GB2105505A - Thiefproof guard fence - Google Patents

Abstract

The fence comprises plastics coated wire netting stretched between posts. Before coating with plastics, a conductive strand of enamelled or varnished copper is woven with the wire of the netting to form conductive loops A-A'. The conductive loop is connected to electronic equipment to detect breakage or modification of the conductive lines and signal an alarm. In an alternative embodiment the conductive lines are formed by metallization on plastics coated wire which is then given a further plastics coating. <IMAGE>

Description

SPECIFICATION Thiefproof guard fence The present invention relates to a thiefproof guard fence comprising plastics-coated wire netting and intended for ensuring detection and signalling to a central watchpost, of any attempt by an intruder to break in or get over.

Thiefproof fence networks are already known.

They employ wire netting specially designed for this purpose and incorporating a special conductor wire having the ordinary outer appearance of the other wires of the netting. This type of netting presents the disadvantage of being very costly. For these devices the alarm is triggered by measurement of the unbalance caused between two identical panels inserted in a Wheatstone bridge circuit.

This method of connection enables compensation of variations in resistance due to atmospheric conditions but it presents the disadvantage of being too sensitive and being triggered for other causes than a break-in (being hit by a ball, closeness of objects causing interference .) and of creating false alarms.

The aim of the invention is to provide a fence which offers very high protection whilst making use of standard wire netting.

In accordance with the invention this result is obtained with a guard fence for detection and alarm signalling comprising at least one plasticscoated metal wire, preferably in the form of netting, stretched between posts and means of detection and signalling of any attempt to break in, characterized in that before coating with plastics, a conductive strand is combined with the metal wire. For example conductive strands of enamelled or varnished copper are woven into the wire netting to form one or more conductive loops.

The conductive line or lines thus formed are closed across a supply circuit and across a detection circuit which enables any breakage or modification of the conductive lines of the netting due to an attempt at shunting to be detected. The present invention in addition offers complete impossibility of the return of information or of interference through earth and thus avoids the risk of false alarms.

An essential advantage of this device is its low cost thanks to the employment of standard commercial welded netting having a rectangular mesh. In addition, after plastics coating of the whole, the outer appearance of the netting is absolutely ordinary and the presence of the conductor wires cannot be detected by a possible intruder.

The invention is explained in greater detail below by means of certain of its embodiments, taken by way of illustration but in no way restrictively, whilst referring to the attached drawings in which: -Figure 1 represents a conductive loop put in place on the supporting netting in accordance with the invention; -Figure 2 is a section of the cable composing the netting after covering with plastics the whole as Figure 1; -Figure 3 illustrates by way of example a way of fencing in an enclosure in accordance with the invention; Figure 4 is a diagram of the preferred electronic detection circuit; Figure 5 is synoptic of the circuit as Figure 4; Figure 6 is a schematic diagram of the alarm transmission and centralization circuit;; -Figure 7 is a special barbed wire which may be employed in combination with the invention; -Figure 8 is a diagrammatic longitudinal section of a conductor element employed in accordance with a variant implementation; -Figure 9 illustrates the realization of a conductive circuit on plastic-coated netting by a surface treatment; -Figure 10 illustrates a particualrly advantageous application of the fence as in Figure 9.

In accordance with Figure 1 the wire netting serving as support is of the type having a welded rectangular mesh. A plurality of strands of enamelled and varnished copper twisted together is put into place so as to form by weaving along the horizontal and then vertical steel cables a long continuous loop the two ends A and A' of which meet at one and the same point on the netting.

The said strands are held along the steel cables by stitching by sewing machine or by stapling or gluing. The whole is then coated with plastics and appears like a normal plastics-coated welded netting.

Figure 2 represents, seen in section, the cable obtained after coating the whole as Figure 1 with plastics. The steel core 1 as well as the plurality of strands of copper 2 are embedded in the plastics coating 3.

Hence the protection of a sensitive point, of a factory or a site, will be achieved by a fence consisting of members of wire netting placed on concrete posts or plastics-coated metal posts, and several metres wide. The said members which may be partially or totally sunk in the ground in order to increase the security, form sections or zones of some tens to some hundreds of metres if necessary, protected specifically and independently in accordance with the plan as Figure 3. Each zone in turn consists of one or more loops identical with the loop A-A' as Figure 1. In accordance with the preferred embodiment of the invention each loop A-A' in turn consists of a plurality of conductive strands of copper, each then forming a conductive loop.In accordance with one variant embodiment, the conductive strand (or strands) forming the loop A-A' may be replaced by a layer of conductive paint or a layer of metallization or any other means of realization of the conductive loop, the said layer being deposited between two layers of plastics coating. Each conductive loop of each zone is connected across an electronic equipment fed either from the local supply circuit or preferably from a solar panel feeding a battery by way of a transistor regulator. This preferred method of supply has the advantage of saving the laying of electric supply lines, of keeping the battery permanently charged, of being largely independent and of being insulated from earth.

Figure 4 represents by way of example a circuit for interrogation of a zone including 8 loops of 8 electrical circuits and able to suit a fence of a maximum length of 250 m. In the case of fences of greater length it is advisable to provide this device as many times as is necessary. The principle of operation of the interrogation circuit is described in the synoptic chart as Figure 5 and in Figure 4.

It comprises a clock (4) controlling the successive excitation of the portions of the eight registers Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q8 of the unit (5).

Each output Q1, Q2, Q3, Q4, 05, Q6, 07, 08 is connected to one of the eight inputs to the interface unit (5). This unit comprises eight electronic circuits each comprising one loop of the fence circuit. Each output from the interface circuit (6) is connected to one input to the eight gates of the comparator unit (7). There exists in addition a direct connection between each output from the register and the second input to each gate of the unit (7).

Thus when the output Q1 sends the signal the gate 1 receives two identical signals over its two inputs and its output remains at rest. If the loop being interrogated is broken its impedance is seen to be modified by a shunt, the gate tilts and communicates a voltage to the alarm circuit.

The portions of fence are thus interrogated in succession during an interval of time less than one second. The circuit comprises in addition a device for resetting the clock to zero when the eight loops have been interrogated so as to recommence the interrogation.

In the event that a shunt had been put in place by an intruder and connected two loops together, for example, the loop 1 and the loop 8, only one of the two gates 1 or 8 would have both its inputs in the same state since the clock sends only one signal at a time and the other gate then triggers the alarm.

The synoptic as Figure 5 represents the detection circuit described above.

Figure 6 represents the schematic diagram of the alarm transmission and centralization circuit.

This circuit offers the advantage of effecting the connection between the central station situated in the guard post and the associated sub-stations at each zone of the fence by means of a single wire; the economy and the ease of laying are not negligible in the case of fences of large dimensions.

A constant current at a regulated voltage flows in the said connecting wire which is also called the busline.

The said busline comprises as much resistance R as there are zones to be supervised, each resistance R being installed in parallel across a closing contact which in the normal position is closed. At the time of an alarm or of a fault in the supply to the substation the corresponding relay drops out and its resistance is in circuit.

The analogue-digital converter sees its values compared cyclically by the central station with a table of values put in store.

When the value from the convertor is equal to a stored value (in the event of alarm) the central unit converts the said value into a zone number which is then dealt with by the various signalling circuits indicated in Figure 6: display, printer, synoptic, audible alarm, alarm report, telephone transmitter... and may be employed either separately or partially or totally combined.

In addition the stored table comprises an instruction corresponding with cutting the wire of the busline.

By way of example a RCA card equipped with a "cosmac" microprocessor (ref. CD P 185604) and a card from the same manufacturer (ref. CD P 183648) are employed for the A/D conversion.

The capacity of this conversion, 8 bits multiplexed over 1 6 lines, enables the system to centralize if necessary on the same principle, other alarms such as: supervision of the burners in the boiler room, temperature of the buildings, or any other industrial application.

This method of transmission and centralization has been described in a non-restrictive way, and one may concaive of employing any other method of transmission (low frequency or the like . . .).

In addition the electronic device may be completed by a circuit for compensation of temperature by thermistance in order to take into account variations in the ohmic resistance of the wires of the netting as function of the external temperature (--200 to +500).

In addition, in accordance with a variant upon the invention, one may conceive of employing instead of the plastics-coated netting or in combination with it one or more barbed wires stretched between the posts. This special barbed wire is composed of a plastics-coated metal wire (9) of the same structure as the cable described in Figure 2 and twisted with a steel wire carrying barbs (10).

A variant implementation will now be described by reference to Figure 8.

A wire treated in accordance with this variant includes: -a central metal core (11); -an insulating layer (12), for example, a layer of plastics; -a conductive layer (13) applied by surface treatment, for example, by metallization; -an outer insulating layer (14).

Hence the conductive element is applied by a surface treatment directly over the plastics sheath coating the central core, the whole being subsequently subjected to another coating with plastics or any other suitable insulating material.

Thus on a plastics-coated wire netting one may achieve a veritable printed circuit, the application of the surface treatment being able to be selective and defining conductive circuits which are then fed and supervised in the same manner as previously.

The surface treatment may be, nonrestrictively, a metallization in accordance with techniques which in themselves are well known and will not be described in greater detail.

In Figure 9 an embodiment is shown of circuits printed over a plastics-coated wire netting (15).

The surface treatment is applied selectively so as to define circuits such as (16, 17, 18). Then insulation is effected either by coating with plastics or by spraying with any adequate insulating constituent.

Finally in Figure 10 there is shown a supplementary security device which detects any break-in tending to make the upper portion of the netting tilt.

On the connecting posts (1 9,20) the dynamometric detection members are arranged, which consist essentially of two symmetrical bars (21,22) mounted to be able to rotate about a point on the posts and controlling inside each post a means of pinching the fence wires, thus triggering the alarm; the means of pinching are equipped with a return spring which brings them back into position when the action upon them ceases.

Finally barbed wires are stretched across the bars (21,22), projecting sideways with respect to the plane of the netting and including a conductive strand produced likewise by metallization.

Thus a complete protection is obtained: detection via the netting for breakage and any deterioration; -dynamometric detection via the bars on the connecting posts; --detection for breakage and deterioration of the barbed wires.

Claims (8)

Claims

1. A guard fence for detection and alarm signalling of the type comprising at least one plastics-coated metal wire stretched between posts and means of detection and signalling of any attempt to break in, characterized in that before coating with plastics a conductive strand is combined with the metal core of the said plasticscoated wire.

2. A guard fence as in Claim 1, characterized in that the plastics-coated metal wire is twisted with a steel wire carrying barbs (10).

3. A guard fence as in Claim 1, in which the metal wire is given the form of plastics-coated wire netting, characterized in that the said wire netting is obtained by the association and plastics coating of on the one hand a standard welded netting having a rectangular mesh, and on the other a continuous loop obtianed by weaving along the steel cables a plurality of strands of enamelled or varnished copper.

4. A guard fence as in Claim 1, characterized in that the conductive element is applied by surface treatment directly over the plastics sheath (12) coating the central core (11), the whole being subsequently subjected to another coating with plastics or any suitable insulating material.

5. A guard fence as in Claim 4, characterized in that the conductive element is applied by metallization.

6. A guard fence as in any one of the Claims 1 to 5, characterized in that it includes a dynamometric detection in the upper portion of the netting, including essentially bars (21,22) mounted to pivot about the connecting posts (19, 20) and operating inside them means of pinching a fence wire.

7. A guard fence as in any one of Claims 1 to 6, characterized in that in addition it includes, stretched between the bars (21,22) barbed wires including a conductive element applied by surface treatment over the plastics sheath coating the central core of one of the wires of the said barbed wire.

8. A guard fence as in Claim 1 substantially described herein with reference to any one of the Figures of the accompanying drawings.