Design and evaluation of a four-DoF upper limb exoskeleton with gravity compensation

Gravity compensation (GC) mechanisms are commonly employed to either support limb gravity in passive exoskeletons or decrease motor power for safe human-robot interaction in active exoskeletons. This paper proposes a four-degree-of-freedom (DoF) upper limb exoskeleton with a theoretically perfect GC system, incorporating three DoFs in the shoulder and one DoF in the elbow. Paired with the anthropomorphic structure, the compact and adjustable GC system housed within the exoskeleton's linkages, reduces limitations on the limb's motion range and facilitates the integration of actuators for an active system. First, the GC system is designed through the analysis of potential energy equations, employing five cable-pulley-spring based units interconnected by parallel and differential mechanisms. Subsequently, the mechanical structure of the exoskeleton is developed, and the theoretically perfect GC in the quasi-static state is verified through numerical calculations. Then, the prototype is fabricated, and its actual performance is evaluated through experiments. The experimental results demonstrate the effectiveness of the exoskeleton with GC. Finally, a potential solution for the integration of actuators is demonstrated, and the limitation of the proposed system is also well discussed.