Design and optimization of 2DOF humanoid dual cross four-bar mechanism for knee joint of an exoskeleton

A 2 degrees of freedom (DOF) humanoid dual cross four-bar for a knee joint is designed, based on the instantaneous center of rotation (ICR) axis motion of a human knee joint. The two DOFs of the joint are flexion/extension and internal /external rotation. The DOF of flexion/extension is realized by the cross four-bar mechanism, and the DOF of internal/external rotation is realized by a set of adaptive roller mechanisms. There is the same humanoid configuration for both the lateral and medial cross four-bar mechanism. The structural parameters are optimized aiming to minimize the displacement trajectory error between the mechanism ICR axis and the human knee ICR axis by the genetic algorithm. The self-adaptive roller mechanism is composed of 3 groups of movable rollers to realize the internal/external rotation of the exoskeleton shank on the tight shank bandage so that it can adapt to different leg shapes. The simulation results show that the ICR synthesis tracking error of the humanoid cross four-bar mechanism is no more than 0.77mm. The wearing experiment shows the motion of the exoskeleton joint can well align that of the human knee joint, verifying the effectiveness of the proposed mechanism.