Hidden models in two degrees of freedom design: {R;G,H} controller

The stable and unstable hidden modes of the {R; G, H} controlled system are characterized by considering cancellations in products of transfer matrices. This characterization is complete because of the assumption that the plant description is controllable and observable, and that R, G and H are implemented via irreducible realizations. Under these assumptions the hidden modes of the controlled system are due exclusively to the system interconnection. The characterizations given show how hidden modes are introduced in the controlled system. This information can be used by the control systems designer to avoid hidden modes if possible; otherwise, the effect of the hidden modes on the system performance can be determined.