Structure design and dynamic analysis of a tensegrity-based carangiform robotic fish

Abstract

<p indent="0mm">Bionic robots are widely used in various fields due to their high flexibility and mobility. When conducting complex underwater operations, it is expected to design a bionic robotic fish with good underwater navigation ability based on the excellent swimming performance of fish. At present, the tail swing mechanism of bionic robotic fish is mostly composed of serial rigid joints. Limited by their own materials and manufacturing process, the joints cannot undergo large elastic deformation, so the corresponding bionic robotic fish is far less flexible than fish in water. A flexible design is a natural choice to better mimic the movement of fish underwater. Tensegrity is a grid-like spatial structural system that consists of pre-tensioned strings and pre-compressed bars. Such structure has the advantages of light weight, flexible deformation, and impact resistance. Introducing tensegrity into the robot field has greater advantages than traditional rigid robots due to its flexibility and other characteristics. Based on a novel 2D tensegrity and the physiological characteristics of fish, this paper proposed a four-joint bionic robotic fish that imitates carangiform swimming motions. Referring to the shape characteristics of fish, the three-dimensional model of robotic fish is established using software. By simplifying several ropes as the external forces of the system, ignoring the parameters of gravity, buoyancy and mass of some parts, the dynamic equations of the two-joint model are established through the coordinate transformation matrix and further extended to the multi-joint system. Taking the single and four-joint systems as examples, the accuracy of the dynamic model is verified by Adams simulations, and the obtained results are consistent with the motion curve of fish during cruise. In order to improve the propulsion performance of robotic fish, some parameters of the system are analyzed. When the stiffness distribution between the joints is changed, the results show that reducing the spring stiffness of the tail fin joint can effectively reduce the influence on the front-end joint. Further analysis is carried out on reducing the stiffnesses of only side springs, or both side and center springs. It is found that reducing the stiffnesses of only side springs is more conducive to the improvement of the driving effect. The influence of the driving strategy on the dynamic performance of the system is discussed, and the application of step function and sinusoidal function to the robotic fish without/with damping is also analyzed. The results show that the fluctuation of the sinusoidal function driven system is larger under the condition of without damping, and the similar dynamic response can be obtained under the condition of damping. This work provides a theoretical basis and application reference for the design of flexible robotic fish. The proposed tensegrity-based carangiform robotic fish holds potential applications as exploration, search and rescue equipment in complex underwater environments.