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In this paper, a nonlinear adaptive robust control method is presented for a single-rod electro-hydraulic actuator with unknown nonlinear parameters. Previous adaptive control methods of hydraulic systems always assume that the system's unknown parameters occur linearly, but, in a practical hydraulic system, unknown nonlinear parameters, which enter the system equations in a nonlinear way, are common; for example, when the original control volumes are unknown or change, uncertain nonlinear parameters will exist. The proposed control method in this paper is to present a nonlinear adaptive controller with adaptation laws to compensate for the uncertain nonlinear parameters due to the varieties of the original control volumes. The main feature of the scheme is that a novel-type Lyapunov function is developed to construct an asymptotically stable adaptive controller and adaptation laws. Furthermore, by combining backstepping techniques and a simple robust control method, the whole system's controller and adaptation laws are presented, which can compensate for all unknown parameters and uncertain nonlinearities. The experimental results show that the nonlinear control algorithm, together with the adaptation scheme, gives a good performance for the specified tracking task in the presence of unknown nonlinear parameters.