Page 297 - Understanding Automotive Electronics

2735 | CH 8 Page 284 Tuesday, March 10, 1998 1:19 PM

8 VEHICLE MOTION CONTROL

Figure 8.17
ABS Braking Action

FPO

the wheels again tend toward lock-up and ABS reduces brake pressure. The
cycle continues until the vehicle is stopped.
It should be noted that by maintaining slip near S , the maximum
o
deceleration is achieved for a given set of conditions. Some reduction in lateral
force occurs from its maximum value by maintaining slip near S . However, in
o
most cases the lateral force is large enough to maintain directional control.
In some antilock brake systems, the slip oscillations are shifted below S ,
o
sacrificing some braking effectiveness to enhance directional control. This can
be accomplished by adjusting the upper and lower slip limits.

Tire-Slip Controller
Another benefit of the ABS is that the brake pressure modulator can be
used for tire-slip control. Tire slip is effective in moving the car forward just as
it is in braking. Under normal driving circumstances, the slip that was defined
previously for braking is negative. That is, the tire is actually moving at a speed
that is greater than for a purely rolling tire. In fact, the traction force is
proportional to slip.
For wet or icy roads, the friction coefficient can become very low and
excessive slip can develop. In extreme cases, one of the driving wheels may be

284 UNDERSTANDING AUTOMOTIVE ELECTRONICS