Quantitative Evaluation of Standing Stabilization Using Stiff and Compliant Actuators

In this paper we evaluate the benefits of series elastic actuation in performing a balancing task on a humanoid robot. By having the possibility to replace the type of transmission at the ankles level, it was possible to repeat the very same experiment in two different conditions: (1) Using series elastic actuators (SEA); (2) Replacing the elastic elements with rigid transmissions. The experiments consisted in perturbing a balanced posture with an impulsive force. Perturbations were applied in two different scenarios: hitting the robot on the upper body and at the support platform, thus acting above and under the actuated joint. The applied perturbations were controlled to be repeatable and the static stiffness of the rigid and elastic actuators were tuned to match. With these assumptions, static responses were the same in the two conditions; differences appeared only during the dynamic response and are motivated by different proportions between active and passive stiffness. In both scenarios, results show that series elasticity simplifies the role of the balancing controller by low pass filtering the dynamics of the zero moment point, consistently observing a more stable balance recovery with SEA through all the experiments.