Optimal Design of a Nonlinear Series Elastic Actuator for the Prosthetic Knee Joint Based on the Conjugate Cylindrical Cam

Active actuation is an essential method to enhance the performance of the prosthetic knee joint. In this paper, a new nonlinear series elastic actuator based on the conjugate cylindrical cam (N3CSEA) is proposed for the actuation of the prosthetic knee in level-ground walking. The N3CSEA, which is optimized with multi-objective of reducing its actuation power and enhancing its mechanical properties, uses two conjugate cylindrical cams to nonlinearly actuate their corresponding helical springs, thus generating required output torques for the prosthetic knee. Unlike conventional SEAs, the motor of the N3CSEA rotates unidirectionally, and its rotation speed is linearly related to the walking speed of the prosthetic knee, thereby reducing the total electric energy consumption of the motor as well as the difficulty of motor control. In addition, the first cam system in the N3CSEA is set responsible for the stance actuation and the second one is set for the swing actuation, which can realize seamless switching of stance and swing in level-ground walking. An N3CSEA-integrated prosthetic knee joint is built for verification, and the experiment shows that the N3CSEA-actuated prosthetic knee joint is of high electric power efficiency.