Automatic Control System:

An automatic control system is a preset closed-loop control system which needs no operator action. This supposes the procedure remains in the normal range for the control system. The automatic control system has two process variables related along with it:  a controlled variable and a manipulated variable.

A controlled variable is the process variable which is maintained at a specified value or inside a specified range.  In the previous instance, the storage tank level is the controlled variable.

A manipulated variable is the procedure variable which is acted on through the control system to manage the controlled variable at the specified value or inside the specified range. In the past example, the flow rate of the water supplied to the tank is the manipulated variable.