Design and Control of a Novel Robotic Knee-Ankle Prosthesis System

In this paper, design and control of a novel robotic prosthetic knee and ankle system is presented. Robotic prosthetic knee and ankle system consists of a robotic knee prosthesis and a robotic ankle prosthesis, connected by a prosthetic pylon, adjusting shank length and alignment between knee and ankle. We present a novel mechanism incorporating a series elastic actuator and crank mechanism, which is applied to transform linear motion of series elastic actuator to rotary motion of knee joint and ankle. The crank mechanism contributes a variable transmission ratio of each joint. This feature could uniquely give the knee both: the torque necessary to assist with standing up from a chair and the speed necessary to swing the leg forward during walking. Because of variable transmission ratio, ankle joint torque is increasing while ankle angle is flexed from plantar flexion to dorsiflexion, whose feature has a similar increase trend with human's ankle joint torque-angle relationship. Robotic knee prostheses and robotic ankle prostheses are mechanically separable integrated system, including actuation, electronics and control respectively and they could be used independently. In order to coordinate movement of knee and ankle as one system in this paper, The knee is controlled as a master and ankle is a slave and they are connected by electrical cable to send control signal from master to slave. Prototype has been built and a preliminary experiment has been implemented with a transfemoral amputee.