Bio-inspired knee joint mechanism for a hydraulic quadruped robot

Over the last few decades, legged robots are becoming a promising solution for rough terrain navigation, however, existing legged machines often lack versatility to perform a wide range of different gaits. To build a highly dynamic and versatile legged robot, it is essential to have lightweight legs with optimized design and suitable actuators for the desired robot performance and tasks. The design goals are to achieve (1) a wide range of motion for bigger foot workspace which will increase rough terrain walking performance by increasing the number of reachable footholds for each step, (2) optimized joint torque curve since torque output is related to joint angle if linear actuators like pistons are used. In this paper, we focus on the knee joint and propose the adaptation and optimization of the so-called isogram mechanism. It exhibits a changeable instantaneous center of rotation (CICR), similar to a human knee joint. We will show how an optimization of design parameters lead to a knee joint design that satisfies the above-mentioned goals. The main contributions of this paper are the kinematic and torque analysis of the isogram mechanism that is actuated by a linear actuator; the optimization of the mechanism´s design parameters; a comparison between the proposed knee joint with the hinge-type knee joint of the quadruped robot HyQ; and experimental results of a proof-of-concept prototype leg featuring the proposed mechanism.