Pitch and Depth Control of a Biomimetic Swimming Robot Using Pectoral Fins

Biomimetic autonomous underwater vehicles (BAUVs) have attracted increasing interest as underwater platforms owing to their high energy efficiency, maneuverability, and low radiated noise compared with conventional propeller-driven vehicles. This study presents an experimental investigation of depth and attitude control for a large-scale BAUV equipped with a buoyancy engine for buoyancy regulation and pectoral fins for roll and pitch control. A 1.9 m BAUV was designed and fabricated, in which a piston–ball screw-based buoyancy engine adjusts the net buoyancy around a near-neutral condition, and the pectoral fins are driven by a PID-based controller to regulate the attitude. The integrated control system was evaluated in an engineering water tank through depth control experiments using the buoyancy engine and attitude control experiments using the pectoral fins. Depth control tests confirmed that the buoyancy engine enables accurate descent to and maintenance of commanded depths while operating near a neutral-buoyancy condition. Pectoral fin experiments demonstrated stable attitude regulation, with a roll RMSE of 1.65° and rapid pitch convergence to the commanded reference angles. Finally, integrated depth tracking using both the buoyancy engine and pectoral fins was validated during waypoint-following experiments in an engineering water tank and field experiments, achieving an RMSE of 0.133 m and 0.257 m, respectively. These results confirm that the proposed BAUV system can achieve stable depth and attitude control in three-dimensional underwater environments, indicating its applicability to practical underwater field exploration.