Note: Descriptions are shown in the official language in which they were submitted.
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INTRUDER/ESCAPEE DETECTION SYSTEM AND METHOD USING A
DISTRIBUTED ANTENNA AND AN ARRAY OF DISCRETE ANTENNAS
DESCRIPTION
TECHNICAL FIELD:
The invention relates to detection systems and methods and, in
particular, to detection systems and methods which are used to detect objects
or people moving in the vicinity of a predetermined path or line defined by a
distributed antenna, for example an open transmission line. The invention is
especially applicable to the detection of intruders or escapees.
BACKGROUND ART:
Known such detection systems use at least one open transmission line,
usually a leaky cable, as a distributed receiving antenna to receive a radio
frequency signal transmitted from an associated antenna; or as a transmitting
antenna to transmit signals for reception by a separate antenna. An intruder
or escapee, or other object, moving in the vicinity of the leaky cable causes
a
perturbation in the coupling of continuous wave RF energy into or from the
leaky cable. Detection of the perturbation indicates an intrusion or escape
attempt. It will be appreciated that there is technically no distinction
between
an intruder traversing the path to enter a protected zone and an escapee
traversing the path to leave a protected zone. For convenience, therefore, in
this specification, the term "intruder" will be used to cover both.
It is desirable to determine, at least approximately, the location of the
intruder along the length of the cable. United States patent number 4,994,789
(Harman) issued February 19, 1991 discloses a detection system in which
several detection zones are provided by interposing phase-shifting modulators
at intervals along the leaky cable. Each modulator can be shunted by a switch.
A signal processor analyzes the signal received from the cable while the
switch
is operated so as to shunt the modulator or connect it in series with the
cable
sections, thereby allowing determination of the section in which the intrusion
occurred. When such a system uses only two zones, it may be relatively
economical. However, when such a system is expanded to many zones, the
interdependence of the modulators, the complexities of switching them, and
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intricacies of signal analysis prohibitively increase cost and reduce
reliability.
United States patent specification number 4,887,069 (Maki) issued
December 12, 1989 discloses a detection system which uses two coaxial
cables, one of them a leaky cable, extending along a perimeter of a protection
zone, one coupled to a transmitter and the other to a receiver. The cables are
subdivided into sections which are interconnected by oscillators and switches
allowing selection of one section at a time. If a section has not been
selected,
the RF signal passes along its inner conductor. When a section is selected,
the RF signal is switched to propagate as an external wave along the outer
sheath of the cable section. Maki also discloses a system in which both of the
coaxial cables are leaky cables, with zones provided by serialized switching,
each zone being powered from a switched local oscillator. In either case,
signal
perturbations caused by an intruder are transmitted through the intervening
sections to a receiver located at one end of the cable. The oscillators and
switches increase complexity and reduce reliability.
In either of these known systems, the radio frequency energy from each
cable section must pass through any preceding sections. Consequently, failure
of an oscillator/switch, or modulator, as the case may be, especially near the
receiver end of the cable, may compromise the system or even render it
inoperative.
The present invention seeks to provide a detection apparatus and method
with a plurality of detection zones, while eliminating or at least mitigating
the
afore-mentioned disadvantages of known detection systems.
SUMMARY OF THE INVENTION:
According to one aspect of the present invention, a detection system for
detecting intruders moving in the vicinity of a defined path comprises a
distributed antenna, for example an open transmission line, extending along
the
path and an array of discrete antennas extending alongside the distributed
antenna and within a predetermined distance therefrom, the antennas being
spaced apart from each other and the distributed antenna and defining a
plurality of detection zones, a radio frequency transmitter connected to one
of
the distributed antenna and the array of discrete antennas, a complementary
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receiver connected to the other of the distributed antenna and the array of
discrete antennas, and control means for controlling the transmitter, receiver
and array of antennas to exchange radio frequency energy several times via the
distributed antenna and selected ones of the array of antennas and to analyze
the energy received from each selected one of the array of antennas so as to
detect perturbations caused by an intruder moving adjacent said path and
adjacent that particular antenna.
The control means may comprise switching means for selecting each one
of the array antennas individually for such energy exchange.
The control means may select the antennas in turn in such a sequence
that, if the energy from a particular antenna when previously selected within
a prescribed time period showed a perturbation, that antenna would be selected
more frequently than those antennas which had not shown such a perturbation
within said time period.
Preferably, the array of antennas are each connected to a respective one
of a plurality of taps distributed along a transmission line extending
alongside
the distributed antenna. The control means then may comprise a plurality of
switching devices for connecting respective ones of the antennas to the
transmission line and switch control means for controlling operation of the
switching devices to select the antennas individually.
The switch control means may comprise a means for transmitting
antenna addresses selectively onto the transmission line and each switching
device then may comprise an address decoder for detecting the address of the
associated antenna and an RF switch operable by the decoder to connect the
antenna to the transmission line.
According to a second aspect of the invention, there is provided a
method of operating an intrusion detection system having a distributed antenna
extending along a path to be monitored and an array of discrete antennas
extending alongside the distributed antenna and within a predetermined
distance therefrom, the method comprising the steps of:
exchanging radio frequency energy several times between the distributed
antenna and each of the array of discrete antennas, in turn, and analyzing
energy received by each selected antenna to detect perturbations in the
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received radio frequency energy indicative of an intruder moving near the path
and adjacent that antenna.
Preferably, the array antennas are selected in turn in such a sequence
that, if the energy from a particular array antenna when previously selected
within a prescribed time period showed a perturbation, that array antenna will
be selected more frequently than those array antennas which had not shown
such a perturbation within said time period.
Thus, the selection sequence may comprise alternate selection of
perturbation-indicating antennas with non-perturbation indicating antennas.
Alternatively, the selection sequence may select all of the perturbation-
indicating antennas after each selection of a non-perturbation-indicating
antenna.
In addition to selecting a perturbation-indicating antenna for which a
perturbation was previously recorded, the selection sequence may also be
arranged to select one or each of the antennas adjacent thereto.
In embodiments of either aspect of the invention, the array antennas
may be fastened to a fence, trees, a wall or a roof and the distributed
antenna
may be a leaky cable or the like buried nearby.
Preferably, each array antenna is a "small" antenna, i.e. no dimension of
the antenna exceeds one tenth of the wavelength of the operating signal and
the antenna has substantially no directivity.
When such a small antenna is mounted upon a fence or other generally
unstable support, it may be susceptible to movement of the fence or other
support. A third aspect of the invention addresses this problem by means of
a series resistance of between 150 ohms and 300 ohms between the tap and
the antenna element, preferably about 220 ohms.
According to a third aspect of the invention, a small radio frequency
antenna suitable for mounting upon an unstable support, such as a wire fence,
comprises an insulating board having first and second opposite surfaces, a
conductive layer on the first surface, means for attaching the antenna to the
support with the second surface opposed to the support, and a resistance
means for connecting the conductive layer to a feedline, the resistance having
a value between about 150 ohms and about 300 ohms.
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BRIEF DESCRIPTION OF THE DRAWINGS:
Further features and advantages of the invention will be apparent from
the following description of preferred embodiments which are described by way
of example only and with reference to the accompanying drawings, in which:-
5 Figure 1 is a pictorial drawing of a detection system;
Figure 2 is a simplified block schematic drawing of the detection system
illustrated in Figure 1;
Figure 3 is a schematic diagram of one of an array of antennas illustrated
in Figures 1 and 2;
Figure 4 is a block schematic diagram of a switching device coupling the
antenna to a transmission line;
Figure 5 is a perspective view of a patch antenna useful in the detection
system illustrated in Figures 1 to 4; and
Figure 6 is a flow chart diagram illustrating a method of operation of the
system.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS:
For convenience of illustration, Figure 1 illustrates only a portion,
including
several detection zones, of the detection system. The system comprises a
leaky coaxial cable 102 or other suitable open transmission line means, either
laid upon the surface of the ground or buried a short distance beneath the
surface, which defines a detection path or line to be monitored. A
transmission
line 103, conveniently a regular coaxial cable, is shown mounted along a
security fence 104 (but alternatively may be buried along the base of the
fence
104). The cable 102 is depicted, for purposes of illustration only, as having
a detection field 102a extending radially around it. It will be appreciated
that,
if the cable 102 is connected to a receiver, the detection field 102a will be
induced rather than generated directly. The transmission line 103 has a
plurality of taps 103/1...103/n spaced apart along its length. The taps are
connected by switching devices 107/1...107/n, respectively, to a
corresponding plurality of small antennas 108/1...108/n, respectively. Each
tap is a T-junction allowing the switching devices to select the antennas
individually for connection to the transmission line 103 without the
continuity
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of the transmission line 103 being interrupted. The patch antennas
108/1...108/n are spaced from the cable 102 to provide a required degree of
coupling therebetween while giving some room for a body to intrude into the
detection zones. In operation, electromagnetic fields between the leoky cable
102 and the plurality of taps 103/1...103/n define a corresponding plurality
of
overlapping detection zones depicted, for purposes of illustration only, by
lines
109/1;110/1...109/n;110/n, respectively.
In one experimental setup, the leaky cable 102 and the transmission line
103 were spaced about 20 feet apart and up to 2 miles in length with the
lo antennas at intervals of 50 feet or so. Thus, typically, each antenna forms
a
perimeter sub-zone about 50 ft long, each sub-zone overlapped with its
neighbouring sub-zone to obtain fuli coverage.
As shown in Figure 2, the transmitter 105 and the receiver 106 are
connected to, and controlled by, a microprocessor 111. The transmitter 105,
receiver 106 and processor 111 may be constructed and operate generally in
a manner known to persons skiiied In this art and so will not be described in
detail here. For examples, the reader is directed to International patent
applications numbers PCT/CA91 /00050, PCT/CA98/0055 9 and
PCT/CA98/00$4Q
The transmitter unit 105 inciudes a radio frequency transmitter
(continuous wave source) 112 and a switch address encoder and transmitter
113 connected in common to the transmission line 103. The switch address
encoder transmitter 113 relays encoded address signals, each comprising an
address unique to one of the antennas produced by the processor 111, for
seiectively operating the switching devices 107/1 ... 107/n and hence
connecting the patch antennas to the transmission line 103 individually and
sequentially. The processor 111 "scans" the patch antennas In turn, i.e. it
causes the RF transmitter 112 to transmit a continuous wave radlo frequency
signal onto the transmission line 103 and the address transmitter 113 to
transmit the address of the selected one of the antennas 10811...108/n. The
switching device associated with the selected antenna deteots the address and
connects the selected antenna to the transmission line 103 to receive the
radio
frequency signal and radiate it towards the leaky cable 102. The receiver 106
~
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detects the corresponding radio frequency signal received by the leaky cable
102 and determines whether or not there is a perturbation indicating that an
intruder has affected the electromagnetic coupling between the selected
antenna and the leaky cable 102. This constitutes "scanning" or "sampling"
of that antenna. How the perturbation is detected will depend upon the
transmitter/receiver chosen. A simple amplitude measurement technique is
preferred for its simplicity, but a "synergistic radar" technique might be
used
instead. If such a perturbation is detected, the microcontroller 111 records
in
memory (not shown) that the selected antenna is in a perturbation-indicating
or "pre-alarm" condition. The microcontroller 111 will not signal an actual
"intruder" alarm until a prescribed number of such "pre-alarm" conditions have
been detected, as will be described later.
Referring to Figures 3 and 4, one of the patch antennas 108/n is shown
mounted on a near side 114 of the fence 104 by a base 115 of an electrically
insulating material. The transmission line 103 is mounted on a far side 116 of
the fence 11. The switching device 107/n comprises an address decoder
117/n and a radio frequency switch 118/n. The switch 118/n is coupled via
a capacitor 119/n to the transmission line 103 and via a decoupling resistance
element 120/n to the patch antenna 108/n. Branches of the T-coupling are
labelled IN and OUT as a matter of convenience but energy may traverse the
coaxial transmission line 103 in either direction. The address decoder 117/n
and switch 118/n are housed within an RF shielding enclosure/shroud 121 /n.
The switch 11 8/n may be either any electromechanical relay or a solid state
device suitable for coupling RF energy to the antenna 108/n. The antenna
address decoder 117/n is connected to the transmission line 103 via an RF
blocking device 122/n to receive DC power and the address signals transmitted
by the address transmitter 113 and controls operation of switch element
1 18/n, via a lead 1 23/n, in response to the receipt of the associated
address
control signal coupled from the transmission line 103. When the switch 11 8/n
is closed, energy is exchanged between the transmission line 103 and the
antenna 108/n via the blocking capacitor 119/n and the decoupling resistor
120/n.
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Alternatively, the transmission line 103 and the switching devices
107/1...107/n may be concealed beneath the earth with short coaxial coupling
lines extending between each of the switches 118/1...118/n and the
associated one of the resistors 120/1 ... 1 20/n, the latter each being
located
with the corresponding one of the antennas 108/1...108/n.
Referring to Figure 5, a suitable patch antenna 108/n comprises a
conductive rectangular patch 130/n, typically having a length of 12" along one
major side 131/n and a width of 4" along a minor side 132/n; in other words
an oblong with a length to width ratio of about 3:1. The patch 130/n is
mounted upon insulating substrate 115/n which is half an inch thick and is
larger than the patch 130/n so as to provide a margin around the patch 1 30/n.
Attachment holes 134/n in the margin facilitate easy fastening, perhaps by
loops of cord, to the fence 104. As shown in Figure 3, the resistor 120/n
extends through an opening in the insulating layer 115/n and is connected at
one end to the opposed surface of patch 130/n and at the other end to the tap.
Alternatively, the insulating substrate 11 5/n can be any of several
structures
which will support the patch 130/n spaced and insulated from any conductive
surface, such as when the fence 104 is made of metallic chain link or the
antenna is connected to some other conductive structure.
Problems of false alarms and missed alarms resulting from movement of the
fence 104 or other supporting structure were substantially mitigated during
experimentation by selecting the value of the decoupling resistors
120/1...120/n to be between about 150 ohms and about 300 ohms.
Eventually it was found that the illustrated embodiment, used with an
operating
frequency of about 80 MHz., was generally optimized with decoupling resistors
120/1...120/n of about 200 to 220 ohms. This gave an antenna gain of
-25dBd with noise immunity to tolerate lateral displacement of the fence
fabric
up to +/- 1/4 inch. This is not to suggest that these values are optimal for
other operating wavelengths or for other forms of patch antennas, for example
circular or triangular. Furthermore, although illustrated separately, the
resistor
120/n may be included in the switching device 119/n or the latter may be so
constructed as to dynamically provide a suitable value of coupling resistance.
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Operation of the system will now be described with reference to the
flowchart shown in Figure 6. In function step 6.1, the processor 111
initialises
the system, e.g. counters, memory, and so on, and then begins the scanning
process by scanning the first antenna 108/1. Such scanning entails the
selection, energisation and received signal detection steps as previously
described. In decision box 6.2, processor 111 accesses its memory (not
shown) to determine whether or not there are any antennas in a "pre-alarm"
state as a result of a previous scanning cycle detecting a perturbation in its
signal. There will be no "pre-alarm" antennas because there were no previous
scans, so, in steps 6.4 and 6.5 the processor 111 will select and scan the
next
antenna 108/2. Step 6.6 determines whether or not that is the last antenna
in the array. Because it is not, loop 6.7 returns the processor to decision
step
6.2, whereupon it checks again whether or not any antennas now are in the
pre-alarm state. So long as none of the antennas are in the pre-alarm state,
this loop will repeat steps 6.2 to 6.6 until all of the antennas have been
scanned, whereupon decision step 6.6 and loop 6.8 cause the processor 111
to repeat the complete scanning cycle beginning with antenna 108/1. The
scanning cycle will repeat until the scanning of at least one of the antennas
detects a perturbation indicating that an intruder is present, i.e. a "pre-
alarm"
condition. When that happens, the processor 111 will record that antenna's
identity in memory. During the next scanning cycle, decision step 6.2 will
show that one or more antennas are in the pre-alarm state and, in function
step
6.3, the processor 111 will cause all of those pre-alarm antennas to be
scanned in turn. In practice, the processor may be programmed to scan not
only each antenna that is in the pre-alarm state, but also its immediate
neighbours. Once these "pre-alarm" and neighbouring antennas have been
scanned, the processor 111 will return to the main path of the flowchart
shown in Figure 6 and perform steps 6.4, 6.5 and 6.6 to scan the next antenna
that is not in the pre-alarm condition. It will be appreciated that, each time
the
processor 111 completes the scanning of one of the antennas that are not in
the "pre-alarm" state, it will scan all of the antennas that are in the pre-
alarm
condition again before it scans the next one of the antennas not in the pre-
alarm state.
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Preferably, the processor 111 processes the results of the scanning
process statistically to determine a probability of an actual intrusion and
raise
an alarm contingent thereupon. If the pre-alarm indication for a particular
antenna turns out to be only a random event, as determined, for example,
5 because neither of its immediate neighbours also registers a pre-alarm, the
processor 111 will not raise an alarm but rather will delete the record from
memory and continue the scanning process with that antenna treated as a
"normal" antenna. This procedure is useful to increase the sensitivity of the
system while avoiding frequent false alarms which might otherwise reduce the
10 response of attendant personnel.
Various modifications to the above-described embodiment are envisaged.
For example, the scanning routine described with reference to Figure 6 could
be arranged to scan the "pre-alarm" antennas alternately with the antennas not
indicating a "pre-alarm".
It is also envisaged that the number of "pre-alarm" antennas selected for
scanning could be limited to a predetermined maximum in order to avoid
reducing their sampling rate to an unacceptable level.
Although, in the above-described detection system, the antennas
transmit and the leaky cable receives, it will be appreciated that antenna
reciprocity applies, meaning that the receiver and the transmitter are
interchangeable.
It should be noted that, because the antennas are isolated electrically
from the leaky cable, mechanical continuity of the coaxial cable 103 can be
maintained, which affords a high level of reliability.
Advantageously, embodiments of the invention allow time- multiplexed
scanning of as many antennas (sub-zones) as possible while maintaining a
sampling rate per antenna high enough to obtain a reasonable resolution of the
intrusion profile signature.
For example, to illustrate the advantages of time multiplexing, the
following practical assumptions may be made:
a) minimum electronic sampling rate:
2 millisec/sample; and
b) typical sample per intrusion profile:
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samples / intrusion;
for a minimum sampling rate per intrusion of:
2 X 5 = 10 millisec/intrusion, or 0.01 sec/int.
c) maximum intruder running speed:
5 8 meters/second (18 miles/hour); and
d) typical width of the detection envelope:
2 meters (6 to 7 feet);
for a minimum intrusion time of:
2 / 8 = 0.25 second
Therefore, the over-sampling ratio for a running intrusion (worst case
scenario)
is:
0.25 / 0.01 = 25
This means that a maximum of 25 sub-zones can be time multiplexed
without sacrificing the probability of detection, even for a fast running
intruder.
As a practical example, a typical correctional facility perimeter might
comprise a dual fence spaced apart by 20 ft and 12 ft high, with 25 antennas
spaced apart by 50 ft, for a total perimeter of 1,250 ft (350 m) maximum per
electronic module with sub-zone resolution of 50 ft.
The concept of a variable sampling rate further improves detection
capability. In the absence of an intrusion (target), the sampling rate per
antenna may be reduced which increases the over-sampling ratio, which
typically increases the number of antennas to "time-multiplex" per electronic
module by a factor of ten (10). Detection of a pre-alarm condition may be
determined according to a pre-alarm threshold, that is normally set midway
between the alarm threshold and background noise. Under quiet operation (i.e.
in the absence of an intrusion) each antenna is switched at a lower rate on an
even basis. But when one antenna response exceeds the pre-alarm threshold,
more time is spent sampling that antenna. This technique presents a variable
sampling rate condition for no intrusion, first intrusion, second intrusion,
etc.,
which means that the probability of detecting a single intrusion is higher
than
detecting a double intrusion and so on. This compromise will usually be
acceptable to increase the number of sub-zones per single electronic module,
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though the acceptance of such a compromise depends on the threat level and
the applications.
Finally, an even finer resolution for the intrusion location may be
obtained by comparing intrusion alarm conditions between adjacent
antennas/sub-zones. This technique allows a resolution accuracy equal to one
third of the antenna spacing to be achieved, for a final location resolution
of
50 ft/3 = 17 ft (5 m).
Advantageously, each antenna/sub-zone may have an individual alarm
threshold, which means that the noise of a difficult sub-zone is not
integrated
with the entire perimeter noise. Also any sub-zone can be temporarily disabled
to allow authorized personnel to cross the perimeter without shutting down the
entire perimeter.
The increased intrusion resolution allows a surveillance video camera,
where applicable, to be pre-set to a finer resolution for more efficient video
assessment.
A wide range of miniature antennas can be used so long as the loaded
impedance is high in reference to the coaxial cable impedance. A high
impedance, like a test probe, extracts a very small portion of the RF signals
that propagate within the coaxial cable in order to minimize the cable loss
along
the array of antennas. Mini-whip antennas, say 10 inches long, or other small,
non-directional antennas, may be used instead of patch antennas.
An advantage of detection systems embodying the present invention
maintain a uniform RF signal strength all along the secured perimeter and
allow
the intrusion/crossing location to be obtained by simply activating only one
antenna at a time, i.e. by time multiplexing the antennas. Therefore, each
individual antenna forms its own detection zone.
