Active IR intrusion detector

Details Active IR intrusion detector Active IR intrusion detector

1996-01-30

1997-10-07

Hannaher, Constantine

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

2503418, G08B 13181

Patent

active

056751508

DESCRIPTION:

BRIEF SUMMARY
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

REFERENCES:
patent: 4068222 (1978-01-01), Treviranus
patent: 4733081 (1988-03-01), Mizukami

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