The Foldable Fin With Dynamic Adjustments for Manta Ray-Inspired Robots

Biological manta rays alter the projected area of their pectoral fins during flapping, achieve different flapping strokes to adjust dynamic generation, such as holding, rising, sinking and rolling strokes. Manta ray-inspired robots possess high motion efficiency and stability, exhibiting broad prospects in underwater applications. A manta ray-inspired robot capable of projected area variations can gain more agility to adapt to complex environments. However, the fin structures of existing manta ray-inspired robots primarily serve to support and drive the flexible skin to flap, lacking the ability to vary the projected area. To achieve diverse flapping strokes, robots typically rely on multiple servos and complex algorithms to adjust each servo’s motion parameters. In this article, we propose a foldable fin for manta ray-inspired robots, structurally varies the robot's span to adjust dynamic generation under simple control. The foldable fin is designed by linkage mechanisms, where three foldable modules are modularly meshed through tooth profiles, enabling synchronized folding and extending along a carbon spar. These design achieve a weight of 95 g with high stiffness, and a $78.75 \%$ foldable ratio with rapid operation. The kinematics of the foldable fin are modeled to describe different strokes. Underwater tests demonstrate that the maximum length of the foldable fin exhibits significant increase in lift and thrust compared to its minimum length in symmetric strokes, while asymmetric strokes exhibit significant adjustments in dynamics. Swimming validations further confirm the propulsion capability of the foldable fin, highlighting its potential to enhance the agility of manta ray-inspired robots.