Process Controls
PRINCIPLES OF CONTROL SYSTEMS
Relationships between these elements and the functions they perform in an automatic control
system are shown in Figure 4. The measuring element performs the measuring function by
sensing and evaluating the controlled variable. The error detection element first compares the
value of the controlled variable to the desired value, and then signals an error if a deviation exists
between the actual and desired values. The final control element responds to the error signal by
correcting the manipulated variable of the process.
Figure 4
Relationships of Functions and Elements
in an Automatic Control System
Feedback Control
An automatic controller is an error-sensitive, self-correcting device. It takes a signal from the
process and feeds it back into the process. Therefore, closed-loop control is referred to as
feedback control.
Rev. 0
Page 5
IC-07
