This thesis proposes a method to simulate the dynamics of an autonomous underwater vehicle towing another autonomous underwater vehicle of equivalent size using a marine cable in the vertical and horizontal plane. There is a coupling effect between the two vehicles because the towed vehicle is of equivalent size. This means that the towed vehicle cannot be modeled as just a payload but rather, must incorporate the forces and moments experienced and acting on it. In this work, only AUVs with symmetrical hulls are considered, where the towing AUV is moving at a constant velocity with a set thrust while the towed AUV has no thrust. The rope system is another important component that needs to be modeled correctly because the rope material and type significantly impact the motion of the vehicles. The rope system in this study is modeled using a numerical approach called the lumped mass spring damper method which is easy to understand and computationally inexpensive. The rope model accounts for buoyancy differences in different ropes and permits cable flexibility. This thesis enables us to study the motion of multiple combinations of different ropes and axi-symmetric types of underwater vehicles with any fixed or movable fin configuration. / M.S. / This thesis studies the motion of an autonomous underwater vehicle towing another autonomous underwater vehicle which is a large as it is. The towed vehicle cannot be assumed to be just a mass attached to the towing vehicle. There is an interaction between the two vehicle. The towed vehicle places a force on the towed vehicle and the towed vehicle likewise places a force on the towing vehicle. This interaction needs to be modeled correctly to fully capture the impact of both vehicles and their appendages. Additionally, the rope system poses a huge impact on the two vehicle depending on what type of rope is selected. Multiple factors affect the performance of a rope such as the shape and the elasticity. Some ropes may also be denser due to their material type and are less buoyant than others. These factors are considered in the modeling of the overall system and allows us to study different combinations of ropes and symmetric hulled autonomous underwater vehicles.

Recent developments in autonomous underwater vehicles (AUV) have created the need for a low cost AUV that is comparable in class and payload capabilities to existing, commercially available, expensive and sub-optimal crafts. The Navy is active in research of autonomous, unmanned, highly efficient, high speed underwater craft. Small, low cost AUVs capable of swarm control are of special interest for military mine applications. No matter the nature of the application or class of craft, a common challenge is the accuracy of maneuvering predic- tions. Maneuvering predictions not only affect design, but also the real time understanding of mission capabilities and endurance. Thus the proliferation of AUVs in recent times for commercial and defense applications have led to the need of higher fidelity of physics based lumped parameter models. The sensor data, along with maneuvering model data can tie into a more accurate trajectory. Multiple such incremental advances in the literature for prediction of maneuvering shall lead to a more accuracy. This work hopes to bridge some important gaps that ensure the creation of such a non-linear LPM to predict the maneuver- ing characteristics of an AUV using non linear hydrodynamic derivatives obtained through static and dynamic CFD. This model shall be implemented for the craft designed for DIVE technologies, our industrial sponsor and an in-house craft, the 690. This model shall also be made generalized for most submerged craft with a torpedo or slender hull form, with cruciform or X configuration of fins. This dissertation looks to provide the framework to identify CFD informed high fidelity dynamic model for AUVs. The model thus created shall be spe- cialized to account for specific important effects such as flow interaction among appendages, effect of using steady and unsteady maneuvers as CFD information and kinematic charac- teristics of captive maneuvers. The specific, innovative contributions in this dissertation are listed below: 1. Definition of a new stability index to incorporate effects of gravity at low-moderate speeds 2. Novel method for identification of hydrodynamic derivatives 3. Systematic and comprehensive study on the parameters affecting VPMM / Doctor of Philosophy / The maneuvering model for an AUV is an indispensable tool that makes the autonomy part of AUVs possible and efficient. The maneuvering model that exists today is mostly linearized and of lower fidelity to increase efficiency. The use of a non linear, higher order hydrodynamic model facilitates better accuracy of maneuvering predictions, essential to mission completion of AUVs applied in research and defense sectors. This higher fidelity can be achieved through informing the model using CFD that is reasonably efficient in computation. This dissertation presents a non-linear, higher order hydrodynamic maneuvering model for the 690 and DIVE crafts, informed with steady and unsteady CFD.

In this dissertation, we seek to improve collision avoidance for autonomous underwater vehicles (AUVs). More specifically, we consider the case of a small AUV using a forward-looking sonar system with a limited number of beams. We describe a high-fidelity sonar model and simulation environment that was developed to aid in the design of the sonar system. The simulator achieves real-time visualization through ray tracing and approximation, and can be used to assess sonar design choices, such as beam pattern and beam location, and to evaluate obstacle detection algorithms. We analyze the benefit of using a few beams instead of a single beam for a low-cost obstacle avoidance sonar for small AUVs. Single-beam systems are small and low-cost, while multi-beam sonar systems are more expensive and complex, often incorporating hundreds of beams. We want to quantify the improvement in obstacle avoidance performance of adding a few beams to a single-beam system. Furthermore, we developed a collision avoidance strategy specifically designed for the novel sonar system. The collision avoidance strategy is based on posterior expected loss, and explicitly couples obstacle detection, collision avoidance, and planning. We demonstrate the strategy with field trials using the 690 AUV, built by the Center for Marine Autonomy and Robotics at Virginia Tech, with a prototype forward-looking sonar comprising of nine beams. / Doctor of Philosophy / This dissertation focuses on improving collision avoidance capabilities for small autonomous underwater vehicles (AUVs). Specifically, we are looking at the scenario of an AUV equipped with a forward-looking sonar system using only a few beams to detect obstacles in our environment. We develop a sophisticated sonar model and simulation environment to facilitate the design of the sonar system. Our simulator enables real-time visualization, offering insights into sonar design aspects. It also serves as a tool for evaluating obstacle detection algorithms. The research investigates the advantages of utilizing multiple beams compared to a single-beam system for a cost-effective obstacle avoidance solution for small AUVs. Single-beam sonar systems are small and affordable, while multi-beam sonar systems are more complex and expensive. The aim is to quantify the improvement in obstacle avoidance performance when adding additional sonar beams. Additionally, a collision avoidance strategy tailored to the novel sonar system is developed. This strategy, developed using a statistical model, integrates obstacle detection, collision avoidance, and planning. The effectiveness of the strategy is demonstrated through field trials using the 690 AUV, constructed by the Center for Marine Autonomy and Robotics at Virginia Tech, equipped with a prototype forward-looking sonar using nine beams.

Precise navigation of autonomous underwater vehicles (AUV) is one of the most important challenges in the realization of distributed and cooperative algorithms for marine applications. We investigate an underwater navigation technology that enables an AUV to compute its trajectory in the presence of unknown currents in real time and simultaneously estimate the currents, using range or distance measurements from a single known location. This approach is potentially useful for small AUVs which have severe volume and power constraints. The main contribution of this work is observability analysis of the proposed navigation system using novel approaches towards uniform observability of linear time-varying (LTV) systems. We utilize the notion of limiting systems in order to address uniform observability of LTV systems. Uniform observability of an LTV system can be studied by assessing finite time observability of its limiting systems. A new definition of uniform observability over a finite interval is introduced in order to address existence of an observer whose estimation error is bounded by an exponentially decaying function on the finite interval. We also show that for a class of LTV systems, uniform observability of a lower dimensional subsystem derived from an LTV system is sufficient for uniform observability of the LTV system. / Ph. D.

This dissertation addresses high-level path planning and cooperative control for autonomous vehicles. The objective of our work is to closely and rigorously incorporate classication and detection performance into path planning algorithms, which is not addressed with typical approaches found in literature. We present novel path planning algorithms for two different applications in which autonomous vehicles are tasked with engaging targets within a stochastic environment. In the first application an autonomous underwater vehicle (AUV) must reacquire and identify clusters of discrete underwater objects. Our planning algorithm ensures that mission objectives are met with a desired probability of success. The utility of our approach is verified through field trials. In the second application, a team of vehicles must intercept mobile targets before the targets enter a specified area. We provide a formal framework for solving the second problem by jointly minimizing a cost function utilizing Bayes risk. / Ph. D.

This thesis presents the derivation of the dynamic model of an autonomous underwater vehicle that tows a large payload. Our analysis is motivated by the fact that the payload is so large that it cannot be modeled by simply appending its dynamics to the dynamics of the autonomous underwater vehicle. Hence, the coupling between the vehicle and payload must be fully modeled. Furthermore, several approximation techniques based on analytic and empirical formulations are investigated for computing the hydrodynamic coefficients of the vehicle. Efficacy and limitations of the approximation techniques are assessed by comparison with hydrodynamic coefficients that are estimated using high-fidelity computational fluid dynamics simulations. / Master of Science / This thesis presents the model to used to predict the motion of an autonomous underwater vehicle that tows a large object. Our analysis is motivated by the fact that the size of the object is so large that it will have a substantial impact on the motion of the vehicle, and likewise the vehicle will have a substantial impact on the object, requiring that the interaction between the two bodies to be fully modeled. The fluid forces and moments acting on the vehicle are approximated using techniques from hydrodynamic theory and experimental results. The accuracy of the approximation is assessed by comparing of the estimated forces and moments with those seen in high-fidelity simulations.

O ambiente aquático tem notória importância para a pesquisa, pela biodiversidade e vastidão, e também do ponto de vista comercial, para a indústria militar e de óleo&gás por exemplo. Entretanto, a sua exploração é prejudicada por diversos fatores, entre eles devido à dificuldade de navegação. Infelizmente, carece-se de sinal GPS (Global Positioning System) embaixo d\'água, o que exige outras técnicas de localização. Assim, este trabalho analisa um sistema de navegação para um veículo submarino autônomo. Graças a sensores de velocidade, girômetros, bússola, entre outros, aplica-se o princípio de dead reckoning para calcular a posição atual do veículo a partir da última posição conhecida. Para tal, é feito inicialmente um estudo dos sensores a serem utilizados e um algoritmo de navegação é proposto, cujos resultados são expressos em coordenadas geodésicas (latitude e longitude), permitindo a visualização da trajetória do veículo em mapas geo-referenciados. Além disso, problemas práticos de medição são tratados. Em seguida, é feito um estudo sobre o ruído dos sensores, utilizando a curva de variância de Allan para caracterização dos sinais dos girômetros e do DVL (Doppler Velocity Logger). Por meio de equações de propagação de erro, os ruídos são recuperados em simulação, permitindo a estimação do erro de posição e de atitude (posição angular) acumulados para uma dada manobra. Finalmente, discute-se um critério de emersão a partir das estimativas de erro de posição. / The main part of our planet is filled with water, so the aquatic environment has notorious research and commercial importance. However, its exploration faces many difficulties. In navigation, the lack of GPS signal (Global Positioning System) during underwater missions requires different techniques, so this document focus on analyzing a navigation system for autonomous underwater vehicles. Thanks to different embedded sensors, like DVL (Doppler Velocity Logger), compass, gyrometers and others, the processes of dead reckoning is applied, witch calculates vehicle\'s current position by using the previously determined position. To do so, a navigation algorithm is implemented, providing geodesic coordinates to plot vehicle\'s trajectories on geo-referenced maps. Also, practical difficulties are discussed and treated. To improve the quality of the results, girometer\'s and DVL\'s errors are analyzed using Allan\'s variance and the navigation errors are estimated using first order time derivative equations in an augmented state space. Lastly, it is discussed a criterion to emerge and correct the vehicle\'s position using GPS signal.

Neste trabalho, estuda-se o sistema de controle distribuído de um veículo submarino não tripulado, assumindo-se um sistema com recursos computacionais limitados. A princípio, são discutidos as fontes e os efeitos dos atrasos de tempo introduzidos pela escassez de recursos. Após o detalhamento da modelagem do veículo LAURS, compara-se o desempenho de alguns compensadores de atraso encontrados na literatura. Para isso, um simulador do sistema de controle do veículo LAURS é desenvolvido com o auxílio de uma ferramenta para Matlab que permite simular os aspectos temporais de um sistema limitado. A partir dos resultados das simulações, incluindo-se o estudo de um cenário, conclue-se que embora os compensadores de atraso melhorem o desempenho do sistema de controle, o custo reduzido de um sistema computacional limitado não justifica a incerteza de desempenho que uma aplicação crítica, como é o caso do veículo submarino não tripulado, apresentará. O resultado das simulações é validado estatisticamente. / This work studies the networked control system of the unmanned underwater vehicle LAURS considering a resource-constrained computer system. At first, delays sources and effects are discussed. The LAURS model is obtained and used for comparison of delay compensation strategies found in the literature. A simulator for the LAURS control system is developed with the aid of a Matlab Toolbox that emulates timing aspects of the limited operating system and network. Considering the simulation results, including a case scenario, it is argued that despite improving performance, the control system with delay compensation strategies remains unpredictable and prone to fail. Therefore, it is not recommended for critical systems such as the unmanned vehicle LAURS, even with the reduced costs of a resource-constrained computer system. The simulation results are statistically validated.

Made available in DSpace on 2017-07-10T17:11:45Z (GMT). No. of bitstreams: 1 DEISY CARDOZO2.pdf: 3855496 bytes, checksum: f8460cbb1dea66b1db54f85149de53c3 (MD5) Previous issue date: 2013-11-29 / Fundação Parque Tecnológico Itaipu / In critical systems such as airplanes, space ships, underwater vehicles, nuclear power plants, among others, failures and faults can bring catastrophic consequences. Therefore, it is required to take into account possibilities of degradation in the performance of its components. This work presents a fault-tolerant control for the propulsion system of the BA-1, an Autonomous Underwater Vehicle (AUV), by using a Fuzzy Allocation Control. This methodology takes advantage of thrusters redundancy, common in autonomous vehicles, to relocate the available thrusters forces in failure situations. That is, it performs an on line reconfiguration of the thrusterforce allocation matrix in the pseudo inverse method. This approach was tested for all the possible cases of horizontal failure of the BA-1 vehicle by means of numerical simulations.Two cases were studied for each failure situation over a linear trajectory in the x axis; the first one studies the case where failures occurs when the vehicle starts its trajectory; and the second one studies the case where the vehicle suffers failures while travelling in its maximum speed.The studies show that the vehicle has an acceptable behavior on most situations, except when it suffers failures of two thrusters which are parallel to the x axis when travelling at maximum speed. However that problem could be solved if the reference trajectory is modified after the failure occurs. In all, the control system developed presents quick responses and an acceptable degradation of performance. / Em sistemas críticos como aeronaves, naves espaciais, veículos subaquáticos, usinas nucleares entre outros, as consequências da falha ou falta de um componente pode ser catastrófica. Portanto, considerar a possibilidade de desempenho degradado por causa de falhas ou faltas em algum dos componentes é um requisito inerente desses sistemas. Este trabalho apresenta um controle tolerante a faltas do sistema de propulsão de um Veículo Autônomo Subaquático, denominado BA-1, utilizando um controle de alocação nebuloso. Esta metodologia aproveita as vantagens da redundância dos propulsores, comum em veículos autônomos, para realocar as forças de propulsão em situações de falta. Assim, este controle realiza uma reconfiguração on line da matriz de alocação de forças dos propulsores utilizada no método da pseudo inversa. Esta proposta foi testada para todos os possíveis casos de falta horizontal do BA-1 mediante simulações numéricas. Foram utilizados dois estudos de casos para uma trajetória reta no o eixo x; o primeiro quando o veículo inicia a sua trajetória com falta nos propulsores, e o se- gundo quando o veículo sofre faltas nos propulsores na sua velocidade máxima de navegação. Os resultados obtidos mostram que o veículo tem um comportamento aceitável em todas as situações excetuando quando o veículo sofre de falta dos propulsores paralelos ao eixo x na sua máxima velocidade, porém poderia ser solucionado modificando a trajetória desejada na ocorrência dessa falta. Contudo o modelo desenvolvido apresenta rapidez nas respostas e desempenho degradado aceitável.

Path-following is a primary task for most marine, air or space crafts, especially during autonomous operations. Research on autonomous underwater vehicles (AUV) has received large interests in the last few decades with research incentives emerging from the safe, cost-effective and practical solutions provided by their applications such as search and rescue, inspection and monitoring of pipe-lines ans sub-sea structures. This thesis presents a novel guidance system based on the popular line-of-sight (LOS) guidance law for path-following (PF) of underwater vehicles (UVs) subject to environmental disturbances. Mathematical modeling and dynamics of (UVs) is presented first. This is followed by a comprehensive literature review on guidance-based path-following control of marine vehicles, which includes revised definitions of the track-errors and more detailed illustrations of the general PF problem. A number of advances on relative equations of motion are made, which include an improved understanding of the fluid FLOW frame and expression of its motion states, an analytic method of modeling the signs of forces and moments and the proofs of passivity and boundedness of relative UV systems in 3-D. The revision in the relative equations of motion include the concept of state relativity, which is an improved understanding of relativity of motion states expressed in reference frames and is also useful in incorporating environmental disturbances. In addition, the concept of drift rate is introduced along with a revision on the angles of motion in 3-D. A switching mechanism was developed to overcome a drawback of a LOS guidance law, and the linear and nonlinear stability results of the LOS guidance laws have been provided, where distinctions are made between straight and curved PF cases. The guidance system employs the unique formulation and solution of the speed allocation problem of allocating a desired speed vector into x and y components, and the course control that employs the slip angle for desired heading for disturbance rejection. The guidance system and particularly the general course control problem has been extended to 3-D with the new definition of vertical-slip angle. The overall guidance system employing the revised relative system model, course control and speed allocation has performed well during path-following under strong ocean current and/or wave disturbances and measurement noises in both 2-D and 3-D scenarios. In 2-D and 3-D 4 degrees-of-freedom models (DOF), the common sway-underactuated and fully actuated cases are considered, and in 3-D 5-DOF model, sway and heave underactuated and fully actuated cases are considered. Stability results of the LOS guidance laws include the semi-global exponential stability (SGES) of the switching LOS guidance and enclosure-based LOS guidance for straight and curved paths, and SGES of the loolahead-based LOS guidance laws for curved paths. Feedback sliding mode and PID controllers are applied during PF providing a comparison between them, and simulations are carried out in MatLab.