Fast Fourier transform for estimating process frequency response

This paper discusses the online technique to estimate the process frequency response without breaking the closed-loop control. The extended method of relay feedback using fast Fourier transform (FFT) is measured input and output responses without disconnecting the controller from the loop, which significantly reduces the time required for the autotuning test. The method utilizes step response information and one period of limit cycle oscillation that induced under tight continuous closed-loop control. The frequency domain identification method is useful for certain critical applications where it is dangerous for the control loop to be broken for tuning purposes. Simulation examples are included to illustrate the efficacy of the proposed method.