Rain sensor

A rain sensor is a technological device that can determine if and how much it rains in order to initiate appropriate action.

For this, the wetting of a surface is detected, and there is a switch signal or a quantitative signal is generated.

The difference from the rain gauge is that not the exact rainfall is measured, but only secondary effects such as wetting or visibility obstruction of an irrigated disc can be registered.

Areas of application

Applications of rain sensors are the automatic control of various processes, including the control of irrigation systems.

Control of windshield wipers on vehicles or ships: the car manufacturer Peugeot offered in 1994 in one of his models for the first time equipped with a standard rain sensor on. Since this development is built from several car manufacturers in vehicles. The sensor is mounted behind the windscreen, usually integrated in the foot of the rearview mirror in the upper region of the windscreen, at least at the outside edge of the field of view. The rain sensor measures the wetting of a small measurement field and enables the control electronics to assess and to activate the wiper when a corresponding wetting is achieved, the wetting of the window. The measuring field usually has a size of 2 cm ( size of a 1-cent piece ).

Other applications include skylights and awnings on houses that are automatically closed or retracted in the rain.

Design and operation

Work rain sensors

• Optoelectronically: Reflection measurement behind a pane of glass
• Electrical: Change in conductivity between open electrodes due to the conductivity of the wetting water
• Change of the capacitance between electrodes insulating protected

Optoelectronic method

Illuminating a glass plate by means of a light emitting diode ( LED), and registering the reflection by a light receiver ( phototransistor ). The physical law of reflection at the boundary is used by the optically thick to the optically thinner material ( refractive index). The angle of the sloping surface of the glass pane the light beam is chosen such that under dry outer disc surface reflects all of the light and the photo receiver runs ( total reflection, the light exits the optically denser medium "glass" is not ). Water drops - for example from rain - on the outer surface of the disk to change the reflection characteristics: it is less reflected part is scattered by the rough surface of the water and escape.

This light at the boundary of glass ( refractive index as n = 1.5) to air ( almost n = 1) is totally reflected, the angle of incidence must be greater than 41 ° ( critical angle for total reflection, measured from the vertical away, so the bigger the flat to the interface running ) can be selected. One possibility would be about 45 °. Located on the glass surface now water ( n = 1.33 ) at which occupies an intermediate position in its optical density, the relevant difference in refractive index is (glass to water) much lower and there is only partial reflection on, so it is less light to reach the detector. Total reflection would occur ( from glass to water) until a higher critical angle of 61 °.

The critical angle for total reflection ( close too thin ) is calculated exactly from the refractive index ratio ( n1/n2 ).

The same also applies to newly joined interface between water and air quite externally: smaller refractive index difference - greater critical angle. The light beam is refracted at a greater angle back and could now be reflected by the water boundary layer to the air. However, the water surface is not flat, which is why a large part of the light coming out: the water is more or less -spread, vaulted drops or by wind and water flow uneven layer on glass. The more glass surface is uneven wetted by water, the more disturbed the total reflection, which in turn is registered by the sensor.

The measured quantity is decreasing in wetting light intensity at the photodetector.

A single small drop in the measuring field should not trigger the wiper, a larger or more but already. The sensor will react to the onset of visual impairment by water drops on the glass.

The proportion of the amount of reflected light can also be used as a control variable for the interval of a wiper, so that the rain sensor can control the speed of the windshield wiper as a function of the amount of rain.

Usually infrared is used. The IR beam is expanded by means of a dispersing lens and guided bonded optical prism at the required angle with the windshield. The prism is necessary to be able to couple the radiation into the pane at all the required angle - without a prism could be due to refraction in the plane-parallel disk access, which is then totally reflected beam no. The light beam is then reflected in the windshield, and is applied to another point on a coupling-out prism prevents the total reflection on the inner surface there leaves the windshield and is focused by condenser lenses on the photoreceptor.

In heavy rain, a sensor system can automatically switch from interval to continuous wiping. During a sudden onset of downpour or in the Gischtfahne a truck, the system immediately switches to the highest speed level. Traps only sporadically raindrops, the electronics control the wiper speed so that the driver always has good insight. Dry rubbing of the wiper blades on the windshield, and therefore the excessive wear of the wiper blade can be avoided. In order to avoid misinterpretation by condensation moisture, an integrated heater can keep the measuring range from dry inside.

Compared with the infrared radiation power of the sun or of tunnel lighting the intensity of the IR diode is extremely low, the quantity to be measured ( be sprinkled light) even vanishingly small. In order to obtain still accurate reading, the signal is operated in pulsed fashion, and evaluated with the aid of a digital lock-in amplifier.

Electrical methods

Electric rain sensors as they are used as a module in building technology or horticulture, having an electrode array. Use the change in the capacitance of the irrigated arrangement or they register the changed electrical resistance due to the conductivity of the rainwater.

Depending on the process the electrodes are either covered with an insulating layer / metallic - protected or open. Open arrays must have corrosion-resistant metal surfaces; selected are often produced as PCB gold plated circuit lines.

Other functions in vehicles

Today (2012 ) connected to the vehicle network (eg CAN or LIN bus) rain sensors often provide even more control options: The signals are used, eg, for the automatic closing of windows and sunroof. Integrated ambient light sensors to control the driving lights to automatically turn on when it is dark or you enter a tunnel. The automatic headlamp cleaning system works on the principle of the optoelectronic measurement used in the rain sensor.

Furthermore, the exposed location is on the windshield to integrate fogging and sun sensors.