Design and Analysis of Hybrid Rigid-Soft Self-Aligning Index Finger Exoskeleton

Abstract

Disrupted hand motor functions may be restored through exoskeleton-assisted rehabilitation training. However, the variability of soft tissue in human joints or across individuals and development of an exoskeleton that combines human machine motion compatibility and dynamic compliance pose persistent challenges. We introduce a hybrid single-motor-driven rigid–soft exoskeleton for the index finger to assist in rehabilitation training. A rigid parallel mechanism directly drives the soft component of the metacarpophalangeal (MCP) joint. In addition, we adopt an interlocking mechanism to induce deformation in leaf springs, enabling the coordinated flexion and extension of multiple joints. A motion analysis based on the modified Denavit–Hartenberg convention confirms that the proposed parallel mechanism can compensate for the misalignment displacement of the MCP joint. Based on the displacement and force applied to the soft component by the designed rigid parallel mechanism, kinematic and static analyses along with dimensional optimization are performed on a dual segment parallel leaf spring. A prototype exoskeleton undergoing tests demonstrated Pearson correlation coefficients of 0.998, 0.991, 0.986, for the MCP, proximal and distal interphalangeal (PIP/DIP) joints, respectively. The corresponding joint flexion angles were 68.19°, 81.91°, and 41.64°. The exoskeleton self-aligns with the index finger joints, properly assisting the natural bending motion of the finger to meet rehabilitation training needs of patients. The proposed exoskeleton can assist with a fingertip force of 6.2 N, thereby satisfying grip requirements, while the reduced force on the dorsal surface of the index finger enhances comfort during use. The proposed solution is promising for developing hand exoskeletons.