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Browsing by Author "Escobar Sanabria, David"

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    Model and Control validation of a high speed supercavitating vehicle
    (2012-08) Escobar Sanabria, David
    Underwater, supercavitating vehicles can achieve higher speeds than conventional submarine vehicles due to the drag reduction result of the vehicle-fluid isolation. Re- search on the control of high speed supercavitating vehicles has led to theoretical so- lutions; however, validation and testing of control laws to drive the vehicle motion are expensive, complex and have not been presented in the open literature. This thesis presents an approach to the experimental validation of control systems for a supercavitating test vehicle in the longitudinal plane. The supercavitating vehi- cle considered in this thesis consists of a cylindrical body with a disk cavitator and two lateral, sweptback, wedge fins. The control validation platform enables the use of the high speed water tunnel located at the Saint Anthony Falls Laboratory to recre- ate realistic flight scenarios including the effect of ocean waves on the vehicle. The test platform uses the hydrodynamic forces produced by the fluid-vehicle interaction, embedded flight computer, and analytical equations of motion to test the closed-loop system performance in real time. The equations of motion for the test vehicle are de- rived based on experiments in which the effect of perturbed flow on the vehicle motion is also considered. A controller for the test vehicle is synthesized using H-infinity op- timization. Water tunnel tests successfully validated the supercavitating vehicle model and controller. The objectives were tracking of pitch angle reference commands and rejection of disturbances produced by an oscillating foil gust generator. The experimental results show the accuracy of the vehicle modeling and control design as well as the effect of the perturbed flow on the closed-loop system performance. The experience gained from this work enabled the introduction of the next generation test platform capable to capture planing phenomena.
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    Modeling, Robust Control, and Experimental Validation of a Supercavitating Vehicle
    (2015-06) Escobar Sanabria, David
    This dissertation considers the mathematical modeling, control under uncertainty, and experimental validation of an underwater supercavitating vehicle. By traveling inside a gas cavity, a supercavitating vehicle reduces hydrodynamic drag, increases speed, and minimizes power consumption. The attainable speed and power efficiency make these vehicles attractive for undersea exploration, high-speed transportation, and defense. However, the benefits of traveling inside a cavity come with difficulties in controlling the vehicle dynamics. The main challenge is the nonlinear force that arises when the back-end of the vehicle pierces the cavity. This force, referred to as planing, leads to oscillatory motion and instability. Control technologies that are robust to planing and suited for practical implementation need to be developed. To enable these technologies, a low-order vehicle model that accounts for inaccuracy in the characterization of planing is required. Additionally, an experimental method to evaluate possible pitfalls in the models and controllers is necessary before undersea testing. The major contribution of this dissertation is a unified framework for mathematical modeling, robust control synthesis, and experimental validation of a supercavitating vehicle. First, we introduce affordable experimental methods for mathematical modeling and controller testing under planing and realistic flow conditions. Then, using experimental observations and physical principles, we create a low-order nonlinear model of the longitudinal vehicle motion. This model quantifies the planing uncertainty and is suitable for robust controller synthesis. Next, based on the vehicle model, we develop automated tools for synthesizing controllers that deliver a certificate of performance in the face of nonlinear and uncertain planing forces. We demonstrate theoretically and experimentally that the proposed controllers ensure higher performance when the uncertain planing dynamics are considered. Finally, we discuss future directions in supercavitating vehicle control.