Disturbance attenuating controller design for strict-feedback systems withstructurally unknown dynamics

We consider the problem of robust controller design for a class of single-i nput single-output nonlinear systems in strict-feedback form with structura lly unknown dynamics and also with unknown virtual control coefficients. Th e unknown nonlinearities in the system dynamics are approximated in terms o f a family of basis Functions, with the only crucial assumption made being that the parameters that characterize such a neural-network based approxima tion lie in some known compact sets. In this setup, we design a robust stat e-feedback controller under which the system output tracks a given signal a rbitrarily well, and all signals in the closed-loop system remain bounded. Moreover, a relevant disturbance attenuation inequality is satisfied by the closed-loop signals. We then extend these results to the case where only t he output variable is available for feedback. In this case, for tractabilit y, the nonlinear functions in the system dynamics are restricted to depend only on the measured output variable, which results in a strict output-feed back form. (C) 2001 Elsevier Science Ltd. All rights reserved.