Finger Grasp Kinematics Towards Exoskeleton Development

Abstract

This paper investigates the relationships between index finger joint rotations to determine whether a one degree of freedom (DOF) exoskeleton could produce natural finger motions for a range of different users and grasping conditions.Four healthy subjects each performed ten trials involving the grasp of two cylindrical objects with diameters of 66.5mm and 47mm. Finger trajectories for each trial were recorded using a motion capture system and were used to obtain joint rotational trajectories for the Metacarpo-phalangeal (MCP) Proximal-interphalangeal (PIP) and Distal-interphalangeal (DIP) joints.Joint ranges of motion (ROM) were largest for all subjects when grasping the smaller diameter object. This effect was also seen where subjects with longer fingers tended to use a larger MCP ROM for the same object. Subject 2 was an exception to this, using the smallest MCP and PIP ROM of all subjects when grasping the large diameter object: this anomaly is thought be caused by difference in palmar engagement with the object.The profile of the MCP-PIP trajectories were similar when normalised to their range of motion, with a maximum y-axis error of 22%. This implies that the MCP-PIP relationship for a cylindrical grasp can be approximated be a general scalable polynomial. We conclude that through parabolic coupling between MCP and PIP joints, a one-DOF exoskeleton is capable of producing functional grasping movements. Through an adjustable coupling mechanism between MCP and PIP, it is also believed that an exoskeleton can be successfully adapted for differing object and finger sizes.