AU2003252846B2 - An Electric Fence Energiser with Overload Detection - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant(s) Actual Inventor(s) Address for Service: Invention Title: Pakton Developments Pty Ltd Paul Thompson CULLEN CO Patent Trade Mark Attorneys, 239 George Street Brisbane Qld 4000 Australian An Electric Fence Energiser with Overload Detection Details of Associated Provisional Application 2002951931 filed 9 October 2002 The following statement is a full description of this invention, including the best method of performing it, known to us: 2 Field of the Invention The present invention relates generally to electric fences and, in particular, to energisers for electric fences.

Although the invention will be described with particular reference to energisers that are used in farm fencing applications, it will be appreciated that the invention may be employed with energisers that are used in other fencing applications.

Brief Discussion of the Prior Art Electric fences are widely used on farms to restrict the movement of both farm and feral animals. Such fences normally include a plurality of electrically insulated posts between which one or more uninsulated wire conductors are strung.

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.

Energisers that are used to energise electric fences in farming applications are usually battery or mains powered. Most modem energisers include a discharge capacitor, a capacitor charging circuit for charging the capacitor to a high potential 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 fence conductors.

The uninsulated conductors of an electric fence are susceptible to being short circuited. When a short circuit does occur the voltage at the output terminal of the energiser drops and the energiser becomes overloaded with the result that the fence becomes ineffective.

Most energisers have some sort of overload detecting circuit for assisting users to readily determine whether an overload condition is present. Probably the most primitive overload detecting circuits consists of a neon lamp connected across the output of the energiser. The lamp remains illuminated while the output voltage of the energiser remains above a predefined threshold level, and turns off when the voltage falls below this level to thereby indicate that the energiser is overloaded.

Other types of overload detecting circuits employ a comparator to compare the output voltage or current levels of the energiser with predefined threshold levels. If the output current exceeds or the output voltage falls below the appropriate threshold level then a visual alert is output via a liquid crystal display or light emitting diode to alert the user to the existence of the overload condition.

Although electric fence energisers which employ overload detecting circuits of the type described in the preceding two paragraphs perform quite satisfactorily they nevertheless suffer from some significant deficiencies. One such deficiency is that since they are directly coupled to the high voltage output of the energiser they must be rated to withstand the high voltages that are generated at the output. This deficiency can significantly increase the cost of energisers which employ overload detecting circuits of the type that compare the voltage or current levels at the energiser output with predefined threshold levels since these types of circuits are particularly expensive to implement in energisers due to the high degree of voltage isolation required between the low voltage circuit and the high voltage output of the energiser.

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

Other objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein, by way of illustration and example, various embodiments of the present invention are disclosed.

Summary of the Invention According to a first aspect of the present invention there is provided an electric fence energiser for energising the conductors of an electric fence, the energiser including an overload detection circuit for detecting whether the energiser is overloaded, a discharge capacitor, and a capacitor discharging circuit for discharging the capacitor to energise the fence conductors, wherein the discharging circuit includes an output transformer having a primary winding which the capacitor is discharged through, the energiser being characterised in that the detection circuit determines whether the capacitor is discharged at a rate which exceeds a threshold rate and outputs an overload alert if it is determined that the capacitor is discharged at a rate which exceeds the threshold rate.

By using the discharge rate of the capacitor to determine the presence or absence of an overload condition the overload detection circuit does not have to be rated to withstand the high output voltages produced by the energiser since the detection circuit can determine the discharge rate of the capacitor while being electrically isolated from the high voltage output of the energiser.

Preferably, the overload detection circuit includes a scaling circuit for scaling the voltage across the capacitor, a filter for filtering the output of the scaling circuit, an analogue to digital converter for digitising the output of the filter, and a microprocessor for processing the output of the analogue to digital converter.

In another preferred embodiment the overload detection circuit includes a sample and hold circuit for sampling the voltage across the capacitor or a scaled version thereof, a timer for triggering the sample and hold circuit to sample the voltage at a known time while the capacitor is being discharged, and a comparator for comparing the sampled voltage with a reference voltage.

According to another preferred embodiment the overload detection circuit includes a differentiator for determining the rate of change of the voltage across the capacitor, and a comparator for comparing the output of the differentiator with a reference voltage.

According to still a further preferred embodiment the overload detection circuit includes a reference capacitor, a reference capacitor charging circuit for charging the reference capacitor to a known voltage, a resistive load, a reference capacitor discharging circuit for discharging the reference capacitor through the resistive load simultaneously with the discharging of the discharge capacitor, and a comparator for comparing the voltage across the reference capacitor or a scaled version thereof with the voltage across the discharge capacitor.

According to a second aspect of the present invention there is provided a method of detecting whether an electric fence energiser including a discharge capacitor which is discharged through an output transformer is overloaded, the method including the steps of: determining whether the capacitor is discharged at a rate which exceeds a threshold rate; and (ii) outputting an alert if it is determined that the capacitor is discharged at a rate which exceeds the threshold rate.

Preferably, the step of determining whether the capacitor is discharged at a rate which exceeds the threshold rate includes the further steps of: determining the rate at which the capacitor is discharged; and (ii) determining whether the discharge rate of the capacitor exceeds the threshold rate.

According to a preferred embodiment the step of determining whether the capacitor is discharged at a rate which exceeds the threshold rate includes the further steps of: determining the voltage across the discharge capacitor or a scaled version thereof at a particular time while the capacitor is being discharged; and (ii) determining whether the capacitor voltage is below a reference voltage.

According to another preferred embodiment the step of determining whether the capacitor is discharged at a rate which exceeds the threshold rate includes the further steps of: determining the percentage drop of the voltage across the capacitor over a predetermined period of time as the capacitor is being discharged; and (ii) determining whether the percentage drop exceeds a threshold value.

Brief Description of the Drawings In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawing, in which figure 1 is a schematic circuit diagram of an electric fence energiser according to an embodiment of the present invention.

Detailed Description of the Preferred Embodiment Referring to figure 1, an electric fence energiser 10 according to an embodiment of the present invention includes an overload detection circuit 11, a discharge capacitor 12, a capacitor charging circuit 13, and a capacitor discharging circuit 14.

The overload detection circuit 11 includes a scaling circuit provided by a resistive divider having a resistor RI connected to the anode of the capacitor 12 and a resistor R2 connecting the resistor RI to ground. The resistive divider functions to apply a portion of the voltage across the capacitor 12 to the input of a filter 15 whose output is coupled to the input of an analogue to digital converter (ADC) 16. The output of the ADC 16 is coupled to an input of a microprocessor 17, while a buzzer 18 and a 2 digit, 7 segment liquid crystal display (LCD) 19 are coupled to respective outputs of the microprocessor 17.

The capacitor charging circuit 13 may for example be an isolated flyback charging circuit. An input terminal 20 of the charging circuit 13 includes contacts 21 and 22 that are respectively coupled to the positive and grounded negative terminals of a suitable power source (not shown) such as a battery. The output of the capacitor charging circuit 13 is coupled to the anode of the capacitor 12 while the cathode of the capacitor 12 is connected to ground.

The capacitor discharging circuit 14 includes a step-up output transformer 23, a silicon-controlled rectifier (SCR) 24, and a timing/triggering circuit provided bythe microprocessor 17. The primary winding of the step-up transformer 23 is connected to the anode of the capacitor 12 and to the anode of the SCR 24. The cathode and gate of the SCR 24 are respectively connected to ground and an output of the microprocessor 17. The secondary winding of the transformer 23 is connected to contacts 25 and 26 of an output terminal 27.

The capacitor discharging circuit 14 periodically discharges the capacitor 12 through the primary winding of the transformer 23 by periodically triggering the SCR 24. In particular, the timing circuit provided by the microprocessor 17 periodically outputs a voltage of short duration to the gate of the SCR 24 to switch the SCR 24 on so that charge which has been accumulated by the capacitor 12 is discharged through the primary winding of the transformer 23 and through the SCR 24. As the capacitor 12 is discharged, a voltage pulse appears across the secondary winding of the transformer 23 and across the output terminal 27. The amplitude of the voltage pulse is much greater than the voltage across the charged capacitor 12 owing to the step-up transformer action of the transformer 23. Once the current flowing through the primary winding of the transformer 23 decreases below the holding current of the SCR 24, which is the minimum current required to maintain the SCR 24 switched on, the SCR 24 switches off with the result that the current path from the capacitor 12 to ground through the primary winding of the transformer 23 is no longer available to discharge the capacitor 12.

Once the capacitor 12 has been discharged by the capacitor discharging circuit 14 the SCR 24 switches off and the charge/discharge cycle is repeated.

The overload detection circuit 11 detects whether or not the energiser 10 is overloaded by firstly determining the rate of discharge of the capacitor 12 during the discharge portion of the energiser's charge/discharge cycle and then comparing the rate of discharge with a predetermined threshold rate to determine whether the discharge rate exceeds the threshold rate. If the discharge rate exceeds the threshold rate then this corresponds to an overload condition.

The operation of the detection circuit 11 is based on the fact that the load applied to the output of the energiser 10, and hence the secondary winding of the transformer 23, is reflected at the primary winding of the transformer 23. Therate of discharge of the capacitor 12 by the discharge circuit 14 is related to the reflected load in that the rate of discharge is proportional to the load. Therefore, if the rate of discharge exceeds the threshold rate which corresponds to the rate of discharge at which the energiser 10 becomes overloaded then the overload condition can be readily detected by comparing the rate of discharge with the threshold rate.

The detection circuit 11 determines the rate of discharge of the capacitor 12 by employing the ADC 16 and the microprocessor 17 to measure the voltage at the junction between resistors RI and R2 which is proportional to the voltage across the capacitor 12 after the voltage has been filtered by the filter 15. The ADC 16 digitises the voltage for processing by the microprocessor 17. At least two measurements are taken, one just before the SCR 24 is turned on when the capacitor 12 is fully charged and then another while the SCR 24 is turned on after a known interval of time has elapsed. The microprocessor 17 then uses the voltage level measurements and the known time interval between them to calculate the rate of discharge of the capacitor 12 to sufficient accuracy to enable an overload to be determined. After calculating the rate of discharge the microprocessor 17 compares the calculated rate to the threshold rate. If the microprocessor 17 determines that the rate of discharge exceeds the threshold rate then this corresponds to an overload condition and the microprocessor 17 activates the buzzer 18 and also outputs the result of the comparison on the display 19.

If enough measurements are performed by the overload detection circuit 11 the peak output voltage of the energiser 10 can be determined by the microprocessor 17 by either calculation or by reference to a stored look-up table. The microprocessor 17 can then display this voltage on the display 19 for the benefit of users.

The foregoing describes only one embodiment of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention. For example, the overload detection circuit 11 may include an analogue sample and hold circuit, timer and analogue comparator. The sample and hold circuit would sample the voltage across the capacitor 12 or a scaled version thereof after being triggered by the timer as the capacitor 12 is being discharged at a known time after the SCR 24 is turned on. The sampled voltage would then be applied to the input of the comparator which would determine if the sampled voltage was over or under a predetermined reference voltage level applied to the other input of the comparator. If the sampled voltage was lower than the threshold voltage then this would mean that the capacitor was being discharged at a rate which corresponded to the energiser 10 being overloaded and the output of the comparator would reflect this.

Another modification that could be made to the overload detection circuit 11 would be to use a differentiator and comparator circuit to determine whether the rate of discharge of the capacitor 12 was such that an overload condition existed. The voltage across the capacitor 12 or output by the scaling circuit would need to be applied to the input of an analogue differentiator such as an op-amp differentiator, the output of which would be proportional to the rate of change of the voltage with respect to time. Since the rate of decay of the capacitor voltage increases as the load increases, the peak negative rate of change of voltage output by the differentiator would also increase. The output of the differentiator would be coupled to one of the comparator's inputs and the other input of the comparator would be coupled to a reference voltage. The reference voltage would be set to equal the output voltage of the differentiator when the rate of discharge of the capacitor 12 was the same as the threshold discharge rate. The comparator would determine whether the rate of change of voltage determined by the differentiator exceeded the threshold rate and would output an appropriate signal if it did.

The overload detection circuit 11 could also be modified to include a reference capacitor charging circuit and a reference capacitor discharging circuit that are both coupled to a reference capacitor. The reference capacitor would be charged to a known voltage by the reference capacitor charging circuit and would then be discharged through a fixed resistive load by the reference capacitor discharging circuit at the same time as the main capacitor 12 is discharged by the SCR 24. The decay rate of the voltage across the discharge capacitor 12 would then be compared with the decay rate of the voltage across the reference capacitor by using a comparator whose inputs are respectively coupled to the capacitor 12 and the reference capacitor to compare the voltages at those points at a particular time during the discharge. If the voltage across the capacitor 12 is less than the voltage across the reference capacitor then this would correspond to an overload condition and a suitable alert signal would be output from the detection circuit 11 The overload detection circuit 11 may also be modified to determine the percentage drop of the voltage across the capacitor 12 over a precise period of time commencing from when the capacitor 12 is fully charged just before the SCR 24 is turned on. The percentage drop is then compared with a predetermined threshold value. If it is determined that the percentage drop exceeds the threshold value then this corresponds to an overload condition and an alert signal is output by the detection circuit 11. This technique has the advantage over other methods which involve a direct comparison of the capacitor voltage or a scaled version thereof with a predetermined threshold in that those other methods can produce errors in mains powered energisers due to the capacitor charging voltage varying as a result of variations in the AC supply voltage. The technique also removes variation in the reference voltage of the ADC 16.

The energiser 10 could also be modified so that the output voltage at the output terminal 27 is altered in response to the load. For example, if the load increases so that the voltage at the output terminal 27 decreases, the overload detection circuit I I would detect this and the energiser 10 would be controlled to boost the output voltage to its proper level.

The energiser may have other means apart from the buzzer 18 and/or display 19 for alerting a user to an overload.

Claims (9)

1. An electric fence energiser for energising the conductors of an electric fence, the energiser including an overload detection circuit for detecting whether the energiser is overloaded, a discharge capacitor, and a capacitor discharging circuit for discharging the capacitor to energise the fence conductors, wherein the discharging circuit includes an output transformer having a primary winding which the capacitor is discharged through, the energiser being characterised in that the detection circuit determines whether the capacitor is discharged at a rate which exceeds a threshold rate and outputs an overload alert if it is determined that the capacitor is discharged at a rate which exceeds the threshold rate.

2. The energiser of claim I, wherein the overload detection circuit includes a scaling circuit for scaling the voltage across the capacitor, a filter for filtering the output of the scaling circuit, an analogue to digital converter for digitising the output of the filter, and a microprocessor for processing the output of the analogue to digital converter.

3. The energiser of claim 1, wherein the overload detection circuit includes a sample and hold circuit for sampling the voltage across the capacitor or a scaled version thereof, a timer for triggering the sample and hold circuit to sample the voltage at a known time while the capacitor is being discharged, and a comparator for comparing the sampled voltage with a reference voltage.

4. The energiser of claim 1, wherein the overload detection circuit includes a differentiator for determining the rate of change of the voltage across the capacitor, and a comparator for comparing the output of the differentiator with a reference voltage.

The energiser of claim 1, wherein the overload detection circuit includes a reference capacitor, a reference capacitor charging circuit for charging the reference capacitor to a known voltage, a resistive load, a reference capacitor discharging circuit for discharging the reference capacitor through the resistive load simultaneously with the discharging of the discharge capacitor, and a comparator for comparing the voltage across the reference capacitor or a scaled version thereof with the voltage across the discharge capacitor.

6. A method of detecting whether an electric fence energiser including a discharge capacitor which is discharged through an output transformer is overloaded, the method including the steps of: determining whether the capacitor is discharged at a rate which exceeds a threshold rate; and (ii) outputting an alert if it is determined that the capacitor is discharged at a rate which exceeds the threshold rate.

7. The method of claim 6, wherein the step of determining whether the capacitor is discharged at a rate which exceeds the threshold rate includes the further steps of: determining the rate at which the capacitor is discharged; and (ii) determining whether the discharge rate of the capacitor exceeds the threshold rate.

8. The method of claim 6, wherein the step of determining whether the capacitor is discharged at a rate which exceeds the threshold rate includes the further steps of: determining the voltage across the discharge capacitor or a scaled version thereof at a particular time while the capacitor is being discharged; and (ii) determining whether the capacitor voltage is below a reference voltage.

9. The method of claim 6, wherein the step of determining whether the capacitor is discharged at a rate which exceeds the threshold rate includes the further steps of. determining the percentage drop of the voltage across the capacitor over a predetermined period of time as the capacitor is being discharged; and (ii) determining whether the percentage drop exceeds a threshold value. An electric fence energiser substantially as herein described with reference to figure 1 of the accompanying drawing. 11i. A method of detecting whether an electric fence energiser is overloaded, the method being substantially as herein described with reference to figure 1- of the accompanying drawing. 13 DATED this 8 th day of October 2003 Pakton Developments Pty Ltd By its Patent Attorneys CULLEN CO.