Type-II Fuzzy Inference System-Based Fractional Terminal Sliding Mode Control for Zero-Force Exoskeleton Robots

Upper-limb exoskeleton robots have a significant impact on rehabilitation, assistive technology, and human augmentation, as they can restore or enhance human physical abilities. This paper presents a novel control approach, called Adaptive Fractional Integral Terminal Sliding Mode (AFITSM), which combines an exponential reaching law with a unique interval type-2 Fuzzy Inference System (FIS). This controller is designed to achieve zero-force control of a 5-degree-of-freedom upper-limb exoskeleton robot, even in the presence of bounded uncertainties. The controller's integral terminal sliding surface ensures that the system converges in a finite time, allowing the exoskeleton to reach its desired state quickly, which is critical in time-sensitive applications. The exponential switching control term reduces chattering and tracking errors, while the AFITSM controller's adaptability, enabled by the interval type-2 FIS, allows it to adjust its parameters in real-time to handle uncertainties and external disturbances. Numerical simulations demonstrate the effectiveness and superiority of the proposed control method over traditional control approaches.