Robotic Locomotion with Structurally Compliant Legs

• Applicant: Professor Dr. C. David Remy
• Subject Area: Automation, Mechatronics, Control Systems, Intelligent Technical Systems, Robotics ; Mechanics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 561973739

Compliance is a highly beneficial feature in legged locomotion. By incorporating elastic elements, legged systems can mitigate impacts from ground contact collisions, store and return energy to avoid negative actuator work, and shape the dynamics of periodic motions. In biological systems, compliance is inherent to the muscles, tendons, and ligaments of the musculoskeletal system, and biological gaits, such as walking or running, are specifically adapted to take advantage of elastic energy storage. In legged robotic systems, compliance is often introduced through so-called Series Elastic Actuation (SEA), where elastic elements are placed in series with the driving motors. SEA offers multiple advantages: it shields gears and motors from damaging impacts and decouples the motors' reflected inertia from that of the joint. In addition, the regulation of the elastic deformation allows for precise control of joint torques and contact forces. This is a crucial ability, as balance in legged locomotion is achieved in large part by modulating the interaction forces between the ground and the feet. If the elastic elements are large enough, they can also be used to temporarily store elastic energy. This concept is known as high-compliance SEA and its goal is to enable the storage and return of energy over the course of the gait cycle. However, traditional SEA systems face challenges. The inclusion of elastic elements increases actuator mass and volume, requiring additional components such as axles and bearings. This mechanical design is particularly challenging, when the springs are designed to store larger amounts of energy. This project introduces a novel interpretation of SEA: instead of embedding compliance within a robot's drive-train, we propose to design the leg itself as the primary elastic component. This approach leverages the mechanical structure of the leg to undergo controlled deformations, allowing it to store and release elastic energy. By modeling, measuring, and controlling the structural deformation, we aim to achieve precise control over ground contact forces. This generalization of SEA will greatly reduce the complexity and mass of robotic legs, it will enhance their mechanical robustness, and it will enable the use of energetically economic gaits. To enable this simplified mechanical design, we must tackle significant challenges in terms of modeling, measuring, and controlling the deflection of the structural compliance. In the proposed project, we will develop new tools and methods to overcome these challenges and we will investigate how to optimally design and manufacture such a new type of robot. In addition, we will identify gaits that take advantage of the elastic energy storage and of the lightweight legs. The projected benefits of improved energetic economy and increased locomotion speed will be demonstrated on a bipedal robot that will be developed and implemented as part of the project.

• DFG Programme: Research Grants