Process Controls PRINCIPLES OF CONTROL SYSTEMSRelationships between these elements and the functions they perform in an automatic controlsystem are shown in Figure 4. The measuring element performs the measuring function bysensing and evaluating the controlled variable. The error detection element first compares thevalue of the controlled variable to the desired value, and then signals an error if a deviation existsbetween the actual and desired values. The final control element responds to the error signal bycorrecting the manipulated variable of the process.Figure 4 Relationships of Functions and Elementsin an Automatic Control SystemFeedbackControlAn automatic controller is an error-sensitive, self-correcting device. It takes a signal from theprocess and feeds it back into the process. Therefore, closed-loop control is referred to asfeedback control.Rev. 0 Page 5 IC-07
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