EcoWalk - biorobotic analysis of the interplay of foot and leg in human walking

• Applicants: Professor Dr. Alexander Badri-Spröwitz; Dr.-Ing. Daniel Renjewski
• Subject Area: Automation, Mechatronics, Control Systems, Intelligent Technical Systems, Robotics ; Biophysics ; Mechanics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449427815

The project EcoWalk aims to investigate and technically reproduce the function of the human leg’s segmental chain, focusing on biarticular muscle-tendon structures. We want to understand the mechanics and control of the ankle joint’s push-off (APO) and energy release and to reproduce this human swing leg catapult mechanism in a bipedal robot. The functional understanding gained will allow us to design light-weight and energy-efficient humanoid robots guiding the improvement of prostheses and exoskeletons supporting human locomotion. By combining computer simulations of human walking with the development of a bipedal walking robot, we studied the function of the plantar flexor muscle-tendon units during the impulsive APO. In a predictive neuromuscular simulation, we replaced active muscle-tendon units with passive linear springs. For our bipedal robot EcoWalker, we used tendon routings, muscle stiffness ratios, and joint angle trajectories derived from simulation results. We proved that a passive ankle joint with elastic tendon support is sufficient to achieve an impulsive APO [1, 2]. We found that the soleus muscle amplifies the ankle joint power, while the gastrocnemius muscle coordinates ankle and knee joint trajectories mechanically during push-off. These simulation results are validated through hardware design and testing as, based on simulation results, we are implementing a robot controller that switches off knee joint torque in selected parts of the stance phase to demonstrate \textbf{passivity} for walking. We identified key topological parameters, which we use in our custom-developed framework to enhance human gait models. Optimisation results guide the design of our real-world robot towards energy-efficient, agile, and robust bipedal locomotion. In recent computer simulations, we started investigating the role of the foot’s morphology leveraging global motion dynamics to generate the push-off behaviour of the ankle, i.e., the influence of toe, arch, and ligament structures. We will further investigate the influence of tendon slack length, joint radii, and muscle-tendon stiffness on APO and cost of transport. The identified parameters will help us tune the next-generation hardware design. With data collected from human walking experiments, we developed a simplified functional model that captures the dynamics of the human foot in walking, the human-specific motion of the centre of pressure, and explains how the rapid leg swing is powered. We analysed the interplay and contribution of functional dynamic contributors to foot motion during walking with our model. Results show an emerging, self-stabilising mechanism that extends the duration of the leg stance phase. We are now designing the next generation EcoWalker robot with improved mechanics and control in order to allow for robot experiments at higher walking speeds.

• DFG Programme: Research Grants