Flexible Electromagnetic Fin: Accurate Electromechanical Modeling for Agile Robotic Design

Bionic fish fins can significantly improve propulsion efficiency, maneuverability, and environmental adaptability in underwater robots. However, limitation exists in those bionic-fins-based robots to achieve high performance. For example, motor-actuated fins generate substantial thrust but are excessively bulky, whereas artificial muscle-driven fins offer biomimetic flexibility yet yield insufficient thrust. In this study, we introduce a miniaturized flexible electromagnetic fin that features high magnetic energy density, compact structure, frictionless elastic joint, and the ability to visually measure the electromagnetic torque. We also develop an electromechanical model that accurately predicts the thrust from an input current by incorporating underwater vibration dynamics. The model is based on three experimentally identified parameters: the electromagnetic torque constant, the underwater damping constant, and the added mass constant. In underwater experiments, the fin achieves a peak thrust of 493 mN, which is remarkable for a bio-inspired fin weighing only 17.2 g. Furthermore, a self-powered robotic fish equipped with this fin reaches a maximum swimming speed of 405.5 mm/s (1.66 body lengths per second) within 3 s, and attains a minimum turning radius of 210 mm (0.86 body lengths). These results provide a reliable theoretical and experimental basis for the design of agile robotic fish powered by flexible bionic fins.