Incorporating input saturation for underactuated surface vessel trajectory tracking control

A Nonlinear Model Predictive Controller (NMPC) for trajectory tracking of surface vessels is presented. NMPC calculates the future control inputs based on the present state variables by optimizing a cost function. The fact that cost function incorporates input constraints as well as state errors in determining the control inputs is exploited. This method can be applied to all systems with input saturation. NMPC formulation and derivation of input constraints are presented. Here the controller is designed based on a 3 DOF nonlinear dynamic model of the vessel. The performance of the controller is demonstrated by simulations. A constant speed sine trajectory is defined as desired path and the simulation results for input saturation case show the control inputs (propeller speed and rudder angle of the vessel) remain within the saturation limits in extreme maneuvers, the vessel recovers from saturation, and the vessels follows the trajectory very closely when the inputs are not saturated.