Generalized design of disturbance observer for non-minimum phase system using an H-infinity approach

In the paper, a generalized design of a disturbance observer formulated in the transfer function domain is presented for a class of single-input single-output (SISO) linear systems. Since conventional approaches seem to have difficulties in handling all general cases and the Linear Matrix Inequalities (LMI) based H-infinity optimization problem can deal with all linear plants, the proposed disturbance observer is generally designed using an H-infinity approach. First, a general frame of a disturbance observer is proposed when control input and system output are available. Then, from the frame, the H-infinity optimization problem for minimizing L₂-norm of a weighted disturbance estimation error is formulated. Furthermore, it will be shown that the proposed design methodology contains conventional approaches by deriving the robust stability condition of overall closed-loop using coprime factorization. Through numerical examples, the effectiveness and advantages of the proposed approach will be demonstrated.