Modeling aspects of nonlinear steering machine of conventional ships

The conventional ships have one main aft thruster, and a rudder actuated by the steering machine (SM), which generates yaw moments to control the yaw angle. SM is a nonlinear system with absolute stability. Considering only yaw angle and mono-variable autopilots, SM can be simplified, taking into account only two limitations: the maximum rudder angle and rudder rate. But for smart autopilots, the nonlinear SM model with tri-positional hysteretic controller (THC) must be considered. In this paper, a new nonlinear SM model, with inertia of hydraulic mass driven by the servomotor, is proposed. Behavior aspects, compared with classic SM from literature, are studied. Both nonlinear and simplified linear proposed models have better behavior, with more realistic responses. Also, limitations and influence on yaw and roll motions are presented.