Page 276 - Understanding Automotive Electronics

2735 | CH 8 Page 263 Tuesday, March 10, 1998 1:19 PM

VEHICLE MOTION CONTROL 8

Figure 8.2
Cruise Control
Block Diagram

Operation of the system can be understood by first considering the
operation of a proportional controller (that is, imagine that the integral term is
not present for the sake of this preliminary discussion). We assume that the
driver has reached the desired speed (say, 60 mph) and activated the speed set
switch. If the car is traveling on a level road at the desired speed, then the error
is zero and the throttle remains at a fixed position.
If the car were then to enter a long hill with a steady positive slope (i.e., a
hill going up) while the throttle is set at the cruise position for level road, the
engine will produce less power than required to maintain that speed on the hill.
The hill represents a disturbance to the cruise control system. The vehicle speed
will decrease, thereby introducing an error to the control system. This error, in
turn, results in an increase in the signal to the actuator, causing an increase in
engine power. This increased power results in an increase in speed. However, in
a proportional control system the speed error is not reduced to zero since a
nonzero error is required so that the engine will produce enough power to
balance the increased load of the disturbance (i.e., the hill).
The speed response to the disturbance is shown in Figure 8.3a. When the
disturbance occurs, the speed drops off and the control system reacts
immediately to increase power. However, a certain amount of time is required
for the car to accelerate toward the desired speed. As time progresses, the speed
reaches a steady value that is less than the desired speed, thereby accounting for
the steady error (e ) depicted in Figure 8.3a (i.e., the final speed is less than the
s
starting 60 mph).
If we now consider a PI control system, we will see that the steady error
when integrated produces an ever-increasing output from the integrator. This
increasing output causes the actuator to increase further, with a resulting speed

UNDERSTANDING AUTOMOTIVE ELECTRONICS 263