Roll damping by rudder control-a new H∞ approach

Roll damping and simultaneous course steering by rudder control is a challenging problem where a key factor is roll damping performance in waves. Roll is a decisive factor for the operation of ships, both due to comfort of crew and passengers and due to requirements from cargo or on-board equipment. In the paper, roll damping and steering performance requirements are described and the controller design problem is formulated in an H_∞ framework. It is shown that the design problem includes zeros on the imaginary axis which cannot be dealt with in standard H_∞ theory. The paper explains the physical origin of the zeros and contributes with new results that can handle problems with zeros on the imaginary axis directly without using approximations in the H_∞ design. Finally, the results of controller design using the new method are analysed, and the controller performance is discussed using an existing LQ design as comparison