Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates to an intrusion warning
or alarm system for indicating the presence of an intruder within
a given area. More particularly, the present invention relates to
an improved intrusion warning system of the type wherein the
presence of an intruder within a given area is detected by
determining the changes caused by the intruder in an electric
field in the area to be protected.
Various types of intrusion warning systems which operate
on the principle of detecting a change in an electric field
caused by the intruder are known in the art. Generally such
systems utilize a high impedance sensing device, for example, an
antenna, which is placed within the area to be protected, and
monitors the electric field within the area5 which electric field
may be either the inherently present electric field or an electric
field specifically produced by the antenna or sensing device.
Any change in the charge on the antenna due to the electric field
being disturbed by the intruder is then detected and converted
to an electrical signal which is used to provide an indication.
One type of intrusion warning or detection system which
also responds to changes in electrical fields caused by an
intruder and which operates in a different manner than that
mentioned above, is disclosed in U.S. Patent No. 3,237,105,issued
February 22nd, 1976, to Henry P. Kalmus. According to the
teachings of this patent, a transmitting electrode and a
receiving electrode are positioned on opposite sides of the area
to be protected, e.g. a doorway, and a quasi-stationary electric
field is produced within the area to be protected by connecting an
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oscillator to the transmitting electrode which oscillates at a
frequency in the range of, for example, 5 to 100 kHz. The
receiving electrode is connected by an amplifier to an AM
detector which detects the modulation of the received field
signal caused by the movement of an intruder between the two
electrodes, and the detected signal is fed to an amplifier
including a bandpass filter with a bandpass in the order of 2 to
20Hz, thereby passing the low frequency component due to movement
of an intruder in the area between the electrodes. This filtered
signal is then fed to an indicating or alarm device to indicate
the presence of an intruder.
Although the system disclosed in this patent operates
satisfactorily in principle, the system is susceptible to a number
of problems when attempts are made to utilize same in practical
applications or to extend the range of the system so that it can
be utilized to cover relatively large areas, thus necessitating
that the sensitivity, and hence the gain, of the system be
increased. One primary problem in practical applications of such
systems is that of false alarms caused by stray electric fields in
the area to be protected, or by transients. One primary source of
stray electric fields normally present is the electric fields
radiated by a.c. power lines or building wiring which would
normally be present in the vicinity of an area to be protected by
such an intrusion warning system. For example, while the primary
a.c. power frequency, e.g~, 60Hz, would be outside of the passband
of the system, the numerous devices and appliances normally
operating from the power line cause extremely rapid current
changes on the line, and these in turn cause energy to be created
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at multiples or harmonics of the power line frequency. If one of
these har~onics should be equal to the frequency of the generated
electric field or sufficiently close thereto to cause a beat
frequency signal to be produced which is within the passband of
the detection system, than a false alarm could result. Similarly,
other transients creating signals within the passbands of the
system could result in false alarms.
SUMMARY OF_THE INVENTION
It is therefore an object of the present invention to
provide an improved intrusion warning system of the above-
identified type and to reduce the susceptibility of such systems
to false alarms.
It is a further object of the present invention to
provide an improved intrusion warning system of the above-
identified type having a greater versatility with regard to its
application, e.g., for the detectionof an intruder anywhere within an
enclosed area such as a room or to provide perimeter protection
for a large area such as a building.
According to the basic concept of the invention, the
intrusion warning system comprises a quasi-stationary electric
field producing means including a field wire located within the
area to be protected and an oscillator circuit connected to the
field wire for producing an output signal having a wavelength
which is very long compared to the length of the field wire and a
frequency which is in the range of from 1 to 40KHz; an antenna
within the area to be protected for receiving the electric field
signal; an amplifier having its input connected to the output of
the antenna and its output connected to an AM detector; a lowpass
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filter connected to the output of the detector for filtering out
signals above approximately 20 Hz; a high gain bandpass amplifier
connected to the output of the lowpass filter for amplifying and
passing only the low frequency component of the detected signal in
the range of from 0.2 to 2Hz due to movement of an intruder in the
area to be protected; a threshold circuit connected to the output
of the bandpass amplifier for producing an output signal whenever
it receives an input signal exceeding a predetermined threshold
value; and an alarm circuit for providing an alarm indicating the
presence of an intruder within the area being protected.
According to further features of the invention, the
susceptibility of the system to false alarms is reduced by
utilizing an oscillator frequency which is approximately midway
between two successive harmonics of the frequency of the a.c. power
existing in the vicinity of the area to be protected and is further
reduced by either locking the frequency of the oscillator to that
of the power line frequency or disabling the alarm circuit whenever
a harmonic of the power line frequency is sufficiently close to the
frequency of the oscillator to produce a beat frequency signal
within or very close to the bandpass of the system by utilizing a
; synchronous detector for the AM detector and/or by providing a
circuit arrangement, e.g., a ramp voltage circuit, which prevents
; the threshold circuit from responding to random or transient
events.
According to still further features of the invention,
the receiving antennas may take various shapes and configurations
and may be remotely located from the input amplifier.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block circuit diagram of a preferred
embodiment of the intrusion warning system according to the
invention.
Figure 2 is a circuit diagram of one embodiment of an
intexline lock module for use in the system of Figure 1.
Figure 3 is a circuit diagram of a further embodiment
of an interline lock module for use in the system of Figure 1.
Figure 4 is a circuit diagram of a preferred embodiment
of the signal detecting circuitry for the intrusion warning
system of Figure 1.
; Figure 5 is a schematic circuit diagram of one embodi-
ment of the receiving antenna and tuned probe circuit according
to the invention which is particularly useful for indoor
applications.
Figure 6 is a schematic diagram illustrating an
application of the antenna and probe circuit arrangement of
Figure 5.
Figure 7 is a schematic circuit diagram illustrating
20 another embodiment of an antenna and probe circuit arrangement
according to the invention which is particularly useful for
outdoor applications.
Figure 8 illustrates the use of the antenna and probe
circuit arrangement of Figure 7 for permieter protection of an
outdoor area.
Figure 9 is a schematic circuit diagram illustrating
an arrangement according to the invention for providing
supervision of the field and antenna wires for an arrangement such
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as shown in Figures 7 and 8.
Figure 10 is a schematic diagram of a circuit
arrangement for providing supervision for the field wire and the
tuned probe circuit cable for an arrangement such as shown, for
example, in Figure 6.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Figure 1, in the intrusion warning or
alarm system according to the invention, in order to produce a
desired alternating electric field within an area to be protected,
a transmitting antenna or field wire 10 is positioned at a desired
location within an area to be protected, and is connected via an
amplifier 11 to the output of a local oscillator 12 which produces
a low frequency output signal whose wavelength is very long
compared to the length of the field wire 10. The frequency of the
oscillator 12 is, for example, in the range of from 1 to 40 KHz,
and preferably in the range of from 2 to 20 KHz.
In order to detect the electric field, a receiving
antenna 13, which may for example be a length of wire or a
conductive plate, is connected to an input of a tuned probe
circuit 14 which includes a resonant circuit tuned to the
frequency of the oscillator 12. The output of the tuned probe
circuit 14 is connected to the input of an amplifier 15, which
preferably provides bandpass shaping for the frequency of the
oscillator 12, and contains sufficient gain to properly drive the
AM detector 16 connected to the output of the amplifier 15. The
AM detector 16 may, for example, be a simple diode detector.
Preferably, as shown and as will be discussed further below, the
AM detector 16 is a synchronous detector.
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The output signal from the AM detector 16 is passed
via a Iowpass filter 17 having a cutoff frequency so that it will
at least filter out signals above about 20 Hz, to the input of a
high gain operational amplifier 18 which is provided with a
bandpass filtering arrangement so that the overall bandpass is in
the order of about 0.2 to 2 Hz, which range constitutes the low
frequency component associated with the motion of an intruder and
is the frequency component of interest. The filtered and
amplified output signal of the operational amplifier 18 is then
fed via a ramp voltage generating circuit l9, which will be
discussed in detail below, to the input of a pulse generator 20
which produces an output pulse whenever the input signal thereto
- exceeds a predetermined threshold value. The output pulse from
the pulse generator 20 is, as is conventional in intrusion
warning systems, féd to the alarm or indicating control circuit 21
which in turn energizes the alarm 22.
In the basic mode of operation of the system, the motion
of an intruder within the electric field produced by the field
wire lO will result in a modulation of the amplitude of the
electric field signal received by the antenna 13 causing the
signal at the output of the AM detector to vary, and result in
the generation of an alarm.
As mentioned above, one primary source of false alarm
in a system of the above-identified type is the electric field
produced by the radiation of the a.c. power line harmonics from
the wiring or power lines present in the vicinity of the area to
be protected. In order to reduce the probability of false alarms
from this source, according to the invention the frequency of the
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oscillator 12 is selected so that it is a non-whole number
multiple or harmonic of the power line frequency and falls
approximately between two successive harmonics of the a.c. power
line frequency. For example, the frequency of the oscillator 12
may be selected ta be 9760 Hz as illustrated which is 162.5 times
the conventional 60 Hz power line frequency. Thus the frequency of
the oscillator 12, and hence that of the generated electric field
- would be clear of any whole number harmonic of the power line
frequency and moreover would generally not result in any beat
frequencies with the harmonics of the power line frequency which
would be within the bandpass of the signal detecting system, and
in particular the amplifier 18.
Although operating the oscillator 12 at a frequency
which is related to the power line frequency in the matter
described above results in a reduction in the probability of false
alarms, this measure by itself would still not substantially
eliminate this source of error since, as is well known, the power
line frequency changes due to load changes and due to intentional
frequency corrections made by the power company to correct
electric clocks. Therefore, in order to further reduce the
probability of false alarms due to this source of error, as shown
in Figure 1 the system is provided with an interline lock module
23 which is connected to the oscillator 12 and its primary purpose
is to reduce the false alarms due to power line harmonics.
One embodiment of such an interline lock module is shown
in Figure 2 and serves the purpose of locking the frequency of the
oscillator 12 to the power line frequency. As shown in this
figure, the output signal from the oscillator 12, which in the
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illustrated example is normally at a frequency of 9750 Hz, is fed
to a divider 25 which divides the frequency of the oscillator 12
down to the power line frequency, i.e., 60 Hz. In the illustrated
embodiment, this means that the divider 25 must produce an overall
division by 162.5 which can be realized in a known manner by, for
example, multiplying by 2 and dividing by 325. Since, the nature
of such a fractional division process does not yield a squarewave
output, but rather a pulse train, this pulse train is then
processed through a circuit including transistors Q2, Q4, and Q5
which serve as a one shot pulse generator to yield an approximately
squarewave output at 60 H-z. The 60 Hz a.c. power line frequency
signal is converted to a squarewave by means of the circuit
containing transistors Ql and Q2 and the output of transistor Q2
is combined with the output of transistor Q5 in a transistor Q6
which operates as a phase comparator. The output of transistor
Q6 therefore contains a d.c. voltage component which depends upon
the phase relationship between the two signals fed to the
transistor Q6. The output voltage from transistor Q6 is then passed
through an active lowpass filter including transistors Q7 and Q8
and the associated components to produce a d.c. error voltage
which is fed to the oscillator 12 in a known manner to control the
frequency and thus keep it phase locked to the a.c. power line
frequency. Thus, false alarms resulting from low frequency beat
note interference from harmonics of the a.c. power line frequency
are avoided.
While the above-described embodiment of the interline
lock module prouides a practical solution to the problem in
question, this solution does have one undesirable feature. That
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is,the locking of the frequency of oscillator 12 to the power line
frequeney results in the operating frequency for the intrusion
warning system continuously changing. This means that the tuned
circuits in the intrusion warning system must be broad enough to
tolerate these frequency changes without resulting in false alarms
eaused by changes in signal amplitudes within the intrusion system
eireuitry. Accordingly to avoid this drawback, a further embodi-
ment of an interline lock module suitable for use in the system of
Figure 1 is shown in Figure 3.
In the arrangement of Figure 3, the frequency of the
oscillator 12 is not locked to the frequency of the a.c. power line,
but rather the oscillator 12 is permitted to operate at a closely
controlled fixed frequency. As shown in Figure 3, the oscillator
is a erystal controlled oscillator again operating at, for example,
9750 Hz which is between the 162nd and 163rd harmonic of the 60 Hz
power line frequency. As shown in the drawing, the oscillator 12
includes a crystal 30 with its associated components and a divider
31 at whose output the desired frequency of 9750 Hz appears. The
divider stage 31 permits an inexpensive quartz crystal operating at
2.496 MHz to provide the desired output frequency. Since the out-
put frequency from the oscillator 12 is closely controlled, no
interference of false alarm problem exists unless the Power line
frequency changes slightly, thus producing a harmonic at or very
near the operating frequency, i.e., 9750 Hz. Such an interference
frequency or harmonic will of course take place in a practical
application because the power line frequency of 60 Hz would only
have to change by 0.185 Hz for its 162nd or 163rd harmonic to appear
at 9750 ~Iz.
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The interline lock module arrangement of Figure 3 pre-
vents false alarms caused by such harmonics in that it continuously
monitors the beat or difference frequency between the power line
harmonics and the output signal from the divider 31 and disables
the alarm as long as this beat frequency falls below a predeter-
mined frequency. This functioning of the interline module of
Figure 3 is accomplished in that the 60 Hz a.c. power line signal
is fed to a transistor Q9 wherein the signal is limited and squared
and then the output of transistor Q9 is differentiated by capacitor
32 and the resistor 33 to form trigger pulses which are fed to the
input and trigger a 50 microsecond one-shot pulse generator formed
by the transistors Q10 and Qll with their associated components.
The train of 50 microsecond pulses appearing at the output of
transistor Qll is fed through a resistor 34 and gated on and off by
a transistor Q12 which is controlled by the output signal from the
divider 31, i.e., the 9750 Hz operating frequency signal. This
gated train of pulses is then demodulated by a diode detector 35 so
that the signal appearing at the output of diode 35 contains fre-
quency components of interst which are the beat frequencies between
the output signal from divider 31 and the 162nd and 163rd harmonics
of the 50 microsecond 60 Hz pulse train appearing at the output of
the transistor ~11. When the-a.c. power line frequency is exactly
60 Hz, the 162nd and 163rd harmonics would be at 9720 Hz and 9780
Hz respectively, and the intruder warning system operating frequency
will lie midway between them at 9750 Hz, resulting in a beat fre-
quency difference of 30 Hz which is outside the passband of the
field detecting circuitry, and in particular the operational
amplifier 18, and hence no false alarm will be produced. However,
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if the power line should change frequency slightly so that one of
its harmonics comes too close to the operating frequency of 9750
Hz, for example, within 3 Hz, a false alarm could be triggered.
In order to prevent such a false alarm, and to provide an addition-
al margin of safety, the output signal from the diode detector 35
is fed to an active lowpass filter, including the operational
amplifier 36 and its associated components which passes beat fre-
quencies below 6 Hz to inhibit the alarm whenever a beat frequency
below 6 Hz, a value which is slightly larger than the bandpass of
the signal detecting circuitry, is present.
While it was pointed out with regard to Figure 1, the AM
detector 16 could be a simple diode detector, the use of such a
detector could result in a false alarm if an extraneous nearby
signal of sufficient magnitude is detected, even if this extraneous
signal has a frequency which is outside the passband of the detec-
tion circuitry. In order to provide false alarm immunity from such
signals, as further indicated with regard to Figure 1, the detect-
or 16 is preferably a synchronous detector to whose other input is
fed the output signal from the oscillator 12 so that the detector
is switched on and off synchronously with the received signal
appearing at the output of the preamplifier 15. The circuit di--
agram for the synchronous detector is shown in Figure 4 and in-
cludes the transistors Q13 and Q14 and the associated components.
The output of the oscillator 12 is connected to the base of the
transistor Q14 while the output signal from the preamplifier 15,
which is shown in Fiugre 4 by transistors Q15 and Q16 and their
associated components, is connected to the base of transistor Q13.
With this arrangement, if a signal appears at the base
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of Q13, it will, depending upon its phase, linearly add to or sub-
tract from the square waveform produced at the collector of tran-
sistor Q13 by the switching on and off of the transistor Q14 by the
output signal from the oscillator 12. If the signal applied to the
base of transistor Q13 is of a different frequency than the signal
from oscillator 12, the collector Q13 will contain the beat fre-
quency difference which will vary the output from the synchronous
detector between plus and minus when there is no input signal to
the transistor Q13, thus producing negligible change in the average
d.c. output of the synchronous detector appearing at the collector
of transistor Q13. This will then result in no false alarm being
produced due to such an extraneous interfering signal.
Alternatively, the desired signal, which is coherent with
the signal from the oscillator 12 appears as a fixed change in the
- average d.c. level at the collector of transistor Q13. Consequent-
ly, any motion of an intruder within the area being protected by
the system according to the invention will cause the received sig-
nal amplitude to change, and consequently the d.c. level at the
collector of transistor Q13 to change and be recognized by the
succeeding circuitry as an intruder produced signal.
It should be noted that the incorporation of a synchron-
ous detector in the intrusion warning system according to the in-
vention requires that the received signal applied to the base of
transistor Q13 be properly phased with respect to the oscillator
signal applied to the base of Q14 because a synchronous detector is
inherently a phase sensitive device and produces no output when its
inputs are 90 out of phase. Since the intrusion warning system
according to the invention operates on the principle of radiating
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an electric field from a field wire and receiving this signal by
means of a receiving antenna spaced from the field wire, there in-
herently is a 90 phase shift resulting from the coupling of the
signal from the field wire 10 to the receiving antenna 13 through
space. This 90 phase shift must therefore be compensated in order
for the synchronous detector to operated at maximum effeciency.
While the compensation for this 90 phase shift may be provided by
shifting the phase of the output signal from the oscillator 12 by
90 prior to applying same to the synchronous detector, according
to a further feature of the present invention, the 90 phase shift
is incorporated in the tuned probe circuit which will be discussed
below.
As indicated above with respect to Figure 1, in order to
eliminate false alarms caused by short term transient responses,
the intrusion warning system according to the invention is provided
with a novel ramp voltage generating circuit 19 whose output volt-
age does not reach the threshold value required to cause the pulse
generator 20 to produce an output pulse until the amplifier 18
produces an output signal indicating sustained motion of an intruder
in a single given direction for a predetermined period of time.
The use of such a ramp signal is based on the realization that in
an electric field type of intrusion warning system as in the present
invention, the amplifier 18 will produce a sustained d.c. signal of
a single polarity for as long as the intruder continues to move in
a given direction. Basically, the ramp circuit operates to begin
the production of a ramp voltage of a given polarity upon the first
detection of a signal at the output of the amplifier 18. This ramp
voltage will continue to increase as long as the output signal of
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the amplifier18 is of the same polarity. The time required for the
ramp to reach the threshold value of the pulse generator 20 is
adjusted so that it is sufficiently long to provide a positive in-
dication of sustained motion and is in the order of approximately
0.75 seconds for average use. If at any time during the ramp time,
the output signal from the amplifier 18 reverses polarity, the ramp
voltage immediately returns to 0, at which point it will begin
again upon the presence of an output signal of either polarity from
the amplifier 18.
Referring now again to Figure 4 there is shown the circuit
for the ramp voltage generator 19 which generally includes the
transistors Q17 to 20 and the charging capacitor 40. The transis-
tor Q17, which has its emitter-collector path connected in parallel
with the capacitor 40, basically acts as a switch which controls
the charging of the capacitor 40 via the resistors 41 and 42 from
the source of d.c. supply voltage, and also the discharging of the
capacitor 40. In particular, whenever the transistor Q17 is con-
ducting, any charge voltage on the capacitor 40 will be immediately
discharged to ground, thus terminating the ramp voltage produced by
the charging of capacitor 40, and further charging of the capacitor
40 will be prevented. Alternatively, whenever the transistor Q17
is in its blocked state, i.e., nonconducting state! the capacitor
40 will be charged by the resistors 41 and 42 to produce the desired
ramp voltage. The transistor Q17 is normally biased so that it is
in the conducting state whenever there is either no output signal
of a given polarity at the output terminal 43 of the bandpass opera-
tional amplifier 18, or whenever the output signal at the terminal
43 passes through 0. The transistor Q18 to 020 which are connected
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between the output terminal 43 of the amplifier 18 and the base of
the transistor Q17, are responsive to a signal of either a positive
or a negative polarity appearing at the output 43 of the amplifier
18 to switch the transistor Q17 to its nonconducting state, thus
permitting the capacitor 40 to be charged to produce the desired
ramp voltage. If the output signal at the terminal 43 maintains a
given polarity for a sufficient length of time (which may be set by
varying the resistance 46 and as indicated above is normally in the
order of 0.75 seconds) capacitor 40 will be charged to a value
10 sufficient to cause the pulse generator formed by transistors Q21
and Q22 and their associated components to be triggered and produce
an outward pulse which initiates an alarm signal.
Alternatively, if the output signal at terminal 43 is
produced by an interference or transient signal having a period
which is short with respect to the time re~uired for the capacitor
16 to be charged to the threshold value required to trigger the
pulse generator, charging of the capacitor 40 would be initiated
whenever the output signal at the terminal 43 is of a positive or a
negative polarity but the transistor Q17 would be rendered conduc-
20 tive, thus immediately discharging the capacitor 40 whenever theoutput signal at terminal 43 passed through 0, and consequently the
capacitor 40 would never be charged to the value required to trigger
the pulse generator and initiate an alarm. It should be noted that
an interference signal of the type mentioned which swings back and
forth between positive and negative for the ramp voltage to reach
the desired threshold value is often encountered in practical appli-
cations. For example, such a signal could be a beat frequency be-
- tween the frequency of the electric field generated by the field
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wire 10 and some other signal or it could, for example, be the re-
sult of vibration of the field wire 10 relative to the antenna 13.
The description of the invention thus far has been
directed to features for improving the immunity of the system to
false alarms. According to further features of the invention,
however, the versatility of the basic instrusion warning system
according to the invention is improved by providing configurations
for the receiving antenna which are different than the simple wire
or rod disclosed in the prior art system and by providing improved
signal input circuitry to the amplifier 15 of Figure 1.
Referring now to Figure 5, there is shown one embodiment
of an improved antenna and tuned probe circuit arrangement accord--
ing to the invention which is particularly useful for indoor appli-
cations, for example, the protection of a room as shown in Figure 6.
The circuit shown in Figure 5, consists of a metal box approximately
one and a half inches high by about 4 inches square with the box
itself serving as the receiving antenna 13 and the remainder of the
circuitry shown in Figure 5 being contained within the metal box.
As shown in Figure 6, such an antenna and probe circuit arrangement
2n may be utilized to provide protection for a room, for example, a
room 16 by 20 feet square, by installing the metal box in the attic
or under the floor in approximately the center of the room to be
protected and by running the field wire 10 substantially around the
perimeter of the room, also within the attic or under the floor.
As further shown in Figure 6, the metal box 13-14 is connected to
the signal input of the intrusion alarm system, which constitutes
the input to the amplifier 15 of Figure 1, via a length of shielded
cable. For reasons, which will become clear from the following
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discussion of the tuned probe circuit, the amplifier 15 is pro-
vided with a low input impedance, which is constituted by the
grounded base input stage transistor Q15 of Figure 4. The probe
; circuit arrangement of Figure 5, which is connected to this low
impedance input basically comprises a parallel resonant LC circuit,
which is tuned to the frequency of the oscillator 12 and which has
a series resistance connected in its inductive branch. The value
of the resistance is chosen to yield the desired Q for the tuned
circuit, with Q values in the order of 10 to 20 be preferred.
Additionally, the impedance of the tuned probe circuit is high
relative to the impedance which it sees looking into the end of
the shielded cable, which in turn is matched to the relatively low
input impedance of the amplifier 15, so that relatively long
lengths of shielding cable may be utilized whereby the receiving
antenna 13 and tuned probe circuit 14 may be remotely located from
the amplifier 15,-and yet provide sufficient gain to enable proper
detection of the signals of interest.
It should further be noted that with the probe and
antenna circuit arrangement of Figure 5, the signal delivered to
the input of the transistor Q15 of Figure 4 is the series signal
circulating within the tuned circuit. This technique results in
the 90 phase shift required to correct for the inherent 90
phase shift which takes place between the field wire 10 and the
antenna 13 due to their capacitive coupling through space, and thus
permits proper operation of the synchronous detector 16. Moreover,
as a result of the low input impedance of the amplifier 15, and its
relationship to the impedance of the shielded cable and the tuned
probed circuit, it is possible to connect a plurality of these
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tuned probe circuits 14 via separate respective lengths of
shielded cable to a single input of the amplifier 15, thus per-
mitting the simultaneous monitoring of a plurality of areas.
Figure 7 shows a modification of the antenna and tuned
probe circuit arrangement of Figure 5 which is particularly useful
for outdoor applications wherein perimeter protection is desired.
The circuit shown in this figure is essentially the same as that
shown in Figure 5 with the exception that the antenna is not con-
stituted by the metal box containing the tuned probe circuit, but
rather is provided by a long length of wire which is connected to
a terminal 70. In applications where such a long wire receiving
antenna is utilized, it is placed parallel to the field wire 10
along a considerable length thereof. One such application is
shown for example in Figure 8. As a result of the close coupling
between the field and antenna wires in this embodiment, the signal
received by the antenna wire is considerably greater in amplitude
than would be received by antenna such as shown in Figure 5, and
accordingly the amplitude of the received signal is tapped down in
the tuned circuit by means of the divider formed by the two series
connected capacitors Cl and C2. Other than this difference, the
circuit shown in Figures 4 and 5 are essentially the same and are
connected to the input of the amplifier 15 in the same manner.
In the application shown in Figure 8, the protection of
the perimeter of field is desired. Accordingly, to provide such
a perimeter protection, the field wire is extended around the per-
imeter of the area to be protected and mounted on fence posts 80,
distributed about the perimeter, by means of insulators 81. The
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field wire 10 in such an application is for example, suspended on
the posts approximately five feet from the ground. The antenna
wire 13 is mounted on the posts 80 in a similar manner so that it
extends around the perimeter parallel to the field wire 10 between
- the field wire and ground. For example, the antenna wire may be
approximately one and a half feet from the ground. With such an
installation, anyone approaching within approximately five feet of
the fence formed by the field and antenna wires will be detected.
It should be noted that with the type of installation shown in
Figure 8 an additional source of false alarm problems is provided
in that wind, birds or the like may cause vibration of the field
and/or antenna wires. In order to reduce the susceptibility of the
system to false alarms from this source, according to a further
feature of the invention as shown in Figure 8, one end of each of
the wires is connected to its associated fence posts 80 via a
spring 82 which places the associated field or antenna wire under
sufficient tension so that any vibration thereof would be at a rate
which is outside of the passband of the amplifier 18. This can be
accomplished for example, by placing the wires under a tension of
approximately 50 pounds with fence posts 80 spaced approximately
twenty-five feet apart~
Finally, in addition to the above-discussed improvements
and modifications of the basic prior art system which extends its
utility for practical applications, it is desirable and usual in
intrusion warning systems to provide some type of supervision for
any lines or wires which are utilized for detection purposes to
determine if the wires are cut, shorted together or grounded so
.~._ .... . . .
~100~8
that the operator will know whether the system is operating
properly. With the outdoor "fence" type of perimeter protection
arrangement shown in Figure 8, it would be relativley easy for
someone to cut, short or ground the field and/or antenna wires
during some period of time when the system is normally not in
operation and accordingly some type of supervision arrangement is
desirable to detect any such damage whenever the operator places
the system in operation. It should be noted that tampering with
the wlres while the system is in operation is not a problem since
such tampering would set off the alarm. A relatively simple
circuit arrangement for providing supervision for a fence type
arrangement such as shown in Figure 8 is shown in Figure 9.
With a fence type arrangement such as shown in Figure 8
wherein the field wire 10 and the antenna wire 13 extend parallel
to one another, passing a current through one wire and then back
through the other wire will suffice to indicate if either wire is
cut or shorted to ground. Moreover, if a series resistance is
connected between the remote ends of the field and antenna wires
when the current is passed therethrough, the basic information
needed to detect if the two wires are shorted together will be
provided. However, merely adding such a resistor across the re-
mote ends of the two wires would interfere with the desired coup-
ling of the electric field between the field and antenna wires,
and accordingly a network must be connected between the remote
ends of the field and antenna wires which will permit the d.c.
monitoring current to perform its supervision function while at
the same tir.e prevent the direct coupling of the field voltage from the field
- 22 -
-`~ 11002~8
wire lO to the antenna wire 13. As shown in Figure 9, the de-
sired resistive termination between the remote ends of the field
and antenna wires is achieved by providing the termination in the
form of two series connected resistors 90 and 91 and by connect-
ing the common junction of the resistors 90 and 91 to ground via
a capacitor 92. This arrangement allows a d.c. monitoring current
applied to the field wire 10 to flow while at the same time shunt-
ing the field voltage to ground via the capacitor~92. Typical
10 values for the resistances 90 and 91 and the capacitor 92 are 1
megohm, 270 kilohms and 0.1 microfarad respectively. Since the
- resistance values for the resistors 90 and 91 are high, this
arrangement does not significantly load or hamper the normal
usage of the field and probe wires. In order to complete the
supervision arrangement, the d.c. current flowing in the loop
formed by the field wire 10, the resistors 90 and 91 and the
antenna wire 13 is monitored by a current detecting circuit 93
which produces an output signal to energize the alarm 22 of Figure
l if the monitored current falls outside of a normal or predeter-
mined range. The reason for this is that if either the fieldwire lO or the antenna wire 13 is cut or shorted to ground the
monitored d.c. current will drop, while if the two wires are
shorted together the current will increase. Consequently, if the
monitored current falls outside of the predetermined range either
by being too high or too low, this is an indication that a
problem exists and the operator is immediately warned of same
when he tries to turn on the system by the fact that the alarm is
sounding.
- 23 -
110~ 8
Although the supervision arrangement of Figure 9 operates satis-
factorily Eor the monitoring of an arrangement wherein a long
antenna wire is utilized, a different problem exists wherein an
installation such as shown for example, in Figure 5 is present.
In such an installation, it is desirable to provide supervision
for the field wire and for the shielded cable connected to the re-
motely located tuned probe circuit and antenna arrangement. Figure
10 shows such a supervision arrangement.
In order to supervise the field wire 10, a d.c. voltage
is applied to the end of the field wire connected to the output of
the oscillator 12 and the other end of the field wire 10 is
connected via a resistance 100 and a capacitance 101 to ground in
order to shunt the output signal from the oscillator 12 to ground.
Connected to the capacitor 101 is the base of a transistor Q23
whose emitter is connected to ground and whose collector is
connected to a source of d.c. voltage and to the alarm circuit 21
of Figure 1. If the probe wire 10 is neither cut nor shorted to
ground, the d.c. voltage applied thereto will appear at the base
of transistor Q23 and hold same in the conducting state. However,
in the event the field wire 10 is cut or grounded at some point,
current will no longer flow to the base of transistor Q23, causing
transistor Q23 to be non-conductive and its collector to go posi-
tive. The positive voltage on the collector of transistor Q23 is
then fed to the alarm circuit 21 of Figure 1 to cause the produc-
tion of an alarm.
In order to be able to monitor the shielded cable connect-
ing the tuned probe circuit 14 to the input of the amplifier 15,
- 24 -
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~,.,.. . ~ .
110~a~8
as shown in Figure 10 a shielded cable having two inner-conductors
102, 103 and a shield 104 is utilized. The conductor 102 is
connected to the high or signal output terminal of the tuned probe
circuit 14 (see Figure 5) and via a voltage divider 105 to the in-
put of the arr~lifier 15. The other inner-conductor 103 of the shield-
ed cable is connected to the low side of the tuned probe circuit
14 (see Figure 5) and via resistor 106 to a source of d.c. voltage.
The shield 104 is connected to the conductor 103 at the location
of tuned probe circuit 14 and is connected to ground only at the
10 end thereof at the location of the amplifier. Thus, the inner-
conductor 103 and the shield 104 fo^m a closed loop extending from
the location of the amplifier to the location of the tuned probe
circuit and back to the location of the amplifier. Connected to
the inner-conductor 104 via a resistance 107 is the base of a
transistor Q24 whose emitter is connected to ground and whose
collector is connected to the base of the transistor Q23. With
the circuit arrangement as shown, which is illustrated for a system
with two identically connected tuned probe circuits connected to
the input of the amplifier 15, if the shielded cable is intact no
20 current will flow through the base of transistor Q24 and consequent-
ly Q24 is non-conducting. Alternatively, if the shielded cable
has been cut, a current will be delivered to the base of transistor
Q24 causing it to become conductive and in turn switch transistor
23 to its non-conductive state to initiate an alarm.
It will be understood that the above description of the
present invention is susceptible to various modifications, changes
and adaptations, and the same are intended to be comprehended with-
in the meaning and range of equivalents of the appended claims.
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