[jimmuller]

Sensors for light, for rain, for mirror dimming? Inside the car or out?

And how does the car know when to turn on the windshield wipers? Us humanoids use Eyewitness Weather. That's like Eyewitness News except that it doesn't require a TV. We look outside and see if it's raining. What technology does the auto-wipers feature in a car use? Or to put it another way, how does the car know what it is raining?

 

[MOG_80TH]

Hi Jim,

It's a mix of all. The main sensors for light and rain sensing are just behind the rear view mirror and pressed up to the glass. Have a look from the outside and you will see what `I mean.

For rain, the sensor detects how much of it's vision is blocked and how quickly to determine wiper speed - which can be tuned of course by the sensitivity dial on the windshield stock. While an Audi vid, it applies to the 718.

 

[Westcoaster]

Sensors for light, for rain, for mirror dimming? Inside the car or out?

And how does the car know when to turn on the windshield wipers? Us humanoids use Eyewitness Weather. That's like Eyewitness News except that it doesn't require a TV. We look outside and see if it's raining. What technology does the auto-wipers feature in a car use? Or to put it another way, how does the car know what it is raining?

It is my understanding that the sensor in the windshield measures the change in relative reflectivity it see's looking out through the glass, as more water drops affect the reflected light the amount of rain is determined to have increased.

 

[MLJJ]

Interesting tidbit my SA offered was to avoid using the rain sensing function exclusively because it can confuse dust/bug splatter with rain and result in dry wipes which could scratch the glass.

 

[Westcoaster]

Interesting tidbit my SA offered was to avoid using the rain sensing function exclusively because it can confuse dust/bug splatter with rain and result in dry wipes which could scratch the glass.

True, or in my case the Clearplex!

 

[jimmuller]

Good answers. Thanks. I asked about sensor location because I have a Fast Lane transponder held to the window with suction cups and I didn't want it to block the sensor.

I asked about sensing rain because I'm curious. Water, clean at least, is transparent to visible light like glass. So sensing a wet windshield would seem to be non-trivial.

 

[ricw]

from Wiki:

The most common modern rain sensors are based on the principle of total internal reflection. At all times, an infrared light is beamed at a 45-degree angle into the windshield from the interior. If the glass is dry, the critical angle for total internal refraction is around 42°. This value is obtained with the total internal refraction formula sin(Oc)=n1/n2 where n1=1 is the approximate value on air's refraction index for infrared [2] and n2=1.5 is the approximate value of the glass refraction index, also for infrared [3]. In that case, since the incident angle of light is 45°, all the light is reflected and the detector receives maximum intensity.
If the glass is wet, the critical angle changes to around 60° because the refraction index of water is higher than air (n1=1.3)[4]. In that case, because the incident angle is 45°, total internal reflection is not obtained. Part of the light beam is transmitted through the glass and the intensity measured for reflection is lower : the system detects water and the wipers turn on.

Simple

 

[Westcoaster]

from Wiki:

The most common modern rain sensors are based on the principle of total internal reflection. At all times, an infrared light is beamed at a 45-degree angle into the windshield from the interior. If the glass is dry, the critical angle for total internal refraction is around 42°. This value is obtained with the total internal refraction formula sin(Oc)=n1/n2 where n1=1 is the approximate value on air's refraction index for infrared [2] and n2=1.5 is the approximate value of the glass refraction index, also for infrared [3]. In that case, since the incident angle of light is 45°, all the light is reflected and the detector receives maximum intensity.
If the glass is wet, the critical angle changes to around 60° because the refraction index of water is higher than air (n1=1.3)[4]. In that case, because the incident angle is 45°, total internal reflection is not obtained. Part of the light beam is transmitted through the glass and the intensity measured for reflection is lower : the system detects water and the wipers turn on.

Simple

Cool, but my explanation was simpler!

 

[jimmuller]

from Wiki:

The most common modern rain sensors are based on the principle of total internal reflection. At all times, an infrared light is beamed at a 45-degree angle into the windshield from the interior. If the glass is dry...

Simple

Got it. Thank you!

 

[GregW]

When I asked my dealer how the rain sensor worked he got a spray bottle of water and used it on the windscreen by the mirror mount. The wipers turned on and I was satisfied. ?

 

[alf_17]

When I asked my dealer how the rain sensor worked he got a spray bottle of water and used it on the windscreen by the mirror mount. The wipers turned on and I was satisfied. ?

Well that is the simple answer

 

[jimmuller]

"How does the rain sensor work?"
"Very well, actually"

 

[andrewhart1976]

from Wiki:

The most common modern rain sensors are based on the principle of total internal reflection. At all times, an infrared light is beamed at a 45-degree angle into the windshield from the interior. If the glass is dry, the critical angle for total internal refraction is around 42°. This value is obtained with the total internal refraction formula sin(Oc)=n1/n2 where n1=1 is the approximate value on air's refraction index for infrared [2] and n2=1.5 is the approximate value of the glass refraction index, also for infrared [3]. In that case, since the incident angle of light is 45°, all the light is reflected and the detector receives maximum intensity.
If the glass is wet, the critical angle changes to around 60° because the refraction index of water is higher than air (n1=1.3)[4]. In that case, because the incident angle is 45°, total internal reflection is not obtained. Part of the light beam is transmitted through the glass and the intensity measured for reflection is lower : the system detects water and the wipers turn on.

Simple

How many doctorates do you have

 

[Barryng]

Thank you all for bringing this question up and the excellent discussion. It is something I have always been curious about but never really knew exactly how it worked.

 

[PeC]

How many doctorates do you have

Basic highschool knowledge is enough for this !