The sensors on the wheels produce a sine wave, with a frequency of, about 50 times the wheel rotation frequency - the exact value is given by exactly how many teeth there are on the ABS reluctor wheel which rotates near to the tip of the sensors.
The ABS controller, by counting the number of zero crossings of this sine wave per unit time, can calculate the wheel's rotation speed.
The ABS controller can compare the speed estimate it obtained during the last calculation cycle with the one it has obtained during the current calculation cycle, and obtain an estimate for the angular acceleration of the wheel.
The angular acceleration is the critical value - a wheel on the point of locking suddenly decellerates (in an angular sense) very quickly - this angular deceleration is used to determine whether to prevent any further build up of brake pressure, or, even to begin pumping fluid back up to the master cylinder and reducing the brake force.
This all happens without **any** reference to the other wheel speeds. After all, what use is it to compare wheel speeds when you don't actually know how much each wheel is slipping. In accurate terms, it's impossible to measure how fast a car is going by measuring wheel speed, because tyre slip on the road is significant - especially when you are developing significant force.
The ABS controller contains 2 micro-processors which take the wheel sensor data in, and perform the calculations - if their answers disagree, the ABS controller is shut down - this is a sensible safety feature, that effectively prevents the ABS from "being silly".
When you are stopped, there is no angular velocity, and no angular deceleration - there's no signal for the ABS controller to act upon, and the brakes just work in a conventional way. There's no need for any motion sensor in the ABS controller - it all works on angular acceleration.
Number_Cruncher
|
I can truthfully say I couldn't have put it better myself ;-)
So ABS could work on a unicycle then!
|
>>So ABS could work on a unicycle then!
Yes!
Turning this on its head, you could implement a form of traction control that would reside entirely within the mapping of the engine ECU. If the angular acceleration of the engine suddenly increased beyond a known threshold, it's a fair guess that you have some wheel slip - by drastically retarding the ignition, you would quickly be able to reduce engine torque output, and reduce the slipping.
I think this is how some F1 teams sneakily implemented a form of traction control after it had been banned.
Number_Cruncher
|
|
|
So ABS could work on a unicycle then!
Well some motorbikes have ABS, and that works on one wheel.
|
|
|
|
Thankyou NC, thats one of your easier to understand comprehensive dscriptions of a very important car function.
Not to say i don't appreciate all of your analyses, but some are so far above me.
|
Yep; that's the way it used to work all right. [But they couldn't leave it alone....]
|
i think the ford escort mk4 had the system just right (not) with lazzy bands being driven off the driveshafts to the abs pick up modules
wonder if the bloke who designed that got the sack when ford had customers saying the abs wasnt working properly?
or did he get a job at jaguar?
anybody? ;-)
|
i think the ford escort mk4 had the system just right (not)
An elegant engineering solution though. [Developed from the Dunlop Maxaret, which was an aircraft ABS]
The belt drove a wheel. Connected to the wheel by a rotary spring was a small flywheel.
When there was no angular deceleration of the road wheel, the wheel and flywheel rotated together. When there was some deceleration, the flywheel would naturally over-run. As there was a rotary spring between the two, the amount of over-run was proportional to the angular deceleration of the wheel - exactly the number that the wheel sensors, and micro-processor of an electronic ABS go to great pains to calculate!
Some hydraulic valving connected to these wheels, and bingo, you have an ABS without all the expensive electronics.
It's one of the reasons why I really respect some of Ford's engineering - without engineering of that sort, motoring would probably be much more expensive than it is.
Just a royal PITA to work on!
Number_Cruncher
|
|
Tch! You beat me to it again NC....
|
|
|
I tried fixing a Maxaret system that was retro-fitted to an armoured Range-Rover many years ago - never again!!
|
i ripped two systems off imported (jersey) fiesta mk3 ghias
and one off an xr3i
might have looked good on the undercarriage of a passenger plane with the wheels out but when you are trying to undo 10 years accumulated grime and corrosion then no thanks
even the belt on my pioneer PL12d packed up at 5 years old
|
|
|
|
i think the ford escort mk4 had the system just right (not)
Please explain your reasoning behind this and why you ripped them out? What were your failure modes? Surely it's just a normal braking system when dealing with routine maintenance.
Just a royal PITA to work on!
NC aren't they sealed units which can't be worked on?
--------------
Mike Farrow
|
my bit to answer is easy
the car had the dashboard light on.,..........mot failure see
cars were ten a penny in the scrappies
take off the lazzy bands,change the master cylinder
remove the abs unit (it was on the bulkhead so as i remember even the pipes could be moved to fit where they used to go
and bobs your doobrie
basically it was a nasty setup that was thraught with problems
thank goodness for relucter rings
|
|
|
Mike,
I was thinking more of the difficulties in working on them in their situation. From above, you are bending right into the back of the engine bay, from below, they're sandwiched between the bulkhead and the back of the engine.
They got in the way when doing clutch jobs. Changing a belt meant taking the driveshaft out.
Number_Cruncher
|
|
|
|
|
|
|
If as number cruncher says it is impossible to measure vehicle speed through the abs sensor how do vehicles with no speedo drive off the transmission measure vehicle speed ? Also from experience the abs module will not always shut down if there is a fault with the sensors, i.e. air gap between sensor and abs ring incorrect, faulty sensor giving spurious readings causing the abs to operate
|
|
Also to add to the discussion why is it then that having tyres with the wrong aspect ratio fitted to one or more wheels will cause the traction control to cut in. Because the sensors compare speed against each other and if it thinks that one corner is turning at a slower speed to the rest it will cut in.
|
|
|
|
Also to add to the discussion vehicles with the wrong sized tyres fitted to a corner will cause the abs/traction control to cut in as it it measures wheel speed on all four corners and if the system thinks that a wheel is spinning/ locking up the appropriate system will cut in. I'm sure i've probably opened a can of worms now but these comments are only from personal experience.
|
|
|
>>measure vehicle speed ?
For the dash mounted speedo, there's no need for an accurate measurement, and the estimate provided by ABS wheel speed sensors, or transmission mounted sensors is absolutely fine for the purpose.
I'm talking about making an accurate measurement of vehicle speed, and because of slip, that's not possible without using something like a doppler radar, a ground tracking laser (think of making your car into a very large optical mouse!), or GPS.
On a dry road, tyres produce their maximum grip at about 20% slip. So, under harsh acceleration or braking, speed measurements based up measuring the rotation speed of gearbox shafts, or of wheels can result in vehicle speed estimates which are 20% in error.
Only if there were gear teeth on the tyres and matching ones on the road would the usual method of vehicle speed measurement be adequate for use in an ABS/ASR system.
As this error only happen when you are putting down a lot of force, it isn't a problem for speedos - the source of inaccuracy isn't a big problem under cruise conditions. But, for an ABS, you are potentialy using a large force, and under these conditions, you simply don't know the vehicle speed.
For some freight locomotives, where the maximum grip is obtained at 2% slip instead of 20%, doppler radar is used to provide vehicle speed input to their wheel slip protections systems - it makes them a bit more immune to the wrong kind of leaves!
Number_Cruncher
|
I don't know much about this subject, so what I'm about to say may be rubbish, so please be gentle with me.
Is there some confusion on this thread between the operation of ABS and that traction control?
|
|
|
>>wrong sized tyres fitted to a corner will cause the abs/traction control to cut in
This is absolutely true - it's one of the later additions to the system that Screwloose alluded to above.
The reason is that there are some conditions where a wheel can either spin, or lock up without the angular deceleration threshold being reached.
Among the possible conditions that can catch out a purely deceleration based system is where you have icy conditions, and the driver brakes very cautiously. If the wheel interia is large, you can obtain a lock up condition without a large deceleration.
So, if a velocity difference persists for some time, the ABS controller will act upon this - which explains your observation with an odd sized tyre. I didn't describe this above, because my post was already sufficiently verbose!
Number_Cruncher
|
NC
So after reluctor rings came active sensors and magnetic wheel bearings; speedily followed by widespread TC - then ASC - then ESP - all trying to outguess themselves and making a simple system into a right dog's breakfast.
Clearly what's needed is a centrally-positioned Dayton Correvit to augment the G-sensors - and adaptive [learning] software that doesn't throw a wobbly, shut down the whole system and light up the dash like a Christmess tree every time a new tyre is fitted!
|
>>Dayton Correvit
That's exactly the sort of sensor I was thinking about when I joked about making a car into a large optical mouse! I don't expect that they come cheap though!
Number_Cruncher
|
NC
I'll bet the bi-axial version doesn't! However; once made in quantity, the unit-cost would come down...
With a true-reference sensor mounted... at the centre of mass[?] then the software could accurately calibrate each wheel and cope with any in-service variance. It could also correct a car that had spun 180 degrees and was going backwards - now that'd be worth seeing.
|
|
|
|
|
|
|
|
