Investigating Batoid-Inspired Propulsion: The Development, Testing, and Performance Analysis of a Tensegrity-Based Robotic Fin for Underwater Locomotion

Unmanned underwater vehicles have become an increasingly important tool for research, industry, and the military to perform surveying, monitoring, exploratory, and other functions. They are conventionally designed with rigid hulls and rotary propellers that utilize steady hydrodynamic principles. This is in contrast to biological swimmers, which generally use flexible bodies and appendages to take advantage of unsteady hydrodynamics. As a result, fish and other swimming animals are notably much more maneuverable and efficient compared to conventionally-designed vehicles. This stark difference motivates the development of biologically-inspired designs that can meet or exceed performance seen in nature. Recently, manta rays (and batoid rays in general) have been identified as an ideal platform for an efficient, high endurance, maneuverable and stealthy underwater vehicle. Towards the goal of creating such a vehicle, this study is specifically aimed at reproducing the major kinematic features of oscillatory batoid rays by developing a tensegrity-based robotic pectoral fin and quantifying the swimming performance of this fin.

In this work, the structural mechanics of cable-clustered active tensegrity beams are experimentally validated, so that analytical predictions for their response to external loads can be used in the design of a tensegrity-based robotic pectoral fin. In order to quantify the relationship between kinematic parameters and performance in ray-like swimming, a tensegrity-based robotic fin, capable of actively producing large span-wise bending and passively producing chord-wise curvature, is developed. Two types of experimental hydrodynamic tests are performed in a water tunnel: constrained tests that measure net thrust and propulsive efficiency; and unconstrained tests that measure velocity and free-swimming economy. Constrained tests demonstrate that the simple fin design can produce significant net thrust that is strongly correlated to flapping frequency. The maximum efficiency of this heaving motion is relatively low, so when compared to a chord-wise rigid fin, this suggests that solely adding chord-wise compliance is not beneficial for maximum propulsive efficiency, although passive flexibility seems to be important for broadening the operating range of highest efficiency. Unconstrained tests demonstrate that free-swimming velocity is correlated to both flapping frequency and amplitude. Importantly though, high swimming velocities come at the cost of low economy, indicating an inherent operational trade-off between transport time and energy usage. Kinematic parameters matching biological observations produce free-swimming velocities that are similar to batoid rays, but increased kinematic complexity is expected to improve both efficiency and economy. Flexibility is shown to be an important design parameter for flapping propulsors, with a compliant artificial skin showing enhanced swimming speeds and economies compared to a stiff skin. The experiments show that while Strouhal number (a nondimensional frequency) is correlated with an operating range of maximum efficiency, it does not uniquely correlate to peak values of economy, indicating that caution should be exercised when statements are made about the role of Strouhal number in free-swimming performance. Overall, this study demonstrates that active tensegrity structures can be effectively used to reproduce biologically-relevant kinematics and shows promise for biologically-inspired flapping fins in the application of underwater vehicles.