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Autonomous underwater vehicles (AUVs) are frequently used for deep-water ocean applications such as surveying and cable laying, where accurate control of vehicle depth and attitude is needed. The water level in the onboard ballast tanks is typically manually set for neutral buoyancy before each mission, while the vehicle is on the surface. The ballast tank contents are not normally adjusted to control vehicle depth and orientation while the AUV is in operation. As a result, vehicle trajectory and orientation is exclusively controlled using the vehicle's control surfaces during a mission. The challenges with controlling the depth and trim of an underwater vehicle include nonlinear hydrodynamic forces, as well as inherent time delays (latencies) associated with water tank level changes and valve adjustments. Furthermore, small changes in the location of the vehicle's center of gravity (i.e., due to the deployment of the AUV's payload equipment) can reduce the control authority of the AUV's control surfaces. To meet these challenges, this paper proposes two unique variable ballast system (VBS) control approaches. The first proposed VBS controller changes the weight of the AUV to help control vehicle depth and vertical (inertial) velocity. The second proposed VBS controller attempts to shift the center of gravity xG along the body-fixed x -(longitudinal)-axis to reduce depth and pitch angle error while restoring control authority to the bowplane and sternplane deflection fins. The ballasting system consists of two water tanks positioned aft and forward of amidships. The ballast tanks are then automatically filled or emptied of ocean water as desired. Numerical simulations have been carried out on a 2-D underwater vehicle simulator to test and compare the performance of the proposed ballast and fin deflection control systems. The simulation results show that, for the assumptions and conditions tested, the proposed controllers are capable of - chieving a setpoint depth and pitch angle with minimal error by effectively utilizing the ballast tanks and deflection fins. As a result, the work presented in this paper helps increase the autonomy of large AUVs on long-duration missions.
Date of Publication: Oct. 2012