Page 304 - Understanding Automotive Electronics

2735 | CH 8 Page 291 Tuesday, March 10, 1998 1:19 PM

VEHICLE MOTION CONTROL 8

Figure 8.22
Semiactive
Electronic
Suspension System

FPO

(from the corresponding sensor’s data). Whenever the weighted amplitude of
the spectrum near the peak frequencies exceeds a threshold, the oil passage
aperture is switched smaller, causing relatively high damping (firm ride).
Otherwise, the aperture is switched to the larger opening, resulting in relatively
low damping (soft suspension).
If in addition the vehicle is equipped with an accelerometer (usually located
in the car body near the center of gravity) and with motor-driven variable-
aperture shock absorbers, then an additional control strategy is possible. In this
latter control strategy, the shock absorber apertures are adjusted to minimize
sprung mass acceleration in the 2 to 8 Hz frequency region, thereby providing
optimum ride control. However, at all times, the damping is adjusted to control
unsprung mass motion to maintain wheel normal force variation at acceptably
low levels for safety reasons. Whenever a relatively large steering input is sensed,
such as during a cornering maneuver, then the control strategy switches to the
smaller aperture, yielding a “stiffer” suspension and improved handling.

ELECTRONIC STEERING CONTROL
In Chapter 1, the steering system was explained. There it was shown that
the steering effort required of the driver to overcome restoring torque generally
decreases with vehicle speed and increases with steering angle. Traditionally, the
steering effort required by the driver has been reduced by incorporating a
hydraulic power steering system in the vehicle. Whenever there is a steering

UNDERSTANDING AUTOMOTIVE ELECTRONICS 291