Method and device for detecting an intruder using a passive infra-red motion detector.

p0001The invention relates to a method for detecting an intruder, which is a moving heat source, by means of a passive infrared motion detector with a sensor head having a plurality of sensor elements which each detect a lobe-shaped area and emit a signal when entering the heat source and a device for performing this method.

p0002To protect premises against unauthorized entry of persons use of the fact that man has a higher temperature as a heat source than its surroundings. The consequent increased emission of long-wave infrared radiation is detected by known per se passive infrared detectors. In such monitoring devices frequently occurs a problem that also temperature changes in the visual field of the infrared detector will be generated when switching on heating and air conditioning or sudden sunlight or air movement. Upon detection of intruders by passive infrared motion fundamental problem is that, to be able to distinguish such stationary temperature changes with great certainty from a moving heat source, as it represents a penetrating into the detection area person.

p0003Passive infrared motion detector or sensors are known per se; they generally have facet mirror or facet lenses to be used as a motion detector can. The facetted optics produced in the visual field of the sensor more narrow zones and maps them to a sensitive infrared radiation difference element. Crossing a heat source one of the zones, the result is a typical signal pattern sen with positive and negative Signalpul which are used for evaluation. Still under construction very differently designed infrared sensors, these known, certain disadvantages. The evaluation strategies for avoiding false alarms are generally based on the selection of certain signal patterns for triggering an alarm. However, it is not possible to make a statement about the distance traveled of the heat source, as appropriate signal patterns are also generated when the heat source varies only in their temperature or even makes only minor oscillations. This is because the entire thermal image information of the space is reduced in this process to a single signal, which permits no clear conclusion about the situation in the monitoring area more. Restricting therefore to avoid false alarms, the alarm in a small class of narrowly defined signal patterns so arise for an intelligent intruder successfully negotiating strategies.

p0004This ambiguity of such a process is also reflected in the fact that can not be distinguished whether an object is warmer or cooler than the surrounding area, or whether it passed the detection area from left to right or from right to left. Furthermore, small animals can cause false alarms by a large sensitivity in the vicinity of the sensor. Neither of the sensor is relatively insensitive to radial movements.

p0005In order to reduce false alarms has already been proposed in EP-A-0107042 to provide two sensor elements. Here, the current, obtained from a first sensor element signal is compared in a correlator constantly with reference signals that are stored in a read only memory, and with the current, obtained by a second short-range monitoring sensor element signals. The correlator provides an output signal corresponding to the correlation of the two signals being compared. An alarm signal is triggered when the correlation exceeds a predetermined value and the amplitude reaches the threshold. The second sensor is here only used as a reference. A spatial representation is not possible with the known infrared detector. It can hereby also not the typical course of a trace that is caused by an intruder, determine.

p0006Object of the invention is to avoid the abovementioned disadvantages and to provide a method and an apparatus which ensures a precise spatial control of the area to be protected and a reliable alarm signal allows, with false alarms and outsmarting to be largely excluded.

p0007This object is achieved with respect to the method with the features of claim 1 and with respect to the device with the features of claim. 4

p0008In the inventive method the area to be protected is monitored by two angle-selective passive infrared sensor heads with different locations. As an angle-sensitive sensor, for example, two heads Pyrodektoren can be used, as are known from EP-A-0245842. In this sensor head, a PVDF film with a series of sensor elements disposed in the focal plane of a spherical or spherical-parabolic mirror. This mirror optics is allocated a corresponding lobe in the detection range, so that an angle-selective detection of infrared radiation source is possible each sensor element.

p0009Such a heat source is thereby targeted from two different directions and their location can be continuously determined by triangulation. Thereby the path of movement of the heat source and thus the trace of the intruder can be recorded in the common detection range of both sensor heads and derived from this trajectory or trace in a reliable alarm criterion for a motion. With the inventive method are false alarms, such as may be caused for example by itself switches on radiators, air movement or sunlight, almost impossible. Due to the spatial resolution is clearly detected in an advantageous manner if the intruder has actually moved a certain distance in the surveillance area. Also a radial movement, ie a movement to right on one of the sensor heads, is clearly recognized as a not to be detected small animal, which is in the immediate vicinity of a sensor is recognized as non-intruder and does not lead to an alarm signal.

p0010Depending on the position and the extent of the heat source or of the intruder different levels of signals are generated in the individual sensor elements, which are for each sensor element amplified and digitized separately. The signals thus obtained are expediently processed in a microprocessor, wherein a direction vector is calculated to the heat source by a geometrically correct weighting of the individual digitized signals for each sensor head. From the intersections of two corresponding direction vectors, the current position of the heat source is continuously calculated and from this the track traveled by the intruder, formed. It is derived an alarm criterion advantageously from the form and length of the track and issued an alarm signal.

p0011In a development of the method, the direction of movement or position of the intruder can be displayed as additional information beyond.

p0012With regard to the apparatus for performing the method erfindungsgemäßens the object is achieved with the features of claim 4th

p0013The invention is illustrated by the drawing in the following. show case<ul><li>FIG. 1 is a conventional PID detector-reflector,</li><li>Fig. 2 shows a known IR sensor with PVDF-foil,</li><li>Fig. 3 basic arrangement of two IR sensors for the inventive method and </li><li>Fig. 4 is a block diagram of a possible embodiment.</li></ul>

p0014In Fig. 1, a conventional facet detector is shown as font drawing in plan view. Such a detector is known for example from DE-OS 2,103,909.

p0015In FIG. 2, a novel infrared sensor is schematically illustrated, which is designed as an angle-selective passive sensor head SK, wherein an optical arrangement is used in which an array of electrically and thermally separate the sensor elements SE is located in the focal plane of a spherical mirror KS as from the above mentioned European patent application 0245842 is known. Each sensor element SE is thereby assigned by the mirror optics (KS), a club-shaped detection area KB. The geometry of the individual beams lobes KB is determined by the focal length of the spherical mirror KS and the number, size and shape of the selective zones.

p0016According to FIG. 3, two such sensor heads SK1, SK2 are used which at a certain distance a, for example, four meters, are located away from each other and to jointly monitor the protected area EB. A sensor head SK1 has at least four sensor elements (SE), the four lobe-shaped surveillance regions KB.

p0017If a heat source, in this case an intruder EN, in the detection region EB of the sensor heads SK1 and SK2, so enter different high signals in the individual sensor elements SE depending on the position and extent of the heat source. After digitizing the signals of the individual signals for each sensor head SK1, SK2 a direction vector RV1, RV2 is calculated to the heat source EN to a computer by geometrically correct weighting. From the intersection of the two direction detectors RV1, RV2 gives the current position of the heat source EN. By continuously calculating this position, the track of the object is traced in the computer and from this the alarm. Moreover, also on a screen associated with the computer and the display device, the path of movement or the position of the intruder presented graphically or otherwise indicated.

p0018In FIG. 4 is shown schematically in a block diagram an embodiment for carrying out the method according to the invention shown. Each sensor head SK1 and SK2 here has for example, four sensor elements SE on whose signals are respectively supplied through an amplifier VER an analog-digital converter AD. The digitized signals of both sensor heads SK1, SK2 or all of the sensor elements SE are a microcomputer uR course Performs who performs the corresponding processing of these signals. The microcomputer uR has a control panel BF and display means ANZ, on an alarm signal is displayed, which can be output as an alarm AL visually or audibly. It can also be displayed on a connected to the display means ANZ colored screen BS the trajectory or position of the intruder.

p0019The invention not only has the advantage that fixed temperature changes do not lead to false alarms, it also has the advantage that can be reduced through better delineation of the monitored zone false alarms. Nor can trigger false alarms small animals in the vicinity of the sensor, because due to the distance information a better estimate of the intensity of the heat source is possible and thus can be detected, whether it is an actual intruder or a fly.