Design and Optimization of Single-degree-of-freedom Six- bar Mechanisms for Knee Joint of Lower Extremity Exoskeleton Robot.

The irregular contact surface between the femur and the tibia of the human body produces J-shaped instantaneous centers of rotation (ICR) of the knee joint. Therefore, two single- degree-of-freedom (SDOF) six-bar mechanisms, which are composed of a trajectory fitting four-bar mechanism and a transmission four-bar mechanism, are used to fit the centers of the human knee joint and increase the load-bearing performance. Firstly, the displacements of ICR of the human knee joint are measured and calculated for the sake of the dimensional synthesis of the trajectory fitting four-bar mechanism. Then, two structural layouts of transmission four-bar mechanism are designed according to different selection of the driving rod in the trajectory fitting four-bar mechanism, and both the transmission four-bar mechanisms are optimized. The results show that after the two- step optimization, the SDOF six-bar mechanism with the crank as the driving rod in the trajectory fitting four-bar mechanism has a better performance in smoothing the knee joint torque curve and superior transmission efficiency.