REDUCTION OF FALSE ALARMS IN ELECTRONIC THEFT DETECTION SYSTEMS

• US 3,868,669 A
• Filed: 04/13/1973
• Issued: 02/25/1975
• Est. Priority Date: 04/13/1973
• Status: Expired due to Term

• Current Assignee: Arthur J. Minasy
• Original Assignee: Arthur J. Minasy
• Inventors: Minasy, Arthur J.
• Primary Examiner(s): Swann, III, Glen R.
• Application Number:
  US05/351,019
• Time in Patent Office: 683 Days
• Field of Search: 340/258C,280,258A 343/5PD,6.8R 325/478,35
• US Class Current: 340/572.1
• CPC Class Codes: G08B 13/2414 using inductive tags
• CPC Class Codes: G08B 13/2471 Antenna signal processing by receiver or emitter
• CPC Class Codes: G08B 13/2474 Antenna or antenna activator geometry, arrangement or layout loop antennae H01Q1/22

Abstract

Electronic theft detection systems are disclosed which have a particular frequency within which protected articles cause electronic response. False alarms are reduced by detecting similar electronic responses outside the frequency range and temporarily deactivating the system when such responses are detected. In a swept frequency system deactivation signals are produced during the portion of the frequency sweep outside the particular frequency and the deactivation remains during the remaining portion of the frequency sweep past the particular frequency.

Claims

1. A method for reducing the occurrence of false alarms in an electronic theft detection system of the type wherein an interrogation signal is varied in frequency and wherein predetermined changes in the electromagnetic field in the vicinity of a checkpoint are produced by the passage therethrough of protected articles carrying special electronic circuits whenever said interrogation signal passes through a given frequency range, and wherein an alarm indication is produced in response to said predetermined changes, said method comprising the steps of monitoring the interrogation signal frequency and preventing alarm indications when said interrogation signal frequency is outside said given range.

2. A method for reducing the occurrence of false alarms in an electronic theft detection system of the type wherein an interrogation signal is varied in frequency and wherein predetermined changes in the electromagnetic field in the vicinity of a checkpoint are produced by the passage therethrough of protected articles carrying special electronic circuits whenever said interrogation signal passes through a given frequency range, and wherein an alarm indication is produced in response to said predetermined changes, said method comprising the steps of monitoring for similar changes in the electromagnetic field in the vicinity of said checkpoint which occur while said interrogation signal is outside said given range and preventing alarm indications in response to detection of said similar changes when said interrogation signal subsequently passes through said given range.

3. A system for detecting the unauthorized passage of articles through a checkpoint, said system comprising a transmitter operative to transmit an interrogation signal which varies in frequency, responder circuits mounted on articles whose unauthorized passage is to be detected, said responder circuits being operative to produce a predetermined change in the electromagnetic field conditions in the vicinity of said checkpoint in response to the incidence on said circuits of said interrogation signal within a given frequency range, receiver mEans operative in response to said predetermined change to produce an alarm actuating signal, alarm means responsive to said actuating signal to produce an alarm indication and means operative to prevent alarm actuation while said interrogation signal is outside said given frequency range.

4. A system according to claim 3 wherein said responder circuits include energy absorbing resonant circuits and wherein said receiver means is connected to respond to energy level changes in the vicinity of said checkpoint caused by the absorption of power in a responder circuit passing through the checkpoint.

5. A system according to claim 3 wherein said means operative to prevent alarm actuation comprises a gate circuit associated with said receiver and arranged in the path of said alarm actuating signal, a frequency sensitive switch connected to receive transmitted interrogation signals and to produce switching signals to open said gate circuit when said interrogation signals are within said given frequency range and to close said gate circuit where said interrogation signals are outside said given frequency range.

6. A system for detecting the unauthorized passage of articles through a checkpoint, said system comprising a transmitter operative to transmit an interrogation signal which varies in frequency, responder circuits mounted on articles whose unauthorized passage is to be detected, said responder circuits being operative to produce a predetermined change in the electromagnetic field conditions in the vicinity of said checkpoint in response to the incidence on said circuits of said interrogation signal within a given frequency range, receiver means operative in response to said predetermined change to produce an alarm actuating signal, alarm means responsive to said actuating signal to produce an alarm indication and means associated with said receiver means for producing alarm inhibit signals in response to said predetermined change which occurs while said interrogation signal is outside said given frequency range and means responsive to the occurance of said alarm inhibit signals for inhibiting alarm indications when said interrogation signal subsequently passes through said given frequency range.

7. A system according to claim 6 wherein said responder circuits include energy absorbing resonant circuits and wherein said receiver means is connected to respond to energy level changes in the vicinity of said checkpoint caused by the absorption of power in a responder circuit passing through the checkpoint.

8. A system according to claim 6 wherein said means associated with said receiver means comprises first and second gate circuits each connected to receive signals from said receiver means, one of said gate circuits being connected to permit signals passing therethrough to produce an alarm indication, the other gate circuit being connected to permit signals passing therethrough to override any opening of said one gate circuit for a predetermined length of time and a frequency sensitive switch connected to receive transmitted interrogation signals and to produce switching signals to open said one gate when said interrogation signals are within said given frequency range and to open said other gate when said interrogation signals are outside said given frequency range.

9. A system according to claim 8 wherein said frequency sensitive switch includes a multivibrator having two output terminals which are alternately energized and which are connected, respectively, to control terminals of said first and second gate circuits.

10. A system according to claim 8 wherein a signal sustaining circuit is connected to the output of said other gate circuit.

11. A system according to claim 10 wherein said transmitter is operative to vary the frequency of said interrogation signal cyclically at a given rate and wherein said signal sustaining circuit has a duration in excess of the cyclic period of frequency variation of said interrogation signal.