A General Design Methodology for Fractional Cascade Control Systems

In this paper we propose a design methodology for a cascade control system where the two controllers are of fractional-order-proportional-integral-derivative type. The tuning of the inner (secondary) controller aims to achieve a high performance (in terms of the integrated absolute error) in the load disturbance rejection task by exploiting the fractional-order derivative action. Then, a fractional-first-order-plus-dead-time transfer function of the inner feedback system in series with the primary process is estimated and the outer (primary) controller is tuned to maintain performance in rejecting load disturbances while achieving a satisfactory set-point response. Simulation results show the effectiveness of the methodology and the key role played by the additional flexibility in the design introduced by the fractional-order controllers.