Analysis of an Unmanned Underwater Vehicle Propulsion Model for Motion Control

The knowledge of dynamic models to describe the behaviors of unmanned underwater vehicles allows to design a precise navigation, guidance, and control system. A practical identification strategy is presented in this contribution, a procedure that can be easily executed during field trials and that is able to obtain a rapid assessment about the characteristics of the vehicle dynamic model. The developed procedure is designed for small-size-class remotely operated vehicles, characterized by low operative speeds. The vehicle is equipped with proprioceptive sensors in charge of measuring data needed to identify its dynamic model. Building on the proposed practical identification experiments, different vehicle propulsion models are considered and analyzed: the first is a general theoretical model accounting for the vehicle hydrodynamics (standard marine Fossen model); the others are modified models introducing additional terms in the produced thrust (torque), namely, a term proportional to the rate of change of the propeller revolutions per minute (r.p.m.), dn/dt (where n is the revolution rate of the propeller), and a delayed copy of the terms connected to the fluid axial flow velocity. For all considered degrees of freedom, the fitting performance of the proposed models resulted to be superior to the first. Moreover, a control system building on the obtained dynamic characterization is described.