EP0107042B1 - Infrared detector for spotting an intruder in an area - Google Patents

Description

The invention relates to a passive infrared (IR) detector according to the preamble of patent claim 1.

US Pat. No. 4,342,987 describes an intrusion detection system with such a detector for optically monitoring an area, in which an alarm is triggered when an intruder moves through the monitored area. The system comprises a passive infrared detector with a sensor element, which has at least two separate areas sensitive to infrared radiation, infrared radiation emanating from an intruder having a temperature that differs from its ambient temperature falling successively on the two sensor areas . The passive infrared detector has optics for focusing the infrared radiation emanating from the intruder onto the sensor element, and an evaluation circuit for monitoring the electrical signal emitted by the sensor element.

A disadvantage of the above-described and other known infrared detectors is that the broadband sensitive sensor elements used, such as pyroelectric crystals or polymers, bolometers or thermocouples, respond to electromagnetic radiation in the entire wavelength range. The result of this is that signals are also generated that originate from IR radiation that are not generated by intruders. Such false alarms must be prevented as much as possible in a good monitoring system.

For this reason, there was a constant search for ways to make the passive IR detectors more secure against false alarms. DE-B-2103909, for example, describes such a monitoring device in which sufficient coverage of a particularly large total area is achieved by means of only one sensor element, which then provides a clearly distinguishable output signal if an intruder exceeds the limit of the coverage area; This is achieved in that a plurality of reflecting surfaces are arranged in such a way that the IR radiation coming from a plurality of separate fields of view is directed onto the sensing element.

To avoid false alarms due to electromagnetic radiation, which is in a wavelength range that does not correspond to that of a black body (intruder) in the temperature range 0-40 ° C, the radiation entrance window of the IR detectors is covered with an optical filter with a pass range of 4-20 µm. As a result, visible light in particular is blocked. Furthermore, the signal emitted by the sensor is amplified by an AC amplifier, which is designed so that only signals in the frequency range are amplified, which corresponds to the passage of an intruder through the different zones of the room to be monitored. This frequency range is preferably between 0.1 and 10 Hz.

To detect intruders in a room to be monitored, it is necessary to cover the entire room, i.e. both near and far to avoid having to install a variety of detectors. A passive IR detector is known from US-A-3480473, in which the IR radiation falls onto the IR sensor via a fine grating arranged in a cylindrical manner around the IR sensor. This enables all-round monitoring and differentiation of background radiation, since a moving body emitting IR radiation generates an electrical alternating signal. In order to distinguish a moving body emitting IR radiation from background radiation, the space to be monitored is generally divided into fan-shaped monitoring areas, for example by zone optics.

In the IR detector of US-A-3829693, thermocouples (or thermistors or pyroelectric detectors) are arranged as IR sensors in different columns such that elements of the same column have the same polarity but differ from the polarity of adjacent columns, so that a moving body emitting IR radiation generates an AC signal. To focus the IR radiation on the IR sensor, the IR detector has two optical systems with different focal lengths, for example a mirror arranged behind the IR detector and having a larger focal length than a germanium lens arranged in front of the IR detector, which covers the close range serves to increase the remote sensitivity.

From EP-C-25983 an IR motion detector is known in which, in order to reduce the sensitivity to electromagnetic radiation penetrating the glass, the optical filter located in front of the input of the IR detector is connected to a heat sink in the form of a solid metal body. Although this arrangement suppresses the secondary IR radiation source, it cannot prevent false alarms caused by heat turbulence in the room, since this turbulence emits radiation in the range of 4 to 20 gm, which also corresponds to that of intruders.

Differential elements are also used in IR detectors, ie the spatial zones are imaged on two closely adjacent sensor elements (for example two electrodes applied to the same element), which are then connected to a differential amplifier. Such a sensor is described for example in US-A-3839640. In the close range, the zones depicted on the individual elements overlap, ie turbulence generates the same electrical signals on both elements, ie the differential amplifier output remains unchanged. With these differential elements it is possible to successfully reduce the turbulence, which is only disturbing if it occurs in the vicinity of the detector to press. At the same time, however, the sensitivity to objects moving in the vicinity is greatly reduced or not recognizable, as is the case with turbulence. In other words: burglars who are very close to the detector cannot be detected; acts of sabotage, e.g. B. masking, overspraying or the like, are not recognized.

EP-C-23354 discloses a pyrodetector with two pyroelectric sensors, one of which is located in the focal point of an IR radiation reflecting concave mirror, while the other is used to compensate for the IR radiation originating in particular from the cover outside the focal point is arranged.

The various known measures to suppress false alarms are effective, but only include part of the problem of false alarms and especially the problem of sabotage. The latter problem relates to the wanton covering of the entry window by an object, e.g. B. hat or board, or by spraying a transparent varnish, which absorbs the IR radiation necessary for the detection of intruders in the wavelength range of 4-20gm. This blinds the detector and intruders can no longer be detected.

Another problem that has not yet been described in the literature is that today's IR detectors must have a signal / noise ratio (S / N) of about 10 before the detector displays an alarm. This ratio had to be chosen high enough to reduce the number of false alarms caused by the detector noise. However, a signal / noise ratio S / N - 10 has considerable disadvantages for the detection of intruders, since the signal generated by the object is proportional to the temperature difference between the object and the background. In addition, the signal in the pyroelectric sensor elements used today is proportional to the speed at which the object moves through the space to be monitored. Because of the high signal-to-noise ratio required to suppress false alarms, it is difficult to detect intruders who move very slowly or / and who reduce the temperature difference to the environment by wearing suitable clothing.

The invention has for its object to avoid the disadvantages of the known IR detectors and an IR detector with increased reliability, ie. H. to create an increased detection probability with a reduced risk of false alarms. Another object of the invention is to provide an IR detector, the electrical circuit of which makes it possible to suppress false alarms which are generated by thermal turbulence and electronic noise and also to detect slowly moving objects with a small temperature difference from the background. Furthermore, it is an object of the invention to provide an IR detector, the evaluation circuit of which generates evaluable signals which make it possible to set the alarm threshold far below the previous signal / noise ratio of 10 without negatively influencing the false alarm suppression. Another task is to create an IR detector which detects acts of sabotage, such as covering the entrance optics with IR-opaque materials (e.g. paper, glass or spray paints), and in which signals distinguishable from hot air turbulence are obtained.

This object is achieved according to the characterizing part of claim 1 in that the output signal of an IR detector of the type mentioned is evaluated not only according to its amplitude but also according to its similarity to a reference signal. For this purpose, reference signals are stored in a read-only memory which correspond to the signals which are generated by an object which moves through the monitoring area of the optics at different speeds. Each signal of the IR detector is then correlated with the reference signals and an alarm is triggered if the similarity with one or more reference signals exceeds a predetermined value and at the same time the amplitude is greater than a fixed threshold value. Since there are still high similarities even with noisy input signals (S / N - 1), this greatly improves the detection probability.

According to one embodiment of the passive IR detector according to the invention, the reference signal is compared with the signal obtained from a second optical system, the monitoring area of which is different from that of the first optical system, in connection with a second detector; The second optical system preferably monitors only the close range of the detector.

According to a preferred embodiment of the IR detector according to the invention, the second sensor element has an optical system, the focal length of which is set up in such a way that the close range (ie housing, window) is imaged on the sensor element, in contrast to the first optical system, which objects at far distances on the maps the first sensor element.

According to a further preferred embodiment of the IR detector according to the invention, the second optical system consists of pinhole diaphragms or mirror segments, which have the effect that the monitoring areas only overlap in the immediate vicinity of the detector.

In a further preferred embodiment of the IR detector according to the invention, the comparison is only carried out with permanently stored references in order to achieve an increase in the probability of detection, a differential sensor element being used for the turbulence suppression. In this case, a second sensor element is unnecessary.

• Figur 1: ein Blockschaltbild einer Ausführungsform eines erfindungsgemässen IR-Detektors,
• Figur 2: die Auftretenswahrscheinlichkeit einer bestimmten Amplitude für verschiedene Ereignisse,
• Figur 3: die Auftretenswahrscheinlichkeit einer bestimmten Ähnlichkeit eines am IR-Detektor auftretenden Signals mit einem der abgespeicherten Referenzsignale für verschiedene Ereignisse,
• Figur 4: die Auftretenswahrscheinlichkeit einer bestimmten Ähnlichkeit zwischen den beiden Signalen, welche von den beiden verschiedenen Optiken für verschiedene Ereignisse erzeugt werden und
• Figur 5: die Ähnlichkeit zwischen den beiden Signalen als Funktion des Abstandes vom Melder für verschiedene Ereignisse.

The invention is explained in more detail below, for example, with reference to the drawings. Show it:

• FIG. 1: a block diagram of an embodiment of an IR detector according to the invention,
• FIG. 2: the probability of occurrence of a certain amplitude for different events,
• FIG. 3: the probability of occurrence of a certain similarity of a signal occurring at the IR detector with one of the stored reference signals for different events,
• Figure 4: the occurrence probability of a certain similarity between the two signals, which are generated by the two different optics for different events and
• Figure 5: the similarity between the two signals as a function of the distance from the detector for different events.

In the block diagram shown in FIG. 1, the IR detector has a first sensor element 11 which is exposed to IR radiation from the room to be monitored by a first optical system 0 ₁ with a certain focal length. Depending on the level of the IR radiation impinging thereon, the first sensor element 11 emits an electrical signal which is amplified by a first amplifier 21. The amplified signal is fed to a first analog / digital converter 31, which converts the analog signal present at the input into a digital signal S, and a correlator K, in which it is compared with reference signals, and a threshold value detector 42, in which the amplitude is determined is feeding. The correlator K and the threshold detector 42 are followed by an alarm stage A, which emits an alarm / sabotage signal as a function of the correlation C determined by the correlator K and the amplitude of the signal S ₁ .

Correlator K uses the signals R _i ... R _n stored in a read-only memory FS, which correspond to different object speeds, as reference signals.

Typically, an object moving through a surveillance area generates a sequence of positive and negative signal pulses, where for example the positive pulse corresponds to the movement into the monitored zone, the negative going pulse corresponds to the movement out of the monitored zone, as for example in FIG GB-A-2047886. The height and width of the pulses depend on the speed of movement and the temperature difference from the object to the background temperature. Pulse sequences can now be selected as reference signals. B. correspond to different typical speeds of movement. However, it is also sufficient to use idealized reference signals, e.g. B. the following rectangular pulses or pulses, which have the known Gaussian shape.

berechnet. r(Ä) ist das abgespeicherte Referenzsignal, S(Ä) das aktuelle vom sich bewegenden Objekt erzeugte Signal, und ^-To/2, ^+To/2 sind Integrationsgrenzen, welche anhand von Experimenten optimiert werden müssen. C(t) ist ein Mass für die Ähnlichkeit der beiden Signale r und S, welche als Korrelation von r und S bekannt ist. (Vergleiche z.B. Introduction to Radar Systems Mc Graw Hill 1962/1980 von M.J.Skolnik). Alarm wird dann ausgelöst, wenn die Korrelation C(t) sowie die Amplitude a(t) im Laufe der Zeit einen bestimmten, vorgegebenen Wert überschreitet. Mit anderen Worten, man setzt in dem erfindungsgemässen Verfahren für die Alarmauslösung zusätzlich eine Schwelle für die Ähnlichkeit von Signalen nebst deren Amplituden. Der Ähnlichkeitsvergleich hat den Vorteil, dass auch bei stark verrauschten Eingangssignalen (Signal/Rausch-Verhältnis - 1), welche bei der konventionellen Methode nicht mehr ausgewertet werden können, eine Korrelation C(t) eindeutig berechnet und mit dem Schwellwert verglichen werden kann. Durch diese Zweikriterienauswertung kann bei vorgegebener Fehlalarmrate die Detektionswahrscheinlichkeit wesentlich vergrössert werden.The current signal S ₁ is now continuously checked for similarity with the reference signals R ₁ ... R _n stored in the read-only memory FS. This is done according to the correlation method known from radar technology, for example, by which the integral is calculated

calculated. r (Ä) is the stored reference signal, S (Ä) the current signal generated by the moving object, and ^-To / 2, ^{+ To} / 2 are integration ^limits that have to be optimized based on experiments. C (t) is a measure of the similarity of the two signals r and S, which is known as the correlation of r and S. (Compare e.g. Introduction to Radar Systems Mc Graw Hill 1962/1980 by MJSkolnik). The alarm is triggered when the correlation C (t) and the amplitude a (t) exceed a certain, predetermined value in the course of time. In other words, a threshold for the similarity of signals and their amplitudes is additionally set in the method according to the invention for triggering the alarm. The comparison of similarity has the advantage that even with heavily noisy input signals (signal / noise ratio - 1), which can no longer be evaluated in the conventional method, a correlation C (t) can be clearly calculated and compared with the threshold value. This two-criteria evaluation can significantly increase the probability of detection for a given false alarm rate.

The results obtained are illustrated in FIGS. 2 and 3. FIG. 2 shows the measured probability of occurrence W _{A of} a certain amplitude A (in relative units) for various current signals S ₁ emitted by sensor element 11 in a logarithmic representation. W _A was determined experimentally by measuring the signals of different nominally identical events again. W _A then denotes the probability that a given signal occurs for a given event. 2: R = electronic noise; LE = slow walking speed, small temperature contrast to the environment; T = close-up turbulence; SE = normal walking speed, temperature contrast AZ to the background = 2 °.

From this it can be seen that with the alarm threshold of S / N = 10 that has been customary up to now, the detection probability is insufficient and that there is still a high probability of false alarms due to hot air turbulence. In particular, intruders with low walking speeds and small temperature differences from the environment are not detected.

3 shows the measured probability of occurrence W _{c of} the maximum correlation C (similarity) of a signal S with the stored reference signals R,... R _n (the greater the value of C, the greater the similarity of the current signal S) , with the reference signal (R, ... R _n ) As can be seen from FIG. 3, the signals caused by a real break-in are shifted to large similarity values and separated from the false alarms.

If the turbulence is to be suppressed more, a differential detector can be used, which suppresses the signals originating from the close range. In this way, a very high false alarm suppression is achieved with a greatly increased probability of detection (intruders with low walking speeds and a small temperature difference to the environment are now detected) when the amplitude of the alarm threshold, for example, to a value of S / N = 2 and similarity, for example a value of C = 0.7 is set. Differential sensors according to EP-A-0086369 (published on August 24, 1983), which are not balanced for high frequencies, are particularly suitable here.

4 and 5, the mode of operation is explained when a further signal S ₂ , which comes from a second sensor element 12, the z. B. is equipped with an optic 0 ₂ , which has a pinhole, which ensures that the monitoring area of the two sensor elements only overlaps in the immediate vicinity of the detector. This signal is also first amplified by a second amplifier 22, then converted into digital form in a second analog / digital converter 23. The signal S ₂ is finally fed to the correlator K (FIG. 1).

According to a preferred embodiment of the passive infrared detector according to the invention, the two sensor elements 11, 12 are located on a chip and, according to one embodiment, are arranged in a housing.

In the correlator K, the current signal S obtained from the first sensor element 11 is continuously compared with reference signals R,... R _n stored in the read-only memory FS and the current signal S _{2 received} from the second sensor element 12.

FIG. 4 shows the correlation C (schematic similarity) of the signals S ₁ and S ₂ as a function of the distance Z from the detector 11, 12 for two different events, such as covering with non-IR transparent material, that is to say a sabotage event S and warm air turbulence T. As can be seen from FIG. 4, the correlation C (similarity) only reaches high values in the immediate vicinity of the detector and the values are different for the two events S and T.

In FIG. 5, the probability of occurrence W _c for the correlation (similarity) of the two signals S ₁ and S ₂ for different events is plotted to further explain this situation. The following mean: R = electronic noise and / or crossing the surveillance area at a large distance from the detector; T = warm air turbulence and S = masking, overspraying in the close range (sabotage event).

As can be seen from FIG. 5, three areas of similarity occur which allow a differentiation of the events and thus enable identification of sabotage.

Claims (12)

1. Passive Infrared (lR) detector for determining the presence of an intruder possessing a temperature differing from the ambient temperature comprising at least one sensor element (11) for generating an electrical signal as a function of infrared radiation impinging upon said sensor element (11) and at least one optical system for focussing the infrared radiation emitted by the intruder upon said sensor element (11), said optical system imaging infrared radiation upon said sensor element (11) which emanates from a number of predetermined seperate fields of view, and an evaluation circuit (K, A) for monitoring the electrical signals delivered by said sensor element (11) dependent upon changes in the impinging radiation caused by the movement of the intruder, characterized in that said evaluation circuit (K, A) comprises a correlator (K), said correlator (K) continuously comparing the actual signal (S₁) from said sensor element (11) with reference signals (R₁ ... R_n) stored in a storage means (FS) and which are representative of typical movement patterns of intruders, said correlator (K) delivering an output signal corresponding to the correlation (C) of the actual signal (S,) and the reference signal (R_i ... R_n) and that said evaluation circuit (K, A) further comprises an alarm stage (A) arranged in circuit after the correlator (K) and structured such that it delivers an alarm signal when the correlation (C) and the amplitude of the actual signal (S_i) simultaneously exceed a predetermined value.

2. IR detector according to patent claim 1, characterized in that it comprises a first sensor element (11) with a first optical system (O₁) and a second sensor element (12) with a second optical system (O₂) both of said optical systems (O₁, O₂) being structured that monitoring regions thereof only overlap in close proximity to the detector and said correlator (K) being structured such that it continuously compares the actual signal (S_i) from the first sensor element (11) with reference signals (R₁ ... R_n) stored in the storage means (FS) and the actual signal (S₂) from the second sensor element (12).

3. IR detector according to patent claim 2, characterized in that the alarm stage (A) is structured such that it delivers a disturbance signal when the correlation (C) between the actual signal (S_i) received from the first sensor element (11) and the actual signal (S₂) received from the second sensor element (12) exceeds a predetermined first threshold value, preferably 0,35.

4. IR detector according to patent claim 3, characterized in that the alarm stage (A) is structured such that it delivers an alarm signal when the correlation (C) between the signal (S_i) received from the first sensor element (11) and the signal (S₂) received from the second sensor element (12) exceeds a predetermined second threshold value, preferably 0,7.

5. IR detector according to anyone of patent claims 1 and 2, characterized in that the alarm stage (A) is structured such that it delivers an alarm signal when the correlation between the signal (S_i) received from the first sensor element (11) and at least one reference signal (R_i ... R_n) from the storage means (FS) exceeds a predetermined threshold value, preferably the value 0.7, and at the same time the amplitude of the signal (S₁) exceeds a predetermined threshold value, preferably twice the rms-value of the noise.

6. IR detector according to anyone of the patent claims 1 to 5, characterized in that the storage means (FS) stores reference signals (R_i ... R_n) corresponding to intruders with different speeds of movements.

7. IR detectors according to anyone of the patent claims 1 to 6, characterized in that as sensor element (11,12) is provided a differential element.

8. IR detectors according to anyone of the patent claims 2 to 7, characterized in that the optical system (O₁) of the first sensor element (11) is structured such that it monitors the room to be monitored in a number of active zones and that the optical system (0₂) of the second sensor element (12) is structured that it only images the radiation inlet window.

9. IR detector according to patent claim 8, characterized in that the optical system O2 of the second sensor element (12) consists of an apertured diaphragm which ensures that the monitoring region of both sensor elements (11, 12) only overlap near to the detector.

10. IR detector according to patent claim 9, characterized in that the optical system O2 consists of mirror elements.

11. IR detector according to any of the patent claims 9 and 10, characterized in that both of said sensor elements (11, 12) are located upon a chip.

12. IR detector according to any of the patent claims 2 through 11, characterized in that both of said sensor elements (11, 12) are arranged in a housing.

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