Relay Feedback Identification with Shifting Filter for PID Control

Abstract

The paper describes a recently introduced relay shifting method for process identification using a single relay feedback test. The aim is to obtain a process model for automatic tuning of PID controllers. This method is applicable for open-loop stable, unstable and integrating systems if there is sustained oscillation in a biased-relay feedback test. For this purpose the identification method uses a filter called a “shifting filter” which enables to estimate the next point on a process frequency characteristic. Furthermore, this additional point can be used to estimate the parameters of the process transfer function with multiple parameters, including static gain, even under a static load disturbance. In the paper, a new more robust algorithm for fitting a second-order time delayed model is introduced. It can be used for the PID controller design of the most processes describable by linear models. For the first time the shifting filter is also applied for the relay feedback identification of unstable systems. The method is demonstrated on examples of stable, unstable and integrating systems.