Analysis and Control of an Intervention AUV with Modified System Parameters
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This report deals with a fully actuated underwater vehicle equipped only with thrusters to produce control forces. The issue addressed is the case where the vehicle picks up an object which is to be transported to another location. When the object is picked up, the center of gravity changes location along the x-axis of the vehicle, and the vessel will incline to a non-zero pitch angle. This thesis investigates the optimal manner in which the underwater vehicle should move through the water with respect to the pitch angle. The results obtained indicate that this is a question with no unambiguous answer. It is dependant on the mass of the vessel, the velocity at which to move forward, and how far the center of gravity moves. Also, the number of thrusters and their characteristics come into play. Calculations show that in some cases it is more efficient to straighten the vessel completely, and then move forward. In other cases, the best decision is to ignore the offset in pitch completely and move forward at an inclination. However, there are cases where it is more efficient to straighten the vessel to an angle other than zero. This optimal pitch angle is, however, quite sensitive to changes in the velocity of the vessel. Finally, a controller based on direct adaptive design is implemented to regulate the pitch to this angle while maintaining a stable speed in the forward direction. This controller is derived by considering the stability properties of the tracking error, and does not require the reference signals to be positively exciting.