Active IR intrusion detector

The infrared detector contains an emitter (4), a receiver (5) and an analysis circuit (2') for obtaining a working signal (Un). The analysis circuit (2') contains a controller (29) for outputting a compensating signal (Ik) superimposed over the incoming signal (Ie), which on the one hand receives the working signal (Un) and on the other hand is connected to the output of the receiver (5). The compensating signal (Ik) is selected so that the working signal (Un) is corrected to the value zero so that the maximum sensitivity is retained at all times.

The present invention lies in the field of infrared detectors, i.e. 
detectors which monitor a room for unauthorised entry and, to this end, 
analyse infrared radiation received by the detector. There are two types 
of such infrared detectors, passive and active. 
With the passive infrared detectors, the detector waits until a radiation 
source, which emits radiation that differs from that of the environment, 
i.e. the temperature of which is other than that of the environment, 
enters into the field of vision. The passive infrared detectors, which are 
relatively low-priced and, today, widespread, can only detect radiating 
objects on the basis of this principle, and reach a limit as soon as 
objects, for example valuable objects, are to be monitored, such objects 
being removable with mechanical, non-detectable means. In addition, with 
the passive infrared detectors, special measures have to be taken to 
prevent so-called masking, i.e. the unnoticed changing or covering of the 
detector's field of vision. 
In contrast to the passive detectors, the active infrared detectors do not 
handle the thermal radiation given off by objects in the field of vision, 
but rather actively irradiate the room to be monitored and react to 
changes in the reflected infrared radiation. In this way, they can also 
detect movements of "dead", i.e. non-radiating, objects. In addition, they 
can only be masked with considerable difficulty because they detect any 
approach. In return, the active infrared detectors have certain problems 
with sensitivity and false alarm reliability, because the reflected 
infrared radiation can be superimposed with such severe interference that 
reliable detection of movements becomes impossible in practice. 
The invention concerns 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 an analysis circuit, 
connected to the receiver, and containing means for obtaining a working 
signal. 
In a detector of this type described in GB-A-2 183 825, the analysis 
circuit contains an operational amplifier, designed as a synchronous 
amplifier, which only amplifies those incoming signals which are in phase 
with the emitted signal. These signals are integrated in two integrators 
having various time constants, wherein, in the non-disturbed state, both 
integrators generate the same voltage, and a difference between these 
voltages indicates an intruder. These infrared detectors are not 
satisfactory with respect to reliability of response because the 
integration of the incoming signal with two different time constants is 
insufficient guarantee that every movement of an object in the monitored 
room will actually be identified. The detector is also not reliable with 
respect to false alarms because the possibility cannot be excluded that a 
difference between the signals from the integrators is caused by causes 
other than the movement of an object. 
The invention is now intended to improve these known active infrared 
detectors with respect to sensitivity, reliability and insensitivity 
towards foreign influences. 
The active infrared detector according to the invention for solving the 
aforementioned problem is characterised in that the analysis circuit has a 
controller for emitting a compensating signal superimposed over the 
incoming signal, the controller on the one hand receiving the working 
signal and on the other hand being connected to the output of the 
receiver, and that the compensating signal is selected so that the working 
signal is corrected to the value zero. 
Correction of the working signal to the value zero has the advantage that 
the maximum sensitivity is retained at all times; the receiver therefore 
works in the same way as a self-balancing scale. The direct result thereof 
is that an unwanted interference signal, provided that it is of the same 
frequency and phase as the emitted infrared radiation, is also compensated 
to zero and does not cause the receiver to be restricted to minimum 
sensitivity. Interference signals of other frequencies are not so critical 
because they can be simply filtered out. 
A first preferred embodiment of the infrared detector according to the 
invention is characterised in that a common optical system is provided for 
the emitter and receiver. The use of a common optical system enables a 
massive reduction in the manufacturing costs and dimensions, and enables a 
maximum range to be obtained for a low power consumption. 
A second preferred embodiment of the infrared detector according to the 
invention is characterised in that the analysis circuit has an 
analogue/digital converter, connected downstream of the controller, the 
digitised signal being obtainable at one output thereof and the other 
output thereof being connected to a digital/analogue converter for 
generating a voltage corresponding to the digital signal value in each 
case, and characterised in that this voltage is used to generate the 
compensating signal. Digitisation of the controller signal has the 
advantage that it enables more differentiated and intelligent signal 
analysis than used to be the case. 
Such signal analysis is possible particularly if, as in a further preferred 
embodiment of the infrared detector according to the invention, one of the 
outputs of the analogue/digital converter is connected to a 
microprocessor. The microprocessor enables, on the one hand, an increase 
in the resolution and, on the other hand, creates the prerequisite for 
coupling the sensor present in the infrared detector to a second sensor 
working according to another detection principal, and analysing the 
signals of both sensors together.

The active infrared movement detector 1 illustrated in FIG. 1 essentially 
consists of an emitter S, which irradiates the room to be monitored with 
pulsed infrared light, of a receiver E for the infrared radiation 
reflected from the monitored room, of an electronic analysis and control 
circuit 2 and of a power supply unit 3. According to FIGS. 2 and 4, the 
emitter S is formed by an infrared light-emitting diode (IRED) 4 and the 
receiver E is formed by a photodiode 5. The emitter S, receiver E, 
electronic circuit 2 and power supply unit 3 are arranged in a common 
housing 6, which is mounted in the room to be monitored at a suitable 
point, for example on a wall or on the ceiling. 
The power supply unit 3 is connected to an external power source and 
contains a fixed voltage regulator (not shown). In the region of the 
emitter S and the receiver E, the housing 6 contains a window 7 which is 
permeable to infrared. In addition, a suitable optical system 8 is 
provided, which naturally need not be arranged between the window 7 on the 
one hand and the emitter and receiver S and E on the other hand, but 
rather can be integrated into the window 7. The optical system 8 can be a 
lens or mirror optical system. 
It is essential that a common optical system be provided for the emitter S 
and receiver E. In other words, this means that the receiver E "looks" 
into precisely those regions of the monitored room that the emitter S is 
covering with infrared radiation. This also enables, for the same power 
consumption, a greatly increased range or, for the same range, a massively 
reduced power consumption. A screen 9 is arranged between the emitter S 
and receiver E in order to prevent a direct light connection between these 
two elements. As can also be seen from FIG. 1, the electronic circuit 2 
has an alarm output 10 for the alarm signals obtained from the signal 
analysis. These alarm signals can activate an internal alarm display 
incorporated into the detector 1 and/or an external alarm display. 
According to FIG. 2, the infrared light-emitting diode 4 is connected 
upstream of a first modulator 11, by means of which the radiation emitted 
by the infrared light-emitting diode 4 is suitably modulated. Preferably, 
this radiation consists of a continuous sequence of pulses and pauses 
between pulses so that the room to be monitored is irradiated with pulsed 
infrared light. It may also be sensible to insert a longer, pre-determined 
emission pause between a sequence of a certain number of pulses and pauses 
between pulses. In this case, the monitored room is irradiated by pulse 
trains or pulse packets which are intermittently emitted and interrupted 
by emission pauses. In this way, the emission pauses can stand in a fixed 
or variable time ratio to the pulse trains. The first modulator 11 is 
controlled by a control stage 12, which obtains its clock pulse from a 
clock pulse generator 13. In particular, the control stage 12 determines 
the time sequence and the length of the signals output to the infrared 
light-emitting diode 4. 
The infrared radiation emitted by the infrared light-emitting diode 4 is 
bundled by the optical system 8 (FIG. 1) and directed into a specific 
region of the monitored room. The infrared radiation reflected from this 
region is collected by the optical system 8 and routed to the 
light-sensitive diode 5. From the diode 5, the received infrared radiation 
is converted into a proportional current (incoming signal) I.sub.e which 
is supplied to the current/voltage converter 14 connected downstream of 
the diode 5 and is converted by the current/voltage converter 14 into a 
voltage (incoming signal) U.sub.e. The converter 14 also acts as a kind of 
filter for uniform light by suppressing light originating from the sun and 
from the room lighting. In a frequency filter 15 connected downstream of 
the current/voltage converter 14, unwanted frequencies are filtered out of 
the incoming signal U.sub.e, whereby interference caused by incandescent, 
fluorescent and discharge lamps, in particular, is suppressed. The output 
of the frequency filter 15 is connected to a separating filter 16 that is 
controlled by the control stage 12 in the clock pulse of the infrared 
light-emitting diode 4 modulation. 
The output signal from the frequency filter 15, which is largely free of 
interference, is supplied via the separating filter 16 alternately to one 
of two integrators 17, 17'. In this way, the separating filter 16 is 
controlled by the control stage 12 so that, for the emission duration of 
the pulses, the incoming signal U.sub.e is routed to one of the 
integrators, for example to the integrator 17, and, for the duration of 
the pauses between pulses, the incoming signal U.sub.e is routed to the 
other integrator, for example the integrator 17'. During any emission 
pauses between the pulse trains or pulse packets, the separating filter 16 
moves into a neutral position in which neither of the two integrators 17 
or 17' receives the incoming signal. The separating filter 16 is 
preferably formed by a controlled switch. 
Since the separating filter 16 is controlled in the modulation clock pulse, 
the integrator 17 only receives the reflected infrared emission signal, 
including any residues of the filtered interference signal, from the 
emission pulse period, and the integrator 17' only receives any residues 
of the filtered interference signal from the period of the pauses between 
pulses, with the result that the reflected infrared emission signal can be 
obtained simply by calculating the difference between the output signals 
from the two integrators 17, 17'. The aforementioned difference 
calculation takes place in a stage 18 connected downstream of the two 
integrators 17, 17'. The output signal from this stage 18 is the infrared 
emission signal U.sub.n, reflected from the monitored room and largely 
freed of interference, which forms the working signal for the signal 
analysis. 
Provided that the conditions in the monitored room remain unchanged, the 
reflected infrared emission signal will also remain constant. However, if 
an object moves in the monitored room, regardless of whether the object is 
a living being, a machine or any other object, then there is a 
corresponding change in the reflected infrared emission signal. Gaseous 
materials only influence the reflected signal if the reflection behaviour 
of the room or room section containing the material concerned changes. 
This means that simple air movements, such as warm air rising from a space 
heater, for example, are not detected by the detector and consequently 
cannot trigger a false alarm, whereas the sudden appearance of vapours or 
smoke and the like does change the reflection behaviour and is therefore 
detected by the detector. 
The working signal U.sub.n is routed, 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 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 compensating current I.sub.k, in 
phase opposition, over the signal from the photodiode 5, wherein the time 
conditions for the superimposition of this compensating current are 
determined by the control stage 12. The controller 19 changes the 
compensating current I.sub.k until the output signal from the stage 18, 
i.e. the working signal U.sub. n, becomes zero. Thus, the maximum 
sensitivity is always retained. 
The control circuit can be compared to a self-balancing scale or to a 
bridging circuit, wherein the zero value of the working signal represents 
the at-rest position. Each infrared signal received, even the unwanted 
basic signal, is compensated to zero. Only in this way is there the option 
of using a common optical system 8 for the emitter and receiver S and E 
(FIG. 1). This is because reflections caused on the emitter side by 
lenses, mirrors and/or infrared windows, which generally exceed by a power 
the reflection signal of a possible object in the monitored room, are 
suppressed by the control circuit. A highly reflective object in the field 
of vision of the detector does not lead to a loss of sensitivity, but 
rather is compensated away, and the maximum sensitivity is retained. 
The comparators 20 and 20' are used for signal analysis. They compare the 
working signal U.sub.n with an upper limit value (comparator 20) and a 
lower limit value (comparator 20') and, if the working signal exceeds 
upper limit value or falls below the lower limit value, sends an alarm 
signal to the alarm output 10. Despite the described working signal 
compensation, this signal analysis can take place because the entire 
control operation is, in fact, so slow that, even in the event of very 
careful and slow intrusion into the monitored room, the infrared signal 
received by the photodiode 5 is not immediately corrected to zero, with 
the result that both comparators 20, 20' still have sufficient time for 
detection. 
On account of the considerable magnitude of the interference reflections 
caused by an imperfect optical system 8 or window 9 (FIG. 1), the 
controller must compensate for a very large amount, generally over 90%, of 
all the reflections, wherein the interference reflections have a fixed 
value, determined by the geometry and material of the optical system and 
window. It would be desirable to equalise this fixed value by means of an 
additional fixed compensating current I.sub.k', which would considerably 
reduce the amount of the total reflections to be compensated by the 
controller 19 and considerably increase the resolution. In this case, the 
controller 19 would have to absorb any deviations caused by production 
tolerances and/or copy tolerances of the infrared light-emitting diode 4, 
in addition to the reflections from the monitored room. 
As can be seen from FIG. 2, a third modulator 22, also controlled by the 
control stage 12, is provided for generating the compensating current 
I.sub.k'. This is either set to a fixed value for the compensating current 
I.sub.k', or is, as shown in the figure, designed to be adjustable. In the 
latter case, the compensating current I.sub.k', can be adjusted so that 
the deviations caused by the infrared light-emitting diode 4 are 
compensated, as well as the aforementioned interference reflections. 
The behaviour of the controller 19 is approximately logarithmic. If it 
requires a certain time t to correct a small change in the working signal, 
then the correction of a change of ten times the magnitude requires only 
twice the time 2t. This behaviour is particularly advantageous when the 
detector is switched on, when the change in the working signal is 100% and 
the time required for the correction is nevertheless not unnecessarily 
long. 
The alarm signal at the alarm output 10 can be further analysed, for 
example tested for plausibility, which can take place in the detector or 
in a control room, or it is routed without further processing to a control 
room where the alarm is then triggered. The alarm signal can additionally 
or alternatively activate a light-emitting diode 23 arranged in the 
detector. According to the illustration, a relay 24 is also provided, the 
contacts of which enable potential-free analysis of the alarm signal. By 
separately testing the output signals from the two comparators 20 and 20' 
for their sign, i.e. by analysing the positive or negative changes in the 
reflections, the direction of movement of an object in the monitored room 
can be determined, either at the detector or away from the detector. 
FIG. 3 illustrates a further option for suppressing or compensating for 
unwanted reflections. In this variant, in which a third modulator 22 (FIG. 
2) is not required, the photodiode 5 forming the actual movement detector 
is connected in parallel to a second photodiode 5', preferably having 
identical data with reversed polarity. In this way, the geometry of the 
arrangement is selected so that one of the photodiodes 5 is arranged in 
the focal point of the optical system 8 (FIG. 1) and the second photodiode 
5' is arranged outside the focal point. In this way, one of the 
photodiodes 5 receives the reflected radiation from the monitored room 
plus any interference reflections, whereas the second photodiode 5' 
receives only the interference reflections. Thus, the difference between 
the photoelectric currents of the two photodiodes 5 and 5' corresponds to 
the desired signal from the monitored room, which can, if necessary, be 
superimposed by interference signals, such as solar radiation or room 
lighting. 
If two identical photodiodes 5, 5' are used, the temperature coefficients 
of the photosensitivity are mutually compensated with respect to the 
common incoming signal. In addition, all those influences and potential 
sources of interference which act on both photodiodes remain without 
effect. Influences or interference of this type are, in particular, copy 
deviations and temperature drifts of the infrared light-emitting diode 4 
and copy deviations and changes over time in the reflection constants of 
the relevant mechanical components, such as varying dyes and surface 
structures. Thus, the controller 19 and the second modulator 21 simply 
have to compensate for the infrared signals reflected from the monitored 
room, whereas around 95% of the total reflections and photoelectric 
currents are compensated by the second photodiode 5'. In this way, the 
influence of the controller 19 can be reduced to around .+-.5%, which 
increases the resolution of the working signal U.sub.n by a multiple of 
approximately ten, which corresponds to around ten times the response 
sensitivity for constant comparator 20, 20' limits. 
The aforementioned checking of the alarm signal for plausibility, which is 
intended to enable false alarms to be suppressed as completely as 
possible, is particularly meaningful in the so-called dual detectors, i.e. 
detectors with sensors working according to two different principles. Such 
known dual passive infrared movement detectors combine the possible 
infrared radiation with ultrasound or microwaves. In the present active 
infrared movement detector, a combination of active/passive infrared is 
feasible. Such a combination would be preferable to the known combinations 
of infrared/ultrasound and infrared/microwaves, not least because the 
infrared radiation behaves in exactly the same way as the visible light 
and is thus controllable with the known optical means on the basis of the 
visible light. The latter advantageous characteristic of infrared 
radiation is particularly important, particularly when protecting easily 
penetrated surfaces with an infrared curtain, for example when protecting 
pictures or sculptures in galleries or museums, or when protecting entire 
window surfaces. 
The analysis circuit 2' illustrated in FIG. 4 differs from the analysis 
circuit 2 in FIG. 2 essentially in that another controller is used and 
that the controller signal is converted from analogue to digital and is 
thus available for analysis in a digitised form. According to the 
illustration, in this embodiment, the first modulator 11 is controlled by 
a program control stage 26 which has, amongst other components, a counter 
27. The program control stage 26 receives its clock pulse from a clock 
pulse encoder 13 and determines the sequence over time and the length of 
the signals output to the infrared light-emitting diode 4. A temperature 
sensor for compensating for the response to temperature changes of the 
control circuit containing the infrared light-emitting diode 4 and the 
photodiode 5 is designated by reference numeral 28. 
The signal processing takes place in a similar manner to that in the 
analysis circuit illustrated in FIG. 2, up to the stage 18 connected 
downstream of the two integrators 17 and 17'. The output signal U.sub.n of 
the stage 18, which forms the working signal for the signal analysis, is 
supplied to a controller 29, which is preferably a so-called PID 
controller, i.e. a controller having a proportional, an integral and a 
differential part, and passes therefrom into a voltage/pulse-width 
converter 30. This generates, from the analogue output signal from the 
controller 29, a pulse-shaped signal, in which the total of pulse plus 
pause between pulses is constant and the width (duration) of the pulse is 
proportional to the signal from the controller 29. The pulse-shaped signal 
from the converter 30 enters the program control stage 26, the counter 27 
of which counts the clock pulses per width of each of the pulses of this 
signal. On account of the proportionality between the pulse-width and the 
output signal from the controller 29, the number of clock pulses per 
pulse-width determined by the counter 27 represents a digital image of the 
analogue output signal from the PID controller 29. 
The pulse-width obtainable at the output from the voltage/pulse-width 
converter 30 will only exactly coincide in very rare cases with a multiple 
of the clock pulse and can vary therefrom by up to .+-.1 d (d=smallest 
information unit). The constant length of pulse+pause between pulses is 
determined by the program control stage 26 and can be approximately 1 ms 
for a clock frequency of 4 MHz and when using a 12-bit counter. Thus, 
1,000 results of up to 12 bits, i.e. 4,096 information units, with a 
precision of .+-.1 d plus any converter 30 error, are available every 
second. 
Since the differential part of the signal supplied to the PID controller 29 
can lead to a certain instability of the digital signal, it is 
advantageous to supply this signal part to a differential controller 31. 
In so doing, the differential part can be divided between the two 
controllers 29 and 31, or the entire differential part can be routed to 
the differential controller 31, or the differential controller can also be 
omitted and only a PID controller 29 used. The essential factor in which 
of these solutions is selected is, not least, the ratio between cost, on 
the one hand, and sensitivity and reliability, on the other hand. It 
should be stressed, however, that all three solutions are fully functional 
and provide satisfactory results. 
The values of the clock pulses 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 counter value is formed, with reference to 
a reference voltage related to the reference voltage source 25, this 
voltage determining the compensating current I.sub.k. Here, a precision of 
.+-.0.001% is achievable without further means, with the result that the 
compensating current precisely corresponds 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 routes thereto the higher-frequency 
parts of the working signal U.sub.n. The output of the converter 32 is 
connected to one of the inputs of the second modulator 21 (FIG. 2), the 
second input of which is connected to the program control stage 26 and the 
output of which is connected to the input of the current/voltage converter 
14. 
The second modulator 21 superimposes the compensating current I.sub.k, in 
phase opposition, over the signal from the photodiode 5, wherein the time 
conditions for this superimposition are determined by the program control 
stage 26. The PID controller 29 changes its output signal and thus the 
pulse/pause ratio such that the output signal from the stage 18, i.e. the 
working signal U.sub.n, become equal to zero. Thus, the level of the 
counter 27 corresponds to the infrared image of the monitored room, up to 
the aforementioned possible deviation of .+-.1 d. 
Although, in practice, this deviation is of no significance, the precision 
can be further increased by calculating the mean of a plurality of 
individual values. Such a mean calculation can, for example, be carried 
out by the counter 27 or by a microprocessor 33 connected downstream of 
the program control stage 26. With this, the infrared signal, which is 
present in the program control stage 26 in a digital form, can be analysed 
in a more differentiated and intelligent manner, which leads to higher 
resolution and thus to improved detection reliability and to improved 
reliability with respect to false messages. In addition, the 
microprocessor facilitates a meaningful coupling of the described 
measurement principle with a second measurement principle in a so-called 
dual detector. The microprocessor 33, which passes the alarm signal, which 
is present in the form of the result of the analysis, to the alarm output 
10, can check the alarm signal for plausibility and thus relieve the 
burden on the control room. 
The described electronic analysis circuit with its control circuit, which 
is comparable to a bridging circuit in which the zero value of the working 
signal represents the at-rest position, offers a range of advantages: 
The electronic compensating circuit suppresses the influence of highly 
reflective objects close to the detector to such an extent that the 
background radiation is still identifiable. Highly reflective objects are 
compensated away and the maximum sensitivity is retained. 
The electronic compensating circuit enables the use of a common 
emission/reception optical system. This is because reflections from 
lenses, mirrors and/or from the infrared window, caused on the emission 
side, which exceed by a power the reflection signal of a possible object 
in the monitored room, are suppressed by the control circuit. 
The digitisation of the signal offers the option of detecting absolute 
infrared radiation values and thus allowing true presence detection, and 
enables the use of a microprocessor with all the associated advantages. 
The detection of the absolute infrared radiation value enables the sign 
thereof to be identified, i.e. identification of whether a positive or 
negative change in the reflection and thus the movement of an object takes 
place close to or away from the detector. 
The recommended analogue/digital converter is substantially less expensive 
than any commercially available A/D converter of the same resolution.