On the Offset-free Nonlinear Model Predictive Control for AUV Docking*

The docking control of AUV is disturbed by model mismatch and sea current. This paper presents a method of AUV docking control based on the application of offset-free nonlinear model predictive control (NMPC) with multiple dynamic constraints. The controller drives the AUV from the initial pose to the entrance of the docking station (DS) through the coordinated motion of the AUV’s surge, sway, heave and yaw. In the process of AUV docking, the control input range, static space and field of view (FOV) of positioning sensor are the constraints of NMPC controller. A state observer is designed to estimate the system state and external disturbance, and the estimation is combined with NMPC controller to ensure the offset-free docking of AUV in the presence of model mismatch and external disturbance. Real-time iteration (RTI) approach is used to solve the on-line NMPC problem in real time. The offset-free NMPC controller is demonstrated in simulation studies using a six degrees of freedom (6-DOF) nonlinear model of AUV. The results obtained demonstrate that the proposed method can realize offset-free AUV docking control even under significant plant-model mismatch and external persistent current disturbance.