GB2050025A

GB2050025A - Intrusion Detector System

Info

Publication number: GB2050025A
Application number: GB8013380A
Authority: GB
Original assignee: Shorrock Developments Ltd
Current assignee: Shorrock Developments Ltd
Priority date: 1979-05-09
Filing date: 1980-04-23
Publication date: 1980-12-31
Also published as: GB2050025B

Abstract

To provide intrusion detection which is particularly versatile, providing good detection capability in adverse weather conditions (such as rain, hail, snow or sandstorm) and over undulating ground, use is made of a combination of a parallel wire intrusion detector device comprising wires 23, 24 carried by supports 25, of which one is shown, so as to be disposed at a different level from a transmitter/receiver detector arrangement comprising transmitter 26 on the illustrated support 25 to a receiver (not shown) on another of the supports 25. The outputs of the two systems may be AND gated to provide for occurrence of an alarm only upon intrusion detection by both systems. A further parallel-wire device may be incorporated with its wires at a level above (Fig.3) or below (Fig. 4) the transmitter/receiver detector arrangement 26. <IMAGE>

Description

SPECIFICATION Intrusion Detector System The invention relates to improvements in external perimeter intrusion detection systems.

There are several intruder detectors currently in use as external perimeter intrusion sensors, and one of the more common is a high frequency transmitter/receiver combination, such as described in U.K. Patent specification 1409658.

A transmitter is set up facing its associated receiver typically over a distance of 100 metres.

The receiver monitors the received signal level, and should an intrusion take place in the area between the transmitter and the receiver, the received signal level will change, which change can be readily detected and used to initiate an alarm.

This type of intrusion sensor can have excellent detection capability particularly against slow moving intrusions and can be operated in rain, hail, snow, sandstorms etc. without giving any weather-induced nuisance alarms.

However, for effective detection down to ground level it is necessary that the ground between transmitter and receiver be substantially level and free from any substantial undulations, as such undulations can provide cover for a skilled intruder who crawls prone aiong a hollow.

A second type of perimeter intrusion detector working on entirely different principles consists of a balanced bridge configuration in which two sensor wires typically 100 metres in length are supported on suitably insulated posts horizontal to each other and parallel to the ground, and connected to the bifilar wound primary windings of a transformer. The wires are energised by an oscillator at a frequency of typically tens of kilohertz, and the configuration balanced by means of a potentiometer and a variable capacitor.

Because the transformer is bifilar wound, when the circuit is balanced there will be no voltage induced in the secondary winding of the transformer. However, when a person encroaches on either wire, a degree of imbalance will be induced into the system according to his proximity to the sensor wire. If the level detector threshold connected to the secondary of the transformer is exceeded, an alarm is given indicating an intrusion.

Such a two wire sensor has the advantage of being able to follow undulations in the ground, but its detection sensitivity is not as good as that of the transmitter/receiver system previously mentioned, its sensitivity being ultimately limited by the effects of inclement weather, particularly rain and wet snow. Further, its maximum detection sensitivity is immediately adjacent to and below the wires and therefore if good ground cover is to be obtained, the sensor wires must be mounted near the ground.

The present invention provides an intrusion detector system combining the two different types of intrusion sensor to produce an enhanced sensor with surprisingly good overall detection characteristics and avoiding the disadvantages of the different types of intrusion detector when used alone.

According to the present invention there is provided an intrusion detector system comprising a parallel wire intrusion detector arrangement and a transmitter/receiver intrusion detector arrangement, said arrangements being disposed in substantially parallel planes one above the other.

Preferably a transmitter of the transmitter/receiver arrangement is adapted to emit microwave electromagnetic radiation in a direction substantially parallel to the wires of the parallel wire detector and towards a receiver. The transmitter/receiver arrangement may be disposed close to the ground to provide detection capability in respect of crawling intruders, whilst the parallel wire arrangement may be disposed at a higher level to provide a further region of detection capability.

Alternatively the wires of the parallel wire arrangement may be disposed closest to the ground with the transmitter/receiver arrangement at a higher level. This latter arrangement is most suitable in cases where the terrain is undulating.

Whatever particular arrangement is employed a distinct advantage over known arrangements may be obtained in respect of the unavoidable compromise between the need for sensitivity and the need to avoid spurious alarms by arranging that an alarm is only given if both detector arrangements are activated. Since the two arrangements work on different physical principles, a false activation is likely to be produced only in one such arrangement. One method of arranging that an alarm is only given if both detector arrangements are activated is to connect the respective outputs of the arrangements to the inputs of an AND gate, the output of the AND gate being operative to activate an alarm.

The invention will now be described further, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a simplified circuit diagram illustrating a parallel wire detector arrangement; Fig. 2 is a schematic illustration of a first embodiment of an intrusion detector system in accordance with the present invention; Fig. 3 is a schematic illustration of a second embodiment of an intrusion detector system in accordance with the present invention; and Fig. 4 is a schematic illustration of a third embodiment of an intrusion detector in accordance with the present invention.

Referring firstly to Fig. 1, parallel wires 10, 11 of a parallel wire intrusion detector arrangement are connected to opposite sides of a balanced bridge oscillatory circuit 12. The circuit 12 comprises, in the arms of the bridge, a capacitor 13, a variable capacitor 14, and two bifilar primary windings 15, 1 6 of a transformer 1 7. An oscillator 1 8 is connected between the midpoint of capacitors 1 3 and 1 4 and a variable resistor 19 itself disposed between windings 1 5 and 1 6. In practice the wires 10, 11 are held above the ground and the circuit 12 is adjusted to a state of balance when the currents flowing through windings 15 and 1 6 are equal and consequently no voltage is induced in a secondary winding 20 of the transformer 1 7. In the event of any imbalance in the circuit 12, produced for example by the approach of an intruder to the wires 10, 11, a voltage will be induced in winding 20 and detected by means of a voltage level detector 21.

Referring now to Fig. 2 a system in accordance with the invention comprises a parallel wire detector arrangement corresponding to that which has been described above with reference to Fig. 1 with its parallel wires 23, 24 (corresponding to the wires 1 0, 11 described above) supported above the ground by means of a pair of supports 25 one of which only is shown. At a lower level on the support 25 is attached a transmitter 26 of a microwave transmitter/receiver detector arrangement. A receiver (not shown) of the microwave detector arrangement is attached on the other support of the pair of supports so as to receive radiation emitted by the transmitter 26, and detection means is adapted to detect any variation in the received radiation, such variation being produced, for example, by the presence of an intruder between the transmitter 26 and the receiver.The arrangement of Fig. 2 is suitable for use over flat terrain where there is no scope for a crawling intruder to avoid the beam of microwave radiations. Typically the transmitter 26 and its associated receiver would be located at a height of one metre above ground whilst the parallel wires 23, 24 (which in effect provide an additional volume of detection capability would be at a height of two metres above the ground.

A variation of the arrangement of Fig. 2 is shown in Fig. 3 in which two pairs of parallel wires 27, 28 and 29, 30 are provided in combination with a microwave detector arrangement. This gives still further height to the effective region of protection given by the system.

In Fig. 4 is illustrated an arrangement suitable for use over undulating or uneven terrain. Here parallel wires 31, 32 of a parallel wire detector arrangement are located closest to the ground thereby ensuring detection of crawling intruders.

A microwave transmitter/receiver detector arrangement is located above the wires 31,32 and parallel wires 33, 34 of an optional further parallel wire detector arrangement may be located above the microwave detector arrangement.

In all the above described arrangements in accordance with the present invention the outputs of the various detector arrangements may be AND gated to require detection from each arrangement before actuation of an alarm. It is thereby possible to reduce the incidence of spurious alarms or to provide increased sensitivity without the penalty of high likelihood of such spurious alarms.

Claims

1. An intrusion detector system comprising a parallel wire intrusion detector arrangement and a transmitter/receiver intrusion detector arrangement, said arrangements being disposed in substantially parallel planes one above the other.

2. An intrusion detector system as claimed in claim 1 wherein a transmitter of the transmitter/receiver arrangement is adapted to emit microwave electro-magnetic radiation in a direction substantially parallel to the wires of the parallel wire detector and towards a receiver.

3. An intrusion detector system as claimed in claim 2 wherein the transmitter/receiver arrangement is disposed close to the ground to provide detection capability in respect of crawling intruders, whilst the parallel wire arrangement is disposed at a higher level to provide a further region of detection capability.

4. An intrusion detector system as claimed in claim 1 or 2 which comprises a further parallel wire intrusion detector arrangement.

5. An intrusion detector system as claimed in claim 4 wherein both of the parallel wire detector arrangements are disposed above the transmitter/receiver intrusion detector arrangement.

6. An intrusion detector system as claimed in claim 4 wherein the transmitter/receiver intrusion detector arrangement is disposed at a height between the two parallel wire intrusion detector arrangements.

7. An intrusion detector system as claimed in any preceding claim wherein the detector arrangements are AND gated to require detection from each said arrangement before actuation of an alarm.

8. An intrusion detector system substantially as hereinbefore described with reference to and as illustrated in Fig. 2, in Fig. 3 or in Fig. 4 of the accompanying drawing.