Page 302 - Understanding Automotive Electronics

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VEHICLE MOTION CONTROL 8

The variation in shock absorber damping is achieved by varying the
aperture in the oil passage through the piston (see Chapter 1 for discussion of
shock absorber configuration). In practical semiactive suspension systems, there
are two means used to vary this aperture size—a solenoid-operated bypass valve
and a motor-driven variable-orifice valve (Figure 8.20).
Figure 8.21 is an illustration of the force/relative velocity characteristics of
a shock absorber having a solenoid-switched aperture.
The control system for a typical semiactive electronic suspension system
has a similar configuration to any electronic control system, as depicted in the
block diagram of Figure 8.22.
The control system typically is in the form of a microcontroller or
microprocessor-based digital controller. The inputs from each sensor are
sampled, converted to digital format, and stored in memory. The sampling is
typically at about 500 Hz. In this control configuration, sensors are provided to
measure body (sprung mass) acceleration, the relative position and motion of
the wheel/body (unsprung/sprung mass), the steering wheel input, and vehicle
speed. The body acceleration measurement can be used to evaluate ride quality.
The controller does this by computing a weighted average of the spectrum of
the acceleration. The relative body/wheel motion can be used to estimate tire
normal force.
Under program control in accordance with the control strategy, the
electronic control system generates output electrical signals to the actuators in

Figure 8.20
Adjustable Shock
Absorber

FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 289