INCORPORATING INPUT SATURATION FOR SURFACE VESSEL CONTROL WITH EXPERIMENTS

Abstract

This paper presents a nonlinear model predictive controller (NMPC) for trajectory tracking of an underactuated surface vessel. The physical control inputs to the vessel are a propeller speed and a rudder angle. The limits of these control inputs are incorporated as input saturation limits. The controller is designed based on a three-degree-of-freedom nonlinear dynamic model of the vessel. The NMPC predicts control inputs by optimizing a cost function. The optimization is done by considering the input constraints. Tests for the identification of the vessel’s dynamic model are performed on a small robotic vessel and the model is verified. The performance of the designed controller is tested via experimentation with the robotic vessel on an outdoor pond. The result shows that under extreme manoeuvres where the saturation of inputs occurs, the control inputs of the vessel stay within the specified limits and the vessel converges towards the desired trajectory after recovering from saturation.