When ABS was first implemented, how did the braking system sense that the wheel had locked?
Posted by CharlesElwoodYeager@reddit | askcarguys | View on Reddit | 43 comments
I'm looking at this from the perspective of an avionics control engineer. We have lots of onboard sensors, and we can calibrate control loop responses to them with a variety of effectors. In the 80s, what kind of information could you reliably get from a wheel?
HobsHere@reddit
Fun trivia: the reason that they are called "tonewheels" is that the technology was borrowed from old Hammond organs that used a series of such wheels, driven by a synchronous motor, to produce the waves that made up the notes. They were called tonewheels as early as the 1930s.
CharlesElwoodYeager@reddit (OP)
HAMMOND?!
HobsHere@reddit
Yep, Hammond.
Paul_The_Builder@reddit
I just want to add that the ABS systems in the 80's and 90's were a lot more crude than they are today. They typically required the wheel or axle to completely lock up before engaging, and would also often just have an on/off solenoid valve on the brake line with slow electronics, making the brake force modulation very jumpy. The sensors and ABS modules were more prone to failure and unwanted behavior.
Here's a video from 1990 that does a pretty good job showing the ABS capabilities of the time.
There was a lot of distrust and skepticism in ABS (and EFI for that matter) in the 80's and 90's, a lot of "car guys" didn't trust the added electronics, which is still true to today. And there was a lot of misinformation about ABS back then, leading a lot of people to believe it was not effective, or even hurt performance. There were some limited tests and anecdotal stores of cars having longer braking distances in good conditions - straight dry road or track. But of course the main point of ABS is not to maximize braking performance in good conditions, but to allow better braking with less user input in bad conditions, and when turning while braking. But still I can't tell you how many times I heard from "car guys" in the 90's and early 2000's that ABS was a scam, and some people would pull the fuse on their ABS controllers, or do something similar to disable the system.
In the mid-late 90's, ABS and active stability control got more advanced, with better more capable electronics, and by the mid 2000's, it was a very capable technology, and in 2012 it became mandatory equipment on all new vehicles.
I think what really sold ABS and convinced "car guys" in the late 90's and early 2000's that it wasn't a scam anymore was active stability control. While pure ABS could be rather crude, active stability control would monitor all 4 wheel speed sensors and have separate control to all 4 brake lines, monitored by a computer. Being able to monitor speed trends in all 4 wheels and brake 1 wheel at a time leads to a lot of enhanced traction control that cannot be replicated by a human. Its kind of like launch control now... in 1990, if you told someone a computer could launch a car better than an experienced drag racing driving, they wouldn't believe you. Now its pretty common knowledge that its true, and the same held true for active stability control, by the late 90's the tech was good enough that it could out perform any skilled human.
Here's a video from I think 1997 from GM explaining active stability control on the C5 Corvette which was a brand new technology at the time. Interesting to see the "advanced computer simulation" graphics hah.
CharlesElwoodYeager@reddit (OP)
not trusting EFI is pure stupidity, but i mean its not like so-called car guys have any real knowledge as to how their machines are coaxed into function anyhow
Paul_The_Builder@reddit
Rebuilt several carbs in my day. I only have 1 carbureted engine left that I own -mercruiser/Chevy 350, which I want to convert to EFI.
I'll say a happy man if I never have to service a carburetor again. Even the crude TBI systems from the 90s are way better than dealing with a carb.
Candid_Ad5642@reddit
I'm fairly certain they have always used a tone wheel to measure the wheel speed on each wheel individually
Also why ABS nearly always have come with some kind of traction control
CharlesElwoodYeager@reddit (OP)
If they compare wheel speeds, in the unlikely event of locking all four wheels, would the ABS fail to reduce brake force?
Astramael@reddit
That’s why modern cars have accelerometers that feed into a vehicle dynamics model. The car understands what’s going on even if all four wheels lock at the same time.
jolle75@reddit
If it’s badly coded, yes. But there are some things you can rule out. A sudden drop to zero is impossible, so, then the system knows to release the brakes a bit to get rotation.
If you want nice info, BMW did a lot of the how’s and stuff with the introduction of motorcycle ABS because riders were very sceptical (some boomers still are). All around the late eighties. Same principles still apply
SailingSpark@reddit
Having owned a BMW, they have some of the most transparent ABS systems around. You barely even notice it is working. The ABS on my mother's old buick would fight you and kick back heavily at the pedal.
KeeganY_SR-UVB76@reddit
Mercedes also has great ABS systems. I tried testing it on an empty road in the rain and it just comes to a stop instantly from 30 MPH. I didn’t feel any locking or slipping.
jolle75@reddit
They use the Bosch systems, on both two and four wheels. The old systems were “only” modulating at 125 times per second, second gen was already 1000 times.
AlwaysBagHolding@reddit
Yes, you see this even in modern cars. There’s plenty of videos every winter of new cars pinballing down an icy hill with all four wheels locked.
HotmailsInYourArea@reddit
It can happen even on modern vehicles if you’re going slow enough on ice. They just have to lock up in such a way to pass the ABS’ parameter check.
Defiant-Giraffe@reddit
Perhaps after 2010 or so. ABS dates back to the '70s and became common in the early '90s. It was rare to find vehicles with traction control until the late '90s.
And at first it was front wheel ABS only, on mostly RWD vehicles.
Bikes-Bass-Beer@reddit
Interestingly enough ABS was developed by the military for helping fighter jets land on short aircraft carrier runways in the 60's
R_Michael_E@reddit
The Boeing B-47 bomber had anti-skid brakes in 1947.
overheightexit@reddit
Second part is backwards. Rear wheel ABS was common for a while. In general, rear wheels lock more easily than front wheels. My father has a ‘95 F-150 with rear ABS.
BoondockUSA@reddit
Locked wheels lose directional control and are less effective at braking. If the rear wheels lock, the rear will want to spin out and become the front. If the front wheels lock but the rear ones don’t, the front will want to remain the front. That’s why preventing the rear wheels from skidding is more important than the front wheels (although four wheel ABS is exponentially better).
Rear wheel ABS was mostly a thing on pickups because unloaded, pickups have relatively light rear ends so a rear wheel skid tends to happen more than with cars.
You can try it yourself if you have a toy car and a table that you can prop at a steep enough angle. Put tape over the rear wheels to prevent them from spinning. You’ll see the rear will spin and become the front. Then switch the tape to the front. The front will stay the front.
Source: Went to a driving school when rear wheel ABS was still a thing and saw the toy demonstration.
QuinceDaPence@reddit
Motorcycles on the otherhand, the front is vital as the most important. The rear locks, you skid but can control it. The front locks and you are falling over, no ifs, ands, or buts about it.
You'll also see offroad/dual-sport/adv bikes occasionally with ABS that can be set between All/front-only/off since offroad on a down hill ABS can potentially prevent you from coming to a stop at all.
BoondockUSA@reddit
Very much correct. You’re gonna have a bad day if your front tire skids on motorcycle.
antonmnster@reddit
If you were lucky you had 3 channel. Two for the front, one for the back.
Kseries2497@reddit
Hey, my '95 Probe GT worked that way. It also couldn't work any other way, since the rear wheels were on the same brake line. One of the few relatively low-tech features of that car, at least by '90s standards.
Otherwise-Ad6675@reddit
The super old school abs was actually rear wheel only and was used up until at least 1993 by dodge for trucks and vans. My 89 D100 has it and its legitimatly worse than just not having abs at all.
BoondockUSA@reddit
Rear wheel ABS was around a lot longer on pickups than 1993. In later years, it was often the standard equipment, while four wheel ABS was the optional upgrade. I had a mid 00’s pickup that still had rear wheel ABS. It was advertised as four wheel ABS but I didn’t discover the listing mistake until it was too late.
Full ABS wasn’t fully mandated on cars and pickups until late 2012.
Defiant-Giraffe@reddit
Yes, but before even that, it was a rare option on some Mercedes and Cadillac models on the front.
jolle75@reddit
This indeed. On most motor bikes you can see the wheel and sensor. Also, don’t have to come with traction control but more a happy free thing with a few lines of code and a bus to the ECU.
Live_Reason_6531@reddit
I don’t know of any prior to the tone ring. Of course it’s absolutely possible there was something before that.
CharlesElwoodYeager@reddit (OP)
what does a tone ring do?
carpediemracing@reddit
https://www.carparts.com/blog/what-is-a-tone-ring-and-can-you-fix-it-in-place/
Basically it's a 1 or 0 sensor reading based on whether or not there's a bit of metal next to it. The ring itself has a series of raised parts (like the top of a generic castle wall) that go past the sensor. If the four sensors don't read the same then something is going on. If one sensor isn't reading either 1 or 0 but not changing then that one wheel isn't spinning.
Or the tone ring is broken, the tone ring might be super dirty (so the low parts are high), the sensor isn't where it's supposed to be, etc.
morosis1982@reddit
Crenellations. Not sure if that's what they're called on the tone ring.
CromulentPoint@reddit
They're actually called merlons. Crenellations are the low parts in between the merlons on a battlement.
Source: Lego castle nerd
Dredgeon@reddit
They may have borrowed the land and valley terminology from hard disks.
jccaclimber@reddit
Different name for a (relative) rotary encoder.
Live_Reason_6531@reddit
https://youtu.be/Y-Hsip4-5Is?si=R7x_7jMgMzJAJUDQ
Slight-Ad4115@reddit
It's always been wheel speed that is measured.
I think real early ABS systems simply compared wheel speeds across wheels.
The current method of measuring wheel speed and calculating acceleration has been around probably since the late 70s when high speed microcontrollers started to become available.
If you had the inclination you could probably get a lot more info from old patents.
jasonsong86@reddit
Probably some kind of wheel speed.
Another_Slut_Dragon@reddit
Information inputs- tone ring- a toothed wheel at each wheel to give an AC signal. Combine all 4 for an overall speed input.
Accelerometer- basic braking G forces.
Brake pedal application switch- lets the ABS know the driver is braking.
For the most part, the car knows roughly how fast it can stop from tables in the controller. The system is just measuring the speed of each wheel and backing off/holding or applying more brake pressure to each wheel based on detected slip just by comparing one wheel speed to the next. Most systems will also take g force into account. If it detects a lot of wheel slip compared to the desired g forces it assumes the road is slippery and backs off on the total braking forces more, and/or it will pulse that wheel more.
Each wheel has a release, hold and add braking pressure solenoid as an output. There is a pump in the ABS module for adding brake pressure. That allows you to decrease, hold or add brake pressure on each wheel.
Pup111290@reddit
I know older pickups had just rear ABS and they just worked off the speed sensor. Downfall with that is both rear wheels needed to lock together for it to kick in
oboshoe@reddit
Yea I had chevy S10 with that arrangement. Basically worked off the speedo sensor.
Able-Woodpecker7391@reddit
Early sensors were a magnetic pickup type sensor and a reluctor wheel mounted usually on the bearing or axle shafts. As the wheel spun, it would generate an analog signal due to the metal moving through the magnetic field of the sensor. If one wheel stops, the signal changes. This is also why when rust would build up under the sensor, the amplitude of the signal would change, causing unwanted ABS activation.
captain_sta11@reddit
Most cars systems in the late 70s into the 80s were electronically controlled like it is today. Sensors watching for speed that didn’t make sense(within a threshold due to circumstances where they would naturally turn at different rates like when turning)to see a wheel was locking up or about to lock up which actuated valves to lower brake pressure. Cars have relied on computers and sensors for a lot of stuff longer than most people think going back 50+ years.