Page 298 - Understanding Automotive Electronics

2735 | CH 8 Page 285 Tuesday, March 10, 1998 1:19 PM

VEHICLE MOTION CONTROL 8

on ice or in snow while the other is on a dry (or drier) surface. Because of the
action of the differential (see Chapter 1), the low-friction tire will spin and
relatively little torque will be applied to the dry-wheel side. In such
circumstances, it may be difficult for the driver to move the car even though
one wheel is on a relatively good friction surface.
The difficulty can be overcome by applying a braking force to the free
spinning wheel. In this case, the differential action is such that torque is applied
to the relatively dry wheel surface and the car can be moved. In the example
ABS, such braking force can be applied to the free spinning wheel by the
hydraulic brake pressure modulator (assuming a separate modulator for each
drive wheel). Control of this modulator is based on measurements of the speed
of the two drive wheels. Of course, the ABS already incorporates wheel speed
measurements, as discussed previously.
The ABS electronics have the capability to perform comparisons of these
two wheel speeds and to determine that braking is required of one drive wheel
to prevent wheel spin.
ELECTRONIC SUSPENSION SYSTEM
In Chapter 1, we described automotive suspension systems as consisting
of springs, shock absorbers, and various linkages to connect the wheel assembly
to the car frame. The purpose of the suspension system is to isolate the car body
motion as much as possible from wheel motion due to rough road input. In
Chapters 1 and 2, it was shown that the performance of the suspension system
is strongly influenced by the damping parameter of the shock absorber.
The two primary subjective performance measures are ride and handling.
Ride refers to the motion of the car body in response to road bumps or
irregularities. Handling refers to how well the car body responds to dynamic
vehicle motion such as cornering or hard braking.
Generally speaking, ride is improved by lowering the shock absorber
damping, whereas handling is improved by increasing this damping. In
traditional suspension design, the damping parameter is fixed and is chosen to
achieve a compromise between ride and handling (i.e., an intermediate value
for shock absorber damping is chosen).
In electronically controlled suspension systems, this damping can be
varied depending on driving conditions and road roughness characteristics.
That is, the suspension system adapts to inputs to maintain the best possible
ride subject to handling constraints that are associated with safety.
There are two major classes of electronic suspension control system: active
and semiactive. The semiactive suspension system is purely dissipative (i.e.,
power is absorbed by the shock absorber under control of a microcontroller). In
this system, the shock absorber damping is regulated to absorb the power of the
wheel motion in accordance with the driving conditions.
In an active suspension system, power is added to the suspension system
via a hydraulic or pneumatic power source. At the time of the writing of this

UNDERSTANDING AUTOMOTIVE ELECTRONICS 285