the transmitting antenna18is sufficient to elicit a response from the RFID device24, whereas a mono-static system must adjust power transmitted to keep a particular RFID device24at threshold, which is slower. However, a mono-static system allows for a second approach to determining the general location of an RFID device24, which may be preferred in certain circumstances.

While 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 be more accurate for monitoring movement of an RFID device from the transition zone14to the detection zone12to prevent false alarms. The determination of the movement of an RFID device may be based upon a comparison of the approximate location of the RFID device at a first time to the approximate location of the RFID device at a later second time. In one exemplary embodiment, an EAS system of the type described above may employed to determine the general position of an RFID device24at a first time, based on the difference in strength or RSSI between return signals received by two receiving antennas (as in the embodiments ofFIGS.2and3) or the difference in power of RF signals emitted by two receiving antennas in bringing the RFID device24to threshold (as in the embodiment ofFIG.3). The same approach may be used to determine the general position of the same RFID device24at a second time, with the difference between the general positions at the first and second times being indicative of the direction in which the RFID device24is moving.

While the EAS systems16and26ofFIGS.2and3may be used to determine the general position and movement of an RFID device24, an EAS system having more receiving antennas will be able to more accurately determine the position and, thus, movement of an RFID device24.FIG.4illustrates an exemplary gate32of an EAS system having four receiving antennas34a-34d, whileFIG.5illustrates an exemplary approach to determining the position of an RFID device24using a system of the type shown inFIG.4. It should be understood that an EAS system may have more than four receiving antennas and that such antennas may be variously positioned (including at different elevations, such as one or more antennas associated with a ceiling and others positioned at ground level) without departing from the scope of the present disclosure.

Regardless of the exact number and position of the receiving antennas of an EAS system, each receiving antenna has a known position within the EAS system and a known position with respect to the other receiving antennas. The approximate distance r1-r4 between the RFID device24and each receiving antenna34(FIG.5) may be determined based on the strength or RSSI of a return signal received by each receiving antenna34or (in the case of receiving antennas configured to also transmit RF signals to the RFID device24) the strength of the RF signal transmitted by each receiving antenna34in bringing the RFID device24to its threshold. By simultaneously solving differential values, the absolute and relative positions of the receiving antennas34a-34dand the distances r1-r4 between the RFID device24and the receiving antennas34a-34dmay be used to determine the two-dimensional position of the RFID device24(i.e., by triangulation).

After the two-dimensional position of an RFID device24has been determined at a first time, the process may be repeated at a later second time to determine the two-dimensional position of the RFID device24at the second time. The positions of the RFID device24at the two times may be compared to determine the direction of movement of the RFID device through the EAS system. As described above, this may be particularly relevant for determining when an RFID device24is moving through the transition zone14and toward the detection zone12, which may be indicative of an attempt to steal a piece of merchandise associated with the RFID device24. The two-dimensional position of the RFID device24may be determined at several times to more accurately and particularly trace the path of the RFID device24through the EAS system. It may be the case that the receiving antennas34are able to more accurately determine the position of an RFID device24at close range, in which case it may be advantageous for the receiving antennas34to be positioned adjacent to the detection zone12to track movement of RFID devices through the transition zone14and toward the detection zone12.

FIG.6A-6Cillustrate movement of an RFID device24through the gate32ofFIG.4, from a first position on one side of the gate32(FIG.6A) to a second position at the gate32(FIG.6B) to a third position on the opposite side of the gate32(FIG.6C). InFIGS.6A-6C, the RFID device24is being monitored by two of the receiving antennas34aand34bof the gate32. The two receiving antennas34aand34bare separated by a distance θ (as inFIGS.2and3). When the RFID device24is a distance of more than four times greater than θ (r>4×θ) away from the gate32(as inFIG.6A), and the RFID device24is moving toward the gate32, the angular difference between a direct line between the first receiving antenna34aand the RFID device24(represented inFIG.6Aby r1) and a direct line between the second receiving antenna34band the RFID device24(represented inFIG.6Aby r2) is small, so the vector distance can be considered largely the separation θ of the two receiving antennas34aand34b. This is similar to the arrangements shown inFIGS.2and3, in which two antennas are treated or assumed to be aligned with an RFID device24.

As the range drops (i.e., as the RFID device24moves closer to the gate32), the difference between the vector distance between the first receiving antenna34aand the second receiving antenna34bdrops, so the range estimate (and the combined RSSI or strength of the return signals received by the receiving antennas34aand34b) then starts to increase. When the RFID device24is exactly the same distance from the first and second receiving antennas34aand34b(as inFIG.6B), the estimated range is essentially infinite. When the RFID device24transits through the gate32, the range estimate (and the combined RSSI or strength of the return signals received by the receiving by the receiving antennas34aand34b) then starts to decrease again, but showing the opposite direction, until the RFID device24is again a distance of more than four times greater than θ (r>4×θ) away from the gate32(as inFIG.6C), at which time the range increases, giving a more accurate measurement of the range. This transit shape is characteristic of transiting the gate32and can be analyzed by looking at the differential of the calculated range change over time and the tendency of the combined RSSI to peak in the center of the gate32(FIG.6B)

InFIGS.6A-6C, only one pair of receiving antennas34aand34bof the gate32ofFIG.4is illustrated as being used to track movement of an RFID device24. If a plurality of gates or pairs of receiving antennas are provided (as inFIG.4), a system controller may select the most appropriate pair of receiving antennas to monitor movement of an RFID device24. The most accurate estimate of range is the minimum value from any pair of receiving antennas (e.g., either receiving antennas34aand34bor receiving antennas34cand34dinFIG.4), representing an RFID device24being most closely aligned with that pair of receiving antennas.

It will be understood that the embodiments described above are illustrative of some of the applications of the principles of the present subject matter. Numerous modifications may be made by those skilled in the art without departing from the spirit and scope of the claimed subject matter, including those combinations of features that are individually disclosed or claimed herein. For these reasons, the scope hereof is not limited to the above description but is as set forth in the following claims, and it is understood that claims may be directed to the features hereof, including as combinations of features that are individually disclosed or claimed herein.