Basic control system definitions

controlled variable:

the quality or condition characterizing a process whose value is held constant by a controller or is changed according to a certain algorithm designed with the interests of the nature of the function the system is performing.

Controlled medium:

the process material in the control system in which the variable to be controlled exists.


An input that is established or varied by some means external to and independent of the feedback system.

Set point or Reference input:

A signal established as a standard of comparison for a feedback control system by virtue of it’s relation to command. The setpoint either remains the same or is varied with respect to time depending on a preset algorithm.

Actuating signal:

An algebraic sum of reference input and primary feedback. It is also called error or control signal.

Manipulated variable:

The quality or condition that is varied as a function of the actuating signal so as to change the value of the controlled variable.

Primary feedback signal:

The function of thecontrolled output which is compared to reference input to obtain the actuating signal.

Error detector:

An element that detects feedback; essentially a summing point which gives the algebraic sum of it’s inputs.


An unwanted variable in the system which tends to affect the system adversely by changing the controlled variable. Disturbance may due to change in set point, supply, demand, environment and the other associated variables.

Feedback element:

An element of the feedback control system which establishes a functional relationship between controlled variable and feedback signal.