Equivalence of different two-degree-of-freedom control structures

Three different approaches in control design are considered, which correspond to different control structures: (1) feedback controller with filtering of the reference signal, (2) two-degree-of-freedom internal model control and (3) generic control scheme. It is shown that all these structures are equivalent if the control aim e.g., the tracking and the disturbance rejection behaviors, respectively, are the same in the three cases. Also inverse unstable and dead time processes are dealt with.