Output feedback control: A state-variable approach

This dissertation deals with the output feedback control of nonlinear systems. The employment of a state-variable approach is the trait d'union which proves to be crucial for the understanding of the limitations of other approaches and the development of a new methodology. The class of systems considered throughout this work is that of incompletely observable nonlinear systems, i.e., systems for which observability is not a global property. In the SISO case, tools leading to the formulation of a separation principle are introduced. Their application to the control of stall and surge in the Moore-Greitzer model of jet engine compressors underlines the advantages of the proposed methodology. The approach is then extended to the stabilization of MIMO nonlinear systems affected by a class of time-varying disturbances. Finally, by introducing the concept of practical internal models, a solution to the output feedback tracking problem is devised. The idea exploits the tools introduced in other parts of the dissertation and represents a unifying framework for other existing approaches to the solution of this problem.