I think I screwed up the address the first time so I am sending this again,
if it arrives twice I apologize.
OK if anybody doesn't want a course in ABS design and Function hit delete now.
<<So, you are saying that ABS systems in general have independent
accelerometers at each wheel? (An accelerometer measures G-forces). As
well as having a computer which integrates the results instantaneously
and also tracks vehicle speed moment to moment?>>
No, the systems do not have an accelerometers. What the systems have are
wheel speed sensors (generally three or four) There is a toothed wheel that
is near the sensor. On the cars I instruct on the wheel has 48 teeth. The
pickup is a magnetic pickup very similar in design to magnetic pickups used
in many distributors. As each tooth passes the sensor an alternating current
is generated in the sensor. (sine wave) Next either in the sensor itself or
in the control unit the signal goes through an analog to digital converter to
convert it to a digital on/off signal that the computer can utilize. This
information is processed constantly. Now by looking at the frequency and how
fast the frequency is changing (and in what direction) G force can be
calculated. I seem to recall that this G force calculation takes place every
10 milliseconds. For example if the R/F wheel is producing a signal of 4800
Hz and that signal then changes to 5000 Hz and this change takes 1 second
there is a very slight positive G force change. (acceleration) If then the
5000 Hz signal goes to 3000 Hz is 1/2 a second there is negative G force
(braking) If the car requires it a circuit can be added that sends a signal
to the speedo for displaying road speed (all of our late model cars do this
BTW)
Now lets look at how this might work in a real world case. You are driving
down a road and for whatever reason have to stop as quickly as possible, to
help avoid the accident you pull to the right and your right two wheels go
into the gutter where there is some loose pebbles and water. Without ABS
either you would have to feather the brake slightly to avoid locking up the
right side wheels, or run the risk of spinning the car as the right side will
not be braking with the same force as the left. Now lets look at ABS the ABS
control unit knows that you have not had your foot on the brakes and the
wheels have been rotating at a steady frequency. Then the control unit sees
a brake pedal application and the frequency start to drop rapidly on all four
wheels. When the two right side wheels start to loose traction the frequency
drops too fast in too short a time to be braking. When this occurs the
hydraulic pressure is modified to the wheel(s) in question to restore
rotation and prevent a skid. Remember the greatest braking efficiency is
obtained with about a 12-15% slip between the tire and the road. Less than
this you are not braking at maximum potential (ABS not needed or active),
more than this number you are in a skid.
Now I've said this before and I will say it again. Just as cars have
different 0-60 times they have different 60-0 times (or distances if you
prefer) My Volvo has brakes that will stop on a dime and give some change.
The rental car I had today had an ABS system that seemed like it worked in
slow motion and was mediocre at best. The best thing any driver can do is to
learn the limits of the car they are driving when there is time, not in the
middle of an accident.
<>
This might be possible however all the systems I have ever examined work on
the above principles. (yes I know more than just Volvo)
<>
Since I am a technical training instructor for Volvo cars you might want to
give me the benefit of the doubt here :-). However, I am willing to copy and
send information from our manuals on design and function if anyone is
interested. E-mail me off list. (I'm on Jury duty it will be a couple of
weeks before I'm in the office again)
Hope this clears up some of the fog
Rick Ewald
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