Fractional-order PI controller with relay auto-tuning method

Fractional-order PID (FOPID) controller has been proven to provide better performance compared to the classical PID by many researchers. So far, the implementation of FOPID especially in the tuning aspect had become an interesting topic in this area. Thus, this paper presents a simple but practical approach in tuning of fractional-order PI (FOPI) controller using relay method. This method is convenient when the process model cannot be determined accurately. Furthermore, it gives clear insight on the input-output response and hence, produced better results within the specified constraints. The FOPI controller was compared with the PI using the same settings on real-time steam temperature control. The FOPI has to be cascaded to an error filter to achieve faster output convergence. This issue was explained in the paper. Both PI and FOPI controllers were subjected to set-point change and load disturbance tests. Integral of squared error (ISE) and Integral of Control (ICO) signal indices indicate the superiority of FOPI over the classical PI.