AU735681B3 - Electric fence monitor - Google Patents

Description

ELECTRIC FENCE FAULT MONITOR FIELD OF THE INVENTION The present invention relates to an apparatus for monitoring the electrical characteristics of an electric fence in order to detect a fault condition of the fence.

BACKGROUND TO THE INVENTION Electric fences are used in a number of applications. Firstly they are widely used to restrict the movement of animals such as farm animals and feral animals. In such an application a common fence fault is a short circuit between an energised wire and ground. Such a fault causes an increase in current or alternatively a decrease in the electrical load (in Ohms) of the fence as calculated from the ratio of voltage to current associated with an energised wire.

There are rural properties using electric fences with very long wire runs. For example the inventor has observed wire runs of up to 16km in length. Since an electric fence is made up of a set of insulated wires hung above the ground, the load presented to a fence energiser connected to the fence includes a capacitive component with respect to earth of approximately nF per wire per kilometre. The load on the energiser presented by a well built and fault free fence is therefore mainly capacitive with the amount of associated capacitance increasing in proportion to the length of the wire runs used. As the output waveform of a typical electric fence energiser has a sharply increasing voltage over a small amount of time, the peak current to charge the capacitive load component associated with the fence may be large. Indeed, where long runs are included in the fence the current associated with the rapidly changing portion of the energiser voltage waveform may typically be of the order of 0.5 A per kilometre of live wire.

It has been observed that hitherto, electric fence fault detectors or monitors have falsely indicated a short-circuit fault in respect of correctly operating electric fences.

This problem is particularly noticeable where the fence in question includes long runs. Accordingly it is an object of the present invention to provide an apparatus that addresses this problem.

Apart from application in the field of agriculture, electric fences are also used to secure a perimeter. Intruders may attempt to defeat an electric fence by cutting it. Cutting electric fence wires may lead to either an open or short circuit on various parts of the fence circuit. At present electric fences used for security applications require a fence monitor to continuously monitor fence voltages in order to detect and warn of breakages in the fences.

An example prior art fence monitoring arrangement is shown in Figure 1. In that Figure a fence energiser 22 and a fence monitor 24 are housed in hut 26. A high voltage cable 28 is connected to energiser 22 and is run, on fence posts, around secured area 30 and back to the hut for connection to fence monitor 24. In the event that there is a breakage in cable 28 then the voltage monitored by monitor 24 will fall. In response to the fall in voltage monitor 24 will trigger an alarm to notify security personnel of a possible breakage in wire 28.

There are a number of disadvantages with prior art arrangements of the type described with reference to Figure 1. Firstly it is necessary to run wire 28 not only from the hut to the perimeter to be secured but also back from the perimeter to the hut in order that it may be connected to the monitor.

Such wiring requires special care in routing and is an added expense.

Furthermore, slowly changing conditions such as grass, shrubs, cobwebs or dust may cause a drain on the fence thereby triggering a false alarm. Another problem is that due to induction between the outgoing and incoming cables the existing systems are also prone to not showing an alarm when fence wires are cut. For example in the event that cable 28 was cut at region 32 as shown, then cable section 28A would remain energised and might induce a voltage in floating cable section 28B sufficiently high to prevent monitor 24 detecting a fault condition.

Where a security fence installation consists of several sections of electrified fence it may be hard to identify which sector has a short circuit fault as load variation due to the short circuit reduces the voltage on all sectors.

Consequently an alarm may be registered on all sectors when this occurs.

It is a further object of the present invention to provide an apparatus which addresses at least some of the above-identified problems.

SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided an apparatus for monitoring the load presented by an electric fence, wherein the apparatus is configured to operatively determine substantially the real load component presented by said fence.

Preferably the apparatus generates a warning indication in the event that the real load (in Ohms) is less than a predetermined value.

In one embodiment the apparatus is arranged to determine the peak voltage between an electrified wire of the fence and a reference potential such as ground, and to determine the current through said wire at the time of the peak voltage and to calculate real load presented by the fence as the ratio of said peak voltage to current.

The apparatus may include means for recording voltage and current readings associated with an electric fence during the period of an electrifying pulse and for comparing said recorded voltage and current readings with subsequent readings in order to detect a change in the electrical characteristics of the fence.

According to a further aspect of the present invention there is provided an apparatus for monitoring voltages and currents associated with an electric fence, the apparatus including: a microprocessor operatively executing a software product containing instructions to calculate the real component of a load presented by said fence.

Preferably said instructions calculate the ratio of the peak voltage during an energisation period of the fence to current through the fence at substantially the time of occurrence of said peak voltage.

According to a further aspect of the present invention there is provided a method for determining a fault condition for an electric fence, the method including the steps of: calculating the real component of a load presented by the electric fence: comparing the calculated real component to a predetermined value; generating a fault indication in the event that the calculated real load is less than a predetermined value.

Preferably the step of calculating the real component of a load presented by the electric fence includes the step of determining the peak voltage on the fence during an energisation pulse of the fence and the current through the fence at the time of said peak voltage.

BRIEF DESCRIPTION OF FIGURES In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate typical preferred embodiments of the invention and wherein: Figure 1 is a schematic diagram of a prior art arrangement for energising and monitoring an electrified security fence.

Figure 2 is a schematic diagram of an apparatus according to the present invention.

Figure 3 is a schematic diagram of two areas each surrounded by an electric fence wire monitored by electric fence monitors according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference to Figure 2 there is depicted a block diagram of an electric fence monitoring apparatus according to the present invention.

The apparatus of Figure 2 includes a current sensing device such as a current transformer or magnetic pick-up coil 2 and a voltage reduction circuit in the form of a capacitively isolated resistive divider network 4. The apparatus is battery powered and includes a power supply regulator 15. In use, pickup coil 2 and divider network 4 are coupled to a wire of an electric fence 14 as shown in Figure 2. Pickup coil 2 and network 4 sense current and voltage signals on wire 14. The fence wire presents a load to an energiser (not shown) connected across the wire. The real component of the load is schematically represented by resistor 1. Signal conditioning circuits 5, 6, 7 and 8 filter current and voltage signals before presenting them to analog to K iz gital converters 16 and 17. The converted digital representation of the 'ai nitudes of the voltage and current is processed by computational module 9 including processing means in the form of microprocessor 18 in communication with memory means in the form of EPROM 19.

Microprocessor 18 executes a program stored in EPROM 19.

(The arrangement of the apparatus of Figure 2 may be varied. For example, in a prototype version of the apparatus the inventors have used a Microchips PIC Device 16C715 IC. This device provides an 8 bit microprocessor, 8 bit Analog to Digital converters and program memory on a single chip and so is a convenient means for implementing the present invention.) lo The program stored in the EPROM includes instructions for the processor to measure the peak voltage and currents associated with wire 1 during the application of energisation pulses to the fence. The microprocessor drives an LCD display 10 to show the peak voltage and currents.

Instructions are also included for determining the time to the peak voltage and the magnitude of the current at the time of the peak voltage. If the time to the peak voltage is greater than a predetermined time period, for example approximately 50 micro-seconds, then the load is calculated from the peak voltage and the current at the time of the peak voltage. The result of such a calculation is an indication of the real component of the load, excluding the reactive component. The program contains instructions for making the calculation according to Ohms law, R=V/I, where R is the real component of the load in Ohms, V is the peak voltage in volts and I is current in amperes at the time of peak voltage. If the pulse is less than the predetermined time period 50 micro secs) then the load is calculated using the peak voltage and the overall peak current being the peak current detected during the pulse.

For short pulses the ratio of peak voltage and overall peak current is a good approximation of the real component of the load (in Ohms).

Alternative instructions for calculating the real component of the load could also be programmed. For example the program may contain instructions for detecting the peak current and peak voltage during an energisation pulse and the phase difference between the two in order to calculate the complex load presented in polar form. The program may further contain instructions converting the complex load to cartesian form in order to

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obtain the real component of the load. Other techniques for calculating the real component of the load may also be used as an alternative and coded into the program operatively executed by microprocessor 18.

By using the real load as calculated above the apparatus is able to disregard large reactive currents flowing in the wire caused by the capacitive load of long fences. Accordingly the false short circuit conditions associated with the long wire runs of some fences are avoided.

In another example of the invention analog circuitry may be used to gate the current circuit off until the peak of the voltage has been detected thus passing only the real component of the current to the analog to digital converter. The fault monitor may also include voltage and current polarity detection circuits to determine the direction of travel of the power in the electric fence.

Where the monitor of Figure 2 is to be used in relation to security fences the program executed by microprocessor 18 includes instructions to measure and store into memory the normal running characteristics of the fence voltage and current waveforms. Such running characteristics include the peak voltage to be recorded at a monitoring point on the fence during an energisation pulse and the current at the time of the peak voltage. The normal running characteristics are produced by averaging many measurements of the fence voltage and current over several energisation pulses of the electric fence. The recorded measurements are then continuously compared to subsequent fence voltage and current readings. If any of the running characteristics detected change by more than a set percentage, then microprocessor 18 sends a command to switch a relay thereby operating a warning alarm alerting security personnel to a possible security breach. The relevant percentage is entered into the memory of the device during installation.

As the apparatus of Figure 2 measures V and I over time it is able to determine the real and reactive components of the load. As previously explained, the capacitance is in direct proportion to the amount of wire connected to the fence, if some of the wire is removed from the circuit, for example by intruders, the monitor will be able to detect a reduction in the Sreactive load. The level of change required to activate main "breach" relay a., 12A can be set by the installer by programming it into a non-volatile memory of the device. A device according to the invention may share the same connection to the fence as the energiser.

Figure 3 depicts two monitors according to the present invention each used to monitor an electric fence in respect of first and second areas. It will be noted that the number of high voltage cable runs to each fenced area may be halved, relative to the arrangement of Figure 1. Furthermore, the arrangement of Figure 3 does not suffer from the problem of induced voltages that was previously described because there is no second return set of wires (28B of Figure 1).

The program executed by microprocessor 18 includes instructions to close main relay 12A when the real component of the load falls below a predetermined level for a predetermined amount of time. An indicator LED-11 is lit to indicate a fault indication in the event that the calculated real load is less than the predetermined value.

The predetermined load level and predetermined amount of time are stored in non-volatile memory accessible to microprocessor 18.

A fence monitored by the apparatus of Figure 2 may also be affected by grass, shrubs, cobwebs or dust gathering against or on energised fence wires. In that case it has been observed that the fence voltage may be reduced so that it is no longer an effective barrier to intruders. A second predetermined load level may be stored in memory of the device of Figure 2.

The apparatus is programmed to close secondary relay 12B to raise an alarm in the event that the load is less than the second predetermined load level.

Accordingly an operator of the system is alerted to the fact that the electric fence monitored requires maintenance to ensure that its operating voltage is returned to its optimal level and to also reduce the likelihood of false alarms.

Adjustable parameters that may be stored in memory of the device of Figure 2 include the short circuit detection load level in Ohms and the time delay from detection of this condition in seconds until the relay is switched.

The maintenance load level in Ohms and the delay from the detection of this condition until a relay is switched, the percentage change in reactive load and the delay from the detection of this condition until a relay is switched, the ercentage change from the averaged normal running characteristics (real load, reactive load, time to peak voltage etc) allowed until a relay is switched may also all be stored.

The table below presents the parameters as stored in a prototype of the monitoring apparatus that the inventor has constructed.

Option Name Value Range Default Description 1 Breach Value 0-9.9kOhm 0.5k Breach limit in kilo Ohms 2 Breach Delay 0-99 sec 3 Delay time elapsed until critical relay closure 3 Envir Value 0-9.9 kOhm 2.0k Environmental load limit in kilo Ohms. Must be greater than Breach Value to operate 4 Envir Delay 0-99 sec 15 Delay time elapsed until non-critical relay closure Signature 0-99% 0 Percentage change of the Change Value energiser pulse shape 6 Signature 0-99 sec 0 Delay from pulse shape Change Delay change until critical relay closure 7 Energiser Fail 0-9.9kV 0.5 Extremely low voltage on Value the fence in kilo volts 8 Energiser Fail 0-99 sec 3 Delay time elapsed until Delay __critical relay closure 9 System Reset 0-99 sec 60 The time interval, during Delay which no pulses are measured, that must elapse before the system is reset System 0-99 sec 15 Time in standby before Standby Time changing to armed 11 Critical Alarm 0-99 1 Whether a critical alarm Reset can be cleared without having to reset the monitoring apparatus (1 means yes, 0 means no) The monitor may include an isolating resistor in series with the flow of power from the energiser, through the monitor to the fence. This isolating resistor stops a short circuit on one sector of the fence from bringing the voltage down on other sectors by limiting the load on the energiser. Since the to monitor uses a comparison of load in Ohms rather then peak. voltage to determine an alarm condition, if the energiser voltage falls the monitor will not necessarily show an alarm thus making it easier to determine the actual sector which has a short circuit fault.

0:C> 9 In one embodiment a fence monitor according to the present invention is combined with the electric fence energiser in a single housing or on a PCB common with components of the energiser.

It will of course be realised that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as set forth in the following claims.

Claims (3)

1. An apparatus for monitoring the load presented by an electric fence, including a micro-processor operatively executing a software product including instructions to calculate substantially the real load component presented by said fence, said apparatus including a current sensor and a voltage sensor wherein by during an energisation period of the fence the time to the peak voltage the peak voltage the magnitude of the current at the time of the said peak voltage (Itp), and the peak current (Ip) are determined and wherein the micro-processor calculates substantially the real load component presented by said fence where the time to the peak voltage (tp) is greater than a predetermined time period according to the formula: Vp/Itp and where the time to the peak voltage (tp) is less than a predetermined time period according to the formula: VIlp.

2. An apparatus according to claim 1 wheirein the predetermined time period is 50 microseconds.

3. An apparatus according to either claim 1 or claim 2 wherein said apparatus activates an alarm where the calculated real load component presented by said fence differs from a predetermined load level. Dated THIS 27 TH day of April 2001 PAKTON TECHNOLOGIES PTY LTD by its attorneys PIZZEYS PATENT AND TRADE MARK ATTORNEYS 0~c1