In this study, a nonlinear tracking controller is designed for a flexible-joint (FJ) robotic manipulator system subject to full-state constraints and external disturbances. First, a nonlinear disturbance observer (NDO) is introduced to guarantee that the respective disturbance can be estimated asymptotically. Then, state-dependent function (SDF) transformations are used to convert the original state-constrained system into a new unconstrained system. Subsequently, a NDO-based tracking controller is proposed to enable the output tracking error to converge to a predefined compact set that can be adjusted to be arbitrarily small. The boundedness of all the closed-loop system signals is rigorously proved. Finally, simulation results are presented that show the effectiveness of the proposed control algorithm.
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