Nonlinear output feedback tracking with almost disturbance decoupling

Addresses the design of global output feedback controls for a class of uncertain nonlinear single-input/single-output systems which are globally transformable into an observable minimum phase system whose nonlinearities depend on the output only. They may be affected by unknown time-varying disturbances or parameter variations entering linearly in the state equations. The proposed dynamic controller is of order ρ-1 (ρ is the relative degree of the given system) and ensures that the closed-loop system enjoys for any initial condition and for any smooth bounded output reference signal (with bounded time derivatives up to order ρ) the following properties: input-to-state stability with respect to disturbance inputs and almost disturbance decoupling, i.e., the influence of disturbances both on the L ₂ and on the L _∞ norm of the output tracking error is arbitrarily attenuated by increasing a positive scalar control parameter. When disturbances are zero the reference signal is exponentially tracked by the output and the equilibrium point corresponding to the reference signal is globally asymptotically stable.