Bio-inspired flexible robotic caudal fin with three-dimensional locomotion

Caudal fin plays significant role in the swimming of bony fishes. It has often been considered as a simple two-dimensional propulsive surface that generates forces in the horizontal plane. However, fish caudal fin has complex origami structures, and is able to produce multiple three-dimensional (3D) kinematics. Based on the biological data from Lauder lab at Harvard University, we developed a multi-material bio-inspired robot prototype with capabilities of both flapping, undulation and shape modulation to experimentally investigate the hydrodynamics of the 3D locomotion of fish caudal fin. The design of the bio-robotic caudal fin was based on detailed analysis of the biological caudal fin for different swimming behaviors. Both flexural stiffness and shape of the fin ray and fin membrane are designed relevant to the caudal fin of Bluegill Sunfish (Lepomis macrochirus). The robotic fin model was actuated at frequencies ranging from 0.5Hz to 2.5Hz. Based on the experimental apparatus, further study will discuss how the fin shape, fin ray flexural stiffness and motion patterns would affect the 3D wake flow of the fish caudal fin propulsion. Actuated by individual fin rays, the fin prototype allows multiple surface shape modulations: flapping of the entire fin, cupping, W-shaped fin motion, fin vertical undulation, which are very close to the three-dimensional motions of the live fishes. The robotic apparatus allows for further investigation of hydrodynamic performance of fish caudal fin during steady swimming, burst-and-coast, braking and backing maneuvers etc.