Complex Modal Analysis of Locomotive Motions of Soft Robotic Fish

In nature, most fish undulate their body and/or caudal fin to generate thrust and swim forward. The specific motion of fish body is usually described as a propulsive wave with growing amplitude. This motion is closely related to the locomotive performance of swimming fish and also used to design a robotic fish. In this study, we develop a soft carangiform robotic fish, which can generate different midline motions by charging air into the body chamber with different pressures. Using the method of complex orthogonal decomposition, the motions of fish body are decomposed into two components: the travelling and standing components. Further, the relative ratio between the magnitudes of these two components is defined as the travelling index, and the experimental results show that the travelling index is independent of the tail-beat frequency and increases with the tail's amplitude. These results agreed well with our previous numerical studies on carangiform fish.