Dynamic Modeling and Optimization of Robotic Fish Based on Passive Flexible Mechanism

This paper proposes a modeling method for a robotic fish with elastic tail, which is driven by a motor in head and uses a spring steel sheet as a passive flexible joint, followed by a rigid caudal fin. The pseudo-rigid-body model (PRBM) is firstly used to analyze the flexible joint, then the fluid force acting on the robotic fish is divided into the pressure on links and cross-sectional resistance. Further, a dynamic model is established by the Lagrangian dynamic method to solve the movement of the robotic fish. And the model is utilized to investigate the influence of joint stiffness on propulsion performance. For different frequencies, the optimal joint stiffness is calculated to maximize the swimming speed. These optimized speeds are all higher than using rigid joints, and a maximum speed increase of 0.33 body length per second is reached at the largest simulation frequency 10Hz. In addition, the swimming speed can be maximized at the same input power by adjusting the stiffness of the joint. These findings prove the effectiveness of passive flexible joints in optimizing the performance of the robot fish.