> >> How can a car with ABS brakes come to a stop in a hill?
> >> Why don't the ABS-system release the brakes when the car stops?
>
> >The ABS system doesn't do the braking. Your foot does.
>
> I didn't say the ABS-system do the braking. I said RELEASE the brakes.
> There's a big difference.
>
> >The ABS uses various methods to effectively make you take your foot off
the
> >brake for very short periods.
> >One way is to pulse a very high pressure onto the line for a short time
> >which pushes your foot off the pedal.
>
> I don't think that's how it's done. If you put very high pressure on the
> line you will also apply this pressure to the brakes, and they would
> brake even harder. But that's not the issue here. I'm not interested
> in the mechanic part of this, what I'm interested in is the electronic
part.
>
> How will the ABS system sense speed when the wheels are locked up?

Taking your last question first.
Simply, it doesn't. It can't.
The sensing of a skid is done by looking at the change in speed. If it
measures the speed of the wheel at 30mph and then 100ms later finds that
the speed of the wheel is now 0mph, it reasons that this doesn't obey
certain laws of physics. Therefore the wheel is skidding.
It's not practical to try and determine what the optimum braking pressure
is going to be. There are too many external variables.
Tyre condition, road surface, a deep puddle 1" in front of the wheel, lines
painted on the road, bumpy surface, and so on.
Regardless of all these, if the speed of the wheel drops suddenly at a high
speed, it can't be doing effective braking.
If the initial speed is very low (say 2mph), the ABS leaves the situation
alone. At that speed the difference in stopping distance between a locked
wheel and one with ideal slip is minimal.
So now there are two simple rules to our algorithm.
1) If the last speed I measured was less than x, don't do anything.
2) If the last speed I measured was greater than x and now it is zero, do
something.

This simple set of rules can be expanded to compare with other wheels and
so on.

Now, what can it do.
We know that the wheel is now at speed=0 so applying more brake pressure is
going to make no difference to the wheel. It can't go any slower. But,
putting more pressure on the line will force the pedal back up against the
drivers foot and lift it up a little bit. When we take our extra pressure
away again, the drivers foot is no longer pressing on the pedal as hard as
it was before it caused the lockup to occur. ie. less braking pressure.
Less braking pressure allows the wheel to start turning again and we can
repeat the process.

This is the system often used in passenger cars where you can feel the
pedal doing odd things. The ABS actuator is inserted in the brake lines of
an otherwise standard system.
The advantage of doing it this way is that it is fail safe. If the ABS
malfunctions, you still have a complete standard braking system.

There are other systems used on things like trucks where the standard brake
system relies on pneumatic servoing. There are also other hydraulic systems
that use a very convoluted means of removing pressure from the brake line.


Steve.

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  Very funny Scotty.  Now beam down my clothes.
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Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680                email: steveb@kcbbs.gen.nz
New Lynn, Auckland           ph  +64 9 820-2221
New Zealand                  fax +64 9 820-1929
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