WO2006063367A1

WO2006063367A1 - Noise reduction on electric fence

Info

Publication number: WO2006063367A1
Application number: PCT/ZA2005/000179
Authority: WO
Inventors: Leslie Sean Hurly
Original assignee: Leslie Sean Hurly
Priority date: 2004-12-07
Filing date: 2005-12-06
Publication date: 2006-06-15
Also published as: ZA200705865B

Abstract

An energiser which includes an apparatus for generating a pulse train which includes a plurality of pairs of pulses at predetermined time intervals, each pair of pulses including a first pulse of a first amplitude and a first polarity and a second pulse, which is produced substantially simultaneously with the first pulse, and which is of a second amplitude which is substantially equal to the first amplitude and a second polarity which is opposite to the first polarity.

Description

NO'ISE REDUCTION ON ELECTRIC FENCE

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to electric fences, used for example for security purposes or for game or animal containment, and more particularly is concerned with electromagnetic interference generated by such fencing.

[0002] An electric fence typically includes one or more conductive wires which extend along a defined boundary and which are connected to an energiser which applies a succession of relatively sharp and high voltage pulses, of controlled energy, to the conductor.

[0003] In one respect an energiser of the aforementioned type resembles a low frequency radio transmitter which is coupled to a long aerial (the conductor). A possible disadvantage of the type of arrangement is that some of the energy

delivered by the energiser to the conductor, which inter alia is a function of impedance matching between the conductor and the energiser, may be radiated into space as electromagnetic noise, referred to as electromagnetic interference (EMI) or radio frequency interference (RFI).

[0004] To some extent the noise problem can be addressed by using suitable filtering techniques. A filter used for this purpose must be capable of handling the power and voltage levels involved and hence tends to be expensive and may absorb a substantial proportion of useful energy which otherwise could go to the conductor. [0005] US Patent No. £,767, 592 relates to the design of an energiser circuit in order to reduce the harmonic content of the generated waveform and hence the resulting EMI.

[0006] Noise problems nonetheless persist particularly on rural telephone networks in which the telephone lines can be long and close to electric fencing. An energiser can have a low frequency component due to the prescribed pulse repetition rate, typically 1 pulse per second, and can also have many high frequency components because of the rapid discharge of energy storage devices used in the energiser.

[0007] There are various ways in which an energiser can be used. In one technique an output terminal of an energiser is connected to earth while a second terminal is connected to a live wire or conductor which is used as a deterrent. The condition of the live wire, or of an earth wire which is connected to the earth terminal, may be monitored for security purposes or for energiser control.

[0008] In a different approach a fence typically has a number of live and earth wires - an arrangement often found in game farming environments. In the situation in which only live strands are attached to an existing structure by means of suitable brackets

the magnitude of a shock which is experienced by an animal or person touching a live strand, is typically determined by the energiser characteristics and the resistance of the earth. Installers are reluctant to use one or more additional earth wires on the brackets because of the cost and perceived waste.

SUMMARY OF INVENTION

[0009] The invention provides, in the first instance, an energiser which includes an apparatus for generating a pulse train which includes a plurality of pairs of pulses at predetermined time intervals, each pair of pulses including a first pulse of a first amplitude and a first polarity and a second pulse, which is produced substantially simultaneously with the first pulse, and which is of a second amplitude which is substantially equal to the first amplitude and a second polarity which is opposite to the first polarity.

[0010] The energiser may be of any suitable form. For example it is possible to make use of two individually controllable and synchronised pulse generators which respectively produce a first pulse train comprising the first pulses and a second pulse train comprising the second pulses. In this respect it can be said that in general a desired pulse train can be obtained by synchronising similar energisers which run at the same frequency and by combining their outputs.

[0011] In a preferred form of the invention the energiser includes a transformer with a primary winding and a secondary winding which includes first and second output terminals and a centre output terminal, the first pulses being produced across the first output terminal and centre output terminal and the second pulse being produced across the centre output terminal and the second output terminal.

[0012] The invention extends, in addition, to an electric fence installation which includes at least first and second elongate conductors and an energiser of the aforementioned kind connected to the conductors so that the first pulses are applied to the first conductor and the second pulses are applied to the second conductor.

[0013] The installation may be such that the first and second conductors are substantially parallel to each other and are fairly close to each other. Preferably the spacing between the conductors is less than 6 inches (150mm). [0014] In the installation the centre output terminal (with the second type of energiser referred to) may be connected to ground.

[0015] The invention further extends to a method of reducing electromagnetic noise in an electric fence which includes at least first and second elongate electrical conductors, the method including the step of applying a first series of first pulses to the first conductor and a second series of second pulses to the second conductor wherein each first pulse has a first amplitude and is of a first polarity and each second pulse is produced substantially simultaneously with a respective first pulse and is of a second amplitude which is substantially equal to the first amplitude and which is of a second polarity which is opposite to the first polarity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention is further described by way of example with reference to the accompanying drawings in which:

Figure 1 schematically illustrates an electric fence installation according to the invention; and

Figures 2 and 3 are graphs of electromagnetic noise generated, respectively, in a conventional electric fence installation and in a fence installation in accordance with the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0017] Figure 1 of the accompanying drawings illustrates an electric fence installation 10 according to the invention which includes a fence 12 and an energiser [0018] The components illustrated in Figure 1 are used to explain the concept of the invention. It will be understood by those skilled in the art that additional mechanical and electrical components would be required for a complete electric fence installation. Such components are however known and therefore are not further described herein.

[0019] The fence 12 includes a plurality of standards or posts 16 which are erected in the ground 18 at regularly spaced intervals. Conventional wire netting 20 may be attached to straining wires, not shown, supported by the posts.

[0020] Elongate electrical conductors 22, which are used for electrifying the fence, are directly mounted to the posts or to stand-off brackets, not shown, so that they are spaced from the wire netting 20.

[0021] The conductors 22 are installed in closely spaced pairs of conductors designated 22A and 22B respectively. In Figure 1 only one pair of conductors is shown but it is to be understood that this is merely for ease of illustration and that the principles described herein have equal applicability to a plurality of pairs of conductors all of which can be energised using the energiser 14 or one or more other energisers.

[0022] In the Figure 1 arrangement the conductors 22A and 22B run along a predetermined length of the fence 12 and are then returned, at different heights from the output paths of the conductors, to a location which is near the energiser 14. Use may be made of a termination resistor 24 at this location. This aspect is also known in the art. [0023] The eπergiser .14 includes a transformer 30 with suitable control circuitry 32. The manner in which this type of transformer is controlled is also known in the art and thus the control circuit 32 is not described in detail. The function of the control circuit is inter alia to control the operation of the transformer 30 and to monitor the condition of the conductors and to provide information which can be used for security and other applications.

[0024] The transformer 30 includes a primary winding 34 which is connected to a suitable electrical source, not shown. The transformer further includes a secondary winding 36 which has a first output terminal 38, and a second output terminal 40. The secondary winding has a centre-tap output terminal 42. The arrangement is such that, as is known to those skilled in the art, the number of windings between the terminals 38 and 42 is the same as the number of windings between the terminals 42 and 40.

[0025] In one form of operation of the energiser the terminal 42 is connected to ground 46. The first terminal 38 is connected to the conductor 22A and the second terminal 40 is connected to the conductor 22B.

[0026] The operation of the transformer 30, controlled by the circuit 32, is such that pairs of pulses are simultaneously and continuously generated by the secondary winding 36 as the power supply to the transformer is switched. The full output voltage of the secondary winding appears across the terminals 38 and 40. However as the terminal 42 is connected to ground the voltage at the terminal 38 is of opposite polarity to the voltage at the terminal 40. Also due to the fact that the terminal 42 is connected to a centre-tap on the secondary winding the voltage pulses appearing across the terminals 38 and 42 on the one hand and 42 and 40 on the other hand have the same magnitude. Thus, in essence, the voltage pulses are complementary with respect to the terminal 42 and if care is taken in the design and operation of the transformer 30 the same impedance characteristics appear across the terminal pairs.

[0027] A practical result of operating the energiser 14 in the manner described is that the conductors 22A and 22B are energised simultaneously with pulses of equal magnitude but of opposing polarity. Electromagnetic fields which are set up in each conductor are, as is known, dependent on the magnitude of the current and the direction of the current flowing through the conductor. There is therefore a natural tendency for the electromagnetic noise which is generated by one conductor to be cancelled by the electromagnetic noise which is generated by the other conductor.

[0028] Figures 2 and 3 contain curves which illustrate the degree of noise reduction achieved by the arrangement of the invention. To obtain the curve shown in Figure 2 an energiser of the type shown in Figure 1 was used but the centre terminal 42 was

left open circuited i.e. it was not connected to earth. The terminal 38 was instead connected to earth and the terminal 40 was connected to a live wire of the fence. The live wires of the fence were all connected in series. The upper and lower curves respectively depict peak and average values of the resulting electric field strength measured at about 2m from the fence.

[0029] The same energiser was then used, running at the same energy level but connected in the configuration shown in Figure 1. Peak and average field measurements were taken at the same points as what were used for the Figure 2 curves, and these are reflected in the upper and lower curves respectively, in Figure 3. Bearing in mind that the Y axes in Figure 2 and in Figure 3 are expressed in decibels it can be seen that the energiser, when operated in accordance with the Figure 1 arrangement, achieves a considerable amount of noise reduction.

[0030] Another aspect of the invention is that a shock is provided to an animal or person contacting the electric fence, irrespective of the earth resistance. If a single live wire only is contacted then the shock will be at a reduced level and will be a function of the earth resistance. However if two of the wires are simultaneously touched then the shock will be at a greater level. This is achieved without the use of a dedicated earth wire and there is no perceived material wastage.

Claims

1. An energiser which includes an apparatus for generating a pulse train which includes a plurality of pairs of pulses at predetermined time intervals, each pair of pulses including a first pulse of a first amplitude and a first polarity and a second pulse, which is produced substantially simultaneously with the first pulse, and which is of a second amplitude which is substantially equal to the first amplitude and a second polarity which is opposite to the first polarity.

2. An energiser according to claim 1 which includes two individually controllable and synchronised pulse generators which respectively produce a first pulse train comprising the first pulses and a second pulse train comprising the second pulses.

3. An energiser according to claim 1 which includes a transformer with a primary winding and a secondary winding which includes first and second output terminals and a centre output terminal, the first pulses being produced across the first output terminal and centre output terminal and the second pulse being produced across the centre output terminal and the second output terminal.

4. An electric fence installation which includes at least first and second elongate conductors and an energiser according to any one of claims 1 to 3 connected to the conductors so that the first pulses are applied to the first conductor and the second pulses are applied to the second conductor.

5. An installation according to claim 4 wherein the first and second conductors are substantially parallel to each other.

6. An installation according to claim 5 wherein the conductors are not more than 150mm apart.

7. An installation according to claim 4, 5 or 6 wherein the energiser is according to claim 3 and wherein the centre output terminal is connected to ground.

8. A method of reducing electromagnetic noise in an electric fence which includes at least first and second elongate electrical conductors, the method including the step of applying a first series of first pulses to the first conductor and a second series of second pulses to the second conductor wherein each first pulse has a first amplitude and is of a first polarity and each second pulse is produced substantially simultaneously with a respective first pulse and is of a second amplitude which is substantially equal to the first amplitude and which is of a second polarity which is opposite to the first polarity.

9. A method according to claim 8 wherein the first pulses are applied between the first conductor and ground and the second pulses are applied between the second conductor and ground.