Mechanism And Experimental Study On The Variable Stiffness Biomimeticcomplaint Fish By Turnning Gas Pressure

Fish with its near-perfect swimming characteristics, such as fast, efficient and good mobility, that has aroused widespread concern of bionics scholars. It is hoped that developing an underwater vehicle with similar swimming characteristics of fish for ocean exploration, water rescue and other occasions. Researchers have made deep studies of fish propulsion mode, propulsion mechanism, mechanism structure, fish stiffness and other aspects. Fish stiffness is a newly established research domain, in this respect there has been an important conclusion: fish can match natural frequency and driving frequency to obtain a higher swimming characteristics by regulating fish own stiffness. Based on this conclusion this thesis designs a variable stiffness biomimetic complaint fish, in order to further study the relationship between fish natural frequency and swimming characteristics.This thesis establishes a variable stiffness model of flexible biomimetic fish based on viscoelastic material and presents the standpoint that the variable stiffness of biomimetic complaint fish tail can be achieved through changing the air pressure in the tail gas chamber. A physical model and mathematical model of complaint tail are builded, then carries on bending vibration analysis of fishtail models using Euler-Bernoulli beam theory and obtains mathematical expressions of bending natural frequency. The thesis presents the mechanism of changing stiffness on the base of turning gas pressure and finite element analysis of the schemes is performed with the finite element analysis software ANSYS/Workbench. The numerical solutions of natural frequency are obtained at different pressures and elastic modulus of tail fin and theoretical analysis showed that the relationship between natural frequency and gas pressure is non-linear. The thesis also presents two schemes of altering stiffness on the base of above study: turning gas pressure and changing the elastic modulus of tail fin.Based on this bionic fish model, a variable stiffness biomimetic complaint fish is designed and manufactured which stiffness can be altered by adjusting the air pressure. The head and tail sections are designed as a rigid and complaint body respectively and manufactured employing methods of 3D printing technology and casting silicone in a mold, finally they are connected using the connecting piece; In order to guarantee the swaying locomotion of the tail, rope drive mechanism is designed. For balancing fish body when swimming in the water, a counterweight program is proposed in terms of "tumbler principle".The relationship between fish body stiffness and swimming characteristics is studied experimentally and the correctness of theoretical analysis is validated simultaneously. The experimental method is that investigating swimming characteristics in each drive frequency while changing natural frequency or and stiffness of the tail fin, this experimental system consists of variable stiffness biomimetic complaint fish, hydrostatic environment with scale displacement, charging\pressure measuring devices, video cameras and other components. The experimental results show that increasing natural frequency of fish, swimming characteristics can be improved and the robofish with complaint tail fin has better swimming characteristics compared with the rigid tail fin. At the same time following conclusions are obtained: both overall stiffness and local stiffness can effect swimming characteristics of the robofish and increasing overall stiffness swimming characteristics can be improved, and what’s more, distributing local stiffness rationally in fish body can maximize the amount of increment in swimming characteristics on this basis.