Abstract
Generally, linear control theories are used in ship autopilot design. When the ship is directionally unstable and therefore nonlinear, the dynamics of the ship should be linearized approximately at its equilibrium point. This will however adversely affect its performnance in proportion to the distance between its operating point and the equilibrium point. Recently there have been many studies made relating to nonlinear control theories with favorable results. One promising result is the external linearization method. This is also referred to as the exact linearization method as differentiated from the approximate linearization method. As described in this paper, we have applied the exact linearization method proposed by Ishidori to the 2nd degree of Freedom Control with path planning and MRAS (Model Reference Adaptive System) and have evaluated its effects by numerical simulation. It is found that when 2nd degree Freedom Control with path planning is applied during fast course alterations, compensation must take into consideration transient response in addition to the nonlinear steady turning characteristics of the ship. An improved design method for the feed-forward controller is also proposed. Further, the affects of ship nonlinear directional instability on the adaptability of MRAS were also studied and it was demonstrated that the MRAS provides adequate adaptability under such conditions.
