A Variable Stiffness Fin for Manta Ray-Inspired Robots With Two Motion Modals

Manta ray-inspired robots exhibit broad application prospects. In nature, different manta rays possess different stiffness of pectoral fins to survive in specific environment. Some rays present stiffer fins that enable an oscillation, achieving large propulsion to adapt to a pelagic lifestyle. While some rays exhibit softer fins that produce a wave-like undulation, enhancing motion efficiency to survive in a benthic environment. Manta ray-inspired robots face the need to switch between maneuverability and endurance in complex environments and diverse tasks. However, existing pectoral fins in manta ray-inspired robots typically possess a simple structure with one kind of stiffness and motion modal. In this article, we design a variable stiffness fin capable of rigid swing and soft bending modals, mimicking the oscillation and undulation observed in manta rays. The fin's moment of inertia and motion constraints can be altered through the rotation of the fin core, enabling it to switch between the two modals. Based on the bending principles of biological fins, a deflection enhancement for soft modal is proposed to concentrate the deflection at the fin tip. Shapes between the two modals are modeled by geometric constraints to theoretically compare their difference. Dynamics tests and swimming validation demonstrate that the variable stiffness fin, with two motion modals, enables switching between large propulsion and high efficiency. These capabilities help adjust the maneuverability and endurance of manta ray-inspired robots for complex environments.