PRINCIPLES OF CONTROL SYSTEMS
Process Controls
Automatic Control System
An automatic control system is a preset closed-loop control system that requires no operator
action. This assumes the process remains in the normal range for the control system. An
automatic control system has two process variables associated with it: a controlled variable and
a manipulated variable.
A controlled variable is the process variable that is maintained at a specified value or within a
specified range. In the previous example, the storage tank level is the controlled variable.
A manipulated variable is the process variable that is acted on by the control system to maintain
the controlled variable at the specified value or within the specified range. In the previous
example, the flow rate of the water supplied to the tank is the manipulated variable.
Functions of Automatic Control
In any automatic control system, the four basic functions that occur are:
Measurement
Comparison
Computation
Correction
In the water tank level control system in the example above, the level transmitter measures the
level within the tank. The level transmitter sends a signal representing the tank level to the level
control device, where it is compared to a desired tank level. The level control device then
computes how far to open the supply valve to correct any difference between actual and desired
tank levels.
Elements of Automatic Control
The three functional elements needed to perform the functions of an automatic control system
are:
A measurement element
An error detection element
A final control element
IC-07
Page 4
Rev. 0
