Design and implementation of thunniform robotic fish with variable body stiffness

Abstract

In nature, undulatory swimmers consume minimum energy by adjusting their body's natural frequency to match the tail-beat frequency. Inspired by this, we study the dynamic model of fish body by considering the body shape and the fluid interactions, and develop a soft thunniform robotic fish. The experimental results show that when the driving frequency is close to the natural frequency, the speed of robotic fish reaches the maximum value, approximately 0.25 body length per second. Moreover, an empty space with a different air pressure is embedded into the body of robotic fish to vary its stiffness, and the natural frequency can be adjusted from 2.0 to 2.8 Hz by modulating the air pressure. The results of robotic fish with variable stiffness show that the forward speed and acceleration are increased with natural frequency, and a smaller stiffness of caudal fin contributes to a better propulsive performance. We also predict that the peak speed and acceleration can be obtained simultaneously by adjusting the stiffness of both fish body and caudal fin properly. These results reveal that the swimming abilities of robotic fish are closely related to the driving frequency and the stiffness property.