Control configured design for smooth, highly-maneuverable, underwater vehicles

Abstract

We describe the development of a new type of robotic underwater vehicle designed specifically for the inspection of critical infrastructures such as boiling water reactor nuclear power plants. These applications require vehicles that can access confined areas, maneuver precisely, and move easily in several directions. In addition, external appendages such as fins or propellers should be avoided in order to reduce the risk of damage through collisions. We propose a smooth, spheroid shaped vehicle that uses an internal propulsion system to generate and direct water-jets for propulsion and maneuvering. Drawing inspiration from aeronautics and ocean engineering, we treat this system as a control-configured vehicle (CCV) and design the system specifically for superior control performance. Like many modern CCV aircraft, our robot is designed to be open loop unstable in order to avoid bulky external stabilizers. We refer to this new type vehicle as a Control Configured Spheroidal Vehicle (CCSV). An integrated pump-valve maneuvering system is developed by combining powerful centrifugal pumps with compact Coanda-effect valves. This system is used to design and construct a compact, multi-degree-of-freedom (DOF) prototype vehicle. To achieve precision orientation control, high speed valve switching is exploited using a unique Pulse Width Modulation (PWM) control scheme. Dead zones and other complex nonlinear dynamics of traditional propeller thrusters and water jet pumps are avoided with use of integrated pump-valve control. Three simple control algorithms for coordinating valve switching and pump output are presented and are verified through experiments. Planar control is complicated by the presence of hydrodynamic instability. A dynamic control system that augments stability and achieves high maneuverability is outlined and implemented. A nonlinear hydrodynamic model is formulated, and its linearized dynamics are analyzed to attain insights into how physical design parameters, such as jet direction and body shape, influence controllability and stability. The integrated design method is implemented and shown to achieve high maneuverability and stability. Finally, this thesis concludes with a discussion on broader CCSV design approaches. The vehicle open loop dynamics are studied and a plant zero is shown to significantly influence closed loop performance. Jet angle and vehicle shape are explored through the lens of optimizing this plant zero location, and design recommendations are presented for both ideal and practical situations. These lessons can be used to design new CCSV systems for a variety of scales and applications.