Tension sensor for an electric perimeter security system

Abstract

A wire tension sensor is provided for an electric fence perimeter or barrier system comprising at least one electric fence wire. The wire tension sensor comprises a housing; an elongate rod accommodated within, and movable relative to, the housing, the rod having a first end that protrudes from the housing and that can be connected to an electric fence wire of the electric fence barrier system that is to be monitored; at least one magnetic field source embodied within or connected to a second end of the rod; and a magnetic field sensor, within the housing and proximate the second end of the rod, to monitor and measure the magnetic field of the at least one magnetic field source, so that any displacement of the at least one magnetic field source, and thus the electric fence wire connected to the first end of the rod, relative to the housing, and in particular, relative to the magnetic field sensor, results in a signal that is proportional to the displacement.

Description

[0001] TENSION SENSOR FOR AN ELECTRIC PERIMETER SECURITY SYSTEM

[0002] FIELD OF INVENTION

[0003] This invention relates to electric perimeter security systems comprising a plurality of electric fence wires, and in particular it relates to a wire tension sensor to detect the the tampering of or parting of one or more of the electric fence wires.

[0004] BACKGROUND OF INVENTION

[0005] Electric perimeter security systems typically comprise a plurality of strands of wire supported between posts. The wires are configured to be part of an electrical circuit, so that should the wires be cut, or grounded, an alarm is raised.

[0006] With reference to Figure 1, an electric fence security system typically comprises a fence energizer 16 to generate high-voltage electrical pulses which are propagated down one or more conducting wires of the electric fence. In Figure 1, four live wires are shown that are looped or connected together at their ends, namely Live Wire A, Live Wire B, Live Wire C and Live Wire D, with an Earth Wire also being provided. When a person or animal touches a conducting wire, a path to ground is created through the person / animal. The electrical potential (and possibly current) is monitored to detect if / when an electrical fault occurs. The electrical potential is typically raised to a level that is extremely painful to touch, so as to provide a very cost-effective physical deterrent with alarm capabilities.

[0007] The excitation of the wire, by a sufficiently high electrical potential in the order of 8 kV, has traditionally provided an effective security barrier. Because of concerns for public safety, under certain circumstances, the excitation voltage may be reduced to very low voltages that are no longer a physical deterrent. Criminals have, however, found ways to separate the wires, by mechanically displacing them, to create a sufficiently large aperture for people to climb through, without disrupting the electric circuit and without raising an alarm. Some mechanical anti-tamper loops have been introduced in the industry, but many of these loops are needed to be effective and take time to install.

[0008] Detecting the displacement of the wire has also been done with electronic circuitry. The use of strain gauges, attached to a rigid structure, typically a pole, and the wire, results in a signal being generated proportional to the relative movement between the structure and the wire. Although these sensors can be extremely effective they have the following disadvantages:

[0009] • The complexity of the electronic circuitry required to amplify the signals.

[0010] • The required rigidity and mechanical stability of the pole. This requires substantial footings and pole dimensions, which is costly.

[0011] • The difficulty in mounting a rigid structure on top of a wall.

[0012] • Strain gauges have maximum sensitivity when they are in a neutral position or the embedded PCB (printed circuit board) is not deflected.

[0013] • The strain gauges cannot be deflected too far before they become physically damaged, which requires ongoing adjustments or replacement of the sensor. This typically also requires that the anchoring poles at the end of the wire also be mechanically rigid. A displacement of +- 4 mm for such sensors is a typical limit.

[0014] • Because the strands of wire of an electric fence undergo expansion and contraction under different temperatures, the ideal location of a strain gauge sensing unit is at the middle point of the span of wire. The contraction or expansion on either side of the sensor would be identical and no deflection of the strain gauge would occur.

[0015] Another form of wire tension detection is a fence circuit switch. In such an application, a spring-loaded mechanical switch is activated when the wire tension changes. The disadvantage of this type of system include deterioration of the switch contacts, the fence must be excited for the detection to be active, and there is no prior warning prior to an alarm condition. It is advantageous to have some type of prior warning indicating a gradual movement, such as when a fence bracing pole creeps, to enable fence maintenance to be performed prior to an alarm condition.

[0016] In view of the limitations of the systems described above, it is object of the present invention to provide a wire tension sensor that addresses many of the problems associated with a strain gauge sensor.

[0017] SUMMARY OF INVENTION

[0018] According to the invention there is provided a wire tension sensor for an electric fence perimeter or barrier system comprising at least one electric fence wire, the wire tension sensor comprising:

[0019] a housing;

[0020] an elongate rod accommodated within, and movable relative to, the housing, the rod having a first end that protrudes from the housing and that can be connected to an electric fence wire of the electric fence barrier system that is to be monitored;

[0021] at least one magnetic field source embodied within or connected to a second end of the rod; and

[0022] a magnetic field sensor, within the housing and proximate the second end of the rod, to monitor and measure the magnetic field of the at least one magnetic field source, so that any displacement of the at least one magnetic field source, and thus the electric fence wire connected to the first end of the rod, relative to the housing, and in particular, relative to the magnetic field sensor, results in a signal that is proportional to the displacement.

[0023] In an embodiment, the wire tension sensor further includes: a control unit in communication with the magnetic field sensor to receive and process the signal from the magnetic field sensor, and to generate an alert signal if the signal exceeds or is outside predetermined thresholds; and

[0024] a communication module to communicate the alert signal.

[0025] The communication module can in addition communicate other parameters, such as magnetic field data, temperature data and a service alarm.

[0026] The at least one magnetic field source typically comprises either at least one permanent magnet or an excitable electromagnetic coil.

[0027] In an embodiment, the magnetic field sensor comprises a semiconductor device, such as a Hall-Effect sensor capable of measuring magnetic field strength along multiple axes. The magnetic field sensor is embedded within a PCB within the housing, the PCB including various additional electronic components.

[0028] An advantage of such a sensor is that the sensor will not be damaged if the mechanical movement of the wire is significant. The displacement of the wire is mechanically constrained in the above example but does not have to be. Because wire displacement can be in the order of tens of millimetres the requirement for extremely rigid fence structures is avoided.

[0029] In an embodiment, the housing includes an elongate primary housing body terminating in an enlarged flange, and a secondary housing body comprising an elongate tube having an inner securing flange that is arranged to be snugly accommodated within and fitted to the enlarged flange of the primary housing body. A pair of securing and guiding arms extends from an inner side of the inner securing flange into the primary housing body, with an opposite, outer end of the elongate tube defining an aperture through which the elongate rod can travel. The housing may be designed and assembled so as to be waterproof. The PCB may have a conformal coating to provide added protection against any moisture. In an embodiment, the elongate primary housing body includes an outer wall, a first end of which defines the enlarged flange, and a second end of which includes an end wall fitted with a connector. The connector can be connected to a spring tensioner, the spring tensioner in turn being connected to the fence post of the barrier system or to an adjacent electric fence wire. The wire tension sensor of the present invention may thus be mounted at the end or at any point along the wire span. If not mounted in the middle of the wire span, compensation would have to be incorporated for unequal wire expansion and contraction. The Hall Effect sensor has an in-built temperature sensor enabling such functionality.

[0030] In an embodiment, the elongate primary housing body further includes an inner wall arrangement that defines a cavity or tube to accommodate the second end of the rod in use.

[0031] In an embodiment, the elongate rod comprises:

[0032] an outer rod body that is housed within the elongate tube of the secondary housing body, an outer end of the outer rod body including an inwardly protruding lip to define the aperture of the elongate tube through which the elongate rod can protrude from the housing;

[0033] an inner rod body that is housed within the elongate primary housing body, an inner end of the inner rod body defining the second end of the rod, which is arranged to accommodate or receive the at least one magnetic field source; and

[0034] a guiding flange between the outer rod body and the inner rod body, the guiding flange being snugly and movably accommodated within the elongate tube of the secondary housing body, as the electric fence wire, and thus the rod, moves relative to the housing.

[0035] In an embodiment a compression spring is accommodated within the secondary housing body of the housing and is arranged around the outer rod body of the elongate rod to enable the elongate rod to move relative to the housing. A first end of the spring acts against the guiding flange of the elongate rod and a second end of the spring acts against the inwardly protruding lip of the outer end of the outer rod body. The elongate rod is thus biased inwardly and towards the elongate primary housing body of the housing.

[0036] BRIEF DESCRIPTION OF DRAWINGS

[0037] The invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings.

[0038] In the drawings:

[0039] Figure 1 shows a section of an electric fence barrier system using the wire tension sensor of the present invention;

[0040] Figure 2 shows an exploded perspective view of the wire tension sensor of the present invention;

[0041] Figure 3 shows a detailed cross-sectional side view of the wire tension sensor shown in Figure 2 of the present invention; and

[0042] Figure 4 shows a perspective view of the wire tension sensor of the present invention.

[0043] DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0044] Referring first to Figure 1, wire tension sensors 10 for an electric fence barrier system 12 are shown secured to Fence Post B. The electric fence barrier system 12 comprises at least one electric fence wire 14, and a fence energizer 16 to generate high-voltage electrical pulses which are propagated down one or more conducting wires 14 of the electric fence 12. In Figure 1, four live wires are shown that are looped or connected together at their ends, namely Live -Wire A, Live Wire B, Live Wire C and Live Wire D, with an Earth Wire also being provided. The energizer 16 comprises a high voltage output 18 connected to, in this case, the Live Wire A, a high voltage return 20 connected to, in this case, the Live Wire D, and a ground return 22 connected to the Earth Wire via connecting wires 24. The energizer 16 further includes an alarm monitor, as is well known in the art.

[0045] Turning now to Figures 2 to 4, the wire tension sensor 10 comprises a housing 30 and an elongate rod 32 accommodated within, and movable relative to, the housing 30.

[0046] The elongate rod 32 comprises a first end 34 that protrudes from the housing 30 and that defines an aperture 36 to enable the elongate rod 32 to be connected to the electric fence wire 14 of the electric fence barrier system 10 that is to be monitored, as shown in Figure 1.

[0047] The wire tension sensor 10 further includes at least one magnetic field source 38 embodied within or connected to a second end 40 of the elongate rod 32. In the version illustrated in Figures 2 and 3, the at least one magnetic field source 38 comprises a permanent magnet, but the magnetic field source 38 may also take the form of an excitable electromagnetic coil.

[0048] The wire tension sensor 10 further includes a magnetic field sensor 44 embodied within a PCB 46 within the housing 30 and proximate the second end 40 of the rod 32, to monitor and measure the magnetic field of the at least one magnetic field source 38. In use, any displacement of the at least one magnetic field source 38, and thus the electric fence wire 14 connected to the first end 34 of the rod 32, relative to the housing 30, and in particular, relative to the magnetic field sensor 44, results in a signal that is proportional to the displacement of the wire 14.

[0049] In an embodiment, the magnetic field sensor 44 comprises a semiconductor device, such as a Hall-Effect sensor 44 capable of measuring magnetic field strength along multiple axes. As such, the mechanical configuration of the magnetic field sensor 44 is not critical. The sensitivity of a Hall-Effect sensor 44 is greater when the magnetic field of the magnetic field source 38 is displaced away from the mid-point of the magnetic field i.e. if the mid-point between the North and South poles of the magnetic field source 38 is not directly above the Hall-Effect sensor 44, which is different to a corresponding strain-based sensing device. The Hall-Effect sensor 44 operating instructions would include rate of change algorithms to ignore relatively slow environmental related effects as well as to detect mechanical failure or a targeted attempt to mechanically defeat the sensor by jamming the mechanical movement of the assembly.

[0050] An advantage of such a sensor 10 is that the sensor 10 will not be damaged if the mechanical movement of the wire 14 is significant. The displacement of the wire 14 is mechanically constrained in the above example but does not have to be. Because wire displacement can be in the order of tens of millimetres the requirement for extremely rigid fence structures is avoided.

[0051] In an embodiment, the wire tension sensor 10 further includes a control unit, also embedded within the PCB 46, in communication with the magnetic field sensor 44 to receive and process the signal from the magnetic field sensor 44, and to generate an alert signal if the signal exceeds or is outside predetermined thresholds. The wire tension sensor 10 further includes a communication module to communicate the alert signal. Advantageously, the wire tension sensing is independent of the electric fence circuit wiring. The communication module may comprise a wireless low power radio transmitter communication module to communicate data wirelessly to the remote location. In an embodiment, each tension sensor 10 has a unique identifier to enable the location of the alert signal to be determined.

[0052] The communication module can in addition communicate other parameters, such as magnetic field data, temperature data and a service alarm.

[0053] Alternatively, as shown in Figure 1, the control unit of each sensor 10 may be connected to a communications box or module 47, which is then arranged to communicate with the remote location.

[0054] In one version, as best shown in Figures 2 and 3, the housing 30 includes an elongate primary housing body 48 terminating in an enlarged flange 50, and a secondary housing body 52 comprising an elongate tube 54 having an inner securing flange 56 (which is best shown in Figure 2) that is arranged to be snugly accommodated within and fitted to the enlarged flange 50 of the primary housing body 48. A pair of securing and guiding arms 58 extends from an inner side of the securing flange 56 into the primary housing body 48, with an opposite, outer end of the elongate tube 54 defining an aperture 60 through which the elongate rod 32 can travel. The housing 30 may be designed and assembled so as to be waterproof.

[0055] In an embodiment, the elongate primary housing body 48 includes an outer wall 62, a first end of which defines the enlarged flange 50, and a second end of which includes an end wall 64 fitted with a connector 66. The connector 66 can be connected to a spring tensioner, the spring tensioner in turn being connected to a fence post 68 of the barrier system 10 or to an adjacent electric fence wire 14. The wire tension sensor 10 of the present invention may thus be mounted at the end or at any point along the wire span. If not mounted in the middle of the wire span, compensation would have to be incorporated for unequal wire expansion and contraction. The Hall Effect sensor 44 has an in-built temperature sensor enabling such functionality.

[0056] In an embodiment, the elongate primary housing body 48 further includes an inner wall arrangement 70 that defines a cavity or tube to accommodate the second end 40 of the rod 32 in use.

[0057] In an embodiment, the elongate rod 32 comprises an outer rod body 76 that is housed within the elongate tube 54 of the secondary housing body 52, an outer end of the elongate tube 54 including an inwardly protruding lip 78 to define the aperture 60 of the elongate tube 54, through which the elongate rod 32 can protrude from the housing 30.

[0058] The elongate rod 32 further comprises an inner rod body 80 that is housed within the elongate primary housing body 48, an inner end of the inner rod body 80 defining the second end 40 of the rod 32, which is arranged to accommodate or receive the at least one magnetic field source 38.

[0059] The elongate rod 32 further comprises a guiding flange 82 between the outer rod body 76 and the inner rod body 80, the guiding flange 82 being snugly and movably accommodated within the elongate tube 54 of the secondary housing body 52, as the electric fence wire 14, and thus the rod 32, moves relative to the housing 30.

[0060] In an embodiment a compression spring 84 is accommodated within the secondary housing body 52 of the housing 30 and is arranged around the outer rod body 76 of the elongate rod 32, as best shown in Figure 3, to enable the elongate rod 32 to move relative to the housing 30. A first end of the spring 84 acts against the guiding flange 82 of the elongate rod 32, and a second end of the spring 84 acts against the inwardly protruding lip 78 of the outer end of the outer rod body 76. The elongate rod 32 is thus biased inwardly and towards the elongate primary housing body 48 of the housing 30.

[0061] The tension sensor of the present invention addresses the shortcomings described above with reference to conventional taut-wire tension sensors. In particular, the tension sensor of the present invention is relatively less expensive to produce and is generally easier to manufacture; it has relatively lower power consumption; and is easier to install. Significantly, the sensor can be easily retrofitted to existing fences without modifying the fence structure.

[0062] The present invention thus discloses a wire tension sensor that addresses many of the problems associated with a strain gauge sensor.

Claims

CLAIMS1. A wire tension sensor for an electric fence perimeter or barrier system comprising at least one electric fence wire, the wire tension sensor comprising:a housing;an elongate rod accommodated within, and movable relative to, the housing, the rod having a first end that protrudes from the housing and that can be connected to an electric fence wire of the electric fence barrier system that is to be monitored;at least one magnetic field source embodied within or connected to a second end of the rod; anda magnetic field sensor, within the housing and proximate the second end of the rod, to monitor and measure the magnetic field of the at least one magnetic field source, so that any displacement of the at least one magnetic field source, and thus the electric fence wire connected to the first end of the rod, relative to the housing, and in particular, relative to the magnetic field sensor, results in a signal that is proportional to the displacement.

2. The wire tension sensor of claim 1 , wherein the wire tension sensor further includes:a control unit in communication with the magnetic field sensor to receive and process the signal from the magnetic field sensor, and to generate an alert signal if the signal exceeds or is outside predetermined thresholds; anda communication module to communicate the alert signal.

3. The wire tension sensor of either claim 1 or claim 2, wherein the at least one magnetic field source comprises either at least one permanent magnet or an excitable electromagnetic coil.

4. The wire tension sensor of claim 1, wherein the magnetic field sensor comprises a Hall-Effect sensor capable of measuring magnetic field strength along multiple axes, the magnetic field sensor being embedded within a PCB within the housing.

5. The wire tension sensor of claim 1, wherein the housing includes an elongate primary housing body terminating in an enlarged flange, and a secondary housing body comprising an elongate tube having an inner securing flange that is arranged to be snugly accommodated within and fitted to the enlarged flange of the primary housing body.

6. The wire tension sensor of claim 5, wherein a pair of securing and guiding arms extends from an inner side of the inner securing flange into the primary housing body, with an opposite, outer end of the elongate tube defining an aperture through which the elongate rod can travel.

7. The wire tension sensor of claim 6, wherein the elongate primary housing body includes an outer wall, a first end of which defines the enlarged flange, and a second end of which includes an end wall fitted with a connector.

8. The wire tension sensor of claim 7, wherein the connector is connectable to a spring tensioner, the spring tensioner in turn being connected to a fence post of the barrier system or to an adjacent electric fence wire.

9. The wire tension sensor of claim 6, wherein the elongate primary housing body further includes an inner wall arrangement that defines a cavity or tube to accommodate the second end of the rod in use.

10. The wire tension sensor of claim 6, wherein the elongate rod comprises:an outer rod body that is housed within the elongate tube of the secondary housing body, an outer end of the outer rod body including an inwardly protruding lip to define the aperture of the elongate tube through which the elongate rod can protrude from the housing;an inner rod body that is housed within the elongate primary housing body, an inner end of the inner rod body defining the second end of the rod, which is arranged to accommodate or receive the at least one magnetic field source; anda guiding flange between the outer rod body and the inner rod body, the guiding flange being snugly and movably accommodated within the elongate tube of the secondary housing body, as the electric fence wire, and thus the rod, moves relative to the housing.

11. The wire tension sensor of claim 10, wherein a compression spring is accommodated within the secondary housing body of the housing and is arranged around the outer rod body of the elongate rod to enable the elongate rod to move relative to the housing.

12. The wire tension sensor of claim 11, wherein a first end of the spring acts against the guiding flange of the elongate rod and a second end of the spring acts against the inwardly protruding lip of the outer end of the outer rod body.

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