Design and Modeling of A Lightweight Lower Limb Exoskeleton With Compliant Knee Joints

Lower limb exoskeletons for gait rehabilitation have been widely studied, including rigid and flexible exoskeletons. In order to improve the human-robot coordination during exoskeleton wearing, avoid the constraints on joints caused by rigid structures, this paper designed a lightweight lower limb exoskeleton with compliant knee joints. By adopting a lightweight design, the main body is composed of carbon fiber and aluminum alloy, resulting in a total weight of only 7KG. Different from the current knee joint cable-driven structure that install the joint motor on the back, we install the knee joint motor on the thigh and use two completely parallel Bowden cables to transmit force. This structure can reduce the mass and rotational inertia at the knee joint, improve the wearable characteristics of the exoskeleton, and reduce the additional frictional losses caused by long-distance force transmission of the Bowden cables. Meanwhile, the exoskeleton adopts a modular design, which facilitates its application in different assistive modes. Finally, through the joint encoders and motor PD control to track the joint trajectory, the measurement results show that the error for the hip and knee joints is within 2° and 3.5° respectively.