Capacitive intrusion detection system

Claim 2

Apparatus for detecting intrusion of a protected area including an object to be protected, said protected object having a capacitance associated with the protected area, comprising first means for defining two distinct time intervals and producing indications of the durations thereof, each of said time intervals being related to a characteristic associated with the security of the protected area, said first-named indicating means comprising means for charging the object, means for discharging said object when the charge thereon exceeds a prescribed threshold level, and a first counter circuit producing a first output signal having one logic level for a predetermined number of threshold crossings of the voltage across said object and defining one of said time intervals, said first counter circuit producing a second output signal having one logic level for the same number of threshold crossings of the charge on said object and defining the other time interval, at least one threshold crossing produced during said first and second time intervals being distinct, and first means for comparing said duration indications and producing an output as a measure of intrusion of the protected area.

Claim 3

Apparatus according to claim 1 wherein said first comparing means comprises: a clock generator producing a train of clock pulses, and second means for comparing the number of clock pulses produced during one of said time intervals with the number of clock pulses produced during the other time interval for detecting intrusion of the protected area.

Claim 4

Apparatus according to claim 2 wherein said second comparing means comprises a second counter circuit responsive to the first output signal of said first counter circuit for first counting the number of clock pulses generated during said one time interval and responsive to the second output signal of said first counter circuit for second counting the number of clock pulses generated during said other time interval, said second counter circuit comparing indications of said first and second countings for producing an output which is a relative indication of the durations of said time intervals.

Claim 5

Apparatus according to claim 3 wherein the output of said second counter circuit is the difference between the first and second countings and thus the difference between the number of clock pulses produced during said time intervals.

Claim 6

Apparatus according to claim 2 wherein said second comparing means comprises an up/down counter responsive to said first output signal of said first counter circuit for counting up the number of clock pulses produced during said one time interval, and responsive to the second output signal of said first counter circuit for counting down from the contents of said up/down counter the number of clock pulses produced during said other time interval, the remainder count in said up/down counter at the termination of the other time interval being an indication of the difference between the number of clock pulses produced during, and of the durations of, said time intervals.

Claim 7

Apparatus according to claim 3 wherein said clock generator produces a train of delayed clock pulses and said first-named indicating means includes a first delay circuit for extending the durations of the output signals of said first counter circuit when a threshold crossing of the charge on the object occurs during generation of a delayed clock pulse.

Claim 8

Apparatus according to claim 3 wherein said charging means comprises a constant current source connected in parallel with said object; and, said discharging means comprises a first threshold circuit connected in parallel with the object.

Claim 9

Apparatus according to claim 3 wherein said first-named indicating means includes means for extending the duration of the time interval between the threshold crossing of the charge on the object and the subsequent initiation of charging of the object.

Claim 10

Apparatus according to claim 7 wherein said first-named indicating means includes second means for indicating when the charge on the object is less than the prescribed threshold level for greater than a predetermined time interval.

Claim 11

Apparatus according to claim 7 wherein said first counter circuit is an m-stage counter in which the first and second output signals are coupled from the Q and Q terminals of the mth stage thereof, said first and second output signals having first and second logic levels, respectively, during counting of 2m 1 threshold crossings of the charge on the object and having second and first logic levels, respectively, during subsequent counting of an additional 2m 1 threshold crossings of the charge on the object.

Claim 12

Apparatus according to claim 5 wherein said first comparison means includes a control circuit responsive to termination of the second output signal of said first counter circuit at the end of the other time interval for producing a control signal, a second threshold circuit responsive to the remainder count in said up/down counter for producing an output signal when this remainder count is outside of prescribed limits, and third means for indicating intrusion of the protected area when an output signal of said second threshold circuit is produced during generation of a number of said control signals form said control circuit.

Claim 13

Apparatus for detecting intrusion of a protected vehicle having a capacitance associated with the security of the vehicle comprising means for charging the vehicle to a prescribed threshold level at a rate that is a function of said capacitance and for discharging the vehicle, means responsive to the operation of said first-named means for producing first and second output signals indicating the durations of first and second distinct time intervals which may have different durations, respectively, that are each defined by the same number of threshold crossings of the charge on the vehicle, and first means comparing said first and second output signals of said indicating means for producing a relative indication of the durations of the time intervals and the security of the protected area. 13 Apparatus for detecting intrusion of a protected vehicle having a capacitance associated with the security of the vehicle comprising means for charging the vehicle to a prescribed threshold level at a rate that is a function of said capacitance and for discharging the vehicle, means responsive to the operation of said first-named means for producing first and second output signals indicating the durations of first and second distinct time intervals, respectively, that are each defined by the same number of threshold crossings of the charge on the vehicle, and first means comparing said first and second output signals of said indicating means for producing a relative indication of the durations of the time intervals and the security of the protected area, said first comparing means comprising a clock generator producing clock pulses, and second means responsive to clock pulses and the first and second output signals of said indicating means for comparing the number of clock pulses produced during the first time interval with the number of clock pulses produced during the second time interval.

Claim 14

Apparatus for detecting intrusion of a protected vehicle having a capacitance associated with the security of the vehicle comprising means for charging the vehicle to a prescribed threshold level at a rate that is a function of said capacitance and for discharging the vehicle, means responsive to the operation of said first-named means for producing first and second output signals indicating the durations of first and second distinct time intervals which may have different durations, respectively, that are each defined by the same number of threshold crossings of the charge on the vehicle, and first means comparing said first and second output signals of said indicating means for producing a relative indication of the durations of the time intervals and the security of the protected area. 13 Apparatus for detecting intrusion of a protected vehicle having a capacitance associated with the security of the vehicle comprising means for charging the vehicle to a prescribed threshold level at a rate that is a function of said capacitance and for discharging the vehicle, means responsive to the operation of said first-named means for producing first and second output signals indicating the durations of first and second distinct time intervals, respectively, that are each defined by the same number of threshold crossings of the charge on the vehicle, and first means comparing said first and second output signals of said indicating means for producing a relative indication of the durations of the time intervals and the security of the protected area, said first comparing means comprising a clock generator producing clock pulses, and second means responsive to clock pulses and the first and second output signals of said indicating means for comparing the number of clock pulses produced during the first time interval with the number of clock pulses produced during the second time interval.

Claim 15

Apparatus according to claim 13 wherein said second comparing means takes the difference between the numbers of clock pulses produced during the time intervals.

Claim 16

Apparatus according to claim 14 wherein said indicating means and said second comparing means each comprise a digital counter circuit.

Claim 17

Apparatus for detecting intrusion of a protected vehicle having a capacitance with respect to the ground comprising means for producing first and second outputs indicating the durations of first and second time intervals, respectively, which are distinct and each a function of the vehicle capacitance, said first nameD indicating means comprising means connected in parallel with the vehicle for charging the vehicle at a rate which is a function thereof, a first threshold circuit having an output, means for connecting said first threshold circuit in parallel with the vehicle, said first threshold circuit operating in one state to discharge the vehicle when the magnitude of the charge thereon exceeds a prescribed threshold level, and a first digital counter circuit responsive to operation of said first threshold circuit for counting the number of threshold crossings of the charge on the vehicle, said first counting circuit having a first output having a particular value for a prescribed number of threshold crossings of the charge on the vehicle for defining the first time interval and having a second output having a particular value for the same number of threshold crossings for defining the second time interval, at least one threshold crossing in each of said time intervals being distinct, and means for comparing the outputs of said indicating means for indicating intrusion of the protected vehicle.

Claim 18

Apparatus for detecting intrusion of a protected vehicle having a capacitance with respect to the ground comprising means for producing first and second outputs indicating the durations of first and second time intervals, respectively, which are distinct and each a function of the vehicle capacitance, said first nameD indicating means comprising means connected in parallel with the vehicle for charging the vehicle at a rate which is a function thereof, a first threshold circuit having an output, means for connecting said first threshold circuit in parallel with the vehicle, said first threshold circuit operating in one state to discharge the vehicle when the magnitude of the charge thereon exceeds a prescribed threshold level, and a first digital counter circuit responsive to operation of said first threshold circuit for counting the number of threshold crossings of the charge on the vehicle, said first counting circuit having a first output having a particular value for a prescribed number of threshold crossings of the charge on the vehicle for defining the first time interval and having a second output having a particular value for the same number of threshold crossings for defining the second time interval, at least one threshold crossing in each of said time intervals being distinct, and means for comparing the outputs of said indicating means for indicating intrusion of the protected vehicle.

Claim 19

Apparatus according to claim 16 wherein said comparing means comprises a clock generator producing clock pulses, and a second digital counting circuit responsive to said clock pulses and said first and second outputs of said first counting circuit for counting the number of clock pulses generated during said first and second time intervals and producing a relative indication of the number of clock pulses generated during said time intervals.

Claim 20

Apparatus according to claim 17 wherein said first connecting means comprises a first delay circuit connected between said first threshold circuit and the vehicle and extending the duration of operation of the first threshold circuit in one state.

Claim 21

Apparatus according to claim 17 wherein said first-named indicating means includes a detector circuit detecting the output of said first threshold circuit for initiating an alarm when the first threshold circuit output is constant for greater than a predetermined minimum time interval.

Claim 28

The method according to claim 22 wherein said step of comparing the clock pulse counts includes the step of taking the difference between the first and second countings of clock pulses.

Claim 31

The method according to claim 24 including the step of periodically charging the object to a prescribed threshold level and discharging the object.

Claim 32

The method according to claim 25 wherein the steps of obtaining the first and second indications comprise counting the same number of charge-discharge cycles of the object for defining the two distinct time intervals.

Claim 33

The method according to claim 26 including the step of producing a train of clock pulses and wherein the step of obtaining said third indication comprises the steps of counting the number of clock pulses produced during one time interval, counting the number of clock pulses produced during the subsequent time interval, and taking the difference between the number of clock pulses produced during the two time intervals.

Claim 34

Apparatus for detecting the compromise of a protected zone, said protected zone having a variable characteristic which undergoes a change in value when said compromise exists, said apparatus comprising means for defining two separate time intervals having durations directly dependent on the value of said characteristic, and means for comparing the duration of one of said intervals with the duration of the other of said intervals and producing an output as an indication of said compromise when either duration changes relative to the other.

Claim 35

Apparatus for detecting the compromise of a protected zone, said protected zone having a variable characteristic which undergoes a change in value when said compromise exists, said apparatus comprising means for defining two separate time intervals having durations directly dependent on the value of said characteristic, and means for comparing the duration of one of said intervals with the duration of the other of said intervals and producing an output as an indication of said compromise when either duration changes relative to the other.

Claim 36

Apparatus according to claim 28 wherein said comparing means comprises means for measuring the difference between said durations of the time intervals.

Claim 37

Apparatus according to claim 29 in which said protected zone comprises a vehicle and said variable characteristic is the capacitance associated with said vehicle.