Abstract: Fish commonly execute rapid linear accelerations initiated during steady swimming, yet the function of the median fins during this process is less understood. We find that the erection/folding time (from the starting time of the linear acceleration (0 s) to the starting time of the folding movement of the fin), as well as the spreading area of the median fins, actively change during the linear acceleration of the live largemouth bass (Micropterus salmoides). To better understand the influence of the folding time and the area change of the median fins on the linear acceleration, we implemented an undulatory biomimetic robotic fish with soft median fins that can be programmed to erect and fold, just like a live fish. To characterize the acceleration performance of the robotic fish, we developed a ‘‘self-propelled’’ experiment technique based on the Kalman filter and Proportional-Integral-Derivative (PID) control algorithm. The experiments on the robotic fish show the acceleration induced by fully-erected median fins increases by 46.3%. Fully-erected median fins positively contribute to propulsion primarily at the onset stage of the linear acceleration while result in a significant decrease in steady swimming speed by 25%, which suggests a large drag force is induced at the steady swimming stage due to the enlarged wetted area. Parametric sweeping experiments on erection/folding time and spreading area demonstrate a proper combination of the erection/folding time and the spreading area enhances the mean linear acceleration by up to 85%. Particle Image Velocimetry (PIV) results reveal that the vortexes shed by the erected dorsal fin are stronger than those shed by the folded fin. As the acceleration process progresses, the thrust generated by the dorsal fins gradually is weakened until only resistance is generated in the end. Our findings may shed light on the realization of controllable surfaces on high performance fish-inspired robotic systems in the future.

Key words: robotic fish, linear acceleration, median fin, instantaneous self-propelled system