Automatic windshield wiper sensors

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When I was a kid of about 9 or 10 years old, my family was on one of it's bi-annual trips through Europe. Being a first-generation American with lots of family (and family friends) throughout Europe, we would go back every two years, flying into Luxembourg and driving through France, Germany, and Austria before making our way into Hungary. On this particular trip, we had a rental car called a Simca. It was a French car, if I remember well, and it had a feature I had never seen before on any of my family's American-made cars at the time: automatic windshield wipers.

As we were driving through France, there was a steady misty-rain (much like today's weather in Houston, TX), and my father was driving. I noticed him turn a lever on stalk extending from the steering column, and the windshield wipers turned on for a moment and then sat silently. I watched as rain accumulated on the windshield to a point where it was almost difficult to see, and then without any intervention on my father's part, the wipers came to life and wiped the water off. This process repeated itself for most of the day, and I put my mind to work trying to figure out what technology was in place to make it work.

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I thought about it and came to the conclusion that it had to be a water sensor that knew when enough water had accumulated on the windshield, and that's when it would wipe the water off. I told my father about how I thought it worked, and he smiled at me and said, "Yep, you figured it out all right." I was proud of myself, since it was quite a complex system. I marveled at the Simca and how space-age it was compared to our American Oldsmobile and Ford Pinto wagon. I couldn't wait to tell my friends about this amazing piece of French technology when I got back to the US.

When we got to Austria, we visited with my father's surrogate parents; a kindly couple who sort of adopted my dad and his friends when they were refugees living in Vienna in the mid-50's. We would visit them every time we went to Europe, and they were like another set of grandparents to me. Azsi Bacsi, as he was known to me, was a former ship captain on the Danube before he defected with his family (who were in a cargo crate on his ship) in Austria after the Soviet invasion of Hungary. He spoke beautiful English aside from his native Hungarian and German. He would always take great interest in conversations with me, and I began to explain to him about the technological marvel I had only recently been made aware of: the automatic windshield wiper that used sensor technology to measure the amount of water on the windshield. He seemed quite impressed and amazed as well until my father started laughing and explained that there was no sensor; it was a delay timer.

I was crushed. I was angry. I was hurt.

Not only did he make me look silly in front of Azsi Bacsi, but I over-thought the problem, coming up with far too complex a solution. I forgot to consider the simplest implementation of a windshield wiper that didn't wipe steadily: a timer. I was also mad at my dad for letting me believe that the car had sensors as part of its windshield wiper system. That put my belief in the whole concept of "cruise control" into a tailspin of doubt. If there were no windshield wiper sensors, how could I believe there was a "cruise control?"

Fast-forward to 2008. I am reading the manual for my 2008 VW Passat when I come across the section for the windshield wipers. I read it, since I have always had an interest in windshield wiper technology ever since that fateful trip to Europe when I read to my amazement that my car's windshield wiper system had a rain sensor! I thought to myself, "Self, we are now driving the car of the future. It's finally here."

I did some searching to find out about the technology behind the sensors, and found this bit on Wikipedia:

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Total internal reflection is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than the critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary no light can pass through, so effectively all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs.

When light crosses a boundary between materials with different refractive indices, the light beam will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e. the ray is closer to being parallel to the boundary) than the critical angle — the angle of incidence at which light is refracted such that it travels along the boundary — then the light will stop crossing the boundary altogether and instead be totally reflected back internally. This can only occur where light travels from a medium with a higher refractive index to one with a lower refractive index. For example, it will occur when passing from glass to air, but not when passing from air to glass.

So, someone finally did it; they figured out how to make the windshield wiper sensor work! This was something I wanted to invent myself, and I would often think about how such a system would work. Of course, the solution now used is far more elegant than those I came up with, so kudos to the engineer(s) who came up with total internal reflection.

Now, I enjoy driving in the rain. Knowing that there's all this cool technology going on to make the windshield wipers wipe only when they need to makes me smile.

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