Robust two-stage non-linear control of a hydraulic servo-system

Unparalleled practical advantages of a hydraulic servo-system, epitomized by high power, fast response and compact size, have led to its widespread use in industrial and military applications, despite its limitations such as leakage, contamination and low efficiency, etc. Classical control approaches have been popularly adopted in most practical hydraulic servo-systems and have enjoyed remarkable success. On the other hand, it is generally agreed that the nonlinear and uncertain dynamics of a hydraulic system must be explicitly addressed in order to achieve robust performance over a range of operating conditions. In this paper, a robust two-stage non-linear control law is proposed, based on the Lyapunov redesign technique. It turns out that the proposed two-stage control strategy guarantees uniform ultimate boundedness for a hydraulic servo-system with more general non-linearities and uncertainties that does not satisfy the matching condition. Furthermore, an explicit stability criterion is provided for such a system. Simulation study shows that the tracking performance of the proposed non-linear controller is superior to those of a popular proportional-plusderivative controller (PDC) or a non-linear controller designed for a nominal system (NC) with respect to rectangular and sinusoidal inputs. In addition, the proposed non-linear controller is shown to be more robust to uncertainties.