Process Controls PRINCIPLES OF CONTROL SYSTEMSA closed-loop control systemis one in which control action is dependent on the output. Figure2 shows an example of a closed-loop control system. The control system maintains water levelin a storage tank. The system performs this task by continuously sensing the level in the tankand adjusting a supply valve to add more or less water to the tank. The desired level is presetby an operator, who is not part of the system.Figure 2 Closed-Loop Control SystemFeedback is information in a closed-loopFigure 3 Feedback in a Closed-Loop Control Systemcontrol system about the condition of aprocess variable. This variable is comparedwith a desired condition to produce the propercontrol action on the process. Information iscontinually "fed back" to the control circuit inresponse to control action. In the previousexample, the actual storage tank water level,sensed by the level transmitter, is feedback tothe level controller. This feedback iscompared with a desired level to produce therequired control action that will position thelevel control as needed to maintain the desiredlevel. Figure 3 shows this relationship.Rev. 0 Page 3 IC-07
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