What´s the difference between a opened and closed loop control?
The signal flow has an start point and an end point. Therefore you also name this as an opened loop control. The problem is hereby: This control cannot react o disturbances.
In order to reach the desired position safely despite of disturbing influences, the circuit has to be complemented to a closed loop control. Which further components you need?
The block diagram of a closed-loop control in general can now completed: