In this paper, a novel robust nonlinear motion controller with disturbance observer (DOB) for positioning control of a nonlinear single-input-single-output (SISO) mechanical system is proposed. The controller is designed in a backstepping manner. At the first step, a PI controller is designed to stabilize the position error. Then at the second step, a novel robust nonlinear velocity controller with DOB is designed to stabilize the velocity error. By using some elegant nonlinear damping terms, the input-to-state stability (ISS) property of the overall nonlinear control system is proved, which leads to a major contribution of construction of a theoretically guaranteed robust nonlinear controller with DOB for the first time in the literature. The performance of the proposed controller is verified through application to a magnetic levitation system. Comparative studies with an adaptive robust nonlinear controller are also carried out. It is shown that the proposed novel controller while being simple is superior over the adaptive robust nonlinear controller for the experimental setup under study.

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