itpro and I ran out of room in the last thread...
ABS
I'll start by quoting from a slightly out of date source, Race Car Vehicle Dynamics by Milliken and Milliken (1995):
"The most recent advance in brake systems involves the addition of wheel speed sensors, and a mechanism for lowering brake line pressure". Current anti-lock brake systems (ABS) cycle the brakes between "near free-rolling" and "near lockup" while attempting to maintain the tire near the friction peak on a variety of surfaces.
Current ABSs use the wheel speed signal for two purposes. First, with the wheel free rolling, the wheel speed defines the vehicle speed. Second, with the brakes applied, the wheel speed determines the percentage slip. The latter is maintained near the best slip ratio for the paticular tyre, often about 10%. Thus the brakes must cycle-off for a small time to update the computer as to the actual vehicle speed. As the computer algorithms are improved, the "off" time is reduced and performance is still further improved.
In the future, vehicles may be fitted with an additional road speed sensor independent of any wheel speed. With this extra information, the ABS computer can be programmed to hold the tire at the correct slip ratio for best braking whenever maximum braking is commanded."
The mechanism back in those days was often just a solenoid valve that either just blocked brake fluid getting to the caliper, or released pressure back to the reservoir. These moved up towards so-called "boosted" systems. More common nowadays is the ABS 3 system and later equivalents, which use an accumulator as a brake booster, and effectively decouples the master cylinder from the wheel cylinders, leading to the "dead" pedal feel that most road cars have nowadays.
I've done a bit of research into the mechanisms used to detect slip, lockup and so on. There's quite a good "Adhesion/slip" curve in the Bosch automotive handbook, with an explanation of how the ABS controller interrogates the sensors. It's based around the maximum braking force being available at 15-20% slip, with braking forces at higher slip values being lower. This means that braking forces below 20% slip are stable, and anything above is unstable, in that using these forces leads to a lockup with no increase in braking force.
The system therefore scans the wheel speed sensors looking for rises in perpheral wheel deceleration that exceed predetermined critical values. If the value is exceeded, the brake pressure is quickly reduced, and then ramped up again until the same thing happens again. The aim seems to be to use the "stable/unstable" quality of the slip curve to ensure that the tyre is working in the area where maximum braking performance is achieved. This will be what's causing small patches of rubber to be left on the road, not because the wheel has locked, but because the slip coefficient of the tyre has become great enough that rubber gets rubbed off the still rolling tyre.
The Milliken quote seems a little self contradictory, and isn't entirely factual. But gives you the gist. The Bosch stuff is about right, and it's worth looking at the graph of the ABS control cycle for high adhesion coefficients. The wheels never lock.
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