AU2005100856A4 - Safety Operation of a High Power Electric Fence Energiser - Google Patents

Description

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AUSTRALIA

Patents Act 1990 INNOVATION PATENT SPECIFICATION Name of Applicant: Pakton Developments Pty Ltd Actual Inventor: Paul Thompson Address for Service: Pakton Developments Pty Ltd PO 1068 Burpengary Q4505 16 Ferrier Rd Narrangba, Q4504 Invention Title: Safety Operation of a High Power Electric Fence Energiser Details of Associated provisional Application: 2004905877 filed 13 October 2004 This invention is described in the following statement: 'n SAFETY OPERATION OF A HIGH POWER ELECTRIC FENCE

SENERGISER

O FIELD OF THE INVENTION The present invention relates to electric fences and, in particular, to energisers for electric fences. Although the invention will be described with In particular reference to energisers that are used in farm fencing applications, it oO 00 will be appreciated that the invention may be employed with energisers that are used in other fencing applications.

In c BACKGROUND TO THE INVENTION Electric fences are widely used on farms to restrict the movement of both farm and feral animals. Such fences normally include a number of uninsulated wire conductors strung on supporting posts from which they are insulated. The conductors are coupled to an energiser that periodically outputs a high voltage pulse to energise the conductors so that an animal will receive a small electric shock if it contacts the energised conductors. Electric fences are also used as a perimeter security system for industrial premises and detention centres.

Energisers that are used to energise electric fences may be mains or battery powered. Most modern energisers include a discharge capacitor, a capacitor charging circuit for charging the capacitor to a high potential (e.g.

several hundred volts), and a capacitor discharging circuit for discharging the capacitor to produce a very high potential output pulse several thousand volts) that is used to energise the fence conductors.

The capacitor charging circuit is typically a voltage converter circuit that converts the relatively low supply voltage powering the energiser to the high voltage required to charge the capacitor.

The capacitor discharging circuit typically includes a semiconductor switch and a step-up output transformer that are both coupled to the capacitor such that the capacitor is able to be discharged through the transformer's primary winding by closing the switch to thereby produce a high voltage pulse across the transformer's secondary winding that can be used to energise the

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C fence conductors.

The semiconductor switch is usually a SCR, these devices are O favoured because of their ability to withstand the high voltage while off and S 5 conduct the high peak currents while discharging the capacitor. The overall control of the energisers charge and discharge cycle can be performed by 0 logic and timers or by a micro-controller.

0O The magnitude and duration of the output pulse are determined by the Opeak charge voltage on the capacitor, the size of this capacitor, the turns ratio S io of the step up transformer and to a lesser extent any filtering or over voltage Slimiting components in the primary or secondary circuits.

Energisers come in many sizes from very small to larger high powered units. Energisers may be rated in stored joules, output joules or by the number of kilometres of fence wire the manufacturer claims that they can effectively energise. For a fixed capacitor size and charge voltage the output voltage measured with respect to a load will fall as the load resistance falls. An electrical load is said to increase (get heavier) when the load resistance measured in ohms is reduced. Electric fences may be loaded by grass, dust or even by the capacitance of the fence wires themselves with respect to earth. It is desirable that the output voltage remains high even when the fence is loaded. So in recent years there has been a tendency for manufacturers to make larger and larger energisers.

The magnitude and frequency of the electric pulse is restricted by safety standards such as IEC60335.2.76. The limitation is specifically intended to ensure that the shock received from the energiser (and hence the fence) is safe for humans. The standard therefore describes tests using the theoretical human body resistance (load) of 500 Ohms. Some standards limit output pulse energy while others limit the voltage or current in the test load.

If an energiser with a fixed charge voltage is designed to deliver a safe pulse at 500 Ohms it's output voltage will fall away as the load resistance is reduced below 500 Ohms, even though the standard does not require it to do so, and it is more desirable that the voltage remain high, that is, have a flat voltage load curve.

O PRIOR ART In order to build an energiser with more desirable load characteristics O larger units now use feedback, either continuously varying the peak capacitor N 5 charge voltage or using a step function to switch more capacitors into circuit as the load resistance is reduced. That is they compare the actual output IND voltage with a reference and increase the pulse energy if the voltage falls 00 below a predetermined level. The level of pulse output energy is limited under Oone safety standard to less than 5 Joules into 500 Ohms. The largest o energisers known to the inventor are rated at 75 Joules stored energy and Sthus are capable of generating pulses which exceed the safe level by at least times. The safety of these units relies on the feedback circuit and the logic in the controlling circuit. Such that they only produce full power when the load on the output is much lower then the 500 Ohms which approximates the resistance of the human body and at which level the standard limits the pulse energy. Since the energiser has a known load performance measuring the output pulse peak voltage and keeping it below a fixed level is a simple way of complying with the standard.

In order to use feedback the actual output pulse magnitude of the energiser must be measured and relayed back to the control circuit. For safe operation there must be a galvanic isolation between the input supply and the output terminals. This isolation barrier is usually between the primary and secondary windings of the output transformer. So a measurement of the output voltage must be passed back across the barrier or deduced from the behaviour of the primary circuit. Several forms of measurement have been used, including extra windings on the output transformer and optically isolated measurement of the actual output voltage and current.

The safety standards specifically describe tests to ensure that failure of any one component in the energiser will not lead to an unsafe output. In practice this testing is limited to creating open circuits or short circuits around any one component at a time. However, in the inventor's experience, it is possible for electronic components to fail gradually over time. For example, it is known that the current transfer ratio of opto-isolators reduces gradually over time. If this occurs in the feedback circuit of a prior art energiser it may cause it to deliver more power than is actually required and lead to an unsafe output,

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Oi.e. the energiser may perceive the voltage on the output to be lower than a predetermined limit when in fact it is not.

O It is an object of the present invention to provide an electric fence N 5 energiser that overcomes, or at least ameliorates, one or more of the deficiencies of the prior art electric fence energisers mentioned above, or that IND provides the consumer with a useful or commercial choice.

tn 00 OSUMMARY OF THE INVENTION S io According to a first aspect of the present invention there is provided an Selectric fence energiser for energising the conductors of an electric fence, the energiser including a charging circuit capable of charging the main discharge capacitor to a variable voltage level, a discharge capacitor, and a capacitor discharging circuit for discharging the capacitor to energise the fence conductors, two or more independent feedback circuits for discerning the actual output pulse magnitude, a micro controller programmed with an algorithm to compare the results of the independent measurements of the output and control the pulse generator circuit power, by varying the charge voltage, so that output pulse is kept below the required safe level even when one of the feedback devices fails or is degraded.

To ensure an energiser using feedback control to increase the output pulse energy as load is increased, remains safe, even with gradual or partial failure in the feedback circuit, the invention uses a scheme of using two or more completely independent forms of feedback. These independent feedback circuits do not, as far as is possible, contain any common components or even rely on the same reference. If one of the feedback circuits is a conventional optically isolated measurement of the peak output voltage or current the second could make a determination of the load on the output (and hence calculate the output pulse magnitude) via the rate of the discharge capacitor voltage drop.

Alternatively, the second or third method may use an extra winding on the output transformer to produce a ratio of the output voltage for measurement by the micro controller.

The algorithm in the micro controller is written in such a way that it will only

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Oincrease power to the pulse generator if all feedback methods agree on the measured output pulse level, within a pre-determined allowable error range.

O If, over time, one of the feedback circuits is damaged or degraded, the micro N 5 controller will detect the disagreement between the measurements provided by the independent feedback circuits. The micro controller will then keep the DNO pulse energy within the safe level and may also show an error to indicate the 00 unit requires maintenance. It should be understood that while varying the Ovoltage on the discharge capacitor is one way of adjusting the pulse energy, io another is to switch more capacitors in or out of circuit.

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 an apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1, an electric fence energiser 20 according to an embodiment of the present invention includes a charging circuit 1, a discharge capacitor 2 and a discharge circuit 11. The charging circuit charges the main capacitor 2 to a voltage as determined by the micro controller 3 which measures the capacitors voltage using the resistive divider 4 and an internal analogue to digital converter. The micro controller 3 fires the main SCR 5 via the gate driver circuit 6 to initiate the discharge of the main capacitor 2 into the primary of transformer 7. The actual output pulse magnitude and perceived load on the output terminals 8 and hence the fence conductors is measured and relayed back to the micro controller via multiple independent paths.

These paths are, the resistive divider 4 which allows the rate of discharge of the main capacitor to be measured, the opto-isolator 9 which relays a voltage proportional to the voltage on the output terminals 8 back to the micro controller 3, and a second opto-isolator 10 which relays a voltage proportional to the output circuit current back to the micro controller 3.

n The actual output pulse magnitude is compared to that of a known safe pulse level (voltage or current) for a 500R load, if the actual pulse magnitude is below this safe level then the stored energy in the discharge capacitor 2 is O increased by the micro controller via the charging circuit to which increases N 5 the output pulse magnitude. This is a classic feedback loop.

Since the load on the secondary is reflected by the output transformer IND back to the primary the rate of discharge of the capacitor is proportional to the

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o00 load. The actual discharge rate for the critical 500 ohm load level can be Sdetermined and saved for later comparison. If the rate of discharge is below f 10o the 500 Ohm rate then the logic in the micro-controller will keep the output Spulse power below a predetermined safe level.

Claims (7)

1. An electric fence energiser for energising the conductors c of an electric fence, the energiser including a discharge capacitor, a O capacitor charging circuit for charging the capacitor, and a discharging circuit for discharging the capacitor through the primary winding of a step up transformer to energise the fence conductors, the energiser INO being characterised by a means of controlling the output pulse power, In o00 two or more circuits for measuring the actual output pulse magnitude or Sload on the output, and a logic circuit to compare the independent Smeasurements and hold the output power below a safe level if any of Sthe measurements indicate that the energiser may be delivering an unsafe output pulse.

2. The energiser of claim 1, wherein one of the circuits measures the actual output pulse magnitude while another measures the load on the output and the logic circuit holds the output power at a safe level if either the voltage is above a predetermined threshold or the measured load resistance is above a predetermined threshold.

3. The energiser of claim 1, wherein the means of controlling the output pulse power level is to vary the stored energy in the discharge capacitor or capacitors.

4. The energiser of claim 1, wherein the logic circuit includes analogue to digital converters and a microprocessor for processing the outputs of the analogue to digital converters.

The energiser of claim 1, wherein if the comparison of the measurements from the independent measuring circuits does not substantially agree then an alert is output and the energiser is stopped.

6. The energiser of claim 1, wherein if the logic circuit allows for a small amount of error without changing the behaviour of the energiser.

7. An electric fence energiser substantially as herein described with reference to Figure 1 of the drawings. 0 DATED this 11th day of October 2005 c, Pakton Developments Pty Ltd 0 By PaulThompson IN 00 0'