Colloquia

Samenvatting

Robotic exoskeletons are increasingly used to improve human gait and reduce the metabolic cost of walking. To meet the demands of safety, low weight, and energy efficiency, the performance of the actuators in the robotic exoskeleton is critical. This research therefore investigates the feasibility of the Maxon High Efficiency Joint (HEJ) compared to state-of-the-art actuators used in lower limb exoskeletons. The comparison focuses on apparent impedance, torque-tracking performance, backdrivability, torque-control bandwidth and inertia.

A set of system identification tests was first carried out to assess whether compensation for friction and magnetic field saturation would improve performance. The analysis confirmed that both effects were significant, motivating the identification and testing of a LuGre friction model and a fifth-order saturation model. 

All compensation models improved the apparent impedance, with the combined LuGre and saturation model yielding the greatest reduction. Relative to the Series Elastic Actuators used in the Symbitron exoskeleton, the apparent impedance decreased from 9 Nm to 0.87 Nm at 7 Hz. Torque-tracking performance was evaluated using a dynamic load case, in which the Maxon HEJ showed smaller errors
both as a percentage of its rated torque and of the required torque compared to state-of-the-art actuators. The static and dynamic backdrive torques were found to be within the same range as those of state-of-the-art actuators.

These results show that the Maxon HEJ, even without compensation models, performs comparably to or better than state-of-the-art exoskeleton actuators and is therefore a feasible option for lower limb exoskeletons. Future work should compare the compensation models used in this research with Maxon’s JPVT mode and validate the findings in a full proof-of-concept lower limb exoskeleton.