CA1086399A - Intrusion alarm - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The invention relates to apparatus for detection of variations in the radiation pattern from a defined space. The apparatus may serve as an intrusion detector and alarm, which senses the presence of an intruder by monitoring light intensity variations in the field of scan. Or it may serve as a fire detector by sensing infrared radiation. One embodiment comprises a rotating mirror, radiation detector, analog-digital converter for deriving a series of digital values for the scanning positions of a complete scan, a memory for storing the digital values for at least one complete scan, and a comparator for comparing incoming digital values with stored digital values for the set of scanning positions. The comparator determines if the incoming value differs from the stored value by more than a predetermined amount. If it does an alarm is actuated and the stored value is retained in the memory.
If it does not, the incoming value is stored in the memory and the alarm is not actuated. Scanning may also be effected by a single line electronic scan-ning device without any mechanical movement.

Description

1~863~g This invention relates to apparatus for detection of variations in the radiation pattern from a defined space and is an improvement on the appar-atus disclosed and claimed in my earlier Canadian Patent Application Serial No.
193,648, filed February 27, 1974. The apparatus is useful, for example, as ~ ., an intrusion alarm which detects the presence of an intruder as a result of detection of a disturbance in the radiation pattern of the defined space being monitored.
Surveillance apparatus for monitoring a predetermined space or area is known in the art. For example, a closed circuit television system may include a TV camera aimed at the area desired to be monitored, and an observer at a remote location may watch a display of the observed area on a television receiver screen. Surveillance apparatus automatically giving an alarm when a static scene changes abruptly, as a result, for example, of the entry into the scene of an unwanted intruder, is also known. For example, United States patent No. 2,493,543 (C.J. Merchant, January 3, 1950) discloses the use of a rotatably mounted photodetector, a magnetic recorder for recording the output signal of the photodetector over a complete revolution, and means for comparing the analog output of the photodetector with the recorded output of the immedia-tely preceding revolution so as to give an alarm if there is a variation between the recorded signal and the signal received direct from the photo-detector.
The apparatus disclosed in United States patent No. 2,493,543 is crude and restricted in operation. The output of the photodetector must pass through slip rings, which tend to generate a noise signal of the same order of magnitude as the photodetector signal, before passing to the recording and comparison apparatus. In an analog system such as disclosed in this patent, there is no versatility possible in the analysis of the information--all that is possible is a real time co~parison of the signal from the photodetectors with the recorded signal for the previous revolution.

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i399 In accordance with my aforementioned Canadian application Serial No.
193,648, apparatus for detection of variations in the radiation pattern from a defined space is provided which comprises means for deriving a digital sig-; nal from the scanned scene and means for storing the digital signal as a series of discrete digital values in a digital memory. A digital comparator is pTO-v;ded for comparing the digital output for one scan with the stored digital values in the memory for a preceding scan of the scene. If there is a dis-crepancy exceeding some predetermined threshold value (to account for noise signals, slow changes in ambient light, etc.) between the digital value for a given portion of the scanned scene and the stored digital value for the same portion of the scanned scene obtained on a previous scan, then an alarm is actuated.
The use of digital signals and a digital memory provides a versa-tility in operation which is not available in an analog system. Use of a digital memory affords the capability of virtually any desired process of the signal, and further affords an improved rejection of false alarms. For example, by selecting a given number of pulses corresponding to the expected size of the target ~in sampling units), spatial discrimination is possible, affording the opportunity to reject spurious pulses. In other words, it is possible to -~
arrange digital logic to ignore a discrepancy between the digital value cor-responding to the immediate scan of a given portion of the scanning area and the stored digital value for the same portion of the scanning area, unless two, three, or some other predetermined number of immediately sequential digital values for immediately sequential scanning angles differ from the corresponding stored digital values by some amount exceeding the predetermined threshold.
This avoids false alarms caused by a single false value. Temporal discrimina-tion is equally possible -- successive scans may be compared to see if an anomaly is repeated.
Also, by recirculating rapidly the memory content over itself dur-ing the dead times between pulse sampling, one can perform various statisti-cal operations ~mean signal value, mean deviation, mean square deviations, etc.) and use this information to adjust the electronic gain and/or the threshold 1~ ;399 alarm level to an optimum value.
The use of the digital memory enables the entire signal processing system to have no moving parts, in contrast with the analog system disclosed for example in the aforementioned United States patent. Indeed, use of a scan-ning mirror which reflects the incident light or other radiation onto a fixed photodetector or other suitable detector enables the signal processing system to be free of moving mechanical parts.
The apparatus can also serve as a fire detector if the radiation sensors are sensitive to infrared radiation. Other uses of the apparatus will be apparent to persons skilled in the art.
In the apparatus according to my earlier application 193,648 a comparison is made of each point in a given trace with each point of a trace ~`
. recorded previously and kept in memory. The most recent trace itself is put in the memory to replace the previous one. Thus, refreshing of the memory is , done all the time, even if an alarm is actuated. In practice this means that the anomaly signal itself is kept in the memory and becomes part of the back-ground: no further alarm is given after the first one.
.~ , -~ An important aspect of the present invention is predicated on the realization that it is advantageous to keep the anomaly out of the memory so ~;~
~ 20 that the alarm continues to be actuated as long as the anomaly is present in - the background. This permits the actuation of a second, permanent, alarm if -~
the anomaly is detected two or three times, and also gives an indication to 1 an observer even if the intruder has passed completely through the field-of-i view.
Thus, in accordance with the broadest aspect of the present invention there is provided monitoring apparatus for the detection of variations in a radiation pattern from a defined space comprising:
a scanning detector for repeated scanning of the defined space and producing an output analog signal in response to the radiation therefrom, 1~)8~399 means for deriving $rom said analog signal a set of sequential digital values corresponding to the radiation pattern of a complete scan of the defined space, a digital memory for storing an initial set of digital values, a comparator for comparing said initial values with a subsequent set of digital values and having a first output on which appears an initial value if the comparison exceeds or equals a predetermined limit value and a second output on which appears a subsequent value if the comparison is less than said predetermined value, said first and second outputs being connected as inputs to said memory and said first output also being connected to an alarm.
The apparatus of my earlier application 193,648 uses a rotating mirror and photocell arrangement as a scanner but, in accordance with another feature of the present invention, the scanner may be a single line electronic - scanning device.
In the accompanying drawing:
Figure 1 is a block diagram representing an embodiment of detection apparatus according to my earlier application Serial No. 193,648;
Figure 2 is a schematic block diagram illustrating an embodiment of the present invention;
Figures 3, 4 and 5 illustrate single line electronic scanning devices in association with cameras; and Figure ~ illustrates use of a single line electronic scanning device in a movie camera.
In the following discussion, it will be assumed that the radiation to be detected is light, and the embodiment illustrated in the drawings will be described in this context. However, it will be understood that other forms of radiation, for example infrared radiation, could be detected and the radia-tion pattern monitored in the same way as herein disclosed, with the exception that infrared radiation reflectors and sensors (say) would be employed.

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10~3~i399 Figure 1 will now be described as an aid to understanding the improvements making up the present invention.
Referring to Figure 1, a rotatable mirror 11, driven by a suitable tor (not shown) reflects incident light from the space to be monitored onto a fixed photodetector 13 which may be a conventional photoelectric cell or other suitable detector. The electrical output of the photodetector 13, which is an analog signal proportional to the intensity of the incident light, is amplified by an amplifier 15 and fed to an analog-digital converter 17. The analog-digital converter 17, under the control of a clock 19 or other suitable synchronizing device, samples the analog signal at sequential scanning angles (sampling positions) at sequential intervals of time, and provides as an out-put a series of digital signals having digitized values proportional to the level of the analog signal at the respective sampling intervals. The series of digital signals for a complete scan of the rotating mirror 11 is stored in a push-through mery 21 which receives the output of the analog-digital con-verter 17. A push-through memory is here defined as one where a new data $ point replaces a related already existing one; this can be achieved by a shift register unit, a random access read/write unit or the equivalent.
The push-through memory 21 has the property of storing a certain number of digital values corresponding to the series of sampled scanning angles. The storage sequence is maintained by an initial synchronization signal provided by the clock 19 or directly from a position sensor responding to an index position of the rotating mirror or its motor, or the like. In-deed, a series of indexing marks associated with the rotating mirror 11 or the drive means therefor may be provided to actuate the sampling of the analog signal by the analog-digital converter 17. When the new sensor signal is fed into the push-through memory 21, the memory is refreshed -- i.e. its load of stored digital values is replaced by an updated load provided as a direct output of the analog-digital converter 17, and the previously stored values are read out of the push-through memory 21 in sequence in the same order as they were originally fed in. Thus, it is possible to store the more recent digital values and simultaneously to feed them to the comparator 23 in the 1t)~6399 proper time sequence with the stored digital values that are being releasedby the memory 21. If the position sensor provides an appropriate electrical output, it may be used as a direct trigger to the analog-digital converter 17, in which case the clock 19 may be completely dispensed with.
A digital comparator 23 receives as two separate inputs each suc-cessive digital signal produced by the analog-digital converter 17 and the counterpart stored digital value read out of the push-through memory 21. If the discrepancy between the value of the output digital signal from the analog-digital converter 17 and the counterpart stored digital value for the immedia-tely preceding scan exceeds a predetermined threshold value, which can beinternally established in the comparator 23, then an appropriate trigger signal actuates an alarm or other suitable indicating device 25. If there is a danger of a false alarm which might be caused by a spurlous discrepancy between a single digital signal and its counterpart stored value, the digital compara-tor can be arranged to compare a series of two, three or more digital values, ;~
so that the alarm will be actuated only if there is a discrepancy in the digi-tal signals corresponding to more than one sampling position of the rotating mirror 11.
The individual elements illustrated in Figure 1 are readily avail-able as shelf items. For simplicity, the rotating mirror 11 has been shown as a plane rectangular mirror, but it is to be understood that a mirror or reflec-tor of any suitable optical configuration can be substituted, depending upon the specific application. A very inexpensive plastic replica mirror having a vacuum deposited chromium reflecting surface will be satisfactory for many intrusion alarm applications. The space to be scanned may require a complete revolution of the rotating mirror 11 or it may require revolution only through a sector. If the latter, suitable indexing and triggering devices (not shown) may be arranged so that the push-through memory 21 and comparator 23 respond - only to the digital signals corresponding to the analog signal received for ~ 30 the sector to be monitored. The scanning rate will of course depend upon the .~

'; , ' ~ ' ;399 application, but it is contemplated that for many applications a complete scan every few seconds will be satisfactory. This enables the use of a photodetector having a relatively slow response.
The use of digital logic to operate on the digital output of the analog-digital converter 17 gives the system a versatility that is absent in a counterpart analog system. It is easy to adjust the digital threshold in the comparator to meet the particular application. If ambient illumination is used, it is necessary that slow changes in illumination caused by sunrise, sunset etc.should not actuate the alarm. This is accomplished by insuring that the scan-ning rate and comparator threshold are set at appropriate values. However, any disturbance in the field of view at the same points on consecutive scans will appear as a strong signal in the comparator and trigger the alarm. In an -~
intrusion alarm system, the anticipated cause of the alarm will of course be the presence of an intruder who acts as a disturbance in the field of view of the scanning device.
Very low cost push-through memories of adequate capacity for most purposes are readily available. For many purposes, a resolution of about one degree in the azimu~h scan is satisfactory for full circle coverage. This would require a push-through memory having a capacity of the order of 360 bytes,which is a relatively low capacity memory.
It is easy, using the digital logicJ to have the push-through memory 21 refreshed not with each sequential scan but, say, every third scan or every tenth scan, or as desired. This would of course necessitate a recycling of the readout of the push-through memory to the input so that the information read out of the push-through memory would immediately be read back in again, except during the updating.
Using digital logic, it is also easy to isolate specific preselected scanning angles for special treatment. For example, if the scanning apparatus is operating a mile away from a lighthouse, at the scanning angle in the direc-tion of the lighthouse there will be a periodic change in the light intensity at that scanning angle. It is relatively easy to devise digital logic which 1~863~9 will cause the comparator to ignore discrepancies in the digital values corre-sponding to that particular scanning angle.
The photode~ector may respond to the ambient incident light, or a special source of illumination may be provided, for example to rotate along with the rotating mirror.
The basic concept of the intrusion alarm rests on the possibility of comparing point-to-point a given trace with one recorded previously and kept in a digital memory. The most recent trace itself is put in the memory to replace the previous one. In the embodiment shown in Figure 1, the refreshing 10 of the memory is done all the time, even if an alarm is actuated. In practicethis means that the anomaly signal itself is kept in the memory and becomes part of the background; no further alarm is given after the first one. -~
The present invention is predicated on the realization that it is far more advantageous to keep the anomaly out of the memory so that the alarm continues to be actuated as long as the anomaly is present in the background. -This permits the actuation of a second, permanent, alarm if the anomaly is detected more than two or three times, and also gives an indication to an observer even if the intruder has passed completely through the field of view.
Figure 2 is a block diagram of an embodiment according to the ; 20 present invention which is capable of achieving anomaly suppression. Signals from sensor 30, scanned by a scanner 31, are fed via an analog signal circuit 32, e.g. an amplifier, to an analog to digital converter 33. The output of the analog to digital converter 33 is under the control of a position r . ~eference 34 associated with the scanner 31 to properly synchronise the output ; signals. The output signals of the analog to digital converter are initially ; fed to a memory 35. During the second and subsequent scans, the values are fed as indicated, into the comparator which also receives an output from the memory 35 and determines whether the signal from the analog to digital con-verter 33 exceeds by a predetermined threshold value the output from the 30 memory 35 for any given position in the scan. If it does, the "yes" output of the comparator feeds the signal previously stored in memory back to the memory, or leaves the value in the memory if a non destructive read memory is ~3gi3~9 used. On the other hand, if the comparator indicates that the threshold value is not exceeded, the "no" output of comparator 36 is actuated and this updated signal is stored in the memory 35.
The symbols shown in Figure 2 may be explained as follows:
Xnt is a point in position 'n' during scan 't' Xtn 1 is a point in position 'n' and kept in the memory ~ is the threshold for alarm Xln is the first scan around directly put in the memory The diagram is self explanatory. A new value Xtn at position 'n' is put directly in the comparator along with the value Xnt 1 kept in the memory. If the alarm is actuated Xnt 1 is kept ~or stored) in the memory; if not, the new value Xtn is put in the memory.
The basic intrusion alarm unit includes an optical sensor and a scanner permitting the sensor to look repetitively at various points in space, each point being identified by a position reference 'n' at which point each analog signal is put in digitized format. During the first scan, the digital value (Xln) is stored in the memory. During the second scan, and each of the following ones, (Xn2, Xn3 ... Xtn) the new value at the 'n' position is compared with the preceding one kept in the memory, and if their difference ~ exceeds a preset threshold value ~1, then an alarm is actuated: this indicates -` the presence of some anomaly in the field-of-view during the most recent scan(s).
It is important not to include this anomaly in the comparison trace so that the threshold for alarm can be exceeded at each scan as long as the anomaly is present. When this happens for once - or continuously for a given number of times - the alarm itself can be then actuated permanently. This is . . .

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63~9 achieved in the co~parator where a decision is made to put in the memory either the ne~ acquired point if there is no alarm; or else, to leave in the memory the point that was read previously for comparison ~henever an anomaly is sensed.The repetition of the alarm is a confirmation that such an alarm is real.
The key elements in the system are the digital memory and the com-parator. The system is not critical in any of its elements except for the accuracy of the sampling position, that should exceed the angular resolution of the sensor; and for the repeatability - or stability - of the sensor itself from trace to trace. The system can follow slight and continuous variations in the emission background - such as a slow day break - but it will respond to a sudden change in any one of the sampled position. Another key element is the continuous detection of the intruder by not including any 'anomalous' point in the memory.
The choice of the sensor, the scanner and the alarm depends on the conditions under which the alarm is to be used. A typical system uses a ; detector operating in the far infrared; this has the advantage of relying on the more stable thermal emission from the surroundings instead of using the visible light reflected from objects. A flat mirror at 45 rotates to scan the horizon, and the sample points are taken at, say, every degree in azimuth.
The alarm is a simple red light that shines whenever an anomaly is detected.
Although in the great majority of cases, passive operation as described previously would seem preferable, active systems are possible. One example might be a room illuminated with a fixed invisible near-infrared lamp matched with an inexpensive Si diode sensor. A second example would be a pulsed GaAs laser (or light emitting diode) mounted on the scanning head, and boresighted with the detector. The detector can have a very narrow optical filter, as well as a narrow electrical filter (coherent detection) if the GaAs operates at a stable frequency. Any change in the amount of reflected light due to some anomaly will trigger the alarm.

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In some circumstances, the use of more than one detector might be advantageous. One example would be to distinguish a crawling animal from a standing human. This is achieved with two detectors to delimitate two differ-ent field-Gf-view in elevation. Each detector has its own memory, and a simul-taneous alarm on both memories would indicate a standing body.
Two detectors operating in different wavelength bands might be useful in some cases to eliminate anomalies that would not be of interest. An example, a unit combining short and long wavelength detection could separate a fire from say, a human being.
If the alarm unit is located at the center of a room, the horizontal scan would cover 360; if loca~ed at one corner, it would cover 90 only for - maximum efficiency. To achieve this, one can divide the smaller angle of interest by the number of positions in the memory and have a position encoder with this precision that will be extended to the full 360. For any sector of the smallest dimension, a sampling is made at each encoder position; for a ~-sector twice as large, sampling is made at every second position, and so on.
Means to adjust the horizontal instantaneous field-of-view of the sensor can be implemented. If not, overlap will result, and this can be advantageous in most circumstances.
An easily implemented option is a counter that delays the alarm for a given number of scan so that the anomaly must be detected in a minimum number . . ~
; of consecutive scans (No). There could be a double warning system such as a light flashing at each scan, and a buzzer actuated only after so many flashes. ' A look at a steady flashing light is a good indication that the anomaly is not accidental: it is a kind of discrimination with time.
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!~ ~ The use of a digital memory and comparator permits the easy imple- i , ~
mentation of a position indicator. This could be obtained by matching memory locations with a series of light emitting diodes (LED's), one LED corresponding -` to a given number of consecutive memory positions. A numerical display could ., also be easily implemented. If the sampling is done at given angular posi-tions, the position location can be as good as the sampling precision. Positionindication would be superfluous for guarding a closed room, but might be very ,:

~ 363~9 useful in an advance observation post, for example.
This type of intrusion alarm depends on the speed of displacement of the intruder, on the length of time he appears in the field-of-view, and on the sampling interval. If needed, it is very simple to use two memories --with a single sensor and scanner. The first memory does trace comparisons at the scanning speed while the second skips a few traces and effectuates the comparison every 20 or 100 scans.
Using the basic concept for the intrusion alarm, that is a digital memory, a comparator and an anomaly suppressor, it is possible to combine such a unit with a single line electronic scanning device (charge coupled device, charge induced devices, Si array, and the like). The detectors in the array can be placed in the focal plane in a camera or any imaging optical device.
Depending on the light intensity the array is self-scanned at a speed suf-ficient to avoid saturating any one of its elements. One scan is stored in the memory for reference and at a given time later another scan is compared point by point (and stored except for the anomalous points). If a preset threshold is exceeded in the comparator, an 'alarm' of some sort is actuated.
The detector array 40 can be at the focal point of the camera lens.
Three examples are shown in Figures 3 - 6 of the drawings. The first method consists in removing the back of the camera and using the device at the film position (Figure 3); the second in redesigning the reflex viewer (single or double lens reflex) to incorporate the array while keeping the viewer itself unobstructed (Figures 4 and 5); the third in using a movie camera (8 or 16 mm) with a film having a square hole at, say, every 20 frames (Figure 6). A
retractable metal plate 41 biased by springs 42 holds the detector array at the film position, through the hole, in the focal plane.
The alarm could be visual or aural. It could also consist in taking a picture of the scene whenever a change occurred. In the first example, a second camera boresighted with the detecting camera would be required; in the second example, a single camera would be needed; and in the third, a change in the scenery would actuate the movie camera for about lO to 20 frames before it is stopped and ready for further observation.

1~8~;3~39 Other points are worth mentioning;
a) The camera should be stably mounted on a tripod and the array scans itself at a speed dependent on the light available through the lens (up -to several lOO KHz). Comparison can be made at any convenient interval (O.Ol to lO seconds or more) by dropping the intermediate scans. In low-light condi-tions, the integrating ability of, say, the charge coupled devices can be used to great profit by utilizing a scan adaptable to the light conditions or as slow as needed to accumulate sufficient charge in the array.
b) Using a wide angle lens, direction of movement can be detected, and a person for example can be photographed under many~aspects when passing from left to right. The system could be used to count objects moving in one or the other direction.
c) The camera is suggested for its availability. A simpler and smaller device could be designed for detecting movement along a road, for advance post detection, for detecting people going in and out of a building, etc.
d) Any imaging device, even of poor quality, could be used with the line array in the focal point to detect intrusion. For example a tele-.
- scope can be fitted with a beam splitter with the eye piece on one side, and r the line array at the focal point on the other side.
e) Using the above principles and the appropriate optics, a non-mechanically scanned intrusion/fire alarm operating in the infrared could be obtained through the development of infrared detectors (PbS, InSb, MCT or ~ PbSnte) directly coupled with an appropriate array of charge-coupled devices .
or the like).

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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-

1. Monitoring apparatus for the detection of variations in a radiation pattern from a defined space comprising:
a scanning detector for repeated scanning of the defined space and producing an output analog signal in response to the radiation therefrom, means for deriving from said analog signal a set of sequential digital values corresponding to the radiation pattern of a complete scan of the defined space, a digital memory for storing an initial set of digital values, a comparator for comparing said initial values with a subsequent set of digital values and having a first output on which appears an initial value if the comparison exceeds or equals a predetermined limit value and a second output on which appears a subsequent value if the comparison is less than said predetermined value, said first and second outputs being connected as inputs to said memory and said first output also being connected to an alarm, whereby intial values on said first output result in no change in the set of values stored in the digital memory while subsequent values on said second output result in a new set of digital values being stored in the digital memory.

2. Apparatus as defined in claim 1, wherein the analog signal is sampled at periodic intervals by the analog-digital converter in response to a trigger signal provided by position sensing means associated with the scanning detector or with drive means for the scanning detector.

3. Apparatus as defined in claim 2 wherein the scanning detector comprises a rotating reflector and a fixed radiation sensor which provides said output analog signal.

4. Apparatus as defined in claim 3, wherein the sensor is a photo detector and the radiation pattern is a light intensity pattern.

5. Apparatus as defined in claim 2 wherein the scanning detector comprises a single line electronic scanning device.

6. Apparatus as claimed in claim 5 wherein the electronic scanning device comprises a single line array of charge coupled devices.

7. Apparatus as claimed in claim 6 wherein the array is held in the focal plane of a camera.