A study on dynamic positioning of an underwater vehicle in task space under environmental disturbances

Abstract

Dynamic positioning is used to maintain the position of a marine vessel at a fixed location or within a prescribed boundary by using its own actuators. In particular, for an underwater vehicle equipped with a manipulator called the underwater vehicle-manipulator system (UVMS), it is important to regulate the end-effector of the manipulator with respect to a given target position in many interactive operations. This thesis presents a systematic procedure for obtaining a mathematical dynamic model of an underwater vehicle and designing a controller that ensures that the endpoint of an underwater vehicle maintains its position in the presence of ocean currents and uncertainties without an explicit use of a disturbance observer. First, 6-DOF dynamic equations of motion considering environmental forces and moments are considered, and semi-analytical/empirical formulae are employed to calculate the associated hydrodynamic coefficients. Then, based on the obtained the dynamic model, a controller design approach to dynamic positioning is proposed. Specifically, an extended Kalman filter (EKF) is used as the state observer and a feedback linearizing control in task coordinates is proposed to be in use. To demonstrate the validity and effectiveness of the proposed approach, numerical simulations and experimental tests in a 2D tank are performed and their results are shown.