In biomechanics and robotics, elasticity plays a crucial role in enhancing locomotion efficiency and stability. Traditional approaches in legged robots often employ series elastic actuators (SEA) with discrete rigid components, which, while effective, add weight and complexity. This paper presents an innovative alternative by integrating continuously compliant structures into the lower legs of a bipedal robot, fundamentally transforming the SEA concept. Our approach replaces traditional rigid segments with lightweight, deformable materials, reducing overall mass and simplifying the actuation design. This novel design introduces unique challenges in modeling, sensing, and control, due to the infinite dimensionality of continuously compliant elements. We address these challenges through effective approximations and control strategies. The paper details the design and modeling of the compliant leg structure, presents low-level force and kinematics controllers, and introduces a high-level posture controller with a gait scheduler. Experimental results demonstrate successful bipedal walking using this new design.

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.

This work introduces a new design paradigm for robotic legs. Our concept extends upon classical series elastic actuation and directly integrates the series compliance into the structure of the leg. This integration reduces the mechanical design complexity and can potentially reduce the overall weight of the leg. In this paper, we introduce a prototype leg with a continuously compliant shank and derive and analyze a non-linear beam model that is used to predict its contact forces. The model is validated in two static experiments: one on the isolated shank and one on the full leg. It shows good agreement with measurements. In our validation, we also studied the influence of the model discretization, showing that about 10 nodes are sufficent. Furthermore, we introduce the concept of a force workspace: the range of forces that can be created by controlling the joint angles of the leg. Due to the coupling of nonlinear deformations and nonlinearities in the kinematics, this workspace is non-trivial. In particular, we demonstrate that it is bounded by a force-singularity in which the force/joint-angle relationship cannot be inverted. The results presented in this work can be applied in the development of state-estimators, set-point filters and controllers, and they can inform the future design of suitable geometries of compliant elements.