Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: INTRUSION ALARM WITH INDEPENDENT TROUBLE EVALUATION
BACKGROUND OF THE INVENTION
The present invention relates to intrusion
detectors and in particular relates to a new arrangement
and method for processing the signals received from sensors
used in intrusion detection systems.
Passive infrared intrusion detectors, microwave
detectors and ultrasonic detectors are often used in paired
combinations to provide a system having a dual technology
which is less prone to false alarms and is generally
considered more reliable. The combination of passive
infrared and microwave detectors is quite common, as the
type of situations which can cause false alarms are
generally not common to each detector, thus reducing the
likelihood of false alarms. The combining of different
detectors improves reliability and increases
sophistication.
A number of existing dual technology intrusion
detection systems make an evaluation of whether the overall
system is working satisfactorily or whether the system,
although not producing false alarms, may be in trouble.
One such assessment of trouble is derived from counting the
number of times one of the sensors produces an alarm output
which is unconfirmed by the other detector. Typically
there is some sort of decay function to decrease the number
of false alarms counted at a certain rate, however, should
the number of false alarms reach a predetermined maximum, a
trouble indication is generated. Other dual technology
systems look at the difference in numbers between the false
alarms of each sensor for a further evaluation of whether
the overall system is working satisfactorily. The
generation of false or unconfirmed alarms as an assessment
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of whether the overall system is working satisfactorily has
the disadvantage, in that a large portion of the
information contained within the signal from the sensor is
not evaluated except to confirm when the signal has
exceeded the alarm threshold condition. This assessment of
trouble is also governed by the alarm criteria, which may
not be the best assessment of whether the system is
operating satisfactorily or operating within a satisfactory
environment.
The signals from the different type of sensors are
well known and are analysed with respect to particular
criteria to derive a signal which is appropriately
processed to determine whether an alarm condition exists.
The prior art systems have focused on alarm
criteria and have included various compromises made to
allow the two technologies to effectively monitor the same
area. These compromises must take into account different
operating environments and to reduce the possibility of
false alarms. A detection system which produces false
alarms is most troublesome and the industry is striving to
produce systems which do not produce false alarms.
Therefore, the industry is faced with the dilemma of trying
to reduce false alarms while also providing a system which
produces an alarm when an intruder enters the monitored
space.
The signal from a passive infrared detector with
respect to the disturbances which occur in the area being
monitored can be characterized as an alternating signal
sometimes considered predominantly sinusoidal whose
magnitude typically varies between 0 and 3.6 volts peak to
peak (5 volts supply) and whose frequency varies from .1 to
10 hertz.
Some approaches for analysing this signal from the
passive infrared detector include the use of two
comparators one for evaluating positive portion of the
signal and the other for evaluating the negative portion.
Pulses are produced when the signal exceeds the threshold
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of the respective comparator and are of a duration
corresponding to the time that the signal remains above the
minimum threshold. Thus positive pulses of variable
duration have been derived by use of two comparators for
evaluating positive and negative portions of the signal
from the infrared detector. It is also possible to rectify
the signal and merely use a single comparator for
evaluation of the signal. The problems with the comparator
approach is that it is difficult to determine what the best
minimum threshold is. A number of factors can affect the
signal from the detector and not all of these disturbances
indicate that a burglar or intruder is present. RF
transient signals produced when switching walkie-talkies
between a receive and transmit mode, or the like RF
transient signals, can produce a very strong, short
duration signal. Heaters coming on within the monitored
area can produce a detectable signal, as well as small
animals such as a cat, etc., crossing through the zone.
Therefore, a problem arises in trying to distinguish
between the presence of a human intruder and a disturbance
in the signal which is not produced by such an intruder.
Use of this alarm criteria includes many compromises and
much of the signal from the detector is ignored (i.e. all
of the signal below and above the threshold).
A different approach has been to integrate the
output signal to provide a measurement of the energy of the
signal and it is believed this measurement is more
indicative of whether an intruder is present.
Unfortunately other factors enter into the consideration
such as the ability of the system to detect the desired
intruder at a long distance from the detector which
typically produces a fairly low frequency signal. Other
problems also occur due to the widely varying ambient
temperature conditions that can occur in the monitored
area. Analysis of the whole signal fails to recognize the
different signals which can be partially evaluated by
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amplitude evaluation alone or in combination with duration
and/or shape evaluation.
Many systems have used a single comparator to
produce a pulse which is counted, and if sufficient pulses
are produced within a certain time period an alarm
condition is produced. Counting arrangements can produce
false alarms as common environmental disturbances such as
blasts of hot air from the heating vents will produce the
same unit of information as the sensing of a valid target.
In order to reduce the occurrence of false alarms it is
possible to increase the comparator trip threshold and/or
increase the number of pulses counted before an alarm is
generated. Both of these techniques will indeed improve
the false alarm immunity however this will be accomplished
at the expense of the detection range of the unit. If the
number of pulses counted before an alarm condition is
produced is increased far detection range will be decreased
since far targets will produce few pulses (due to low
amplitude and frequency). If the thresholds are increased,
far response will again be reduced since the far signals
are of lower amplitude. It is for these reasons that
maximum pulse setting allowed is typically 3.
In one prior art arrangement the output signal from
the detector is fed into an absolute value circuit and
subsequently to a voltage controlled pulse generator
subsection. When the signal reaches a minimum amplitude
the voltage controlled pulse generator begins to produce
constant width pulses at a repetition rate proportional to
the amplitude of the signal typically in the hundreds of
hertz. These pulses are counted or integrated and stored
by the means of a capacitor. When the stored energy
reaches a preset level an alarm signal is generated. This
system suffers the same basic draw backs as a window
comparator system in that slowly changing low amplitude
transients which barely cross over the threshold generate
full amplitude pulses which are integrated towards a
possible alarm generation.
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Since the slow transients are allowed to produce
the same unit of information as valid distance targets, the
low frequency response of the amplifier has been set to de-
emphasize low frequency response to reduce the probability
of false alarms. Unfortunately since distant valid targets
produce low frequency signals the overall pattern coverage
is decreased as a result.
According to a different arrangement the sinusoidal
signal is fed into an absolute value circuit and when this
signal exceeds a minimum threshold its amplitude is used to
vary the charge current of a capacitor which is used as a
energy storage device. The charging current equation is
Icharge=(Vsignal~Vminimum threshold)/Rcharge
When a certain amount of energy over time (in volt seconds)
has been accumulated in the capacitor the unit will signal
an alarm. This technique is an improvement over previous
methods in that the effects of low amplitude transients
which barely cross over the minimum threshold are reduced.
This is accomplished as their energy over time is low and
thus their contribution to the accumulated total energy is
low. This technique does require the gain of the amplifier
to be excessively high to quickly generate an alarm
condition by far-off targets moving at low speed. This
presents a problem for RF induced transients which are
greatly amplified as a result of this excessive gain
requirement.
The present invention seeks to overcome the
problems associated with the prior art techniques and
provide a system having improved information processing
allowing more accurate evaluation of the signal. The
invention in the simplest form is relatively inexpensive
but the system is also capable of a high degree of
sophistication and evaluation of the signal for more
demanding applications. The invention recognizes that the
alarm criteria is not necessarily the most appropriate to
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determine a trouble signal indicative of changes in the
working environment or whether the environment is such that
the alarm criteria must be changed or the overall operation
of the system reassessed.
SUMMARY OF THE INVENTION
An intrusion detection system according to the
present invention has at least one sensor for determining
the presence of an intruder, alarm processing means for
processing the signal from the at least one sensor and
produces an alarm based on the characteristics of the
signal from the at least one sensor which characteristics
are indicative of an intruder in the monitored space. A
supervisory signal analysis arrangement is included which
evaluates the signal from the at least one sensor for
changes in the environment of the monitored space which
could give rise to a higher probability of false alarms and
produces a trouble indication when the evaluation of the
signal indicates the environment has reached a
predetermined condition where false alarms are likely to
occur. This allows corrective steps to be taken prior to
false alarms occurring. The supervisory signal analysis
arrangement applies different criteria for signal
processing and analysis than the criteria of said alarm
processing means whereby changes in the environment are
analysed by a criteria appropriate for an assessment of the
operating environment.
According to an aspect of the invention, the
supervisory signal analysis arrangement processes the
signal by means of a series of comparators having different
stepped minimum thresholds within the normal amplitude
range of the signal of interest whereby the output from the
comparators allows the magnitude of the amplitude of the
signal to be assessed relative to the stepped minimum
thresholds. The arrangement also assesses the rate at
which the amplitude of the signal increases and includes
means for producing a signal of predetermined amplitude
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from each comparator and of a duration corresponding to the
duration the signal is above the respective minimum
threshold. Different weighting factors are applied to the
signals from each comparator and the weighted signals are
combined and evaluated.
According to an aspect of the invention, a method
for processing an output signal from a detector of an
intrusion detection system which output signal corresponds
to the changes in infrared energy in the area being
monitored is disclosed. The method comprises dividing said
signal into a first and second division with the first
division being processed for alarm condition analysis and
the second division processed for trouble in the operating
environment assessment. The signal is processed by the
second division to produce at least first and second sets
of pulses, with each pulse of the first set of pulses being
produced when the signal is of an amplitude exceeding a
first predetermined level and being of a duration
corresponding to the duration the signal is maintained
above the first predetermined level. Each pulse of the
second set of pulses is produced during a pulse of the
first set of pulses when the signal exceeds a second
predetermined level which is higher than the first
predetermined level and of a duration corresponding to the
duration the signal is maintained above the second
predetermined level. The set of pulses are analysed to
evaluate whether a trouble condition exists.
A method is also disclosed for separate assessment
of alarm conditions and trouble or working environment
conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown in
the drawings wherein:
Figure 1 is a schematic of a system using separate
alarm and trouble assessment processing;
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Flgure 2 is an illustration of analysis of the
signal from a detector at various amplitude levels for
alarm and trouble determination;
Figure 3 is a schematic of an alarm system using a
feedback arrangement from trouble assessment to vary the
alarm processing steps;
Figure 4 is a schematic of a dual detection system
where each sensor has separate trouble assessment and there
is cross feedback to additionally vary alarm criteria of
each sensor based on the trouble assessment of the other;
Figure 5 is a schematic of a system similar to
Figure 4 with additional trouble assessment analysis based
on the combined trouble assessment of the sensors;
Figure 6 is a schematic layout of the passive
infrared detector;
Figure 7 is a schematic of an alternate arrangement
showing a system of four comparators;
Figure 8 is a schematic of the response from a
generally symmetrical pulse of a full wave rectified signal
when processed by the four comparator system with the
resulting pulses being shown; and
Figure 9 is a time vs. amplitude chart showing the
pulses produced from the arrangement of figure 7 analysing
the full wave rectified signal produced from a transient RF
disturbance.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention allows increased flexibility
for allowing valid motion to be distinguished from
naturally occurring disturbances. The invention recognizes
that the signals from the sensors contain information from
which an assessment of the working environment of the
sensor can be made to preferably automatically adjust the
alarm criteria if necessary to reduce false alarms or to
allow evaluation of the environment on an ongoing basis and
at various levels whereby more meaningful data is
available. The technique may be implemented using
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conventional means such as analog circuit design
techniques. The technique is also readily implemented
using digital techniques to take advantage of the long term
product stability, manufacturability and design flexibility
offered by Digital Design.
A schematic of the processing of the signal from a
sensor used in an intrusion detection system is shown in
Figure 1. The sensor 2 outputs a signal sent on line 4 to
the alarm processing unit 6 as well as to the trouble
assessment unit 8. The alarm processing unit 6 can process
the signal from the sensor in any conventional manner or at
various amplitude levels and weighting factors, as
subsequently discussed. The trouble assessment 8 is a
separate evaluation of the signal using somewhat different
criteria than the alarm processing, as it is additionally
seeking out information with respect to the environment and
somewhat longer term factors which can contribute to false
alarms. The output from the alarm processing unit 6 is
shown as 10 and the output from the trouble assessment 8 is
shown as 12. In some cases, the trouble assessment may
merely be a number or an index indicating the level of
trouble in which the sensor 2 is operating. In other
cases, it can be a number of outputs whereby the degree of
trouble and the position of the trouble may be more
accurately determined.
To consider how this signal 4 from the sensor 2 can
be evaluated, the various threshold levels of different
comparators are shown in Figure 2. As can be seen in the
lefthand side of Figure 2, five different alarm thresholds
are shown as A1 through A5. Preferably, the alarm
processing unit 6 applies different weighting factors to
the different threshold levels A1 through A5, and in this
way, the signal from the sensor can be varied depending
upon the criteria for determining whether an alarm exists.
In some cases, when the signal exceeds the threshold A1, a
very low weighting criteria could be applied, whereas if
the signal exceeds A4, a much higher weighting factor could
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be applied. In this way, signals which are more typical of
human motion can be determined and distinguished from
signals which are less likely to be caused by humans. For
example, there may be some nonhuman type signals which may
influence levels A1 through A3, whereas if you know the
signal is in A4, it is more important and should be
afforded a higher weight. An A3 condition may still be
important, but should require additional detections or
confirmation relative to an A4 condition. In any event,
the various levels of threshold allow customizing of the
alarm processing and the application of different weighting
factors as may be appropriate for the particular user or as
may be required by the particular environment.
Trouble assessment typically begins at lower
thresholds, as this is the portion of the signal which can
include much of the environmental effects. Other transient
type environmental effects can show up in the higher
threshold levels and can overlap with low alarm factors.
The assessment of trouble can have much tighter divisions
to allow further discrimination of the signal. The
assessment of trouble is often looking for longer term
effects and need not be as concerned with the criteria for
alarms to quickly produce a signal indicating a human has
entered the environment. Therefore, longer term analysis
of the signal can take place, some dampening or
approximating of the signal can take place, and different
weighting factors can be applied. Therefore, the system of
Figure 1 recognizes that trouble assessment will involve
different criteria than alarm assessment, although there
can be overlap in the analysis of the signals and each
signal can be analysed by the multi-level approach.
With the arrangement of Figure 3, there is a
feedback mechanism whereby the trouble assessment can be
used to vary the alarm criteria. For example, in some
cases it may be desirable to maintain a certain head room
or spacing between what is considered the average
environmental signal and the first level of alarm or a
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particular level of the alarm having a relatively
significant waiting factor. In a one or single threshold
alarm system, you may merely want to space that level a
certain additional amplitude above the average
environmental effect. This is accomplished in Figure 3 by
the feedback loop 14. In this case, the alarm processing
unit 6 includes variable criteria which can be influenced
by the feedback system 14.
In Figure 4, a dual detection system is shown
having a first sensor 18, a second sensor 20, a first alarm
processing unit 22, a second alarm processing unit 24, a
trouble assessment for the first sensor 18 labelled 26, and
trouble assessment for the second sensor 20 labelled 28,
with each of the alarm processing units 22 and 24 including
cross feedback from the opposite trouble assessment 28 and
26, respectively. Each of the alarm processing units 22
and 24 also include feedback from their own trouble
assessment. In this way, depending upon the particular
assessment of trouble and the signals, the alarm processing
of the particular sensor may be varied, and in some cases,
the alarm processing of the other sensor may be varied.
This can be carried out by increasing or decreasing the
particular alarm processing sensitivities. Sensors with
adjustable sensitivities are known, however, their
adjustment is generally adjusted and thereafter remains at
the adjusted level. For example, if the first sensor 18
was a microwave sensor and sensor 20 was a passive
infrared, if a heater came on within the environment of the
sensor 20 sufficient to raise the trouble assessment, the
sensitivity of the alarm processing unit 24 may be moved
upwardly whereby the possibility of a signal alarm may be
less. In this case, because the environment of the passive
infrared sensor has become more troublesome, it may be
necessary to increase the sensitivity of the microwave
system. On the other hand, what might occur is that the
microwave system may have previously been operating at a
very high sensitivity due to the fact that the passive
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infrared sensors seldom malfunctioned and, therefore, the
sensitivity of the microwave sensor must be decreased now
because the chance of the passive infrared sensor producing
an alarm output would increase because the environmental
effect for that particular sensor has been raised.
Therefore, with this system, there can be cross feedback
between the two systems to customize the variations.
Typically, in any dual detection technology there is some
judgement in setting the sensitivities of these units, and
with the arrangement of Figure 4, this judgement can be
exercised, on an ongoing basis as opposed to a single
assessment or fixed assessment.
In the arrangement of Figure 5, the dual sensor
technology of Figure 4 has been combined with an additional
system trouble evaluation identified as 30. In this case,
trouble assessment is forwarded to the overall system
trouble evaluation. Based on the trouble assessment from
each of the detectors, the system can then evaluate a
strategy for varying each of the alarm processing units 22
and 24. This allows ongoing management based on the
combined findings of both trouble assessments, where both
trouble assessments can be inputted and modify each of the
alarm processing criteria 22 and 24.
It can be appreciated with the systems of Figures 4
and 5 that there need not be a feedback loop and it may be
sufficient merely to provide a record of the trouble
evaluation for subsequent trouble shooting if false alarms
occur, or it may merely allow people to continue to monitor
trouble evaluation and they can manually adjust the system
of alarm processing or trouble assessment, if desired. In
any event, the evaluation of trouble at the various levels,
as shown in Figure 2, allows customizing of the system for
the particular sensors being used, i.e. passive infrared,
microwave or ultrasonic, as well as customizing due to the
known environment in which the sensor is going to be
placed. Various weighting factors can be applied to the
outputs from the various threshold levels, and as discussed
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in the previous application, the duration of the signals
above those thresholds can also contribute to the
evaluation.
A schematic of the infrared motion detector system
52 is shown in figure 6. The system includes a lens
assembly 54 which focuses infrared energy originating from
sources within the area of coverage on the two dual element
passive infrared detector shown as 56. The resulting
output is amplified and band pass filtered by the band pass
amplifier shown as 58. The band width of the band pass
amplifier 58 is approximately 0.1 hertz to 10 hertz.
The signal is then passed through an absolute value
convertor 60 which is a full wave rectifier. This
technique is used to conveniently analyse average energy
content in the cyclic signals.
The full wave rectified signal is then fed into an
alarm processing unit 53 and into a trouble or supervisory
signal processing unit 55. Alarm processing unit 53 may be
of the conventional type or use an arrangement similar to
trouble processing unit 55 modified for alarm detection
criteria. Trouble processing unit 55 includes an n level
multicomparator stage 62 which has been preconfigured to
analyse the maximum dynamic range of the signal by
evaluating the signal at n predetermined minimum threshold
values stepped throughout the maximum dynamic range. As
the input signal crosses each of these thresholds, a
corresponding output pulse is generated at the
corresponding comparator output. The pulse has a fixed
magnitude, which is a function of the dynamic range of the
system. The rectified signal can be characterized by
accumulation of the energy of the output pulses from multi
stage comparator 62. The more levels that are used, the
more information that is extracted from the input signal.
The output pulses from the n level multi comparator 62 are
then passed through the pattern or shape detector 64 which
analyses the signal for certain characteristics. Part of
this analysis, which is carried out by a microprocessor
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based on the information from the multilevel signals,
includes pulse symmetry evaluation which is sensitive to
the instantaneous change in the number of comparator
outputs tripped. If the rate is too high due an an RF
induced transient event for example, the result in output
pulses to the next section, i.e., the pulse amplitude
weighting stage 66 are reduced in duration reducing their
effect on the energy accumulation storage mechanism 70.
The pattern or shape detector 64 in one analysis is
tailored to detect the symmetry of an RF induced transient
signal which is shown in Figure 8 and is characterized by a
sharp initial transition followed by an exponential decay.
For normal signals of a nonrepetitive nature, the output
pulses from the pattern or shape detector 64 are identical
to the pulses originating from the n level multi comparator
62.
The pattern or shape detector 64 can identify RF
transient signals, and in some cases the weight thereof is
reduced. It is also possible to increase the weight
thereof to produce a trouble alarm, if desired by the user,
so that the source of RF transient signals can be
investigated and appropriately dealt with.
Certain signals, such as a rotating ceiling fan,
produce a recognizable repetitive pattern which is detected
by detector 64 which can appropriately modify the pulse
amplitude weighting stage 66 to reduce the impact of this
signal and/or modify the results of the weighted signals by
the addition of a modified signal at the energy
accumulation/storage device 70.
In some cases, the Pattern or Shape Detector 64
will not be required and can be deleted from the trouble
processing unit 55. Similarly, in some cases, pulse
symmetry detection may not be required and can be deleted.
The N level comparator and weighting factors alone can
provide significant improvements in the assessment of
trouble and adaptability for particular environments. This
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is also true where the N level comparator and weighting
factors are used for evaluation of alarm conditions.
It has been found that it is desirable to apply
different weighting factors to the pulses from different
stages. For example, although the information which is of
relatively low amplitude may include some false
information, the information is certainly valuable and
cannot be ignored. However, when the signal is above this
minimum level by a certain amount detected by the next
comparator this information is a much clearer indication
that a valid intruder motion detection has occurred.
Therefore, different weighted factors may be applied to the
different stream of pulses coming from the n level
comparator. It can also be appreciated that custom
tailoring of the response and weighting factors can make
adjustments for particular ambient conditions or particular
needs of the area being detected. Thus, it allows
selection, variation and tailoring of the system to the
particular environment in which it is being placed or the
application that it is intending to protect. For example,
it could allow customization to effect a system which is
more sensitive to slow movement versus fast movement or
more sensitive to near targets versus far targets. For
example, far off detection may be enhanced without
increasing the probability of false alarms due to heaters
by increasing the weighted factors used on the second and
third level comparator outputs while decreasing the
weighting factor of the first level comparator outputs
which is typically the minimum level of interest. The
weighting factors directly effect the rate of charging the
energy accumulation storage device 70 per recognized event.
The pulses which are most often produced by human motion
near or far, moving slow or fast will be given the most
weight while those most often produced by common transients
will be given a lower weight. The more comparators
implemented the higher the degree of sophistication
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possible and the increased ability to distinguish between
various disturbance sources throughout the monitored range.
This in effect allows a low or overall weight to be
assigned to "average" signal energy produced by transients
and high overall weight to the average signal energy
produced by valid motion to minimize the probability of
false alarms while enhancing the detection capability of
the detector. This capability is not possible via
traditional single time constant single threshold systems.
This weighting factor provides a further degree of freedom
and allows the amplifying requirements to be less
demanding.
The weighted pulses and any modified signals (if
any) are then literally added by the voltage to current
converter 68. The output signal represents a weighted
modification of the input signal energy. The weighting
factors can also be adjusted to more accurately reflect the
energy under the curve, if desired, or in contrast may be
used to provide a more accurate assessment of the detector
signal or detector signals by increasing or decreasing
certain portions thereof to emphasize or de-emphasize
certain portions of the signal. The point of this system
is to allow additional freedom with respect to customized
assessment of the signal to validly detect targets within
the area being monitored. This system allows tailoring of
the response to achieve this result and tailoring of the
system to affect the environment in which it is placed.
The counted weighted pulses from the voltage to
current converter 68 are stored in the energy accumulation
storage device 70. If a signal is of an energy sufficient
to accumulate energy quicker than it is discharged by the
constant energy decay device 72, then the trouble
comparator/timer 74 is tripped, signalling a trouble
condition to the trouble output device 76. The actual
detection of a trouble condition could light an LED or
produce an audible trouble signal or be recorded in some
manner. This recording step can also include recordal of
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the weighted pulses to allow user evaluation and possible
lead to the identify of an element in the environment
contributing to this condition.
After a fixed duration output devices 76 and 78 are
reset as is the energy accumulation storage device 70 by
the alarm comparator/timer 74.
The components and functions contained within the
microprocessor outline 49 can be carried out by a
microprocessor or by analogue techniques. As the levels of
analysis, increase the benefits of using a microprocessor
are more easily justified.
The constant energy decay device 22 decays at a
rate suitable to facilitate "memorizationT' of recent events
for some minimum time duration.
The prior art alarm systems typically trigger their
detection mechanism at some predefined threshold. This is
done in order to minimize the probability of false alarms
and results in essentially 30% of the information contained
in the area under the signal being ignored. This is done
as the algorithms that are used are unable to properly
discriminate the information as only one time constant is
used. The present technique, particularly in the
microprocessor based environment, can utilize this
information for background thermal "noise monitoring" which
may be used to evaluate the working environment of the
detector.
The different weighting factors may be dynamically
altered to enable the detector to adapt itself to
temperature or environmental changes and thus maintain high
sensitivity.
The information sensed and produced by the
algorithm may be interpreted and processed using FUZZY
LOGIC processing techniques. Fuzzy logic is a form of
artificial intelligence which enables decisions to be made
based on imprecise, non-numerical information, much the
same way as humans do. This technique could facilitate
"intelligent'T, dynamic alteration of the weighting factors
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by embedding the intelligence of the product designer into
each detector. Any source of information produced by the
system which may be described by a "linguistic variable"
may be processed using fuzzy logic techniques. For
example:
1. The "weighting_factor" may be defined as VERY
LOW/LOW/MED/HIGH/VERY HIGH
2. The "ambient temperature" may be defined as
COLD/COOL/COMFORTABLE/WARM/HOT
3. The "weight_change" may be defined as
NEGATIVE-LARGE/NEGATIVE-SMALL/NONE/POSITIVE-
SMALL/ POSITIVE-LARGE
By using a set of "IF-THEN" rules ( A Fuzzy
Inference System), a particular weighting factor
(:weight_n") may be adjusted according to the following
rule:
if AMBIENT TEMPERATURE is COLD and the
WEIGHTING_FACTOR for "weight_n" is LOW THEN
WEIGHT_CHANGE for "weight_n" is NEGATIVE SMALL
Although the above example is based on three data
sources, it will be appreciated that any variable sensed or
produced by a motion detection system (single or dual
technology) which may be assigned a "Linguistic variable"
may be processed using Fuzzy Logic techniques.
The major advantage of using fuzzy logic techniques
is to further reduce susceptibility to false alarms caused
by the fixed thresholds in the motion detection system by
offering an accurate means of adapting the detector's
coefficients to suit its environment.
Figure 7 shows a voltage supply g0 supplying each
of the four comparators 92, 94, 96 and 98. These
comparators receive the full wave rectified signal
indicated as 100. The four level comparators have
different minimum thresholds (V1-V4) with comparator 92
producing the first pulse indicated in Figure 8 as 102 and
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comparator 94 producing pulse 104 and comparator 96
producing pulse 106 and comparator 98 producing pulse 108.
In this case the output from a full wave rectified
symmetrical pulse so indicated at the top of Figure 8 is
being analysed. Four pulses are produced indicated as
pulses 102, 104, 106 and 108. The first pulse 102 is of
the longest duration and each of the pulses 108, 106 and
104 occur within the duration of pulse 102. Similarly,
pulses 106 and 108 occur within the duration of pulse 104
and pulse 108 occurs within the duration of pulse 106.
It can also be appreciated from a review of the
pulses of Figure 8 that an approximation of the symmetrical
signal so shown at the top of the figure has been
reproduced. By adding more comparators, additional
accuracy can be achieved. Furthermore, the applying of the
weighting factors to the different stages can allow further
discrimination of the events causing these disturbances.
Figure 9 show the pulses produced when a full wave
rectified transient RF signal indicated as 110 is being
processed by the comparators. As can be seen there is an
almost instantaneous tripping of the various comparators
92, 94, 96 and 98 (indicated by signals 112, 114, 116 and
118) followed by a staged reset corresponding to the decay
function of the full wave rectified signal. With this
information the pattern or shape detector 64 can
distinguish this as an RF signal which is to be reduced in
importance or filtered out. As previously described with
respect to Figure 6, different weighting factors can be
applied to the pulses once it has been recognized as an RF
signal or the signal can be ignored. The microprocessor
based system allows the weighting factors to be changed as
an RF transient signal is recognized to reduce or eliminate
the importance thereof.
With this system, non linear complifying of the
signal from the detector occurs to allow adjustments for
frequency characteristics of the signal detector while the
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weighting factors accommodate adjustments based on signal
amplitude.
The system has been described with respect to an
analogue arrangement, however, it can easily be carried our
digitally using a microprocessor. This arrangement is more
suitable for higher levels of evaluation for example 4 or
more levels of analysis or where the ability to alter
weighting factors during processing is desired.
Although the invention has been described herein in
detail it will be understood by those skilled in the art
that variations may be made thereto without departing from
the spirit of the invention or the scope of the appended
claims.
