Comments on ``Nonlinear Output Feedback Control of Dynamically Positioned Ships Using Vectorial observer Backstepping''

• 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 in the above paper (1) and by Grovlen and Fossen, where an observer-based backstepping method is used. This note points out that the observer design in the above-mentioned papers does not cover unstable ship dynamics and suggests a remedy for an extended class of ships. The proof for the nonlinear observer used in the design in the above-mentioned papers only applies to ships with stable sway yaw dynamics. In the above-mentioned papers an example concerning thruster assisted mooring of a tanker is given, which does not fulfill the needed stability properties, so an extension to this case is highly 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.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Elektroteknik och elektronik -- Reglerteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Electrical Engineering, Electronic Engineering, Information Engineering -- Control Engineering (hsv//eng)

Nyckelord

Marine systems

nonlinear systems

observer design

stability

state estimation.

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