Nonsingular fast terminal sliding mode control based on a novel nonlinear disturbance observer for robotic systems

This paper delves into the problem of high-precision tracking control for robotic systems, specifically addressing challenges posed by model uncertainties and external disturbances. To this end, a new nonsingular fast terminal sliding mode surface (NFTSM) with an unified structure is developed to circumvent singularity, without segmenting sliding manifold into multiple segments, which can improve steady-state accuracy and reduce the complexity of the sliding surface. Following that, a Lyapunov stable controller characterized by a fast convergence law has been developed to stabilize the tracking errors to zero with bounded time. To address model uncertainties and external disturbances, a novel nonlinear disturbance observer (DO) is devised. The DO only has one adjusted parameter and can effectively estimate lumped disturbances to provide a feedforward compensation. Crucially, there is no requirement for upper bounds of disturbances and their derivatives during the design steps. Finally, the effectiveness and advantages of the proposed scheme are verified by extensive simulations and experiments conducted on a 7-DOF robot manipulator. The results sufficiently illustrate that the proposed DO-based sliding mode control scheme has high tracking accuracy, good robustness, and disturbance-rejection abilities.