rec.autos.simulators

I'm wondering if any of you out know the details of how these really
work on real cars.  Here's what I already know, and don't know:

ABS:  I know systems detect wheel slip on individual tires, and then
start pulsing the brakes.  What I don't know is exactly how wheel slip
is detected, and how the brakes are pulsed.  From an implementation
viewpoint, when does car/wheel enter ABS mode, and how is brake
force/torque pulsed when in ABS mode?

Traction Control:  This is actually probably easier to implement than
ABS, because as soon as wheel slip is detected for a powered wheel, I
can just cut engine power, or apply brakes to those wheels, or both.
That's what I've read actually happens.  Anybody know otherwise?

Yaw Control--Stability Management:  This is the big question mark.  So
far, I've read that based on many input conditions (steer angle,
speed, lateral gees, etc.), the computer can apply brakes to
inside/outside wheels to control the yaw of the vehicle to match the
desired yaw.  It can also adjust drive torque to inside/outside wheels
through the differential to achieve the desired yaw.  Now, what I've
read so far has been very vague to say the least.  Does anyone know
the details of how this works?

Thanks in advance for your help!

-Dave Gierok

Any way on my car there are what's known as an e***r ring on each wheel
hub or axle. These rings have a ribbed OD. There is a small pick up that
senses these ribs. Now how the computer uses this info I'm really not sure.

MadDAWG

The systems which I have been trained on work in this fashion, and each
wheel effectively has a separate brake system, so that each wheel is able to
maintain a slip of no more than 20%.  The systems will try and maintain a
20% slip though, as that is what has been determined to be the most
effective in terms of control and braking distance.

Traction control can work two ways, with most systems being one or the
other, and some even using both.

Some of this technology may have changed, as I have been out of the business
for about 6 years now, but that is what was being used then.

Glen Pittman

Now back to how my 95 Mustang works there is basically a speed sensor on
each wheel. If it was seeing an increase of speed due to slip (my style)
then there would be no need for the Anti-LOCK to kick in because the wheel
is not locked or close to it.  So back to the question of game programming I
would think it would be similar too, but opposite of traction control.

I'm not questioning if your right or wrong, just trying to make sence of it
is all.

Thanks
MadDAWG

<snip>

About 6 or 7 years ago we met one of the field engineers responsible for
tuning an anti-lock brake system for a particular car model.  This was a
6-week process with several engineers assigned to each car to be tuned.  We
did not meet the programmers that actually wrote the ABS control program.

Tuning this system was pretty hairy, he used a laptop to adjust parameters
(integer math in the micro-controller at that time) while the car was
tested on all sorts of surfaces.  The controller program had ~1000
adjustable parameters, of which about 200 were used in the tuning process,
the other 800 were generally left at the original "default" settings.

It seemed that much of the ABS controller program was dedicated to
producing a reliable estimate of vehicle speed when various wheels are all
slipping.  Another large part of the code was full of logic that branched
to different control algorithms after it determined the type of road
surface that the tire was running on.

While the intent is to hold wheels at some ideal slip ratio for max
braking, the reality is that wheel speeds are jumping all over the place
during an ABS stop.  I watched from the side of the road when a Corvette
did some braking tests and each wheel came to a visible stop several times
(and then immediately spun back up to speed) in the course of a 60-0 mph
stop.

So my conclusion is that you don't want to re-create an actual anti-lock
program.  These are all trade secrets, no manufacturer that I know has
exposed ABS algorithms or source code.

Your sim must know the actual velocity over the road, which should make it
easy to keep wheels from locking up.  Your tire model better include
friction circle effects<grin>.

-- Doug Milliken
   www.millikenresearch.com

Bill Berry

Thats pretty much what I was figuring, but it seems odd to call it that. Oh
well guess thats why they didn't ask me.

MAdDAWG

Let me know if you have any more insight on the algorithm specifics of ABS.

Thanks,
Dave

Of course, of course.  I am trying to use your book as a bible of sorts :).

Yikes, ~1000 parameters.  I believe I'll 'cheat' a little bit and use our
known vehicle speed, and slip values for each wheel.  Actually, I already
tried this and got very nice results (for ABS and Traction Control).  I'll
probably leave it at that.

Yaw/Stability control is still difficult though.  I plan on braking outside
wheels in an oversteer situation, and braking inside wheels in an understeer
situation.  But detecting oversteer & understeer is trickier than I
originally thought.  Any ideas?  Keep in mind we can 'cheet' again and use
all current physical attributes that are already calculated in the
simulation.

Thanks,
Dave

Thanks for the input.  Any idea on how to detect understeer/oversteer
situations such that we know when to brake inside/outside wheels?  I can't
immediately see how.  Hmmm, anybody else?

-Dave Gierok

Braking the outside wheels for oversteer?  I suspect that is not what you
want to do.  You don't really want to apply a yaw moment to the car IMO,
that would confuse the driver.  Plus braking the outside rear could actually
make things worse.  What you want is to reduce the rear slip angle and/or
increase the front.  The trick is how do you do that using only the brakes,
without causing so much forward weight transfer that it actually increases
the oversteer?  Don't know for sure, but I think if you just brake the front
wheels a bit and leave the rears alone it might work.  Or maybe just the
outside front.  Just thinking out loud really.  ;o)

Don't have the actual source here, but I did an understeer/oversteer
indicator (with the amount) for Racer (an unreleased version still).
Gives you an indication of the amount of u/o steer as well. Quite nice
and not often seen. :)

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art  : http://www.marketgraph.nl/gallery/

...

BTW In the book 'AI Game Programming Wisdom' it uses the
understeer/oversteer calculations to do countersteering for AI (as the
amount of steer is a worthy value as well as its sign). Lovely chapter
on racing AI, although a bit short (but worth buying alone for it).

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art  : http://www.marketgraph.nl/gallery/