Electronic article surveillance system with adaptive filtering and digital detection

• US 5,602,531 A
• Filed: 05/07/1996
• Issued: 02/11/1997
• Est. Priority Date: 04/07/1995
• Status: Expired due to Term

• Current Assignee: 3M Company
• Original Assignee: 3M Company
• Inventors: Nelson, John E., Rude, Michael J., Tao, Samuel H.
• Primary Examiner(s): SWANN III, GLEN R
• Application Number:
  US08/646,005
• Time in Patent Office: 280 Days
• Field of Search: 340/572, 340/551, 340/552
• US Class Current: 340/572.4
• CPC Class Codes: G08B 13/2471 Antenna signal processing by receiver or emitter
• CPC Class Codes: G08B 13/2482 EAS methods, e.g. description of flow chart of the detection procedure
• CPC Class Codes: G08B 13/2488 Timing issues, e.g. synchronising measures to avoid signal collision, with multiple emitters or a single emitter and receiver

Abstract

An electronic article surveillance (EAS) systems includes an asynchronous noise suppression adaptive filter which removes asynchronous interference from the signal of interest with minimum distortion of the transients emitted when a sensitized tag is interrogated. The system also includes a synchronous noise suppression filter which removes interrogation synchronous noise from the signal of interest. The resulting EAS system increases the likelihood that a sensitized tag will be detected when one is present and reduces the occurrence of false alarms.

Claims

1. An electronic article surveillance system in which a drive field signal is generated in an interrogation corridor to detect presence of a sensitized tag, comprising:

means for acquiring a passive snapshot of an environment in the corridor;

means for acquiring an interrogation snapshot of the environment in the corridor; and

an adaptive filter, connected to receive the passive snapshot and to receive the interrogation snapshot, and adapted to adaptively filter the passive snapshot and subtract the filtered passive snapshot from the interrogation snapshot to produce a recovered signal.

2. The electronic article surveillance system of claim 1, further including:

means for acquiring an active snapshot of the environment in the corridor; and

a synch filter, connected to receive the active snapshot and to receive the interrogation snapshot, and to subtract the active snapshot from the interrogation snapshot.

3. The electronic article surveillance system of claim 2, wherein the means for acquiring an active snapshot is further adapted for acquiring a plurality of active snapshots and creating therefrom a composite active snapshot.

4. The electronic article surveillance system of claim 2, wherein the active snapshot is acquired so as to reduce power line synchronous interference in the active snapshot.

5. The electronic article surveillance system of claim 1, further including at least one linear phase bandpass filter adapted to reduce to the level of asynchronous interference in the passive snapshot and the interrogation snapshot.

6. The electronic article surveillance system of claim 1, wherein the adaptive filter conditions the passive snapshot such that the least squares error residual between the filtered passive snapshot and the interrogation snapshot is minimized.

7. The electronic article surveillance system of claim 1, wherein the passive snapshot and the interrogation snapshot are acquired so as to reduce power line synchronous interference in the passive snapshot and the interrogation snapshot.

8. The electronic article surveillance system of claim 7, wherein the passive snapshot and the interrogation snapshot are acquired at like points with respect to the phase of the power line signal, and wherein the like points are portions of the power line signal with the minimum amount of power line synchronous interference.

9. The electronic article surveillance system of claim 1, further including means for identifying a sensitized tag signal.

10. The electronic article surveillance system of claim 9, wherein the means for identifying is connected to receive the recovered signal, and further includes:

means for finding zero crossings of the drive field signal;

means for defining tag timing gates associated with the zero crossings of the drive field signal;

means for making a first assessment of the recovered signal within the tag timing gates;

means for making a second assessment of the recovered signal in an associated region outside of the tag timing gates;

means for comparing the first and second assessments to each other; and

means for identifying a sensitized tag signal based on a favorable comparison.

11. The electronic article surveillance system of claim 10, wherein the means for making the first assessment further includes means for determining a maximum value of the recovered signal within each of the tag timing gates;

and

wherein the means for making the second assessment further includes means for determining a maximum value of the recovered signal in an associated region outside of each of the tag timing gates.

12. The electronic article surveillance system of claim 11, wherein the means for comparing further includes means for determining the ratio of the maximum value of the recovered signal inside the tag timing gates to the maximum value of the recovered signal in an associated region outside of the tag timing gates.

13. The electronic article surveillance system of claim 9, further including means for deciding that a sensitized tag is present in the corridor upon identification of the sensitized tag signal.

14. The electronic article surveillance system of claim 9, further including means for rejecting the identified sensitized tag signal based on an early switch inhibit test.

15. The electronic article surveillance system of claim 9, further including means for rejecting the identified sensitized tag signal based on an asymmetry test.

16. The electronic article surveillance system of claim 9, further including means for rejecting the identified sensitized tag signal based on a switch decay test.

17. An electronic article surveillance system, comprising:

at least one interrogation signal generator;

at least one signal sense detector;

a receiver, connected to receive a signal from the signal sense detector, the receiver further including;

synch means for filtering interrogation synchronous interference from the received signal;

asynch means for filtering asynchronous interference from the received signal and producing therefrom a recovered signal; and

detection means, connected to receive the recovered signal, for identifying whether the recovered signal contains a sensitized tag signal.

18. The system of claim 17 wherein the system is a pulsed system.

19. The system of claim 18 wherein the average magnetic energy produced by the interrogation signal generator is less than 1.0 Oe.

20. A method of detecting presence of a sensitized tag in an interrogation corridor of an electronic article surveillance system, wherein a drive field signal is generated in the interrogation corridor comprising the steps of;

(a) sampling the environment in the corridor in the absence of the drive field signal to acquire a passive snapshot;

(b) sampling the environment in the corridor during presence of the drive field signal to acquire an interrogation snapshot;

(c) adaptively filtering the passive snapshot;

(d) subtracting the filtered passive snapshot from the interrogation snapshot and producing therefrom a recovered signal; and

(e) assessing the recovered signal for presence of a sensitized tag.