Combination of C-Turn and Online Variable Stiffness Enables Sharp Turning Maneuvers for Robotic Fish

The ability of fish to turn sharply is one of the key skills for surviving in water, allowing them to more effectively tackle multiple survival challenges, including evading predators and pursuing prey. Imitating the sharp turning maneuvers of fish to improve the maneuverability of robotic fish is extremely challenging, but important for practical applications. This article proposes a motion control strategy for sharp turning maneuvers of robotic fish, which combines fish-like C-turn behavior and a variable stiffness mechanism. Based on the developed dynamic model, with the optimization goal of minimizing turning time, the parameters of C-turn motion and fishtail stiffness are optimized to improve the turning efficiency of robotic fish. Simulation and experimental results demonstrate that the proposed turning strategy for robotic fish performs well in shortening turning time and improving turning efficiency. Compared with the strategies for the case of no C-turn and fixed stiffness, the case of no C-turn and adjustable stiffness, and the case of C-turn and fixed stiffness, the turning time for the proposed turning strategy is decreased by 53.2%, 41.1%, and 27.9%, respectively.