Approaching a target position and holding station in flowing water is a fundamental and critical capability for robotic fish operating in natural aquatic environments. Despite decades of advances in enhancing swimming efficiency and maneuverability, this capability remains underdeveloped, largely owing to the insufficiently characterized, highly nonlinear fluid-structure interactions inherent to freely swimming robotic fish in flows. To bridge this gap, we propose the SWiFT framework, a Swimming With Flow Toolbox that enables the efficient exploration of an egocentric station-holding policy for a body and/or caudal fin (BCF) robotic fish in unknown and turbulent background flows via reinforcement learning (RL). Our SWiFT integrates a free-swimming flow-tank experimental setup with a highly efficient, physically consistent computational fluid dynamics (CFD)-based simulator and a systematic sim-to-real transfer pipeline. The resulting policy achieves substantial improvements over state-of-the-art methods across all metrics, most notably root-mean-square error (RMSE) of distance. Furthermore, we validated that egocentric feedback alone, without any explicit flow sensing, enables station-holding in unknown turbulent flows, closely mirroring the biological phenomenon of rheotaxis. Accordingly, the success of this egocentric station-holding policy not only advances robotic fish control toward real-world deployment, but also highlights SWiFT's promise as a foundation for tackling complex swimming tasks for underwater robots.
Xiaozhu Lin, Xuejiao Huang, Hongru Dai et al.· 0 citations
Navigating in unsteady wake flows, such as Kármán vortex streets, presents a formidable challenge for biomimetic autonomous underwater vehicles. Biological fish achieve this by utilizing their lateral line sensory systems to perceive local flow gradients and adopting an energy-efficient swimming pattern known as the Kármán gait. To translate this biological phenomenon into a practical robotics engineering solution, this paper proposes a fully computational framework focusing on the modeling and simulation of a spatio-temporal sensory system to autonomously generate the Kármán gait. To overcome the unrealistic assumption of full-state observability common in existing reinforcement learning studies, we model a multi-point lateral line array coupled with a frame-stacking mechanism. This allows the simulated agent to reconstruct the spatio-temporal topology of the surrounding unsteady flow relying exclusively on local pressure and velocity gradients. The sensory model is integrated with a spatio-temporal perceptual twin delayed deep deterministic policy gradient (STP-TD3) algorithm, which drives a Hopf-oscillator-based central pattern generator. Through rigorous high-fidelity computational fluid dynamics simulations, we quantitatively evaluate the autonomous emergence of the Kármán gait by assessing the agent’s kinematic energy proxy-mapped from joint actuation effort. Results reveal that the agent expends significantly less mechanical effort navigating through the turbulent vortex street compared to swimming in steady water, suggesting the active exploitation of the local wake dynamics. The results theoretically underscore the necessity of distributed STP for biomimetic robots, providing a robust algorithmic blueprint for future physical deployments in complex aquatic environments.
Xinqi Wang, Ming Wang, Xinyan Liu et al.· Bioinspiration & Biomimetics· 0 citations
