Using ____ loop control, an error signal is generated when the output signal varies from the reference input signal, and the error signal is fed to the controller to reduce the error.

A control system manages, commands, directs, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large industrial control systems which are used for controlling processes or machines. They are studied in Control Engineering classes.
 

There are two types of control systems:
Open loop control systems (non-feedback control systems)
Closed loop control systems (feedback control systems)