Oceanographic submersibles

The tasks Autonomous Underwater Vehicles (AUVs) are expected to perform are becoming more and more challenging. Thus, to be able to address such tasks, we implemented a high maneuverability propulsion system: a vectored thruster. The design of a vehicle equipped with such a propulsion system will be presented, from a mechanical, electronic and software point of view. The motion control of the resulting system is fairly complex, and no suitable controller is available in the literature. Accordingly, we will present the derivation of a novel tracking controller, whose adaptive properties will compensate for the lack of knowledge of the system's parameters. Computer simulations are provided and show the performance and robustness of the proposed control algorithm to external perturbations, unmodelled dynamics and dynamics variation. We finally illustrate the advantage of using an adaptive controller by comparing the presented controller to a Proportional Integral Derivative controller.

The inviscid hydrodynamic coefficients of an underwater vehicle (Ocean EXplorer), including the nonlinear effects of the wave surface, are computed using a boundary-integral method. A mixed Eulerian-Lagrangian formulation (Longuet-Higgins and Cokelet, 1976) is used for the treatment of nonlinear free-surface conditions. The algorithm is validated using the work-energy theorem (Yeung, 1982) and experimental data. Results, in the form of free-surface elevations and hydrodynamic coefficients, are obtained for a range of body geometries and maneuvers. The open-loop dynamics of underwater vehicles are then investigated by solving the 3DOF rigid-body equations of motion (OXY plane). The advantages and possible usage of the developed methodology for the design and control of underwater vehicles, as well as topics for further research, are addressed in the conclusion chapter of the thesis.

Boundary integral algorithms are developed to analyze three-dimensional inviscid fluid-body interactions, including the nonlinear free-surface effects. Hydrodynamic coefficients are computed for various body geometries, some corresponding to that of small underwater vehicles, in deep waters and near the free surface. The fully nonlinear unsteady wave-radiation problem corresponding to forced submerged-body oscillations and forward translation are solved using the mixed Eulerian-Lagrangian formulation (Longuet-Higgins and Cokelet, 1976). By implementing the leading-order free-surface conditions on the calm surface, linear time-domain solutions are also obtained. The nonlinear and linear results are compared to quantify the nonlinear free-surface effects. Linear frequency-domain analysis of the wave-body interactions is also carried out using a boundary-integral method based on the simple-source distribution (Yeung, 1974). The linear time-domain and the latter frequency-domain results are also compared for a validation of the algorithms.

Application of a small autonomous underwater vehicle (AUV) is described as a platform for measurement of oceanic turbulence in coastal waters during cold atmospheric fronts. The turbulence package, mounted on the AUV, allows horizontal profiling and measurement of small-scale fluctuations of velocity and temperature and other characteristics of the flow in the ocean mixed layer. The turbulence measurements were made in conjunction with current profile measurements, conductivity, temperature, and depth measurements, providing the background conditions. The navigation and tracking data from the ship and the underwater vehicle are also presented. The primary focus of this research was to collect and analyze data from the ocean in order to resolve the turbulent velocity fluctuations and the dissipation rates of turbulent kinetic energy. The aim of this thesis is to explain the approach for measurement and analysis of ocean data. It includes the manufacture of the measurement probes, the preparation of the electronic system, the coding of the acquisition software and use of several algorithms for detecting the presence of turbulence and mixing. Two observational oceanographic experiments are described as a basis for illustrating the techniques and methods in data acquisition and analysis of the oceanographic and turbulent quantities.

Accurate Autonomous Underwater Vehicles positioning requires an appropriate control design which takes into account the nonlinear coupling between the different degrees of freedom. Assuming a vehicle equipped with two side-thruster modules including two tunnel thrusters each, the control problem will be split into an outer control loop handling the motion of the vehicle, and an inner control loop designed to track the thrust commanded to each thruster. A multivariable Lyapunov function based approach, characterized by robustness properties with respect to parametric uncertainties and linearly bounded control output, will be proposed for the outer-loop and simulation results will be discussed. Regarding the low-level control framework, the performance of nine different controllers including conventional PI, sliding mode fuzzy controllers, and adaptive schemes such as model reference and sliding mode adaptive controllers, will be compared through theoretical derivations and experimental results. Such a comparison will show the advantages of the adaptive schemes in terms of tuning, robustness, and tracking performances.

When a control system for an Autonomous Underwater Vehicle (AUV) requires thrust, it is common to apply a simplified model to estimate the force generated. Even though this model takes into account several parameters, it will never recover the real value. Our challenge is to directly measure the force, in real time, from the tunnel thrusters used in the positioning control of the Mini AUV known as Morpheus. Therefore, a force sensor system has been designed, optimized, machined and tested, that supports the thruster assembly. The sensor implements strain gages to measure the deformation in a beam. To optimize the capabilities of the sensor, a finite elements analysis has been run. The sensor has been fabricated and tested to determine the static and dynamic characteristics. This thesis discusses the design implementation, optimization, fabrication and testing of the force sensor. The discussion begins with an overview of the problem, then explains the fabrication, optimization, testing and concludes with recommendation for future work.

The complex dynamics of the oceans are only beginning to be understood. There is a wide range of dynamic scales in the ocean from the Gulf Stream, with scales as large as the ocean itself, to the microstructure scales of turbulent dissipation. The program of work presented in this dissertation involves the implementation of a turbulence measurement package on board a recently developed small autonomous underwater vehicle (AUV), as well as the design of an optimized AUV platform and the development of new oceanographic sensors for measurement of micro-structure velocity. Attention is focused on ensuring that the platform is sufficiently quiet since small-scale, low level measurements are easily contaminated by the measurement process, structural vibrations, rigid-body motions and electrical interference; particularly so with the requisite machinery of a self-propelled AUV. Successful measurement entails making suitable modification to the AUV and its mode of operation. In addition to optimization of the measurement platform, consideration is given here to the optimization of the sensors for flow measurement using an AUV. Included in the research are laboratory tests of the new probes and a successful mission in making high quality measurements of ocean turbulence. Modern adaptation of the well-known Pitot tube shows promise in being less sensitive to vehicle self motion as well as yielding a greater spectral range, thereby facilitating more accurate measurement. Comparisons with shear probes and hot film probes, conducted in an axisymmetric water jet and in a wind tunnel, suggest that the pressure probe, developed as part of the work presented here, resolves the dissipation scales more fully than the shear probe. Additionally, the pressure probe does not suffer from the spectral distortion of the signal observed in measurements using a shear probe. In addition to measurement of velocity microstructure, consideration is given to the implementation of modern signal processing hardware in designing a method for the direct measurement of density microstructure. This basic property of the ocean has never before been measured directly. Results, obtained off the Florida coast in 18 meter deep water with the Ocean Explorer AUV; Cook, reveal a complex mixing event. Simultaneous measurement of two components of the velocity microstructure and measurements with a CTD package are analyzed and the instantaneous rates of viscous dissipation of turbulent energy are calculated. The dissipation rate was not stationary and showed a gradient vertically with depth as well as horizontally. The AUV platform, modified for low vibration noise, allowed measurement of dissipation rates of O(10^-8 W/kg).

The developing subsurface structure of a shallow sub-tropical water column during the passage of cold low-pressure atmospheric front is characterized through synoptic and in-situ observations during the passage of three separate fronts over South Florida. Subsurface distribution of current, salinity, temperature, density and dissipation rates were examined using an autonomous underwater vehicle (AUV), ship-based instruments, moored instruments and an Ocean Surface Current Radar (OSCR) as the fronts passed through the region. Airfoil shear probes mounted in a package on the nose of the AUV were used to measure the level and distribution of small-scale turbulence in the water column and to estimate the in-situ dissipation rate of turbulent kinetic energy. Prevailing meteorological conditions were determined from two NOAA C-MAN stations and, for two of the experiments, from a local Air Sea Interaction Spar buoy (ASIS). The first atmospheric front examined was in December 1998. A significant 10°C drop in air temperature was recorded. The AUV carried out several pre-programmed surveys over a 6-day period. A turbulent kinetic energy dissipation rates of O(10-6W/kg) were observed in the water column during the passage of the front. Fetch-limited, offshore, wind-induced surface and subsurface currents were identified during the passage of the front on April 9, 2000. As the winds increased in magnitude and shifted direction, a change in surface current was apparent in the OSCR observations. A bottom-mounted ADCP and an AUV-mounted ADCP both recorded distinct corresponding contributions to the subsurface current due to the winds. Clockwise rotation of the current profile in the water column, consistent with wind-generated currents, was observed. A third low-pressure cold front passed through the region on April 18 an 19, 2000. AUV surveys were carried out as the front passed over the region for 19 hours within a 24-hour period. Dissipation rates reached O(10 -6W/kg) during the period of the survey and decreased to O(10 -8W/kg) subsequently. The distribution of dissipation rate appeared to agree with the characteristic log law for wind-induced turbulence at the start of the passage of the front, but was significantly higher subsequently and more dependent on the combination of convective fluxes and wind stress.