Open Loop Control System
A control system in which the control action is totally independent of output of the system then it is called open loop control system. Manual control system is also an open loop control system
Closed Loop Control System
Control system in which the output has an effect on the input quantity in such a manner that the input quantity will adjust itself based on the output generated is called closed loop control system. Open loop control system can be converted in to closed loop control system by providing a feedback. This feedback automatically makes the suitable changes in the output due to external disturbance. In this way closed loop control system is called automatic control system. Figure below shows the block diagram of closed loop control system in which feedback is taken from output and fed in to input.
Also Read: Basics of Alarms & Trips in Control Systems