A dynamic positioning (DP) system includes different control functions
for automatic positioning and guidance of marine vessels by means of
thruster and propeller actions. This paper describes the control funct
ions which provide station-keeping and tracking. The DP controller is
designed using model-based control, where a new modified LQG feedback
controller and a model reference feedforward controller are applied. A
reference model calculates appropriate reference trajectories. Since
it is not desirable nor even possible to counteract the wave-frequency
movement caused by first-order wave loads, the control action of the
propulsion system should be produced by the low frequency part of the
vessel movement caused by current, wind and second-order mean and slow
ly varying wave loads. A Kalman filter based state estimator and a Lue
nberger Observer are used to compute the low-frequency feedback and fe
edforward control signals. Full-scale experiments with a supply vessel
demonstrate the performance of the proposed controller.