Like how does the car know how heavy is the current rainflow? What is the feedback loop involved?
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There are several systems that are commonly used.
1. Variable speed, continuous action, manual control. You wiper controls have several speed settings. You select the speed manually. The wipers oscillate continuously at the speed you selected. The speed of the wiper blade itself is determined by the speed you select.
2. Full speed, intermittent. Every swipe of the blade is done at full speed. Between swipes, the system pauses for some interval. This ensures that the wipers always move quickly but avoids the scenario where they are wiping a dry windshield.
3. Rain sensing: a sensor on the windshield shines light from the backside of the glass and determines whether there is rain, based on reflection and refraction. This system adjusts wiping speed and frequency based on the presence of water. It’s generally only found on premium cars.
I’m not sure exactly how it works, but there is a sensor in my windshield and I believe it consists of a camera that registers drops on the glass, so the more drops, the fastest setting.
It’s a bit clumsy at times, btw, sometimes it will set itself to the fastest speed for some seconds with a little rain, and then slow down to one wipe every 3 or 4 seconds.
[Here is a nice explanation from the Volkswagen website](https://www.volkswagen.co.uk/en/technology/comfort/auto-rain-sensors.html)
In the windshield there is a camera that sees only in infra-red, and really only a few hundred pixels of it.
When water hits the windshield, some of it will be over the camera’s field of view and it will change the amount of infra-red light that gets through the windshield (and is seen by the camera). The more rain there is, the less IR there will be. The camera senses this change in IR and alters the wipers accordingly.
It also will measure the rate of change of the IR – if it is decreasing very quickly after each wipe, that means it’s raining quite hard and it’ll up the wiper speed. If the amount isn’t getting low enough before the next wipe, it’ll assume it’s wiping too hard and slow it down.
It is a fairly dumb system, all told, and fooled by a number of things, but it works well enough in most cars. It’s also why the camera is almost always where the rear-view mirror is – it’s the least obtrusive place that is swept by the wipers.
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