GB2394282A

GB2394282A - Rain sensing apparatus

Info

Publication number: GB2394282A
Application number: GB0223927A
Authority: GB
Inventors: Gavin Robert Geoffrey Erry
Original assignee: Qinetiq Ltd
Current assignee: Qinetiq Ltd
Priority date: 2002-10-15
Filing date: 2002-10-15
Publication date: 2004-04-21
Also published as: GB0223927D0

Abstract

A rain sensor is described which emits light from a source (14) into a target volume under investigation. A detector means (30, 32, 34) is arranged to detect any light back-scattered from at least two locations within the target volume, preferably different planes (20, 22, 24) along the same axis. Detection of back-scattered light can be used as an indication of water droplets in the target volume. The frequency of flashes of back-scattered light and intensity thereof may be determined and processed to distinguish between different conditions.

Description

1 2394282

Rain Sensing Apparatus This invention relates to a rain sensing apparatus for detecting rain droplets, especially through a transmissive substance.

s Some vehicles, for example cars, are now being provided with automatic rain sensors to determine when there is rain falling onto the windscreen. This can be used to automatically operate the windscreen wipers, lights etc. as appropriate. Ideally the rain sensor will also detect how hard it may be raining and adjust the windscreen 0 wipers as appropriate.

One known form of rain sensor for cars uses total internal reflection of light within the windscreen. For instance, as described in US patent 4, 620, 141 an optical source is coupled to the glass windscreen and directs light into the windscreen at an angle of 5 incidence that means the light will undergo total internal reflection at the outer glass-

air interface. A detector is also coupled to the windscreen to detect the reflected light.

In dry conditions the emitted light will be reflected internally and be detected by the detector. The intensity of the detected light will therefore be that of the emitted light less standard losses.

In wet conditions however, droplets of rain will form on the windscreen. If a droplet is located over the windscreen where the rain sensor is located the emitted light may not experience a glass-air interface but a glass-water interface. As the refractive index of water is higher than that of air the emitted light in this case may not undergo total 25 internal reflection. Therefore there will be a noticeable drop in intensity of the transmitted signal indicating that water is forming on the windscreen. This can be used to operate the windscreen wipers and lights of the car automatically.

One problem with this type of rain sensor however is that the sensor needs to be 30 coupled to the windscreen and needs to have accurate alignment to ensure total internal reflection which adds to the manufacturing and assembly cost and complexity. Also it only works for water droplets formed at the point of reflection.

An extension of this system is to locate the detector such that the detected light has undergone multiple reflections, both from the glassoutside air interface and the glass-

inside air interface. This increases the probability of the emitted light reflecting at a point where there is a water droplet in wet conditions. However the reflection from 5 the inner glass-air interface will be affected by condensation forming on the inside of the windscreen, i.e. misting.

It is therefore an object of the present invention to provide a rain sensing apparatus which mitigates at least some of the above mentioned disadvantages.

Thus according to the present invention there is provided a rain sensor comprising a light source for emitting light into a target volume and a detector means arranged to detect light scattered from water droplets from at least two different locations in the target volume.

The present invention therefore projects light into an area outside of the vehicle and as such does not need to be mounted on the windscreen. In dry conditions the light is simply emitted and no signal is detected at the detector means. In wet conditions however there will be water droplets within the target volume. These rain droplets 20 will back scatter some of the emitted light which can be detected by the detector means. As the detector means looks at more than one location in the target volume a single event causing back-scattering, such a scattering from an insect or aerosol, can be discriminated from rainfall where there will be rain droplets throughout the target volume. 2s It should be noted that as used herein the terms droplet and rain droplet are taken to mean a variety of forms of water/ moisture and includes all forms of precipitation such as rain, snow and hail as well as fog and mist or water droplets present through other means such as road spray etc. The present invention then does not necessarily interrogate the surface of the windscreen to determine its condition but instead interrogates a target volume outside of the vehicle and detects water droplets therein. False positives due to internal misting are therefore avoided.

Conveniently the detector means is adapted to determine the intensity of the back-

scattered light and/or the frequency of incidences of detection of backscattered light.

In other words the frequency of scattering events may be determinedspatial s frequency or temporal frequency or both. This may be used to give an indication of the density and size of the water droplets in the target volume, which information could be used to adapt other car systems, such as the windscreen wipers, accordingly.

Each time emitted light interacts with a droplet some light will be backscattered.

This will give a 'flash' which can be detected. The frequency of flashes can be lo counted and will give an indication of the number of water droplets in the air. Also the amount of scattering will be linked to the droplet size and form - for instance large water droplets will have a different scattering profile to, say, snow flakes.

Processing of the detected signal could be used to give an indication of the type of moisture. The detector means preferably comprises a detector focussed on each location in the target volume. Each detector conveniently comprises a two dimensional array, such as a CCD array, such that scattering flashes due to water droplets can be detected.

20 The light source is preferably an infrared light source such as an infrared LED.

Infrared is used as it would not be visible to the driver although other wavelengths could be used. The scattering from water droplets will generally be by Mie scattering and as such is wavelength independent and therefore a range of wavelengths could be considered. As will be understood by a person skilled in the art, the term light is not 25 to be limited to radiation in the visible spectrum but should be read as including any suitable radiation which could be scattered by the water droplets expected.

Conveniently the different locations are located at different distances from the detector means on the same axis. Looking at different locations at different distances 30 from the detector means on the same axis allows a simple illumination system as only a beam of light need be projected. Imaging locations on different axes is entirely possible but would require either wide angle illumination, which can have an effect on illumination intensity, or multiple illumination sources. Further having one 'look

direction' means that the device can be oriented in a vehicle with the look direction in a convenient direction such as in the expected direction of vehicle movement.

A convenient way of imagine multiple distances along a single axis is to use a 5 diffraction grating in concert with a lens to provide spatially separate images with differing amounts of defocus. As explained in International patent application No. WO99/46768 the use of a suitable diffraction grating with a lens allows multiple images to be produced of different object planes. Pages 14 to 16 of WO99/46768, the contents of which are incorporated herein by reference thereto explains, with lo reference to figure 10 of that document, how multiple object planes can be imaged simultaneously to a single image plane using a quadratically distorted grating. If a detector were to be located in the each of the zero and +1 and -1 diffraction orders each would see an in focus image of a different object plane.

5 Significant rain would result in water droplets being present in each object plane and so each detector would detect back-scattered radiation from the particular object plane. The number of object planes, or different locations which are imaged may vary but 20 three can be used to give reasonable discrimination without the apparatus being unduly expensive or complex.

The rain sensor may conveniently comprise a processor responsive to the output of the detector means adapted to determine when it is raining. Conveniently the processor 25 utilises a neural network or genetic algorithm. The processor may also be adapted to send appropriate control signals to other systems.

In a general sense then the present invention allows the imagine of multiple planes to identify the number of scattering particles present. In another aspect of the invention 30 therefore there is a particle counting apparatus comprising a source of light for illuminating a target volume, a detector means for detecting back-scattering of light from any particles in the target volume from at least two locations and a processing means responsive to the detector means for determining the amount of particles in the field of view.

s The invention will now be described by way of example only with reference to the following drawings of which; Figure 1 shows a rain sensor according to the present invention mounted in a vehicle, s and Figure 2 shows a rain sensor of the present invention.

Referring to figure 1 part of a vehicle is shown. A rain sensor, generally indicated 2, lo is mounted on the vehicle, in this case on the rear view mirror support 4 behind the rear view mirror 6. Placing the rain sensor behind the rear view mirror 6 means that the rain sensor is not in the line of the driver's vision and so it causes no obstruction to vision. However the skilled person would appreciate that the sensor could be located in other positions, for instance on the dashboard or near a window corner.

5 Indeed the sensor need not be located in the driver's compartment at all and could be located in a headlight cluster or even externally mounted on the vehicle. However mounting the sensor such that it looks through the windscreen is preferred as this gives the best indication of what conditions are like in the line of the driver's vision.

For this reason mounting the sensor by a rear view mirror can be advantageous.

The sensor is arranged so as to illuminate the outside environment is a generally forward direction, along the general expected direction of travel, in this case generally along axis 8. Infrared light is used so as to prevent any distraction for the driver from any reflection from the windscreen 12 or scattering from any water droplets 10 l 2s outside. An infrared LED is used as the source with a wavelength of around 880nm although the skilled person would appreciate that other wavelengths could be used.

The sensor is mounted such that the emitted light passes through the windscreen 12.

Therefore the angle of the incidence of the light on the windscreen is arranged away 30 from grazing incidence. In passenger vehicles such as cars some windscreens may have infrared absorbing components either incorporated in the glass or coated thereon to reduce interior heating. In this case it may be necessary to ensure that during manufacture of the windscreen the infrared absorbing component is omitted in a small

window where the sensor is designed to operate. Alternatively the wavelength of operation could be selected to be one which is not absorbed by the coating.

In dry conditions the light is emitted and substantially no light is scattered back to the 5 rain sensor 2. However in wet conditions, such as when it is raining, the presence of water droplets 10 in the illumination beam will cause some of the emitted light to be scattered back towards the rain sensor 2. Detection of this scattered light can be used to indicate wet conditions. If the rain sensor is connected to a control unit (not shown) this could be used to automatically activate some of the vehicle systems such 10 as operation of the windscreen wipers, activation of the vehicle lights or closure of windows etc. Other objects in the line of sight may cause scattering however. For instance airborne particles of dirt may be of sufficient size to cause scattering. Grit etc. thrown up from 5 the road could also cause a scattering event and the presence of insects will give rise to false signals too. However all of these scattering events will be single instance scattering events, i.e. the scattering will occur at one location only. In wet conditions there will be water droplets throughout the environment. The rain sensor of the present invention therefore looks in more than one location and uses the presence of 20 scattering events in each location as being indicative of wet conditions.

In the embodiment shown in figure 1 the rain sensor looks at three distinct planes 20, 22, 24 within the illuminated area as will be described below. This has the advantage that a single low power LED can be used to illuminate the scene in a generally 25 forward direction and a simple detector arrangement can be used. However the skilled person would understand that other arrangements are possible. For instance separate detectors could be provided with separate collection optics focussed at different areas or in different directions, although the latter option might require more than one illumination source.

Figure 2 shows more detail of the preferred embodiment of the invention. The IR source, LED 14 is arranged to illuminate the target volume. A lens 16 is positioned so as to collect light back-scattered from the target volume and a distorted grating 18 is also provided. The distorted grating applies a differing amount of defocus to each

diffraction order. The principle of grating design is explained in WO99/46768 page 8 and specifically for defocus gratings on pages 9 - 12. The use of this arrangement for imaging multiple planes is described on pages 14 to 16 and the principles of grating location are described on pages 7 to 8. The contents of WO99/46768 are incorporated s herein by reference thereto.

Detectors 30, 32 and 34 are therefore positioned so as to receive light from the 0 and 1 and +1 diffraction orders, each of which has a different degree of defocus. In effect therefore each detector receives an in focus image from one of the three planes 20, 22, lo 24. The image received at each detector is therefore independent from the effect of any misting on the windscreen. However if desired the system could be arranged such that an additional detector does receive an image from a plane corresponding to the windscreen so that the condition of the windscreen can also be assessed.

is The rain sensor is typically arranged to image planes within about 200mm and is therefore conveniently located near to the windscreen but other arrangements are possible. Each image plane is typically somewhere between 20mm and 80mm apart.

Each detector 30, 32, 34 is a two dimensional CCD array and generates an image of 20 the respective plane 20, 22, 24. Of course the skilled person will appreciate that a single two dimensional CCD array could be used with parts of the array corresponding to detectors 30, 32, 34. As mentioned the effect of rain in the object plane will be to back-scatter some of the light emitted by LED 14. Therefore rain will result in the image at each detector comprising flashes or streaks corresponding to Is light scattered from the rain drops.

The detectors 30, 32, 34 are therefore connected to a control unit 36. This detects the presence of flashes in each of the images to give an indication that water droplets have been detected. The control unit may also be adapted to count the number of flashes to 30 give an indication of the number of water droplets in the scene. The size or intensity of the flash can also be used to determine the size or nature of the water droplets. All this information can be used to modify the vehicle systems accordingly. For instance based on the number and size of water droplets the speed of the windscreen wipers could be adjusted or the degree of illumination of the vehicle lights could be

controlled. The control unit could also activate closure mechanisms for windows, sunroofs or the like. The scattering characteristics of snow say will be different to rain, and that of rain different to a fine mist. The control unit could identify the moisture type and control vehicle systems accordingly.

The processing scheme used in the control unit to determine whether it is raining or not, and what the conditions are like, is based on a genetic algorithm or neural network. lo The rain sensor according to the present invention therefore provides a relatively simple and low cost rain sensor with a flexibility of use. The sensor can operate from a range of different locations making it utilizable in a range of vehicles and in a range of positions. It also can discriminate between different conditions allowing more versatility in control of other vehicle systems.

The invention has been described above in relation to one particular embodiment.

Other embodiments and arrangements of the invention would be apparent to the skilled person without departing from the spirit of the invention.

Claims

Claims

1. A rain sensor comprising a light source for emitting light into a target volume and a detector means arranged to detect light scattered from water droplets from at least two different locations in the target volume.

2. A rain sensor as claimed in claim 1 wherein the detector means is adapted to determine the frequency of instances of detection of backscattered light.

3. A rain sensor as claimed in claim 1 or claim 2 wherein the detector means is adapted to determine the intensity of the back scattered light.

4. A rain sensor as claimed in any preceding claim wherein the detector means comprises a detector focussed on each location in the target location.

5. A rain sensor as claimed in claim 4 wherein each detector comprises a CCD array. 6. A rain sensor as claimed in any preceding claim wherein the light source comprises a source emitting light in the infrared spectrum.

7. A rain sensor as claimed in any preceding claim wherein the light source comprises a light emitting diode.

8. A rain sensor as claimed in any preceding claim wherein the detector means is arranged to detect light scattered from different locations at different distances from the detector along the same axis.

9. A rain sensor as claimed in claim 8 wherein the detector means comprises a diffraction grating adapted to provide spatially separated images with differing amounts of defocus.

l 10. A rain sensor as claimed in any preceding further comprising a processor responsive to the output of the detector means adapted to determine when it is raining. A rain sensor as claimed in claim 10 wherein the processor utilises a neural network or genetic algorithm.

12. A rain sensor as claimed in claim 11 wherein the processor is adapted to send appropriate control signals to other systems.

13. A particle counting apparatus comprising a source of light for illuminating a target volume, a detector means for detecting backscattering of light from any particles in the target volume from at least two locations and a processing means responsive to the detector means for determining the amount of particles in the field of view.