GB2221990A

GB2221990A - Vibration-sensitive intrusion detection system

Info

Publication number: GB2221990A
Application number: GB8819550A
Authority: GB
Inventors: Roger William Leslie Hoyle
Original assignee: Shorrock Ltd
Current assignee: Shorrock Ltd
Priority date: 1988-08-17
Filing date: 1988-08-17
Publication date: 1990-02-21
Anticipated expiration: 2008-08-17
Also published as: GB2221990B; GB8819550D0

Abstract

An intrusion detection system for detecting attempts to penetrate a vertical barrier such as a fence or wall includes a strain sensitive cable 10 which generates an alarm signal in response to movement of the barrier. The alarm signal is amplified 12 and filtered 14 and passed to a detector circuit 16 when it is rectified and converted to an envelope signal characteristic of the time taken for the alarm signal to rise from zero to its peak value (the rise time). The envelope signal is differentiated 26 to remove slow rise time signals and then supplied to a threshold detector 32. Signals which exceed the pre-set threshold value are counted 34 as to their frequency in a pre-set time period determined by a timer 36. Repeated signals in excess of the threshold value lead to activation of an alarm circuit 38 as being indicative of an intrusion. Any long term continuous signal generated by the cable 10 is detected by a second threshold detector 44 which will also activate the alarm circuit 38 if the signal persists for longer than a pre-determined period measured by a timer 46. <IMAGE>

Description

INTRUSION DETECTION SYSTEM This invention relates to an intrusion detection system especially for detecting attempts to penetrate a fence or wall.

It is known to attach an insulated electrical cable to a fence or wall structure to be protected such that when the cable is flexed, or a pressure is applied thereto, an alarm signal is generated. In a typical system the signal generated by the cable, which is usually an electric, piezoelectric or triboelectric type, is amplified, filtered and then passed to a threshold detector. An alarm signal which exceeds the threshold value causes a pulse which starts a timing circuit which would usually have a timing period of from 5 seconds to 1 minute.

The number of pulses, generated by individual alarm signals exceeding the threshold value, are counted by a pulse counter and if a pre-set count is exceeded within the timing period an alarm is activated. The timing circuit and the pulse counter would then be reset. If the timing circuit reaches the end of its set timing period, and the pre-set pulse count is not exceeded, then the pulse counter is re-set and no alarm is generated.

The counting of pulses and the requirement that the alarm signal exceeds a threshold value is intended to prevent the alarm being activated by flexing of the cable caused by either random "one-off" events or by relatively small amounts of movement of the cable such as could be caused by animals or the passage of a motor vehicle close to the fence. However, a single, substantially continuous alarm signal generated over a long time period would not cause activation of the alarm in which case it would be possible for an intruder to climb over the fence or wall without alerting anyone to that fact. In order to spot such signals, and activate the alarm where appropriate, it is usual to provide a further alarm signal processing circuit where the output pulse from the threshold detector is timed as to its duration.The measured duration of the pulse is then compared with a pre-set time value and the alarm activated where the measured duration exceeds the preset value. Such a system is decribed in U.S. Patent No.

4365239.

Whilst such systems are successful in preventing alarm activation as a result of non-intrusion signals generated by, for example, small animals, the signals caused by environmental changes or conditions such as wind, rain etc. cannot be differentiated from those caused by a genuine intruder. In fact under conditions of heavy rain and/or high winds such systems as previously described would be in a state of almost continuous alarm activation.

It is an object of the present invention therefore to provide a signal processing circuit for a fence mounted intruder detector system which can differentiate between alarm signals caused by an intruder and those caused by environmental conditions.

With this object in view the present invention provides an intrusion detection system comprising a strain sensitive cable to be disturbed by an intuder and serving to generate an alarm signal in response to movement of the barrier, characterised in that the system also comprises signal processing means to which the alarm signal is supplied and operative to produce a pulsed output related to the time taken for the alarm signal to traverse a pre-determined excursion, the pulsed output being supplied to a counter wherein the number of pulses occurring over a pre-set time period are counted, compared to a pre-determined count value and an alarm activated where the pulses counted exceeds the pre-determined value.

A convenient excursion for the production of the pulsed output is from zero to a peak value, say of the first alarm signal, which will usually be the largest peak of any disturbance with a rise time indicative of an intrusion.

Preferably the system also includes a threshold detector to which the pulsed output is supplied, said detector having stored therein a pre-deterincd threshold value to which the pulsed output is compared, the detector producing an output signal for the counter if the pulsed output exceeds the threshold value.

The signal processing means preferably includes a detecting circuit operative to rectify the alarm signal generated by the cable and to output a signal proportional to the alarm signal envelope. This envelope signal is advantageously supplied via an amplifier to a differentiating circuit which modifies the envelope signal to produce the pulsed output signal which is then supplied to the counter.

Advantageously the system also includes a second processing circuit including a timer and comparator which compares the alarm signals amplitude with a preset threshold value and activates an alarm when the amplitude exceeds the threshold value for a predetermined time period measured by the timer.

Preferably the second processing circuit is supplied with the rectified envelope signal to compare with the threshold value.

The invention will be described further with reference to the accompanying drawing in which the single figure is a block diagram illustrating a preferred embodiment of the intruder detector system according to the invention.

In the single figure a strain sensitive cable 10 is mounted on or in a vertical barrier structure such as a wall or fence (not shown) which is to be protected.

The cable 10 may be an electric, piezoelectric or triboelectric type and is such that flexing of the cable 10 caused by vibrations in the wall or fence to which it is attached generates a small, but detectable, electric current. These types of cables are generally known as microphonic cables and are usually of co-axial construction.

It will thus be appreciated that any movement of the barrier such as might be caused by physical contact with the fence of any intruder or an animal, cutting of the individual links of a fence or movement by environmental conditions such as wind or rain, will result in an alarm signal being generated in the cable 10. The alarm signal is amplified by amplifier 12 and then filtered (typically in the frequency range of 300 to 3,000 Hz) by a band pass filter 14. The output from the filter 14 is then rectified in a detector circuit 16 comprising a diode 18 and connected in parallel, a capacitor 20 and a resistor 22.

The detector circuit 16 has a short charging time constant of, for example, microseconds and a relatively long discharging time constant of, for example, milliseconds. The effect of these two time constants is to output from the detector circuit 16 an envelope signal which is characteristic of the rise time of the alarm signal, that is the time taken for the alarm signal to rise from zero to its peak value.

The envelope signal is then amplified by a first envelope amplifier 24 and then supplied to a differentiator 26 which comprises a second capacitor 28 and a second resistor 30. The differentiated signal is then passed to a threshold detector 32 which is provided with a pre-determined threshold value. It will be appreciated that an alarm signal generated in the cable 10 will actually be in the form of a series of signals having a frequency equal to the frequency of vibration of the cable 10 itself. The differentiated signal output from the differentiator is characterised solely by the maximum rise time recorded in any one series of alarm signals generated in the cable 10. If the differentiated signal value exceeds the threshold value then the threshold detector 32 generates an alarm pulse.

The alarm pulse is supplied to a counter 34 and timer 36 circuit such that the number of such pulses in a pre-determined time period can be counted and compared to a pre-set number. If the number of alarm pulses exceeds the pre-set number in the time period an alarm circuit 38 is activated. Once the alarm 38 has been activated the counter 34 and timer 36 are re-set to their respective starting values. The counter 34 and timer 36 are also similarly re-set if there is an insufficient number of alarm pulses counted in the timing period.

It will be understood that, in use, alarm signals generated by the cable 10 occur as a series of signals according to the frequency of vibration of the cable 10 itself. When the alarm signal has been converted to the envelope signal by the detector circuit 16 the signal consists of a number of pulses, each pulse corresponding to one of alarm signal series. Furthermore, the envelope signal reflects proportionately the rate at which the alarm signal rises from zero to its peak value. Where there is a long series of alarm signals relatively close together each one having a peak value only slightly larger than the previous signal the envelope output is proportional only to the change in the peak value from one signal to the next and is hence lower in its value than an output based on a rapidly rising alarm signal.This relationship between the alarm signals and the signal in the processing circuitry is accentuated by the differentiator 26 such that the differentiator output practically only consists of pulses caused by short, sharp alarm signals.

It will be apparent therefore that the alarm circuit 38 will only be activated as a consequence of these short sharp alarm signals exceeding the threshold value stored in the threshold detector 32 and exceeding the pre-set count stored in the counter 34. Such alarm signals are only caused by an intruder either cutting or otherwise breaking the fabric of the barrier to which the cable 10 is attached or an intruder attempting to climb thereover. Signals caused by other sources such as animals or adverse weather conditions are effectively filtered out either by the threshold detector 32 or the detector circuit 16 and the differentiator 26 respectively.

It is possible to defeat such a system by applying a mechanically generated signal to the barrier and slowly increasing its amplitude until it is substantially equal to a signal which would be caused by an intruder. The slow increase of this signal would ensure that its rise time is insufficient to trigger the threshold detector 32. Once this condition has been achieved it is possible to either cut or climb the barrier without causing an alarm as the signals caused by such an intrusion would be masked by the continuous signal and the differentiator output would be of insufficient amplitude to exceed the threshold value of the threshold detector 32.

To prevent this occurring an additonal processiriy circuit 40 is included in the system which is supplied with the envelope signal output from the detector circuit 16. The envelope signal is first amplified by a second envelope amplifier 42 and then compared with a second threshold value held in a second threshold detector 44. If the amplified rectified envelope output exceeds the second threshold value a second timer 46 is started. If the envelope output is maintained in excess of the second threshold value for longer then a pre-set timing period (usually of the order of seconds) the timer 46 will generate a signal to activate the alarm circuit 38. It will be appreciated that the continuous signal necessary to "mask" a genuine intrusion signal is much greater than that normally caused by an intruder or any environmental factors which may be applied to the barrier.

Variations are possible within the scope of the invention for example the additional processing circuit 40 may be omitted and the counter 34 and timer 36 could be replaced by an up/down counter connected to a clock oscillator signal or any other suitable method of determining the number of pulses in a given period of time.

It is also possible that the cable itself may constitute the barrier i.e. as a trip wire or similar.

The excursion of the alarm signal used to determine the pulsed output need not be the rise time of the first alarm signal from zero but may be calculated on a peak to peak basis or use the second or subsequent alarm signals generated.

Claims

1. An intrusion detection system comprising a strain sensitive cable to be disturbed by an intruder and serving as to generate an alarm signal in response to movement of the barrier, characterised in that the system also comprises signal processing means to which the alarm signal is supplied and operative to produce a pulsed output related to the time taken for the alarm signal to traverse a pre-determined excursion, the pulsed output being supplied to a counter wherein the number of pulses occurring over a pre-set time period are counted, compared to a pre-determined count value and an alarm activated when the counted pulses exceeds the pre-determined value.

2. An intrusion detection system as claimed in claim 1 further including a threshold detector to which the pulsed output is supplied, said detector having stored therein a pre-determined threshold value to which the pulsed output is compared, the detector producing an output signal for the counter if the pulsed output exceeds the threshold value.

3. An intrusion detection system as claimed in claim 1 or claim 2 in which the signal processing means includes a detector circuit operative to rectify the alarm signal generated by the cable and to output an envelope signal proportional to the alarm signal's envelope.

4. An intrusion detection system as claimed in claim 3 in which the signal processing means further includes a differentiating circuit which is supplied with the envelope signal from the detector circuit via an amplifier and is operative to modify the envelope signal to produce the pulsed output signal which is supplied to the counter.

5. An intrusion detection system as claimed in any preceding claim which further comprises a second processing circuit including a comparator which compares the alarm signal's amplitude with a pre-set threshold value and activates an alarm when the amplitude exceeds the threshold value for a pre-determined time period measured by a timer.

6. An intrusion detection system as hereinbefore described with reference to and as illustrated in the accompanying drawing.

6. An intrusion detection system as claimed in claims 3 and 5 in which the envelope signal produced by the detector circuit is supplied to the second processing circuit for comparison with the threshold value.

7. An intrusion detection system as hereinbefore described with reference to and as illustrated in the accompanying drawing.

Amendments to the claims have been filed as follows 1. An intrusion detection system comprising a strain sensitive cable to be disturbed by an intruder and serving as to generate an alarm signal in response to movement of the cable, characterised in that the alarm signal is supplied to a detector circuit operative to rectify the alarm signal generated by the cable and to output an envelope signal proportional to the alarm signal's envelope, the envelope signal being supplied to signal processing means operative to produce a pulsed output related to the time taken for the alarm signal to traverse a pre-determined excursion, the pulsed output being supplied to a counter wherein the number of pulses occurring over a pre-set time period are counted, compared to a pre-determined count value and an alarm activated when the counted pulses exceeds the predetermined value.

3. An intrusion detection system as claimed in claims 1 or 2 in which the signal processing means further includes a differentiating circuit which is supplied with the envelope signal from the detector circuit via an amplifier and is operative to modify the envelope signal to produce the pulsed output signal which is supplied to the counter.

4. An intrusion detection system as claimed in any preceding claim which further comprises a second processing circuit including a comparator which compares the alarm signal's amplitude with a pre-set threshold value and activates an alarm when the amplitude exceeds the threshold value for a pre-determined time period measured by a timer.

5. An intrusion detection system as claimed in claims 3 and 5 in which the envelope signal produced by the detector circuit is supplied to the second processing circuit for comparison with the threshold value.