Propeller shaft speed controller design based on nonlinear models of nondimensional thrust and torque coefficients

This paper presents a nonlinear control system based on state observer for underwater vehicle thrust control. For this purpose first nonlinear thrust and torque models is approximated with a quasi-steady state. Then dynamic equations are represented by using a three-state model of propeller shaft speed, forward speed of the vehicle, and the axial flow velocity. At this stage, a control system including nonlinear observer for estimation of axial flow velocity and a nonlinear shaft speed controller for compensation of thrust losses is presented. Stability is proven in the framework of Lyapunov stability theory. The proposed control system is simulated for an underwater vehicle that shows high performance compared with a control system without observer for axial flow velocity.