Path following of underactuated autonomous underwater vehicles in the presence of ocean currents

A control strategy allowing underactuated underwater vehicles to accomplish 3D straight line path following tasks in the presence of ocean currents is developed. The ocean current is considered constant, irrotational and acting in any direction of the inertial frame. The control technique is based on a modified three-dimensional Line-of-Sight (LOS) guidance law with two integrators and three feedback controllers. The integral effect is introduced to successfully counteract the drifting caused by the unknown current. The integration laws are chosen to reduce the risk of wind-up effects. Furthermore, due to the irrotational nature of the ocean current, it is seen that the vehicle dynamics can be defined in terms of relative velocities only. This significantly simplifies the control system compared to approaches based on absolute velocities, and as a result no adaption laws are required. Uniform global asymptotic stability and uniform local exponential stability for the closed loop system are proven and explicit bounds on the guidance law parameters are given.