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Comment by yareal | Hacker News Reader

Autowiper sensors are some of the simplest, well understood technology around. You place an infrared led at a low angle to the windshield, you place an ir collector so the light from the LED hits it. (Remember that at shallow angles, light reflects off glass.)

When water is on the sensor, the angle of refraction and reflection change, and the ir collector suddenly sees much less light.

Two components, plus a microcontroller. Usually two such sensors are on the windshield. Absolutely dead simple and cheap.

Why the fuck would you replace that with a camera trying to parse video?! That's so much more complicated. That's like trying to determine vehicle speed from a camera rather than sensors on the tires or whatever.

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Because petulant manchild thinks he is best car engineer on the planet. Its why FSD in its current state is such trash, its why a lot of the car is leaps and bounds better than most other cars, and basics that should have been accepted as learned lessons have to be relearned at the expense of the consumer and ridicule by the rest.

Autowipers do not work. Cannot work, will never work. My tesla can't see a full downed tree in my lane, gets spooked by dips in the road and pops collision alerts, how the hell would it see rain.

There are plenty of tasks that become impossible when using the wrong tools. It's like trying to move a lot of sand with a fork.

> That's like trying to determine vehicle speed from a camera rather than sensors on the tires or whatever.

Don’t give him any ideas. Seems like he has the “not built here” complex

Actually I'm somewhat certain that the vehicle speed is determined by the motor RPM, the known reduction gear ratio, and a known tire size. That's why speed can be inaccurate if you switch to incorrectly sized tires.

No. The vehicle speed is determined by a single sensor, the Vehicle Speed Sensor. It measures the rpm of the driveshaft (or one of them), the shaft that is the output from the transmission. You don't need the gear ratio.

But yes, it does assume that the tires are a certain size, so if you put the wrong size tires on, it will be wrong. Also, it's not super-accurate: tires sizes differ slightly by brand/model, plus they change their outer diameter as they wear down. That error accumulates.

I wonder what you mean by "accumulates". If I change the tires, the slight difference due to the previous brand and the slight difference due to the new brand do not compound. I only get the last one. And if the older ones were worn, the new ones will have less error, as they are less worn.

If the errors really accumulated, once in a while we would need to recalibrate the speed sensor, else we would be doing 90 and showing 10. In fact the error is limited to some reasonable amount.

Accumulate in this context means that there are several small errors, and it's a function of summing or multiplying those accumulated errors. It does not mean that they accrue over time.

They are not this simple; if so, an RLS (Rain / Light Sensor) assembly would cost 50 cents. To do a proper job, you need an array of IR emitters, and an array of detectors; and they need to be properly integrated into your windshield glass.

The RLS is usually a part of the rearview mirror supporting structure.

It also has to also work for hail and snow/sleet. Again, it's not as simple as you suggest.

Anything water based on top of the light emitter will have the same effect. You just need to use a formula describing its refractive index with temperature and density inputs. The temperature is known by the car, the density can use a look up table for various states the water could be in.

Is that not what I described, more or less? None of the components are particularly complex or expensive except the part where they join with the glass in the structure. Trying to do it with video is a much, much harder problem.

there is a hugely important implied assumption here - that the size of the water droplets are large enough for refraction to be detected. Any place with frequent drizzles or mists, these rain detectors do not work well or reliably.

> That's like trying to determine vehicle speed from a camera rather than sensors on the tires or whatever.

Don’t give him any ideas

It reminds me of that old story where NASA spent millions to develop a pen that works in space, while the Soviets simply used a pencil.

https://www.snopes.com/fact-check/the-write-stuff/

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