Control System in which output quantity has no effect on the input quantity is called Open Loop Control System. Open Loop Control System has no facility to correct automatically the error generated in the output. From output of the system no feedback is given back to the input for correction.
In Open loop control system the output can be varied by varying the input. But due the external disturbance system output may change. Any variation in the output from the desired once again attained by varying the inputs manually.