ELECTRONIC SECURITY SYSTEM

• US 3,810,147 A
• Filed: 12/30/1971
• Issued: 05/07/1974
• Est. Priority Date: 12/30/1971
• Status: Expired due to Term

• Current Assignee: CHECKPOINT SYSTEMS, INC. (CCL Industries Incorporated)
• Original Assignee: George Jay Lichtblau
• Inventors: Lichtblau, George Jay
• Primary Examiner(s): Caldwell, John W.
• Assistant Examiner(s): Wannisky, William M.
• Application Number:
  US05/214,361
• Time in Patent Office: 859 Days
• Field of Search: 340/280,258R,152T 343/6.5SS,6.5R,726-728
• US Class Current: 340/572.3
• 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 13/2488 Timing issues, e.g. synchronising measures to avoid signal collision, with multiple emitters or a single emitter and receiver

Abstract

An electronic security system especially adapted for use in retail stores and employing a multi-frequency resonant tag circuit having distinct frequencies for detection and deactivation. A transmitting system provides an electromagnetic field within a controlled area at a frequency which is swept through a range including the detection frequency of the resonant circuit. In the presence of a tag circuit within the controlled area and operative at the detection frequency, pulses are detected by a receiver which includes noise rejection circuitry for discriminating true signals from noise. The receipt of a predetermined number of signal pulses within a prescribed interval of time causes an alarm actuation. The resonant circuit is formed by printed circuit techniques as a relatively small tag which can be affixed to an article of merchandise. The tag includes a fusible link integrally formed as part of the circuit and which can be fused upon application of an electromagnetic field of predetermined magnitude at the deactivation frequency of the resonant circuit. Deactivation of the tag destroys the resonant properties of the tag at the detection frequency such that a deactivated tag produces no alarm when passing through a controlled area.

Claims

1. An electronic security system comprising:

transmitter means for providing an electromagnetic field in a predetermined area at a frequency repetitively swept through a predetermined range;

a multi-Resonant tag circuit having a first resonant frequency within said predetermined range of frequencies and a second frequency outside of said predetermined range of frequencies;

receiver means for detecting the presence of said first resonant frequency from a tag circuit present in said predetermined area; and

deactivation transmitter means for providing an electromagnetic field in a predetermined area at said second frequency thereby to induce a current flow in a fusible link of said tag circuit sufficient to destroy the resonant properties of said tag circuit at said first resonant frequency.

2. An electronic security system according to claim 1 wherein said receiver means includes:

an RF front-end for receiving the electromagnetic field from said transmitter means and for providing an RF output signal in response thereto;

detector means operative in response to said output signal to provide output pulses in response to the presence of the first resonant frequency of said tag circuit in said electromagnetic field;

means for suppressing noise received with said output pulses; and

means operative in response to said output pulses for providing an output indication representing the presence of a tag circuit having said first resonant frequency in said predetermined area.

3. An electronic security system according to claim 2 wherein said noise suppression means includes:

means for passing only the signal spectrum of said RF output signal; and

means for digitally processing said pulses from said detector means to provide output pulses only in response to pulses of predetermined rate and duration.

4. An electronic security system according to claim 1 wherein said transmitter means includes:

an oscillator for providing a modulation signal at a predetermined frequency;

a voltage controlled oscillator operative in response to said oscillator to provide a carrier frequency varied through a predetermined frequency range in response to the modulation signal of said oscillator;

an amplifier coupled to said voltage controlled oscillator; and

an antenna coupled to said amplifier for providing said electromagnetic field.

5. An electronic security system according to claim 1 wherein said means for electrically deactivating said tag circuit includes:

an antenna for providing said electromagnetic energy at said second resonant frequency; and

transmitter means for driving said antenna to provide said electromagnetic energy.

6. An electronic security system according to claim 5 wherein said antenna includes a balanced loop antenna matched to the output of said transmitter means by a balanced lead structure.

7. An electronic security system according to claim 5 wherein said antenna includes a balanced loop antenna having one terminal coupled to the output of said transmitter means, said loop antenna being balanced thereto by a balanced lead structure, the other end of said loop antenna being capacitively coupled to ground potential.

8. An electronic security system according to claim 1 wherein said receiver means includes:

detector means operative in response to said first resonant frequency in a received electromagnetic field from said transmitter means to provide output pulses in response thereto;

filter means for suppressing noise in the presence of such pulses;

means for amplifying said output pulses;

means for shaping said amplified pulses; and

means for digitally processing said shaped pulses and to provide an alarm indication only in response to a predetermined number of shaped pulses occurring within a predetermined time interval.

9. An electronic security system according to claim 8 wherein said filter means includes:

a low pass filter operative to pass the signal spectrum of received pulses;

a notch filter operative to remove the signal component caused by said modulating frequency; and

a bandpass filter operative to remove noise components outsidE of said signal spectrum.

10. An electronic security system according to claim 8 wherein said means for digitally processing said detected pulses includes:

means operative in response to said shaped pulses to provide first output pulses of predetermined width greater than the width of expected received pulses;

staircase generating means operative in response to said first output pulses to generate a staircase signal which increases in amplitude in the presence of each of said first output pulses;

means operative in response to said first output pulses to provide counter pulses of a width slightly less than the period of expected signals;

gate means operative in response to said first output pulses for providing a gate signal to said counter means defining an interval of time within which a predetermined number of counter pulses must be received;

counter means operative in response to said counter pulses to provide an output indication in response to the presence of a predetermined number of counter pulses within an interval of time specified by the gate signal of said gate means;

said staircase generating means being operative to provide a reset signal upon the exceedance of a predetermined threshold level by said staircase signal to reset said counter means, said staircase generating means and said gate means.

11. An electronic security system according to claim 1 wherein said receiver means includes:

detector means operative to provide output pulses in response to the presence of said tag circuit in said electromagnetic field; and

a cross correlation filter operative in response to said output pulses to provide an output signal for indicating the presence of a tag circuit only when the spectral content of said output pulses is of predetermined configuration.

12. An electronic security system according to claim 1 wherein said receiver means includes:

means for detecting pulses produced by said multi-resonant tag circuit being swept by the electromagnetic field of said transmitter means;

means for discriminating signal pulses from said tag circuit from spurious signals; and

means for providing an alarm indication in response to receipt of a predetermined number of said signal pulses occurring within a predetermined time interval.

13. An electronic security system according to claim 1 wherein said multi-resonant tag circuit includes:

a first tuned circuit resonant at said first frequency within said predetermined range;

a second tuned circuit resonant at said second frequency outside of said predetermined range; and

a fusible link in circuit with said tuned circuits and operative to fuse upon application of electromagnetic energy of predetermined power at said second frequency.

14. An electronic security system according to claim 13 wherein said multi-resonant tag circuit includes:

a planar substrate of electrically insulative material, said first and second tuned circuits being formed on said substrate in planar circuit configuration.

15. An electronic security system according to claim 1 wherein said multi-resonant tag circuit includes:

a planar substrate of electrically insulative material;

a first conductive path formed on a surface of said substrate in a configuration to define a first inductor;

a second conductive path formed on said substrate in a configuration to define a second inductor;

a plurality of pairs of conductive areas each pair formed of conductive areas in alignment on respective opposite surfaces of said substrate, the conductive areas on the substrate surface containing said first and second conductive paths being electrically connected thereto at selected points to define a plurality of capacitors for said tag circuit; and

a conductive path of predetermined size provided in circuit with said tag circuit on said substrate and operative to fuse upon application of electromagnetic energy at said second resonant frequency.

16. For use in an electronic security system which includes means for providing an electromagnetic field of a frequency which is swept within a predetermined range, means for detecting a first frequency within said range, and means for providing an electromagnetic field at a second frequency outside of said range, a multi-frequency resonant tag circuit comprising:

a first tuned circuit resonant at said first frequency within said predetermined range;

a second tuned circuit resonant at said second frequency outside of said predetermined range; and

a fusible link in circuit with said first and second tuned circuits and operative to fuse upon application of an electromagnetic field of predetermined power at said second frequency to thereby destroy the resonant properties of said tag circuit at said first frequency.

17. The invention according to claim 16 wherein said multi-frequency resonant tag circuit includes:

a planar substrate of electrically insulative material, said first and second tuned circuits being formed on said substrate in planar circuit configuration.

18. The invention according to claim 17 wherein said fusible link is also formed in planar circuit configuration with said first and second tuned circuits.

19. The invention according to claim 16 wherein said multi-frequency resonant tag circuit includes:

a planar substrate of electrically insulative material;

a first conductive path formed on a surface of said substrate in a configuration to define a first inductor;

a second conductive path formed on said substrate in a configuration to define a second inductor; and

a plurality of pairs of conductive areas fromed on said substrate each pair having conductive areas in alignment on respective opposite surface of said substrate to define a capacitor, the conductive areas on the substrate surface containing said first and second conductive paths being electrically connected to said paths at selected points to define said first and second tuned circuits.

20. The invention according to claim 16 wherein said multi-frequency resonant tag circuit includes:

a planar insulative substrate having formed on one surface thereof a first conductive area centrally formed on said substrate surface;

second and third conductive areas formed in adjacent spaced relation along one side of said substrate surface;

a first conductive path arranged on said substrate surface between and in electrical connection with said first and second conductive areas;

a second conductive path formed on said substrate surface between and in electrical connection with said second and third conductive areas; and

wherein the opposite surface of said substrate includes;

fourth, fifth and sixth conductive areas each in alignment and substantially coextensive with said respective first, second and third conductive areas formed on said one substrate surface;

a conductive path connecting said forth and fifth conductive areas;

said first and second conductive paths and plurality of conductive areas comprising said first and second tuned circuits; and

wherein said fusible link includes a conductive path interconnecting said fifth and sixth conductive areas and dimensioned to fuse upon application of an electromagnetic field of predetermined power at said second frequency.

21. The invention according to claim 16 wherein said first and second tuned circuits have a positive mutual coupling coefficient.

22. The invention according to claim 16 wherein said first and second tuned circuits have a negative mutual coupling coefficient.

23. An electronic security system according to claim 1 wherein:

said multi-resonant tag circuit has a third resonant frequency operative when the resonant properties of said tag circuit at said first resonant frequency have been destroyed by provision of an electromagnetic field thereto; and

further including;

means for detecting the presence of said third resonaNt frequency from said tag circuit.

24. The invention according to claim 16 wherein said multi-frequency resonant tag circuit includes:

a third tuned circuit resonant at a third frequency when the resonant properties of said tag circuit at said first frequency have been destroyed by application of said electromagnetic field at said second frequency.