GB2164183A - Intruder detecting fences - Google Patents

Abstract

A fence comprises a plurality of posts interconnected by a filament, and means for detecting a change in the tension of the filament. In one form a mechanical trip switch is sensitive to increase or decrease of tension to change its state, in which changed state on electric or optical sensor is operated. Preferably the trip switch moves an optical diaphragm located between the ends of adjacent optic fibres. In an alternative form, an optic fibre is abraded (whereby its transmissivity is changed) upon change in tension of the filament. The filament may be constituted by the optic fibre itself. The filament may be housed within a coaxial flexible sheath, which sheath comprises the visible fence wire. <IMAGE>

Description

SPECIFICATION Security system When a boundary to be kept secure is long, it is often impractical to provide an absolute barrierto intruders along the whole boundary. Instead, a security system is provided which is sensitive to the passage of an intruder past the barrierso that guards can be alerted and directed to the point of entry ofthe intruder.

It has been suggested that a chain linkfence could be made secure in thisway by providing a transducer at regular intervals along thefence sensitive to vibration in the fence which might be caused by an intruderclimbing or cutting the fence. Howeverthe problem of systems such as this is thatfalse alarms can be caused by wind or animals.

According to one aspect ofthe invention there is provided a security system comprising a series of spaced posts, a line between adjacent posts and a trigger switch mounted within a post operable by the line when the tension does not lie within a given range of values. The line may be contained in a non-rigidtube extending between adjacent posts. The trigger switch can be connected to actuate an alarm at a central pointtoindicatethe point of entry of an intruder.

Preferablythe trigger switch comprises a spring loaded device, the spring being released when the tension in the line reduces to zero (for example when the line is cut or breaks) or when the tension in the line exceeds a threshold value (for example when an intruderclimbsthebarrier),the release of the spring causing activation ofthe switch.

Activation of the switch can be caused by a mechanical connection which is moved on release of the spring-loaded device. It could be caused by an optical connection, for example when the surface of an optical fibre is abraded by the release ofthe spring loaded device, causing lightto escape from the fibre and the light output ofthe fibre to drop. Atransducer responsive to the light output operates the switch.

The drop in output at the end of the optical fibre can be used to actuate the alarm.

In another aspect the invention provides a trigger switch operable by a line (for example the line of the above security system), the switch comprising a first hinged member biassed in one direction aboutthe hinged axis, a second hinged member which in the 'set' position ofthetriggerswitch extends substantially perpendicularto and in engagement with the first hinged memberto prevent movement of the first hinged member under its bias, the arrangement being thatwhenthetension on the line lies within a given rangethetriggerswitch when set remains in that position, but when the tension reduces belowthe given range, the bias ofthe second hinged member causes itto hinge about its axis away from the position substantially perpendicularto the first hinged mem berandwhenthetension increases above its given range the second hinged member moves against its bias in the other direction away from the position substantially perpendicular to the first hinged member, in both cases allowing the first hinged memberto hinge under its bias. Movement of either hinged member underthe biases can be caused to actuate a transducer, such as an electrical switch by mechanical connection or a radiation sensor by interruption of a beam ofthe radiation bymovementofa hinged member across the beam, or a sound sensor responsiveto the noise of a hinged member striking a stop plate.

Examples ofthe invention will now be described with reference to the accompanying drawings in which: Figure lisa side elevation of part of a security system, Figure 2 is a cut-away detail ofthe system of Figure 1, Figure 3 is a perspective view of the detail of Figure 2, Figure 4 is a side elevation, and Figure 5 is a perspective view, of a trigger switch which can be substituted for the switch of Figures 2 and 3, Figure 6 is a side elevation of an alternative trigger switch, Figure 7 illustrates another aspect of the invention, Figure 8 shows a section through the junction of a post with a tube showing a spring-loaded device in its tensioned position, and Figure 9 shows the device of Figure 8 when the applied tension has exceededthethresholdofthe device.

Figure lisa side elevation of a security system incorporating one aspect ofthe invention. A series of vertical posts 11 of plastics material support a series of horizontal tubes 12 between adjacent posts. A line runs in each tube, anchored at one end and connected at the other end to a trigger device housed in one post 11 which is hollow. The tubes 12 are of plastics material and are self supporting, but not able to resist human forces (e.g. a weight exceeding 7kg). Figure 2 is a section through one such hollow post 11.

A line 13 in each tube 12 is secured to atrigger device 14which comprises a spring hinged member secured to the post 11 and biassed to move to a position across the width of the post, as indicated atA, but normally restrained by the line 13. Down the centre of the post is a further line 15 suspended atthe top 16 and supporting an annulus 17 atthe lower end, held down by a spring 18. A radiation source 19 directs a beam of radiation through the centre hole ofthe annulusto a receiver 21. A guide 22 may be provided forthe annulus.

When an intruder penetrates the system he may cut atube 12 and the line 13, or he may climb onthetube 12 causing the line 13 inside to break. In either case the trigger device 14 is released and moves underthe bias to strike the line 15, causing the annulus 17to be raised and interruptthe radiation falling on the receiver 21 which then activates an alarm (not shown).

The components 15 to 22 can be replaced by a radiation source at one endofthe postdirecting a beam of radiation to a radiation receiver at the other end ofthe post, the receiver op~} á.ng an alarm when the trigger device passes th rough the beam.

Figures 4and 5 show a trigger switch including a pair of spring hinged members 31,32. In this case the members are biassed towards the wall ofthe post on which they are mounted. In the set position as illustrated the bias Fofthe uppermember31 has a component in the horizontal direction which is balanced bythesum ofthe horizontal componentofthe bias F of the lower member 30 and the normal tension in the line 13. When the tension in the line increases, the upper member 31 is lifted and releases the lower member 32 to strike the wall of the post. When the tension drops, the upper member31 forces the lower member32 to the left until eventually the members disengage and both strike their supporting walls.The noise ofthe striking is picked up buy a microphone (not shown) which operates an alarm. Astriker plate, not shown, can be provided to generate a loud noise on impact.

In a further alternative arrangement, illustrated in Figure 6, the line 13 beyond the tube 12 travels longitudinally in the post 11 to be attached to a lower hinged member 41. In the set position an upper hinged member 42 is parallel to the wall of the post and is biassedawayfromthatwall and the lower hinged member is substantially perpendicularto the upper hinged member and is biassed upwardstowardsits supporting wall. In an alternative arrangement the line would travel down the tube to the lower hinged memberwhich would be biassed downwards towards it supporting wall. In the illustrated arrangement, a variation in tension causes the lower hinged member to move away from its position substantially perpendicularto the upper hinged member,thus releasing the upper hinged member.The movementofthe upper hinged member (and in the case of a reduction in the tension, ofthe lower hinged member as well) can be sensed by any ofthe methods already described.Thetension in the line 13 in the set position can be adjusted by a turnbuckle 43.

Figure7 shows another example ofthe invention in which a security device comprises a fixed structure on which is mounted a source of radiation and a detector sensitive to said radiation and separated from said source by a gap, the device further comprising a movable structure which can be placed in said gap and which includes a tube whose interior defines a path for the radiation from the source to the detector. A barrier 71 is provided with a gap 72 which can be closed by a door 73 hinged to the structure 71 defining one side of the gap 72. Recessed within one side of the barrier71 is a source 74 of radiation such as light and recessed within the barrier71 on the other side ofthe gap is a detector 75 for detecting that radiation. Electric cables 76 connectthe source 74 and the detector75to a suitable alarm system.The barrier portions 71 are formed with aligned holes 77 along which radiation can pass from the source74 across the gap and to the detector75. The door 73 is provided with a hole 78 which is aligned with the holes 77 when the door73 has been hingedto its position inwhich itclosesthe gap 72. In this position,there is no outward sign ofthe present ofthe security device, so that an intruder would receive no warning that he is aboutto activate the security device when he opens the door 73. As soon as the door 73 is opened,the hole 78 becomes misaligned with the holes 77, breaking the passage of radiation from the source 74to the detector 75 and the change of outputfrom the detector 75 along the wires 76 will activate the alarm.

If the door 73 is flexible or is even replaced by a flexible barrier, then the security device will be activated when the door or barrier is flexed so thatthe hole 78 ceases to contain a straight line between the source 74 and detector 75. With this arrangement, or the arrangement of Figure7, an attempt to cutthrough the door or barrier would cause the alarm to be activated as soon as the blade obscured the hole 78.

In the system of Figure 1, it is not necessary for the posts 11 to be vertical and the tubes 12 may be of any flexible material. The line 13 may be fixed to a trigger device at either end, rather than being anchored atone end.

Figures 8 and 9 show a modification of the arrangement of Figure 2. In this arrangementthe line 15 is moved sideways to activate the switch at its lower end by a spring-loaded device 81, comprising a V-shaped member 82 connected at its apex to the line 13 inthetube 12 and a spring 83forced bythe arms of the V-shaped member 82 againstthe wall of the post 11 around the tube 12. When the spring 83 is released, it extends to a natural length greaterthan the distance between the postwall and the normal position ofthe line 15, displacing the line 15 and thus activating the switch atthe lower end of the line 15.

The spring 83 can be released in two ways. When the line 13 is cut or broken, the member 82 moves with the free end ofthe spring 83 to the left in Figure 1; whenthetension in the line increases so thatthe out-turned ends ofthe arms of the member 82 are no longerableto restrain the end ofthe spring 83 and the member 82 is pulled into the interiorofthespring 83. It will be noted that the arms ofthe V-shaped member 82 touch the sides of the tube 12. As the tension in the line 13 increases, the member 82 is drawn into the tube 12 and the tube sides will force the arms ofthe member 82 together, thus releasing the spring 83 from their ends.

The line 15 has been illustrated as a mechanical connection, but it could be replaced in any ofthe embodiments by an optical fibre. The spring-loaded device 81 can be provided with an abrasive end piece which, on release of the device, would abrade the surface ofthe optical fibre so that light travelling along the fibre could escape. The drop in output atthe end of the fibre is sensed byatransducerwhoseoutput operates the trigger switch. The drop in output atthe end ofthefibre can be caused by breakage of the fibre, ratherthan by abrasion of its surface. An optical fibre mightalso be used as the of line 13, any interference with the security system being arranged to cause a drop in outputfrom the end of such a fibre so as to operatethetriggerswitch. The interference may causethe optical fibre to be abraded or broken.

According to another aspect of the invention, there is provided a security system comprising a movable barrier across a gap, a light source for directing a beam of lightsensor, and a lightsourcefordirecting a beam of light means responsive to the output ofthe sensorto indicate movement of said passage defining means beyond a predetermined value.

Claims (8)

1. A security system comprising a series of spaced posts, a line between adjacent posts and means mounted within a post, operable bythe line when the tension does not lie within a given range of values.

2. A system as claimed in Claim 1 comprising an non-rigid tube extending between adjacent posts, said line lying within a said tube.

3. Asystem as claimed in Claim 1 or Claim 2 wherein said means comprises a trigger switch comprising two spring biased members in mutual engagement, with the line connected to one of said members, the bias of the springs being chosen such thatthe members are in equilibrium when the tension on the line is within said given range, the members moving when the tension lies outside said range until they reach a position which they disengage, allowing the spring bias to move the members to a released position.

4. A system as claimed in Claim 3 wherein said members are parallel in said equilibrium position.

5. A system as claimed in Claim 3 wherein said members are substantially at right angles in said equilibrium position.

6. A security system as claimed in any one of Claims 3 to 5 comprising an optical fibre which is broken or abraded on operation of said trigger switch, a light source supplying light to the optical fibre and means responsive to a drop in light output at the end ofthefibre.

7. A security system as claimed in Claim 1 and Claim 2 wherein said line comprises an optical fibre, said system comprising a device to abrade the fibre when the tension does not lie within said given range of values and said means comprises a lightsourcefor supplying lightto the optical fibre and means respon size to a drop in light output at the end ofthe fibre.

8. A security system substantially as herein described with reference to Figures 1 to 3 or Figure 7 alone oras modified by Figures4 and 5 or by Figure 6 or by Figures 8 and 9 of the accompanying drawings.