Design and control of a bioinspired underwater robot with hydrogel-based flexible pectoral fins

Bioinspired flexible propulsion offers a promising approach for improving the efficiency and maneuverability of underwater robots. Inspired by the undulatory locomotion of median and/or paired fin organisms, this article presents a flapping propulsion system based on flexible pectoral fins fabricated using hydrogel materials. Coordinated actuation of multiple fin rays generates continuous traveling-wave deformation of the pectoral fins. To address the difficulty of establishing accurate hydrodynamic models for flexible flapping propulsion, a path following control framework combining offline data-driven modeling and online adaptive control is developed. Experimental measurements are used to establish the mapping between flapping motion parameters and hydrodynamic forces, which is implemented as a lookup-table-based feedforward compensation. An adaptive super twisting sliding mode control strategy is further incorporated to enhance robustness against external disturbances and measurement noise. Numerical simulations and pool experiments demonstrate the stability and reliability of the proposed approach for flapping-based path following. This work provides practical insights into the design, modeling, and robust control of flexible flapping propulsion systems.