A Kinematic Analysis of Palm Degrees of Freedom for Enhancing Thumb Opposability in Robotic Hands

TL;DR

This paper quantitatively analyzes how palm degrees of freedom enhance thumb opposability in robotic hands, highlighting the importance of palm and finger DoF distribution for improved hand kinematics.

Contribution

It introduces a framework to evaluate palm DoF effects on thumb opposability and provides design guidelines for robotic hands considering palm motion.

Findings

Palm DoF significantly improves thumb-finger opposability.

Redistributing DoF to the palm involves trade-offs between workspace and redundancy.

Repositioning finger bases enhances opposability more than extending reach.

Abstract

This study investigates the kinematic role of palm degrees of freedom (DoF) in enhancing thumb opposability in a five-finger robotic hand. A hand model consisting of a five DoF thumb and four fingers with three to four DoF is analyzed, where palm motion is introduced between adjacent fingers. To quantitatively evaluate thumb-finger interaction, the overlap workspace volume is defined based on voxelized fingertip reachable regions. Seven cases are considered, including configurations with increased total DoF and configurations in which the total DoF is maintained by redistributing DoF from the fingers to the palm. The results show that palm DoF significantly improves opposability, particularly for the ring and little fingers, by repositioning their base locations rather than simply extending their reachable range. However, when the total DoF is constrained, redistributing DoF to the palm leads to trade-offs between overlap workspace expansion and kinematic redundancy. These findings indicate that palm DoF and finger DoF play distinct roles in hand kinematics and should be considered jointly in design. This study provides a quantitative framework for evaluating palm-induced opposability without relying on object or contact models and offers practical design guidelines for incorporating palm motion in robotic hands.