Preliminary thruster control experiments for underwater vehicle positioning

To improve the maneuverability of underwater vehicles, accurate thruster control is required based on a precise thrust model. Especially, in real systems, the ambient flow velocity and angle effects on the thrust force are very important, because vehicles are continuously moving and/or hovering with thruster actions. In this paper, we execute preliminary experiments with the model containing ambient flow effects as CAR (Critical Advance Ratio) and CIA (Critical Incoming Angle). The distinguished characteristics of the above model is the three axial flow states classification. The whole thrust map is divided into three states according to the state of ambient flow velocity, angle, and propeller shaft velocity. The several controllers are tested using the three flow states information. The preliminary experimental results show the best performance can be obtained by the open loop control with accurate model, because the thrust force cannot be measured directly, so the force map from the propeller shaft velocity to thrust force plays important roll in control performance. However, the DOB (Disturbance Observer) with closed loop controller is realistic solution, because it can compensate various unknown disturbances in real systems.