Feed back control:
Feed back control involves the detection of the controlled variable and counteracting of charges its it’s value relative to set point, by adjustment of a manipulated variable. This mode of control necessities that the disturbance variable must affect the controlled variable itself before correction can take place. Hence the term 'feedback' can imply a correction 'back' in terms of time, a correction that should have taken place earlier when the disturbance occurred.
Feed forward control :
Feed forward control system is a system in which corrective action is based on measurement of disturbances inputs into the process. This mode of control responds to a disturbance such that is instantly compensates for that error which the disturbance would have otherwise caused in the controlled variable letter in time. Feed forward control relies on a prediction. As can be seen from the figure of feed forward control a necessary amount of input goes to the process. This measurement goes to the controller which gives output to the control valve. The control valve regulates the flow.
Feed back control :
In feed forward control no difference between the desired result and actual result need exist before corrective action is taken in feed back control a difference must exist. Hence, open loop or feed forward control is capable of perfect control, but feed back is not. Due to economic impartibility of precision , predicting the amount of correction necessary to achieve satisfactory results with feed forward control, feed back control is most often used. In order to properly choose the type of feed back controller for a particular process application, two factors time and gain must be considered.