Differentiable Dynamics and Fast Simulation of Continuous Elastic Robotic Fish
arXiv:2509.16145v3 Announce Type: replace Abstract: Body flexibility plays a critical role in fish-like swimming, as the spatial distribution of stiffness governs body deformation, hydrodynamic loading, and propulsive performance. Exploiting this mechanism in robotic fish requires dynamic models that capture continuous body elasticity, fluid-structure interaction, and the resulting self-propelled motion. Existing approaches often prescribe body kinematics, approximate the body using discrete rigid or compliant segments, or incur high computational costs that limit their use in design optimization. In this letter, we present a differentiable full-body dynamics model and fast simulation framework for motor-actuated elastic robotic fish based on Hamilton's principle. The proposed formulation represents the robot as a continuously deformable elastic body and couples its structural dynamics with hydrodynamic forces without prescribing body kinematics. The resulting simulator is differentiab