Capacitive intrusion detection system with balanced resonant circuits

Claim 2

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.

Claim 3

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.

Claim 4

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

Claim 5

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.

Claim 6

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.

Claim 7

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.

Claim 8

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.

Claim 9

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.

Claim 10

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.

Claim 11

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.

Claim 12

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.

Claim 13

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.

Claim 14

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.

Claim 15

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.