GB2503201A - Electric fence having low power detection mode - Google Patents

Abstract

An electric fence sensor system has a low power detection mode. When the presence of a body near the fence is detected, the system will switch from the sensing mode to electrified mode for a preset time interval. During this time a high voltage pulse will be present, before the system resumes the detection mode. The power and/or frequency may be dependent on the size of the perceived object. The system may comprise an energizer 2, a wire 3 and a sensing device 1. The sensing device may use a change in capacitance to detect the presence of a body. An LC or RC oscillator may be used where the wire to be electrified forms part of the capacitance of the circuit. Microprocessor a measures the frequency of the oscillator-wire circuit several times a second, where changes in the frequency are caused by small changes in the capacitance arising from the presence of a person or animal. The microprocessor uses an algorithm to decide whether to switch into the electrified state. A single wire may be used for both detection and electrification, alternatively separate wires may be used.

Description

Electric fence sensor system (for agricultural applications)

Description

This invention describes a device for sensing objects near to a fence and the subsequent switching of the energizer mechanism.

Existing electric fences have several disadvantages. They are the cause of fires during dry periods as they short circuit through vegetation. They are electrically noisy, often causing interference to nearby communications systems. Power is consumed through heat loss by constantly maintaining the high voltage in the charger unit. A danger is posed to vulnerable persons, children and perhaps domestic animals.

An electric fence that employs an element of detection and is therefore energized only when a "threat" is detected would overcome many of these problems. Possible implementations of this include PIR, vibration sensing, ultrasonic distance measurement and seismic ground sensors.

The system proposed here is a detection system that employs a single wire for both detection and electrification. This can be easily retro-fitted to existing single-wire systems. A system of multiple wires, such as in a mesh arrangement, where each wire is a dual purpose detect/electrified element or alternate wires with detect-only wire(s) could also be employed, based on the proposed system.

r" During normal operation, whilst in detection mode, the "fence" remains in a low power, non-electrified state. When a change in capacitance (frequency) is noted the circuit will make a decision T whether to switch from "sensing" mode to electrified mode for a preset time interval. During this T electrified period, a high voltage pulse will be present, before the system resumes the detection mode. The power and/or frequency of electric pulse may be adjusted by the microprocessor *" according on the size of the perceived object, depending on configuration.

Advantages of this approach include -Low power. As the fence is hardly ever electrified, far less power is required providing a substantial saving in power consumption. A smaller battery or power source than used in conventional systems may therefore be employed with or without a solar panel. For the latter, the system could be in effect maintenance free.

Low cost. The smaller battery and associated solar panel would significantly reduce hardware and running costs. Only a single wire is needed.

Intelligent. Numerous possibilities including 1. Varying the power and/or frequency of the electrified pulse depending on the size and associated signature of the animal. This could work in multiple ways -for example, administering a mild or null shock to humans, dogs etc and a larger shock to larger animals such as cows, horses. Or an inverse relationship whereby smaller animals such as foxes or sheep are identified for shock in preference to larger animals and humans.

2. Prevention of fire from accidental shorting. As the fence is only energized after detection, the risk of shorting from grass, trees etc is greatly reduced. Furthermore, the system will revert to sensing mode after a few seconds of the electrified state. On resuming sensing mode, the system will compare the before and after frequency and make a decision whether to re-e'ectrify. It may be configured to shutdown if a constant change, due to a fallen branch for example, is though to be present If this condition is persistent a warning indicator may be flagged to alert personnel.

3. Automatic adjustment of power and frequency to ambient conditions detection such as soil moisture.

4. Noise. Existing systems routinely cause interference to surrounding electrical systems such as telephone wires or wireless communications. By employing the proposed system, where the fence is energized perhaps less than 0.1% of the time, interference will be almost eradicated.

5. Timed. The microprocessor could increase shock by adjustment of frequency and/or power at sunset and reduce at sunrise to protect livestock from nocturnal predators. Or it could be off during the day and on at sunset.

The system will now be described with reference to attached figures; Figure 1 illustrating the integration of the technology with an existing system. Figure 2 is a basic schematic showing the main components in situ (a microprocessor, isolating switch/re'ay, protective diodes and earthing mechanism). Figures 3 and 4 show the performance of the system and accompany the description of the example algorithm.

C') The design uses an LC or RC oscillator where the wire to be electrified forms part of the r-capacitance in the circuit. The frequency of the oscillator-wire circuit is measured several times per second by the microprocessor. Changes in this frequency are caused by small changes in T capacitance arising from the presence of an object such as an anima' or person and ground. The r wire, in effect, acts an aerial.

The microprocessor measures the change in frequency and by means of an algorithm decides T whether to switch into the "electrified" state. A simple algorithm is discussed shortly.

The basic setup may be seen in Figure 1 where the sensing device is labelled a, the electrification circuit (or energizer) , a wire that attaches to the existing energized fence wire y and the main fence wire itself (6).

A little more detail is shown in Figure 2 to aid description. Note that the power source or battery is not shown. The power consumption of this proposed unit is tiny and the existing supply to the energizer may be used. Alternatively a smaller, cheaper battery could be used for the whole system.

Referring to Figure 2, the wire usually electrified (3) is connected as an antenna to the oscillator circuit. The output frequency from this is input to a microprocessor(la) that counts the oscillator frequency over a fixed timing interval. The output frequency from the oscillator will change slightly due to the additional stray capacitance if a body in contact with ground approaches the wire.

This change in frequency is detected by the microprocessor, processed to produce a smoothed background and compared to an adaptive threshold value. If this trigger value is exceeded, the system isolates itself from the wire by means of a normally closed switch, relay or solid state device (ib) for a fixed duration and during this time switches the wire to the electrified state before reverting to the "sensing" mode. The microprocessor (la) may be protected by a fuse in case of isolation failure.

An earth probe may sense the moisture content of the ground and set the power and frequency of the electrified circuit automatically.

On startup, and periodicafly, the system auto-calibrates to account for frequency drift and changing ambient conditions. A rapid change, characterized by an approaching object, would cause the system to energize whereas a slow, gradual change would be interpreted as natural ambient variation and incorporated into the background signal. The threshold trigger value would move accordingly with changing background. The sensitivity is configurable for different applications.

Figure 3 shows a moving average background of frequency count per time period. The large spike is caused by a person's hand approaching and then grasping the wire before retreating. After this has occurred, the background can be seen to gently increase monotonically.

Description of algorithm

A very simple a'gorithm is presented here. Numerous imp'ementations are possible (see note at end).

On startup, the system will collect ambient data for a fixed period, for example 10 seconds, to characterize the background. During this time a moving average will be calculated which is used to set a detect" threshold. At the end of this calibration period, the current state will be checked against the detection threshodd.

C') The detection threshold will track the signal for gradual chaiiges owing to changing ambient " conditions, as in Figure 3 after about 40 seconds have elapsed.

In addition to the detection threshold, the rate of change of the signal moving average is monitored.

If the threshold is exceeded and the change in signal is significant (such as the near step-change at r-around 33s in Figure 3), the fence will isolate the sensing electronics and energize the wire. From this point the signal is not updated for the duration of the energized period; in Figure 3 this is 7 seconds but the duration is configurable. After this energizing period, the energizer is switched off and a slight delay (eg. 500 rns) follows before detection resumes.

As the system is essentially!blind during this time, there is no implementation of hysteresis using an adaptive recovery level. This could of course be implemented in a system using separate wire(s) for detection and electrification. Figure 3 shows the continuous signal, which would not be available to a single-wire system.

After the electrified period, the system reverts to sensing. The latter portion of the signal buffer containing the values that caused the a'ert are discarded and the threshoki is re-calculated using the values remaining in the buffer pius new values as they become available.

Figure 4 illustrates what the single-wire system would "see" in operation. At approximately 33s, the system energizes and receives no further data. Seven seconds later (in this example) the system resumes and new data begins to overwrite the buffer.

At this point, an assumption has been made that the animal has retreated from the fence. If this is not the case and the reading indicates the presence of animal (a repeat, or new, offender) then the fence is re-energized for another period in the same manner.

If a new higher state persists, this may be due to a fallen branch or other object. Depending on the configuration, the fence may restart the background characterization process or set a warning indicator (eg. flashing LED) to the user.

This algorithm is just one implementation and there are many ways of implementation such as signal processing, filtering of data, a confidence-level approach or statistical testing against proven data using methods such as a Chi-squared test. C') r r r

Claims (3)

1. Claims 1. A detection system for electric fences for use in agricultural applications that uses the same wire for both detection and electrification.
2. 2. Multiple wires could also be used in addition to the pmposal in (1) where either each wire is dual purpose or alternate wires are used for detection and electrification.
3. 3. The system (1) will work with existing energizer systems or may be incorporated into a single unit, integral with the electrification circuit.