Page 277 - Understanding Automotive Electronics

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8 VEHICLE MOTION CONTROL

Figure 8.3
Cruise Control Speed Performance

increase. In this case the actuator output will increase until the error is reduced to
zero. The response of the cruise control with PI control is shown in Figure 8.3b.
The response characteristics of a PI controller depend strongly on the
choice of the gain parameters K and K . It is possible to select values for these
I
P
parameters to increase the speed of the system response to disturbance. If the
speed increases too rapidly, however, overshoot will occur and the actual speed
will oscillate around the desired speed. The amplitude of oscillations decreases
by an amount determined by a parameter called the damping ratio. The
damping ratio that produces the fastest response without overshoot is called
critical damping. A damping ratio less than critically damped is said to be
underdamped, and one greater than critically damped is said to be overdamped.

264 UNDERSTANDING AUTOMOTIVE ELECTRONICS