EP0711442B1 - Active ir intrusion detector - Google Patents

Description

The present invention is in the field of infrared detectors, which are detectors, who monitor a room for unauthorized entry and for this purpose one from Evaluate the infrared radiation received by the detector. There are two types of such infrared detectors, the passive and the active.

With passive infrared detectors, the detector waits until one radiation source, the one emits different radiation from the environment, i.e. a different temperature than shows their surroundings, penetrates into the visual field. The passive infrared detectors that relatively inexpensive and widespread today, in principle only radiant Detect and hit objects as soon as objects, such as valuables, to be monitored using mechanical, undetectable means are removable. In addition, special measures are required for passive infrared detectors against the so-called masking, that is the unnoticed changing or covering the field of view of the detector.

In contrast to the passive ones, the active infrared detectors do not process the im Objects in the field of view are emitted heat radiation, but rather irradiate them actively monitor the room to be monitored and react to changes in the reflected infrared radiation. As a result, they can also move from "dead", that is, not radiating Detect objects. In addition, they are very difficult to mask because they are notice any approach. The active infrared detectors have certain difficulties sensitivity and false alarm security because of the reflected infrared radiation can be overlaid by such strong interference that reliable detection movement becomes almost impossible.

The invention relates to an active infrared detector for the detection of movements in a surveillance room with a transmitter for transmitting modulated infrared radiation in the surveillance room, with a receiver for those from the surveillance room reflected infrared radiation, and with one connected to the receiver and means for obtaining an evaluation circuit containing a useful signal.

In the case of a detector of this type described in GB-A-2 183 825, the evaluation circuit contains an operational amplifier designed as a synchronous amplifier, from which only those receive signals are amplified that with the transmitted signal are in phase. These signals are in two integrators with different time constants integrated, both integrators having the same voltage in the undisturbed state generate, and a difference between these voltages indicates an intruder. This infrared detector can not satisfy in terms of responsiveness because the integration of the received signal with two different time constants is not sufficient Guarantees that every object movement in the surveillance room too is actually recognized. The detector is also not false alarm-proof because it is not excluded that can be a difference between the signals of the integrators causes other than caused by object movement.

By means of the invention, this known active infrared detector with regard to sensitivity, Reliability and insensitivity to external influences improved become.

The inventive active infrared detector according to the preamble of claim 1 is to achieve the object characterized in that the evaluation circuit on the one hand with the useful signal acted upon and on the other hand connected to the output of the receiver for delivering a compensation signal superimposed on the receiver signal, and that the compensation signal is selected such that the useful signal is regulated the value is zero.

The adjustment of the useful signal to the value zero has the advantage that always the maximum Sensitivity is retained; the recipient thus acts as one self-balancing scales. It follows immediately that an undesirable Interference signal, provided that this has the same frequency and phase as the emitted infrared radiation has to be compensated to zero and not to throttle the receiver leads to minimal sensitivity. Interference signals of other frequencies are not so critical because they can be easily filtered out.

An active infrared detector is also known from FR-A-2 290 671, whereby a useful signal is adjusted to zero. The However, the evaluation circuit evaluates the received and broadcast signal.

This is a first preferred embodiment of the infrared detector according to the invention characterized in that common optics are provided for the transmitter and receiver is. The use of a common optic enables a massive reduction the manufacturing costs and dimensions, as well as achieving a maximum range with low power consumption.

A second preferred embodiment of the infrared detector according to the invention is characterized in that the evaluation circuit is connected downstream of the controller Analog / digital converter has, at one output, the digitized controller signal available and its other output with a digital / analog converter for generation a voltage corresponding to the respective digital signal value and that this voltage is used to generate the compensation signal. The digitization of the controller signal has the advantage that it is more differentiated than previously and smarter signal evaluation becomes possible.

Such a signal evaluation is particularly possible if as with another preferred embodiment of the infrared detector according to the invention the one output of the analog / digital converter is connected to a microprocessor. The microprocessor on the one hand enables an increase in resolution and on the other hand creates the prerequisite, after the existing sensor in the infrared detector with a second another detection principle to couple the sensor and the signals of both Evaluate sensors together.

Fig. 1

eine schematische Schnittdarstellung eines erfindungsgemässen Infrarotmelders,

Fig. 2

ein Blockschaltbild eines ersten Ausführungsbeispiels der Auswerteschaltung des Infrarotmelders von Fig. 1,

Fig. 3

eine Detailvariante der Schaltung von Fig. 2; und

Fig. 4

ein Blockschaltbild eines zweiten Ausfürungsbeispiels der Auswerteschaltung des Infrarotmelders von Fig. 1.

The invention is explained in more detail below on the basis of exemplary embodiments illustrated in the drawings; shows:

Fig. 1

2 shows a schematic sectional illustration of an infrared detector according to the invention,

Fig. 2

2 shows a block diagram of a first exemplary embodiment of the evaluation circuit of the infrared detector of FIG. 1,

Fig. 3

a detailed variant of the circuit of Fig. 2; and

Fig. 4

2 shows a block diagram of a second exemplary embodiment of the evaluation circuit of the infrared detector of FIG. 1.

The active infrared motion detector 1 shown in Fig. 1 consists essentially of a transmitter S which irradiates the room to be monitored with pulsating infrared light, from a receiver E for the infrared radiation reflected from the monitoring room, from evaluation and control electronics 2 and from a power supply 3. 2 and 4, the transmitter S is an infrared light emitting diode (IRED) 4 and the receiver E is formed by a photodiode 5. Transmitter S, receiver E, electronics 2 and power supply 3 are arranged in a common housing 6, which in the monitoring room at a suitable location, for example on a wall or on the Ceiling is mounted.

The power supply 3 is connected to an external supply and contains a fixed voltage regulator (not shown). The housing 6 contains in the area of the transmitter S and of the receiver E an infrared-transmissive window 7. In addition, there is suitable optics 8 provided, of course not between window 7 on the one hand and the transmitter and receiver S or E must be arranged on the other hand, but also in the window 7 can be integrated. The optics 8 can be a lens or a mirror optics.

It is essential that common optics are provided for transmitter S and receiver E. is. In other words, this means that the receiver E is exactly in those areas of the surveillance room "sees" that the transmitter S is currently applying infrared radiation. And this enables a multiple range with the same power consumption or massively reduced power consumption with the same range. Between transmitter S and receiver E is a shield 9 for preventing a direct light connection arranged between these two elements. As can also be seen in FIG. 1, the electronics 2 has an alarm output 10 for those obtained in the signal evaluation Alarm signals on. These can be an internal one installed in the respective detector 1 and / or activate an external alarm display.

2, the infrared light emitting diode 4 is preceded by a first modulator 11 a suitable modulation of the radiation emitted by the infrared light emitting diode 4 he follows. This radiation preferably consists of a continuous sequence of pulses and pulse pauses so that the room to be monitored is irradiated with pulsating infrared light becomes. It can also be useful to run a sequence of a certain number insert a longer, predetermined transmission pause of pulses and pulse pauses. In this In this case, the monitoring room is irradiated by intermittently emitted and pulse trains or pulse packets interrupted by pauses in transmission. The Transmission pauses to the pulse trains in a fixed or in a variable time ratio stand. The first modulator 11 is controlled by a control stage 12, which receives its clock from a clock 13. The control stage 12 determines in particular the time sequence and the length of the signals emitted to the infrared light-emitting diode 4.

The infrared radiation emitted by the infrared light-emitting diode 4 is bundled by the optics 8 (FIG. 1) and directed into a defined area of the monitoring room. The infrared radiation reflected from this area is collected by the optics 8 and thrown onto the light-sensitive diode 5. The received infrared radiation is converted by the diode 5 into a proportional current (receiver signal) I _e , which is fed to a current / voltage converter 14 connected downstream of the diode 5 and is converted by this into a voltage (received signal) U _e . The transducer 14 also acts as a type of filter for uniform light by suppressing light from the sun and room lighting. In a frequency filter 15 connected downstream of the current / voltage converter 14, undesired frequencies are filtered out of the received signal U _e , as a result of which interference caused by incandescent, fluorescent and discharge lamps is suppressed. The output of the frequency filter 15 is connected to a switch 16 controlled by the control stage 12 in time with the modulation of the infrared light-emitting diode 4.

The output signal of the frequency filter 15, which is largely free of interference, is alternately fed to one of two integrators 17, 17 'via the switch 16. The switch 16 is controlled by the control stage 12 in such a way that the received signal U _{e is} transmitted to one integrator, for example to the integrator 17, during the duration of the pulse transmission and to the other integrator, for example to the integrator 17 ', during the duration of the pulse pauses. , is directed. During any pauses in transmission between the pulse trains or pulse packets, the switch 16 remains in a neutral position in which neither of the two integrators 17 or 17 'is acted upon by the received signal. The switch 16 is preferably formed by a controlled switch.

Due to the control of the switch 16 in time with the modulation, the integrator 17 receives only the reflected infrared transmission signal, including any residues of the filtered interference signal from the time of the transmission pulses, and the integrator 17 'only receives any residues of the filtered interference signal from the time of the pulse pauses, so that the reflected infrared transmission signal can be obtained by simply forming the difference between the output signals of the two integrators 17 and 17 '. The aforementioned difference formation takes place in a stage 18 connected downstream of the two integrators 17, 17 '. Its output signal is the infrared transmission signal U _n , largely cleaned of interference and reflected from the monitoring room, which forms the useful signal for the signal evaluation.

As long as the conditions in the interstitial space do not change, this will also be reflected Infrared transmission signal remain constant. However, one takes place in the monitoring room Movement of an object, regardless of whether it is a living being, a machine or is something, then there is a corresponding change of the reflected infrared transmission signal. Gaseous substances influence the reflected Signal only when the reflective behavior of the substance in question Room or room section changes. The latter means that mere air movements, such as warm air rising from a radiator, from the detector cannot be detected and therefore cannot trigger a false alarm, whereas that sudden occurrence of vapors or smoke and the like the reflection behavior changed and therefore detected by the detector.

The useful signal U _n is supplied on the one hand to a controller 19 and on the other hand to two comparators 20 and 20 '. The output of the controller 19 is connected to the one input of a second modulator 21, the second input of which is connected to the control stage 12 and the output of which is connected to the input of the current / voltage converter 14. The second modulator 21 superimposes a compensation current I _{k on} the signal of the photodiode 5, the timing conditions for the superimposition of this compensation current being determined by the control stage 12. The controller 19 changes the compensation current I _k until the output signal of stage 18, that is to say the useful signal U _n, becomes zero. This means that the maximum sensitivity is always maintained.

The control loop can be with a self-balancing scale or with a bridge circuit are compared, the value zero of the useful signal representing the rest position. Each received infrared signal, also the unwanted basic signal, is compensated for zero. This is the only way to open the possibility for sender and receiver S or E (Fig. 1) to use a common optics 8. Because reflections caused by the transmitter of lenses, mirrors and / or infrared windows that reflect the possible reflection signal Object in the surveillance room are usually exceeded by potencies suppressed by the control loop. A highly reflective object in the detector's field of vision does not lead to a loss of sensitivity, but is compensated for, and the maximum sensitivity is retained.

The comparators 20 and 20 'are used for signal evaluation. They compare the useful signal U _n with an upper limit value (comparator 20) and a lower limit value (comparator 20 ') and deliver an alarm signal to the alarm output 10 when the limit value is exceeded or undershot. This signal evaluation can take place despite the described compensation of the useful signal, because the whole control process is so slow that the infrared signal received by the photodiode 5 is not immediately compensated for zero even with very careful and slow entry into the monitoring space, so that the two comparators 20, 20 'sufficient time remains for a detection.

Because of the considerable size of the interfering reflections caused by imperfect optics 8 or windows 9 (FIG. 1), the controller 19 has to compensate a very large amount of generally more than 90% of the total reflections, the interfering reflections being caused by the geometry and material of the optics and windows have a fixed value. It would be desirable to compensate for this fixed value by means of an additional, fixed compensation current I _{k '} , as a result of which the amount of total reflections to be compensated for by the controller 19 would decrease sharply and the resolution would increase considerably. In this case, the controller 19 would have to accept not only the reflections from the monitoring room, but also any deviations caused by manufacturing tolerances and / or specimen variations of the infrared light-emitting diode 4.

As can be seen in FIG. 2, a third modulator 22, which is also controlled by the control stage 12, is provided for generating the compensation current I _{k '} . This is either set to a fixed value of the compensation current I _{k '} , or, as shown in the figure, it is adjustable. In the latter case, the compensation current I _{k 'can} be adjusted so that not only the mentioned interference reflections but also the deviations caused by the infrared light-emitting diode 4 are compensated.

The controller 19 has an approximately logarithmic behavior. If he is to regulate a small change in the useful signal requires a certain time t, then the Correction of a change ten times larger only twice the time 2t. This behavior is particularly advantageous when switching on the detector, where the change in the useful signal Is 100% and still does not waste an unnecessarily long time for the adjustment.

The alarm signal at alarm output 10 can be further evaluated, for example for plausibility be checked what can be done in the detector or in a control center, or it is sent to a control center without further processing, where an alarm is then triggered. The alarm signal can additionally or alternatively be a light-emitting diode arranged in the detector 23 activate. As shown, a relay 24 is also provided, whose contacts enable potential-free evaluation of the alarm signal. By separate examination the output signals of the two comparators 20 and 20 'on their sign, by evaluating the positive or negative changes in the reflections the direction of movement of an object in the monitoring room, towards the detector or away from this, be determined.

In Fig. 3 is another way to suppress or compensate for unwanted Reflections shown. In this variant, in which no third modulator 22 (Fig. 2) is required, the photodiode forming the actual motion detector 5 a second photodiode 5 'with preferably identical data with reversed polarity connected in parallel. The geometry of the arrangement is chosen so that the a photodiode 5 in the focus of the optics 8 (Fig. 1) and the second photodiode 5 'outside is arranged by this. This receives the one photodiode 5 from the Monitoring room reflected radiation plus any interfering reflections, against the second photodiode 5 'only receives the interference reflections. So the difference corresponds the photo currents of the two photo diodes 5 and 5 'the signal sought from the monitoring room, which at best from interference signals such as solar radiation or Room lighting can be overlaid.

When using two identical photodiodes 5, 5 ', the temperature coefficients of the photosensitivity are mutually compensated for with respect to the common received signals. In addition, all those influences and potential sources of interference that have an effect on both photodiodes remain ineffective. Influences or disturbances of this type are in particular specimen scatter and temperature drifts of the infrared light-emitting diode 4, as well as specimen scatter and changes over time in the reflection constants of the relevant mechanical components, such as varying colors and surface structures. The controller 19 and the second modulator 21 thus only have the compensation of the infrared signals reflected from the monitoring room, whereas around 95% of the total reflections and photo currents are compensated by the second photo diode 5 '. As a result, the influence of the controller 19 can be reduced to approximately ± 5%, as a result of which the resolution of the useful signal U _{n increases} to approximately ten times what corresponds to approximately ten times the sensitivity for constant limits of the comparators 20, 20 '.

The aforementioned checking of the alarm signal for plausibility, which is as complete as possible Suppression of false alarms should be possible, especially with so-called dual detectors are detectors with two people working on different principles Sensors, useful. Combine known dual passive infrared motion detectors of this type passive infrared radiation with ultrasound or with microwaves. In the present active infrared motion detector, a combination of active / passive infrared is conceivable. Such a combination would be the known combinations of infrared / ultrasound and Infrared / microwaves are preferable not least because the infrared radiation behaves exactly the same as visible light and therefore with those from visible light known optical means is controllable. The latter advantageous property Infrared radiation is particularly useful for protecting easily penetrable areas with an infrared curtain, for example when protecting pictures or sculptures in Galleries or museums, or when protecting entire window areas, particularly important.

The evaluation circuit 2 'shown in FIG. 4 differs from the evaluation circuit 2 of FIG. 2 essentially in that another controller is used and that the controller signal is converted analog / digital and thus for evaluation in digitized Form is available. As shown in this embodiment the control of the first modulator 11 by a program control stage 26 which has a counter 27, among other things. The program control stage 26 receives its clock from a clock 13 and determines the time sequence and the length of the infrared light-emitting diode 4 signals emitted. With the reference numeral 28 is the first modulator 11 assigned temperature sensor to compensate for the temperature response of the the infrared light emitting diode 4 and the photodiode 5 containing the control circuit.

Up to the stage 18 downstream of the two integrators 17 and 17 ', the signal processing proceeds analogously to that in the evaluation circuit shown in FIG. 2. The output signal U _n of stage 18, which forms the useful signal for the signal evaluation, is fed to a controller 29, which is preferably a so-called PID controller, that is to say a controller with a proportional, integral and differential component, and reaches a voltage from it / Pulse width converter 30. This generates a pulse-shaped signal from the analog output signal of the controller 29, in which the sum of the pulse plus the pulse pause is constant and the width (duration) of the pulses is proportional to the signal of the controller 29. The pulse-shaped signal from the converter 30 reaches the program control stage 26, the counter 27 of which counts the clock clocks per width of each of the pulses of this signal. Because of the proportionality between the pulse width and the output signal of the controller 29, the number of clock cycles determined by the counter 27 per pulse width represents a digital image of the analog output signal of the PID controller 29.

The pulse width available at the output of the voltage / pulse width converter 30 is only in rare cases and can exactly match a multiple of the clock cycle up to ± 1d (d = smallest unit of information) next to it. The constant length pulse + Pulse pause is defined by the program control stage 26 and is at a clock frequency of 4 MHz and approx. 1 ms when using a 12 bit counter. So stand pro Second 1,000 results with a maximum of 12 bits, that's 4,096 pieces of information with accuracy of ± 1d plus the possible error of converter 30 are available.

Since the differential portion of the signal supplied to the PID controller 29 to a certain extent Unstability of the digital signal can result, it is advantageous to share this signal Differential regulator 31 supply. You can do a division of the differential portion on the two controllers 29 and 31, or the entire differential component lead to the differential controller 31, or you can also omit the differential controller and only use the PID controller 29. Deciding which of these solutions one chooses not least the relationship between effort on the one hand and sensitivity and reliability on the other. However, it should be emphasized that all three solutions are fully functional and deliver satisfactory results.

The values of the clock clocks determined by the counter 27 pass from the program control stage 26 into a pulse width / voltage converter 32, in which a voltage corresponding to the respective counter value, which determines the compensation current I _k , is formed with reference to a reference voltage obtained from the reference voltage source 25. An accuracy of ± 0.001% can easily be achieved here, so that the compensation current corresponds exactly to the level of the counter 27. The output of the differential controller 31 is also connected to the pulse width / voltage converter 32 and supplies it with the higher-frequency components of the useful signal U _n . The output of converter 32 is connected to one input of second modulator 21 (FIG. 2), whose second input is connected to program control stage 26 and whose output is connected to the input of current / voltage converter 14.

The second modulator 21 superimposes the compensation current I _{k on} the signal of the photodiode 5 in the opposite phase, the time conditions for this superimposition being determined by the program control stage 26. The PID controller 29 changes its output signal and thus the pulse / pause ratio in such a way that the output signal of the stage 18, ie the useful signal U _n , becomes zero. The status of the counter 27 thus corresponds to the infrared image of the monitored room except for the already mentioned possible deviation of ± 1d.

Although this deviation will be irrelevant in practice, accuracy can be determined further increase by averaging from a large number of individual values. A Such averaging can be done, for example, by the counter 27 or by a the microprocessor 33 downstream of the program control stage 26. With this the infrared signal present in the program control stage 26 in digital form can be differentiated and be evaluated more intelligently, resulting in higher resolution and thus to improved detection reliability and improved security False positives. The microprocessor also facilitates a meaningful coupling of the measuring principle described with a second in a so-called dual Detector. The microprocessor 33, the alarm signal present as a result of the evaluation outputs to alarm output 10, the alarm signal can check for plausibility and thereby relieve the head office.

• Durch die kompensierende Elektronik wird der Einfluss stark reflektierender Objekte nahe beim Melder so weit zurückgedrängt, dass die Hintergrundstrahlung nach wie vor erkennbar ist. Stark reflektierende Objekte werden wegkompensiert und die maximale Empfindlichkeit bleibt erhalten.
• Durch die kompensierende Elektronik wird die Verwendung einer gemeinsamen Sende/Empfangsoptik ermöglicht. Denn sendeseitig verursachte Reflexionen von Linsen, Spiegeln und/oder vom Infrarotfenster, die das Reflexionssignal eines möglichen Objekts im Überwachungsraum in der Regel um Potenzen übertreffen, werden durch den Regelkreis unterdrückt.
• Die Digitalisierung des Signals bietet die Möglichkeit, Absolutwerte der Infrarotstrahlung zu erfassen und dadurch eine echte Präsenzdetektion zu ermöglichen, und sie ermöglicht den Einsatz eines Mikroprozessors mit all seinen Vorteilen.
• Die Erfassung der Anbsolutwerte der Infrarotstrahlung ermöglicht die Bestimmung von deren Vorzeichen, also die Feststellung, ob eine positive oder negative Änderung der Reflexion und damit eine Objektbewegung auf den Melder zu oder von diesem weg stattfindet.
• Der vorgeschlagene Analog/Digital-Wandler ist wesentlich preisgünstiger als jeder käufliche A/D-Wandler gleicher Auflösung.

The evaluation electronics described, with their control circuit comparable to a bridge circuit, in which the value zero of the useful signal represents the rest position, offer a number of advantages:

• Due to the compensating electronics, the influence of strongly reflecting objects near the detector is suppressed so far that the background radiation can still be seen. Highly reflective objects are compensated for and the maximum sensitivity is retained.
• The compensating electronics enable the use of a common transmission / reception optics. This is because the control circuit suppresses reflections from lenses, mirrors and / or from the infrared window caused by the transmission, which generally exceed the reflection signal of a possible object in the surveillance room by potencies.
• The digitization of the signal offers the possibility of acquiring absolute values of the infrared radiation and thereby enabling real presence detection, and it enables the use of a microprocessor with all of its advantages.
• The detection of the absolute values of the infrared radiation enables the determination of their sign, that is to say whether there is a positive or negative change in the reflection and thus an object movement towards or away from the detector.
• The proposed analog / digital converter is considerably cheaper than any commercially available A / D converter with the same resolution.

Claims (21)

1. An active infrared detector for detecting movements in a monitored room, having an emitter for emitting modulated infrared radiation into the monitored room, having a receiver for the infrared radiation reflected from the monitored room, and having an analysis circuit connected to the receiver, containing means for obtaining a working signal and analysing only received signals, characterised in that the analysis circuit has a controller (19, 29) for outputting a compensating signal (I_k) which is superimposed over the incoming signal (I_e), which on the one hand receives the working signal (U_n) and on the other hand is connected to the output of the receiver (5), and that the compensating signal is selected so that the working signal is corrected to the value zero.
2. An infrared detector according to claim 1, characterised in that a common optical system (8) is provided for the emitter (S, 4) and receiver (E, 5).
3. An infrared detector according to claim 2, characterised in that the analysis circuit has a first modulator (11), connected to a control stage (12, 26), for the pulse-shaped modulation of the signal emitted by the emitter (S, 4), a controlled separating filter (16) connected to the control stage, two integrators (17, 17') connected downstream of the separating filter and means (18) for calculating the difference between the output signals from the integrators.
4. An infrared detector according to claim 3, characterised in that the incoming signal (U_e) is routed to the integrators (17, 17') via the separating filter (16) in phase with the modulation of the emitted signal so that integration of the incoming signal takes place in one of the integrators (17) for the duration of the pulses and takes place in the other integrator (17') during the gaps between pulses.
5. An infrared detector according to claim 4, characterised in that at least one comparator (20, 20') in which the working signal (U_n) is compared with at least one limit value is connected downstream of the means (18) for calculating the difference.
6. An infrared detector according to claim 5, characterised in that two comparators (20, 20') are provided in which the working signal (U_n) is compared with an upper or a lower limit value.
7. An infrared detector according to claim 6, characterised in that the output signals from the two comparators (20, 20') are tested for their sign in order to determine the direction of movement of an object detected in the monitored room.
8. An infrared detector according to one of claims 3 to 7, characterised in that a second modulator (21), controlled by the control stage (12), is connected downstream of the controller (19), the modulator (21) superimposing, in phase opposition, the compensating signal (I_k) over the incoming signal (I_e).
9. An infrared detector according to claim 8, characterised in that the control behaviour of the controller (19) is approximately logarithmic.
10. An infrared detector according to claim 8, characterised by a third, preferably adjustable, modulator (22) for generating an additional compensating signal (I_k') for compensating for reflections caused by the optical system (8) or by an infrared-permeable window (7) of the detector (1).
11. An infrared detector according to claim 8, characterised in that a second diode (5') is provided, connected with reverse polarity in parallel to the first diode (5) forming the receiver (E) and having preferably identical data, and that the difference between the photoelectric currents of the two diodes forms the incoming signal (I_e).
12. An infrared detector according to claim 11, characterised in that the first diode (5) receives the infrared radiation reflected from the monitored room and any interference radiation reflected by the optical system (8) or by an infrared-permeable window (7) of the detector (1), and that the second diode (5') receives only the aforementioned interference radiation.
13. An infrared detector according to claim 12, characterised in that the first diode (5) is arranged in the focal point of the common optical system (8) and the second diode (5') is arranged outside the focal point.
14. An infrared detector according to one of claims 1 to 4, characterised in that the analysis circuit has an analogue/digital converter (26, 30) connected downstream of the controller (29), at one of the outputs of which the digitised controller signal is obtainable and the other output of which is connected to a digital/analogue converter (25, 32) for generating a voltage corresponding to the digital signal value in each case, and that this voltage is used to generate the compensating signal (I_k).
15. An infrared detector according to claim 14, characterised in that one of the outputs of the analogue/digital converter (26, 30) is connected to a microprocessor (33).
16. An infrared detector according to claim 15, characterised in that the controller (29) receiving the working signal (U_n) is formed by a PID controller.
17. An infrared detector according to one of claims 14 to 16, characterised in that the analogue/digital converter is formed by a signal converter (30) for converting the controller signal into a pulse-shaped signal and by a stage (26), connected downstream of the signal converter, for obtaining numerical values corresponding to the magnitude of the individual pulses.
18. An infrared detector according to claim 17, characterised in that the signal converter (30) is formed by a voltage/pulse-width converter which generates a pulse-shaped signal from the analogue output signal from the controller, in which the pulse plus pause between pulses duration is constant and the width of the pulse is proportional to the controller signal.
19. An infrared detector according to claim 18, characterised in that the stage (26) connected downstream of the signal converter (30) has a counter (27) and a clock pulse encoder (13), wherein clock pulses corresponding to the width of the individual signal pulses are counted by the counter.
20. An infrared detector according to claim 19, characterised in that the digital/analogue converter is formed by a pulse-width/voltage converter (32) connected to a reference voltage source (25), each value of the counter (27) being converted into a voltage in the pulsewidth/voltage converter (32).
21. An infrared detector according to claim 20, characterised in that the working signal (U_n) is routed, in parallel to the PID controller (29), to a differential controller (31) for the differential part of the signal, and that the output of the differential controller is connected to the pulse-width/voltage converter (32).

Images