A ship stabilizing fin controller based on the Internal Model Control (IMC) method is described in this paper. The proposed method directly shapes the output sensitivity function which relates the wave disturbance to the ship roll motion to achieve good disturbance rejection. The adopted IMC method gives a controller that depends explicitly on the model parameters and a design parameter that specifies the desired system response. Moreover, under the IMC structure, the sensitivity functions depend linearly on the design transfer function and this makes it easy to satisfy the internal stability condition. A reference conditioning technique is employed in the design that takes the limited actuator authority into the controller design consideration to avoid unknowingly pushing the actuator beyond its saturation and slew rate limitation boundaries. Numerical results indicate that very good roll reduction is achieved for sinusoidal disturbance and reasonable roll reduction is obtained for a narrow band type of disturbance generated by passing a white noise sequence through a second order shaping filter.

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