Output feedback design for exact state stability of flat nonlinear systems

The notion of strong observability is generalized for nonlinear systems. It allows to write the state of the system as a function of the measurable outputs and their time derivatives only, therefore allowing the construction of an unknown-input observer once such derivatives are estimated. It is shown that the output based control design problem for robust finite-time state stability for nonlinear systems can be solved in two steps: the design of an unknown-input observer and the design of a full state controller computed from a set of new outputs with suitable invertibility properties. The problem of finding those new outputs can be tackled, for instance, for systems that are flat, or equivalently fully linearizable by dynamic feedback.