A novel design method of an unknown disturbance observer for non-minimum phase plants is proposed in the present paper. In order to improve the estimation performance, we introduce the approach as a virtual augmented plant by adding a parallel model to the non-minimum phase real plant. The parallel model is designed so that the virtual augmented model becomes the minimum phase. Thus, it is possible to design the unknown disturbance estimator for the minimum phase plant but for the non-minimum phase plant. As the result, it is possible to improve the estimation performances. In this case, it is important to clarify the relationship between the unknown disturbance estimation signal for the real plant and the virtual augmented plant. In the present paper, the unknown disturbance estimation signal of the real plant is re-constructed by using the disturbance estimation of the virtual plant. And the parallel model design method is also proposed. The effectiveness of the proposed method is verified by numerical simulations for several mechanical vibration systems. The results show that the proposed method can improve estimation performances in comparison with conventional methods.

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