Controllers:

A controller is a device which generates an outcome signal based on the input signal it receives. The input signal is really an error signal that is the difference among the measured variable and the desired value or setpoint.

This input error signal represents the amount of deviation among where the process system is really operating and where the process system is desired to be operating. The controller gives an output signal to the last control element that adjusts the procedure system to decrease this deviation. The features of this output signal are dependent on the type, or mode, of the controller. This lesson describes the simplest type of controller that is the two positions, or ON- OFF, mode controller.

Figure: Process Control System Operation