PRINCIPLES OF CONTROL SYSTEMS Process ControlsAutomaticControlSystemAn automatic control system is a preset closed-loop control system that requires no operatoraction. This assumes the process remains in the normal range for the control system. Anautomatic control system has two process variables associated with it: a controlled variable anda manipulated variable.A controlled variable is the process variable that is maintained at a specified value or within aspecified 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 maintainthe controlled variable at the specified value or within the specified range. In the previousexample, the flow rate of the water supplied to the tank is the manipulated variable.FunctionsofAutomaticControlIn any automatic control system, the four basic functions that occur are:MeasurementComparisonComputationCorrectionIn the water tank level control system in the example above, the level transmitter measures thelevel within the tank. The level transmitter sends a signal representing the tank level to the levelcontrol device, where it is compared to a desired tank level. The level control device thencomputes how far to open the supply valve to correct any difference between actual and desiredtank levels.ElementsofAutomaticControlThe three functional elements needed to perform the functions of an automatic control systemare:A measurement elementAn error detection elementA final control elementIC-07 Page 4 Rev. 0
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