Page 299 - Understanding Automotive Electronics

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8 VEHICLE MOTION CONTROL

book, commercial suspension systems are primarily semiactive. The active
suspension system has potential to be in production in the future and is
explained in Chapter 11. In this chapter, we explain only the semiactive system.
The primary purpose of the semiactive suspension system is to provide a
good ride for as much of the time as possible without sacrificing handling.
Good ride is achieved if the car’s body is isolated as much as possible from the
road. A semiactive suspension controls the shock absorber damping to achieve
the best possible ride.
In addition to providing isolation of the sprung mass (i.e., car body and
contents), the suspension system has another major function. It must also
dynamically maintain the tire normal force as the unsprung mass (wheel
assembly) travels up and down due to road roughness. Recall from the discussion
of antilock braking that cornering forces depend on normal tire force. Of course
in the long-term time average, the normal forces will total the vehicle weight plus
any inertial forces due to acceleration, deceleration, or cornering.
However, as the car travels over the road, the unsprung mass moves up
and down in response to road input. This motion causes a variation in normal
force, with a corresponding variation in potential cornering or braking forces.
For example, while driving on a rough curved road, there is a potential loss of
steering or braking effectiveness if the suspension system doesn’t have good
damping characteristics.
Figure 8.18 illustrates typical tire normal force variation as a function of
frequency of excitation for a fixed-amplitude, variable-frequency sinusoidal
excitation (see Chapter 2 for a discussion of sinusoidal frequency response).
The solid curve is the response for a relatively low-damping-coefficient shock
absorber and the dashed curve is the response for a relatively high damping
coefficient.
In Figure 8.18, the ordinate is the ratio of amplitude of force variation to
the average normal load (i.e., due to weight). There are two relative peaks in
this response. The lower peak is approximately 1 to 2 Hz and is generally
associated with spring/sprung mass oscillation. The second peak, which is in
the general region of 12 to 15 Hz, is resonance of the spring/unsprung mass
combination.
Generally speaking, for any given fixed suspension system ride and handling
cannot both be optimized simultaneously, as explained in Chapter 1. A car with a
good ride is one in which the sprung mass motion/acceleration due to rough road
input is minimized. In particular, the sprung mass motion in the frequency region
from about 2 to 8 Hz is most important for good subjective ride. Good ride is
achieved for relatively low damping (low D in Figure 8.18).
For low damping, the unsprung mass moves relatively freely due to road
input while the sprung mass motion remains relatively low. Note from Figure
8.18 that this low damping results in relatively high variation in normal force,
particularly near the two peak frequencies. That is, low damping results in
relatively poor handling characteristics.

286 UNDERSTANDING AUTOMOTIVE ELECTRONICS