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

Relationships between these elements and the functions they perform in an automatic control system are shown in Figure 4. The measuring element performs the measuring function by sensing and evaluating the controlled variable. The error detection element first compares the value of the controlled variable to the desired value, and then signals an error if a deviation exists between the actual and desired values. The final control element responds to the error signal by correcting the manipulated variable of the process.

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Figure 4 Relationships of Functions and Elements in an Automatic Control System

Feedback Control

An automatic controller is an error-sensitive, self-correcting device. It takes a signal from the process and feeds it back into the process. Therefore, closed-loop control is referred to as feedback control.

Summary

Basic process control terms are summarized below.

Process Control Term Definitions Summary

A control system is a system of integrated elements whose function is to maintain a process variable at a desired value or within a desired range of values.

Control system input is the stimulus applied to a control system from an external source to produce a specified response from the control system.

Control system output is the actual response obtained from a control system.

An open-loop control system is one in which the control action is independent of the output.

A closed-loop control system is one in which control action is dependent on the output.

Feedback is information in a closed-loop control system about the condition of a process variable.

A controlled variable is the process variable that is maintained at a specified value or within a specified range.

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.