The purpose of this paper is to present a general framework for the design of a nonlinear disturbance observer for Euler-Lagrange systems, in particular, for mechanical, electro-mechanical, and power electronic systems. The generalized momentum plays a crucial role in realizing the proposed method, and the global stability is guaranteed under certain conditions. In the absence of parameter variations and/or model uncertainties, the proposed method guarantees global exponential stability. Otherwise, model uncertainties and parameter variations are merged with the input disturbance into a “lumped disturbance term”. Then under boundness assumption on the lumped disturbance term, the observer can asymptotically estimate to any desired accuracy the lumped disturbance. In the sequel of this paper, motivated by the proposed nonlinear disturbance observer, a robust tracking control for robot manipulators is proposed. Again, in the absence of parameter variations and/or model uncertainties, the global stability is guaranteed. Otherwise, using tools from singular perturbation theory, the proposed method ensures arbitrary disturbance attenuation, small tracking error, and boundness of all closed loop signals. The theoretical results are illustrated on friction compensation and robust tracking of two degrees of freedom planer robot manipulator with short comparison with a classical, linear disturbance observer.
