Structurally stable nonlinear regulation with step inputs

It is shown that for a nonlinear regulator to be structurally stable in its response to step reference and disturbance signals, it is both necessary and sufficient for the controller to be of feedback type and to incorporate integrating action in each error channel. These results constitute an extension to nonlinear systems of the well-known method of “integral control” used in the design of linear regulators; and as such they provide an additional instance of the Internal Model Principle of control theory.