Adaptive sliding mode tracking control of hydraulic servosystems with unknown non-linear friction and modelling error

This paper deals with the use of adaptive discrete-time sliding mode tracking control in order to assure good tracking performance as well as to guarantee robustness against non-linear frictional forces and modelling error. The control scheme ensures that the absolute value of the sliding function decreases when it is outside the sliding boundary layer and the steady state value of the sliding function is bounded by the sliding boundary layer. Application of the scheme to a hydraulic servosystem has shown that adaptively estimated frictional forces compare favourably with those obtained from direct measurement. A significant reduction in tracking error is achieved through the use of non-linear friction compensation.