EP0107042A1 - Infrared detector for spotting an intruder in an area - Google Patents

Abstract

In the case of a passive infrared detector, in order to reduce the susceptibility to false alarms and to increase the probability of detection, the current signal (S1) received from a first sensor element (11) is continuously processed in a correlator (K) with reference signals (R1... FS) are stored, and / or compared with the current signal (S2) received from a second sensor element (12) monitoring the short range. The correlator (K) emits an output signal which corresponds to the correlation (C) of the two signals (R1 ... .Rn or S2) compared with one another. An alarm signal is triggered when the correlation (C) reaches a predetermined value, e.g. B. 0.7, and the amplitude reaches the threshold. The IR detector not only has a high level of security against false alarms and a high detection probability in the case of heavily noisy signals, but also triggers in the event of sabotage, e.g. B. Covering the input optics, an alarm signal.

Description

The invention relates to an IR detector for detecting a body which has a temperature which deviates from its ambient temperature, with at least one sensor element which serves to generate an electrical signal as a function of IR radiation incident thereon and at least one for focusing those emanating from the body IR radiation on the optics serving the sensor element, as well as an evaluation circuit used to monitor the electrical signal emitted by the sensor element.

It is known to use IR detectors which respond to the IR radiation emanating from a body, in particular from people, so-called “passive IR detectors”, in monitoring devices for the detection of intruders in rooms to be monitored. A disadvantage of these IR detectors and the broadband sensitive sensor elements used today, such as pyroelectric crystals or polymers, bolometers or thermocouples, is that these elements 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-AS 2 103 909, for example, describes such a monitoring device in which adequate coverage of a particularly large overall area is in each case only one sensor element is reached, which then delivers a clearly distinguishable output signal when 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, ie both the near and far areas, in order to avoid having to install a large number of detectors. A passive IR detector is known from US Pat. No. 3,480,473, in which the IR radiation falls onto the IR sensor via a fine grating arranged in a cylindrical manner around the JR sensor. This enables all-round monitoring and differentiation of background radiation, since a moving body emitting IR radiation generates an electrical alternating signal. To distinguish a moving body emitting IR radiation from background radiation, all together the space to be monitored is divided into fan-shaped monitoring areas, for example by zone optics.

In the IR detector of US Pat. No. 3,829,693, 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 neighboring 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-PA 25 983 an IR motion detector is known, in which, in order to reduce the sensitivity to electromagnetic radiation penetrating 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 thermal turbulence in the room, since this turbulence emits radiation in the range of 4 to 20 µm, 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 Pat. No. 3,839,640. At close range they are on the Zones shown overlapping individual elements, ie turbulence generate the same electrical signals on both elements, ie the differential amplifier output remains unchanged. With these differential elements it is possible to successfully suppress the turbulence, which is only disturbing if it occurs in the vicinity of the detector. 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, such as masking, over-spraying or the like, cannot be recognized either.

From EP-PA 23 354 a pyrodetector with two pyroelectric sensors is known, one of which is located in the focal point of an IR radiation reflecting concave mirror, while the other serves 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 willful covering of the entry window by an object, e.g. Hat or board, or by spraying on a transparent lacquer, which absorbs the IR radiation necessary for detection or intrusion in the wavelength range of 4 - 20 µm. This blinds the detector and intruders can no longer be detected.

Another problem not yet described in the literature is that today's IR detectors Signal / Noise Ratio (S / N) must be around 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 very significant 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 and / or who reduce the temperature difference to the environment with suitable clothing.

The invention has for its object to avoid the disadvantages of the known IR detectors and to create an IR detector with increased reliability, ie increased detection probability with reduced susceptibility to 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. It is also an object of the invention to provide an IR detector, the evaluation circuit of which generates evaluable signals which allow the alarm threshold to be set well below the previous signal / noise ratio of 10 without negatively influencing the false alarm suppression. Another task is to create an IR detector in which acts of sabotage, such as covering the entrance optics with IR-impermeable materials (for example paper, glass or spray paints), are detected and in which signals distinguishable from hot air turbulence are obtained.

This object is achieved according to the invention in that the output signal of an IR detector of the type mentioned at the outset is evaluated not only in terms of its amplitude but also in terms of its similarity with 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 resemblance to one or more reference signals exceeds a predetermined value and the amplitude is at the same time greater than a predetermined threshold value. Since there are still high similarities even with noisy input signals (SIN - 1), this greatly improves the probability of detection.

According to an embodiment of the IR detector according to the invention, the reference signal is obtained from a second optical system, the monitoring range of which is different from the first optical system, in connection with a second detector; The second optical system preferably monitors only the near area 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 cause the Only overlap monitoring areas 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 Aehnlichkeit eines am IR-Detektor auftretenden Signals mit einem der abgespeicherten Referenzsignale für verschiedene Ereignisse.
• Figur 4: Die Auftretenswahrscheinlichkeit einer bestimmten Aehnlichkeit zwischen den beiden Signalen, welche von den beiden verschiedenen Optiken für verschiedene Ereignisse.
• Figur 5: Die Aehnlichkeit 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:

• Figure 1: A block diagram of an embodiment of an IR detector according to the invention.
• Figure 2: The probability of occurrence of a certain amplitude for different events.
• Figure 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 probability of occurrence of a certain similarity between the two signals, which of the two different optics for different events.
• 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 ₁ 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 _I.

The _K orrelator _K uses the signals R1 ... Rn, which are stored in a read-only memory FS and which correspond to different object speeds, or the signal S ₂ , which is obtained from a second sensor element 12, which has a second optic 0 ₂ different from 0 ₁ .

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

berechnet. r ist das abgespeicherte Referenzsignal, s das aktuelle vom sich bewegenden Objekt erzeugte Signal, und -T_o/2, +To/2 sind Integrationsgrenzen, welche anhand von Experimenten optimiert werden müssen. C(t) ist ein Mass für die Aehnlichkeit 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 Aehnlichkeit von Signalen nebst deren Amplituden. Der Aehnlichkeitsvergleich 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 ... Rn stored in the read-only memory S ₁ . This takes place according to the correlation method known from radar technology, for example, according to which the integral is obtained

calculated. r is the stored reference signal, s the current signal generated by the moving object, and -T _{o / 2} , + To / 2 are integration limits that must 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 MJ Skolnik). 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, in the method according to the invention, an additional trigger is used ne threshold for the similarity of signals and their amplitudes. The similarity comparison has the advantage that even with heavily noisy input signals (signal / noise ratio ~ 1), which can no longer be evaluated with 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 of a certain amplitude A (in relative units) for various current signals S ₁ emitted by sensor element 11 in a logarithmic representation. W 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 ΔT to the background = 2 °.

From this it can be seen that with the alarm threshold of S / N = 10 which 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 a small temperature difference to the environment are not detected.

FIG. 3 shows the measured probability of occurrence W _{c of} the maximum correlation C (similarity) of a signal S ₁ with the stored reference signals R ₁ ... Rn (the greater the value of C, the greater the resemblance of the current signal S ₁ with the reference signal (R ₁ ... Rn) 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 C = _0, 7 is placed. Differential sensors according to CH-PA 884/82, which are not balanced for high frequencies, are particularly suitable here.

The mode of operation is explained in FIGS. 4 and 5 when a further reference signal S ₂ , which originates from a second sensor element 12, is equipped, for example, with an optical system 0 ₂ , which has a pinhole, which ensures that the monitoring range of the the two sensor elements only overlap 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 reference signal S ₂ is supplied as the correlator K.

The correlator K is the correlation C of the obtained from the first sensor element 11, signal S ₁ to the signal S _l received by the sensor element. 11

In the figure 4, the correlation C (schematic resemblance) 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, so a tamper event S and _W armluftturbulenz ( T) applied. 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.

To further explain this fact, FIG. 5 shows the probability of occurrence W for the correlation (similarity) of the two signals S1 and S ₂ for different events. The following mean: R = electronic noise and / or passing through the monitoring area at a large distance from the detector; T = hot 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. IR detector for the detection of an intruder which has a temperature which deviates from its ambient temperature, with at least one sensor element which serves to generate an electrical signal as a function of an IR radiation incident thereon and at least one for focusing the IR emanating from the intruder. Radiation onto the sensor element serving optics, which directs IR radiation coming from several specific, separate fields of view onto the sensor element, and an evaluation circuit that monitors the electrical signal emitted by the sensor element and that detects a change in the incident radiation caused by movements of the intruder Output signal, characterized in that the evaluation circuit has a correlator (K) in which the current signal S _{1 received} from the sensor element (11) continuously with reference signals (R ₁ ... Rn) stored in a read-only memory (FS), which e correspond to typical movements of intruders, is compared, and which emits an output signal which corresponds to the correlation (C) of the current signal (S ₁ ) and reference signal (R ₄ ... Rn), and an alarm stage connected downstream of the correlator (K) (A), which is set up so that it emits an alarm signal when the correlation (C) and the amplitude of the signal (S ₁ ) simultaneously exceed a predetermined value. 2. IR detector according to claim 1, characterized in that it has a first sensor element (11) with first optics (0 ₁ ) and a second sensor element (12) with second optics (0 ₂ ), the two optics ( 0 ₁ , 0 ₂ ) are set up in such a way that their monitoring areas only overlap in the immediate vicinity of the detector, so that the correlator (K) is set up so that it continuously includes the current signal (S ₁ ) received by the sensor element (11) in the Read-only memory (FS) compares stored reference signals (R _i ... Rn) and / or the current signal (S ₂ ) received from the sensor element (12). 3. IR detector according to claim 2, characterized in that the alarm stage (A) is set up in such a way that it emits a fault signal when the correlation (C) between the signal (S _I ) received by the first sensor element (11) and the signal (S _Z ) received from the second sensor element (12) exceeds a predetermined, first threshold value, preferably 0.35. 4. IR detector according to claim 3, characterized in that the alarm stage (A) is set up in such a way that it emits an alarm signal when the correlation (C) between the signal (S ₁ ) received by the first sensor element (11) and the signal (S ₂ ) received by the second sensor element (12) exceeds an opposite, second threshold value, preferably 0.7. 5. IR detector according to one of claims 1 and 2, characterized in that the alarm stage (A) is set up so that it emits an alarm signal when the correlation (C) between the signal received by the first sensor element (11) (S ₁ ) and at least one reference signal (R ₁ ... Rn) from the read-only memory (FS) a predetermined threshold value, preferably the value 0.7 and at the same time the amplitude of the signal (S ₁ ) a predetermined threshold value, preferably twice the rms value of noise, exceeds. 6. IR detector according to one of the claims 1 to 5, characterized in that the read-only memory (FS) contains stored reference signals (R ₁ ... Rn) which correspond to intruders at different speeds. 7. IR detector according to one of the claims 1 to 6, characterized in that a differential element is provided as the sensor element (11, 12). 8. IR detector according to one of the claims 2 to 7, characterized in that the optics 0 _{1 of} the first sensor element (11) is set up in such a way that it monitors the space to be monitored in several active zones and that the optics 0 _{2 of} the second Sensor element (12) is set up so that it monitors the space to be monitored in several active zones and that the optics 0 _{2 of} the second sensor element (12) is set up so that it only shows the radiation entry window. 9. IR detector according to claim 8, characterized in that the optics 0 _{2 of} the second sensor element (12) consists of a pinhole, so that it is ensured that the monitoring areas of the two sensor elements (11, 12) overlap only in the immediate vicinity of the detector . 10. IR detector according to claim 9, characterized in that the optics o ₂ consists of mirror elements. 11. IR detector according to one of claims 9 and 10, characterized in that the two sensor elements (11, 12) are on a chip. 12. IR detector according to one of the claims 2 to 11, characterized in that the two sensor elements (11, 12) are located in one housing.

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