Robust nonlinear control design for dynamic positioning of marine vessels with thruster system dynamics

Abstract

Considering the dynamics characteristics of thruster systems for dynamically positioned marine vessels with model parameter uncertainties and unknown time-varying ocean disturbances, the dynamic positioning (DP) control problem becomes a mismatched nonlinear control problem. To solve this problem, a DP robust nonlinear control law is developed using the command filtered vectorial backstepping combined with a disturbance observer (DO) and an auxiliary dynamic system (ADS). The DO provides the estimates of mismatched compound disturbances induced by model parameter uncertainties and unknown ocean disturbances. The ADS addresses the effects of thrust saturation constraints. The command filtered vectorial backstepping obviates differentiations of intermediate control function vectors due to introducing command filters and compensates for filtering errors of intermediate control function vectors caused by command filters by means of designed dynamic systems. The developed DP control law is computationally simple and the DP control performance is improved. Under the developed DP control law, the positioning errors of vessels are made as small as desired and all signals in the DP closed-loop control system are uniformly ultimately bounded. The high-fidelity simulation results on a supply vessel validate the developed DP control law.