Frequency response identification with cascade relay feedback

Abstract

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 effectively excite a process not only at the process critical frequency ω c but also at 0.5 ω c and 1.5 ω c . One necessary condition for the existence of stationary oscillations with two different fundamental frequencies is derived. The process frequency response at multiple frequencies can be accurately estimated from one relay test using the FFT algorithm. The proposed method is insensitive to noise and step-like load disturbance. Simulation shows its effectiveness. The technique can also be easily extended to identify other frequency response points of interest.