The design of a self-propelled flexible hull undersea vehicle

Abstract

There is interest in designing biologically inspired underwater vehicles that propel themselves with flapping foils like fish, because fish performance suggests a desirable combination of maneuverability and efficiency. Bluefin Tuna, for example, are known to migrate long distances while also being able to perform high acceleration maneuvers and swim at speeds in excess of 100 km/hr. To achieve performance on par with biological swimmers, a machine must replicate the fluid dynamic interactions between a fish's body and water as well as the efficient actuation of its control surfaces. It isn't feasible to design actuators that are well suited for such devices without a relationship between the kinematics of a flexible body in water and the forces on it. It is difficult to derive these forces analytically, however, since this would require a solution to Navier-Stokes equations. It is also challenging to design a mechanical drivetrain that fits within the envelope of a fish and can generate comparable forces and displacements. This thesis explores an experimental approach to designing an efficient, self-propelled underwater vehicle that is modeled after a .681 m long Skipjack Tuna. Analytical models are used to estimate the joint torques required to maintain biological swimming kinematics, to select optimal actuators, and to size the components of a drivetrain for the device. The drivetrain of this prototype has rigid transmission elements and is designed with low friction drive components. These properties will allow us to measure the actuator torque profiles and from them estimate the torque applied to each segment of the device to inform the design of specialized actuators. The machine is mostly assembled, and position control loops were designed and tuned based on the measured open loop transfer functions of the joints in air. Unfortunately, we were unable to finish assembly of the device and test it in fluid.