How do i reset my windshield wipers

[Thorbuilt]

I recently had a new glass installed and i was told i need to reset my rain sensor.... I tried today but don't know how to do it? Anyone know how?

2012 Audi a3 TDI

thanks

 

[Zenerdiode]

The rain sensor works by measuring the changes of the refractive index of the glass when rain drops are present. The only thing that could upset this when the glass is replaced is if the 'tint' is different from your previous screen. I don't think that it can be 'reset' though.

Please also post an Auto-Scan so we may see the type of rain sensor and any associated coding values.

 

[ElSupremo]

The rain sensor works by measuring the changes of the refractive index of the glass when rain drops are present. The only thing that could upset this when the glass is replaced is if the 'tint' is different from your previous screen. I don't think that it can be 'reset' though.

Hmm.. as an electro-optical engineer I have a hard time buying your "how it works" explanation. First of all, the refractive index is an inherent material property, and will change SLIGHTLY with temperature and pressure but...err...not from a water drop or two. It is the square root of the dielectric constant and for most glasses is about 1.5. I could be wrong, but I would guess that the REAL way it works is much like a capacitive sensor- much like a capacitive proximity sensor works, or a Theremin. But I agree, it's not obvious how it could be reset. And if a tint was put on (especially if it was slightly conductive, but that is not a requirement), that could change whole calibration or prevent it working.

I have noticed that, at least on my 2012 Tiguan, I need to clean the surface over the senor every once in a while, or it simply doesn't sense rain. Try that extremely simple solution first.

 

[Sebastian]

It's not about refraction at all, it's only about transparency at certain wave length. The rain light sensor uses one wave length for rain sensing and another one for light sensing, both differ based on the windshield type/color used and also slightly depending on the installation position for each of the individual models. That said, the transparency is being adjusted by coding the rain light sensor accordingly. The more important thing though is that the actual wiper module needs to be recoded before the wipers can be put back on. This is to ensure the alternat parking position (APS) is deactivated - in order to properly align the wiper arms.

 

[Zenerdiode]

You're correct Steve, I wrote that a bit too quickly last night. The refractive index of a material is an inherent property. What I was trying to explain, is that the emmiter (infra-red LED) and sensor (photo-diode or whatever) are arranged behind the glass so they are beyond the critical angle of the refractive index of a glass to air transition, so total internal reflection occurs and the infra-red lands on the photodiode(s). When a rain drop lands on the surface of the glass, you have a glass to water transition, this alters the critical angle at that point, you don't get total internal reflection and some light escapes into the water bead. This causes a reduction of light falling on the sensors and triggers the wiper.

I therefore beg to differ with you Sebastian, it's all about refraction. Otherwise it wouldn't work in total darkness or you'd need an emitter on the outside of the glass.

 

[Sebastian]

You are correct that there is a reflection which is part of the concept, yet that has (per my understanding and VWs documentation of the technique) not much relevance for the sensitivity of the sensor - which is what we are trying to correct here. But then I am neither a physics major nor an engineer - so I have little clue of what I am talking about in the first place.

 

[ElSupremo]

You're correct Steve, I wrote that a bit too quickly last night. The refractive index of a material is an inherent property. What I was trying to explain, is that the emmiter (infra-red LED) and sensor (photo-diode or whatever) are arranged behind the glass so they are beyond the critical angle of the refractive index of a glass to air transition, so total internal reflection occurs and the infra-red lands on the photodiode(s). When a rain drop lands on the surface of the glass, you have a glass to water transition, this alters the critical angle at that point, you don't get total internal reflection and some light escapes into the water bead. This causes a reduction of light falling on the sensors and triggers the wiper.

I therefore beg to differ with you Sebastian, it's all about refraction. Otherwise it wouldn't work in total darkness or you'd need an emitter on the outside of the glass.

Well, you're close ZD - but no cigar. It does have to do with refraction but not the critical angle. I did a little research and here's the straight skinny.

The "critical angle" has nothing to do with it since the sensor works with an EXTERNAL IR LED emitter and photodiode sensors. If light can get into the glass, it can always get out (do a Wikipedia on "total internal reflection").

What REALLY happens is that the photodiode sensors look for the reflection of the LED emitter light off the windshield/"screen" combination. The reflection back into the photodiodes comes from the "first surface" (inside the car) and the second surface (outside on the windshield) and for 2 surfaces of UNCOATED glass is maybe 7--8% (but the screen obviously changes that). It's that 7-8% in your home windows that lets you see your reflection in a window when it's dark outside.

When water accumulates on the outside surface - water has a refractive index of about 1.2 - it acts as an ANTI-REFLECTION coating, which almost eliminates the second surface reflection (rather than the LED emitter light getting reflected it passes right through...) and hence REDUCES the amount of light getting back into the sensors. That "reduction in reflected intensity" is what is detected. For you techies, the reflection from a coated surface is minimized when the coating has an index of refraction of the square root of the product of the material being coated (here, glass) and the outside medium (here, air, whose index is 1).

Obviously I've left out a lot of details but you should get the jist of it - rain coats the outside surface, acts as an AR coating, reduces the amount of reflected (LED-emitted) light getting back to the photodiodes. The sun doesn't have much of an effect because the LED emitter is relatively monochromatic and the photodiode sensors have narrow bandpass filters on them tuned to the wavelength of the emitter), but if Sebastian is correct (a high probability based on past posts...), a second "unfiltered" photodiode sensor could be used as a "sunlight monitor" and used to offset external sources of light.

Whatever wasn't covered in this class will be covered on the quiz...although for "extra credit" see http://en.volkswagen.com/en/innovation-and-technology/technical-glossary/regensensor.html

Now can I finish putting out ?