Page 285 - Understanding Automotive Electronics

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8 VEHICLE MOTION CONTROL

Figure 8.8
Digital Cruise Control Configuration

and an integral term that is formed by a summation. In performing this task the
controller continuously receives samples of the speed error e , and where n is a
n
counting index (n = 1, 2, 3, 4, . . .). This sampling occurs at a sufficiently high
rate to be able to adjust the control signal to the actuator in time to compensate
for changes in operating condition or to disturbances. At each sample the
controller reads the most recent error. As explained earlier, that error is
multiplied by a constant K that is called the proportional gain, yielding the
P
proportional term in the control law. It also computes the sum of a number of
previous error samples (the exact sum is chosen by the control system designer
in accordance with the desired steady-state error). Then this sum is multiplied
by a constant K and added to the proportional term, yielding the control
I
signal.
The control signal at this point is simply a number that is stored in a
memory location in the digital controller. The use of this number by the
electronic circuitry that drives the throttle actuator to regulate vehicle speed
depends on the configuration of the particular control system and on the
actuator used by that system.

272 UNDERSTANDING AUTOMOTIVE ELECTRONICS