A Portable Upper-Limb Exoskeleton With Pneumatic-Driven Variable-Stiffness Units for Long-Term Working in Fixed Overhead and Ahead Postures

Long-term working in fixed postures often leads to muscle fatigue and musculoskeletal disorders. Existing passive exoskeletons for overhead tasks introduce resistance during arm lowering, while active exoskeletons face limitations due to their weight and lack of comfort. To address these challenges, this study introduces the design of three pneumatic-driven variable-stiffness units: a bending actuator, a tensile actuator, and a teeth-engagement clutch. Based on them, a portable semi-active upper-limb exoskeleton was developed for sustained overhead and ahead postures. The exoskeleton is controlled by a portable pneumatic system and the total mass is 3.35 kg. Feasibility of applying the proposed variable-stiffness units in exoskeleton design was verified by promising results of performance evaluation, e.g., the bending variable-stiffness actuator achieved 50.98-Nm/rad bending stiffness and 87.9-fold stiffness variation ratio at a relatively low vacuum pressure of 40 kPa. Compared to the condition without wearing the exoskeleton, averaged muscular activation level reduced by 42.3 ± 2.4% over twelve healthy subjects while maintaining 4 commonly-used postures in assembly work, indicating the promise of applying the proposed exoskeleton for long-term working assistance in fixed overhead and ahead postures.