Note: Descriptions are shown in the official language in which they were submitted.
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INTRU13ION DETECTION APPARATUS
This invention relates to intrusion detection
apparatus a:zd particularly to the signal processing
portion of such apparatus.
A wide variety of intrusion detection equipment
is known. Some of it is for use in perimeter security
systems where for example, a special taut wire
perimeter fence m.ay be installed which, when cut or
jarred, generates a signal which in turn triggers an
alarm. Enclosed buildings or spaces within such
buildings oi'_ten have intrusion detection sensors for
detecting tree opening of windows or doors, the cutting
of electric circuits, etc. and may be provided with
infra-red o~_° other movement-detecting sensors within
confined areas. Sensors are known for responding to
seismic vibration:> ; platform sensors are available for
placement around railway tracks to sense unwanted
human intrusion upon such tracks.
Conventionally, the signal processing circuitry
following the particular intrusion detection sensor
employed wi~_1 be tailored to the specific sensor and
specific adaptation at hand.
If the intrusion detection system designer
perceives that more than one type of intrusion is
expected, then several detectors may be utilized, each
with its own associated circuitry. In some cases, the
outputs of two or more such circuits are compared
against some standard or threshold, as for example in
U.S. patent 4,223,304 (Barowitz, 16 September, 1980)
and U.S. patent 4,107,660 (Chleboun, 15 August, 1978).
A difficulty with the known intrusion detection
signal processors is that they are relatively
inflexible, being adapted for use with particular
sensors operating within particular frequency ranges.
However, in a particular application, a number of
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different types of intrusion may be expected, and
consequently several different types of sensor may be
required to be employed.
A further difficulty with many of the known
systems is that they are sensitive to transients and
spurious signals that may cause false alarms. To some
extent the known systems have circumvented these
problems by incorporating delay circuitry that rejects
signals above a particular amplitude threshold but
whose duration is too short to be likely to represent
an intrusion. However, these systems tend not to be
able to discriminate persistent signals of sufficient
amplitude caused by unwanted intrusion from those
caused by an increase in vibration level generally.
By way of example, suppose that a secured military
area within a perimeter fence attached to an intrusion
detection system, is a site for frequent helicopter
landings. If the sensitivity of the perimeter fence
detector and associated circuitry are set at a high
enough level to detect unwanted human intrusions, the
vibration due to a helicopter landing may be
sufficient to trigger an unwanted false alarm.
To overcome the foregoing problems, the present
invention comprises signal-processing circuitry for
use in an intrusion detection system that provides
both flexibility and adaptability to many different
intrusion detection situations, and also provides an
automatic means for rejecting a multiplicity of
signals that would otherwise trigger a false alarm,
which are caused by a general increase in the
prevailing level of noise or vibration in the vicinity
of the sensor or sensors used.
Commonly, a complete intrusion detection system
includes one or more sensors and one or more alarm
devices or warning devices. The signal processing
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circuitry to which the present invention is directed
may be provided as part of a complete system including
such sensor or sE~nsors and alarm or warning devices,
or may be provided separately, with the user of the
system then able to select sensing devices and select
alarm or warning devices suitable to the situation.
In onE~ aspect of the invention, a number of
signal processors are provided each including a
bandpass filter tuned to a selected frequency range.
Each filter receives the output of at least one of the
sensors usE:d fox- the system. The output can be
received direct from the sensor or via some
intervening signal processing circuitry. (It is to be
understood that i.t is conventional in the electrical
design arts to include a variety of circuit elements
having discrete functions that may be desirable or
even necess~3ry to proper operation of the system, but
yet which have no direct relationship to the special
signal processing apparatus being described. For
example, if the output signal from the filter has to
travel long distance before reaching the remaining
signal processing circuitry, it may be desirable to
incorporate an amplifier for the signal in the
vicinity of the filter so that the strength of the
amplified signal at the input of the rest of the
signal processing circuitry is adequately high. When
reading this specification, the reader should
understand that the electrical engineer designing the
circuit may elect to provide conventional signal-
modifying circuit elements, e.g. line frequency reject
notch filters, preamplifiers, delay or equalizing
circuits, variab7_e gain controls, etc. as may be
suitable. Accordingly, when in this description it is
stated that one circuit element receives as an input
the output of some other circuit element upstream, it
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is to be understood that there may well be intervening
elements between the two specified elements which
process or massage the signal in some way according to
the perceived design objectives of the designer.)
Each such bandpass filter is accordingly responsive to
sensed movement having vibration frequency components
within the associated passband. These signal
processors each provide an output affirmative signal
if the amplitude of the constituents of the input
signal within the associated frequency range exceeds
a predetermined level for a predetermined time
interval.
The threshold amplitude level at which an output
affirmative signal is generated by the signal
processor can, if desired, be varied in response to
prevailing ambient noise or any other criterion
selected by the circuit designer. To this end, the
apparatus may include a threshold bias adjustment
circuit receiving an output signal from one or more of
the bandpass filters which receives the sensed signal.
This threshold bias adjustment circuit raises the
threshold signal above which an output affirmative
signal is produced by one or more other
bandpass filters in response to an increase in ambient
noise.
Where the intrusive movement is expected to be
reflected in typical frequency nodes, then for
increased rejection of false alarms, it may be
desirable to combine the various bandpass filter
outputs in a logic circuit, preferably after
digitizing these outputs, in order to establish
whether the pattern of detected signal frequency
components corresponds to patterns that are known or
expected by the designer to be associated with
unwanted human intrusion. To that end, several
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different affirmative output signals may be provided within
different frequency channels; equally, one or more
inhibiting output signals could be provided in response to
detected frequencies within other frequency channels for
the purpose of adjusting the threshold bias of the other
channels or even for outright rejection of the affirmative
signals that otherwise might trigger an alarm.
In the drawings,
Figure 1 is a block diagram of intrusion detection
signal processing apparatus in accordance with one
embodiment of t;he invention;
Figure 2 is a block diagram of signal processing
apparatus for i~se in association with a second embodiment
of the invention.
Referring to Figure 1, a sensor 11 is placed within or
near a secured space for detecting intrusive movement. The
sensor 11 is a sensor for sensing or tracking movement of
a barrier. The sensor can include a movable barrier member
such as a noise coaxial cable.
The sensor output passes through a line-frequency
notch reject filter 13, so as to reject spurious signal
components at t;he line frequency (typically 60 Hz in North
America, 50 Hz in Europe, for example).
The output= of the notch filter 13 is applied to each
of a number of bandpass filters. The number of bandpass
filters to ~~e chosen will be dependent upon the
application. In F_Lgure 1, three bandpass filters are
included for reasons of simplicity, although in many
applications more ba;ndpass filters would be expected to be
included.
(Throughout thi:~ description it is understood that any
conventional s_~gnal :processing devices may be
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inserted in the circuitry where desired to massage or
modulate the sign~.l in some suitable way. Equally, in
some cases electrical engineers will recognize that
the sequence of various elements could be reversed.
Notch filters cou7_d, for example, follow the bandpass
filters. But it is easier and less expensive to have
a single notch filter precede all of the bandpass
filters.)
Specifically, the output from the notch filter 13
is passed via adjustable gain controls (or adjustable
attenduators) 15, 17, 19 to respective bandpass
filters 21, 23, 25, which have been designated for
convenience the channel A filter, the channel B filter
and the channel C filter respectively. Each filter
21, 23, 25 :is independently tunable to a particular
passband selected by the designer.
In the particular example being discussed, it is
assumed that channels A and C are tuned to two
separate frequency ranges in which signal components
representative of unwanted human intrusion are likely
to occur. Channel B by contrast is an ambient noise
channel; thE~ bandpass filter 23 is tuned to that
frequency r~inge in which ambient noise, especially
occasionally occurring ambient noise of fairly strong
amplitude, is expected to occur.
Although only a single sensor is shown providing
a split input to each of the three bandpass filters
21 , 23 , 25 , it is to be understood that a number of
different sensors could be employed, each of which
could provide an output to one or more channels.
Sensors may be as:~ociated with bandpass filters on a
one-to-one basis, or otherwise as the designer may
choose.
The outputs of the three bandpass filters 21, 23,
25 are applied as inputs to delay circuits 27, 29, 31
WO 94/20937 PCT/CA93100092
respectively. Preferably, these delay circuits are
adjustable as to the time interval during which
effective delay of the output signal of the associated
bandpass filter i.s subjected. These delay circuits
have the e:Efect of rejecting very short, transient
signals, even if they exceed a particular threshold
amplitude, so that such signals, which typically are
spurious signals not caused by unwanted human
intrusion, :may be rejected.
The outputs of delay circuits 27 and 31 from the
channel A ~~andpa:~s filter 21 and channel C bandpass
filter 25, which are expected to reflect different
types of intrusive human movement, are passed to
comparator circuits 33 and 35 respectively. These
comparator circuits compare the output signal received
from their associated delay circuits against a
threshold amplitude. If that amplitude is exceeded,
then the comparator generates an alarm signal which is
passed to a suitable alarm or warning device 37.
Because occasional high ambient noise levels may,
unless countermeasures are taken, provoke spurious
signals and therefore false alarms as a consequence of
relatively high signal levels passing through the
channel A and channel C bandpass filters 21, 25
respectivel~r, they channel B ambient noise bandpass
filter 23 provides its output to a threshold bias
circuit 39 which in turn varies the threshold level
operating in comparators 33 and 35 respectively. The
variation is effected via adjustable gain controls 41,
43 respectively. As the signal passed by the channel
B bandpass filter 23 increases, so does the threshold
amplitude acTainst which comparators 33 and 35 test the
input signa:L that they receive within channels A and
C respectively. So as ambient noise increases,
accordingly a higher level of signal component within
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WO 94/20937 PCT/CA93/00092
channels A and C is required to trigger an alarm.
To take again the helicopter example, if a
helicopter is landing, it may be expected to create a
general increase in signal level within all or many
frequency ranges to which various bandpass filters may
be tuned. Consequently, every time the helicopter
landed there would be an alarm signal triggered,
unless a suitable countermeasure were taken. The
countermeasure taken is to increase the threshold
amplitude at which the comparators 33 and 35 generate
an alarm signal when the landing noise is occurring.
The threshold amplitude governing comparators 33 and
35 increases in response to ambient noise, which will
be received by the channel B bandpass filter 23 and
passed on to the threshold bias circuit 39 so as to
raise the threshold amplitudes against which
comparators 33 and 35 test the output signal from the
channel A and channel C bandpass filters 21, 25
respectively. Alternatively, the circuit 39 (or some
substitute circuit) could simply nullify the alarm
trigger signal when ambient noise level is above some
specific level. (It may be tolerable to have the
intrusion detection circuit rendered inoperative when
the secured area is the site of activity by authorized
personnel.)
To give a more specific working example, the
sensor 11 might be a strain-sensitive cable mounted on
a perimeter fence, say a chain-like fence. Human
intrusive movement in the vicinity of the perimeter
fence causing flexing or stretching of the coaxial
cable, would be expected to produce characteristic
nodes at relatively low frequencies (typically less
than 30 Hz) and thus the coaxial cable would be
expected to produce a reasonably strong output signal
in relatively low frequency ranges under about 30 Hz.
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So if the channel A bandpass filter were a low-pass
filter tuned to a frequency-range below about 30 Hz,
the output from that filter could be used to generate
a useful affirmative signal which, if higher than the
established threshold amplitude, would trigger the
alarm.
The channel C bandpass filter might be tuned for
example, tc> the range l5KHz to 30KHz, it has been
found that with_Ln this frequency range there are
characteristic amplitude nodes reflective of the
cutting of a wire. Consequently, if the perimeter
fence wire were being cut or even if the coaxial cable
itself were being cut, a characteristic signal would
be expected to occur within channel C (lSKHz to
30KHz). This too could trigger the alarm if the
amplitude of the sensed signal within this
frequency range e:KCeeds the threshold established for
the comparavor 35.
The channel B bandpass filter could be a very
broad frequency range filter; indeed in some cases
the channe7_ B filter might encompass all useful
frequencies, In other cases, it may be desirable to
tune the channel B filter to the most prevalent
frequency r<~nges of ambient noise expected to occur,
or to particular types of ambient noise that reflect
certain typES of acceptable intrusion (the helicopter
landing, in.the example previously used, or perhaps
the opening of a gate under the protection of a guard,
or somethin~~ of that sort). If the ambient noise
detected wit=hin channel B is sufficiently high, the
threshold b:Las adjustment circuit 39 will raise the
amplitude levels above which the output signals from
channel A and channel C filters 21, 25 respectively
must be in order too trigger an alarm.
The specific circuits used to implement the block
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diagram of Figure 1 are entirely within the
conventional choice of the designer. Bandpass filter
design is a straightforward exercise; both active and
passive filters are known, any suitable ones of which
may be chosen for the particular type or types of
installation for which the system has been designed.
The delay circuits may be conventional resistance-
capacitance circuits. The comparator circuits may use
conventional differential amplifiers or similar
analog-circuit devices. The threshold bias circuit
may simply be applied as a bias to the comparator via
a resistor or voltage divider. The alarm circuit may
be any suitable conventional alarm or warning device.
The circuit of Figure 1 may be entirely analog in
character. However, for more complex arrangements, it
is usually desirable to digitize the signals
corresponding to selected frequency ranges before
performing comparisons. To this end, the circuitry of
Figure 2 is suitable.
The Figure 2 circuitry can be substantially
identical to the Figure 1 circuitry up to the point of
output of the delay circuits 27, 29, 31. However,
instead of having the delay circuit outputs fed to
bias circuits or comparators, their outputs,
preferably following full-wave rectification, are
digitized by a conventional analog-digital converter
45. The analog-digital converter preserves the
frequency sensitive information and provides as many
digitized outputs as there are analog inputs, each
digital output corresponding to a particular frequency
range passed by a particular bandpass filter. These
digital outputs are all provided to a logic circuit 47
which performs the required comparisons.
The comparisons required are those chosen by the
designer for any particular installation. If the only
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comparisons chosen are the same as those described
with refere:zce to Figure 1 , then one of the digital
signals corresponding to the analog signal within the
frequency range passed by the channel B bandpass
filter 23 will be used to raise the effective
threshold at: which the digitized signal corresponding
to channels A and C respectively generate an output
alarm signal. In more complex installations, the
designer may decide that, for example, if affirmative
signals are capable of being generated by, say, half
a dozen bandpass filters, that it will be necessary
for affirmative sugnals to be present in at least four
of the six channels before an alarm is triggered.
Equally, the amp_Litude level at which an alarm is
triggered may be raised, depending upon whether one or
more ambient noise channels are providing sufficiently
strong signals. I:t can be seen that the complexity of
the logic circuitry can quickly escalate as the number
of channels being examined increases in number. The
choice of the number of channels to be examined and
their role with reference to the frequency ranges
selected will be highly dependent upon the particular
security installation for which the system is chosen
and will equally be dependent upon the system
designer's expectations. In appropriate cases, the
circuitry could be absolutely standard with only the
gain, tuning, and time adjustments being set for a
particular __nstallation and the logic circuit being
governed by a particular replacement EPROM memory chip
selected for the particular type of installation in
question.
The alarm device 49 can be any conventional alarm
or warning devicE~ responsive to a suitable output
provided by the logic circuit 47. The only difference
between the alarm 37 of Figure 1 and the alarm 49 of
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Figure 2 is that the Figure 1 alarm 37 is triggered by
either of two analog signals, whereas the alarm 49 is
triggered by a single output signal of the digital
circuitry 47.
Further variants in the circuitry will be
apparent to those skilled in the art without departing
from the scope of the invention, which is as defined
in the accompanying claims.
