Frequency domain analysis of plant model using closed-loop step response

In this paper, a simple scheme to estimate frequency transfer functions of linear time-invariant single-input singleoutput (SISO) systems based on closed-loop step response data is investigated. The method uses only a single set of step response data of plants under closed-loop operating conditions. The input and output of a plant are filtered by a simple bandpass filter, and the frequency transfer function of the plant is obtained with the Discrete Fourier Transforms (DFTs) of the filtered input and output signals. The validity of the proposed method is illustrated through simulations for some plants with a time delay, an unstable pole, or resonant frequencies and anti-resonant frequencies.