GB2267375A - Detection system comprising a fence and detector. - Google Patents

Abstract

The detection system comprises a strand 5 usually of galvanised wire and a detector 1. The strand can be stretched between fence posts and eyelets used to guide the free movement of the strand and a detector located at one end of the length of the strand. If the strand is stretched by a small amount, the resulting stretch is passed to the detector and the electrical change in the resistance of the detector is received in the electronic controller and an alarm generated. The strand may be electrically conductive and connected in series with its detector so that breakage of the strand may also be detected. The detector comprises carbon-impregnated, electrically conductive synthetic rubber. <IMAGE>

Description

DETECTION SYSTEMS COMPRISING A FENCE AND DETECTOR. ~~DETEC~ION~SYSTEMS~COMPRISING~A~FENCE~AND~DETECTOR.

This invention relates to detection systems and sensors for use therewith.

The invention relates particularly to detection systems consisting of a fence comprising least one strand of material, typically a metallic wire, together with a sensor adapted to detect movement or breakage of the strand such as may occur for example when a person or animal disturbs it.

Detection systems are frequently used in which penetration of a perimeter fence or area causes a remote alarm to operate without alerting the intruder. Detection systems are known which employ infra-red sensing, fibre optics or electrically charged wire. These generally suffer from a combination of high cost,false alarms, and high operating and maintenance requirements. Many are unsuitable for areas in which the public may access and protected fences need to be unobtrusive.

Accordingly to the present invention there is provided a detection system consisting of a fence comprising at least one strand of material together with a sensor comprising a component having a variable electrical resistance and capable when used in conjunction with electronic circuitry, providing a controlled retention of the alarm level threshold,of detecting movement or breakage of the strand by a change in the said variable electrical resistance.

According to another aspect of the invention there is provided a sensor comprising a component having a variable electrical resistance,said element being located between two attachment points such that a change in the distance between the attachment points brings about a change in the said electrical resistance. Preferably the said component is capable of,or capable of being adapted to detect both an increase and/or a decrease in the distance between the said attachment points.The electrical resistance of the said component is preferably caused to change by alteration of its cross-section, which may be brought about for example by stretching or relaxation of the component.

The invention may conveniently be described with refer -ence to the accompanying drawings,which illustrate but do ne restrict or limit the invention.

Figure 1 shows diagrammatical part-section, the structure of the embodiment of the sensor which comprises an element 1, which is a length of carbon-impregnated electrically conductive synthetic.rubber which varies as the element is stretched;in this illustrated embodiment,the element is tubular cross-section, outside diameter being 5mm, inside 3mm and a length of 9cm.Element 1 is located within a resilient and insulating rubber tube 2 of bore 5mm and outside diameter 13mm., which provides the necessary elastricity to retain the strand in continual tension.Metal plugs 3 and 3' at each end of the tube 2 provide mechanical coupling and also enable electrical contact to be made with the conductive rubber. Water-repellent electrically insulating nylon cords 4 provide means of attachment to the strand and/or anchorage points 5 when the sensor is in use, and the whole is enclosed within a waterproof PVC sleeve 6.

Nylon is used in this embodiment as it is non-porous and does not become electrically conductive with damp.

Monofilament nylon is preferred to avoid retention of water between the fibrils such as could happen with a woven filament.

It will be appreciated that in use water should be excluded from the element 1 and its connections, and for some applications, particularly when the sensor is to be used out-of-doors, it may be advantageous to enclose the sensor within a waterproof or substantially waterproof enclosure.

Figure 2 ahowa diagrammatically,in longitudinal section.

the structure of another sensor according to the invention.

It is sometimes advantageous to hide or disguise con -ponents of the security system .to reduce the likelihood of interference, and this embodiment provides a sensor adapted to be embedded within a cavity in,for example,a support -ing post.The sensor is shown in situ within such a cavity.

It may also be necessary to increase the tension on the strand and in this application a helical spring may be used instead of the rubber tube described in figure 1. It will be appreciated that arrangements, methods of attachment etc., can be employed without departing from the invention.

The sensor comprises an element 21 which is a length of carbonimpregnated electrically-conductive synthetic rubber,the electrical resistance of which varies as the element is stretched. Metal grips 22 and 22' are attached at the ends of element 21 providing means whereby the element can be mechanically coupled to the strand and/or anchorage with which,in use,it will be functionally associated,and providing also means whereby electrical contact can be made between the element 21 and the electrical conducting means e.g. wires 24 and 24', connecting the element 21 to he sensing circuitry distant or otherwise,capable of reacting appropriately to the change in resistance of element 21. Element 21 and its associated grips 22 and 22' are located within the bore of a spring 25 which is retained by the insulating bushes 23 and 23' so as to electrically insulate the spring from the element 21. The spring provides the elasticity for the component. The nylon loop 26 electrically isolates the component from the strand 27.

Figure 2 shows a sensor in which the element 21 is insufficiently robust to provide when in use both the resilience necessary to maintain tension in the strand and the variable electrical resistance which is basic to the functioning of the sensor. If it is required to have a system with a very small tension then a very weak spring can be used,but this conductive rubber element is very weak in its elastic properties and it could be insufficient to use the element alone for tensioning. Should the physical properties of the conductive rubber improve with future developments then it may be possible to use it without additional tensioning. It will be apparent to the skilled man that,as in the sensor shown in Figure 1, these functions can be separated,.wholly or in part,and it is within the scope of the invention to employ an element of variable resistance which itself has insufficient resilience to maintain the desired tension in an attached strand but in association with a second component to impart the necessary resilience. For example element 21 itself may comprise plurality of components,which optionally may be bonded together, one providing the variable resistance and possibly some resilience,and the other providing any necessary additional resilience.

The drawing shows the sensor in situ in a cavity,a conveniently sized hole,in the post 28.Plugs 23 and 23' are employed to seal the ends of the hole to exclude water and to locate the sensor in the hole. The static end of the element is held by means of a plug 22 and 22' which is held in position by a washer and screw combination.

The sensor may conveniently comprise a means for signalling a change in the electrical resistance of the element thereof consequent on the movement or breakage of a strand whereby such change can be detected,for example by the activation of an alarm. Alternatively,or additionally,the sensor may comprise means for the recording change in the said resistance. The sensor may transmit a change in the said resistance along wires,appropriately connected to the element,or it may comprise a transmitter so that an alteration in resistance can be signalled remotely to an appropriately tuned receiver. The sensor can also signal through a telephone encoder so that the sensing can call a preset telephone number.

In its simplest embodiment the detection system of the invention comprises a strand of material, conveniently a metal wire, supported at the appropriate height and attached at one end to an anchorage and at the other to a sensor which itself is secured such that disturbance of the strand shortens or lengthens the element of the sensor, so alter -ing its electrical conductivity and increasing or de -creasing the current passing through it, which change in current can be detected and appropriately signalled or recorded.

In addition to supports at or near its ends, the strand may be supported by additional intermediate supports which may be,for,example, posts with loops or eyelets at approp -riate points through which the strand passed and moves freely, so that movement at any point in its length is transmitted to its associated sensor. The strand may be single or multi-core,and it may be part of a fence comprising, over part or all of its length,a plurality of lengths of material,for example,wire.A strand to which the sensor is functionally connected(that is,connected in such manner that movement or breakage of the strand is detected by the sensor) may,but may not necessarily itself,extend the whole length of the fence provided-that movement or breakage in the detection system of which the strand is a component is capable of being detected. In another embodiment the strand may form part of a multistrand system in which one or more sensors are present attached to different sensors and the sensors are electrically connected so that a movement of any one of the strands will be detected.

The material and dimensions of the strand will be appropriate to its purpose and will be apparent to the man skilled in the art. Thus the material and strength should be such that in use the degree of extension by stretching or thermal expansion is not so great that the effectiveness of the system is unduly affected, and its flexibility should be such that it will, if necessary, bend and operate around corners in the fence line. Some degree of long-term stretching, effect of temperature, and movement resulting from ageing of the supporting posts may be com -pensated for in the associated electronics.

The controlled retention of the alarm threshold adjusts the detection circuitry to the steady state voltage across the resistive element,so that when a strand is installed and the sensor or sensors put under tension, the electronics will adjust to this steady state and only respond to a change in this resistance.By this means the system does not respond to slow changes such as environmental changes, ageing etc. In the initial installation of the system as a result of this design feature, after an very short initial settling period, the detection system will therafter respond to changes in resistance of any of the elements and require not adjustment.The sensitivity of the system can be readily adjusted if required.

A further embodiment of the invention is shown diagrammatically in figure 3 of the attached drawings, which shows in side elevation a detection system comprising three strands 31, one end of each strand being attached to post 32 and the other end of the strand to one of three sensors 33. Each of the sensors is attached to post 34.Inter -mediate supporting posts have eyelets through which the strands are able to slide freely,so that disturbance at any point of the strand is conveyed to the sensor by movement of the strand. Theoretically twenty sensors can be connected in series to has the effect of halving the sensitivity of each sensor, but this can be conpensated for by adjust -ment of the system sensitivity control.

In use each strand is tensioned sufficiently to ensure that movement of the strands such as would be induced by contact with them of anything which it is desired to detect, produces a response in the sensor by altering the electrical resistance of the element thereof.Conveniently the strand is tensioned so that the element is slightly stretched.In this state either further stretching or re -laxation of the element,corresponding to movement of the strand or to its breakage, will produce a change in the resistance of the element which may be detected, signalled or recorded as required.The system is able to differentiate in the signalling between these events.

Location of the sensors may be at any convenient point, consistent with the effective operation of the system.

We have found that each strand may be up to about 50 metres in length or even more if conditions are appropriate e.g.if the corners are minimal.Leads 35, from the connection of the sensors in series, are taken to the de -tector electronics.

Optionally the system may be designed such that short period changes in the tension of the strand, such as may result for example from the brief alighting of a bird, or a horse jumping the fence, is not registered. In an improvement of the system, the strand can itself be or include the return wire from the sensor,so that any attempt to interfere with the detection system by cutting the strand can be detected.The system may respond if this wire should be electrically bypassed before this cutting of the strand.

Fig.4 shows the circuitry. 2 is the resistive element which is supplied from a regulated voltage source 1. 2 is the resistive detector element in series with which is the open circuit detector 4 and the earth bypass detector resist -or 3.( If the fence wire which is the bottom end of 2 is bypassed to earth,the voltage at 2 will change sufficiently to be detected).The voltage is input to the voltage compar -ator 9. The output of 9 is the result of the comparison with this input and the voltage derived from the time constant 8.The voltage from 8 is almost equal to the input voltage as 9 is high gain.When there is a change in the input due to the sensor detection,there is a large change in the voltage from 9 as the time constant 8 cannot follow at that rate. This change triggers the monostable 5,and the analogue gate 7 is opened thereby isolating 8 from any further change. If the disturbance has a duration less than the reset time of 5,the bistable 6 will not be set, but if the duration is greater than the reset time of 5, 6 will be set,due to the fact that the D input to 6 bistable is set at the time of the trigger. The bi -stable 4 will then trigger the pre-alarm monostable 10, and when it resets the main alarm 11 is set. The pre -alarm can set an RF transmitter or recorder, and the main alarm can drive a bell,siren or transmitter as required.

The detection system can be expanded to comprise a principal detection system as described and anciliary units which have individual sensors. In the present design there is a principal unit and three anciliary units.By this tec -nique the perimeter covered is increased by a factor of four.The power taken has been restricted so that the de -tection system can be battery driven, if required,so it may be used in a country area where mains power is not available. A 12v. 50AHr.battery will keep the system operating for at least a year between recharges.

Claims (10)

1. A detection system consisting of a fence, comprising at least one strand of a material, sensor means, comprising at least one sensor coupled individually to a strand and having a variable electrical resistance related to tension in the strand, and circuit means, responsive to a change in electrical resistance of a sensor indicative of a change in tension in a strand, to provide a detection signal.

2. A detection system as claimed in Claim 1 in which the sensor means is arranged to be located between two attachment points such that a change in the distance between these points brings about a change in the electrical resistance.

3. A detection system as claimed in Claim 1 or Claim 2 in which the sensor means comprises a plurality of sensors connected in electrical series.

4. A detection system as claimed in any one of the preceding Claims in which each said strand is arranged to maintain the coupled sensor in tension.

5. A detection system as claimed in any one of the preceding Claims in which each sensor includes an elongate element of electrically conductive rubber.

6. A detection system as claimed in Claim 5 including resiliant means in parallel with the element and operable to limit the tension applied thereto.

7. A detection system as claimed in any one of the preceding Claims in which the circuit means includes a voltage comparator operable to compare the instantaneous voltage across a sensor element with an average of that voltage.

8. A detection system as claimed in any one of the preceding claims in which the circuit means includes discrimination means operable to inhibit response to disturbances of less than a minimum duration.

9. A detection system as claimed in any one of the preceding claims in which at least one strand comprises an electrical conductor connected electrically in series with the associated element and the circuit means is operable to provide different alarm signals for a break in the strand and disturbance of the strand.

10. A detection system substantially as herein described with reference to the accompanying drawings.