Nonlinear Terminal Model‐Based Second‐Order Sliding Mode Control for Robust Tracking in Robotic Exoskeletons

ABSTRACT This paper introduces a novel second‐order sliding mode control (SOSM) strategy that employs a nonlinear terminal model‐based switching surface to achieve precise and reliable trajectory tracking for a two‐DoF upper‐limb exoskeleton robot. By integrating the robot's dynamic model into the switching manifold, the proposed approach simplifies the torque control law and effectively decouples chattering between the joint axes. The inclusion of a nonlinear terminal sliding term endows the closed‐loop dynamics with terminal‐type behavior, leading to fast convergence of the tracking errors and improved transient performance and disturbance attenuation. Unlike classical sliding mode control (SMC), which can suffer from significant discontinuities and stress actuators, the proposed SOSM–NTSM scheme provides smoother control actions. In line with high‐order sliding‐mode results, it achieves a significant reduction of chattering and asymptotic convergence of the model‐based sliding variable in the presence of bounded modeling uncertainties and external disturbances, together with locally exponential convergence of the tracking errors once sliding is established. Validations and a comparative study conducted on the iTbot exoskeleton robot demonstrate significant improvements in tracking accuracy, torque smoothness, fast convergence, and overall robustness. This makes the proposed approach particularly well suited for ensuring safe and comfortable human‐robot interactions in rehabilitation scenarios.