Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to improvements in
electric fencing monitoring systems, particularly for mul-
tiple fence arrangements of relatively long lengths.
Electric fences utilizing an energizer at one end
for producing a pulse along the length of the fence have
been known for many years and are presently being used more
frequently due to a more general awareness of their advan-
tages over conventional fences. The pulse generated is of
such a level and frequency that it will deter passage of
10 animals and the like through the fence. The advantages of
electric fences are particularly significant in farm fencing
arrangements where temporary paddocks are desired and elec-
tric fences are becoming more extensively used ~or this purp-
ose.
Electric fences permit a simpler, lighter and there-
fore cheaper construction than conventional fences as there
is no longer any need to make design allowances for animals
endeavouring to push through the fence to get to pastures on
the other side.
However~ problems do arise if a power loss of a
certain dimension, or a complete power loss occurs along
the fence; because of ~he light construction of the fence
stock losses through the fence can occur or damage to crops
can result frorn intrusion of ones own stock or foreign an-
25 imals-into crop growing areas. Therefore there is a need
for some means of simply determining when a fault occurs
along a conductor of a sufficiently significan~ nature to
make the conductor ineffective for its design purpose and
when such fault occurs an alarm is raised.
So~e proposals have been made for alarm systems
for these types of fences~ however, these have generally
been complex or ineffective in operation, particularly for
fences of considerable length. An electric fence system
suitable for len~thY~systems has been proposed by us as
35 described in our~ a~ ~nt ~ æ~ No. 509,325. This system
operates with a monitoring device including an alarm and
a reflector means in the fence which reflects pulse signals
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to the monitoring device which will activate the alarm if
th~ pl~lses are absent or are weak. This system has been
used in the field and found to work satisfactorily. The
present invention aims to remove the need for providing a
reflector device in an electric fence alarm system and yet
operates effectively.
There is provided according to the present inven-
tio~ a monitoring system for an electric fence having an
energizer means for applying an electric pulse to a fence
10 con~uctor, said system comprising means for sensing the vol-
tag~ of sai~ pulse in said conductor, means for sensing the
current in the conductor, the value of current and voltage
being related to the magnitude of said pulse and ambient
conditions at the conductor, monitoring means for computing
;J' 15 sig~als related to said current and voltage and adapted to
operate alarm means when the computed signal falls outside a
predetermined range of values or to indicate a reading on
indlcator meams.
Conveniently said monitoring means includes means
20 for de~ecting a rapid fall in current level (as may be caused
by ~ breakage) whereby said alarm or indicator means is ac-
tiv~ted. Said monitoring means includes means for measuring
-or detecting a rise or fall in current level ~as may be
caused by an unacceptable drop in potential in the fence
25 through insulator breakdown or moisture leads to earth) where-
by said alarm is activated in the case of rise beyond pre-
determined limits~ Additionally or alternatively an indicat-
or is provided to give a reading of current levels at any
given moment in operation.
The invention will be described in greater detail
havlng reference to tlle accompanying circuit diagrams in
which Figure 1 is a schematic block diagram of an electric
fence circuit.
Figure 2 shows details of voltage current measure-
35 ment, curvature and mixing circuits.
Figure 3 shows an alternative mixing circui~.
Figure 4 shows a detailed switching circuit.
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Figure 5 shows details of a detector circuit.
The fence and energize~ construction m~y be of
conventional type and of course may take many forms as is
known in the art. For exa~ple the fence may be either a
single wire energized with respect to the earth or many wires
so energized. The fence may also consist of a wire or wires
connected to the earth together with the insulated active
wire or wires.
Referring to Figure 1, the voltage of each pulse
10 emitted by the energizer 21 is measured by volt meter 10 and
is modified or linearized by curvature circuit 11. The cu-
rrent in the earth return to the energizer is measured by
amrneter 12 which in turn is linearized by curvature circuit
130 The linearized values of the voltage and current are
15 received by a mixing circuit 14 to give an output measured by
the indicating device 15. The indicating device lS gives at
any instant a measurement of the apparent voltage level at
the remote end of the fence although the monitoring device is
connected into line in juxtaposition with the energizer.
In operaltion the linearized current signal is fed
into a distance control circuilt 16 which includes a potentiom-
eter whose resistance has been selt according to the length
of the fence such that under certain conditions given similar
voltage pulses the indicating device will indicate the same
25 value as a voltameter reading taken at the remote end of the
fence. Thus where there is a variation in conditions such
as dampness, long grass, the current measurement figure will
increase (caused by a drop off in voltage in the fence) lead-
ing to a signal mixed with the incoming voltage signal in
30 mixer 14 which gives rise to a fall in the indicator reading.
If this reading falls below a predetermined value the switch-
ing circuit 17 is switched to activate alarm 18. In any
event the indicating device gives the operator an indication
of fence operation of low voltage acceptable and high voltage.
The rnonitoring device includes a resistance/capa-
citance amplifier detector circuit 20 for detecting a sudden
decrease in current caused for example by a breakage in the
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fence. If the decrease is gradual, say by drying under the
sun, the circ:uit does not operate.
Thus the monitoring system of the present invention
indicates the effectiveness of the fence at a distance from
the energizer as set or determined by the operator. However,
the device can be connected and mounted in juxtaposition
to the energizer without the need for reflectors or operating
parts at remote parts of the fence. The exception to this
is that it is preferable for a terminating resistor 30 to
10 be inserted inLo the fence circuit at the end remote from
the energizer to act as a passive load in the fence circuit.
The valuP of the resistor 30 may be of the order of the natur-
al impedance of the fence so that the value of the electrical
pulse generated by the energizer will not be degraded. Thus
15 ~he monitoring~ system will be sensitive to a fence breakage
at a point near the remote end of the fence. This is because
the termination resistor increases the total current in the
system so that a breakage near the remote end of the fence
will cause a sudden larger decrease in current which is eas-
20 ily detectable even in extremely lengthy fences of up to 40or 5Q kilometers or longer.
Thus the monitoring system of the invention
includes three essential functions:-
1. An indicator 16 for indicating the peak voltage
25 in the wire at a predetermined distance from the energizer;
2. An alarm operable if the indicated peak voltageas said predetermined distance falls below a predetermined
minimum; and
3. An alarm which detects a sudden shortening
30 of the fence~
The functions are achieved without the need for
reflectors or transponders in the fence remote from the en-
ergizer.
The circuits of the monitoring system will now be
35 described in more detail having reference to the circuit
diagrams in Figures 2 to 5.
One form of the alarm is shown in Figure 2.
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Across the output transformer of the fence en-
ergizer is connected two resistors in series Rl and R2 con-
stituting voLtage measurement and curvature circuits 10 and
11. Approximately 1~500 of the output pulse from the en-
ergizer appears across R~. This is rectifi,ed by diode Dland charges up capacitor Cl which discharges very slowly
through R5. The DC voltage appearing across Cl,R5 is propor-
tional to the output voltage of the energizer. One end of
the energizer output transformer is connected to the fence
10 whilst the other is connected in series with V.D.R. (voltage
dependant re~;istor) and R10 to the earth terminal to provide
the current measurement and curvature circuits 12 and 13.
The output pulse of the energizer causes current to pass
into the fence via V.D.R. and R10. The voltage appearing
15 across V.D.R. (due to the ience current) is divided 'by R3
and potentiometer R4 constituting the distance control circ-
uit 16. This voltage is not linear to the current (distort-
ed) because of the characteristic of the V.D.R. A portion
of the voltage across R4 is selected by the tap and is rec-
' 20 tified by D2 and charges C2 which very slowly discharges
', through R6. The voltage across R5 is connected via R7 to
the positive :Lnput of ~he operational amplifier ICl. The
voltage appearing across R6-is connected to the negative
input of the same operational amplifier via R8. R8 and R9
~5 form a negative feedback divider so that the operational
amplifier operates with a small amount of gain. The resul~
is the output of the amplifier is the difference of the two
, input signals multiplied by a small fixed factor representing
the voltage of the fence energizer minus the current in the
30 fence circuit. A volt meter Rm and M connected across the
,' output of the operational amplifier can be made to indicate
', the value of the voltage that appears at some point or the
end of the fence circuit by the adjustment of R4. As the ,,
fence circuit now changes its loading on the energizer by
extra or less leakage (changes of current into the fence
, circuit) the output volt meter will also change and indicate
; the voltage to be found at the measuring point on the fence
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circuit. As the operational amplifier is fed with a single
power supply its output cannot go below zero.
A second form of the alarm is shown in Figure 3.
A special high voltage transformer T2 is connected
across the output of the fence energizer. This special high
voltage tra~sformer has a turns ratio of 1000 to 1 so that
in its secondary 1/1000 of the output of energizer occurs.
The secondary output is rectified by the bridge rectifier
BRl and charges capacitor C4 to the peak secondary voltage.
The output pulse of the energizer causes current to pass
into the fence circuit via V.D.R. ~voltage dependant resist-
or) and R10. The voltage appearing across V.D.R. is rec-
tified via bridge rectifier BR~ and charges capacitor C5.
The potentiometer R23 connected across C5 allows a portion
of this voltage to be taped off. The voltages across C5
and C~ are connected together but opposing via D8. This
allcws an indicating volt meter (Rm,M) to be connected to
the circuit reading the voltage across C4 Ithe output voltage
of the energizer) minus a portion of the voltage across C5
(the current in the fence circuit). The volt meter can be
made to indicate the measured voltage that appears at some
point, or at the end of the fence circuit by adjusting
potentiometer R23. As the fence circuit now changes its
loading on the energizer by extra or less leakage (changes of
current into the fence circuit), the output volt meter will
also change and indicate the voltage to be found at the mea-
suring point on the fence circuit. Diode D8 prevents the
output voltage to fall below zero.
This circuit represents an alternative to Figure
1 and has the following advantages:-
(1) The output pulses from the energizer may be
of either positive or negative polarity. I
(2) If only an indicator is required (no alarm)
no external power is required~
(3) It may be used also as a volt meter used at
any point along the fence by adjusting R23 to zero or adding
a switch to effectively do the same.
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(4) An add on electronic alarm may be added at
a later date.
This circuit has the disadvantage of using a
special high voltage transformer.
The output from the voltage measuring circuits
may also be connected to a simple electronic voltage level
switch shown in Figure 4~ constituting switch circuit 17,
so that a voltage level may be set so that once the indicated
voltage falls below the set point some alarm device (bell
lights etc.) connected may operate bringing to the noticeof the operator that this condition has occurred. The alarm
may be connected to plug 18a.
A simple DC power supply is formed by Tl, D3, D4,
and C3. This DC supply is connected across IC 1,2,3,4 and
also a fixed current is supplied to D7 (zenner diode) via
the current regulator R2 and TRl (junction field effect
transistor). As D7 is supplied with a constant current not
dependant on the power supply a constant voltage appears
across Rl9, R14B and R200 A portion of this voltage across
20 R20 and the tap of po~entiometer R14B is connected via R18
to the positive input of operational amplifier IC4.
As the output of IC4 is connected directly to its
own negative input its output voltage is exactly the input
voltage at its positive input. The output of IC4 is also
connected to the positive input of IC2 via R17. The output
! voltage of the previous described voltage measuring circuits
(Figures 2 and 3) is connected via Rll to the negative input
of IC7 (operating as a voltage comparator).
The output of IC2 will be zero if either the two
input voltages are the same or if the voltage on the negative
input is greater than the positive input. If the voltage
of the positive input is greater than the voltage of the
negative input the output voltage of IC2 will rise to nearly
the same as the DC supply voltage to IC2. This output is
connected via R12 to the positive input of the operational
amplifier IC3.
Also connected o the positive input of IC3 is
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a zenner diode D6 so that the input voltage to IC3 will not
rise above the opera~ing point of D6. As IC3 has its output
also connected to its own negative input its output voltage
is exactly the input vol~age supplied ~o its positive input
; 5 fixed to a peak value by zenner diode D6.
The voltage output of IC3 causes current to flow
through R13 and the light emitting diode D8 which is op-
~ tically coupled to the light dependant resistor LDR. When
;~ light falls upon l,DR current flows via R22 into the gate
10 of the triac TR2 thus causing current to flow from the
electrical supply through the alarm connected via the alarm
~ s~cket. The output voltage from IC3 also causes current
;~ to flow through R14A and R15. Because the values of D6 and
D7 (which are the same) and Rl9, R14B, R20 and R14A,R15 the
;` ~ 15 voltage across the tap of potentiometer R14A,R15 the voltage
~, across the tap of potentiometer R14A and R15 will be greater
than the voltage across the tap of potentiometer R14B and R20
- (R14A and R14B are mechanically connected together) when IC2
~; and IC3 have an output voltage. This causes a current to
20 flow via R16 and D5 into the positive input of IC2 thus prov-
iding positive feedback. This positive feedback which is
constant regardless of the position of the coupled potentiom-
eters R14A and B causes hysteresis so that positive switching
!- ~ of IC2 occurs. The amount of positive feedback is sufficient
i~ 25 to cause reliable switching to occur with minimal dead band.
An indicating volt meter (Rm,M) may be connected to the two
- inputs to the IC2 via a changeover push button PB so that in
the normal position it indicates the voltage on the fence but
when pushed the volt meter indicates the "set" voltage set by
30 the position of R14B that the alarm will operate at. As the
volt meter is connected to low impedance sources it has no
loading effect on the circuit. As the operational amplifiers
ICl, 2,3,4 and the voltage reference D7 are independant of
~` the supply voltage (within limits) the circuit will operate
!, 35 with a ten fold change in supply voltage~
p By the additivn of the detector circuit 20 shown
} in Figure 5 the alarm will also activate the alarm should
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a break occur in the wire of the fence circuit. A hreak
in the fence circuit is a sudden occurrence and is detected
by a sudden decrease in the fence circuit current. This
will be different from a decrease in the fence circuit cu-
rrent by natural causes as these occur much slower (wet fencedrying in the sun etc.). As stated in the circuits of Fig-
ures 3 and 4 the fence circuit current causes a voltage drop
across V.D.R. and R10. Small charges in current cause a
much larger charge in voltage across R10 then V.D.R. It
is also desirable but not necessary to place at all ends
of each fence wire a terminating load. This will cause a
definite fence circuit current associated with the end of
each fence wire. Should only a very short piece of wire
be broken off a very long fence it will be detected easily.
This terminating load may be many more times the natural
impedance of the fence so that no degradation of the system
will result due to their use.
The voltage across R10 and V.D.R. is rectified
by D9 and charges C6. C7 is also charged but through R24.
, 20 C6 discharges through R23 but C7 must discharge through R23
and R24 so that the time constant for C7, R23 and R24 is
` ~ much larger. C6 is connected to the negative input of the
operational amplifier IC5 while C7 is connected to the pos-
itive input.
The output of IC5 remains zero unless the voltage
on C7 is larger than the voltage on C6. This occurs when
there is a sudden decrease in the current in the fence circ-
uit because oE the different discharging time constants of
C6 and C7. The output of IC5 is connected via R25 to the
gate of a smaLl silicon controlled rectifier TR3 which
latches into conduction. The current passing from the alarm
system power supply through the high voltage fixed current
light emitting diode D10 causes TR3 to remain switched on.
TR3 remains conducting until IC5 has no output and the reset
(RESET PB) is operated. As diode Dll is connected to the
negative input of IC2 (Figure 4) TR3 also forces IC2 into
operation by forcing the negative input of IC2 to a low
potential. This causes the alarm circuit to operate until
TR3 is reset to the non-conducting state. Light emitting
diode D10 indica~es to the operator when the alarm operates
that the fault condition is a breakage in the fence circuitO
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