CAPACITIVE INTRUSION DETECTION SYSTEM WITH BALANCED RESONANT CIRCUITS

• US 3,778,807 A
• Filed: 12/13/1972
• Issued: 12/11/1973
• Est. Priority Date: 12/13/1972
• Status: Expired due to Term

• Current Assignee: William J. Ralston
• Original Assignee: William J. Ralston
• Inventors: Ralston, William J.
• Primary Examiner(s): Caldwell, John W.
• Assistant Examiner(s): Swann, III, Glen R.
• Application Number:
  US05/314,904
• Time in Patent Office: 363 Days
• Field of Search: 340/258C,276
• US Class Current: 340/564
• CPC Class Codes: G08B 13/26 by proximity of an intruder causing variation in capacitance or inductance of a circuit

Abstract

Each of a pair of detector wires is strung along a portion of a boundary to be protected so that both wires are exposed to similar general environmental conditions. The wires are interconnected in respective resonant circuits which are similarly driven at a common frequency so that each resonant circuit provides an a.c. signal having a phase and amplitude which are variable as a function of the capacitance of the respective detector wire. By synchronously demodulating the difference between the two signals, a difference amplitude signal is obtained which is relatively sensitive to differential changes in the capacitances. By employing the rate of change of this demodulating difference signal as an indication of intrusion, a system is obtained which is relatively insensitive to gradual and environmental changes.

Claims

1. Intrusion detection apparatus comprising:

a pair of capacitive detector elements exposed to the environment in which intrusion is to be detected, the capacitance with respect to ground of each element being variable as a function of changes in the respective environmental area;

interconnected with each detector element, a respective inductance forming therewith a resonant circuit;

means for applying a preselected a.c. drive to each of said resonant circuits thereby to obtain from each circuit an a.c. signal having an amplitude and phase which are variable as a function of the respective detector element capacitance, both of said resonant circuits being driven at the same frequency;

a differential amplifier operative to reject common mode components of signals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtain an a.c. difference signal;

detector means operating synchronously with said drive means for demodulating said a.c. difference signal to obtain a phasesensitive amplitude signal, said amplitude signal being sensitive to intrusions into the environment of said detector elements.

2. Apparatus as set forth in claim 1 in which said detector elements are wires of substantially equal length supported adjacent respective portions of a boundary relative to which intrusion is to be detected.

3. Apparatus as set forth in claim 2 in which said wires are strung in spaced relation to a grounded metallic fence.

4. Apparatus as set forth in claim 3 in which said inductances said differential amplifier and said detector means are mounted in close proximity to said wires thereby to minimize stray capacitances.

5. Apparatus as set forth in claim 1 in which said drive means provides a phase reference signal to said detector means and in which said detector means is of the type providing an output signal Vo KDVRVD cos theta where:

Vo the d.c. output of the detector;

KD a gain constant associated with the said detector;

VR the amplitude of said phase reference signal;

VD the amplitude of said a.c. difference signal; and

theta thE phase angle between the phase reference signal and the a.c. difference signal.

6. Intrusion detection apparatus comprising:

a pair of capacitive detector elements exposed to the environment in which intrusion is to be detected, the capacitance with respect to ground of each element being variable as a function of changes in the respective environmental area;

interconnected with each detector element, a respective inductance forming therewith a parallel resonant circuit resonant essentially at a preselected frequency;

means for driving each of said resonant circuits at a frequency approximately equal to a half-power point of said circuits, thereby to obtain from each circuit an a.c. signal having an amplitude and phase which are variable as a function of the respective detector element capacitance;

a differential amplifier operative to reject common mode components of signals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtain an a.c. difference signal;

detector means interconnected with said drive means and operated synchronously therewith for synchronously demodulating said a.c. difference signal to obtain a phase-sensitive amplitude signal; and

means providing an indication of intrusion detection when the rate-of-change of said difference amplitude signal exceeds a preselected level.

7. Apparatus as set forth in claim 6 in which said drive means provides a phase reference signal to said detector means and in which said detector means is of the type providing an output signal Vo KDVRVD cos theta where:

Vo the d.c. output of the detector;

KD a gain constant associated with the said detector;

VR the amplitude of said phase reference signal;

VD the amplitude of said a.c. difference signal; and

theta the phase angle between the phase reference signal and the a.c. difference signal.

8. Intrusion detection apparatus comprising:

first and second detector wires, each supported along a respective portion of a boundary relative to which intrusion is to be detected, the capacitance with respect to ground of each wire being variable as a function of changes in the respective environmental area;

a respective inductance connected in parallel with the capacitance exhibited by each wire thereby to form a circuit resonant at a preselected frequency which is common to both circuits;

resistance means for feeding each parallel circuit, each resultant circuit having a Q which is substantially equal to a preselected value;

a constant-amplitude signal source driving both of said resonant circuits through the respective resistance means at a frequency approximately equal to the lower half-power points of said resonant circuits to obtain from each an a.c. signal having an amplitude and phase which are variable as a function of the respective detector wire capacitances;

a differential amplifier operative to reject common mode components of signals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtain an a.c. difference signal;

detector means controlled by said signal source means for synchronously demodulating said a.c. difference signal to obtain a phase-sensitive amplitude signal;

means for obtaining from said amplitude signal a rate-of-change signal; and

means providing an indication of intrusion detection when the rate-of-change signal passes out of a preselected range of values.

9. Intrusion detection apparatus comprising:

first and second detector wires, each supported along a respective portion of a boundary relative to which intrusion is to be detected, the capacitance with respect to ground of each wire being variable as a function of changes in the respective environmental area;

a respective inductance connected in parallel with the capacitance exhibited by each wire therEby to form a circuit resonant at a preselected frequency which is common to both circuits;

a transistor for feeding each parallel circuit, the parallel resonant circuit being connected to the collector of the respective transistor;

a constant-amplitude signal source driving both of said resonant circuits through the respective transistors at a frequency approximately equal to the lower half-power points of said resonant circuits to obtain from each an a.c. signal having an amplitude and phase which are variable as a function of the respective detector wire capacitances;

a differential amplifier operative to reject common mode components of signals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtain an a.c. difference signal;

detector means controlled by said signal source means for synchronously demodulating said a.c. difference signal to obtain a phase-sensitive amplitude signal;

means for obtaining from said amplitude signal a rate-of-change signal; and

means providing an indication of intrusion detection when the rate-of-change signal passes out of a preselected range of values.

10. Intrusion detection apparatus comprising:

a metallic fence;

first and second detector wires, each supported adjacent a respective portion of said fence, the capacitance with respect to said fence of each wire being variable as a function of changes in the environment adjacent that wire;

a respective inductance connected in parallel with the capacitance exhibited by each wire thereby to form therewith a circuit which is resonant essentially at a preselected frequency, said preselected frequency being common to both circuits;

a constant-amplitude signal source driving both of said resonant circuits, through respective isolating means, at a frequency approximately equal to the lower half-power points of said resonant circuits thereby to obtain from each resonant circuit an a.c. signal having an amplitude and phase which are variable as a function of the respective detector wire capacitances;

a differential amplifier operative to reject common mode components of signals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtain an a.c. difference signal;

detector means controlled by said signal source means for synchronously demodulating said a.c. difference signal to obtain a phase-sensitive amplitude signal;

filter means for obtaining from said amplitude signal a rate-of-change signal; and

means providing an indication of intrusion detection when the rate-of-change signal passes out of a preselected range of values.