Abstract
The paper describes the use of a recently introduced relay shifting method for estimation parameters of a second order time delayed model. The aim is to obtain a process model for setting PID control parameters. For this purpose, an algorithm is designed for estimation of model parameters from two frequency response points obtained from a single relay feedback test without any assumptions about a model transfer function. The relay shifting method is slightly modified here by using an integrator to receive the frequency response points in positions more suitable for model fitting. This modification enables to better estimate the static gain even under constant load disturbance. The proposed solution is demonstrated on simulated examples.
Highlights
If we want to control a process, we need to determine its properties and we can design its control
Åstrom and Hägglund were the first ones who proposed this method [1]. They proposed a relay feedback experiment where a process is under a relay control for finding the critical gain and the critical frequency of the closed loop process
This technique postulates a stable oscillation after the time tL in the relay feedback experiment with the period Tp (Tp=T1+T2, T1≠T2, see Fig. 3) and that the process can be described by a linear time invariant SISO model
Summary
If we want to control a process, we need to determine its properties and we can design its control. The method using the relay feedback control is one of the most commonly used methods for tuning PID controllers. Åstrom and Hägglund were the first ones who proposed this method [1] They proposed a relay feedback experiment where a process is under a relay control for finding the critical gain and the critical frequency of the closed loop process. This relay feedback approach enables to calculate the same parameters like the Ziegler-Nichols method [2] but without a priory information about the process, in a shorter time and in a controlled manner. Some relay identification methods do not consider problems caused by the influence from measurement noise, load disturbances and nonzero initial process conditions that are in practical applications often encountered
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