Nonlinear Observers and Output Feedback Control with Application to Dynamically Positioned Ships

The decomposition of nonlinear output feedback control into an observer and a state feedback control is an open problem. A solution for dynamic positioning of ships has been proposed by Fossen and Grøvlen (1998) and Grøvlen and Fossen (1996), where an observer-based backstepping method is used.

The suggested observer design in Fossen and Grøvlen (1998) and Grøvlen and Fossen (1996) does not cover unstable ship dynamics and we suggest a remedy for an extended class of ships. Typically thruster assisted mooring of large tankers does not fulfill the needed stability properties, so an extension to this case is motivated. We propose a method to extend the results, under a detectability condition. This condition implies stable surge dynamics, which is a natural assumption for ships. Finally, we discuss a nonlinear separation principle for stabilizing this class of ship dynamics, where we can reuse our nonlinear observer design.