This paper presents the design of an infinite-order disturbance observer (IFDOB) to suppress disturbances including a periodic disturbance. An IFDOB includes the dynamics of a periodic disturbance to suppress it. However, a conventional IFDOB finds it difficult to suppress disturbances in the low-frequency domain. In contrast, a disturbance observer (DOB) can suppress disturbances in the low-frequency domain, however, it has difficulty suppressing a periodic disturbance. Therefore, this paper proposes a designed IFDOB that can suppress both a periodic disturbance and low-frequency disturbances. The proposed method is constructed by designing parameter γ of an IFDOB considering sensitivity and complementary sensitivity functions. Simulations were conducted, and the validity of the proposed method in a practical system was confirmed by experiments using a multi-axis manipulator.
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