Toward a Miniaturized Hybrid-Driven Swimming Robot for Water–Air Cross-Domain Locomotion

The flyingfish has long inspired the design of bioinspired robots capable of traversing both aquatic and aerial environments. This study introduces an untethered, miniaturized hybrid-driven robotic platform designed for multimodal motion across aquatic and aerial environments. Compared to previous designs, the proposed robot features a substantially reduced form factor while employing a synergistic propulsion strategy—combining high-frequency fish-like undulation with propeller-based thrust—to enable agile maneuvering on the water surface, underwater, and during aerial phases. Experimental results demonstrate that the swimming robot can attain a peak speed of 6.88 body lengths per second on the surface, which significantly outperforms single-mode counterparts while maintaining robust attitude control during rapid transitions. To enable high-speed pitch regulation and seamless water-to-air transitions, we derive a comprehensive dynamic model and develop a reverse-dynamics-based sliding-mode control scheme, integrated with$z$–$\theta$plane line-of-sight guidance and a state observer. The robot's full cross-domain mobility is experimentally verified, including the complete transition sequence from submerged standby to attitude adjustment and aerial acceleration. In a safe indoor laboratory environment, the highest recorded aerial flying height exceeds approximately 5.5 times the body length. This work provides critical insights into the design and control paradigms for next-generation miniaturized aquatic robots, highlighting the promise of bioinspired hybrid propulsion and adaptive control strategies for cross-domain applications.