Kinematic and hydrodynamic modeling of a wire-driven robotic fishtail: An experimental study

Robotic fish have been extensively developed for scientific research and engineering applications, with the robotic fishtail serving as a crucial component. Due to the complex deformation mechanism and hydrodynamic interaction, it remains a significant challenge to establish a real-time precise kinematic and hydrodynamic model for flapping robotic fishtails. This paper presents a kinematic and hydrodynamic model for a wire-driven robotic fishtail, incorporating experimental investigation. The wire-driven mechanism featuring a continuously compliant backbone and position-variable vertebrae is developed, enabling more flexible fishtail flapping and adjustable fish morphology. The kinematic and hydrodynamic models are established through the quasi-static method with proposed corrections accounting for flapping velocity and nonlinear backbone deformation during a flapping stroke, precisely assessing real-time hydrodynamic responses. The proposed models have been validated through physical tests in an open-water environment, and exhibit superior response predictive ability compared to traditional methods. A significant force peak is observed at the beginning of a flapping stroke, while small amount of reversed thrust occurs at its end. In a high-frequency flapping motion, the soft caudal fin generates greater thrust than a hard fin. Comprehensively, this study presents a systematic research methodology for modeling and experimental investigation of novel robotic fishtails and biomimetic propellers.