How do Rain-Sensing Windshield Wipers Work?

[freshprince1]

So I was driving my trusty Mazda 3 the other day when it started to rain and my rain-sensing wipers came on. I got to thinking, how do they work?

Just did a quick search on Wiki and Answers and couldn't find anything...anyone know?

[Mibz]

http://www.jdpower.com/autos/articles/Rain-Sensing-Wipers/

tl;dr: A sensor bounces light off the windshield and measures how much comes back. Wet or dirty windshields reflect less light and so the wipers are triggered. More advanced sensors vary speed based on the amount of light that comes back.

[freshprince1]

Well, I kept looking and it seems I found some info:

"...a new type of wiper system is starting to appear on cars that actually does a good job of detecting the amount of water on the windshield and controlling the wipers. One such system is made by TRW Inc., here is a PDF describing their rain sensor system. TRW Inc. uses optical sensors to detect the moisture. The sensor is mounted in contact with the inside of the windshield, near the rearview mirror.

The sensor projects infrared light into the windshield at a 45-degree angle. If the glass is dry, most of this light is reflected back into the sensor by the front of the windshield. If water droplets are on the glass, they reflect the light in different directions -- the wetter the glass, the less light makes it back into the sensor.

The electronics and software in the sensor turn on the wipers when the amount of light reflected onto the sensor decreases to a preset level. The software sets the speed of the wipers based on how fast the moisture builds up between wipes. It can operate the wipers at any speed. The system adjusts the speed as often as necessary to match with the rate of moisture accumulation. "

http://auto.howstuffworks.com/wiper4.htm

Link to Manufacturer's PDF (http://static.howstuffworks.com/pdf/rain-sensor.pdf)

This is the system for GM and most Cadillac's. I can't recall seeing the sensor in my Mazda, but at least I've got an idea how it works now. Pretty neat stuff. I wouldn't have guessed how it worked.

[dr_jared88]

The sensor on the mazda I believe is in the back of the rear view mirror (the part stuck to the windshield).

[speedog]

Just remember to turn them off when you're driving on a country road during the summer and there's lots of grasshoppers flying around - just one splatting on the sensor area will end up in quite a bigger mess on your windshield.