Autonomous Underwater Vehicle Control in Presence of Waves

Abstract

In this paper, the 6 degrees of freedom equations of motion of an autonomous underwater vehicle (AUV) are described as a linear model and divided into three non-interacting (or lightly interacting) subsystems for speed control, steering and diving. In addition to the model of the AUV dynamics, the first and the second order wave force disturbances, i.e. the Froude-Kriloff and diffraction forces are introduced. Based on the principle of superposition it is possible to represent the AUV dynamics as the sum of low and high frequency motions. An algorithm of non-linear regression for the rationalization of the sub-surface sea spectrum is provided. Two different control designs, based on H2 and H∞ methodologies, were applied to the diving and course control of the vehicle considering the presence of the wave disturbances. The work is based on the slender form of the Naval Postgraduate School AUV, considering that the subsystems can be controlled by means of two single-screw propellers, a rudder, port and starboard bow planes and a stern plane. The wave effect on the corresponding motions of the underwater vehicle is analyzed and evaluated considering the AUV operating at different depths and different sea states using both controllers. The model presented here can be a useful simulation tool to predict the underwater vehicles behavior in different mission scenarios.