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

During the underwater vehicle R&D process, a crucial task in the beginning phase is to decide the specifications of sensors and actuators. The designers need to make decisions if an off-the-shelf product will meet the requirement, or more efforts should be devoted to the development of a component. The communication format undertaken between the controller and the subsystems is another important design issue worth of close attention. Once these specifications are settled, it will be very troublesome to change them afterwards in case a design flaw is discovered. It will be even worse if the problems are found after the prototype vehicle is constructed. In order to ensure the flexibility and shorten the development time, this paper proposes an architecture for general-purpose low level controller suitable for underwater vehicles. We suggest using the idea of ¡§tiers¡¨ to construct a vehicle controller with multiple layers. Generally speaking, there are many different paths of information flow in a vehicle control system. It can be high-level tier and abstractive intention of the human operator interpreted by the man-machine interface; or the mid-level tier control commands to maneuver the vehicle to a specific direction; down to the low -level tier as the raw commands fed to the thrusters. The performance and the reliability of the system deeply depend on the flow of these information and commands. High- and mid-level tiers information can be modeled mathematically, but the low-level tier is product-dependent. In other words, once a new sensor or actuator is installed, the control software related to these components need to be revised accordingly. The modification of the software might exist at multiple places if the structure is not organized as tiers. In order to maintain full flexibility of the vehicle controller structure throughout the R&D period, the high- and mid-level will be implemented in the man-machine interface for ROV case, and in the mission planner in the AUV case. The low-level tier is implemented in the onboard computer. The onboard low-level controller covers a variety of communication format of physical ports, such as serial line, D/A, A/D, D/IO and PWM. Port setting parameters, such baud-rate or DA range, can be specified remotely on the surface. The physical connecting ports of the sensors can be changed freely without rewiring or reprogramming. Taking the stability of the controller as the top priority, we used DOS operating system as the platform to implement our concepts. DOS has been in the market for more than two decades, but it has the merits of fast in booting, highly stable, efficient in computation. We use its timer interrupt service INT 0X1C to construct a realtime thread to poll the readiness of sensory channels, and uploads the data to the surface via a channel-driven packet. The packets delivered to the surface are split into channels and reconstructed back to their original raw data format. The other necessary service routines, such as DA, AD and DIO, are also embedded inside this thread for its promptitude. We constructed an experimental platform with this low-level controller to verify if the vehicle alitude control can be accurate enough as the carrier of the Seafloor Laser Scanner developed by our lab. Prior to the experiments, issues, such as whether the bouyancy of the system is pro or con for driving the vechile, were studied with Simulink. The poorness of altitude control caused by the deadzone effect of the thruster failed to be duplicated as in the simulation, while the alitude control gave a tracking error within ¡Ó 5cm.

In the development of underwater vehicles, it is necessary to conduct performance test in the water tank. However, different factors need to be considered depending on different cases. The purpose of this paper is to construct a simulation platform in the air to study the scenarios like side current or constant height profiling. Although these tests are difficult to be performed in the water tank, we can get some solutions from observing the dynamics of simulation platform. The simulation platform we used consists of three links to constraint the motion in a polar coordinate system. It carries a wireless micro-camera, and two DC motor-driven propellers. At the end of the distal link and metal disk is attached on the other side of the pivot of the last link to provide counter-weight which can simulate different status of the "buoyancy" of the platform. The encoder which is uses to trace the motion of the simulation platform, is mounted at each join between two links. The control program has two parts: servo control of propellers and target tracking. In order to approach to the real-time searching, we derived image with gray scale instead of color form to increase image refreshing rate during the tracking process. For the current experiment, the target is at dot generated by an LED. The location of the bright dot is detected by a histogram-based threshold, and the actual location is further refined with intensity-weighted algorithm. The offset between of the target and the center of the image is used as the feedback to command the propellers to drive the platform. The goal is to keep the target at the center of the image as close as possible. A linear PD control (proportional - derivative) is implemented to drive the propellers. Preliminary experiments show that the simulation platform can track a target with about 15 frames per second refreshing rate under the condition that the target does not move too fast and vanishes in the image. When ROV with laser scanner syetem, seafloor away from ROV's accuracy is necessary. In this eassay, we use tracking angle and tracking bright dot to qualify and quantify the influence of buoyancy and propeller on the altitude control in different cases.

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).

abstract: A novel underwater, open source, and configurable vehicle that mimics and leverages advances in quad-copter controls and dynamics, called the uDrone, was designed, built and tested. This vehicle was developed to aid coral reef researchers in collecting underwater spectroscopic data for the purpose of monitoring coral reef health. It is designed with an on-board integrated sensor system to support both automated navigation in close proximity to reefs and environmental observation. Additionally, the vehicle can serve as a testbed for future research in the realm of programming for autonomous underwater navigation and data collection, given the open-source simulation and software environment in which it was developed. This thesis presents the motivation for and design components of the new vehicle, a model governing vehicle dynamics, and the results of two proof-of-concept simulation for automated control. / Dissertation/Thesis / Masters Thesis Computer Science 2020

Oceanographers are currently investigating small-scale ocean turbulence to understand how to better model the ocean. To measure ocean turbulence, one must measure fluid velocity with great precision. The three components of velocity can be used to compute the turbulent kinetic energy dissipation rate. Fluid velocity can be measured using a five-beam acoustic Doppler current profiler (VADCP). The VADCP needs to maintain a tilt-free attitude so the turbulent kinetic energy dissipation rate can be accurately computed to observe small-scale ocean turbulence in a vertical column. To provide attitude stability, the sensor may be towed behind a research vessel, with a depressor fixed somewhere along the length of the towing cable. This type of setup is known as a two-part towing arrangement. This thesis examines the dynamics, stability and control of the two-part tow. A Simulink simulation that models the towfish dynamics was implemented. Through this Simulink simulation a parametric study was conducted to see the effects of sea state, towing speed, center of gravity position, and a PID controller on the towfish dynamics. A detailed static analysis of the towing cable's effects on the towfish enhanced this dynamic model. The thesis also describes vehicle design and fabrication, including procedures for trimming and ballasting the towfish. / Master of Science

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

There exists a need for a new class of underwater vehicle that can perform both close control tasks, as well as long-range exploration, without manual reconfiguration. A tetherless underwater vehicle (TUV) with acoustic communications to an operator station has potential to fulfill this need, while also removing much of the operating costs associated with tether management. The problem with TUVs is the limited communications bandwidth and time lag increasing with range from the transmitter. This thesis introduces a new class of controller for TUV vehicles that isolates the operator from the time-varying delay. This isolation is achieved through the formation of a continuum of control comprised of existing control paradigms, such as predictive and autonomous control. A smooth evolution through the continuum is formulated based on the time delay. The resulting controller permits operator close control for extended ranges without manual reconfiguration of the vehicle or controller.

Precise positioning is crucial for the success of navigation of underwater vehicles. At present, different instruments and methods are available for underwater positioning but few of them are reliable for three-dimensional position sensing of underwater vehicles. Long baseline (LBL) positioning is the standard method for three-dimensional underwater navigation. However, the accuracy of LBL positioning suffers from its own drawback of relatively low update rates. To improve the accuracy in positioning an underwater vehicle, integration of additional sensing measurements in a LBL navigation system is necessary. In this study, numerical simulation and experiment are conducted to investigate the effect of interrogate rate on the accuracy of LBL positioning. Numerical and experimental results show that the longer the interrogate rate, the greater the LBL positioning error. In addition, no reply from a transponder to transceiver interrogation is another major error source in LBL positioning. The experimental result also shows that the accuracy of LBL positioning can be significantly improved by the integration of velocity sensing. Therefore, based on Kalman filter, this study integrates a LBL system with vehicle dynamic model to improve the accuracy of positioning an underwater vehicle. For conducting the positioning experiments, a remotely operated vehicle (ROV) with dedicated Graphic User Interface (GUI) is designed, constructed, and tested. To have a precise motion simulation of ROV, a nonlinear dynamic model of ROV with six degrees of freedom (DOF) is used and its hydrodynamic parameters are identified. Finally, the positioning experiment is run by maneuvering the ROV to move along an ¡§S¡¨ trajectory, and Kalman filter is adopted to propagate the error covariance, to update the measurement errors, and to correct the state equation when the measurements of range, depth, and thruster command are available. The experimental result demonstrates the effectiveness of the integrated LBL system with the ROV dynamic model on the improvement of accuracy of positioning an underwater vehicle.