EP0421119B1 - Passive infrared motion detector - Google Patents

Abstract

It is proposed to arrange attenuation means (6) in the known passive infrared motion detector having an azimuthal detection angle of 180 DEG in order to increase the detection sensitivity for laterally incident infrared beams directed via deflection mirrors (8) onto the sensor surfaces (9) and to extend the detection region. As a result, the transmittance for an infrared beam (SI) incident from a frontal detection region (I) is smaller than the transmittance for an infrared beam (SII) incident from a lateral detection region (II). As a result, the detection region definable by means of r and phi (r = detection range, phi = azimuthal detection angle) can be extended. The azimuthal detection angle can be widened to 220 DEG in combination with non-central Fresnel lens segments (4). <IMAGE>

Description

The invention relates to a passive infrared motion detector according to the preamble of claim 1.

Passive infrared motion detectors are essentially devices which trigger a switching process as a function of the detected infrared radiation of an object emitting heat radiation. They are used to monitor a room area for moving objects, whereby the passive infrared motion detectors e.g. react to the change in heat radiation in the detection area to be monitored. Such an infrared radiation object is e.g. a person who moves in a room to be monitored. A passive infrared motion detector only works as a receiver of infrared heat radiation, whereas other types of infrared motion detectors have an active infrared transmitter.

From the European patent EP 0 113 468 a passive infrared motion detector with an azimuthal detection angle of 180 ° is known, which has an infrared sensitive sensor inside a housing. There are window-like openings in the housing in which there is a wide-angle collecting optic, in particular a Fresnel plastic lens. In this case, frontally incident infrared rays from the detection area to be monitored are bundled directly onto the infrared sensor, whereas laterally infrared rays incident from the detection area are focused on the infrared sensor only after intermediate reflection on a deflecting mirror system.

Apart from cases in which an individual adaptation of the detection characteristics to the local conditions is desired depending on the local conditions, the largest possible detection range is aimed at with passive infrared motion detectors and at the same time high detection sensitivity. Due to the fact that in the passive infrared motion detector known from the prior art, the infrared rays incident laterally from the detection area are focused on the infrared sensor by means of intermediate reflection at a deflecting mirror system, the intensity of the incident on the infrared sensor is ultimately due to scattering losses Radiation is reduced compared to the radiation emitted directly by the object. This leads to a narrowing of the detection characteristic in the lateral area.

The passive infrared motion detector known in the prior art is highly sensitive, in particular to temperature changes occurring in the front detection area. However, this fundamentally desirable fact is disadvantageous if switching operations are already triggered by objects which are not intentionally detected, in particular small animals, or by air currents passing by. As a remedy for the latter, triggering unwanted switching operations, the measure known in the prior art has proven itself to be arranged on the sensor side, at a distance behind the plastic Fresnel lens, to arrange a plastic film which transmits infrared radiation. This has two effects. Firstly, the entire frontal and lateral incident infrared radiation is attenuated in intensity and secondly, a heat-insulating air cushion is generated between the plastic Fresnel lens and the plastic damping film, which creates the heat generated inside the passive infrared motion detector due to the evaluation electronics there does not let outside. This stabilizes a temperature gradient of approximately + 2 ° C compared to the outside detection area. Without this measure, the thermal energy dissipated to the outside via the Fresnel lens would be carried away by air flowing past, which could lead to the triggering of undesired switching operations. With this arrangement of the plastic film, which in particular brings about thermal insulation, it is disadvantageous that the infrared rays incident from the lateral spatial sectors and inter-reflected by the deflecting mirror system are also attenuated, which further reduces the detection sensitivity in the lateral area. The invention attempts to remedy this.

It is an object of the invention, starting from the prior art of the generic type and avoiding the aforementioned disadvantages, to provide a passive infrared motion detector which increases the detection sensitivity and increases the detection range for laterally incident infrared rays which are inter-reflected via a deflecting mirror guaranteed.

This object is achieved by the features characterized in more detail in the characterizing part of claim 1. Advantageous embodiments of the invention are characterized in more detail in the subclaims.

According to the invention, additional attenuation means are provided in the beam path of the infrared rays incident on the infrared sensor from the front detection area compared to the infrared rays incident from the lateral detection area. This has the effect that the transmittance for the infrared rays incident from the front detection area is lower than the transmittance of the infrared rays incident on the infrared sensor from the lateral detection area. Here, the transmittance is defined as the ratio of the radiation intensity weakened after passing through a medium to the initial radiation intensity, in which case the initial radiation intensity in the beam path in front of the Fresnel lens and the weakened radiation intensity directly in front of the infrared sensor are measured. The measure according to the invention of additionally attenuating only the frontally incident infrared rays, but not the laterally incident infrared rays, decisively reduces the noise level received on the infrared sensor due to the heat background radiation which is always present. This improves the signal-to-noise ratio of the laterally incident infrared rays compared to the frontally incident infrared rays, as a result of which the infrared radiation changes of the former can be better detected. As a result, not only are the constrictions of the detection characteristic in the area of the laterally incident infrared rays present in the prior art eliminated, but it can be increased by a sensible arrangement of the Fresnel lens centers for the laterally incident infrared rays, the detection area beyond that detection area, as is known in passive infrared motion detectors without or complete coverage by means of plastic film.

An infrared-permeable plastic film is advantageously provided as a damping means in the beam path of the frontally incident infrared rays, the transmittance of which is known to be essentially a function of the infrared wavelength to be transmitted, the material and the film thickness. In practice, films have proven their worth, the transmittance of which is between 58% and 68% at a wavelength of 10 »m. However, the damping effect can also be achieved with other measures, e.g. by means of a varnish applied to the Fresnel plastic lens or by a different thickness of the Fresnel plastic lens. However, the use of a plastic damping film offers the advantage that a heat insulation layer is present in the front detection area, in which the detection sensitivity is particularly high, because of the aforementioned air cushion, due to which undesired switching operations can be largely prevented.

According to an expedient embodiment, the Fresnel lens system used as a wide-angle collecting optic is realized by a plastic Fresnel lens which is bent in a semicircular convex manner into the detection area. Here, strip-like segments covering the azimuthal detection area are semicircular arranged side by side, which have Fresnel lenses stamped into the plastic film.

According to a further embodiment, at least two zones of segmented central Fresnel lenses, preferably eleven, are provided for the detection of the infrared rays incident from the front detection area. This makes it impossible to undermine the frontal detection area.

According to a particularly preferred embodiment of the invention, segments with acentric Fresnel lenses are arranged laterally next to the segments assigned to the frontally incident infrared rays. The teaching according to the invention combined with the acentric Fresnel lenses described above thus offers the decisive advantage that the azimuthal detection range of the passive infrared motion detector extends beyond 180 ° to e.g. Can be extended by 220 °.

Fig. 1

eine Frontalansicht des erfindungsgemäßen Passiv-Infrarot-Bewegungsmelders durch einen Fensterdurchbruch in der Gehäusewandung mit Kunststoff-Fresnellinsen-Folie,

Fig. 2

eine mediane Schnittansicht durch den Passiv-Infrarot-Bewegungsmelder gemäß der Schnittlinie II-II in Fig. 1,

Fig. 3

die Erfassungscharakteristik eines im Stand der Technik bekannten Passiv-Infrarot-Bewegungsmelders mit bzw. ohne Dämpfungs-Folie

Fig. 4

die Erfassungscharakteristiken des erfindungsgemäßen Passiv-Infrarot-Bewegungsmelders gemäß zweier Ausführungsformen,

Fig. 5

eine Gegenüberstellung diverser aus dem Azimut lateral oder frontal einfallender sowie auf den Infrarotsensorflächen empfangener Infrarot-Nutzsignal mit gleicher Ausgangsintensität I₀ bei verschiedenen Dämpfungsanordnungen,

Fig. 6

eine segmentierte Fresnellinsen-Kunststoff-Folie mit azentrischen lateralen Fresnellinsen.

The invention is described and explained in more detail below with reference to the schematic drawings. Show it:

Fig. 1

2 shows a frontal view of the passive infrared motion detector according to the invention through a window opening in the housing wall with plastic Fresnel lens film,

Fig. 2

2 shows a median sectional view through the passive infrared motion detector according to section line II-II in FIG. 1,

Fig. 3

the detection characteristic of a passive infrared motion detector known in the prior art with or without a damping film

Fig. 4

the detection characteristics of the passive infrared motion detector according to two embodiments,

Fig. 5

a comparison of various infrared useful signals incident laterally or frontally from the azimuth and received on the infrared sensor surfaces with the same output intensity I₀ with different damping arrangements,

Fig. 6

a segmented Fresnel lens-plastic film with acentric lateral Fresnel lenses.

1 shows a frontal view of the passive infrared motion detector 1 according to the invention with a window-like housing recess 3 in a housing 2. A Fresnel lens film 4 made of plastic is clamped in a semicircular convex manner in the housing recess 3. Infrared rays S _I incident frontally from a first detection area I are bundled directly onto infrared sensors 9 by means of the Fresnel lens film 4. Infrared rays S _II incident laterally from a second detection area II are focused on the infrared sensors 9 with the help of the Fresnel lens film 4 only after intermediate reflection at two deflecting mirrors 8.

2 shows a median side sectional view through the passive infrared motion detector 1 according to the section line II-II in FIG. 1. The Fresnel lens plastic film 4 is convex in a semicircular shape into the detection areas I, II to be monitored and is held in a Fresnel lens mount 5 of the housing 2. For damping the frontally incident from the detection range I infrared rays S _I-plastic film Fresnel lens 4 is permeable to infrared damping plastic film 6 provided in the beam path behind the and supported film version cushion 7 in a. An air cushion 10 is formed behind the Fresnel lens plastic film 4 and the damping plastic film 6, which is largely sealed off from the ambient air with the aid of end webs 11. Within the passive infrared motion detector 1, the infrared sensors 9 are arranged centrally on a circuit board 12, onto which the infrared rays S _II incident laterally from the second detection area II are focused via two deflection mirrors 8. Frontal incident from the first detection area I _I S infrared rays are bundled without intermediate reflection directly to the infrared sensors. 9

FIG. 3 shows a first detection characteristic 13, known in the prior art, of the passive infrared motion detector 1 without a damping plastic film 6. An indented constriction 15 for the laterally incident infrared rays can be clearly seen. This is essentially due to the fact that, due to the intermediate reflection at the deflecting mirrors 8, the infrared rays S _{II are} scattered on the material of the deflecting mirrors 8, which is associated with a loss of energy and thus an intensity. A second detection characteristic 14 arises if both those incident from the front from the first detection area I and those incident laterally from the second detection area II Infrared rays S _I , S _II can be additionally damped with the damping plastic film 6. It can be seen that the shape of the detection characteristics 13, 14 is essentially retained, whereas a detection range r which is variable over an azimuth angle φ is reduced in the detection characteristic 14 compared to the detection characteristic 13.

4 shows a third detection characteristic 16 which has been improved according to the invention. It is distinguished by the fact that the constricted constrictions 15 in the detection characteristics 13, 14 are not only compensated for, but even overcompensated. A fourth detection characteristic 17 results when the teaching according to the invention is used in combination with a Fresnel lens film 4 with laterally acentric Fresnel lens segments 18, which is described in more detail below. As a result, the azimuthal detection area can then be expanded by a third detection area III to a total of 220 ° (cf. also FIG. 2).

Fig. 5 shows a comparison of various laterally and frontally incident infrared useful signals at different noise levels and different arrangements of the damping foils 6. First of all, the conditions in a passive infrared motion detector 1 without any damping foil 6 are to be considered. A resulting noise level R₂ is relatively high. A useful signal emitted from the lateral direction with the intensity I₀ is received in accordance with signal a _II on the infrared sensor surface 9 only with the intensity reduced by ΔI _d (d = dissipation = scattering), whereas the incident from the front and represented by the signal a _I Infrared useful signal essentially reaches the infrared sensor surface undamped. The damping by the Fresnel lens plastic film is not considered in this qualitative discussion. A comparison of the curves a _{I, II shows} that the signal / noise ratio of the frontally incident useful signal a _{I is} greater than that of the laterally incident useful signal a _II . As is well known, an increased signal / noise ratio means higher sensitivity.

A passive infrared motion detector, in which both the frontal and the laterally incident infrared rays S _I , S _{II are covered} by a damping film 6, has a significantly reduced background noise R₀. In this case, laterally incident useful signals C _{II are} not only attenuated by the already mentioned proportion ΔI _d , which is due to the scattering at the deflection mirrors 8, but additionally attenuated by ΔI _a by the damping plastic film 6. The damping measure ΔI _a (a = attenuation) naturally depends on the transmitted infrared wavelength and the thickness and material of the damping plastic film 6 used.
In contrast, a frontally incident useful signal represented by curve C _I has an intensity which is only reduced by the damping measure ΔI _a . When comparing the curves C _I, C _II it can be seen that the frontally incident useful signal C _{I has} a more favorable signal / noise ratio than the laterally incident useful signal C _II .

. Es ist somit festzuhalten, daß aufgrund der zusätzlichen Dämpfung der frontal einfallenden Infrarotstrahlen S₁ die lateral einfallenden Infrarotstrahlen S_II gegenüber den frontal einfallenden Infrarotstrahlen S_I erstmals ein günstigeres Signal/Rausch-Verhältnis aufweisen. Hieraus resultiert die zuvor beschriebene verbesserte dritte Erfassungscharakteristik 16.As a result of the partial coverage of the infrared rays S _I incident frontally from the detection area _I , there is only a slight increase in that due to the heat background radiation always present noise level R₁ compared to the aforementioned noise level R₀. It can be seen that the laterally incident useful signal b _{II is} attenuated only by the amount ΔI _d , whereas the frontally incident useful signal b _I is attenuated by the attenuation amount ΔI _a . The following applies ${\text{ΔI}}_{\text{d}}{\text{<ΔI}}_{\text{a}}\text{= I₀-I₂ = R₂-R₀}$

. It should therefore be noted that due to the additional attenuation of the frontally incident infrared rays S _1, the laterally incident infrared rays S _{II have} a more favorable signal-to-noise ratio than the frontally incident infrared rays S _I. This results in the improved third detection characteristic 16 described above.

6 shows a Fresnel lens plastic film 4, which is divided into individual strip-shaped Fresnel lens segments 18. Each segment 18 represents a Fresnel lens, which bundles infrared rays S _I , S _II , S _III incident from different spatial sectors. In the present case, the frontally incident infrared rays S _{I are assigned} two zones of Fresnel lens segments 18 arranged one above the other. This has the advantage that it is no longer possible to undermine the frontal detection area I. What is decisive in the present Fresnel lens plastic film 4 is that the, preferably four, laterally arranged Fresnel lens segments 18 are designed as eccentric Fresnel lenses, the respective lens centers 19 of which lie outside the Fresnel lens segment 18. When using such a known Fresnel lens plastic film 4 in combination with the teaching according to the invention, the azimuthal detection area can be expanded to a total of 220 °, since incident areas from a third detection area III (FIG. 2) third infrared rays S _III can still be detected because of the improved detection sensitivity according to the invention.

Claims (5)

1. Passive infrared movement detector with at least one infrared sensor arranged inside a housing and also with at least one housing opening which is provided in the housing and has a Fresnel lens system, first infrared rays, frontally incident from a first sensing area and concentrated in assigned Fresnel lenses of the Fresnel lens system, being focused onto the infrared sensor, and second infrared rays, incident laterally from a second sensing area and concentrated in assigned Fresnel lenses of the Fresnel lens system, being cast onto the infrared sensor after interreflection at a system of deflecting mirrors, characterized in that damping means (6) are provided, on account of which the transmittance (T_I) for the first infrared rays (S_I), incident on the infrared sensor (9) from the first sensing area (I), is lower than the transmittance (T_II) of the second infrared rays (S_II), incident on the infrared sensor (9) from the second sensing area (II).
2. Passive infrared movement detector according to Claim 1, characterized in that an infrared-transmissive damping plastic film is provided as the damping means (6).
3. Passive infrared movement detector according to Claim 1 or 2, characterized in that there is provided as the Fresnel lens system a plastic Fresnel lens film (4), designed as a plastic Fresnel lens, the strip-shaped Fresnel lens segments (18) of which, which cover the azimuthal sensing area (I, II) and have impressed Fresnel lenses, being arranged semicircularly next to one another.
4. Passive infrared movement detector according to Claim 3, characterized in that for sensing the first infrared rays (S_I) incident from the first sensing area (I) there are provided at least two zones, arranged one above the other, of segmented central Fresnel lenses, preferably 8 in each case.
5. Passive infrared movement detector according to one of the preceding claims, characterized in that for sensing the infrared rays (S_II, S_III) incident from the second and third sensing area (II, III) there are provided Fresnel lens segments (18) which are in each case arranged laterally and are designed as acentric Fresnel lenses with Fresnel lens centres (19) lying outside the Fresnel lens segments (18).

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