Feasibility Study of an Active Ball Joint Mechanism for a Wheelchair-Mounted 2-DOF Shoulder Exoskeleton

Wheelchair users with upper-limb muscle weakness can benefit from exoskeletons to support them in their activities of daily living (ADL) when, for example, shoulder assistance is needed for reaching and lifting tasks and tasks with prolonged positions against gravity. For this technology to be accessible to wheelchair users, exoskeletons must be compact, easy to mount, lightweight, and cost-effective. Novel ball-joint designs have been proposed as a compact solution for robotic actuators and joints. In this work, we evaluate the feasibility of using an active ball joint mechanism for the actuation of shoulder flexion/extension and abduction/adduction in a 3D-printed 2 degrees of freedom (DOF) exoskeleton. On average, three healthy participants wearing the exoskeleton could reach 100% range of motion (ROM) of the required ADL workspace in shoulder flexion and 81% in shoulder abduction. Moreover, they had a delayed onset of muscular fatigue by 144s (113%) during muscular endurance tasks. During the exoskeleton's validation in two ADL tasks, the muscular activity in the anterior deltoid decreased by 48% during drinking tasks and 51% during combing tasks. Our results show that the proposed mechanism used in shoulder exoskeletons can provide a compact and accessible solution for assisting the upper limbs.