A 2-DoF Elbow Exoskeleton with Spherical Scissor Mechanism for ADL Assistance

This paper presents the design and experimental validation of a 2-DoF elbow exoskeleton with a 2-DoF spherical scissor mechanism (SSME) to assist chronic stroke patients with upper limb impairments in activity-of-daily-living (ADL) tasks. Since forearm pronation/supination was missing in most portable exoskeletons and the existing designs with forearm rotation were too bulky, we propose a 2-DoF elbow exoskeleton with a spherical mechanism for elbow and forearm assistance. Benefiting from the spherical scissor and cable-driven mechanisms, the SSME can assist in elbow flexion/extension and forearm pronation/supination, up to 97.467% ADL workspace, with a mechanical design as light as 0.543 kg, practical for daily use. Besides, the proposed sensing method and kinematic model for the spherical scissor mechanism can measure human joint motions, solving the sensing challenge for forearm motion in most existing upper limb exoskeletons. Furthermore, with inverse kinematics and robot-joint-space impedance control, the SSME can independently control the elbow and forearm angle with minimal influence on another, regardless of its coupled nature. Moreover, our experiments have validated the proposed exoskeleton design in terms of the force control performance of actuators, robot joint impedance control response, and robot assistance in an ADL task.