Variable stiffness ankle actuator for use in robotic-assisted walking: Control strategy and experimental characterization

Abstract Despite the increasing interest in using powered ankle-foot orthoses for assistive purposes, their development and benchmarking still present core challenges. Powered orthoses have to be safe and provide adequate torque while keeping limited size and weight. The discordance of these requirements is a challenge for the development of these devices. This paper describes the control strategy and characterization of a compact variable stiffness actuator, to be used in an assistive ankle-foot orthosis for impaired subjects. The results of the characterization experiments show the advantageous behavior of the actuator and its performance in providing different relevant assistive torque profiles, with different actuator stiffnesses, during emulated walking experiments. However, some divergences in the results obtained in different testing conditions highlight the need for more general benchmarking techniques. Towards this objective, the paper also proposes a novel performance indicator that can be used to better evaluate the performance of robotic actuators both in quasi-static and in dynamic conditions. The article concludes with a call for research on new benchmarking techniques, to understand more in-depth series elastic’s actuators behavior under dynamic conditions.