Attitude control of an underwater vehicle subjected to waves

Abstract

This paper presents a method for estimating the spectra of water wave disturbances on five of the six axes of a stationary, slender body underwater vehicle in an inertia dominated wave force regime, both in head seas and in beam seas. Inertia dominated wave forces are typical of those encountered by a 21 inch diameter, torpedo shaped underwater vehicle operating in coastal waters and sea state 2. Strip theory is used to develop transfer function phase and magnitude between surface water waves and the slender body pitch, heave, and surge forces and moment for the vehicle in head seas, and for pitch, heave, yaw, and sway forces and moments in beam seas. Experiments are conducted which verify this method of transfer function calculation, and demonstrate the effects of vehicle forward motion in the head seas case. Using known sea spectra and linear time invariant systems theory allows for estimation of the water wave disturbance spectra for these forces and moments. Application of sliding control techniques are then developed for the underwater vehicle longitudinal plane equations of motion. Computer simulations are used to demonstrate the dependence of underwater vehicle depth control upon the pitch control, and adaptive pitch control is shown to provide good performance in the presence of substantial parametric uncertainty. Pitch disturbance rejection properties of variations of the sliding controller are investigated. Both single frequency and stochastic disturbances are used, and the stochastic disturbance is developed using the results of the earlier investigation.