Abstract:
An intrusion detector for supervising a region including a sensor which views a plurality of fields-of-view of the region and provides an output responsive to motion of an infrared radiation source between the fields-of-view, a first filter which provides a first filtered output based on a first, predetermined, detection pulse frequency range of the sensor output, a second filter which provides a second filtered output based on a second, predetermined, detection pulse frequency range of the sensor output and processing circuitry which receives the first and second filtered outputs and detects, in either or both of the filtered outputs, a sequence of detection pulses indicating an intrusion condition.

Description:
FIELD OF THE INVENTION 
     The present invention relates to intrusion detectors in general and, more particularly, to signal processing in passive infrared detectors. 
     SOFTWARE APPENDIX 
     Submitted herewith is a software appendix. 
     BACKGROUND OF THE INVENTION 
     Passive infrared detectors are widely used in intruder, e.g. burglar alarm systems. The infrared detectors of such systems generally respond to radiation in the far infrared range, preferably 7-14 micrometers, as typically irradiated from an average person. A typical passive infrared detector includes a pyroelectric sensor adapted to provide an electric output in response to changes in radiation at the desired wavelength range. The electric output is then amplified by a signal amplifier and processed by signal detection circuitry. 
     To detect movement of a person in a predefined area, typically a room, passive infrared detectors are provided with a discontinuously segmented optical element, e.g. a segmented lens or mirror having at least one optical segment, wherein each segment of the lens or mirror collects radiation from a discrete, narrow, field-of-view such that the fields-of-view of adjacent segments do not overlap. Thus, the pyroelectric sensor receives external radiation through a segmented field-of-view, including a plurality of discrete detection zones separated by a plurality of discrete no-detection zones. The system detects movement of a person from a given zone to an adjacent zone by detecting, for example, a relatively sharp drop or a relatively sharp rise in the electric output of the pyroelectric sensor. 
     It is appreciated that abrupt changes in ambient temperature may result in abrupt changes in the output of the pyroelectric sensor and, thus, false alarms may occasionally be detected. To avoid this problem, most intruder alarm systems use a dual-element pyroelectric sensor having two, adjacent, pyroelectric sensor elements. The two elements are arranged vis-a-vis the segmented optics such that the two elements have interlaced, non-overlapping, fields-of-view. The two elements are electrically configured to provide opposite polarity electrical outputs, such that the net signal received from the sensor is substantially zero when both sensor elements simultaneously detect radiation from the same source. The net signal is greater than zero when the radiation is detected by the two elements non-simultaneously, for example a moving source will generally be detected first by one of the elements and then by the other element. 
     Intrusion detectors using dual-element sensors are generally more reliable and have a better detection resolution than corresponding single element sensors. However, even dual-element sensor systems occasionally generate false alarms due to uncontrolled effects of noise including, inter alia, internal system noise, radio frequency (RF) and other external noise, or random noise known as &#34;spikes&#34;. These uncontrolled effects are generally overcome by increasing detection thresholds or by using pulse-counting techniques known in the art, thereby decreasing the detection sensitivity. 
     As long as there are no intruders in the supervised area, the amplified sensor output consists of a substantially constant, typically zero, signal which is subject only to the above mentioned effects. However, in an intrusion situation, the amplified sensor output includes a series of pulses responsive to movement of the intruder across a series of adjacent detection zones. Since pulses in the amplified output may also result from occasional noise, genuine intrusions are typically verified by detecting a series of pulses, typically at least three pulses, to avoid false alarms. 
     In existing systems, detection of intruder motion is generally dependent on two factors, namely, the distance of the intruder from the detector and the angular velocity of the intruder relative to the detector. The distance of the intruder generally controls the magnitude of the received IR energy and thus of the amplified sensor signals, whereby a close intruder will normally generate a stronger signal than a far intruder. The angular velocity of the intruder, i.e. the rate at which the intruder moves from one detection zone to the next, generally controls the frequency of pulses in the amplified sensor output. Thus, the frequency of detection pulses generated by a &#34;fast sweeping&#34; intruder is higher than the frequency of detection pulses generated by a &#34;slow sweeping&#34; intruder. It should be noted that the &#34;sweeping&#34; rate, i.e. the angular velocity, of a given intruder is a function of the linear velocity of the intruder, the direction of motion of the intruder and the distance of the intruder from the intrusion detector. 
     For optimal coverage of most intrusion situations, wide range amplifiers are generally used to amplify the signals produced by the pyroelectric sensor. Such amplifiers respond to a wide range of detection pulse frequencies. However, at very high angular velocities, typically more than approximately 10 degrees per second, wide range amplifiers do not provide sufficient separation between consecutive detection pulses whereby there are overlaps between adjacent edges of consecutive pulses. Thus, detection signals corresponding to fast sweeping intruders typically include super-peak structure, each such structure consisting of series of local peaks superposed on a single, wide, base pulse. In a fast sweeping intrusion situation, where the amplified signal exceeds the detection threshold across entire super-peak structures, the structures are misidentified as single detection pulses and intrusions are not detected. 
     Another problem of detectors using wide frequency range amplifiers is their poor amplification at extreme, i.e. very high or very low, frequencies. It should be noted that distant intruders generally produce weak signals having a low detection pulse frequency and, therefore, such intruders are often ignored by detectors using wide range amplifiers. 
     Passive infrared intrusion detectors are described, for example, in U.S. Pat. No. 4,709,153, U.S. Pat. No. 4,752,768, U.S. Pat. 4,242,669, U.S. Pat. No. 4,982,094, U.S. Pat. No. 5,084,696, U.S. Pat. No. 5,077,549 and U.S. Pat. No. 4,764,755. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a passive infrared intrusion detector capable of detecting intruders having a high angular velocity, i.e., a high sweeping rate, relative to the detector. It is a further object of the present invention to provide a passive infrared detector capable of detecting intruders, particularly distant intruders, having a low angular velocity relative to the detector. 
     According to one aspect of the present invention, the detector identifies multi-peak pulses, also referred to herein as super-pulses, in an amplified output of a pyroelectric sensor. Each such super-pulse includes a series of narrow local peaks superposed on a wide base pulse. The detector preferably uses local detection thresholds to discriminate between the local peaks in the super-pulses. The local detection thresholds are preferably dynamically adjusted according to the time intervals between consecutive peaks. This dynamic threshold adjustment improves the ability of the detector to discriminate between local peaks in the super-pulses. 
     According to another aspect of the present invention, the intrusion detector includes at least two amplifiers adapted for amplifying at least two, respective, detection pulse frequency bands of the pyroelectric sensor output. Preferably, in accordance with this aspect of the present invention, the detector includes a high frequency range amplifier and a low frequency range amplifier. The high frequency range amplifier responds to sensor signals of fast sweeping intruders, for which a finer separation between pulses is required. The low frequency range amplifier provides enhanced amplification of sensor signals of slow sweeping and/or distant intruders. 
     There is thus provided, in accordance with a preferred embodiment of the invention an intrusion detector for supervising a region comprising: 
     a sensor which views a plurality of fields-of-view of the region and provides an output responsive to motion of an infrared radiation source between the fields-of-view; 
     a first filter which provides a first filtered output based on a first, predetermined, detection pulse frequency range of the sensor output; 
     a second filter which provides a second filtered output based on a second, predetermined, detection pulse frequency range of the sensor output; and 
     processing circuitry which receives the first and second filtered outputs and detects, in either or both of the filtered outputs, a sequence of detection pulses indicating an intrusion condition. 
     Preferably, the processing circuitry comprises a first comparator which compares the first filtered output to at least one first threshold and a second comparator which compares the second filtered output to at least one second threshold. 
     Preferably, the first comparator comprises a first window comparator and the at least one first threshold comprises first upper and lower thresholds and wherein said second comparator comprises a second window comparator and the at least one second threshold comprises second upper and lower thresholds. 
     Preferably, the first and second thresholds are dynamically adjusted based on ambient conditions. 
     Preferably, said processing circuitry comprises a digital processor. 
     In a preferred embodiment of the invention the first frequency range comprises a high frequency range and the second frequency range comprises a low frequency range. Preferably, the first frequency range is between about 3 Hz to about 10 Hz. Preferably, the second frequency range is between about 0.1 to about 3 Hz, preferably between 0.1 and 2 Hz. 
     Preferably, the detector includes an alarm circuit which provides a sensible indication when said processing circuitry detects said sequence of detection pulses. 
     In a preferred embodiment of the invention the first and second filters comprise respective first and second amplifiers, such that the first and second filtered signals are amplified signals. 
     There is further provided in accordance with a preferred embodiment of the invention, a method of supervising a region, comprising: 
     viewing a plurality of fields-of-view of the region; 
     sensing incident infrared radiation from the region and providing a sensor signal responsive to motion of an infrared radiation source between the fields-of-view; 
     detecting a series of extremum values in the sensor signal; and 
     detecting motion of the infrared radiation source based on time and amplitude differences between at least some of the extremum values in said series. 
     In a preferred embodiment of the invention detecting motion of the infrared radiation source comprises thresholding a given extremum value of the amplified sensor signal using a threshold dependent on the time interval between the given extremum value and the last previous extremum value. 
     The method preferably comprises dynamically adjusting said threshold in accordance with ambient conditions. 
     There is further provided, in accordance with a preferred embodiment of the invention, an intrusion detector for supervising a region comprising: 
     a sensor which views a plurality of fields-of-view of the region and provides an output responsive to motion of an infrared radiation source between the fields-of-view; 
     a processor which detects a series of extremum values in the sensor output and determines motion of the infrared radiation source based on time and amplitude differences between at least some of the extremum values in said series. 
     Preferably, the processor detects motion of the infrared radiation source by thresholding a given extremum value of the amplified sensor signal using a threshold dependent on the time interval between the given extremum value and the last previous extremum value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from the following detailed description of preferred embodiments of the present invention, taken in conjunction with the following drawings in which: 
     FIG. 1A is a schematic, block diagram, illustration of intrusion detection circuitry in accordance with one preferred embodiment of the present invention; 
     FIG. 1B is a schematic, block diagram, illustration of intrusion detection circuitry incorporating digital processing in accordance with another preferred embodiment of the present invention; 
     FIGS. 2A and 2B schematically illustrate a low frequency component and a high frequency component, respectively, of a typical low frequency signal in the circuitry of FIGS. 1A or 1B; 
     FIGS. 3A and 3B schematically illustrate a low frequency component and a high frequency component, respectively, of a typical high frequency signal in the circuitry of FIGS. 1A or 1B; 
     FIGS. 4A and 4B schematically illustrate a flow chart of a preferred algorithm for the digital processing incorporated by the circuitry of FIG. 1B; 
     FIG. 5 is a block diagram of intrusion detection circuitry incorporating digital processing, in accordance with yet another preferred embodiment of the present invention; 
     FIG. 6 is a graph generally illustrating the responsivity of a typical pyroelectric sensor as a function of the frequency of detection pulse generated thereby; 
     FIGS. 7A and 7B are a schematic flow chart of a preferred algorithm for the digital processing incorporated by the circuitry of FIG. 5; and 
     FIGS. 8A and 8B are schematic illustrations of a &#34;normal&#34; detection pulse frequency signal and a high detection pulse frequency signal, respectively, which may be processed by the circuitry of FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1A which schematically illustrates intrusion detection circuitry 10 in accordance with one preferred embodiment of the present invention. Circuitry 10 is connected to a far infrared sensor 12, preferably a pyroelectric sensor, which produces an electric output in response to radiation in a far infrared wavelength range. Sensor 12 is preferably responsive to infrared radiation in a wavelength range of between approximately 7 micrometers and approximately 14 micrometers, which is a typical radiation range of the human body. Sensor 12 preferably views a plurality of fields-of-view of a supervised region, preferably through segmented optics (not shown in the drawings) such as a segmented Fresnel lens. As known in the art, the plurality of fields-of-view of sensor 12, also referred to herein as detection zones are preferably discrete, i.e., non-overlapping zones. The electric output produced by sensor 12, which preferably includes a dual element sensor, comprises a pulse for each time a far infrared source exits one of the detection zones or enters an adjacent zone. 
     It is appreciated that the frequency at which detection pulses are generated by sensor 12 is dependent on the angular velocity, i.e. the sweeping rate, of the infrared source being detected. In a preferred embodiment of the present invention, as shown in FIG. 1, the output signal produced by sensor 12 is amplified by a low frequency range amplifier 14 or a high frequency range amplifier 16, which are both connected to the output of sensor 12. 
     When sensor 12 generates a low frequency signal, for example a signal responsive to a distant, slow moving, intruder, the signal is efficiently amplified by low frequency range amplifier 14 to produce an amplified signal component V L . The gain of amplifier 14 at low detection pulse frequencies, typically frequencies of between 0.1 and 1 pulses per second, is higher than that of wide range amplifiers, ensuring enhanced amplification of the typically weak signals generated by distant intruders. 
     When sensor 12 generates a high frequency signal, for example a signal responsive to a near, fast moving, intruder, the signal is efficiently amplified by high frequency range amplifier 16 to produce an amplified signal component V H . At high detection pulse frequencies, typically between 2 Hz and 10 Hz, amplifier 16 has a higher detection pulse resolution, i.e. a better separation between adjacent detection pulses, than that of wide range amplifiers. This enables detection of fast sweeping intruders which are generally not detected by conventional intrusion detectors. 
     Reference is now made also to FIGS. 2A and 2B, which schematically illustrate amplified signal components V L  and V H , respectively, generated in response to a typical low frequency signal from sensor 12. Reference is also made to FIGS. 3A and 3B which schematically illustrate amplified signal components V L  and V H , respectively, of a typical high frequency signal from sensor 12. 
     The output of amplifier 14, V L , is received by a first far-infrared-signal window comparator 18 and the output of amplifier 16, V H , is received by a second far-infrared-signal window comparator 19. The outputs of window comparators 18 and 19, which are responsive to changes in the outputs of amplifiers 14 and 16, respectively, are provided as inputs to a main controller 20. Comparators 18 and 19 use detection &#34;windows&#34;, ±U L  and ±U H , to evaluate the changes in outputs V L  and V H , respectively. The comparison between signals V L  and V H  and windows ±U L  and ±U H , respectively, is shown schematically in FIGS. 2A-3B. The detection windows used by comparators 18 and 19 are preferably continuously updated by controller 20 using feedback signals U L  (t) and U H  (t), respectively. Window update signals U L  (t) and U H  (t) are preferably generated by a window update circuit in controller 20 based on inputs responsive to changes in ambient conditions, particularly changes in temperature, which may affect the output of sensor 12. 
     In particular, as the background temperature increases, the difference in radiation between an intruder and the background decreases. This requires lower values of U L  and U H  to insure detection of intruders. However, such lower values also make the system more vulnerable to false alarms. Thus, the threshold levels are adjusted to take account of the required sensitivity required to assure detection of intruders, giving a minimum sensitivity as required by the expected difference between the background and the potential intruder. 
     When an intruder crosses the segmented field-of-view of the intrusion detector, the output of amplifier 14 and/or 16 changes abruptly and, consequently, window comparator 18 and/or 19 generates an intrusion detection signal to controller 20. An intrusion alarm circuit in controller 20, activated in response to the intrusion detection signal, provides an intrusion alarm signal which operates an audible or other alarm indication near the detector or at a remote monitoring station. 
     Additional, optional, features of the intrusion detector of the present invention are described in US. Pat. Nos. 5,237,300 and 4,604,524 and in Israel Patent Application 110,800, filed Aug. 28, 1994, which was filed in the PCT as application number PCT/EP95/01501, which are assigned to the assignee of the present application, the disclosures of all of which are incorporated herein by reference. For example, devices for detecting attempts to tamper with the intrusion detector may be used in conjunction with the present invention. The execution of such additional features is preferably also controlled by main controller 20. 
     Reference is now made to FIG. 1B which schematically illustrates intrusion detection circuitry 25 in accordance with another preferred embodiment of the present invention. Circuitry 25 is connected to far infrared sensor 12, as in the embodiment of FIG. 1A, which produces an electric output in response to radiation in a far infrared wavelength range typical of the human body. As described above, sensor 12 views a plurality of fields-of-view of the supervised region, preferably through a segmented Fresnel lens. Thus, as described above, the electric output produced by sensor 12 includes a pulse for each time a far infrared source exits one of the fields-of-view and enters an adjacent field-of-view. 
     As in the embodiment of FIG. 1A, the output signal produced by sensor 12 in FIG. 1B is amplified either by low frequency range amplifier 14 or by high frequency range amplifier 16, which are both connected to the output of sensor 12. When sensor 12 generates a low frequency signal, for example a signal responsive to a distant, slow moving, intruder, the signal is amplified by amplifier 14 to produce amplified signal V L . When sensor generates a high frequency signal, for example a signal responsive to a near, fast moving, intruder, the signal is amplified by high frequency range amplifier 16 to produce an amplified signal V H , 
     The output of amplifier 14, V L , is received by a first analog-to-digital (A/D) converter 22 and the output of amplifier 16, V H , is received by a second A/D converter 24. The outputs of A/D converters 22 and 24, which correspond to the outputs of amplifiers 14 and 16, respectively, are provided as inputs to a signal processor 26, which preferably includes a microprocessor. Processor 26 generates an intrusion detection signal to a controller 28. An intrusion alarm circuit of controller 28, activated in response to the intrusion detection signal, provides an intrusion alarm output which operates an audible alarm or some other indication, near the detector or at a remote monitoring station. A preferred intrusion detection algorithm to be carried-out by processor 26 will now be described with reference to the schematic flow chart illustrated in FIGS. 4A and 4B. 
     In a preferred embodiment of the present invention, the algorithm carried out by processor 26 begins by initial setting or resetting of the following parameters: 
     N L  --the number of detection pulses detected in low frequency component V L  ; 
     N H  --the number of detection pulses detected in high frequency component V H  ; 
     T L  (ref)--reference time for pulses detected in low frequency component V L  ; and 
     T H  (ref)--reference time for pulses detected in high frequency component V H . 
     Once the initial parameter values are set, processor 26 proceeds to set window thresholds ±U L  and ±U H , which are preferably determined in accordance with ambient conditions such as temperature, as described above with reference to comparators 18 and 19 in the embodiment of FIG. 1A. Once the thresholds are set, processor 26 compares the digitized and amplified signal components V L  and V H  to window thresholds ±U L  and ±U H , respectively. When |V L |&gt;U L , processor 26 determines the time, T L , of a potential detection pulse in signal V L . Similarly, when |V H  |&gt;U H , processor 26 determines the time, T H , of a potential detection pulse in signal V H . 
     If the time interval between the pulse detection time, T L  or T H , and the respective reference time, T L  (ref) or T H  (ref), is within a time range Tmin L  or Tmin H  and Tmax L  or Tmax H , the respective detection pulse count, N L  or N H , is increased by one. If the time interval, T L  -T L  (ref) or T H  -T H  (ref), is shorter than its respective minimum time interval, processor 26 proceeds to search for the next detection pulse. If time interval T L  -T L  (ref) is longer than Tmax L , the low frequency pulse count, N L , remains unchanged and processor 26 proceeds to evaluate the high frequency pulse count N H . If time interval T H  -T H  (ref) is longer than Tmax H , pulse count N H  and reference time T H  (ref) are reset to zero and processor 26 proceeds to search for the next high frequency pulse. 
     To avoid false alarms, a minimum number of high frequency detection pulses, N T , are required for generating an intrusion alarm signal. As illustrated in FIG. 3B, when a near, fast moving intruder crosses the segmented field-of-view of the intrusion detector, a number of high frequency detection pulses are generated, e.g. at times T 1  &#39;, T 2  &#39;, T 3  &#39; and T 4  &#39;. In some preferred embodiments of the present invention, the threshold number of detection pulses, N T , required for intrusion detection is set to a value between 2 and 4. As shown in FIG. 3A, only one low frequency detection pulse is expected to be generated in response to the fast moving intruder and, thus, only one low frequency detection pulse is preferably required for generating an intrusion alarm signal. Thus, in a preferred embodiment of the present invention, an intrusion alarm signal will be generated only when N H  &gt;N T  and N L  &gt;0, as illustrated in FIG. 4B. 
     As illustrated in FIG. 2A, when a far, slow moving intruder crosses the segmented field-of-view of the intrusion detector, a number of low frequency detection pulses are generated, e.g. at times T 1 , T 2 , T 3  and T 4 . As described above, the threshold number of detection pulses, N T , may be set, for example, to a value of between 2 and 4. As shown in FIG. 2B, no high frequency detection pulses are expected to be generated in response to a far, slow moving, intruder and, thus, no requirement is set on detection of high frequency pulses for generating an intrusion alarm signal. Thus, in a preferred embodiment of the invention, an intrusion alarm signal will be generated whenever N L  &gt;N T , as illustrated in FIG. 4B. 
     Reference is now made to FIG. 5 which schematically illustrates intrusion detection circuitry 30 in accordance with yet another, preferred embodiment of the present invention. The circuitry of FIG. 5 includes a far infrared signal amplifier 34, preferably a wide range amplifier as is known in the art, which amplifies the output of far infrared sensor 12. The output of amplifier 34 is received by a signal processor 38 whose operation is different from that of prior art signal processors. The output of signal processor 38, which is responsive to variations in the output of amplifier 34, as described in detail below, is connected to an input of a controller 40. When an intruder crosses the segmented field-of-view of sensor 12, the output of amplifier 34 changes and, based on analysis of the amplified signal, processor 38 generates an intrusion detection signal to controller 40. An intrusion alarm circuit of controller 40, activated in response to the intrusion detection signal, then provides an intrusion alarm signal which operates an audible alarm or some other indication, near the detector or at a remote monitoring station, as described above. 
     Reference is now made to FIG. 6 which schematically illustrates the responsivity of pyroelectric sensor 12, R, calculated as the electric power output of sensor 12 divided by the far infrared power illuminating the sensor, as a function of the frequency of detection pulses produced by the sensor. It should be noted that the responsivity of sensor 12 drops dramatically as the detection pulse frequency rises. This results in generation of low power, non-distinct peaks at high detection pulse frequencies, as described in detail below. 
     Reference is now made also to FIGS. 8A and 8B which schematically illustrate a &#34;normal&#34; detection pulse frequency signal and a high detection pulse frequency signal, respectively, both of which may be processed by the circuitry of FIG. 5. Note that the scales of FIGS. 8A and 8B are different with FIG. 8A showing about 10 seconds of a typical low frequency signal and FIG. 8B showing about one second of a typical high frequency signal. When the detection pulse frequency generated by sensor 12 is relatively high, typically more than about one pulse per second, the amplified detection pulses are not completely isolated, due to overlaps at the edges of adjacent pulses. Thus, at high detection pulse frequencies, the output of amplifier 34 includes a multi-peak pulse, hereinafter referred to as a super-pulse, which includes a series of narrow, local, detection peaks superposed on a single, wide, base pulse. An example of such a super-peak pulse is shown in FIG. 8B. Although each local peak in the super-pulse corresponds to a distinct sensor pulse, i.e. a distinct rise and drop in the output of sensor 12, wide range amplifier 34 cannot reproduce distinct detection pulses due to the inherent overlapping between consecutive peaks. Thus, typically, super-pulses generated by wide range amplifiers in response to detection pulse frequencies on the order of 2-4 Hz or higher, have the shape of a &#34;rising staircase&#34;, whereby each local detection peak corresponds to a step in the &#34;staircase&#34;. This is in contrast to the distinct detection pulses generated in response to slower moving intruders, as shown schematically in FIG. 8A. 
     It should be noted that the local peaks in the super-pulses are not detectable by the thresholding methods used in existing detectors. In prior art detectors, super-pulses are not distinguishable from isolated, single detection pulses because super-pulses and single pulses are both characterized by a single rise above a threshold and a single drop below the threshold. Since intrusion detection is preferably confirmed by detecting a number of consecutive pulses, to avoid false alarms, multi-peak super-pulses are generally ignored by existing detectors because they are mistaken to be single, isolated pulses. The present invention provides a method, preferably executed by hardware or software in signal processor 38, which overcomes this problem. A preferred digital processing algorithm for processor 38 will now be described with reference to the schematic flow chart illustrated in FIGS. 7A and 7B. 
     As shown at the top of FIG. 7A, the preferred algorithm begins by initial setting or resetting of the following parameters: 
     N P  --the number of detection pulses; 
     T--the time between consecutive detected intrusions. 
     Once the initial parameter values are set, processor 38 proceeds to calibrate signal amplitude thresholds V D  min and V T  (T D ), which are defined below, preferably in accordance with ambient conditions such as temperature, as described above with reference to preceding embodiments. After calibrating the signal amplitude thresholds, processor 38 searches for local extrema V i  in the digitized and amplified signal V(T). Processor 26 then determines the time, T D , which lapsed from the last previous local extremum, V i-1 , in signal V(T). 
     Processor 38 also determines the absolute value of the amplitude change, V D , between the last previous extremum, V i-1 , and the present extremum, V i . If |V D  |≦V D  min, extremum V i  is ignored and extremum V i-1  is maintained as reference for the next extremum found in the search. If |V D  |&gt;V D  min, processor 26 proceeds to evaluate the time interval between extrema V i  and V i-1 . If time interval T D  is longer than a minimum time interval, T D  min, and shorter than a maximum time interval, T D  max, processor 38 proceeds to perform a finer evaluation of difference signal V D , as described below. 
     It will be appreciated from FIG. 8B that the change in amplitude between consecutive extrema is generally dependent on the time interval between the consecutive extrema. Thus, in a preferred embodiment of the present invention, processor 38 determines a time-interval-dependent threshold, V T  (T D ), based on the predetermined relationship which generally exists between the time interval and amplitude change across consecutive extrema. The time-interval-dependent thresholds may be determined based on a look-up-table stored in a memory of processor 38. If V D  |≦V T , extremum V i  is ignored and extremum V i-1  is maintained as reference for the next extremum found in the search. However, if V D  &gt;V T , the number of detected pulses is raised by one, i.e. N=N+1. Then, the time interval between consecutive detection pulses, T(N P )-T(N P  -1), is compared to a predetermined threshold, Tmax. 
     If T(N P )-T(N P  -1)≦Tmax, processor 38 proceeds to determine whether a threshold number of detection pulses, N T , has been reached. If N P  is greater than threshold number N T , which is typically between 2 and 5, processor 38 generates an intrusion detection signal to controller 40 which operates an alarm circuit as described above. If T(N P )-T(N P  -1)&gt;Tmax, the number of detection pulses, N P  is reset to zero and the entire detection procedure described above is repeated to detect new pulses. 
     It should be appreciated that the present invention is not limited to what has been thus far described with reference to preferred embodiments of the invention. Rather, the scope of the present invention is limited only by the following claims: ##SPC1##