GB2063537A - Sensor for use with intrusion detection circuitry - Google Patents

Abstract

A sensor for use with intrusion detection circuitry includes a hollow electrically conductive tube 10 having a substantial length and at least two electrically conductive wires 12, 14 extending loosely inside the tube along the length thereof, each of the wires having alternating areas of electret sensitivity 16, 18 and insensitivity. The electric signals generated by vibration of the wires in the tube are subtracted by a processor and the resultant electric signal fed to an alarm. <IMAGE>

Description

SPECIFICATION Intrusion detection device Intrusion detection is becoming more important, both from a military standpoint and for commercial applications. Military bases and installations, factories, power stations, stores and even private homes are being protected by sophisticated electronic surveillance systems which are capable of monitoring the periphery of the system so as to warn against unwanted intrusion.

One type of intrusion detection which has become more important in this security-conscious world is that where a sensing element is concealed in the ground around the outer edges of an area to be protected. The sensing element, of whatever type, is adapted to monitor the movement of persons or objects approaching the outer edge, usually producing a signal indicating such an intrusion.

While intrusion detection of the aforementioned type is likely to giave a reasonable degree of assurance, additional protection is desireable if it can be achieved without greatly increasing costs and technical difficulties. Use of such a sensor which is capable of eliminating far field disturbances such as thunder, passing jet airplanes, and heavy traffic at a distance substantially remote from the protected area permits far greater efficiency since false alarms are substantially eliminated.

According to the invention there is provided an improved sensor for use with an intrusion detection circuitry, the sensor comprising a hollow electrically conductive tube having a substantial length; at least two electrically conductive wires extending loosely inside said tube along the length thereof, each of said wires having alternating areas of electret sensitivity and insensitivity; and means connecting each wire electrically to said tube, the sensor producing electric signals generated by vibration of said wires in said tube.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a partially cut-away schematic view of a preferred embodiment of sensor according to the present invention, and Figure 2 is a block diagram of intrusion detection circuitry employing the sensor of Fig. 1.

As shown in Fig. 1, the sensor includes a hollow electrically conductive tube 10 of substantial length which has contained within it loosely fitted electrically conductive wires 1 2 and 1 4. Each wire has alternating areas of electret sensitivity and insensitivity, wire 1 2 having sensitive areas 1 6 and wire 1 4 sensitive areas 1 8. As shown in Fig. 1, the wires are disposed so that the sensitive areas of one wire are opposite the insensitive areas of the other wire. Both wires 1 2 and 14 are connected electrically to the tube 10 through wires 1 9 and 20.Electric signals from wire 1 2 to tube 10 and from wire 14 to tube 10 can then be processed to determine whether or not there has been an intrusion.

As shown in Fig. 2, the sensor 10 provides signals in both wires 1 2 and 1 4 which are transmitted to processor 24. Typically, processor 24 is adapted to electronically subtract the signals provided by lines 1 2 and 14. The net signal from the processor 24 is transmitted through line 1 7 to alarm 26.

A modification of the present sensor, includes the use of more than two wires inside the tube 10. Typically the tube is at least 1 5 metres in length and preferably up to a hundred metres or longer. The lengths of the electret and insensitive areas on each of the wires range from one-half metre to two metres. It is preferred, of course, that the length of the electret areas is approximately the same as the length of insensitive areas on the wires.

The above described sensor can be installed in any length and in any location where seismic disturbances are likely to occur which would indicate the presence of an intruder. All that is required is that the sensor 10 be positioned so that seismic vibrations caused by compression of the surface of the ground in which the sensor is buried or vibrations transmitted through the sensor can be mea stirred. The signals produced by vibration of the wires in the tube are transmitted to the signal processor 24 and the on to the alarm 26. In order to eliminate far field effects, the signal processor 24 is adapted to subtract the signal of one wire from the signal of another wire.When the subtraction of the signal from one wire from the signal of another wire remains below a certain threshold value, which can be calibrated depending upon the particular installation, any far field effect which would tend to influence all of the various electret sensing areas of both of the wires 1 2 and 14, or at least a major portion of them, will be eliminated since the numerous sections will tend to cancel one another during the subtraction process. On the other hand, when intrusion takes place quite close to the sensor only one of the individual sensitive electret areas 1 6 or 1 8 will be substantially disturbed. Thus, the subtraction of one signal from the other wi!l produce a relatively high value signal in line 17, either positive or negative, above the threshold value, which causes operation of alarm 26 and permits detection of the intruder.

1. A sensor for use with an intrusion detection circuitry, the sensor comprising a hollow electrically conductive tube having a substantial length; at least two electrically con

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Intrusion detection device Intrusion detection is becoming more important, both from a military standpoint and for commercial applications. Military bases and installations, factories, power stations, stores and even private homes are being protected by sophisticated electronic surveillance systems which are capable of monitoring the periphery of the system so as to warn against unwanted intrusion. One type of intrusion detection which has become more important in this security-conscious world is that where a sensing element is concealed in the ground around the outer edges of an area to be protected. The sensing element, of whatever type, is adapted to monitor the movement of persons or objects approaching the outer edge, usually producing a signal indicating such an intrusion. While intrusion detection of the aforementioned type is likely to giave a reasonable degree of assurance, additional protection is desireable if it can be achieved without greatly increasing costs and technical difficulties. Use of such a sensor which is capable of eliminating far field disturbances such as thunder, passing jet airplanes, and heavy traffic at a distance substantially remote from the protected area permits far greater efficiency since false alarms are substantially eliminated. According to the invention there is provided an improved sensor for use with an intrusion detection circuitry, the sensor comprising a hollow electrically conductive tube having a substantial length; at least two electrically conductive wires extending loosely inside said tube along the length thereof, each of said wires having alternating areas of electret sensitivity and insensitivity; and means connecting each wire electrically to said tube, the sensor producing electric signals generated by vibration of said wires in said tube. An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a partially cut-away schematic view of a preferred embodiment of sensor according to the present invention, and Figure 2 is a block diagram of intrusion detection circuitry employing the sensor of Fig. 1. As shown in Fig. 1, the sensor includes a hollow electrically conductive tube 10 of substantial length which has contained within it loosely fitted electrically conductive wires 1 2 and 1 4. Each wire has alternating areas of electret sensitivity and insensitivity, wire 1 2 having sensitive areas 1 6 and wire 1 4 sensitive areas 1 8. As shown in Fig. 1, the wires are disposed so that the sensitive areas of one wire are opposite the insensitive areas of the other wire. Both wires 1 2 and 14 are connected electrically to the tube 10 through wires 1 9 and 20.Electric signals from wire 1 2 to tube 10 and from wire 14 to tube 10 can then be processed to determine whether or not there has been an intrusion. As shown in Fig. 2, the sensor 10 provides signals in both wires 1 2 and 1 4 which are transmitted to processor 24. Typically, processor 24 is adapted to electronically subtract the signals provided by lines 1 2 and 14. The net signal from the processor 24 is transmitted through line 1 7 to alarm 26. A modification of the present sensor, includes the use of more than two wires inside the tube 10. Typically the tube is at least 1 5 metres in length and preferably up to a hundred metres or longer. The lengths of the electret and insensitive areas on each of the wires range from one-half metre to two metres. It is preferred, of course, that the length of the electret areas is approximately the same as the length of insensitive areas on the wires. The above described sensor can be installed in any length and in any location where seismic disturbances are likely to occur which would indicate the presence of an intruder. All that is required is that the sensor 10 be positioned so that seismic vibrations caused by compression of the surface of the ground in which the sensor is buried or vibrations transmitted through the sensor can be mea stirred. The signals produced by vibration of the wires in the tube are transmitted to the signal processor 24 and the on to the alarm 26. In order to eliminate far field effects, the signal processor 24 is adapted to subtract the signal of one wire from the signal of another wire.When the subtraction of the signal from one wire from the signal of another wire remains below a certain threshold value, which can be calibrated depending upon the particular installation, any far field effect which would tend to influence all of the various electret sensing areas of both of the wires 1 2 and 14, or at least a major portion of them, will be eliminated since the numerous sections will tend to cancel one another during the subtraction process. On the other hand, when intrusion takes place quite close to the sensor only one of the individual sensitive electret areas 1 6 or 1 8 will be substantially disturbed.Thus, the subtraction of one signal from the other wi!l produce a relatively high value signal in line 17, either positive or negative, above the threshold value, which causes operation of alarm 26 and permits detection of the intruder. CLAIMS

1. A sensor for use with an intrusion detection circuitry, the sensor comprising a hollow electrically conductive tube having a substantial length; at least two electrically con ductive wires extending loosely inside said tube along the length thereof, each of said wires having alternating areas of electret sensitivity and insensitivity: and means connecting each wire electrically to said tube, the sensor producing electric signals generated by vibration of said wires in said tube.

2. The sensor of Claim 1, wherein two wires are disposed so that the electret area of one wire is opposite an insensitive area of the other wire.

3. The sensor of Claim 1 or 2, wherein said tube is at least 1 5 metres in length and the electret and insensitive areas range in length from one-half metre to two metres.

4. Intrusion detection circuitry including a sensor according to any one of the preceding claims.

5. The circuitry of Claim 4, including means for subtracting the signals produced by the wires and for feeding the resulting signal to an alarm.