Experimental comparison of nonlinear motion control methods for a variable stiffness actuator

Variable compliant actuators play a key role in the development of efficient biomechatronic systems since energy can be stored in the compliant element thus leading to consumption reduction. In this paper, experimental results comparing passivity-based control (PBC) and feedback linearization (FL) for motion control of an actuator with variable torsional stiffness (VTS) aiming at applications like prosthetic knee joints are presented. The concept of VTS and the experimental setup are described and a mathematical model of the latter one is derived. Based on this, a control architecture consisting of an extended Kalman filter (EKF) to estimate the velocities, a friction compensation as well as the mentioned controller types is developed. Both control methods are analyzed in terms of accuracy, dynamics and their control torque. FL and PBC lead to a stable control with high performance whereas the robustness is low by reason of the model-based control design. FL is superior to the PBC in terms of accuracy and control torque, which is mainly due to the high sensitivity of PBC regarding the discrete position signals. In addition, it is shown that FL can be applied for stable operation near the second natural frequency for different stiffness values.