|Standalone controller (PID)|
A controller is an instrument used for controlling a process variable (measurement). Its continuously monitors the error signal and gives a corrective output to the final control element.
a measurement variable
Measurement variable: It is the demand variable measured and controlled.
Desired Variable: it is the demand signal (set point) to which the process variable is controlled.
Deviation: it is the error signal caused by the difference between the measurement and the demand signal.
Output: It is the corrective signal from the controller to the final control element.