A Design on New Error Compensator for Fractional PI Controller and Its Application

Fractional-order controller was proven to perform better than the integer-order controller. However, the absence of a pole at origin in the approximated transfer function produced marginal error in fractional-order control system. This study demonstrated the design of an error compensator that comprises of a very small zero and a pole at origin to produce a zero steady-state error for the closed-loop system. Some modification on the error compensator was suggested for different order fractional integrator that can improve the overall phase margin. The proposed method can eliminate the steady-state error and additionally improved the percentage overshoot of the closed-loop response. The study was validated on steam temperature control of a steam distillation process that exhibits an s-shape step response. Hence, the PI controller was tuned using the ZieglerNichols rules and applied for both integer and fractional PI. Significant improvement of the fractional PI and the enhancement of the proposed error compensator was discussed evidently through simulation and supported by experimental applications. Key-Words: Fractional-order control; Fractional PI; Oustaloup’s Recursive Approximation; Ziegler-Nichols tuning; steam temperature control; hydro-steam distillation.