Page 293 - Understanding Automotive Electronics

2735 | CH 8 Page 280 Tuesday, March 10, 1998 1:19 PM

8 VEHICLE MOTION CONTROL

These coefficients depend markedly on slip, as shown in Figure 8.14. The solid
curves are for a dry road and the dashed curves for a wet or icy road. As brake
pedal force is increased from zero, slip increases from zero. For increasing slip,
µ increases to S = S . Further increase in slip actually decreases µ , thereby
b
b
o
reducing braking effectiveness.
On the other hand, µ decreases steadily with increasing S such that for
L
fully locked wheels the lateral force has its lowest value. For wet or icy roads, µ
L
at S = 100% is so low that the lateral force is insufficient to maintain directional
control of the vehicle. However, directional control can often be maintained
even in poor braking conditions if slip is optimally controlled. This is
essentially the function of the ABS, which performs an operation equivalent to
pumping the brakes as done by experienced drivers before the development of
ABS. In ABS-equipped cars under marginal or poor braking conditions, the
driver simply applies a steady brake force and the system adjusts tire slip to
optimum value automatically.
In a typical ABS configuration, control over slip is effected by controlling
the brake line pressure under electronic control. The configuration for ABS is
shown in Figure 8.12. This ABS regulates or modulates brake pressure to
maintain slip as near to optimum as possible (e.g., at S in Figure 8.14). The
o

Figure 8.14
Braking
Coefficients versus
Tire Slip (Solid
curves for dry road,
dashed curves for
wet or icy road):

FPO

280 UNDERSTANDING AUTOMOTIVE ELECTRONICS