Experimental Evaluation of a Stiffness-Fault-Tolerant Control Strategy on an Elastic Actuator for Wearable Robotics

Abstract

Elastic actuator design is primarily motivated by safety in human-robot interaction and energy efficiency. However, the increased complexity when compared to direct drives, makes the system prone to technical faults in elastic and kinematic elements. On the other hand, Variable Stiffness Actuators (VSA) often introduce non-linear torque-deformation relationships. We propose a stiffness-fault-tolerant control strategy for elastic actuators with non-linear compliant characteristics, capable of adapting to stiffness changes. The control strategy is demonstrated experimentally in a MACCEPA-based elastic actuator designed to power a knee exoskeleton. We develop the general model-based control scheme based on impedance control and adapt it to the experimental actuation system. Experiential evaluations under oscillatory and sigmoid trajectories are considered to analyze performance. Results demonstrate that the control system is capable of accurately tracking a reference trajectory under stiffness variations and interaction disturbance.