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This paper presents a study wherein we experimentally characterize the flight dynamics and control system of a lab-scale ocean kite, then refine, validate, and extrapolate this model for use at full-scale. Ocean kite systems harvest ocean current resources through high-efficiency cross-current flight, enable energy extraction with an order of magnitude less material requirements than stationary systems with same rated power outputs. However, an ocean kite represents a nascent technology that is characterized by relatively complex flight dynamics and requires sophisticated flight control algorithms. To characterize the flight dynamics and control of ocean kite systems rapidly, at a relatively low cost, the authors have developed a lab-scale, closed-loop prototyping environment for characterizing tethered systems, whereby 3D printed systems are tethered and flown in a water channel environment. While this system has been shown to be capable of yielding dynamically similar flight characteristics to some full-scale systems, there are also fundamental limitations to the geometric scales and flow speeds within the water channel environment, making some real-world scenarios impossible to replicate from the standpoint of dynamic similarity. To address these scenarios, we show how the lab-scale framework is used to refine and validate a scalable dynamic model of a tethered system, which can then be extrapolated to full-scale flight. In this work, we present an extensive case study of this model refinement, validation, and extrapolation on an ocean kite system intended for operation in the Gulf Stream or similar current environments.

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This paper presents the closed-loop experimental framework and dynamic model validation for a 1/12-scale underwater kite design. The pool-based tow testing framework described herein, which involves a fully actuated, closed-loop controlled kite and flexible tether, significantly expands upon the capabilities of any previously developed open-source framework for experimental underwater kite characterization. Specifically, the framework has allowed for the validation of three closed-loop flight control strategies, along with a critical comparison between dynamic model predictions and experimental results. In this paper, we provide a detailed presentation of the experimental tow system and kite setup, describe the control algorithms implemented and tested, and quantify the level of agreement between our multi-degree-of-freedom kite dynamic model and experimental data. We also present a sensitivity analysis that helps to identify the most influential parameters to kite performance and further explain remaining mismatches between the model and data.

The immense potential for ocean current energy harvesting is being actively explored as the push for renewable energy becomes more urgent. This paper demonstrates a tow testing platform built to examine and validate mathematical models related to the performance of tethered, underwater, hydrokinetic devices in development to harvest energy from ocean currents, and presents experimental results illustrating how such a system can be used. The platform has been modularly designed to allow for the testing of various tethered, underwater energy-generation systems, including kite-based systems, coaxial turbines, and duct sails. Previous research on tethered energy-generating systems has primarily been focused on airborne systems. Additionally, the limited experimental research on tethered, underwater energy-generation systems has relied on a rigid rod for mechanical and electrical connections between the test articles and instrumentation. The tow testing platform presented in this paper accomplishes the mechanical and electrical connection through a dual-function tether, featuring internal conductors and an external sheath to support towing loads. The goal of this platform was to enable the emulation of ocean currents, which are largely unidirectional, and to retrofit a regular pool into a cost-effective, adaptable, small-scale tow tank test bed. The capabilities of the system include two-way communication with tethered devices, multiple towing profiles in order to emulate different flow regimes, real-time control, and differential velocities via dual winch operation.

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LEONARD, ZAK. Sensor Fusion Observer Design and Experimental Validation of an Underwater Kite. (Under the direction of Dr. Chris Vermillion).Underwater energy-harvesting kites possess the capacity, under appropriate control algorithms, to deliver more than an order of magnitude more power per unit mass than stationary devices. However, in order to perform path-following control of an underwater kite, it is essential to maintain accurate estimates of the kite's position and velocity. Obtaining these measurements is complicated by the facts that (i) the kites operate in GPS-denied environments and (ii) tether curvature renders simple line angle sensor-based measurements inaccurate in numerous flow conditions. In this thesis, a closed-loop observer that fuses line angle sensor and accelerometer measurements with a dynamic tether model to maintain a real-time estimate of an underwater kite's states is presented. The observer design including a robustness to observer tether drag coefficient error was analyzed in simulation. The observer design was experimentally validated on a 1/10-scale kite model, using a customized tow testing system deployed in the NC State Aquatic Center. The observer was validated in an approximately straight tether regime and a curved tether regime. In the straight tether regime, the results were close to a straight-line estimate as expected. In the curved tether regime, the results were compared against underwater photograph. There, the observer estimated position matched the calculated position from the photograph and therefore outperformed the straight-line estimate.