I own a 2002-model UK-specification Vauxhall Vectra.  The car is
equipped with a rain-sensitive windscreen wiper system and I'm curious
as to how it works.  I'd particularly like answers to the following
points:

1. How does the system know when it's raining at all?
2. How does the system know how hard it is raining?
3. What is the heuristic by which the system decides how much wiping
to do for a given amount of rain?
4. Is the amount of wiping performed related to the speed of the car at all?

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Request for Question Clarification by juggler-ga on 15 Apr 2004 02:06 PDT

I've located some general explanations of how rain-sensitive wipers
work. The information is not specific to the Vectra, but it'd give you
a basic idea of how these systems know when it's raining and how much
it's raining. However, the information doesn't cover either "the
heuristic by which the system decides how much wiping to do" or
whether the Vectra's system is speed-sensitive.

Would you be interested in the general explanation?

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Clarification of Question by fraserspeirs-ga on 15 Apr 2004 02:59 PDT

Thanks for your response, but parts 3 and 4 are the real heart of the
matter that I'm interested in.

I don't necessarily require Vauxhall's canonical word on how the
system works, however.  If you can find, say, owner's group
discussions or some such that cover #3 and #4, I'd be happy with that.

Thanks.

Hello.

Okay, after reviewing the sources below, I feel that they do answer
parts #1-4 of your question.

For parts #1-3...

Rain-sensitive wipers are typically controlled by a sensor near the
windscreen (often on the back of the rearview mirrow).

"The sensor projects infrared light into the windshield at a 45-degree
angle. If the glass is dry, most of this light is reflected back into
the sensor by the front of the windshield. If water droplets are on
the glass, they reflect the light in different directions -- the
wetter the glass, the less light makes it back into the sensor.
 The electronics and software in the sensor turn on the wipers when
the amount of light reflected onto the sensor decreases to a preset
level. The software sets the speed of the wipers based on how fast the
moisture builds up between wipes."
source: How Stuff Works: Wipers
http://auto.howstuffworks.com/wiper4.htm

Also see: 

"TRW Rain Sensor Allows Drivers to 'Set and Forget' the Wipers"
http://www.accidentreconstruction.com/news/mar00/030100e.html

TRW: Rain Sensor
http://www.trw.com/extlink/1,,,00.html?ExternalTRW=/images/rain_sensor.pdf&DIR=2
(This document is in PDF format, so the Adobe Acrobat Reader is
required. If you don't have that, visit:
http://www.adobe.com/products/acrobat/readstep2.html )

As for part #4...
Obviously, under certain circumstances, vehicle speed will affect how
much rain accumulates/remains on a windscreen, so, if it's raining,
vehicle speed will have some effect on the rain sensor in any case.
Additionally, though, it turns out the previous-generation (before
2003) Vectra's wiper system was speed-sensitive.  I located a 2001
press release from General Motors (concerning the Holden Vectra, which
is the same car built at the same factories, but shipped to Australia)
that the wipers were "road speed-sensitive."
Source: General Motors
http://media.gm.com/division/holden/news/releases/010306_ltd_equipe.html
 

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search strategy:
"rain * wipers" work
"rain sensor" wipers work
"rain sensor" wipers work light

I hope this helps. If anything is unclear or needs amplification,
please let me know via the "request clarification" feature. Thanks.

hi
if the procedure used to set the wiper speed is that mentioned in the
answer by juggler, then you don't need to make the wipers speed
sensitive. if the vehicle is moving faster, more water gets deposited
on the wind shield and the wipers naturally move faster because more
ir rays are dispersed. so, why do we need to take the speed of the
vehicle into consideration for the wiper design?