ALBERT

Description: n/a

Description: Published

Description: 125-127

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: restricted

Description: Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution September 1994

Description: Closed loop control of an unmanned underwater vehicle (UUV) in the dynamically difficult environment of shallow water requires explicit consideration of the highly coupled nature of the governing non-linear equations of motion. This coupling between an UUV's six degrees of freedom (6 DOF) is particularly important when attempting complex maneuvers such as coordinated turns (e.g. simultaneous dive and heading change) or vehicle hovering in such an environment. Given the parameter and modelling uncertainties endemic to these equations of motion, then a robust 6 DOF sliding controller employing six-element vector sliding surfaces provides a framework in which satisfactory UUV control can be achieved in shallow water. The vehicle equations of motion are developed and cast in a form that is amenable to non-linear sliding control design. A complete 6 DOF sliding controller with vector sliding surfaces is then formulated via a Lyapunov-like analysis. The sliding controller is then modified via a weighted least-squares approach to work with a particular UUV which has only 4 DOF control authority available. The modified controller is shown to work well for a variety of commanded UUV maneuvers in the presence of significant environmental disturbances and vehicle hydrodynamic parameter uncertainties via numerical simulation. Use of the signals generated by the controller are shown to be of utility in vehicle buoyancy control.

Description: The financial support of the Office of Naval Research under Contract No. N00014-90-J-1912 is gratefully acknowledged.

Description: Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution September 1994

Description: This paper presents a method for estimating the spectra of water wave disturbances on five of the six axes of a stationary, slender body underwater vehicle in an inertia dominated wave force regime, both in head seas and in beam seas. Inertia dominated wave forces are typical of those encountered by a 21 inch diameter, torpedo shaped underwater vehicle operating in coastal waters and sea state 2. Strip theory is used to develop transfer function phase and magnitude between surface water waves and the slender body pitch, heave, and surge forces and moment for the vehicle in head seas, and for pitch, heave, yaw, and sway forces and moments in beam seas. Experiments are conducted which verify this method of transfer function calculation, and demonstrate the effects of vehicle forward motion in the head seas case. Using known sea spectra and linear time invariant systems theory allows for estimation of the water wave disturbance spectra for these forces and moments. Application of sliding control techniques are then developed for the underwater vehicle longitudinal plane equations of motion. Computer simulations are used to demonstrate the dependence of underwater vehicle depth control upon the pitch control, and adaptive pitch control is shown to provide good performance in the presence of substantial parametric uncertainty. Pitch disturbance rejection properties of variations of the sliding controller are investigated. Both single frequency and stochastic disturbances are used, and the stochastic disturbance is developed using the results of the earlier investigation.

Description: This research was sponsored in part by ONR Contract N00014-90-J-1912.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution May 1994

Description: This thesis compares classical nonlinear control theoretic techniques with recently developed neural network control methods based on the simulation and experimental results on a simple electromechanical system. The system has a configuration-dependent inertia, which contributes a substantial nonlinearity. The controllers being studied include PID, sliding control, adaptive sliding control, and two different controllers based on neural networks: one uses feedback error learning approach while the other uses a Gaussian network control method. The Gaussian network controller is tested only in simulation due to lack of time. These controllers are evaluated based on the amount of a priori knowledge required, tracking performance, stability guarantees, and computational requirements. Suggestions for choosing appropriate control techniques to one's specific control applications are provided based on these partial comparison results.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution January 1994

Description: Closed loop control of an underwater vehicle in an acoustic position net requires an accurate hydrodynamic model of the vehicle. The model is essential to the control algorithm design process. Further, it is an integral part of the observer used to generate complete state estimates from the position measurements. An experimental apparatus and numerical analysis technique for vehicle system identification during the thruster induced hover limit cycle are described. Detailed comparisons to other techniques are made and extension of the technique to four degrees of freedom with coupling is discussed. A model of the Remotely Operated Vehicle Hylas is determined. The model determined by the system identification procedure is then used in the designs of a state estimator and controller for trajectory following by the vehicle. The algorithms are initially evaluated in a numerical simulation. Tests are made for stability, trajectory following performance, and accuracy of the state estimator under varying system and environmental conditions. Finally, the results of vehicle trials are presented. System stability and accurate trajectory following under the control of the algorithms are demonstrated using ROY Hylas. The high accuracy level of the simulation is also demonstrated by the trials and directions for continued research are discussed.

Description: The financial support of the National Science Foundation on Grant No. OCE- 8820227 and the Office of Naval Research under Contract No. N00014-90-J-1912 is gratefully acknowledged.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution February 1994

Description: Ambient noise in the sea has been observed for over 100 years. Previous studies conclude that the primary source of microseisms is nonlinear interaction of surface gravity waves at the sea surface. Though this source relationship is generally accepted, the actual processes by which the wave generated acoustic noise in the water column couples and propagates to and along the sea floor are not well understood. In this thesis, the sources and propagation of sea floor and sub-sea floor microseismic noise between 0.2 and 10 Hz are investigated. This thesis involves a combination of theoretical, observational and numerical analysis to probe the nature of the microseismic field in the Blake Bahama Basin. Surface waves are the primary mechanism for noise propagation in the crust and fall into two separate groups depending on the relative wavelength/water depth ratio. Asymptotic analysis of the Sommerfeld integral in the complex ray parameter plane shows results that agree with previous findings by Strick (1959) and reveal two fundamental interface wave modes for short wavelength noise propagation in the crust: the Stoneley and pseudo-Rayleigh wave. For ocean sediments, where the shear wave velocity is less than the acoustic wave velocity of water, only the Stoneley interface wave can exist. For well consolidated sediments and basalt, the shear velocity exceeds the acoustic wave velocity of water and the pseudo-Rayleigh wave can also exist. Both interface waves propagate with retrograde elliptic motion at the sea floor and attenuate with depth into the crust, however the pseudo-Rayleigh wave travels along the interface with dispersion and attenuation and "leaks" energy into the water column for a half-space ocean over elastic crust model. For finite depth ocean models, the pseudo-Rayleigh wave is no longer leaky and approaches the Rayleigh wave velocity of the crust. The analysis shows that longer wavelength noise propagates as Rayleigh and Stoneley modes of the ocean+crust waveguide. These long wavelength modes are the fundamental mechanism for long range noise propagation. During the Low Frequency Acoustic Seismic Experiment (LFASE) a four-node, 12- channel borehole array (SEABASS) was deployed in the Blake Bahama Basin off the coast of eastern Florida (DSDP Hole 534B). This experiment is unique and is the first use of a borehole array to measure microseismic noise below the sea floor. Ambient background noise from a one week period is compared between an Ocean Bottom Seismometer (OBS) and SEABASS at sub-bottom depths of 10, 40, 70 and 100 meters below the sea floor. The 0.3 H z microseism peak is found to be nearly invariant with depth and has a power level of 65 and 75 dB rel 1 (nm/ s2)2)/ H z for the vertical and horizontal components respectively. At 100 m depth, the mean microseismic noise levels above 0.7 Hz are 10 dB and 15-20 dB quieter for the vertical and horizontal components respectively. Most of this attenuation occurs in the upper 10 m above 1.0 Hz, however higher modes in the spectra show narrow bandwidth variability in the noise field that is not monotonic with depth. Dispersion calculations show normal mode Stoneley waves below 0.7 Hz and evidence of higher modes above 0.8 Hz. A strong correlation between noise levels in the borehole and local sea state conditions is observed along with clear observation of the nonlinear frequency doubling effect between ocean surface waves and microseisms. Particle motion analysis further verifies that noise propagates through the array as Rayleigh/Stoneley waves. Polarization direction indicates at least two sources; distant westerly swell during quiescent times and local surface waves due to a passing storm. Above 1.0 Hz the LFASE data shows little coherence and displays random polarization. Because of this, we believe scattered energy is a significant component of the noise field in the Blake Bahama Basin. A fully 3-D finite difference algorithm is used to model both surface and volume heterogeneities in the ocean crust. Numerical modeling of wave propagation for hard and soft bottom environments shows that heterogeneities on the order of a seismic wavelength radiate energy into the water column and convert acoustic waves in the water into small wavelength Stoneley waves observed at the borehole. Sea floor roughness is the most important elastic scattering feature of the ocean crust. Comparisons of 2D and 3D rough sea floor models show that out-of-plane effects necessitate the use of 3D methods. The out-of-plane energy that is present in the LFASE data comes from either heterogeneities in the source field (i.e. mixed gravity wave directions) or, equally likely, scattering of the source field from surface or volume heterogeneities in the sea floor.

Description: This research was supported by Office of Naval Research grants N00014-89-C-0018, N00014-89-J-1012, N00014-90-C-0098, N00014-90-J-1493 and N00014-93-1-1352.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1991

Description: The oceanographic community is moving towards unmanned autonomous vehicles to gather data and monitor scientific sites. The mission duration of these vehicles is dependent primarily on the power consumption of the propulsion system, the control system and the sensor packages. A customized propulsion thruster is designed. This includes a specialized propeller tailored to ABE and a matched motor and transmission. A non-linear lumped parameter model of the thruster is developed and experimentally verified. The model is used to predict thruster performance and compare the design thruster with other variants of propeller and motor/transmission combinations. The results showed that there is a trade-off between rapid dynamic response and power conservation. For the typical ABE trajectory, the designed thruster provides good dynamic response and the lowest power consumption of all the modelled thruster units.