Page 72 - Understanding Automotive Electronics

2735 | CH 2 Page 59 Tuesday, March 10, 1998 10:55 AM

THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION 2

The control electronics generates the electrical signal for the actuator in
response to the control input and in accordance with the desired relationship
between the control input and the system output. The operation of the plant is
directly regulated by the actuator (which might simply be an electric motor).
The system output may also be affected by external disturbances that are not an
inherent part of the plant but are the result of the operating environment.
One of the principal drawbacks to the open-loop controller is its inability
to compensate for changes that might occur in the controller or the plant or for
any disturbances. This defect is eliminated in a closed-loop control system, in
which the actual system output is compared to the desired output value in
accordance with the input. Of course, a measurement must be made of the
plant output in such a system, and this requires measurement instrumentation.
Closed-Loop Control
It is the potential for change in an open-loop system that led to feedback,
or closed-loop, control. In a closed-loop control system a measurement of the
output variable being controlled is obtained via a sensor and fed back to the
controller, as illustrated in Figure 2.23.
The measured value of the controlled variable is compared with the
desired value for that variable based on the reference input. An error signal
based on the difference between desired and actual values of the output signal
is created, and the controller generates an actuating signal that tends to
reduce the error to zero. In addition to reducing this error to zero, feedback
has other potential benefits in a control system. It can affect control system
performance by improving system stability and suppressing the effects of
disturbances in the system. Later chapters will include numerous examples of
closed-loop control, such as idle speed control.
The generic closed-loop control system illustrated in Figure 2.23 has
some of the components found in an open-loop control system, including the

Figure 2.23
Closed -Loop Control
Configuration

FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 59