Thruster Modeling and Controller Design for Unmanned Underwater Vehicles (UUVs)

Thruster modeling and control is the core of underwater vehicle control and simulation, because it is the lowest control loop of the system; hence, the system would benefit from accurate and practical modeling of the thrusters. In unmanned underwater vehicles, thrusters are generally propellers driven by electrical motors. Therefore, thrust force is simultaneously affected by motor model, propeller map, and hydrodynamic effects, and besides, there are many other facts to consider (Manen & Ossanen, 1988), which make the modeling procedure difficult. To resolve the difficulties, many thruster models have been proposed. In the classical analysis of thrust force under steady-state bollard pull conditions, a propeller's steady-state axial thrust (T) is modeled proportionally to the signed square of propeller shaft velocity (Ω),T=c1Ω|Ω| (Newman, 1977). Yoerger et al. (Yoerger et al., 1990) presented a one-state model which also contains motor dynamics. To represent the fourquadrant dynamic response of thrusters, Healey et al. (Healey et al., 1995) developed a twostate model with thin-foil propeller hydrodynamics using sinusoidal lift and drag functions. This model also contains the ambient flow velocity effect, but it was not dealt with thoroughly. In Whitcomb and Yoerger's works (Whitcomb & Yoerger, 1999a; Whitcomb & Yoerger, 1999b), the authors executed an experimental verification and comparison study with previous models, and proposed a model based thrust controller. In the two-state model, lift and drag were considered as sinusoidal functions, however, to increase model match with experimental results, Bachmayer et al. (Bachmayer et al., 2000) changed it to look-up table based non-sinusoidal functions, and presented a lift and drag parameter adaptation algorithm (Bachmayer & Whitcomb, 2003). Blanke et al. (Blanke et al., 2000) proposed a three-state model which also contains vehicle dynamics. Vehicle velocity effect was analyzed using non-dimensional propeller parameters, thrust coefficient and advance ratio. However, in the whole range of the advance ratio, the model does not match experimental results well. In the former studies, there are three major restrictions. First, thruster dynamics are mostly modeled under the bollard pull condition, which means the effects of vehicle velocity or ambient flow velocity are not considered. However, while the thruster is operating, naturally, the underwater vehicle system is continuously moving or hovering against the current. In addition, the thrust force would be degraded by up to 30% of bollard output due to ambient flow velocity. Therefore, the bollard pull test results are only valid at the O pe n A cc es s D at ab as e w w w .in te ch w eb .o rg