Self-adaptive under actuated biomimetic soft robotic hand exoskeleton to improve dexterity and grasping capabilities

This paper introduces a soft hand exoskeleton based on an underactuated biomimetic pulley differential mechanism for fingers flexion including the thumb. The mechanism is powered by a single motor, enabling adaptive movement of the thumb, index and middle fingers to accommodate irregularly shaped objects. Unlike the existing pulley differential mechanisms, the proposed pulley differential design couples the thumb with the index and middle fingers and enforces a mechanical force constraint, where the thumb force is always equal to the combined forces of the index and middle fingers. This provides equilibrium grasp across infinite hand poses without individual finger control. To achieve a range of grasps, the device is equipped with three solenoids that allow deactivation of individual fingers. The grasping kinetics for the proposed mechanism are tested experimentally on eight able bodied subjects. The results demonstrate that the mechanism achieves a stable grasp on both regular and irregular shaped objects, with a minimal and acceptable alteration in the finger positions compared to their normal human poses. Power grasps exert grasping forces of 10.3 N and 11.5 N on regular and irregular shapes, respectively, with maximum finger pose changes of approximately 9.8% and 8.9% from the full range of joint motion. Pinch grasp deactivates the middle finger, enabling the thumb and index finger to provide a sufficient grasp of approximately 7 N on both sides. This mechanism concept show potential for performing diverse daily activities requiring various hand poses.