An intrusion detection system and a method therefor

Abstract: A dual sensing intrusion detection system comprising a passive infrared radiation detection sensor (12) which generates a first output signal (32) in response to the detection of an intruder in the volume of space, a second detection sensor (6) which is directed to the same volume of space and generates a second output signal (46) in response to detection of the intruder and a switch (38) which activates the second detection sensor in response to the detection of the intruder by the infrared radiation detector. Logic circuit (36) receives the first and second output signals and produces an alarm signal in response thereto to indicate the detection of the presence of the intruder in the volume of space.

Description: The present invention relates to a dual sensor intrusion detection system and is more particularly concerned with the problem of reducing the power consumption of a dual sensor intrusion detection system. Dual sensor intrusion detection systems are well known in the art. See for example, U.S. Patent No. 4,401,976 or 4,437,089. A typical dual sensor intrusion detection system comprises a passive infrared radiation (PIR) sensor and a microwave sensor. The sensors are directed to detect an intruder from the same volume of space. To trigger an alarm, however, both of the sensors must simultaneously detect the presence of an intruder. The use of two different types of energy sensing devices directed at the same volume of space to detect the presence of an intruder, renders such a dual sensing intrusion detection system highly immune to false alarms. Increasingly, however, it is necessary to mount or install intrusion detection systems in locations where it is difficult or expensive to supply wires for electrical power or alarm conditions. Thus, the intrusion detection system must be self-contained. This requires the use of batteries. If a battery powered dual sensor intrusion detection system is constantly on it is useless, because as a practical matter, batteries must be changed so frequently. U.S. Patent 4,437,089 discloses two detectors with the sensitivity of one detector increased when the other detector detects an intruder. However, that reference does not disclose or teach activating a second detector only when there is a detection by the first detector in order to reduce power consumption. German patent DE-C1-3,410,888 disclosed a dual sensor intrusion detection system. The system comprises a first, infrared, intrusion detector and a second intrusion detector of the active ultrasonic type that irradiates the volume of space monitored thereby. The respective detection signals from the first and second intrusion detectors are applied, after some processing of the signals, to a circuit which generates an alarm signal if both detection signals are simultaneously present. To reduce power consumption from a battery power source, the first intrusion detector is continuously operable and the second intrusion detector together with other circuitry is normally turned-off. The first intrusion detector includes a timer which is triggered by detection of an intrusion to activate the second intrusion detector and other circuitry so that dual sensor operation is provided over a period. If no alarm signal is given within the timer period the system reverts to the first intrusion detector only being operative. The present invention enables us to provide a dual sensor intrusion detection apparatus in which a first, passive intrusion detector is continuously operable and the activation of the second intrusion detector is controlled in a manner that reduces power consumption where the apparatus is monitoring a zone of high traffic so that many people are being detected by the first intrusion detector. In the apparatus to be described, a subsequent detection by the first detector occurring less than a predetermined time - say 2 minutes - after the previous defection is inhibited from activating the second intrusion detector. Furthermore this period is restarted with each subsequent detection. As will be explained below a reduction of power consumption can thereby be achieved during periods when there is high traffic in the zone monitored. Broadly stated the present invention provides an intrusion detection apparatus comprising first and second intrusion detectors operable to provide first and second detection signals respectively upon detection of an intrusion into a volume of space monitored by each intrusion detector, said first intrusion detector being a continuously operable passive detector and said second intrusion detector being normally inoperable to generate said second detection signal, means responsive to said first and second signals to generate an alarm signal upon an intrusion being detected by both said first and second intrusion detectors, and a timer responsive to each first detection signal to generate a control signal of predetermined duration that is applied to said second intrusion detector to render it operable to generate said second detection signals for said predetermined duration, characterized by: a further timer triggered in response to each of said first detection signals to provide a further control signal of a predetermined duration that is applied to said second intrusion detector to inhibit same from being rendered operable to generate said second control signal in response to a subsequent first detection signal being generated during the duration of said further control signal. In an embodiment of the invention described below, the second intrusion detector is an active detector of the microwave type. That is, it irradiates the volume of space being monitored with microwaves. The second detector is not turned off but is normally maintained in an idle state where it is ready for detection on being activated but it is not capable of generating the second detection signals. On activation to an active state detection signals may be generated. The power consumption in the idle state is less than that in the active state. In an embodiment of the invention described hereinafter, the second intrusion detector - of the active microwave type - is operable at a pulse rate which is too low for intrusion detection but which maintains its circuit elements properly biased in readiness for operation when it is switched to the active state. The active state is one in which the intrusion detector is pulsed at a higher pulse rate to enable intrusion detection to be performed. This higher pulse rate requires a great power consumption than the active state but is only applicable following an intrusion detection by the first intrusion detector. In an another aspect of the invention there is provided an intrusion detection apparatus comprising first and second intrusion detectors operable to provide first and second detection signals respectively upon detection of an intrusion into a volume of space monitored by each intrusion detector, said first intrusion detector being a continuously operable passive detector and said second intrusion detector being an active intrusion detector for irradiating said volume of space with microwaves and being normally inoperable to generate said second detection signals, means responsive to said first and second detection signals to generate an alarm signal upon an intrusion being detected by both said first and second intrusion detectors, and a timer responsive to each first detection signal to generate a control signal of predetermined duration that is applied to said second intrusion detector to render it operable to generate said second detection signals for said predetermined duration, characterised in that: said second intrusion detector is switchable between an idle state in which it is normally operable and an active state into which it is put in response to said control signal; said idle state being one in which said second intrusion detector is operable at a pulse rate which is too low for intrusion detection but which maintains its circuit elements properly biased in readiness for operation when it is switched to the active state; and said active state being one in which said intrusion detector is pulsed at a higher pulse rate to enable intrusion detection to be performed and in which the power consumption is higher than in said idle state. The present invention and its practice will now further described with reference to the accompanying drawings, in which: Fig. 1 is a schematic block circuit diagram of an intrusion detection system of the present invention. Fig. 2 is a detailed block diagram of the passive infrared detector portion of the intrusion detection system shown in Fig. 1. Fig. 3 (A-C) is a detailed circuit diagram of the microwave detector portion of the intrusion detection system shown in Fig. 1. Fig. 4 is a flow chart diagram showing the operation of the intrusion detection system of Fig. 1. Referring to Fig. 1, there is shown a schematic block diagram of the intrusion detection system 10 of the present invention. The system 10 comprises a passive infrared detector portion 4, which generates a first signal in response to the detection of an intruder in a volume of space at which the passive infrared detector 4 is directed. The system 10 also comprises a microwave sensor detector portion 6. The microwave sensor detector portion 6 emits microwave radiation and is directed at the same volume of space at which the passive infrared detector portion 4 is directed. In the event an intruder in the volume of space at which the passive infrared portion 4 and the microwave radiation portion 6 are directed is detected by both the passive infrared radiation detector 4 and the microwave radiation detector 6, then an alarm signal 50 is generated by the system 10 of the present invention. The passive infrared radiation detector portion 4 is well known in the art and can be found embodied in the passive infrared radiation sensor detector portion of C&K Systems, Inc.'s Dual Tech Intrusion Device. A typical passive infrared radiation portion comprises (as shown in Fig. 2) a dual element pyroelectric infrared sensor 12 which generates a first signal in response to the detection of an intruder crossing a plurality of zones in the volume of space at which the portion 4 is directed. The first signal is then amplified by a first amplifier 14 and is passed through a band pass amplifier 16. The first signal is then processed by the processing circuit 18 which comprises a negative threshold detector circuit 20A and a positive threshold detector circuit 20B. The first signal is applied simultaneously to both the negative threshold circuit 20A and the positive threshold circuit 20B. From the negative threshold detector circuit 20A, the signal is supplied to an invertor 22A and a diode 24A and is passed to a three-second pulse stretcher 26A. From the positive threshold detector circuit 20B, the signal is supplied to a diode 24B and a three-second pulse stretcher circuit 26B. The output of the three-second pulse stretcher circuit 26A and the three-second pulse stretcher circuit 26B are supplied to an AND gate 28. The signal from the AND gate 28 is then supplied to an eight-second pulse stretcher circuit 30 and the output signal 32 thereof is the output of the passive infrared radiation sensor portion of the system 10 of the present invention. The first output signal 32 is supplied to a timer circuit 34 as well as to an alarm signal processing circuit 36. The timer circuit 34 generates a control signal 35 in response to the first signal 32 supplied thereto. The control signal 35 is supplied to the mode select control circuit 38 of the microwave detection sensor 6. The microwave detector portion 6 of the system 10 comprises a microwave generator/sensor 44, which emits microwave radiation and is directed at the same volume of space at which the infrared radiation sensor 12 is directed. A typical microwave generator/sensor 44 is a Gunn diode and a Schottky diode. The microwave is generated by a microwave driver circuit 42, which is under the direction and control of the mode select control circuit 38. The microwave reflected from the volume of space is then collected by the same microwave sensor/generator 44 and is supplied to the microwave detect circuitry 40. The microwave detect circuitry 40 is also under the control of the mode select control circuit 38. From the microwave detect circuit 40, a second signal 46 is then supplied to the alarm processing circuit 36. If a first signal 32 and a second signal 46 are both supplied to the alarm signal processing 36 within a predefined period of time, then an alarm signal 50 is produced by the alarm signal processing circuit 36. The alarm signal 50 is the alarm output of the system 10 of the present invention. Referring to Figs. 3A, 3B and 3C, there is shown in greater detail the circuit for the timer circuit 34, the alarm signal processing circuit 36, the mode select control circuit 38, the microwave detect circuit 40, the microwave drive circuit 42, and the microwave transceiver 44. The circuit diagrams shown in Figs. 3B and 3C are connected at the points A, B, C, D, and E. The circuit diagram shown in Fig. 3B is connected to the circuit diagram shown in Fig. 3A at the point F. The circuit diagram shown in Fig. 3C is connected to the circuit diagram shown in Fig. 3A at the point G. Figure 3A shows the timer circuit and the alarm signal processing. U4 is a "one shot". When the PIR detect circuitry detects the presence of an intruder, pin 3 of the PIR connector goes low. This falling edge activates this first one shot. The output of this one shot stays low for five seconds. This is the time that the microwave transceiver drive is activated. Activation of this timer also begins the activation of the sample and hold, microwave amplifier circuitry, and the alarm signal processing. This is accomplished through signal F. If detection occurs by the microwave sensing circuitry, the return signal is present on line G. U7 forms the AND gate of the PIR and the microwave signals. The output of U7 is then used to relay the alarm information to the control panel. At the end of detection by the microwave detector, the other half of U4 (designated U9) then inhibits reactivation of the microwave detector for two minutes. If there are additional PIR detections during this time, the two minute period is restarted. In this fashion, in high traffic areas, the power consumption of the unit is kept to a minimum. Figure 3B shows the mode select logic and the microwave drive circuitry. When the microwave drive circuitry is activated, the voltage at F goes low. This turns on Q5. This then changes the feedback capacitance in the oscillator formed by R18 and C15 and C16. The two oscillating frequencies are the fundamental difference in the idle state and the active state of the microwave detection circuitry. In the idle state, the band width of the detect circuitry is not high enough to detect the presence of an intruder, however, all of the capacitors in the microwave amplifier and signal processing circuitry are charged up allowing for rapid detection when necessary. U7 then forms two pulses from the basic oscillator frequency. The first pulse is for the microwave drive transistor, Q6. The second pulse C is slightly delayed. This is used for the sample and hold transistor. The actual return doppler shifted signal is present on line B. Figure 3C also includes some of the select control circuitry, the sample and hold, the microwave amplifier, and the microwave alarm signal processing. Signal line E in this figure does two things, first it changes the sample and hold capacitance to one that will respond to the frequencies of interest, and secondly it takes the microwave amplifier out of the low current mode and into a more responsive mode (that also draws more current). The first two stages of U8 and the associated circuitry is the microwave amplifier, and CR12, CR13, and C30, C32 and the last stage of U8 along with associated resistors make up for the alarm signal processing. When an alarm is declared, the signal at G goes low (to a logic zero). The operation of the system 10 of the present invention can be understood by referring to the flow chart shown in Fig. 4. Initially, the microwave sensor/generator 44 is placed in an idle state. By an idle state, it is meant that the microwave sensor/generator 44 is supplied pulses at the rate of approximately 1 Hz. At approximately 1 Hz, the microwave sensor/generator 44 is unable to detect any intruder in the volume of space at which the microwave portion 6 is directed. However, at 1 Hz, all of the circuit elements in the microwave portion 6 are properly biased. Thus, although the microwave portion 6 is unable to detect the presence of an intruder, the microwave portion 6 is nevertheless in a state whereby it can be switched on rapidly. In the absence of the microwave portion 6 being in an idle state, i.e., the microwave portion 6 were in a completely off state, it would take approximately two minutes for the microwave portion 6 to reach steady state whereby it is able to detect an intruder, from an off state. This is due to the capacitance and resistance in the system 10 and the frequency involved. The figure of 1 Hz rate is chosen because an intruder walking at 1 mile per hour will have the frequency rate of approximately 30 Hz. Thus, for the microwave portion 6 to detect an intruder operating at 1 Hz, the intruder must be moving less than 1/30th mile per hour (or is moving slower than .6 inch per second). In normal operation, i.e., active state when the microwave portion 6 is on, the microwave circuit portion is pulsed at the rate of 2 KHz. Initially, the infrared radiation sensor portion 4 of the system 10 is on. However, since the infrared radiation sensor portion of the system 10 is a passive device, very little power is consumed by this device. Thus, initially, the only power consumed by the system 10 is the power to the electronics to process the infrared radiation detected and to maintain the microwave sensor portion 6 in the idle state. The infrared radiation sensing device 12 senses the presence of an intruder in the volume of space to which it is directed. This is shown as block 102. If an intruder is not detected, then system 10 reverts to the initial state 100. If an intruder is detected in that volume of space, the first signal 32 is produced. The first signal 32, as previously stated, is provided to the timer circuit 34. The timer circuit 34 determines if the signal 32 is received within a preset period of time from when the last first signal 32 was received. If the current first signal 32 is received within the timing period of when the last first signal 32 is received, then the timing circuit is reset as shown by block 106 and the system 10 returns to the initial state 100. On the other hand, if the timing circuit 34 has timed out, i.e., the present first signal 32 is received after the preset period of time from the last first signal 32 received, then the timing circuit 34 issues the control signal 35 to the mode select control circuit 38. The control signal 35 is sent to the mode select control circuit 38 to switch the microwave drive circuit 42 from an idle state to an active state and to turn the microwave detect circuit 40 from off to on. As previously discussed, by an active state it is meant that the microwave drive circuit 42 issues pulse signals to the microwave transceiver 44 at the rate of approximately 2 KHz. Once the microwave drive circuit 42 is placed in an active state, and the microwave detect circuit 40 is placed in the on state, the microwave detect circuit 40 attempts to determine if an intruder is detected by the transceiver 44. If an intruder has not been detected by the microwave transceiver 44, then no second signal 46 is generated by the microwave detect circuit 40. In that event, the system 10 can reset the timer 34 and is returned to the idle state 100. On the other hand, if an intruder is detected by the microwave transceiver 44 and the second signal 46 is generated by the microwave detect circuit 40, then the alarm signal processing circuit 36 generates the alarm signal 50. There are many advantages to the intrusion detection system 10. First and foremost is that power consumption is extremely low. Secondly, the immunity to false alarm of the dual sensor detection system is preserved. It should be noted that only idle power is supplied to the microwave intrusion sensor portion 6 of the detection system 10. The microwave intrusion sensor portion 6 is activated only when a passive infrared radiation detection portion 4 has detected an intruder and only when the detection of the intruder is after a preset period of time. The benefit of the latter will be explained hereinafter. Thus, the intrusion system 10 can be used with a battery source and can be placed in any remote or inaccessible location. Furthermore, since power consumption is extremely low, on the order of 100 microamp, a rechargeable battery with a small solar collector can be used. The solar collector can be used to recharge the battery in the daytime in ambient light. The recharging of the rechargeable battery virtually assures the detection system 10 having an indefinite lifetime. Alternatively, a nine volt battery would have an operational functional capability for lasting almost a year. The timing circuit 34 of the system 10 provides the following function. During the daytime, for example, if the system 10 is directed in a normally people intensive place, such as a retail store, the microwave portion 6 of system 10 should not be switched to activated state at all. Thus, the timing circuit 34 provides that if one first signal 32 is detected followed by a second first signal 32 detected within the preset time period of the timing circuit 34, then the microwave sensor portion 6 is not activated. This would indicate that there are many people milling about or being detected by the system 10 and is presumably normal activity and should not cause an alarm state. This further saves battery power drain. Although the intrusion detection system 10 of the present invention has been described with respect to a passive infrared radiation detection sensor to trigger a microwave intrusion detection sensor, the invention can be practiced with any combination of dual sensors - provided that the first sensor, the sensor to initially detect the presence of an intruder is of the passive type. A passive intrusion detection sensor can be an infrared radiation detect sensor, such as that shown in Fig. 2 or it can also be an acoustic detection sensor which generates an output signal in response to an increase in acoustic energy in a volume of space. The second detection sensor can be an active or a passive detection sensor. An active detection sensor can be the microwave radiation detection sensor shown in Fig. 1, or it can be a photoelectric sensor, or even an ultrasonic detection sensor. The invention can be practiced by using any passive detection sensor to detect an intruder to generate an output signal, which turns on a second detection sensor. Further, the second detection sensor need not have an idle state and an active state - if a microwave detector is not used. If the active detection sensor is, for example, a photoelectric sensor, the sensor has an on state and an off state. This greatly reduces power and is immune to false alarms due to dual sensing nature of the system.

Claims: An intrusion detection apparatus comprising first (4) and second (6) intrusion detectors operable to provide first (32) and second detection signals (46) respectively upon detection of an intrusion into a volume of space monitored by each intrusion detector, said first intrusion detector (4) being a continuously operable passive detector and said second intrusion detector (6) being normally inoperable to generate said second detection signals, means (36) responsive to said first (32) and second (46) detection signals to generate an alarm signal (50) upon an intrusion being detected by both said first (4) and second (6) intrusion detectors, and a timer (34) responsive to each first detection signal to generate a control signal (35) of predetermined duration that is applied to said second intrusion detector (6) to render it operable to generate said second detection signals (46) for said predetermined duration, characterized by: a further timer (U9) triggered in response to each of said first detection signals (32) to provide a further control signal (U9-10) of a predetermined duration that is applied to said second intrusion detector (6) to inhibit same from being rendered operable to generate said second control signal (46) in response to a subsequent first detection signal (32) being generated during the duration of said further control signal (U9-10). An intrusion detection apparatus as claimed in Claim 1 in which the input of said further timer is connected to the output of the first-mentioned timer. An intrusion detection apparatus as claimed in Claim 1 or 2 in which said second intrusion detector is normally maintained in an idle state in which its circuitry is powered to maintain it in readiness and in which it is activatable by a first-mentioned control signal to enter an active state in which it is operable to generate said second intrusion detection signals, and wherein the power consumption of said second intrusion detector in the idle state is less than that in the active state. An intrusion detection apparatus as claimed in Claim 1 or 2 in which said second intrusion detector is normally maintained in an off state and is activatable by a first mentioned control signal to enter an on state in which power consumption is greater than in the off state. An intrusion detection apparatus as claimed in Claim 3 in which said second intrusion detector is an active intrusion detector operable to irradiate the volume of space monitored thereby with microwaves, and the microwave radiation is pulsed at a frequency rate which is lower in said idle state than in said active state. An intrusion detection apparatus as claimed in Claim 1, 2 or 4 in which said second intrusion detector is an active intrusion detector operable to irradiate the volume of space monitored thereby. An intrusion detection apparatus as claimed in Claim 6 in which said second intrusion detector is of the microwave, ultrasonic or photo-electric type. An intrusion detection apparatus as claimed in any preceding claim in which said first intrusion detector is a passive infrared detector. An intrusion detection apparatus as claimed in claim 1 or 2 in which said second intrusion detector is an active intrusion detector for irradiating said volume of space with microwaves and is switchable between an idle state in which it is normally operable and an active state into which it is put in response to said control signal; said idle state being one in which said second intrusion detector is operable at a pulse rate which is too low for intrusion detection but which maintains its circuit elements properly biased in readiness for operation when it is switched to the active state; and said active state being one in which said intrusion detector is pulsed at a higher pulse rate to enable intrusion detection to be performed. An intrusion detector apparatus as claimed in claim 9 in which said pulse rate in the idle state is 1Hz and in the active state is 2kHz. An intrusion detection apparatus comprising first (4) and second (6) intrusion detectors operable to provide first (32) and second detection signals (46) respectively upon detection of an intrusion into a volume of space monitored by each intrusion detector, said first intrusion detector (4) being a continuously operable passive detector and said second intrusion detector (6) being an active intrusion detector for irradiating said volume of space with microwaves and being normally inoperable to generate said second detection signals, means (36) responsive to said first (32) and second (46) detection signals to generate an alarm signal (50) upon an intrusion being detected by both said first (4) and second (6) intrusion detectors, and a timer (34) responsive to each first detection signal to generate a control signal (35) of predetermined duration that is applied to said second intrusion detector (6) to render it operable to generate said second detection signals (46) for said predetermined duration, characterised in that: said second intrusion detector is switchable between an idle state in which it is normally operable and an active state into which it is put in response to said control signal; said idle state being one in which said second intrusion detector is operable at a pulse rate which is too low for intrusion detection but which maintains its circuit elements properly biased in readiness for operation when it is switched to the active state; and said active state being one in which said intrusion detector is pulsed at a higher pulse rate to enable intrusion detection to be performed and in which the power consumption is higher than in said idle state.

Inventors: JOHNSON RICHARD ADAM, JOHNSON, RICHARD ADAM
Applicants: C & K SYSTEMS INC, C & K SYSTEMS, INC.
Application year: 1989
Main classification: G08B13
IPC codes: G08B13, G08B29