Unlike many terrestrial applications, GPS is unavailable to autonomous underwater vehicles (AUVs) while submerged due to the rapid attenuation of radio frequency signals in seawater. Underwater vehicles often use other navigation technologies. This thesis describes a range-based acoustic navigation system that utilizes range measurements from a single moving transponder with a known location to estimate the position of an AUV in geodetic coordinates. Additionally, the navigation system simultaneously estimates the currents acting on the AUV. Thus the navigation system can be used in locations where currents are unknown. The main contribution of this work is the implementation of a range-based navigation system in geodetic coordinates for an AUV. This range-based navigation system is implemented in the World Geodetic System 1984 (WGS 84) coordinate reference system. The navigation system is not restricted to the WGS 84 ellipsoid and can be applied to any reference ellipsoid. This thesis documents the formulation of the navigation system in geodetic coordinates. Experimental data gathered in Claytor Lake, VA, and the Chesapeake Bay is presented. / Master of Science

No description available.

Thesis (S.M.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Ocean Engineering; Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2005. / Includes bibliographical references (p. 78-79).

The Virginia Tech self-mooring autonomous underwater vehicle (AUV) is capable of mooring itself on the seafloor for extended periods of time. The AUV is intended to travel to a desired mooring location, moor itself on the seafloor, and then release the mooring and return to a desired egress location. The AUV is designed to be an inexpensive sensor platform. The AUV utilizes a false nose that doubles as an anchor. The anchor is neutrally buoyant when attached to the AUV nose. When the vehicle moors it releases the false nose, which floods the anchor making it heavy, sinking both the anchor and AUV to the seafloor. At the end of the mooring time the vehicle releases the anchor line and travels to the recovery location. A prototype vehicle was constructed from a small-scale platform known as the Virginia Tech 475 AUV and used to test the self-mooring concept. The final self-mooring AUV was then constructed to perform the entire long duration mission. The final vehicle was tested successfully for an abbreviated mission profile. This report covers the general design elements of the self-mooring AUV, the detailed design of both the prototype and final AUVs, and the results of successful field trials with both vehicles. / Master of Science

Underwater autonomous vehicles are frequently used for deep-water ocean applications such as surveying and cable-laying, where accurate control of vehicle depth and attitude is needed. The water level in the on-board ballast tanks are typically manually set for neutral buoyancy before each mission, while the vehicle is on the surface. The resulting weight of the water level is not normally adjusted while the unmanned vehicle is in operation to control vehicle depth and orientation. As a result, vehicle trajectory and orientation is exclusively controlled using the vehicle’s control surfaces during a mission. The challenges with controlling the depth and trim of an underwater vehicle include nonlinear hydrodynamic forces as well as relatively slow response times and inherent time delays (latencies) associated with water tank level changes and valve adjustments. To meet these challenges, this thesis proposes two unique variable ballast system control approaches. The proposed control approaches may be suitable for large autonomous underwater vehicles with both small (volume = 0.027 m3, each) and large (volume = 0.216 m3, each) ballast tanks. The first proposed variable ballast system controller uses the current parameters of the ballast tanks to determine the appropriate action to be implemented. This controller was designed change the weight of the AUV to help control vehicle parameters such as depth and vertical (inertial) velocity. The second proposed variable ballast controller attempts to shift the center of gravity x_G along the body-fixed x-(longitudinal) axis by changing the weight in the ballast tanks. By shifting the center of gravity, the controller attempts to reduce depth and pitch angle error while regaining control authority to the bowplane and sternplane deflection fins. The ballasting system consists of two water tanks positioned aft and forward of amidships. The ballast tanks are then automatically filled or emptied of ocean water as desired. Setpoint depth control and x_G shifting numerical simulations have been carried out on a two-dimensional underwater vehicle simulator to test and compare the performance of the proposed ballast and deflection control systems. The simulation results show that, for the assumptions and conditions tested, the proposed controllers are versatile and capable of achieving a setpoint depth and pitch angle with minimal error by effectively utilizing the ballast tanks and deflection fins. As a result, the work presented in this thesis helps increase the autonomy of large AUVs on long duration missions.

Thesis (M.S.)--Joint Program in Oceanography/ Applied Ocean Science and Engineering, Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution, 2005. / Bibliography: p.124-125.

Submitted by Diego Cesar (rasecg3@gmail.com) on 2017-06-26T19:47:01Z No. of bitstreams: 1 main_compressed.pdf: 16459769 bytes, checksum: b7838aeb4e94120d45daddb2c1b3c80e (MD5) / Approved for entry into archive by Flávia Sousa (flaviabs@ufba.br) on 2017-06-28T14:27:38Z (GMT) No. of bitstreams: 1 main_compressed.pdf: 16459769 bytes, checksum: b7838aeb4e94120d45daddb2c1b3c80e (MD5) / Made available in DSpace on 2017-06-28T14:27:38Z (GMT). No. of bitstreams: 1 main_compressed.pdf: 16459769 bytes, checksum: b7838aeb4e94120d45daddb2c1b3c80e (MD5) / Underwater navigation is affected by the lack of GPS due to the attenuation of the electromagnetic signals. Thereby, underwater robots rely on dead reckoning as their main navigation systems. However, localization via dead-reckoning raises uncertainties over time. Consequently, visual and acoustic sensors have been used to increase accuracy in robotic systems navigation, specially when they move in relation to a target object. This level of precision is required, for instance, for object manipulation, inspection, monitoring and docking. This work aims to develop and assess a hybrid visual controller for an autonomous underwater vehicle (AUV) using artificial fiducial markers as reference. Artificial fiducial markers are planar targets, designed to be easily detected by computer vision systems and provide means to estimate the robot’s pose in respect to the marker. They usually have high detection rate and low false positive rate, which are desirable for visual servoing tasks. On this master thesis was evaluated, from among the most popular and open-source marker systems, one that presents the best performance in underwater environments in terms of detection rate, false positives rate, maximum distance and angle for successful detection. Afterwards, the best marker was used for visual servoing purposes in an underwater robot. The firsts experiments were performed on the Gazebo robot simulation environment and, after that, on a real prototype, the FlatFish. Tests on a saltwater tank were performed in order to assess the controller using static and adaptive gains. Finally, sea trials were performed, using the controller that best behaved on the controlled environment in order to assess its performance on a real environment. The tests have shown that the visual controller was able of station-keeping in front of an artificial fiducial marker. Additionally, it was also seen that the adaptive gain brings improvements, mainly because it smooths the robot’s motion on the beginning of the task. / Navegação submarina é afetada pela falta de GPS, devido à atenuação de ondas eletromagnéticas. Por causa disso, os robôs submarinos baseiam-se em sistemas de navegação via odometria e sensores inerciais. Contudo, a localização via esse tipo de abordagem possui uma incerteza associada que cresce com o passar do tempo. Por isso sensores visuais e acústicos são utilizados para aumentar a precisão da navegação de veículos submarinos. Nesse contexto, a utilização de um controlador visual aumenta a precisão dos sistemas robóticos quando se locomovem em relação a um objeto alvo. Esse tipo de precisão é requerida para manipulação de objetos, inspeção, monitoramento e docagem submarina. Esse trabalho tem como objetivo projetar e avaliar um controlador visual híbrido para um veículo submarino autônomo (AUV) utilizando como referência marcos visuais artificiais. Os marcos artificiais são alvos planares projetados para serem facilmente detectados por sistemas de visão computacional, sendo capazes de fornecer meios para estimação da posição do robô em relação ao marco. As suas características de alta taxa de detecção e baixa taxa de falsos positivo são desejáveis para tarefas de controle servo visual. Este trabalho analisou, portanto, dentre os marcos mais populares e de código aberto, aquele que apresenta o melhor desempenho em ambientes submarinos, em termos de taxa de detecção, número de falsos positivos, máxima distância e ângulo para detecção. Posteriormente, o marco que apresentou melhor performance foi utilizado para aplicação de controle visual em um robô submarino. Os primeiros ensaios foram realizados na plataforma de simulação robótica Gazebo e, posteriormente, em um protótipo de AUV real, o FlatFish. Testes em um tanque de água salgada foram realizados visando avaliar a solução proposta utilizando um ganho estático e um ganho adaptativo para o controlador visual. Finalmente, testes no mar foram realizados utilizando o controlador que apresentou os melhores resultados no ambiente controlado, a fim de verificar seu desempenho em um ambiente real. Os testes mostraram que o controlador visual foi capaz de manter o veículo em frente aos marcos visuais artificiais e que o ganho adaptativo trouxe vantagens, principalmente por suavizar a movimentação do robô no início da missão.

The focus of this work is to design and construct a version of the secondary propulsion units used on US Navy submarines for the Virginia Tech 690 autonomous underwater vehicle. These units were used to demonstrate a control system developed in a separate study which allowed the vehicle to autonomously perform advance maneuvers such as course-keeping, mooring and obstacle avoidance. The study of the miniaturized thrusters prompted an in-depth look into two thruster designs. The first was a retractable rimdriven propeller design which was found to be too power inefficient for implementation. The final design was an azimuthing ducted propeller capable of vectoring thrust 360 degrees. Two body sections containing an implementation of the ducted propeller design were constructed and mounted to the 690 vehicle. Tests were successfully conducted in a pool. / Master of Science

Conventional streamlined autonomous underwater vehicles (AUVs) with a single thruster and stern planes are typically trimmed to be somewhat buoyant or heavy in water. To maintain depth, they must generate a constant hydrodynamic force which requires that they swim at a constant pitch angle. Although tail fins are the typical mechanism for generating this control moment, they become ineffective at low speeds. To enable an existing AUV to travel at lower speeds, one may easily incorporate a modular moving mass actuator. In some cases, it may also be advantageous to include a fixed wing. The equations of motion and equilibrium conditions to regulate depth are derived, and the effectiveness and low-speed efficiency of a fixed wing is evaluated. The effect of the vertical offset of the moving mass is analyzed to establish the relation between the control angle and the moving mass linear position. A description of the design of a one degree of freedom moving mass actuator module and preliminary experiments using the Virginia Tech Miniature AUV is provided. Data is presented for a series of fixed MMA position experiments as well as a dynamic position test. The results illustrate the effectiveness of a moving mass actuator at generating low-speed control moments. With the collected data, parameter identification is performed to get an estimate of the hydrodynamic parameters. / Master of Science

Buoyancy driven underwater gliders, which locomote by modulating their buoyancy and their attitude with moving mass actuators and inflatable bladders, are proving their worth as efficient long-distance, long-duration ocean sampling platforms. Gliders have the capability to travel thousands of kilometers without a need to stop or recharge. There is a need for the development of methods for hydrodynamic modeling. This thesis aims to determine the hydrodynamic parameters for the governing equations of motion for three autonomous underwater vehicles. This approach is two fold, using data obtained from computational flight tests and using a semi-empirical approach. The three vehicles which this thesis focuses on are two gliders (Slocum and XRay/Liberdade), and a third vehicle, the Virginia Tech Miniature autonomous underwater vehicle. / Master of Science