Design of Novel Knee Joint Mechanism of Lower-Limb Exoskeleton to Realize Spatial Motion of Human Knee

The rotation axis of human knee joint varies according to knee flexion angles. That is, human knee movement is spatial with dominant flexional rotation. Knee joints of most lower-limb exoskeletons were, however, realized with a simple revolute pair for design simplicity. Wearing the knee joint with a simple revolute pair constrains inevitably natural parasitic motion of human knee joint. Rigid constraints imposed due to the simple knee joint lowers wearability and comfortability. In addition, it may act as potential risks to harm the knee joint structure of the wearer. The concept of polycentric knee is a well-known approach to mimic the variation of knee rotation center. Polycentric knees, however, realize only the planar trace of projected points of rotation axis. That is, parasitic knee rotations of varus and internal rotation occuring naturally during knee flexion cannot be realized by polycentric knees. In order to resolve this, a novel spherical knee joint of exoskeleton is introduced in this paper to realize knee spherical movements. For the design, change in instantaneous rotation axes during knee flexion is formulated to a spherical trace as a function of knee flexion angles. A spherical four-bar linkage is suggested to realize the axis trajectory, not the point trace. A method to find instantaneous rotation axis and rotation matrix of coupler link is derived, when the angle of input link is given. Using the kinematic relations, kinematic parameters of the mechanism are optimized to minimize angle difference between the instantaneous axis of coupler and the required axis of knee rotation. Finally, based on the synthesized kinematic parameters, a prototype design was introduced in this paper.