Integrated direct/indirect adaptive robust control of a class of nonlinear systems preceded by unknown dead-zone nonlinearity

This paper presents an integrated direct/indirect adaptive robust control (DIARC) scheme for a class of nonlinear systems preceded by unknown non-symmetric, non-equal slope dead-zone nonlinearity. Due to the inherent nonlinear parametrizaton nature of the unknown dead-zone nonlinearity, existing robust adaptive control methods have been focusing on using various linearly parametrized models with on-line parameter adaptation for an approximate inversion of the unknown dead-zone nonlinearity. As a result, even in the absence of other uncertain nonlinearities and disturbances, asymptotic output tracking can never be achieved. Departing from those approximate dead-zone compensations, this paper makes full use of the fact that, though not being linearly parametrized globally, the unknown dead-zone nonlinearity can be linearly parametrized within certain known working ranges. Thus indirect parameter estimation algorithms with on-line condition monitoring can be employed for an accurate estimation of the unknown dead-zone nonlinearity. With these accurate estimates of dead-zone parameters, perfect dead-zone compensation is then constructed and utilized in the development of an integrated direct/indirect adaptive robust control algorithm for the overall system. Consequently, asymptotic output tracking is achieved even in the presence of unknown dead-zone nonlinearity. Furthermore, the proposed DIARC achieves certain guaranteed robust transient performance and final tracking accuracy even when the overall system may be subjected to other uncertain nonlinearities and time-varying disturbances. Comparative simulation results obtained validate the effectiveness of the proposed approach as well.