Like how does the car know how heavy is the current rainflow? What is the feedback loop involved?
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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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