Re: principles of ABS questions

[steveb@falcon.kla.com (Steven Buchholz)]

> I understand that ABS works by measuring angular velocity -- in a
> nutshell the braking pressure is reduced at the wheel that rotates
> slower.  My question is, what happens when all four wheels are locked?
> That is, how does the ABS computer "know" whether I am sliding or fully
> stopped on a icy, 20% slope, driveway?
> 
> A maybe related question: it seems that making ABS work with fulltime
> 4WD is more difficult; on some part time 4WD, engaging 4WD disconnects
> the ABS.  In my 92 100, blocking the rear diff disconnects the ABS --
> of course there can not be any speed difference for the rear wheels,
> but couldn't ABS still work wrt the front?
> 
I believe that most ABS systems actually monitor the rate of change in the 
rotational velocity between the four wheels, to determine when "impending 
lockup" is present.  At this time it decides to modulate the pressure applied
to the brake that is approaching lockup.  I'm pretty sure that the older ABS
systems only used information WRT the rotation of the wheels ... many newer
systems include some sort of accelerometers to attempt to provide more info-
rmation to the modulation algorithm.  I think that many ABS systems actually 
stop modulating the brakes when they sense that the vehicle has slowed to 
nearly a stop (5MPH?).  Audi has stated that ABS is more difficult to do on 
an AWD car.  Of course a vehicle that has locked differentials doesn't 
have to worry so much about individual wheels locking up (true of the rear
wheels on quattros with diffs locked).  Of course many of the newer cars 
actually have open diffs and count on the ABS system to minimize wheelspin
as well as provide the anti-lock braking function ...

I believe that there are a number of references that you could consult to 
satisfy your curiosity ... I would imagine that the Bosch Automotive Hand-
book would have a top level description of how ABS systems work.  I 
recommend this book to anyone that has an occasional "how does that work?" 
question regarding automobiles (including FI systems).  It doesn't delve 
too deeply into the details ...

HTH!
Steve Buchholz
s_buchho@kla.com
San Jose, CA (USA)