Process Controls
PRINCIPLES OF CONTROL SYSTEMS
A closed-loop control system is one in which control action is dependent on the output. Figure
2 shows an example of a closed-loop control system. The control system maintains water level
in a storage tank. The system performs this task by continuously sensing the level in the tank
and adjusting a supply valve to add more or less water to the tank. The desired level is preset
by an operator, who is not part of the system.
Figure 2 Closed-Loop Control System
Feedback is information in a closed-loop
Figure 3 Feedback in a Closed-Loop Control System
control system about the condition of
a
process variable. This variable is compared
with a desired condition to produce the proper
control action on the process. Information is
continually "fed back" to the control circuit in
response to control action. In the previous
example, the actual storage tank water level,
sensed by the level transmitter, is feedback to
the level controller.
This feedback is
compared with a desired level to produce the
required control action that will position the
level control as needed to maintain the desired
level. Figure 3 shows this relationship.
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