ELECTRONIC SECURITY SYSTEM HAVING IMPROVED NOISE DISCRIMINATION

• US 3,863,244 A
• Filed: 06/14/1972
• Issued: 01/28/1975
• Est. Priority Date: 06/14/1972
• 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:
  US05/262,465
• Time in Patent Office: 958 Days
• Field of Search: 340/280,258R,258C,152T 343/6.5SS,6.5R,6.8R 328/108 325/474,473
• US Class Current: 340/572.3
• CPC Class Codes: G06K 19/0672 with resonating marks
• CPC Class Codes: G08B 13/2414 using inductive tags
• CPC Class Codes: G08B 13/2431 Tag circuit details
• 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 adapted for use in a controlled area such as a retail store and employing a multi-frequency resonant tag circuit having distinct frequencies for detection and discrimination. An electromagnetic field is provided within the controlled area at a frequency which is swept through a range including the detection frequency of the resonant tag circuit. In the presence of a tag circuit within the controlled area, pulses are detected by a receiver which includes noise rejection circuitry for discrimination true signals from noise. The noise rejection circuitry is sensitive to expected signals and operative to discriminate true signals fromnoise on the basis of several different criteria, resulting in substantially reduced false alarm actuation.

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;

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

means for discriminating between relatively steep pulses from said detector means caused by the presence of a tag circuit in said electromagnetic field and less steep pulses caused by noise;

noise rejection circuitry operative in response to pulses from said discriminating means for providing output signals and including means operative in response to pulses from said discriminating means of first and second polarity to provide respective first and second pulses of predetermined pulse height and pulse width;

means for providing third pulses in response to said first and second pulses of alternating polarity;

means defining a time interval in which said third pulses must be received in order to provide an output indication;

means operative in response to said third pulses to provide an output indication of alarm actuation in response to a predetermined number of received pulses.

2. The system according to claim 1 including means for electrically deactivating said tag circuit by application of electromagnetic energy at said second resonant frequency to destroy the resonant properties of said tag circuit at said first resonant frequency.

3. The system according to claim 1 wherein said noise rejection circuitry includes:

a first threshold detector for providing an output pulse in response to a received pulse of positive polarity and predetermined height;

a second threshold detector operative to provide an output pulse in response to a received pulse of negative polarity and predetermined height;

a first pulse width discriminator operative in response to pulses received from said first threshold detector to provide output pulses of predetermined width;

a second pulse width discriminator operative in response to pulses received from said second threshold detector to provide corresponding output pulses of predetermined width;

gate means operative in response to the output pulses received from said first and second pulse width discriminators for alternately inhibiting said first and second discriminators;

means for counting the output pulses provided by said first and second pulse width discriminators and for providing an output indication upon receipt of a predetermined number of pulses from said discriminators; and

gate means for providing a time interval within which said output indication from said counting means must be present and operative to reset said inhibiting means and said counting means at the end of said time interval.

4. The system according to claim 1 wherein said noise rejection circuitry includes:

means operative in response to pulses from said pulse discriminating means for providing first output pulses of predetermined height and width in response to received pulses of one polarity, and second output pulses of predetermined height and width in response to received pulses of opposiTe polarity;

means for providing third output pulses in response to received pulses of alternating polarity;

means for counting said third pulses and for providing an output indication upon a predetermined count thereof; and

means for providing a gate signal representing a time interval within which said third pulses must be received in order to provide said output indication.

5. The system according to claim 1 wherein said pulse discriminating means includes a high pass filter for passing only a predetermined higher frequency portion of the signal spectrum of the output pulses of said detector means.

6. The system according to claim 4 wherein said noise rejection circuitry includes:

multivibrator means operative in response to input pulses thereto to provide output pulses of predetermined width slightly less than the period of the modulation rate of said transmitter means;

integrator means operative in response to the output pulses of said multivibrator means for providing an output signal of a level respresentative of the average value of said multivibrator output pulses;

means for providing a threshold level of predetermined amplitude; and

means for providing an output signal upon exceedance of said threshold level by said integrator output signal;

said integrator means having a time constant to provide an output signal of a level which exceeds said threshold level only after receipt of a predetermined minimum number of multivibrator output pulses.

7. The system according to claim 1 wherein said pulse discriminating means includes filter means for passing a portion of the signal spectrum of the output pulses of said detector means which is higher than twice the modulation frequency of said transmitter means.

8. The system according to claim 1 wherein said transmitter means includes an antenna operative to provide said electromagnetic field in said predetermined area, said detector means being additionally connected to said antenna.

9. The 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 modulation signal to provide a carrier frequency varied through a predetermined frequency range;

an amplifier coupled to said voltage controlled oscillator; and

a non-resonant antenna coupled to said amplifier for providing said electromagnetic field;

said detector means being additionally connected to said antenna.

10. The system according to claim 1 wherein said transmitter means includes:

an oscillator for providing a carrier signal;

a resonant antenna coupled to said oscillator; and

oscillator means coupled to said antenna for providing a frequency modulation signal thereto to vary said electromagnetic field through said predetermined range;

said detector means being additionally connected to said antenna.

11. The 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;

said detector means being operative in response to said RF output signal.

12. The system according to claim 11 wherein said transmitter means and said RF front end each include an antenna for respectively radiating and sensing said electromagnetic field.

13. The system according to claim 1 wherein said transmitter means and said receiver means each include an antenna for respectively radiating and sensing said electromagnetic field;

said antennas being closely spaced and coupled together by means of respective windings of a transformer.

14. 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.

15. An electronic security system according to claim 2 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.

16. An electronic security system according to claim 15 wherein said antenna includes a balanced loop antenna matched to the output of said transmitter means by a balanced lead structure.

17. An electronic security system according to claim 3 wherein said noise rejection circuitry further includes:

means operative in response to said output indication from said counting means 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.

18. 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.

19. An electronic security system according to claim 18 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.

20. 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;

first and second 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 first and second capacitors for said tag circuit;

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

a conductive path provided through said substrate and interconnecting one of said first and second conductive paths and one of said conductive areas to complete said tag circuit.

21. 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 resonant tag circuit having at least one resonant frequency within said predetermined range of frequencies;

receiver means for detecting the presence of said at least one resonant frequency from a tag circuit present in said predetermined area;

said receiver means including detector means operative to provide output pulses in response to the presence of said at least one resonant frequency of said tag circuit in said electromagnetic field;

means for discriminating between relatively steep pulses from said detector means caused by the presence of a tag circuit in said electromagnetic field and less steep pulses caused by noise;

noise rejection circuitry operative in response to pulses from said discriminating means for providing output signals in response to received pulses of predetermined pulse height, pulse width, pulse spacing, polarity, sequence and repetition rate; and

means operative in response to the output pulses of said noise rejection circuitry to provide an output indication of alarm actuation in response to a predetermined number of received pulses.

22. The system according to claim 21 wherein said noise rejection circuitry includes:

means operative in response to pulses from said pulse discriminating means for providing first output pulses of predetermined height and width in response to received pulses of one polarity, and second output pulses of predetermined height and width in response to received pulses of opposite polarity;

means for providing third output pulses in response to received pulses of alternating polarity;

means for counting said third pulses and for providing an output indication upon a predetermined count thereof; and

means for providing a gate signal representing a time interval within which said third pulses must be received in order to provide said output indication.

23. The system according to claim 22 wherein said noise rejection circuitry includes:

multivibrator means operative in response to input pulses thereto to provide output pulses of predetermined width slightly less than the period of the modulation rate of said transmitter means;

integrator means operative in response to the output pulses of said multivibrator means for providing an output signal of a level representative of the average value of said multivibrator output pulses;

means for providing a threshold level of predetermined amplitude; and

means for providing an output signal upon exceedance of said threshold level by said integrator output signal;

said integrator means having a time constant to provide an output signal of a level which exceeds said threshold level only after receipt of a predetermined minimum number of multivibrator output pulses.

24. The system according to claim 21 wherein said pulse discriminating means includes a high pass filter for passing only a predetermined higher frequency portion of the signal spectrum of the output pulses of said detector means.