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UBC Theses and Dissertations

Simulation, modelling, and control of a near-surface underwater vehicle Field, Adrian Ivery

Abstract

This thesis presents a theoretical study into the dynamics and control of underwater vehicles. Problems related to simulation, modelling and control are examined for a submarine operating near the free surface. Two particular issues in this study are the effects of surface waves on the submarine and the cable tension effects of a body towed by the submarine. Fundamental rigid body mechanics and hydrodynamic principles are used to construct the dynamic equations of motion for a submarine. External control, hydrodynamic, restoring and disturbance forces are recognized and included in the model. A specific study of the results of and the current methods for the evaluation of hydrodynamic added mass and damping for a submarine is included. The equations of motion are expressed for the particular application to the autonomous underwater vehicle (AUV) DOLPHPN. A guidance system based on global positioning system (GPS) waypoints is incorporated in the simulation. Two tow cable models are presented and included in the simulation model, one for straight flight and one dynamic model for tracking maneuvers. A system identification method is presented for the realization of the dynamic tow model. The superposition of linear free surface waves is used to represent a given sea state for simulation. The effects of these waves on the submarine are included in simulations. Based on the information resulting from the equations of motion and simulation environment, two suitable control system algorithms are developed and compared. The open loop characteristics of the submarine are studied. The control systems designs are based on the Linear Quadratic Gaussian (LQG) method, and use the Loop Transfer Recovery (LTR) design process. Design based on a linear model is used as a basis, while two augmentations of the model are compared for effectiveness. The tracking performance for ramp and step input commands are compared. Then a turning maneuver is simulated with the tow models. Finally, two long crested sea states and three relative wave directions are simulated with each of the controllers for a single commanded velocity. The effects of sensor noise and the filtering of this noise are also presented. Both augmentations using the LQG control approach show satisfactory performance results for an AUV operating near the surface while towing. Actuator saturation is observed in simulations with waves. Non-linear effects degrade the performance of the compensators. Some improvements are suggested for this application.

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