Luo, Yinnan: Study on bipedal running on compliant ground using hybrid zero dynamics controller

Abstract in Englisch:

This research contributes to the development of energy-efficient bipedal running gaits on compliant ground using the hybrid zero dynamics control framework, which has been predominantly applied to rigid ground in existing literature. The robot model is composed of five rigid body segments connected by four actuated revolute joints using electric motors. The periodic running gaits at a constant average velocity consist of single support and flight phases with discrete transitions between them. During the single support phase, the compliant contact between the stance foot and the ground is modeled as a nonlinear viscoelastic interaction. Feedback controllers are developed for the continuous phases to enforce holonomic virtual constraints, thereby synchronizing the joint angles with a reference trajectory on the actuated joints, resulting in reduced-order zero dynamics. A multiple shooting technique is employed to ascertain periodic solutions of the hybrid zero dynamics. The formulation of energy-efficient gaits is conceptualized as an optimization problem, wherein the parameters of the reference trajectory are optimized. The stability of the gait is evaluated a posteriori by computing the Floquet multipliers of the periodic solutions. The magnitude of these multipliers is considered as optimization constraints, enabling the generation of stable and energy-efficient running gaits on compliant ground. Comparison with rigid ground demonstrates that ground compliance does not necessarily reduce energy efficiency. In certain scenarios, the elastic properties of the ground can be harnessed to enhance overall efficiency despite the energy dissipation due to damping.