Page 275 - Understanding Automotive Electronics

2735 | CH 8 Page 262 Tuesday, March 10, 1998 1:19 PM

8 VEHICLE MOTION CONTROL

Figure 8.1
Cruise Control Configuration

Whenever the actual speed is less than the desired speed the throttle opening is
increased by the actuator, which increases vehicle speed until the error is zero, at
which point the throttle opening remains fixed until either a disturbance occurs
or the driver calls for a new desired speed.
A block diagram of a cruise control system is shown in Figure 8.2. In the
cruise control depicted in this figure, a proportional integral (PI) control
strategy has been assumed. However, there are many cruise control systems still
on the road today with proportional (P) controllers. Nevertheless, the PI
controller is representative of good design for such a control system since it can
reduce speed errors due to disturbances (such as hills) to zero (as explained in
Chapter 2). In this strategy an error e is formed by subtracting (electronically)
the actual speed V from the desired speed V :
d
a
e = V – V a
d
The controller then electronically generates the actuator signal by combining a
term proportional to the error (K e) and a term proportional to the integral of
P
∫
the error (that is,K etd ). The actuator signal u is a combination of these two
I
terms:
∫
d
u = K e + K et
I
P
The throttle opening is proportional to the value of this actuator signal.

262 UNDERSTANDING AUTOMOTIVE ELECTRONICS