Comparison of Torque Controllers for an Ankle Exoskeleton with a Series Elastic Actuator Driven by a Uni-directional Bowden Cable during Walking

Exoskeletons have been used for human performance restoration and enhancement for many years. Due to the dynamic nature of human locomotion, torque control is widely used in lower-limb exoskeleton assistance during stance phases of walking. In these systems, series-elastic actuators (SEA) are commonly used to provide low error torque tracking in the presence of unknown and changing human dynamics. Good torque tracking performance is important to the fidelity of gait-related biomechanical experiments and the effectiveness of clinical gait recovery training. Control of lower-limb exoskeletons is normally hierarchical, with high level controllers determining behavior-related desired torques and torque control lying at a lower level. Many low-level control methods have been proposed for existing exoskeleton systems. However, relatively little has been reported on the relative performance of different torque controllers on the same platform under practical walking conditions, making differentiation among candidate methods difficult. Moreover, the interactions between highand low-level controllers are unknown. This study aimed to compare the torque-tracking performance of prominent torque controllers, under realistic experimental conditions, with multiple high-level controllers, in a single exoskeleton platform [1]. These results are expected to help guide the selection and tuning of lower-limb exoskeleton torque controllers for locomotion assistance, which will benefit gait-related biomechanics research and clinical training.