A Manta Ray Robot with Tunable Two-Dimensional Wing Stiffness

Abstract

Manta rays achieve efficient and maneuverable swimming through flapping of their large pectoral fins, where stiffness plays a critical role in hydrodynamic performance. Most existing manta-ray robots employ fixed or one-dimensional compliance, limiting their ability to replicate the two-dimensional stiffness variation essential for traveling wave propulsion. This paper presents a manta ray–inspired robot equipped with an active stiffness control mechanism that enables reconfigurable, two-dimensional stiffness distributions in its pectoral fins. The design integrates a cable-driven actuation system with anisotropic disks, providing multiple distinct stiffness states that can be locked during operation. Mechanical characterization confirms periodic stiffness variation, with spanwise stiffness increasing by more than 30% and chordwise stiffness decreasing by about 10% as the disk rotates from 0degree to 90degree, then recovering from 90degree to 180degree. Robot experiments evaluate the influence of stiffness on fin kinematics, thrust generation, and free-swimming performance. Thrust tests demonstrate that stiffness substantially affects steady-state thrust; under certain conditions, the optimal setting produces up to five times more thrust than the least effective one. Free-swimming trials further reveal that stiffness alters swimming speed, with up to 20% variation observed in low-frequency, large-amplitude flapping. These results highlight the potential of active stiffness control to enhance the performance of bio-inspired underwater robots and provide new insights into the role of structural compliance in aquatic locomotion.