Design and Modeling of Elastic Variable Stiffness Robotic Fish Tail

Fish have attracted a huge number of researchers in recent years due to their exceptional swimming abilities, and many imitation fish propulsion robots have been produced. Numerous studies have demonstrated that fish obtain the best swimming speed and efficiency by adjusting the stiffness of their bodies when the tail beats at different frequencies, yet most contemporary robotic fish neglect the stiffness of the fish body. In this article, we create a fishtail with elastic materials, which is combined with a hard silicone cylinder and several regularly arranged balls in the cylinder. We achieve changeable stiffness by squeezing the inner elastic balls to enhance total stiffness, which is controlled by a stepper motor, with the reason that second moments of cross section can influence the stiffness of beam. After modeling and analyzing the system, we build it into a composite cantilever beam made up of a hollow outer cylinder and a periodic heterogeneous core. The stiffness of this complicated beam is calculated using a explicitly formula. Finally, in order to achieve a considerable stiffness increase, we construct a test platform and undertake experimental verification.