Contact Force Control with Continuously Compliant Robotic Legs

This paper presents a novel robotic leg design and an associated control approach, which aims at providing an extension to the classical series elastic actuation concept. We propose to directly integrate the series compliance into the structure of the robotic leg itself, as opposed to co-locating spring and motor as done in traditional series elastic actuators. Our approach will eliminate mechanical design complexity and lead to a reduction of mass in the legs. This will, as a secondary benefit, improve the energy efficiency of locomotion. The primary contribution of this work is a model-based controller that can stably and precisely regulate the ground contact forces during stance. This control approach is demonstrated in a set of test-bench experiments, in which we control the contact forces of a modified version of the robotic leg ScarlETH. Here, the rigid shank is replaced by a continuously compliant element made of spring steel. This work presents the first step towards a new generation of robotic legs with structural compliance.