H∞ robust control design for dynamic ship positioning

Prerequisites to the successful application of the H_∞ control design technique to the marine dynamic positioning (DP) control problem are a realistic nominal model, relative bounds on modelling uncertainty, a set of frequency-dependent weighting functions to translate the control design specification into a control solution and a performance index to quantify the robustness. The authors propose procedures and algorithms to achieve these objectives. The H_∞ robust control design technique is then used to calculate a robust controller which has superior properties to the conventional DP systems. The trade-off between track keeping and station keeping is undertaken via appropriate weighting functions and a two-degree-of-freedom control structure is employed. The technique proposed shows that effective wave filtering can be obtained with guaranteed stability robustness. The advantage of the proposed control design technique is demonstrated using ship nonlinear simulations