Linear active disturbance rejection control (LADRC) has been paid much attention in academic and industrial fields. However, the selection of the order for LADRC controller design and the choice of parameter b0 in system correcting are key facts which are still being faced by designer. In this paper, an effective method about how to select the order of LADRC and the parameter b0 is given first. Frequency analysis often used by engineers for designing controller, the normal transfer function form of LADRC is constructed, so the loop gain, close loop transfer function and disturbance transfer function to a general high-order system are presented and can be easily used. The example shows that the proposed method is easy to apply and verified the lower-order LADRC can obtain the better effective than PID and high-order LADRC, and the frequency response analysis of a thermal power plant is elaborated and the simulation result indicates that LADRC has a strong robustness against the large variation of parameters in the plant mode.
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