Three-Dimensional Fast-Start and Posture Control of Multijoint Robotic Fish

The ability to execute rapid 3-D turning maneuvers is essential for robotic fish to evade threats and perform complex underwater tasks. This article proposes a 3-D fast-start posture control method inspired by the hunting maneuvers of the pike. Initially, we develop a four-joint robotic fish equipped with a gravity-center adjustment mechanism and a flexible tail. Building on this design, we establish the dynamic and hydrodynamic models of the robotic fish, offering a reliable simulation environment for validating the proposed method. Subsequently, taking into account the pitch moment generated by the tail fin during fast-start maneuvers, we introduce a sliding-block position control model based on model predictive path integral (MPPI) control. A novel slider-position compensation approach is further proposed to address gravity-center instability observed in physical experiments. Finally, simulation and underwater experimental results demonstrate that the proposed method effectively predicts pitch disturbances caused by tail-fin motion and adjusts the slider position in real time to maintain stability. Compared with existing approaches, our method substantially improves the efficiency and stability of 3-D fast-start maneuvers, providing valuable guidance for enhancing the high maneuverability of underwater robots in complex underwater environments.