Kinematic study of a soft-and-rigid robotic digit for rehabilitation and assistive applications

This paper presents the kinematic study of a pneumatically actuated soft-and-rigid robotic digit designed to be used in exoskeleton-based hand rehabilitation and assistive applications. The soft-and-rigid robotic digit is comprised of three inflatable bellow-shaped structure sections (soft sections) and four semirigid sections in an alternating order which correspond to the anatomy of a human finger. The forward and backward bending motions at each soft section (joint) are generated by pressure and vacuum actuation, respectively. The goal here is to investigate the compatibility of the soft robotic digit's kinematic parameters such as range of motion, center of rotation, and lengthening at the joints with the required anatomical motion of the human finger to ensure proper function and safe interaction. The soft robotic digits were fabricated using silicone rubber materials in a compression molding process for the experimental study. The kinematic parameters of both a human and soft robotic index finger were measured using a motion capture system. The obtained results show that the robotic digit was able to provide the required range of motion: 0-90° at the metacarpophalangeal (MCP) joint, 0-100° at the proximal interphalangeal (PIP) joint, and 0-70° at the distal interphalangeal (DIP) joint. Furthermore, the data show the center of rotation of each soft section (robotic joint) was remotely coincident with that of the corresponding index finger. The lengthening of the three soft sections of the robotic digit were measured to be 7mm, 7mm, and 2mm for the MCP, PIP, and DIP, respectively. The corresponding values for the dorsal skin lengthening of a human index finger is 11mm, 15mm, and 5mm and are longer than the achieved lengthening in the robotic digit.