Robust control of nonlinear strict-feedback systems with measurement errors

This paper presents a new method for robust control of a class of uncertain nonlinear systems in strict-feedback form with state measurement errors. The measurement feedback control problem is solved by recursively designing input-to-state stability (ISS) induced nonlinear state estimators and virtual control laws. With the gain assignment technique, the closed-loop system can be transformed into an interconnection of ISS subsystems, the ISS and input-to-output stability (IOS) of which can be guaranteed by the cyclic-small-gain theorem. Moreover, the IOS gain from the measurement error of the system output (the first state) to the system output can be designed to be linear and arbitrarily close to the identity function.