Wearable Shoulder Exoskeleton with Spring-Cam Mechanism for Customizable, Nonlinear Gravity Compensation

Wearable, mechanically passive (i.e. spring-powered) exoskeletons may be more practical and affordable than active, motorized exoskeletons for providing continuous, home-based, antigravity movement assistance for people with shoulder disability. However, the biomechanical moment due to gravity is a nonlinear function of shoulder elevation angle and, thus, challenging to counteract proportionally across the shoulder elevation range of motion with a spring alone. We designed, fabricated, and tested an integrated spring-cam-wheel system that can generate a nonlinear moment to proportionally compensate for the expected antigravity moment at the shoulder. We then incorporated the proposed system in a benchtop model and a novel wearable passive cable-driven exoskeleton that was intended to counteract half of the gravitational moment during shoulder elevation movements. The rotational moment measured from the benchtop model closely matched the theoretical moment during simulated positive shoulder elevation. However, a larger moment (up to 12.5% larger) was required during simulated negative shoulder elevation to stretch the spring to its initial length due to spring hysteresis and friction losses. The wearable exoskeleton prototype was qualitatively tested for assisting shoulder elevation movements; we identified several aspects of the prototype design that need to be improved before further testing on human participants. In future studies, we will quantitatively evaluate human kinematics and neuromuscular coordination with the exoskeleton to determine its suitability for assisting patients with shoulder disability.