Rain, sun or something else perhaps?

It was a French car manufacturer that first used a rain sensor in a series production vehicle back in 1994. That was when the triumphant advance of the component began, and today the rain sensor is a standard feature. It registers the wetting of the windshield with water and transmits an appropriate signal to a control unit programmed to activate the windscreen wipers. The speed of the wipers varies depending on the intensity of the rain. An exact measurement of the amount of rain is not possible, but also not necessary. HELLA was a pioneer in the development of rain sensors and is now one of the market leaders both in original equipment and the aftermarket.

A rain sensor including its additional functions is an optoelectronic component. It sits discreetly hidden behind the blackened area of the windshield, at the level of the rear-view mirror. If the windshield has to be replaced because of damage by a stone or gravel throw, the old sensor can be glued or clipped into the base of the new windshield.

Today, a combination of rain and light sensor is usually used. Depending on the vehicle model, a distance and environment detection sensor (radar/lidar) and also a camera, for example for recognising traffic signs or for implementing lane-keeping systems, are placed in the base behind the windshield.

The rain sensor

The rain sensor registers water drops on the windshield by means of an optoelectronic process. Strictly speaking, it detects the refraction of light caused by the water drops. To that end, light-emitting diodes in the optical unit send infrared light to the glass surface. If the pane is dry, it reflects the infrared light almost completely and a photodiode, also housed in the sensor system, measures the reflected light intensity. If the pane is wet with moisture (water drops), the infrared light escapes from the front pane as a result of the refraction caused. Less light is then reflected to the photodiode – this is called the scattered light principle. The sensor electronics can evaluate the results and send the appropriate signal to the windscreen wiper control unit. In this way various wiping frequencies up to continuous wiping in heavy rain can be put into operation at varying speeds. And there is yet another advantage: the wiper switches off automatically when there are no water drops on the windshield. This avoids "dry wiping", which is harmful to the wiper blades. Even in winter, modern rain sensors work. However, if a layer of ice has formed on the windshield, the electronics will stop the damaging "scrubbing” of the ice. In order to avoid condensation in the optics of the sensor, the interior can also be heated.

The light sensor

The light sensor works with two independent sensors: one has a wide angle of detection so that it captures all ambient light regardless of the angle of incidence. The second sensor measures the road surface light. It is aligned in such a way that it only detects the light intensity directly in front of the vehicle. The automatic brightness adjustment of a head-up display, for example, is based on this measurement method. The automatic activation of the low beam, on the other hand, is based on data from both sensors.

Light sensors form the basis for other interactive measures as regards the regulation of vehicle lighting. Here oncoming vehicles are just as relevant as those driving up ahead. The day/night mode also plays a part in the vehicle lighting as does the recognition of street lighting including the reflecting of traffic signs, for example, when the vehicle is driven into built-up areas. Intelligent assistance systems such as adaptive dynamic cornering light, adaptive light distribution (selective illumination of danger spots) or glare-free high beam (adaptive cut-off line) are also included depending on the headlamp technology involved. The HELLA Group, for example, supplies the relevant systems such as the Advanced Frontlighting System (AFS). In the case of LED headlamps from HELLA, individual LED modules can be switched on and off based on the information from the light sensor (and other ambient parameters), depending on the ambient conditions and the traffic situation.