A Practical Hydrodynamic-Based Model of AUV Thruster Dynamics for Use in Closed-Loop Control of Vehicle Motions

This report documents two novel improvements in the finite-dimensional nonlinear dynamical modeling of marine thrusters. Previously reported models, which fail to capture many of the characteristic nonlinear reponses that occur during unsteady operations, assume that the lift and drag forces on the propeller blades are proportional to the sine and cosine of the angle of attack where the angle of attack is a function of the axial flow velocity and the propellers angular velocity. We have found that the lift and drag forces are not sinusoidal. We have also incorporated the effects of rotational fluid velocity and inertia on thruster response. The force curves-and model parameters are identified using experimental data from the load cell and acoustic doppler current meters. The accuracy of the model is determined by comparing experimental performance with numerical simulations. The results indicate that thruster models with nonsinusoidal lift and drag curves provide superior accuracy in both transient and steady-state response. Incorporating rotational fluid velocities into our model gave an insignificant improvement for our case. However, rotational fluid flow may be important for other types of thrusters. The research performed under this grant was reported in 9, 4, 14, 3, 5 and is referenced at the end of the text.