GB2137789A - Intrusion detector installation - Google Patents

Abstract

To minimise the risk of spurious alarms, the installation comprises both a two wire detector including the wire pair 4,4 (or multiples thereof) which detects perturbations in a field, and a physical fence sensor including microphonic cable 42 which provides a signal when stressed. The detector and sensor are so coupled electronically that an alarm signal is given only when signals are generated by them both within a predetermined period, whilst only an alert signal is given if one only thereof is actuated or they are actuated successively outside said predetermined period. <IMAGE>

Description

SPECIFICATION Intrusion detector installation This invention relates to intrusion detector installations, particularly, but not exclusively, for outdoor use.

It is already known to provide various types of intrusion detector comprising two wires, for example 100 metres length, extending parallel to one another over an area to be protected. One of these wires is a field wire which is energised by an oscillator which operates at a high frequency, of the order of kilohertz, and the other wire is a sensor wire which senses the field of the field wire. Any intrusion into the area by a human being has the effect of disturbing the field generated by the field wire, and this disturbance is detected by appropriate circuitry and is used to give an alarm indication. Such a detector will be referred to herein as "a two-wire intrusion detector".

If it is reliably to detect a skilled human intruder, such an intrusion detector tends to be prone to producing spurious alarms from other causes, such as the approach of animals or birds, or the occurrence of inclement weather, It is an object of the present invention to overcome problems of this nature.

With this object in view, the present invention provides an intrusion detector installation which comprises a two-wire intrusion detector as above discussed, in combination with a physical fence sensor arrangement adapted to produce a signal upon occurrence of physical contact with a fence, the intrusion detector and the fence sensor being so coupled that successive intruder detection by each of them within a predetermined period of the order of a number of seconds produces an alarm whilst actuation thereof either separately, or in succession outside said predetermined period, produces only an alert signal.

The physical fence sensor arrangement may comprise, for instance, one or more vibration switches fixed in appropriate locations upon a supporting fence structure. More conveniently, such arrangement may comprise one or more lengths of so-called "microphonic" cable (i.e. a cable incorporating a dielectric which is sensitive to stress) incorporated into a fence structure so that it will be stressed by anyone scaling the fence structure, thereby causing modification in a current passing through or field generated by the cable.

The two-wire intrusion detector in the installation of the invention may be so set up as to provide for directional sensing of the approach of an intrusive body, thereby further to minimise the incidence of spurious alarms.

To achieve this latter effect, the two wire intrusion detector may have its wires arranged so as to form part of a respective balanced detection circuit, these circuits being balanced relative to one another and coupled to an oscillator, detecting means for detecting any imbalance in said detection circuits as a results of movement of an intrusive body in hte vicinity of said wires, and thereupon to provide an alarm, said detecting means including a transformer in which two coils form primary windings, each said coil forming part of a respective one of said detection circuits so that a current is induced in a secondary winding of the transformer only when the currents flowing through the primary windings are unequal, and directional sensing means for deriving directional information concerning the movement of the intrusive body in the vicinity of said pair of wires.

This arrangement helps to discriminate between intrusion andspurious effects or acceptable approaches to said pair of wires.

The directional sensing means may comprise a synchronous demodulator operative to provide a positive or a negative output according to which wire the intrusive body is nearest, and positive and negative output detectors whose respective outputs are arranged to provide an activating output only when a sequence of outputs of predetermined polarity, corresponding to intrusion, is detected.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic end view illustrating a practical embodiment of the intruder detector installation of the invention; Fig. 2 is a block diagram illustrating a practical circuitry arrangement suitable for the intrusion detector installation of Fig. 1; and Fig. 3 is a view similar to Fig. 1 but illustrating another embodiment of the intrusion detector installation of the invention.

Referring firstly to Fig. 1, an intruder detector installation conforming to the invention is illustrated very diagrammatically therein and comprises a two-wire intruder detector comprising a parallel pair of sensor wires 4 carried by any suitable support structure such as is illustrated at 41 so as to extend side by side along the region to be protected. Disposed side-by-side with the sensor wires is a perimeter fence which is represented for convenience only as posts 40 and whose fencing wires, mesh or the like is not illustrated in the figure. What is shown in relation to the fence, however, is a physical fence sensor which is illustrated as microphonic cable 42 which may be incorporated into the fence in any suitable way such that it will be subjected to mechanical stress if an intruder should attempt to scale the fence.Thus, it may be present as longitudinal runs along the fence and to which mesh of the like protective material is anchored.

Referring now to Fig. 2 which illustrates, very diagrammatically the circuitry of the intrusion detector installation of Fig. 1, an oscillator 1 is connected between ground and a moving contact of a potentiometer 2. The opposing ends of the potentiometer 2 are, in turn, connected to two coils 7 and 8 forming primary windings of a balanced transformer 3, opposing ends of said coils 7 and 8 being connected respectively to the sensor wires 4 of the parallel pair of such wires 4.

A small fixed capacitor 5 is connected to one of the sensor wires 4 and a variable capacitor 6, of value comparable with the capacitor 5, is connected to the other sensor wire 4. Both of the capacitors 5, 6 are connected to ground. The two coils 7 and 8 of the transformer 3 are so wound that, by adjusting the potentiometer 2 and the variable capacitor 6, the current in the coils 7 and 8 is balanced and a secondary winding 9 of the transformer 3 will normally have no oscillatory signal induced into it. The secondary winding 9 is connected to succeeding electronics signal processing equipment by way of an amplifier 10 whose gain may be controlled by a variable resistor 11 whose setting determines the overall sensitivity of the two-wire intruder detector.

Should an intruder now approach either of the sensor wires 4, then due to the fact that, upon his approach, he will come closer to one of the sensor wires 4 than he will to the other of the sensor wires 4, an imbalance of the sensor configuration will result, and a voltage will be induced into the secondary winding 9 in proportion to the extent of imbalance introduced into the system.

The signal induced in the secondary winding 9 is amplified by the amplifier 10 whose output feeds a synchronous demodulator 12 to which is also fed a reference voltage from the oscillator 1.

The output from the synchronous demodulator 12 will be either a positive-going voltage or a negative-going voltage depending upon which sensor wire 4 has been encroached upon first.

The output from the synchronous demodulator 12 is fed to a band pass filter 1 3 which rejects signals construed as being too slow or too fast to be indicative of intrusion by a human being.

The output from the band pass filter 1 3 is now fed to a positive-going signal detector 14 and a negative-going signal detector 15, and, providing the signal excursion exceeds the threshold of the corresponding detector, an output will be given from one or the other of the signal detectors 14 and 1 5. The output from the one or the other of the signal detectors 14 and 1 5 is used to actuate an audible or visual two wire alerting section 1 6 of an alarm which has been indicated diagrammatically at 1 9 and can indicate three intrusion conditions, that is to say a two-wire detector alert signal at 16, a fence sensor alert signal at 19 and a full alarm signal at 18.

The signal outputs from the pair of sensor wires 4 may, of course, be combined in a number of ways. As any intrusion of significance will be in a given direction, for example from right to left, the outputs from the signal detectors 14 and 1 5 will occur in sequence. Whether the positive-going signal occurs before the negative-going signal is dependent upon which sensor wire 4 is connected to which transformer primary winding 7, 8, and matters may be readily arranged to provide the desired sequence. Assuming that connections have been made so that the positive-going signal excursion occurs first, then the arrangement may be such that the output from the signal detector 14 is applied to a flip flop 20, thus enabling the latter.

When the negative going signal excursion takes place the output from the signal detector 1 5 is applied to a trigger input of the flip flop 20, causing it to change stage and generate an output to operate an alert relay 21 which keeps the two wire alert signal 1 6 energised to indicate that a signal intrusion detection appears to have taken place.

A timer 22, activated by an enable output of the signal detector 14, resets the flip flop 20 after a predetermined time, said time being variable. This is to ensure that should the flip flop 20 be enabled for whatever reason, unless it is triggered within the set time, it will be reset to its original stage.

With such an arrangement a non-intrusive approach, e.g. a bird landing on one of the sensor wires 4 or a legitimate approach and retreat of a person to the outer sensor wire 4 can generate an enable signal at the flip flop 20, yet no alarm would be given and the flip flop 16 would be reset at a specific time thereafter.

Referring now to the microphonic cable 42 incorporated in the fence 40, as shown in the lower part of Fig. 2, this is connected to a signal processor 43 the function of which is sense stresses occurring in the cable 42 which latter may be, for instance, of the kind incorporating a stress-sensitive dielectric sheathing or concentric layer so as to create or vary an electrical signal generated by or sensed by the cable upon being subjected to stress. The signal from the cable 42, indicative of stressing of the cable, upon receipt by the processor 43 is effective to energise the respective alert signal 1 7 to provide an audible warning that an intrusion at the fence has been sensed, a respective alert relay 44 serving to hold the signal 17 "on".

A connection is provided between the relays 44 and 21 and the processor 43 senses energisation thereof and includes means for sensing the time delay occurring between the respective energisations. It will readily be understood that with an effective intrusion, successive effective signals will normally be generated at the two-wire detector and at the physical fence sensor constituted by the cable 42. Accordingly, the processor 43 may be set to energise the alarm signal 18 and provide not just an alert signal, but a full alarm, upon the two alert signals 16 and 17 being generated within a predetermined period (e.g. a small number of seconds). If there is only one alert signal, or if the two alert signals occur outside the preset period of time, they manifest themselves only as the alert signals, and do not provide the full alarm. Naturally, the processor, which is indicated only diagrammatically at 43, may incorporate manually-adjustable means for setting or varying the present period of time, e.g.

according to the practical requirements of the particular installation.

Although the intrusion detector installation as described above uses only one pair of sensor wires 4 any one of a variety of sensor wire configurations may be used. Preferably at least two pairs of parallel sensor wires are employed, each pair of wires being connected to respective signal processors as described above. A typical sensor wire configuration having three pairs of sensor wires 4 is illustrated in Fig. 3.

As an example, the upper pair of wires 4 may be disposed at a height of two metres above the ground, and the lower pair of wires 4 may be disposed at a height of one metre above the ground so that an intruder, whether crawling, jumping or vaulting, must make a close approach to one of the pairs of wires.

With this arrangement of sensor wire configurations and signal processing, in combination with the physical fence sensor, a very effective intrusion detector can be achieved capable of detecting most attempts at intrusion.

A further variation can be obtained by AND gating the outputs of the flip flops 1 6 of each of the pairs of wires 4 by respective AND gates which, in turn, feeds respective alert relays 21. In this mode of operation, an alert will only be given when two pairs of the sensor wires 4 detect a undirectional intrusion. Such a mode of operation virtually eliminates unwanted activations of the two-wire intrusion detectors due to animals or ground-foraging birds yet, by appropriate adjustment of the sensitivity of the signal processors, and maintains the two-wire intrusion detector's ability to detect reliably human intrusions.

Other variations are possible, for instance, in relation to the physical fence sensor. Thus, for example, the fence 40 may incorporate, instead of or in addition to the microphonic cable, other physical stress sensing devices, such as vibration switches, compression switches and/or transducers, and the like.

Claims (7)

1. An intrusion detector fnstalfatfon comprising a two-wire intrusion detector in combination with a physical fence sensor arrangement adapted to produce a signal upon occurrence of physical contact with a fence, the intrusion detector and the fence sensor being so coupled that successive intruder detection by each of them within a predetermined period of the order of a number of seconds produces an alarm whilst actuation thereof either separately, or in succession outside said predetermined period, produces only an alert signal.

2. An intrusion detector installation as claimed in claim 1 in which the physical fence sensor arrangement comprises one or more vibration switches fixed in appropriate locations upon a supporting fence structure.

3. An intrusion detector installation as claimed in claim 1 in which the physical fence sensor arrangement comprises one or more lengths of microphonic cable incorporating a dielectric which is sensitive to stress incorporated into a fence structure so that it will be stressed by anyone scaling the fence structure.

4. An intrusion detector installation as claimed in claim 1,2 or 3 in which the two-wire intrusion detector provides for directional sensing of the approach of an intrusive body.

5. An intrusion detector installation as claimed in claim 4 in which the two wire intrusion detector has its two wires arranged so as each to form part of a respective balanced detection circuit, these circuits being balanced relative to one another and coupled to an oscillator, detecting means for detecting any imbalance in said detection circuits as a result of movement of an intrusive body in the vicinity of said wires, and thereupon to provide an alarm, said detecting means including a transformer in which two coils form primary windings, each said coil forming part of a respective one of said detection circuits so that a current is induced in a secondary winding of the transformer only when the currents flowing through the primary windings are unequal, and directional sensing means for deriving directional information concerning the movement of the intrusive body in the vicinity of said pair of wires.

6. An intrusion detector installation as claimed in claim 5 in which the directional sensing means comprises a synchronous demodulator operative to provide a positive or a negative output according to which wire the intrusion body is nearest, and positive and negative output detectors whose respective outputs are arranged to provide an activating output only when a sequence of outputs of predetermined polarity, corresponding to intrusion, is detected.

7. An intrusion detector installation substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2 or in Fig. 3 of the accompanying drawings.