Linear Multi-controller Structure for Control of a Nonlinear MIMO Model of a Drill Ship

In the paper a linear control system structure with modal controllers for multi-input, multioutput (MIMO) nonlinear dynamic processes is presented. The problems under study are exemplified by synthesis of a system for control a position and yaw angle of drillship described by a 3DOF nonlinear mathematical model of its low-frequency (LF) motions over the drilling point. The equations of the ship motions include the impact of the sea current and wind forces on the ship's hull. In the proposed control system structure a set of (stable) linear modal controllers is used that create an adaptive linear controller with variable parameters tuned appropriately to operation conditions chosen on the basis of two measured auxiliary signals. These are: the ship's current forward speed measured with respect to the water and systematically calculated the difference between the yaw angle and the sea current angle. The system synthesis is carried out by means of Diophantine polynomial matrix equations method for system pole placement after having linearized model of low-frequency motions made by the vessel at its nominal “operating points” in steady states. Furthermore, the control system has been extended by an additional integrating path to compensate influences of environmental disturbances. The final part of the paper includes simulation results of system operation with an adaptive controller of (stepwise) varying parameters along with conclusions and final remarks.