Magnetic surveillance system with odd-even harmonic and phase discrimination
First Claim
1. Apparatus for detecting the presence within a surveillance zone of a body of high permeability material, the latter being constructed, when linked in said zone with both a magnetic field varying at a fundamental frequency and the substantially constant magnetic field of the earth, to produce a detectable signal containing both odd and even harmonics of said fundamental frequency, said apparatus comprising means for establishing in said zone said varying magnetic field, means for coupling to said zone to detect signals produced by said body, means coupled to said coupling means for separately determining for a detected signal the respective amplitude of a first and second component thereof whose respective frequencies are equal to two different harmonics of said fundamental frequency, one harmonic being odd and the other being even, said determining means including means for determining the phase of said first component, and means coupled to all of said determining means for furnishing an indication of the presence of said body within said zone whenever said amplitudes and phase simultaneously fall within respective predetermined limits.
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Accused Products
Abstract
Apparatus for detecting the presence within a surveillance zone of a body of high permeability material creates a surveillance field varying sinusoidally at a fundamental frequency. Parallel filter channels select the second and third harmonic components of the perturbations created by said material when present in the field and the phase of the second harmonic relative to a reference signal is compared. The signal level of each of the second and third harmonic components as well as the aforesaid phase congruency cooperatively control an alarm circuit. Rhomboid shaped transmitter coils surrounding figure "8" receiver coils improve coupling to the high permeability body.
58 Citations
16 Claims
- 1. Apparatus for detecting the presence within a surveillance zone of a body of high permeability material, the latter being constructed, when linked in said zone with both a magnetic field varying at a fundamental frequency and the substantially constant magnetic field of the earth, to produce a detectable signal containing both odd and even harmonics of said fundamental frequency, said apparatus comprising means for establishing in said zone said varying magnetic field, means for coupling to said zone to detect signals produced by said body, means coupled to said coupling means for separately determining for a detected signal the respective amplitude of a first and second component thereof whose respective frequencies are equal to two different harmonics of said fundamental frequency, one harmonic being odd and the other being even, said determining means including means for determining the phase of said first component, and means coupled to all of said determining means for furnishing an indication of the presence of said body within said zone whenever said amplitudes and phase simultaneously fall within respective predetermined limits.
- 10. Apparatus for detecting the presence within a surveillance zone of a body of high permeability material, the latter being constructed, when linked in said zone with both a magnetic field varying at a fundamental frequency and the substantially constant magnetic field of the earth, to produce a detectable signal containing both odd and even harmonics of said fundamental frequency, said apparatus comprising means for establishing in said zone said varying magnetic field, means for coupling to said zone to detect signals produced by said body, first filter means coupled to said coupling means for attenuating all signals having a frequency less than the second harmonic of said fundamental frequency, second filter means coupled to an output of said first filter means for passing only those signals having a frequency substantially equal to the third harmonic of said fundamental frequency, third filter means coupled to an output of said first filter means for passing only those signals having a frequency substantially equal to the second harmonic of said fundamental frequency, means coupled to an output of said second filter means for determining if the signals therefrom have an amplitude within a given range, means coupled to an output of said third filter means for determining if the signals therefrom have an amplitude within a given range, further means coupled to an output of said third filter means for producing squarewave signals therefrom and determining whether the deviation in phase of said squarewave signals from the phase of a reference signal falls within a given range, and means coupled to all of said determining means for furnishing an indication of the presence of said body within said zone whenever all of the signals therefrom fall within their respective given ranges.
Specification