AI summary
Problem
Most manta ray robots rely on fixed or one-dimensional fin compliance, limiting their ability to replicate the two-dimensional stiffness variations needed for efficient 3D traveling-wave propulsion.
Approach
The researchers designed a cable-driven, bioinspired manta ray robot with anisotropic disks that allow active, tunable two-dimensional (spanwise and chordwise) stiffness control in its pectoral fins.
Key results
- Spanwise stiffness increased >30% and chordwise stiffness decreased ~10% via disk rotation
- Optimal stiffness settings generated up to five times more thrust than the least effective setting
- Stiffness modulation altered swimming kinematics, yielding up to 20% speed variation
- Validated through mechanical characterization, thrust testing, and free-swimming trials
Why it matters
This work provides a practical testbed for studying active stiffness control in aquatic locomotion and offers actionable design principles for next-generation bio-inspired underwater robots.
Abstract
Manta rays achieve efficient, highly maneuverable three-dimensional swimming by flapping their large pectoral fins in both the spanwise and chordwise directions, where fin stiffness likely plays a critical role in hydrodynamic per- formance. However, most existing manta-ray robots use fixed or one-dimensional compliance, which limits their ability to reproduce the two-dimensional stiffness variations necessary for authentic 3D traveling-wave propulsion. This paper presents a bioinspired manta ray robot equipped with an active stiffness control mechanism that enables tunable two-dimensional stiff- ness in its pectoral fins. The design integrates a cable-driven actuation system with anisotropic disks, providing multiple distinct stiffness in both the spanwise and chordwise directions that can be locked during operation. Mechanical character- ization confirms periodic stiffness variation, with spanwise stiffness increasing by more than 30% and chordwise stiffness decreasing by about 10% as the disk rotates from 0° to 90°, then recovering from 90° to 180°. 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.