Nonlinear Control and Disturbance Compensation for Underactuated Ships Using Extended Linearisation Techniques

Both in maritime and inland water transport, sophisticated nonlinear control is of increasing importance to contribute to a growth in transport capacity while maintaining safety and reliability. In this paper, two alternative nonlinear control approaches are presented that are based on the technique of extended linearisation and lead to gain-scheduled feedback controllers: whereas in the first approach a fixed assignment of the closed-loop eigenvalues is made, the second approach follows the idea of linear-quadratic control (LQR), which results in a varying eigenvalue configuration in the complex plane. The feedback control part is extended by appropriate feedforward control to reduce the tracking error further. To achieve accurate trajectory tracking control tasks despite external disturbances by cross currents or wind forces in lateral direction a nonlinear observer-based approach to disturbance compensation is proposed in this paper. As a consequence of the use of extended linearisation techniques, the model-based design of the whole control structure avoids time-consuming heuristic parametrizations of gain-scheduled controllers. The efficiency and the achieved tracking performance are shown by simulation results. Here, both tracking behaviour and rejection of disturbance forces in lateral direction are considered.