Editorial Feature

Rain-Sensing Wipers


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A rain sensor is a switching device incorporated into four-wheel automobiles. There are two types of rain sensors: a water conservation sensor that supports an irrigation system – this works by closing the system during an excessive amount of rainfall; and a second type of device used to support the activation of windshield wipers during rainfall. The following article discusses the structural and functional principles to rain-sensing wipers and the principle of working to help protect the interior of a car.

The main functional purpose of a windshield wiper is to sweep off the rain and snow from a viewing safety glass. The windshield wipers rely on sensors in order to facilitate the protection of an automobile in extreme weather conditions, with the rain sensor being the major point of control in initiating the mechanisms involved in this system.

How Does a Rain Sensor Work?

A rain sensor is typically located at the top of a windshield screen and positioned in the center of this viewing quadrant behind the rearview mirror. The complete sensor unit is made up of LED, photo diode, light sensor, and an ambient light sensor. This light sensor system works to transmit infrared light onto the windshield at a 45° angle via an optical component. Typically, in dry conditions, the windshield reflects the infrared light in the direction of the sensor. However, when raindrops fall on the windshield, moisture content begins to accumulate. This process interferes with the amount of infrared light reflecting back to the rain sensor (i.e., the infrared light begins to travel through the viewing quadrant, which means that there is less light traveling back to the rain sensor).

Parameters such as high moisture content on the windshield and the degree of infrared light being reflected serve as main factors in indicating the level of rainfall onto the moving vehicle. By measuring the amount of light being reflected back to the rain sensor, the device can determine how frequently to sweep the wipers on the windshield. The following video demonstrates rain-sensing wipers on a Toyota vehicle. This video animates exactly how fluctuations in weather conditions can manipulate the rain-sensing system in a car.

Toyota Rain Sensing Wipers

Activating the Wiper System

The electronic system in cars switches on the windscreen wipers when the infrared light is reflected back onto the rain sensor is below a certain pre-set level.  A standard windscreen wiper is controlled by an electrical motor that turns a module near the steering wheel. The intermittent nature of the wiper system is controlled by a potentiometer. The potentiometer is a device with a three-terminal resistor element and a contact that can change the voltage divider and measure the electrical potential. It is commonly used as a position transducer. The use of a potentiometer can increase the torque power required to push the wipers across the windshield.
Rotation of the intermittent control to the module alters the resistance value.  Saturation of the capacitor inside the potentiometer activates the electronic switch that delivers an electrical current to the motor, controlling the movement of the wiper. There is also a park switch that opens and closes to control a start and stop weep by the wiper. As soon as the park switch opens, the wipers stop moving. If you have ever driven in rainfall that increases in intensity, you probably have noticed that the frequency of sweeps made by the wiper system also increases. The amount of time between each sweep is usually based on how long it takes for the capacitor in the potentiometer to saturate.
One problem with rain sensor control is the sensitivity in activating this system. A moving vehicle often has to pick up speed to activate this sensor, and often, a washer system is switched on to alert this rain sensor. The rain sensor system by TRW, Inc. uses optical sensors that are sensitive to the moisture content on the windshield of a vehicle. This automatic sensor has full control over the wiper system and can determine the contact force required for the rain to hit the windshield, and can then control the speed at which to move the motor to the wiper. During interruption to the infrared beam that projects through the windshield, an analog and digital signalling system measures the strength of rainfall on the screen and, through electrical signalling, controls the wiper motor automatically.

Sources and Further Reading

  • Erjavec, J. (2010). Automotive Technology: A Systems Approach. USA, New York: Delmar, Cengage
  • Streather, A. (2012). Porsche 996 The Essential Companion: Supreme Porsche. England, Dorset: Veloce Publishing Ltd.
  • Sobey, E. (2009). A Field Guide to Automotive Technology. USA, Chicago: Chicago Review Press, Incorporated.
  • TRW Automotive. Body Control Systems Europe and Emerging Markets.

This article was updated on 14th February, 2020.


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