Variable Stiffness Actuator applied to an active ankle prosthesis: Principle, energy-efficiency, and control

Series elastic actuators are very popular in rehabilitation robotics. Among other advantages, elastic elements between the actuator and the load permit to store and release energy during the task completion, such that the energy balance is improved and the motor power peak is decreased. In rhythmic tasks like walking, this reduces to design the spring stiffness such that it works at resonance. To comply with different gaits and cadences, it is therefore necessary to design Variable Stiffness Actuators (VSA). This paper proposes three contributions: (i) we apply a particular concept of VSA to an active ankle prosthesis; (ii) we discuss the relevance of using VSA to change the stiffness also within the gait cycle; and (iii) we elaborate some control strategies for this device. Our guideline is to track a mechanical design and a controller maximizing energy efficiency. We establish that a promising approach is simply to control the amount of energy stored in the elastic element.