Optoelectronic sensor for detecting moisture on a windshield with means to compensate for a metallic layer in the windshield

A sensor device for detecting the degree of wetting of a multi-layer transparent pane, made in particular of glass, by an, in particular, drop-shaped precipitation. The sensor device is provided with a beam guide element which comprises two beam windows and cooperates with a beam transmitter and a beam receiver. In order to prevent spurious beams caused by an extremely thin intermediate layer located in the pane from having a noticeable influence on the signal used to control the windscreen wiper system, the beam guide element comprises a further beam exit window in the central region of its rear surface facing away from the pane. The region is located between the two beam windows. The further beam exit window is allocated to the measuring path and cooperates with an additional beam detector. Only those irrelevant beams reflected at the extremely thin metallic intermediate layer located in the pane exit therefrom. The signal resulting from the existing irrelevant beams which is detected by the additional beam detector is used to correct the signal supplied by the actual beam detector. Alternatively; signal correction can be achieved by an electrical circuit.

TECHNICAL FIELD 
This invention relates to a sensor device designed for detecting the degree 
of wetting of a transparent windscreen by a mainly drop-shaped 
precipitation. 
BACKGROUND OF THE INVENTION 
The particular purpose of optoelectronic sensor devices is to detect the 
quantity and/or quality in a representative form of the moisture 
precipitating per unit of time on the front or rear windscreen of a motor 
vehicle and in dependence thereupon, to influence automatically a 
windscreen wiping system allocated to the screen. 
Devices for controlling a motor-driven windscreen wiping device have become 
known from DE 32 43 373 A1, DE 33 14 770 C2, DE 40 06 174 C1, DE 40 06 420 
A1, DE 40 17 063 A1 and DE 40 19 066 A1, wherein by way of a beam guide 
element attached to the inner surface of a transparent windscreen, beams 
emitted from an associated beam transmitter are coupled into the 
transparent windscreen and after at least one reflection at the outer 
surface of the windscreen are decoupled again by way of the beam guide 
element and are directed to an associated beam receiver. 
Some multi-layer windscreens installed nowadays in motor vehicles have an 
integrated intermediate metallic layer which is extremely thin and thus 
hardly impairs the transparency of the windscreen. This said intermediate 
layer is provided on the one hand to provide possible electrical heating 
of the windscreen and on the other hand to provide thermal insulation 
against infra-red rays which effect the motor vehicle from the outer side. 
This type of windscreen leads to the problem that the beams emitted by the 
beam transmitter are likewise reflected to a considerable extent at this 
intermediate layer and they are thus not available for the measuring 
process. Moreover, these beams stray as spurious beams in the beam guide 
element and they can thus have a negative influence on the signal supplied 
by the beam receiver. It has also been established that the spurious beams 
change in dependence upon the wavelengths of the beams emitted by the beam 
transmitter, these wave-lengths varying with the different temperatures. 
SUMMARY OF THE INVENTION 
In order to increase the size of the measuring area to be scanned with 
respect to achieving a representative result, it is possible for several 
measuring paths to be achieved in adjacent portions of the beam guide 
element which are preferably combined to form one block unit. This is 
attached in various ways for example by means of adhesive to the 
transparent windscreen. 
The aim of the present invention is to develop further a sensor device in 
such a way that the spurious beams caused by an extremely thin 
intermediate layer located in the windscreen can have at least no 
noticeable influence on the signal used to control the windscreen wiper 
system. 
This aim is achieved according to the invention by providing an 
optoelectronic sensor device wherein a front surface on a beam guide 
element is connected to the inner surface of a windshield beneath the 
wiping area covered by a motor-driven wiper device. The beam guide element 
has at least one measuring distance with a beam entry window and a beam 
exit window which is spatially separate-therefrom and is aligned at an 
angle of approximately 90.degree. thereto. A beam transmitter is allocated 
to the beam entry window. A beam receiver is allocated to the beam exit 
window. Beams emitted by the beam transmitter are reflected in dependence 
upon the precipitation present on the windshield and directed to the beam 
receiver which in each case provides a signal dependent upon the 
associated quantity of precipitation. 
Compensating means are provided in order to correct the signal supplied by 
the beam detector and resulting from the quantity of mixed beams. Such 
correction is dependent upon the quantity of spurious beams reflected at 
an extremely thin and thus partially transparent metallic intermediate 
layer located within the windshield. 
The advantage with such a development is that by measuring the pure portion 
of spurious or irrelevant beams and by measuring the portion of 
spurious/useful beams it is possible to produce a relationship between 
these variables. This relationship, if necessary, can be used to balance 
the variations occurring during the manufacture of the windscreen, such as 
of the thickness of the windscreen or the characteristics of the layer. 
Further particularly advantageous developments of the device according to 
the invention are indicated in dependent claims and are described in 
greater detail with reference to two embodiments, which are illustrated in 
the drawings. 
Also disclosed is a method for improving the sensitivity of a rain-sensing 
system based on infra-red light in connection with a heated windscreen, 
windshield, or electriclear glass.

Corresponding components in the figures have been provided with the same 
designations. 
BEST MODE FOR CARRYING OUT THE INVENTION 
As the drawings show, a sensor device for detecting the degree of wetting 
of a preferably glass pane 1 by, in particular, drop-shaped precipitation 
comprises two beam transmitters 3', 3" (one only indicated for the sake of 
simplicity) and two beam receivers 4', 4" (likewise one only indicated for 
the sake of simplicity), a beam guide element 2 formed from two parallel 
lying portions 2a, 2b, which is attached by means of optical adhesive to 
the inner surface 1' of the pane 1 not exposed to the precipitation. 
The pane 1 is in particular the windscreen of a motor vehicle on which the 
sensor device, disposed in a housing (also not shown for the sake of 
simplicity), is provided at an exposed point, i.e. a point of the inner 
surface 1' which does not impair vision but is predetermined for detecting 
the precipitation. The pane 1 is, as is usual nowadays for safety reasons, 
a composite pane comprising two layers 1a and 1b as well as a film 1c 
located between the two. In order with a pane of this type to provide 
insulation against infra-red rays which strike the outside of the pane and 
to provide heating for the pane, an extremely thin metallic intermediate 
layer 1d is installed in the pane 1 in the region of the film 1c. 
The beam guide element 2 consisting of the portions 2a, 2b comprises two 
support parts 2a', 2a"which are mechanically interconnected and optically 
separated from each other. In each case a beam lens 2b*, 2b** and 2c*, 
2c** is disposed with its planar base surface at the support parts 2a', 
2a" lying opposite in each case the beam windows 2b', 2b" and 2c', 2c". 
The identically sized beam windows 2b', 2b" and 2c', 2c" are disposed at 
the support parts 2a', 2a" in such a way that the center lines of the beam 
lenses 2b*, 2b** and 2c*, 2c** are offset with respect to each other by an 
angle of approximately 90.degree.. The beam lenses may be either fixed by 
their planar bases on the beam windows for example with the aid of a 
respective centering dowel and optical adhesive or may be integrally 
formed directly on the support parts. 
An additional beam exit window 2d', 2d" is provided in the respective 
central region of the two support parts 2a', 2a", i.e. between the actual 
beam windows 2b' and 2c' and 2b" and 2c" and each beam exit window is 
allocated an additional beam detector 5' (5"), 6. The beam exit windows 
2d', 2d" are disposed parallel to and directly adjacent to the actual beam 
exit windows 2c', 2c". These windows are, however, owing to spatial 
restrictions located in different planes. 
As shown in FIGS. 1 and 2, the two additional beam exit windows 2d', 2d" 
correspond in width to the width extension of the beams in the respective 
measuring paths. The height of the said beam exit windows is dependent 
upon the spacing of the metallic intermediate layer 1d with respect to the 
surface 1' of the pane 1. The beam exit windows are provided in each case 
with an additional beam lens 2d*, 2d** formed as a correspondingly 
designed portion of a non-spherical lens configuration. An additional beam 
detector 5', 5" is then allocated in each case to the additional beam exit 
windows 2d', 2d" and the additional beam lenses 2d*, 2d**. As is evident 
from the schematic graph of the beam paths in FIG. 2, only those spurious 
beams reflected at the metallic intermediate layer 1d are received by the 
two additional beam detectors 5', 5", whereas the two actual beam 
receivers 4', 4" are influenced by a mixture of beams which comprise 
useful beams reflected by the outer surface 1" of the pane 1 and spurious 
beams reflected at the metallic intermediate layer 1d. 
The arrangement of the additional beam windows 2d', 2d", the additional 
beam detectors 5', 5", and the allocation with respect to the actual beam 
exit windows 2c', 2c" and the actual beam receivers 4', 4" produces a 
quasi clear geometrical relationship between the amount of spurious beams 
emerging at the additional beam exit windows 2d', 2d" respectively. 
Consequently, it is possible with the aid of an analogue or digital 
compensating stage (not illustrated for the sake of simplicity) downstream 
of the actual beam receivers 4', 4" and the additional beam detectors 5', 
5" to correct correspondingly the signal which is emitted by the actual 
beam receivers 4', 4" respectively, which signal is dependent upon the 
transmission characteristics and varies in wave length with the 
temperature. 
In accordance with the embodiment shown in FIGS. 3 and 4, a simplified 
construction is facilitated by the use of a sensor device comprising two 
measuring paths are produced by a common fresnel-lens-type beam lens 2e" 
which is allocated to the two additional beam windows 2d', 2d". The beam 
lens cooperates with an additional beam detector 6. In this type of 
configuration, the two measuring distances are interrogated successively 
and the output signals of the two actual beam receivers 4', 4" are 
influenced in dependence thereupon. 
The present invention also discloses a method for improving the sensitivity 
of a rain-sensing system based on infra-red light in connection with a 
heated windscreen, windshield, or electriclear glass. 
1. Description Of The Measuring Technique 
The disclosed rain-sensing system functions as follows. The rain sensor is 
attached at the windshield inside the motor vehicle in the wiping region 
of the windscreen wiper. An infra-red ray is transmitted by an LED and 
directed through the windscreen. The light beam is totally reflected at 
the boundary layer windscreen-air owing to the acute angle. The light so 
reflected is then received again by a photodiode. Should a wetting of the 
screen occur, then a portion of the light beam is deflected to the outside 
and no longer reflected at the boundary layer. Consequently, less light 
arrives at the photodiode and a wiping cycle is eventually triggered. The 
transmitting diodes work with a predetermined frequency which is filtered 
out again at the receiving side with a phase-selective rectifier. This 
feature renders possible a rain-sensing system of this type which to a 
large extent is independent of any influences from the surrounding light. 
2. Problems With A Heated Windscreen 
It has been established when installing this type of rain-sensing system on 
a heated windscreen that the sensitivity severely reduces in comparison to 
a normal windscreen. 
The cause of this effect is the metallic layer in the center of the 
composite glass (see FIG. 5). The boundary layer glass-air is not the 
first place where the light beam of the rain sensor is reflected, as the 
light beam has already been reflected at the metallic inner layer in the 
composite glass. Consequently, a relatively high constant portion of noise 
signal which does not change when wetting occurs, is measured in the 
output signal of the rain sensor. 
Accurate measurements have shown that approximately 70% of the infra-red 
light is already reflected inside the screen. 
3. Description Of The Active Compensating Circuitry 
Turning now to FIGS. 6-8, FIG. 6 illustrates the block diagram of a 
rain-sensing system with active compensation. In that figure, there is 
depicted a current/voltage converter 3 for the purpose of phase-selective 
rectification. The time graph of the currents, such as occur when using an 
active compensation, is illustrated in FIG. 7. 
Item 1 (FIG. 7) 
This illustrates the receiving current. It consists of a constant noise 
portion of approximately 70% which is caused by the direct reflection at 
the reflective layer internal to windscreen and a variable portion which 
can lie from 0% to 30%. Full modulation occurs in FIG. 5, whereas there is 
no modulation in Diagram 1a (FIG. 7). Using a windscreen without a 
reflective internal layer, where the signal is completely decoupled at the 
outside surface, it is not possible to measure any current at all. 
However, owing to the direct reflection at the internal reflective layer 
of the windscreen, the current does not fall below 70%. 
Item 2 (FIG. 7) 
This illustrates the compensating signal. This signal is generated which is 
phase-shifted by 180.degree. relative to the receiving currents and has an 
amplitude which corresponds exactly to the amplitude of the constant 
portion in the receiving current. 
In Item 1 (FIG. 7), the receiving current and the compensating current are 
superimposed at the current summation point S (shown in FIG. 8). This 
represents mathematically an addition, with the constant portion being 
compensated exactly in the receiving current. It is important to couple in 
a current actively (which is related to the transmitted current) at the 
current summation point, since in this way only the useful portion is 
maintained in the receiving current. Should the voltage be simply divided 
at this point, then the small useful voltage portion would also be 
subdivided. 
Item 3 (FIG. 7) 
This illustrates the summation of the receiving current and the 
compensating current. The amplitude of the signal amounts only to 
approximately 30% of the original receiving current. However, this current 
can vary from zero to full amplitude in dependence upon an external 
wetting. 
Item 4 (FIG. 7) 
The original signal amplitude is achieved again by increasing the 
amplifying factor of the current/voltage converter. It must, however, be 
fully understood that by virtue of this circuitry with only approximately 
30% of the available light, a signal amplitude of almost 100% is produced 
"artificially." The result of this feature is a deterioration in the 
signal-to-noise ratio. The rain-sensing system would usually provide a 
sufficient signal-to-noise ratio when using a compensating circuitry of 
this type. 
A method for improving the sensitivity of a rain-sensing system based on 
infra-red light in connection with a heated windscreen (heated windshield 
or electriclear glass) is depicted in FIG. 8, which illustrates 
representative circuitry suitable for implementing the present invention. 
FIG. 8 depicts two current components, I.sub.R and I.sub.C, which are 
summed and introduced at the input portion (point "S") of the current to 
voltage convertor circuit (indicated as I/U). I.sub.R represents an 
oscillating current which is a combination of useful and noise signals 
received by LED 1. I.sub.C represents an oscillating compensation current 
generated by circuitry which adjusts the current magnitude relative to the 
expected signal as presented earlier. I.sub.R and I.sub.C are phase 
shifted 180.degree. relative to each other. U.sub.REF, voltage 
proportional to the source for the received signal current, I.sub.R, is 
introduced at the second input to the current to voltage convertor circuit 
I/U. 
The output from I/U, V.sub.S, is an oscillating voltage which represents a 
noise reduced/eliminated signal.