I’m not ashamed to admit that rain-sensing wipers are one of those things that I think are clever, but have never felt the need for. I mean, I can see when there’s rain on my windshield, and it’s just not that hard to hit a little stalk on the steering wheel column to turn the wipers on. I don’t exactly see the point of them, except perhaps if you subscribe to the notion that true luxury means the car should do everything for you without you having to debase yourself by touching a control: shift, turn on lights, and, yes, turn on the windshield wipers.
Modern cars use a pretty sophisticated and clever system that uses LEDs that shine infrared light into the interior surface of the windshield, and then light-sensitive sensors detect how much light is reflected back. The amount of light reflected back is different on a dry windshield than on one with water on it, based on the principle of total internal reflection. Essentially, this phenomenon alters the reflection and refraction of light on a wet windshield, and the sensors can send this information to a computer within the car’s CAN bus to activate the wipers and adjust their speed depending on just how wet the windshield is.
Here, this video will give a more detailed explanation than I’m currently willing to:
Now, one thing to realize about this modern method is just how dependent on advanced electronics like computers and small integrated sensors and infrared LEDs it is, and just how much math is involved. It’s a non-trivial solution to the problem, and it’s only quite recently that we as a species have been able to make such components cheap and reliable enough for a system like this to make sense.
That’s why when I tell you that the first mass-produced dynamically-adjustable rain-sensing wiper system was introduced way back in 1970, you should realize that we’re about to talk about something nice and strange, because 1970s technology was not remotely capable of a rain-sensing system like a modern car, at least not without filling most of the car up with clunky humming boxes of computer hardware.
The first mass-produced car that came with any sort of rain-sensing wiper system was the Citroën SM, introduced in 1970.
The SM was a sort of technological showcase for Citroën; it started out as a planned sportier version of Citroën’s DS, but soon developed into its own unique car, though it did adopt a lot of the SM’s advanced features, like the hydropneumatic self-leveling suspension and steerable headlights. The resulting car felt like an Earthbound spaceship, powered by a 2.7-liter Maserati V6 and so much ’70s sex appeal you’d think someone shoved Burt Reynolds into a Cuisinart and then brushed the resulting slurry over the leather seats.
In a car like the SM, an innovation like rain-sensing wipers made complete sense.
So, if there were no infrared LEDs or small computer modules or CCD cameras or any of the other components of modern car tech, how did they pull it off?
By being clever, that’s how. And, let’s be honest here, with this sort of engineering, being French probably didn’t hurt, either.
Citroën was sort of oddly circumspect about their rain-sensing wiper system, never really mentioning it specifically. In the owner’s manual, the system is part of what they just refer to as “intermittence.” And that’s not wrong – the rain sensing was part of their intermittent wiper system – but I do think it’s a bit odd they didn’t call it out more specifically.
That could be because the problem they seemed to be trying to solve was less about the obvious detect-rain-turn-on-wipers problem, and more about the more subtle how-do-we-make-better-intermittent-wipers problem. You see, where most intermittent wiper systems just use variable timing to adjust how often the wipers wipe, Citroën wanted a system that adapted how often the wipers wiped to how much rain was falling, and that meant that their system had to know when it was raining, and roughly how much.
Here’s how it worked: when you put the wipers in intermittent mode, the wiper system began to monitor the electrical current drawn by the wiper motor. If the current draw was high, that meant the windshield was dry, because there’s a lot more friction when trying to drag a rubber wiper over a dry glass windshield. When this condition was detected, the wipers moved less frequently, because there was less rain.
But when the current draw from the wiper motor dropped, that meant that there was less friction between the wiper blade and windshield, a condition that would be caused by the windshield becoming wet, likely from rain, (though to be fair, I suppose if a squirrel climbed on your roof and peed down your windshield or someone dumped a full bottle of Yoo-Hoo on your window, that would work, too) and the wiper speed would get faster until the current draw increased again, indicating a drier windshield.
So, look at what we have here! A simple, completely electromechanical system that is not just detecting when there’s rain on the windshield, but adjusting the wiper speed dynamically based on that feedback. Instead of detecting rain via total internal refraction or whatever I was going on about before, it was detecting rain via its lubricating qualities on the surface of the windshield. And you know what? They both work! Both methods are capable of detecting the presence of rain, but one requires zero computers or sensors.
Here’s the system in action; it’s maybe not especially dramatic to see, because the wipers just, you know, work, but I think it’s still worth showing:
I love this solution so much. It’s so clever and simple, and you can almost picture a room full of Citroën engineers sitting around a table thinking about ways to detect rain, when someone just remembered the visceral sensation of running a squeegee over a dry windshield, and how much different that was than sliding one across a wet one. It’s an engineering solution that seems like something based on feel, on the experiences of our senses in the real world, not on the understanding of complex math or physics, but gut-level engineering.
This approach may largely be gone, but I think it’s still worth appreciating.
Top graphic images: Bring A Trailer; Citroënvie
The automatic wipers on our 2018 MDX were a constant annoyance for me. My wife would just leave the wipers on and sometimes when I got in the car on a sunny, completely dry day, they would go off when I least expected them to. Maybe it was a layer of dust deposited when Saharan sand was blowing over that little armpit of America and falling down to Earth. I don’t know. But I much preferred the variable intermittent wipers on my simpler Accord. It, like my A4 Jetta before it, was clever enough to extend the interval between wipes when I was at a stop.
You embedded a photo of the SM from an angle I’d never seen before, and I absolutely love the Citroen logo on top of the hood vent.
I really like my automatic wipers. And my automatic headlights. The transmission is not automatic, though.
The big advantage of the modern system is of course that it can detect rain even when the wipers are not working at all.
We have a ’24 Kia Niro Hybrid with the sensing wipers that work amazing, until they get confused by a bit of gooey bug or bird leavings they can’t clean off with spray. They will wipe that windshield til they wear the glass through…
We’ve got a contender for the next Autopian member T-Shirt:
An image of the Citroen SM.
And the following quote in stylized letters:
“So much ’70s sex appeal you’d think someone shoved Burt Reynolds into a Cuisinart and then brushed the resulting slurry over the leather seats.”
Think about it.
The auto wipers on my 2013 Crown are overly sensitive in light rain on the “high” setting but not sensitive enough in heavy rain on the “low” setting, so I’m constantly adjusting their sensitivity. Sometimes I wonder if regular intermittent wipers would have been better.
The system on my old 2006 330i was much more set-and-forget, though it would get confused during intermittent rain showers, getting stuck on maximum even as the rain eased.
The system on my brand new Jaecoo isn’t too bad, but the sensitivity adjustment is buried in the touchscreen, so I just manually engage it as needed during light rain.
This has been my automatic wiper journey. Thank you.
Am I missing the auto “on” aspect? It’s really just auto intermittent adjustment.
Because the system has no way of knowing the rain has started or ended.
A picky edit: (The SM was a sort of technological showcase for Citroën; it started out as a planned sportier version of Citroën’s DS, but soon developed into its own unique car, though it did adopt a lot of the SM’s advanced features), I think that the second reference to the SM was meant to be DS. If I had money to burn the first 2 cars I would buy are a DS and an SM. Weird is good, French weird is fantastique.
I had a couple of Peugeots that had some uh, interesting ways of doing things, but never as French weird as Citroën.
Back in my automotive service days I’d always prepare my team on the mornings of wet snow/ice days for the inevitable wave of people coming in with broken wipers, wiper arms, fried motors, and broken linkages because they never turn off the automatic wipers. And nothing in that list is inexpensive on European luxury vehicles. Sorry, that’s not covered under warranty.
Oh, it would happen occasionally in nice weather when they would head on into the automatic car wash and the wipers would spring up to do their duty, only to be beaten back down into submission by the angry tentacles of cleanliness. Some of the washes around here have signs or the attendants remind you to turn them off. Still happens though.
Rain sensing wipers are the worst. Always over sensitive with a mind of their own, dry wiping a light spritz.
My GTI has a sensitivity adjustment
This is not only clever, but something that would be helpful even today. Intermittent wipers usually don’t quite hit the proper frequency (and that ideal frequency changes as conditions do), so something like this would be great. I’d like this more than my current rain-sensing wipers, I think.
I love my rain sensing wipers if only because I am extremely OCD about my wipers wiping at exactly the right speed for the current conditions and spend way too much time with my hand on the stalk adjusting it haha. Now the car just does it all on it’s own! Magic!
Most new Toyotas have 5 different intermittent speeds in addition to low and high speeds. While not fully automatic, you can usually get it close. I don’t like it when my wipers are too fast, it almost feels like cruising in 3rd gear for no reason.
I get what he’s saying though. Where I live, rain storms can often change very frequently in intensity in a short period of time. One second it’s pouring, the next, barely a drizzle, followed by a deluge. One would have to constantly be changing between the various intermittent speeds, and thus, still have their hand on the stalk the entire time, constantly adjusting it.
With the auto sensing, it tends to “know” that the rain has slowed significantly quicker than I’d be able to get my hand to the controls and slow them down anyway.
I had an old Cadillac with a slider switch for the intermittent wipers. The ultimate in luxury: infinite wiper settings!
Every car I’ve had has had at least 5 intermittent settings as well, but that’s kind of the problem for me because I’m always fiddling with it to make sure it’s wiping at exactly the speed it needs to. I swear my 95 Town Car had at least 10 intermittent setting with 1 through 8 being exactly the same speeds haha.
Yeah, the old Ford and GM stalks were kind of lame. Plus, you have to go through all of the intermittent settings to get to Low.
This bothers my wife to no end. She never once criticizes my driving, except to tell me to adjust the wipers.
I have a hard time believing there aren’t more of us wiper OCDers on this site. We have taillight fetishists and engineering nerds. Let us have our perfectly-wiped windshields.
I wish Tesla had done this instead of the vision based approach. Which works okay when it’s pouring but not so well in lighter rain. It does figure it out eventually. Although who knows what little oddities may be in the code.
good reason to burn it.
This is brilliant, all cars should have this.
Also, one feature I’ve always wanted in a car: whenever i am playing music, analyze it and figure out the BPM, and make my turn signals and wipers go in sync w the music to some degree. Would be so cool.
When I was in high school, a friend and I built an electronic device from a kit that would synchronize a strobe light to the music that was being plugged into it. You had a dial that set the beat to pick out, and a sensitivity level to adjust, and it would be like a disco scene, before disco was a thing! Quite a lot of fun at parties!
This is a feature I never wanted until I had it. It’s great in semi-drizzly conditions where otherwise I would constantly have to adjust the intermittent speed.
I’m surprised how useful it is myself.
Also great when it’s not raining, but the roads are wet. When you’re behind someone, they’re kicking up water and the wipers clear it. Come to a stop light, and they take a break.
I like articles about old Citroens. I’d be happy for any excuse you use to write about them, even daily. How about CitroWednesday?
I’m still waiting on about 400 or so Mercury Monday posts and I don’t need Torch getting distracted.
I’d rather Torch get back to his “‘World’s Worst Cars’ Book Redemption” series. It’s been 2 years!!!
Totally forgot about that one. That was fun.
I’ll also take some Glorious Garbage while he’s at it.
Please, you’re standing between a man and the delicious Citroen content he craves. Have a heart, guys.
This is how you get scurvy!
See! You all are gonna cause me to get the scurvy! I need the Citroen content because, as everyone knows, Citroen is the cure for the scurvy. Just like the cereal people put vitamins in the cereal, Autopian can provide us with our daily Citroen to enjoy with our breakfast and save the lives of children, probably.
Look, if we had a day that started with C-, I’d be totally game. But when reading The Autopian, I need alliterative, themed content. I love the Citroen content dearly, but I have expectations that were set by this here establishment, and they need to be met.
Torch – I desperately need 10,000 words on the Mercury Montego on Monday. Not before. Not after. MONDAY.
I would have been delighted to have this system in my Comanche when I spent 16 hours driving in varying levels of mist over the past two days.
Can any engineers out there explain how the citroen detected the higher current draw when the windshield got dryer, and how it then turned off the wiper motor?
Also, how would it detect lower resistance (rain) unless it was always checking with an occasional wipe whenever the car was running?
Glorified flasher relay. The frequency of cycling changes based on the current draw.
The harder an electric motor has to work, the higher the current draw. This results in a change in voltage for a fixed resistance DC motor which is easily detected even in the 70’s.
A motor’s current draw, assuming sufficient power supply, is inversely proportional to its speed. The lower the speed the higher the current draw.
The power supply is fed through a self resetting bimetallic (thermal) breaker, ie a flasher as TheDrunk mentioned below.
The more heat that is put into the breaker the longer it takes to cool down and self reset.
If you look at the picture near the red coil of wire you can see the “start” or normally closed and “finish” or normally open contacts and the moveable arm between them.
The normally closed contact bypasses the motor’s internal park switch while the “tripped” or normally open contact supplies power through the park switch which keeps the motor running until it opens when it reaches the park position.
At least that is the best I can do based on the physics and what I can see of the device in the picture above.
They weren’t auto on/off, you still had to turn the wipers on, but it would set the delay automatically
My new daily has rain-sensing wipers and I’m still trying to figure them out so I’m not sure yet whether I like them or not.
The Citroen SM, one of my all-time favorite cars – they still look like the future. Somehow I’m not surprised rain-sensing wipers debuted on this. While staring at one you could tell me it used a gravatronic magnometer to levitate six feet off of the ground in service mode and I’d believe it.
My ’98 Eldorado was the only car I’ve ever owned with rain-sensing wipers. I have no idea how those worked, but they did function fairly well and were actually kind of nice on long-drives with variable weather.
I always hate it when I intermittently wipe, especially in instances where continuous wiping is necessary.
In very light rain I prefer to just tap the stalk up when I want a wipe. Granted, this only works well with STD Japanese Car (TM) ergonomics. Fuggedaboudit with old Ford knob operated systems, etc…
All my previous cars have been VWs or a Peugeot, where you push the stalk up to go through the various levels of wipe, so if you just want one wipe, you push down. Now I have a Mazda and I’m still trying to re-wire my brain for upside-down wiper ergonomics.
Thats cool. I like clever mechanical or electromechanical solutions like that. They seem so much more interesting than “a computer does it”. Like the old cam lobe based rear blinker setups from Ford and Mercury.
To clarify here… despite what it says in the article (hopefully that talk of “1970s technology was not remotely capable” was hyperbole for effect rather than just straight-up misinformation) you don’t need any kind of computer for the optical sensor… it’s a clever and simple design that uses a light source and a light detector, and because there’s less internal reflection when the windshield is wet, less gets from the source to the sensor. The patent for that design was filed in the 1970s and used parts (like an IR LED) that had been commercially available since the early 1960s. You could do your sensor threshold detection entirely via simple analog circuit.
The only designs that need a ton of computing power are the dumb ones like Tesla’s approach that tries to use the cameras (that were not designed with this in mind) and some software to do it and it is notoriously bad at its job compared to a few dollars’ worth of LED and brightness sensor. IMO, the normal optical sensor and this current sensor are both clever, simple solutions to the problem compared to “we threw 300 watts of AI hardware and some cameras at the problem”.
I was going to say that they did some amazing stuff in the “before times” and sometimes constraints make your design better.
I loved when Citroen marched to their own drummer and came up with their own solutions with little to no regard to how the rest of the industry did something.