Design of a passive spatial 2-DoF singularity-configurable gravity compensator for forearm and shank with roll-pitch motion

Abstract

Passive gravity compensators (PGCs), distinguished by their compact and lightweight design, have been extensively applied in rehabilitation exoskeleton systems. However, existing spatial PGCs developed by pseudo-spatial gravity compensation mechanisms encounter singularities when the limb is oriented vertically, hence severely restricting their workspaces. This research presents a PGC specifically designed for the forearm and shank with roll-pitch motion, capable of positioning its singularity outside the limb's workspace by adjusting the twist angle while preserving perfect gravity compensation. A static balancing condition considering all masses is established, followed by an angle mapping mechanism utilizing a bevel differential and a timing-belt drive to achieve the required angular relationship. The perfect gravity balance is validated through simulations. Natural rubber bands (NRBs), characterized by their lightweight design, are utilized in the fabrication of zero-free-length springs (ZFLSs), with the maximum compensation error recorded at 8.42 % for the NRB measuring 30 mm in width and 3 mm in thickness. A wearable 2-DoF PGC prototype utilizing NRB-ZFLSs has been developed. The quantitative experiment revealed that the maximum compensation error is 10.43 % when the maximum gravity compensation torque is 5.04 Nm.