h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

How do they work. What are they sensing?

(Yes, yes...water, obviously ...But how?)

It's little windscreen wipers gnomes sitting on the car, and they sense rain just like we do, really.

Rubbish, as the gnomes put up their umbrellas the spokes trip a series of micro switches that progressively speed up the wipers.

Firstly, the wipers work in the dark as well as in daytime, so I presume they don't have anything to do with light. Secondly, there is nothing visible on the car that can measure moisture.

I notice my "automatic wipers" don't start automatically. I have to nudge them by switching them to manual and back to auto. Once wiping, they will continue to wipe until the windscreen is dry and then will stop. If a few minutes later it starts raining again, I have to nudge them again.

Furthermose, if there's a sudden increase in rain, they don't adapt by speeding up until after the next wipe. That is, the next wipe comes when you would have expected it before the sudden increase in rain happened.

I conclude that they work by measuring the friction with the windscreen at each wipe and calculating the time to the next wipe based on it. The less friction, the wetter the windscreen is considered to be, so the shorter the time to the next wipe.

bounces an IR beam off the windscreen.
Wetter the screen is the less IR is returned to the sensor and thus the wipers are activated. Clever ones do variable control depending.

It should thus be possible to trigger it with anything on the windscreen that reduces the IR reflected.

My brain is telling me 'refraction' so with glass/water having a different refractive index to glass/air the ir beam will be bent by a different amount and thus miss the sensor. Presumably it averages this over a certain portion of the screen to work out how heavy the rain is.

The other option would be some sort of electronic sensor built into the windscreen. The obvious one would be a grid of fine wires (similar to those used to heat the windscreen) arrange those right and you could build a capcitance based sensor. When the water falls and spread out between the wires the capacitance would change and then some other electronic magic could switch on the wipers (or off or change speed etc).

From W***pedia: -

The most common modern rain sensors are based on the principle of total internal reflection: an infrared light is beamed at a 45-degree angle into the windshield from the interior — if the glass is wet, less light makes it back to the sensor, and the wipers turn on.

So...as Ictoan said, infrared.

On my UK spec Fiesta (high spec for a Fiesta) the rain sensing wipers adjust their speed according to how much it is raining. They also have 6 sensitivity settings - everything from frantic wiping in drizzle (common here in Wales) to the occasional desultory wipe in a monsoon (Capel Curig - http://www.metoffice.gov.uk/weather/uk/wl/capel_curig_latest_weather_graphs.html )

t. - note the rain glasses...

just been browsing t'web on this. Seems they did try something like that Phil but it wasn't reliable. They also tried one which measured the vibration of rain drops hitting the screen - also not reliable. And in reverse there was one which used high frequency vibrations of the windscreen to shake the water up and cause it to dissipate - strangely that did not work either. Seems the IR one is the only current reliable technology. Although I'm intrigued by Gnomon's hypothesis - if you'd care to mention make and perhaps model of your car Gnomon I could try and verify it - but understand if you don't want to.

Anecdotally, and interestingly (perhaps), mine often come on when I drive under tree cover, even on dry days. I concluded that there must be some sort of light sensing going on, which would support the IR explanation.

They are bloody infuriating though. Often they need a little nudge to begin with (as Gnomon says) and then wipe away at the slightest hint of moisture. I spend more time adjusting their sensitivity now than I would turning them on and off manually.

Mine is a Peugeot 406.

yep 406 seems to be IR based. There should be a control/sensor on the back of the rear view mirror and/or the ceiling light dome apparently.

There's a big sensor there, but I thought it was for the automatic headlights. If it the wipers use IR, it doesn't explain why they won't start when it starts to rain. Perhaps it goes into a sleep mode after a while to save irradiating the planet.

various bods on forums seem to get the same. All down to fine tuning the responses. You'll have one trigger level to turn on and one trigger level to turn off again. They also have problems with grease and dirt on the screen. Although I'm sure an upstanding fella like you has a spotlessly clean car

The first few drops of rain might not land on the sensor. After the first wipe, the whole windscreen will be wet and they will keep going. I suspect that simply measuring absolute reflectivity wouldn’t work very well anyway, for example when the windscreen gets dirty; they probably trigger on change.

Control systems such as this also usually incorporate some kind of hysteresis or adaptive damping, to ensure they don’t go straight to monsoon mode when one big drop lands right on the sensor, and conversely don’t stop immediately no water is detected after half-an-hour of heavy rain.

it's not 'a' sensor in that sense though, as far as I can tell it 'scans' a section of the windscreen and averages the response. Hence the ones which do variable speed and can tell the diffference between a light shower and a monsoon.

"for example when the windscreen gets dirty"
as per previous post this is actually a complaint about them, particularly if the screen is greasy rather than just dirty.

“it's not 'a' sensor in that sense though”

Well, the little bit of the windscreen where the emitted light either reflects or leaks out. The entire ‘sensor’ in mine (a VW) is within the mirror-mount, a circle of about 50mm diameter. I assume both emitter and receiver are contained within this, as light travels in a straight line. It probably measures quantity of water by averaging over several samples.

I’ve never had any trouble with dirt, only with sudden changes in ambient light levels. This clearly means it’s compensating for ambient conditions by assessing the differences between successive samples rather than absolute reflectivity.

So do you have a Peugeot too, Gnomon? Sounds like yours work the same as mine.

In my car, there's a sort of black cutout in the windsreen, in front of the rear view mirror. This, I'd guess, houses the photocell which tells my mirror to darken at night or when I drive through the Clyde Tunnel (). Presumably it also houses Ic's IR gubbins.

My favourite feature of my car is the automatic headlights. The number of times I've driven off in other cars at night and not remembered to switch the lights on!

yeah but I was under the impression (not found any schematics yet) that it looked at more than just the area immediately ahead of it. Could be wrong though.

I'm assuming that the sensor and emitter must be in different places as the IR beam is shot at 45 degrees to the screen.

nope, seems it is just a small path - not a direct point though. The manufs seem somewhat reticent about putting up any real explanations of the actual design.

See, apparently they work on total internal reflection -
http://www.absoluteastronomy.com/topics/Total_internal_reflection

and the IR beam hits the screen at 45degrees (apparently) and that would mean that if the screen is at 45degrees to the horizontal and the IR beam is therefore horizontal from the back of the mirror then the beam will be reflected straight down?

Think I am misunderstanding this as that requires another detector.

The whole light sensor thingy is rather big and bulky and could easily have a transmitter and a receiver in it.