Frequency response identification with cascade relay feedback

The standard relay arrangement can only identify one point (on the process frequency response) from the resultant stationary oscillations. In this paper, a new technique for process frequency response identification is presented, which can identify multiple points on the process frequency response from a single relay feedback test. A new relay, called cascade relay, is proposed and it can ectively excite a process not only at the process critical frequency ω_c but also at 0.5ω_c and 1.5ω_c. Guidelines for relay parameters selection are suggested. The process frequency response at multiple frequencies can be accurately estimated from one relay test using the FFT algorithm. The proposed method is robust to noise and step-like load disturbance. The technique can also be easily extended to identify other frequency response points of interest. Simulation shows its ectiveness.