Improved critical point estimation using a preload relay

In this paper, we present a technique to automatically estimate the critical point of a process frequency response. The method yields significantly and consistently improved accuracy over the relay feedback method, pioneered by Astrom and co-workers, at no significant incremental costs in terms of implementation resources and application complexities. The proposed technique improves the accuracy of the conventional approach by boosting the fundamental frequency in the forced oscillations, using a preload relay which comprises a normal relay with a parallel gain. In addition, we will show empirically the other benefits of the proposed method in terms of assessment performance based on an improved estimate, applicability to other classes of processes when the conventional relay method fails, a shorter time duration to attain stationary oscillations, and possible application to extract other points of the process frequency response. The effectiveness of the proposed technique is verified by simulation results, and also demonstrated via real-time experimental results in the critical point estimation of a coupled-tanks system.