Observer based 3-D path following control of underactuated underwater vehicle in the presence of ocean currents

Spatial path following is an important but difficult technology for underwater vehicle in the complicated ocean environment, especially for an underactuated vehicle. This paper addresses the problem of 3-D Path Following of an underactuated underwater vehicle in the presence of constant unknown ocean currents. The nonlinear controller including dynamics and kinematics levels assuming that there is no current are derived firstly. The controller steers the vehicle along the path and make position and velocity errors between vehicle and virtual reference point on path converge to zero. Then, considering currents as the disturbance to vehicle kinematics, an observer is designed to obtain the unknown current estimate. The kinematics controller is revised to compensate the errors caused by the currents. Simulation results with and without currents are presented. The results show that the controllers and observer designed in this paper are effective and performing well.